back bay boston, part ii: groundwater levels · back bay boston, part ii: g r oundwater levels...

CIVIL ENGINEERING

PRACTICE •

Fall 1986 Volume 1, Number 2 ISSN: 0886-9685

Back Bay Boston, Part II:G roundwater Levels

Man-made structures that permanently lower groundwaterlevels can have adverse effects on

buildings with water table sensitive foundations.

HA R L P. AL D R I C H & JA M E S R. LA M B R E C H T S

1CIVIL ENGINEERING PRACTICE FALL 1986

JO U R N A L O F T H E BO S T O N SO C I E T Y

O F CI V I L EN G I N E E R S SE C T I O N/ASCE

Reprinted from Civil Engineering Practice:Journal of the Boston Society of Civil Engineers

Section/ASCEVolume 1, Number 2

Fall 1986Copyright @1986 BSCES

CIVIL ENGINEERING PRACTICE: JOURNAL OFTHE BOSTON SOCIETY OF CIVIL ENGINEERSSECTION/ASCE (ISSN: 0886-9685) is published twiceyear ly by the Boston Society of Civi l EngineersSection/ASCE (founded 1848). Editorial, circulation andadvertising activities are located at: Boston Society of CivilEngineers Section/ASCE, The Engineering Center, 236Huntington Ave., Boston, MA 02115-4701; (617) 536-2576. Known as The Journal of the Boston Society of CivilEngineers Section/ASCE until 1985.



Man-made structures that permanently lower groundwater lev-els can have adverse effects on build-ings with water table sensitive foundations.

HA R L P. AL D R I C H, JR. &JA M E S R. LA M B R E C H T S

The temporary or permanent lowering ofthe groundwater table can adverselya ffect both natural and constructed envi-

ronments, causing ground subsidence, floodingand damage to structures. The Back Bay sectionof Boston serves as an excellent site for thestudy of the causes and effects of groundwaterlevel diminishment, and provides ample reasonsfor the need to monitor and maintain groundwa-ter levels to preserve building foundations.

The second in a series of studies on BackBay, this article summarizes groundwater levelsin Back Bay since the area was filled more than100 years ago, and traces the effects of construc-tion of sewers and drains, subways and othertransportation corridors, and buildings on thegroundwater table. Part I described the geologyof Back Bay as well as subsurface soil condi-tions and the topographic development of the

area, concluding with a discussion of buildingfoundation practice through the turn of the cen-t u r y, a practice based primarily on untreatedwood piles.(1) Part III, now in preparation, willcomplete the series, documenting foundationdesign and construction practice from 1900 tothe present.

B a c k g ro u n dThis study focused on the geographical area

bounded by the Massachusetts BayTransportation Authority's Southwest CorridorProject (south), Charles Street (east),Massachusetts Avenue (west) and the CharlesRiver Basin (north). This area currently encom-passes the Back Bay Historic District (primarilybetween Boylston Street and Beacon Street) andthe central spine across Back Bay where majorprojects have been constructed during the past30 years. The South End neighborhood, locatedto the south of the Southwest Corridor, wasexcluded, primarily because little data ongroundwater levels exist for this area.

During the nineteenth century, a tidal estuaryof the Charles River known to Boston residents asthe Back Bay (see Figure 1) was filled to createland for an expanding population. Most of thehomes, churches and other buildings constructedprior to 1900 were founded on wood piles driventhrough fill materials and organic soils to bear inthe underlying sand and gravel or clay stratum.For the most part, the tops of these piles were

Case Study

Back Bay Boston, Part II:G roundwater Levels

32 CIVIL ENGINEERING PRACTICE FALL 1986

Figure 1. Amap of Back Bay Boston showing the location of the colonial coastline.


cut off below the water table at the time of con-struction with the expectation that they would bepreserved if permanently immersed below thegroundwater table.

With construction of sewers, drains, sub-ways and the basements of buildings below thewater table, some of which leak, the groundwa-ter level has dropped in Back Bay. Where woodpiles have been exposed to air for some time, thepiles have rotted when attacked by fungi, borersand other organisms. A few buildings have set-tled and cracked, requiring owners to underpintheir structures at great cost in order to restorethe foundations.

The Gro u n d w a t e r Ta b l eThe Webster dictionary defines the ground-

water table as the level below which the groundis saturated with water. In geotechnical engineer-ing, it is the stabilized static water level in anopen excavation, or in a shallow well orpiezometer, as illustrated by well A in Figure 2.In Back Bay, the water table generally occurswithin the fill stratum from 10 to 15 ft. below theground surface.

Three principal water bearing aquifersoccur in Back Bay, separated by impervioussoils. The lowest aquifer, a relatively thin, butapparently continuous stratum of outwash sandand gravel or glacial till underlying the Bostonblue clay, is relatively pervious. The middleaquifer is a compact gravelly sand stratum up to20 ft. in thickness confined between the blueclay and a near continuous stratum of org a n i csilt and peat. This pervious outwash materialoccurs primarily over the western and northernsections of Back Bay. It does not exist in theCopley Square area. The top aquifer is the arti-ficial fill, commonly a silty coarse to fine sand,placed during the nineteenth century. T h egroundwater level in the fill, the top aquifer, isthe principal concern in Back Bay.

In the westerly section of Back Bay, wherethe sand outwash stratum occurs below the rela-tively impervious organic soils, a second "watertable" may be present - one that may dif f e rfrom the water table in the fill. This situation isrepresented by well B in Figure 2. If the waterlevel in all wells or piezometers in the figureswere equal, then the groundwater would behydrostatic with depth.

"Normal" Groundwater Levels. If therewere no loss of groundwater by pumping andleakage into sewers and drains, and no addi-tions to the groundwater from leaking watermains and other sources, the probable "normal"water table in Back Bay would be as shown inTable 1.

In colonial times, Back Bay was a tidalestuary that had a mean water level approximat-ing the mean tide in Boston Harbor (el. 5.65Boston City Base (2)). Following the comple-tion of the Mill Dam along Beacon Streetacross Back Bay in 1821, and until 1880 whenmost of Back Bay was filled, water levels in thereceiving basin east of Massachusetts Av e n u ewere variable and generally below mean tide.

After Back Bay was filled, the groundwa-ter table would have been expected to riseabove mean tide level. The land mass wasbounded on the north by the Charles River andon the south by South Boston Bay, both tidal.Surface water from rainfall and snowmelt thatpercolated into the ground would have beenexpected to raise the water table unti l a

Figure 2. Atypical soil profile, with ground-water table. Well A shows the static waterlevel.


horizontal gradient in the fill was established toconduct groundwater by seepage toward the adja-cent bodies of open water. In the latter part of thenineteenth century, the groundwater level in BackBay was, in fact, approximately el. 8.0 ft.(3)

The construction of the Charles River Damin 1910 raised the mean water level in theCharles River Basin to el. 8.0. The Back Baygroundwater table would have then been expect-ed to rise further, perhaps to el. 9.0 or higheralong Boylston Street. Normal groundwater lev-els in the area between Charles Street andStorrow Drive would have then been from el.10.0 down to 8.5, as a result of groundwaterrunoff from the west side of Beacon Hill.

Major sources of groundwater in Back Bayare infiltration of rainfall and snowmelt, leakagefrom water mains, and recharge from man-madegroundwater recharge systems. The sand out-wash receives water from the fill by seepagedownward through the organic soil and by directflow from the fill through holes, trenches andother manmade "openings" excavated throughthe organic stratum.

Only a fraction of the annual precipitationactually enters the ground because more than 80percent of the Back Bay is covered by impervi-

ous surfaces such as streets, sidewalks and build-ings. Even in open, unpaved areas, only part ofthe precipitation enters the ground. A l t h o u g hmost of the precipitation in Back Bay becomesr u n o ff and is carried away by storm drains andsewers, the seasonal variations in the type, andlevel, of precipitation cause an annual fluctua-tion in groundwater levels up to about 2 ft. insome areas.

The Charles River may become a source ofgroundwater in the Back Bay when the watertable falls. However, seepage through the fill isseverely impeded by remnants of the Mill Damand the West Side Interceptor along BeaconStreet, and by the Boston Marginal Conduitunder Storrow Drive. Because the river level ismaintained at el. +7.5 to +8.0, its effect ongroundwater levels is essentially constant. T h eriver's influence on water levels in the filldecreases rapidly with distance from the river.

The relatively pervious sand outwash stra-tum also underlies the Charles River. The MillDam and Boston Marginal Conduit would notimpede recharging in this stratum. However,since the river bottom is also blanketed byorganic soils, its influence on piezometric waterlevels in the Back Bay outwash is uncertain.

TABLE 1

Probable “Normal” Groundwater Levels

Probable AverageTime Period Groundwater Level Comments

Pre-1800; before El. 5.7 (BCB) Back Bay was a tidalBack Bay was filled (mean tide level) estuary of the

Charles River

1880-1910; El. 5.8 at Above mean tide levelafter filling Charles River; due to infiltration of

el. 8± interior rainfall & snowmelt

1910-present; El. 8 at Charles River Basinafter Charles River Charles River; maintained at el. 8.0Dam completed el. 9.5± interior

Note: Assumes no loss of groundwater by pumping or by leakage into sewers, drains, foundations or basements; and no additions to groundwater from leaking water mains andother man-made sources.


Leaky pipes, particularly water mains, canbe significant localized sources of groundwater.Cotton and Delaney provided groundwater con-tours that indicated several mounds where waterlevels were as much as 5 to 10 ft. above sur-rounding areas.(4) The overall contribution tothe water table from leaking water mains may beabout equal to that from precipitation. Cottonand Delaney reported that Boston Wa t e rDepartment data from the early 1940s indicatedthat water main leakage would have provided anequivalent recharge of 0.73 million gallons perday (gpd) per square mile. This amount isapproximately equal to the recharge from 50 in.of precipitation per year, assuming a 30 percentinfiltration rate. Storm and sanitary sewers locat-ed above the groundwater table can also leak andcontribute to groundwater. Because they are notunder pressure, their effect is probably minor.

In several areas, permanent recharge sys-tems have been installed to help maintaingroundwater levels. Notable examples are ther e c h a rge systems at Copley Square and Tr i n i t yChurch. In these systems, surface drainage fromprecipitation is collected and directed to drywellsor reverse drains. Water then seeps back into theground through special piping systems.Temporary recharge systems have been used inareas adjacent to construction projects, notablythe Prudential Center, to prevent or correct low-ered groundwater levels caused by deep excava-tions and construction dewatering.

Loss of groundwater, and the resulting low-ered water levels in Back Bay, occur primarilyfrom leakage into sewers and drains, leakagethrough walls and floors of subway tunnels,underpasses, building foundations and otherstructures below the water table, and by pumpingfrom sumps. In addition, water levels may belowered temporarily by pumping from excava-tions in order to facilitate construction.

Adverse Effects of L o w e red Gro u n d w a t e r L e v e l s

Temporary or permanent lowering of thegroundwater table from man-made or natural caus-es have been shown to adversely affect buildings,

streets, underground utilities and other structures,as discussed by Aldrich.(5) Potential problemsapplicable to the Back Bay are illustrated in Figure3 and include:

• Deterioration of wood piles • Ground subsidence • Negative friction (drag) on piles

Deterioration or decay of wood piles is clear-ly the most serious potential problem associatedwith lowered water levels. As long as the watertable remains above the tops of the piles, and thewood and surrounding soil remain saturated, thewood will not rot. Under these conditions, untreat-ed wood piles can be considered to be permanent.

H o w e v e r, if the groundwater level dropsbelow the tops of the piles, favorable conditionsmay be present for plant growth and insectattack. A greatly increased supply of oxygen,combined with moisture and moderate tempera-tures, facilitate the growth of fungi. Grubs orwood borers, termites and other insects may alsoattack the "exposed" wood.

The butts of piles that are surrounded byfill, in particular sand and gravel as well as ashesand cinders, are more prone to rotting

FIGURE 3. Principal adverse effects of low-ered groundwater levels: (1) decay of woodpiles when exposed to oxygen; (2) groundsubsidence due to compression of organicsilt and peat; and (3) negative friction(drag) on piles when the ground settles


than are piles that are embedded in organic silt,peat and other relatively impervious soils. Whenthe water table drops, the fine-grained soilsremain saturated for a time, thus protecting thepiles from immediate deterioration.

The time required for significant deteriora-tion to occur, following a drop in groundwaterlevel below the tops of wood piles, is highly vari-able. It depends on the species of wood, the type ofsoil in which the piles are embedded, the amountof moisture, temperature and other factors.Exposure for a few months is not considered seri-ous. However, serious deterioration will probablyoccur after a drawdown period of 3 to 10 years.

Ground Subsidence. When the groundwaterlevel is lowered, the effective stress on soils thatoccur below the water table is increased.Buoyancy in the zone of drawdown is lost. Ifunderlying soils are compressible organic soilsor soft clays, these materials will consolidate asthe soil structure adjusts to the increase in theoverburden load. Settlement will also occur ifthe upper soils dry out and shrink when thewater table is lowered.

Most areas of the Back Bay have experi-enced one or more significant temporary ground-water drawdowns for the construction of sewersand drains, subways, foundations for buildings,and other excavations that have required pump-ing. For this reason, ground subsidence due tofuture temporary or nominal permanent loweringof the water table is not considered to be a seri-ous concern.

Negative Friction. All buildings in BackBay that are supported by piles driven throughfill and organic soils - whether they are woodpiles bearing in the sand and gravel outwash ormarine clay, or are long piles driven to bear inthe glacial till or bedrock - will experience nega-tive friction or drag loads when the ground sur-rounding the piles settles. The building may set-tle as a result. The potential adverse effects aremost pronounced for wood piles that derive theirsupport by skin friction in the marine clay.

While significant negative friction undoubt-edly developed in the nineteenth century fromthe compression of organic soils under theweight of overlying fill, and from

temporary groundwater drawdowns, this factor isnot likely to be a serious concern in the future.

Construction in Back BayThe construction and maintenance of

embankments, sewers and drains, transportationcorridors and buildings throughout Back Bayhave affected groundwater levels. The impact ofthis complex interconnected underground sys-tem, shown in Figure 4, on the water table can-not be appreciated without some knowledge ofeach component.

Mill Dam. The first significant filling inBack Bay took place in 1820 when the Mill Damwas constructed along Beacon Street fromCharles Street to Sewall's Point in Brookline,near the present Kenmore Square. From adescription given by Howe, a cross section of thedam can be developed as shown in Figure 5.

As a dam, the structure was relatively imper-vious to the flow of water from one side to theo t h e r, except where it has been breached locallyby construction in the past 100 years. However, ina longitudinal direction along Beacon Street, thestructure is probably very pervious.

Filling of Back Bay began in 1858 at thePublic Garden, continuing westward toMassachusetts Avenue by 1880. From 10 to 20 ft.of sand and gravel fill were placed over soft org a n-ic soils that were underlain by a deep clay stratum.Considerable ground subsidence occurred over along period of time from the compression of theo rganic soils and, to some extent, the clay.

Concurrent with the filling, a sea wall wasconstructed along the Charles River to createBack Street that parallels Beacon Street on thewater side. The top of this wall is clearly visiblefrom Storrow Drive. A similar wall was con-structed in about 1865 behind homes on thewater side of Brimmer Street on Beacon Hill.Both walls were composed of dry-laid graniteplaced on a timber platform and supported onwood piles (see Figure 6). It is probable thewalls were ballasted with stone or gravel, similarto the Mill Dam walls.

Construction of buildings followed closelybehind the Back Bay fill ing. All of these


FIGURE 4. The locations of sewers and drains, and major transportation alignments.


buildings were founded on untreated wood pilescut off typically at el. 5.0, approximately 2 to 3ft. below the groundwater table.(1)

Sewers and drains in Back Bay have con-tributed to at least localized depressions in thegroundwater table. Furthermore, dewatering forsewer construction undoubtedly caused extensivetemporary lowering of the water table in someareas. Plans of the principal existing sewers andconduits in the Back Bay are shown in figures ina report by Camp, Dresser & McKee.(7)

The earliest sewers and drains in Boston dis-c h a rged by gravity from the hills to adjacent tidalareas. Flow velocities were high and there werefew problems. With the development of the lowfilled-land areas like the Back Bay, the extension of

the sewer system created serious drainage problemsin Back Bay because of the area's flat gradients andground settlement.

Most house drains and sewers were belowbasement level, and when minimum slopes tostreet sewers and interceptors were provided, theoutfalls were rarely above low tide. As a result,the contents of the sewers were dammed up bythe tide during the greater part of every day.( Tide gates were commonly adopted to preventsalt water from flooding the lower reaches of thesewers.) Settlement of the filled land causednumerous breaks in sewer connections andreversals of slope. Deposits of sludge and debriswithin the sewers and in tidal areas accumulatedr a p i d l y, with their attendant health and odorproblems.

By 1868, the State Board of Health recog-nized a serious public health problem and, in 1875,the City Council authorized the Mayor to appoint acommission to study the sewage system and toplan for future needs of the city. The plan adoptedbecame the Boston Main Drainage System.

The Boston Main Drainage System, wasconstructed from 1877 to 1884. The principalfeature of these works was a system of intercept-ing sewers along the margins of the city toreceive the flow from the already existing sew-ers. These intercepting sewers drained to apumping station located at Old Harbor Point onDorchester Bay (Calf Pasture on ColumbiaPoint) where sewage was pumped to Moon

FIGURE 5. A typical cross section of the Mill Dam.

FIGURE 6 The sea wall along Back Street.


Island and discharged into Boston Harbor onoutgoing tides.

Existing combined sewers (storm water anddomestic sewage) in the northerly section of theBack Bay that formerly discharged into theCharles River at Beaver, Berkeley, Dartmouth,Fairfield and Hereford Streets were connected tothe West Side Interceptor that was constructedalong Beacon Street. Other sewers locates southof the railroads drained into the East SideInterceptor that follows Albany Street.

Design and construction of the West SideInterceptor is of articular interest (see Figure 4).It travels down Charles Street to Beacon Street,where it turns westerly down Beacon toHereford Street, then turns southerly downHereford and Dalton Streets to Falmouth Street,and then westerly to Gainsborough Street. In theBeacon Street area, the invert grade varies fromapproximatel el. 0 at Beacon and A r l i n g t o nStreets, to el. -2.4 at Beacon and HerefordStreets and to el. -4.7 at Huntington Avenue andGainsborough Street.

Excavation and dewatering for the con-struction would have been required to at least 2ft. below these grades, into the fill and org a n i csoil on Beacon Street; and to approximately el. -6.0 in the sand outwash stratum in Dalton andFalmouth Streets. So, over 100 years ago, if notbefore, the outwash stratum experienced its firstsignificant temporary drawdown. Significantground subsidence and negative friction on woodpiles undoubtedly occurred.

The intercepting sewers and the mains e w e r, from the upper reaches to the pumpingstation at Calf Pasture, varied in size fro 3 to10.5 ft. in diameter. The larger ones were circu-lar and the smaller ones were generally egg-shaped. The West Side Interceptor was egg-shaped, 57 in. wide and 66 in. high (see Figure7). Sewers were constructed with double or triplerows of mortared brick, and where piles wererequired, a timber platform was constructed andthe sewer was cradled on mortared granitemasonry. It is of considerable importance to notethat the intercepting sewers were constructedwith an underdrain pipe varying from 8 to 12 in.in diameter that was placed below the sewer to

control groundwater during construction. Observation wells in Back Bay at that time

indicated water levels similar to those measuredbefore sewer construction, but within 10 years,in 1894, areas were found where the groundwa-ter was as low as el. 5 or lower, indicating thatthere was leakage into low-level sewers or thatgroundwater was being pumped.

The Boston Main Drainage System wasdesigned with sufficient capacity to carry the esti-mated dry weather flow of sanitary sewage and asmall volume of storm water. Excess storm flowand diluted sewage from the West Side Interceptorwere discharged into the Charles River at numer-ous overflow outlets. Boston Marginal Conduit.With the construction of expensive homes alongthe Charles River, there were increasing demandsto eliminate the odors and nuisance of the tidalbasin. Under the Acts of 1903, a half-tide dam

FIGURE 7. A typical cross section of theWest Side Interceptor. Invert elevations atselected locations are noted.

was completed in 1910 at the location of the for-mer Craigie's bridge, where the Museum ofScience is now located. The dam was constructedwith gates and a lock to maintain the water level inthe Charles River basin at approximately el. 8.0.

As part of the dam project, the BostonM a rginal Conduit was constructed along theBoston side of the basin to collect flows fromStony Brook, and mixed sewage and storm wateroverflows from the West Side Interceptor that for-merly discharged into the river at the sea wall (seeFigure 4). Water was to be maintained at a lowlevel in the conduit by means of tide gates con-structed at the outfall below the Charles River dam.

The marginal conduit was constructed in a100-ft. wide earth fill embankment placed imme-diately north of Back Street, beyond the old dryrubble retaining wall. Presently, the conduit liesbeneath Storrow Drive. Over most of its length,it is a reinforced concrete horseshoe-shaped sec-tion 76-in. wide by 92 in. high, supported onwood piles, as shown in Figure 8.

The structure was constructed level with aninvert grade estimated at el. -1.5. Drawings indi-cated that it was built within a double row oftongue and groove wood sheeting that was dri-

ven into the organic silt and left in place. Again,a large diameter underdrain pipe was placed justbelow the marginal conduit to facilitate dewater-ing during construction.

When the Storrow Drive underpass wasbuilt in 1951, a portion of the conduit was relo-cated inland, away from the river. The relocatedsection, from Dartmouth Street to Mt. Ve r n o nStreet, was an 8-ft. diameter reinforced concretepipe with an invert grade at el. -1.5. An under-drain pipe was placed beneath this new pipe, andit was apparently connected to the old under-drain when the relocated section was tied in.

The Mill Dam, West Side Interceptor andthe Boston Marginal Conduit act as dams imped-ing the flow of groundwater from the CharlesRiver basin into the Back Bay. Furthermore,while relatively impervious perpendicular totheir axes, they can conduct groundwater withrelative ease in a longitudinal direction.

The present systeni of low level sewers wasconstructed throughout Back Bay between 1910and 1912. Underdrain pipes again were used asshown in Figure 9, which presents a section throughthe St. James Avenue storm and sanitary sewers. Bythat time, nearly 75 years ago, there was little


FIGURE 8 A typical cross section of theBoston Marginal Conduit.

FIGURE 9 A typical cross section of the St.James Avenue sewer.

doubt that groundwater leaked into sewers, that theproblem was widespread and that groundwaterlevels in Back Bay were controlled primarily bythis leakage.

Two major subways have been constructedacross Back Bay by the Boston Tr a n s i tCommission (now known as the MassachusettsBay Transportation Authority). Between 1912 and1914, the BoyIston Street subway tunnel wasbuilt, and from 1937 to 1940 the HuntingtonAvenue subway was added. Their locations areshown on Figure 4 on page 37.

The Boylston Street subway crosses BackBay from Massachusetts Avenue to Charles Street.Within this area, the bottom of the subway variesfrom approximately el. +3.0 at MassachusettsAvenue, to el. -19.0 between Arlington Street andHadassah Way (its lowest point) to el. -10.0 atCharles Street. Table 2 presents elevation and soilcondition data for the subway. A c r o s s - s e c t i o nthrough the structure between Berkeley andClarendon Streets is shown in Figure 10.

The structure was supported on a wide vari-ety of soils including the fill, organic silt, and nat-ural sand and gravel outwash. Where peat wasencountered, approximately between HadassahWay and Charles Street (a distance of 460 ft.),wood piles were driven to support the structure.

L.B. Manley, Asst. Engineer for the Tr a n s i tCommission at the time, reported on soil condi-t i o n s : ( 8 )

"As is well known, the land reclaimedfrom the Back Bay consists of sand and gravel filling resting on a bed of silt whoseupper surface lies at about grade 0, BostonCity base, or grade 100, Boston T r a n s i tCommission base. This layer of silt is continu-ous throughout the length of the subway, andattains a thickness of about 17 ft. at DartmouthStreet, and over 20 ft. in the Fens. BetweenExeter Street and Charlesgate East andbetween Clarendon Street and Charles Street,where it finally disappears, it averages about 8ft . in thickness. Below the sil t betweenMassachusetts Avenue and Hereford Street,and at Exeter Street, are pockets of peat from2 to 4 ft. in thickness. Another extensive body

of peat occurs between Arlington and CharlesStreets, where it attains a great depth.

"Below the silt and peat is a stratum of sandand gravel which also extends throughout thelength of the subway excavation except for alength of about 1,600 ft. between Exeter andClarendon Streets. This sand and gravel carriesl a rge quantities of water laden with sulphuratedhydrogen, which has been offensive to passersbyand injurious to the health of those working in it.This gas, as it leaves the surface of the water, isparticularly destructive to metal, and copperfloats in several of the temporary pump wellshave been corroded through at the surface of thewater in a few weeks' time by the action of thisgas. It is supposed that this layer of gravel is thesame as that which appears in the bed of theCharles River and affords an underground watercourse which tends to equalize the level of thegroundwater in the Back Bay."

During construction, a temporary draw-down of water levels both in the fill and in thesand-gravel stratum would have occurred. Wherethe subway route passed opposite to what is nowthe Prudential site, drawdown in the sand stra-tum to el. -10.0 is estimated.

Constructed between October 1937 and1940, the Huntington Avenue subway crossesunder Massachusetts Avenue as it enters BackBay and joins the BoyIston Street subway atExeter Street (see Figure 4). Within this area, the bottom grade of the subway structure


FIGURE 10. A cross section of the BoylstonStreet Subway at Sta. 58+00 betweenBerkeley and Clarendon Streets. (Ref. 21).


varies from el. -10.0 at Massachusetts Av e n u edown to el. -19.0 where the structure passesbelow the rai lroad tracks (and under theMassachusetts Turnpike Extension). Table 3presents elevation and soil condition data.

The subway was founded on the outwashstratum that extends from 5 to 12 ft. below thebot tom of concrete f rom Massachuset tsAvenue to the Turnpike. North of theTurnpike to Boylston Street, the structurebears on clay and organic soils, without pil-ing. During construction, the outwash stratumwas dewatered for the entire length of thesubway along Huntington Avenue to grades aslow as, or even below, el. -20.0. A very signif-icant drawdown of water level occurred over a

wide area, for a period of 2 to 3 years. A nobservation well at Massachusetts andCommonwealth Avenues, 0.4 miles away, wasreported to have dropped from el. 7.0 to el. 0in 1939.

Construction for the Huntington Av e n u esubway required extensive and prolonged dewa-tering to levels below any known constructionbefore or since. In addition, drains installed inthe tunnels of both subway lines have undoubt-edly collected groundwater that leaked into thestructure.

Construction of Storrow Drive in the early1950s included an underpass and traffic inter-change in the Berkeley Street area. This under-pass is approximately 1,300 ft. long between

TABLE 2

Boylston Street Subway

Approximate El. Top Soil ConditionsLocation Station of Rail at Bottom of SubwayKenmore Street 0+00 16.2 Sand & gravel fill(at Commonwealth) underlain by silt

Charlesgate West 6+00 -8.7 Silt underlain by sand(at Commonwealth) & gravel

Charlesgate East 10+00 -18.9 Sand & gravel; short(at Newbury St.) section of clay

Massachusetts Ave. 19+32 7.5 Sand & gravel fill(at Newbury St.) underlain by silt

Hereford Street 27+15 7.7 Silt over sand & gravel

Gloucester Street 31+55 1.9 Sand & gravel

Fairfield Street 37+15 -4.8 Sand & gravel

Exeter Street 43+75 -6.4 Silt over sand & gravel

Dartmouth Street 49+80 -6.5 Silt over thin peat overthin sand & gravel

Clarendon Street 56+05 -8.8 Silt over thin peat

Berkeley Street 62+25 -13.0 Sand & gravel

Arlington Street 69+10 -14.5 Clay

Hadassah Way 73+35 N -14.0 Fairly hard blue clayS -11.5 (peat between Hadassah Way

& Charles Street)

Charles Street 78+00 N -4.2 Blue clay & gravelS -5.0

Notes: Information was obtained from Boston Transit Commission Plans No. 10219, 10386, 10091, 10418,11157, 11159,11161 and 11162 of “Boylston Street Subway.” The bottom of subway structure varies from 4 to 5.5 ft. below top of rail. The subway is supported on wood piles from Station 71+82 to Station 76+41.


portals, with 300-ft. long approach ramps at eitherend. The road surface descends as low as about el.-4.0, about 15 to 17 ft. below the ground surface.

The underpass was designed to preventgroundwater lowering by the extensive use ofwaterstops. The structure was designed as a boatwith sufficient weight to resist hydrostatic upliftpressure. Invert slabs were up to 2-ft. 8-in. thick.Precipitation and other surface water is collectedin catch basins and cross drains that feed intopipes below the slab. These pipes transport waterto wet wells near each portal where the water isthen pumped into the Charles River.

Soon after completion, leaks were reportedin the reinforced concrete walls. In order to col-lect the infiltrating groundwater and improve the

appearance, gutters and false walls wereinstalled. The leaks were evidently neverrepaired. A significant volume of groundwater isapparently infiltrating into the underpass asrecent dry weather pumping volumes have beenreported to be about 20,000 gallons per day fromeach wet well.

The Massachusetts Turnpike Extension, asix-lane limited access highway, crosses theBack Bay. The highway was constructedbetween 1963 and 1966, and is located just northof the Conrail (formerly Boston and A l b a n y )railroad alignment (see Figure 4). The roadwaywas depressed 15 to 20 ft. below adjacent citystreets and developed areas. The road surfacedescends from about el. 11.0 at Massachusetts

TABLE 3

Huntington Avenue Subway

Approximate El. Top Soil ConditionsLocation Station of Rail at Bottom of Subway

Massachusetts Ave. 13+85 -6.0 12 ft. hard packedcoarse sand

Cumberland Street 21+50 -10.9 11 ft. hard packedcoarse sand & gravel

West Newton Street 26+65 -13.0 7 ft. hard packed sand& gravel

Garrison Street 32+00 -13.1 4 ft. hard packed coarsesand

B&A Railroad Tracks 37+50 -13.6 Hard yellow clay (sand(Mass. Turnpike pinches out at StationExtension) 37+50±)

Blagden Street 41+30 -10.7 4 ft. silt over medium(& Exeter Street) blue clay & sand

Boylston Street 44+50 -6.9 4 ft. peat over 8 ft.(& Exeter Street) fine sand over stiff

blue clay

Notes: information was obtained from City of Boston Transit Department Plans No. 17947,17943, 17936,17933 and 17914 of “Huntington Ave. Subway, Plan & Profile.” The bottom of thesubway structure varies from 3.5 to 6 ft. below top of rail. Footings, pedestrian passageway(Mass. Ave.) and bottoms of catch basins are deeper.


Avenue down to el. 6.0 at Tremont Street. The turnpike was designed to prevent a per-

manent lowering of groundwater levels belowabout el. 6.5 to 8.5, depending on the location.West of Huntington Avenue, an underdrain sys-tem was used to limit uplift pressures on theslab, Through the Prudential Center site, twolines of steel sheetpiling driven 5 ft. into the clayinhibit the flow of groundwater to the turnpikeunderdrain.

Because the road surface east of HuntingtonAvenue was lower, underdrains were not used. T h eturnpikestructure was designed for uplift as a boatsection, using a thick concrete slab to preventflotation. A drain was provided along the northwall to prevent groundwater levels from exceedingel. 8.5. Existing drains in the railroad alignment tothe south maintain water levels at about el. 7.0.

Southwest Corridor Project. This new trans-portation structure was constructed between1981 and 1985. It has two tracks for the relocat-ed Massachusetts Bay Transportation A u t h o r i t yOrange Line subway and three tracks for com-muter rail and Amtrak service. Through BackBay, the alignment followed parts of two originalrailroad embankments that were constructedacross the Receiving Basin in the mid-1830s (seeFigure 4 on page 37). From MassachusettsAvenue to Dartmouth Street, the new concretestructure was below ground in a 3,000-ft. Icingcut-and-cover tunnel that required excavations asdeep as 38 ft.- East of Dartmouth Street, thestructure extended about 10 ft. below formergrade. Depths of excavations and other data aresummarized in Table 4 on page 46.

Reinforced concrete slurry walls were usedfor lateral support of the sides for about 2,100 ft.of the tunnel excavation (see Figure 11). T h econcrete walls were 3-ft. thick and penetrated 8to 15 ft. into the clay stratum. They were used asthe tunnel's permanent outside walls. A l t h o u g hwater leakage did occur through some of the ver-tical joints between wall panels, there was noappreciable lowering of groundwater levels inadjacent areas.

In other deep excavation areas where adja-cent structures were further away from the exca-vation or absent, steel sheet-piling was used for

temporary lateral support of the excavation. Eastof Dartmouth Street, excavations were shallowerand soldier piles with wood lagging were used.Water seepage into these excavations temporari-ly lowered groundwater levels in adjacent areasas much as 12 ft.

Where concrete slurry walls were used, thetunnel was supported on a thick concrete invertslab bearing on compacted sand and gravel fillthat was used to replace unsuitable organic soils.East of this portion of the tunnel, the structurewas supported on precast-prestressed concretepiles driven through the clay to end bearing onglacial till or bedrock.

In order to allow groundwater movementacross the corridor structure, a groundwaterequalization underdrain system was installed.This system consisted of longitudinal drainsplaced 2 to 4 ft. below the pre-constructiongroundwater level on either side of the structure.Where slurry walls formed the tunnel walls, 8-in.diameter header pipes surrounded by crushedstone were connected to 8-in. galvanized steelpipes cast into the walls and connected beneaththe invert slab. In other areas, rectangular drainsof crushed stone wrapped in filter fabric wereconstructed beneath the invert slab and up theoutside of each wall in order to allow water toflow between longitudinal drains on tither side.

Major Buildings. The first major buildingswith deep basements in Back Bay were theLiberty Mutual and New England Life buildings,constructed in the late 1930s. Since that time,other buildings requiring excavation well belowthe groundwater table have been erected.

Te m p o r a ry Effects of BuildingConstruction Dewatering

Where excavations have been carried belowthe water table for building construction in BackB a y, the water table in nearby areas has been low-ered, in some cases by a significant amount. Ta b l e5 on page 48 summarizes pertinent information -dates of construction, location, foundation type,elevation of the deepest excavation, dewateringand drawdown - for major construction projectsgathered from the available literature, reports


and construction records. The locations of majordeep excavations for both buildings and sewerand transportation projects, and the approximateelevations of the bottoms of these excavations areshown in Figure 12 on page 50.

Copley Square Area. The sand Outwash isshown in Figure 4 on page 37 to be absent aroundand for some distance north, east and south ofCopley Square, which is generally near the intersec-tion of Dartmouth and Boylston Streets. T h e r e f o r e ,the principal source of water to excavations in thearea is by seepage from the fill. Drawdown in thefill is limited to the distance between normalgroundwater level and the top Of tile underlyingo rganic silt, generally less than 7 to 10 ft.Drawdowns of this magnitude were usually con-fined to areas near the excavation. The volume ofwater entering excavations has been small and tem-porary recharging has generally not been practiced.

Excavations for deep foundations haveadvanced from the early use of unsupportedslopes, often combined with deeper laterally sup-ported soldier piles and wood lagging, to themore recent use of steel sheetpiling and concretediaphragm walls installed in slurry trenches.

In the case of the New England Mutual LifeInsurance Co. building, a nearly 40-ft. deep exca-vation was opened in 1939 Over the western two-thirds of the block bounded by Clarendon andBerkeley Streets, and Boylston and NewburyStreets. Steel soldier piles and wood lagging wereused to support the sides of the excavation in theo rganic silt stratum, and the overlying fill was cutback to a stable slope. Surface water and ground-water were collected in troughs cut into the top ofthe organic silt stratum at the toe of slope behindthe sheeting, and sumps were used to dewater theexcavation, The building was founded on larg espread footings and mats bearing on the stiff crustof the clay at el. -17.0 to -22.0, in early classicexample of a "floating" foundation. Adjacent tothe excavation, groundwater levels in the fillwould have been lowered to the top of the under-lying organic silt, about el. 0. Water levels inobservation wells located immediately west ofthe site, at Clarendon Street, dropped 4 to 6 ft. toabout el. 2.0. There were no reports of adversee ffects to surrounding buildings.

The excavation, in 1947, for the JohnHancock Berkeley building was very

FIGURE 11. A cross section of the Southwest Corridor Project tunnel, taken at FollenStreet.


similar to that for the New England Mutualbuilding. The excavation was opened onBerkeley Street between St. James Avenue andStuart Street. Again, soldier piles and woodlagging were used to support the sides of theexcavation in the organic silt, while the overly-ing fill was cut to a slope of about 1.5 horizon-tal to 1 vertical. In order to intercept ground-water and surface runoff, an 8-in. pipe was

installed around the sides of the excavation in asand-filled trench located just above the org a n-ic silt. The excavation was dewatered usingthree caisson wells.

During construction, groundwater levelsin the fill adjacent to the excavation wouldhave been lowered to the top of the org a n i csilt, as they were during the construction atthe New England Mutual building. A 1 0 - f t .

Table 4

Southwest Corridor Project

Elevation of Elevation of Bottom of Structure Bottom of

Subway Amtrak Deepest Excavation

Gainsborough Street 6.0 3.3 3.6

Massachusetts Avenue 6.0 -6.0 -7.5

Blackwood Street -8.2 -14.4 -23.0*

West Newton Street -10.4 -16.7 -24.5*

Harcourt Street -10.4 -16.4 -24.5*

Yarmouth Street -4.7 -12.4 -14.4 -18.4**

Dartmouth Street 3.6 -4.0 -5.6 -12.4**

Clarendon Street 3.6 -5.0 -6.4 -9.6**

Berkeley Street 1.6 -5.0 -6.4 -8.4**

Chandler Street (to east end) -2.1 -3.0 -4.4

Notes: Information was obtained from 1981 and 1982 Massachusetts Bay Transportation Authorityplans for Contracts No. 097-115 and 097-120.*Removed organic silt to top of clay stratum.**Lower elevation for trench excavated for track drain pipe.


drawdown, to about el. -2.0 on St. JamesAvenue, would have occurred. Casagrandereported a 10-ft. drawdown in 1947 acrossBerkeley Street at the Liberty MutualBuilding.(9) To the west of the excavation,groundwater levels in the fill were also low-ered by 4.5 and 2.5 ft. at distances of 125 and300 ft., respectively.

An increase in the rate of settlement of

the Liberty Mutual building, located acrossfrom the site at Berkeley Street, was attributedto an increase in the effective stress in the claystratum caused by lowered groundwater levels,and to the effects of disturbance to the struc-ture of the clay from the driving of steel H-piles.(10) The building settled an additional0.5 in., about half of which was recovered inrebound when the groundwater returned to

Lateral Earth Support System Type of Foundation

None, excavation sloped Belled caissons bearing on clay crust

Steel sheet piling Rectangular pedestals bearing on clay crust

Concrete diaphragm wall Slab on compacted gravel borrowreplacement fill

Concrete diaphragm wall Slab on compacted gravel borrow replacement fill

Concrete diaphragm wall Slab on compacted gravel borrow replacement fill

Steel sheet piling Precast-prestressed concrete piles to glacial till/bedrock

Soldier piles & lagging Precast-prestressed concrete piles to south (Amtrak) side. to glacial till/bedrockNone to north side.

Soldier piles & lagging Precast-prestressed concrete to south side. None to piles to glacial till/bedrocknorth side.



ion


Table 5Temporary Effects of Major Back Bay Construction on Groundwater Levels

Site Dewatering

Probable Method Construction Years of Lowest or Project Location Dewatering Elevation RemarkSewers & Drains

West Side Charles St. to Beacon 1877-1884 Varied el. -2.0 Underdrains Interceptor to Hereford to Falmouth to el. -7.5 & sumps sheeting

to Gainsborough StLow Level Sewers Throughout Back Bay 1910-1912 Underdrains & sumps

West Side At Christian 1968-1969 el. -7.5 Wellpoints Interceptor Science Church in sandRelocation CenterSubways

Boylston Street Boylston St. from 1912-1914 el. -19.0 Deepest between Subway Arlington St. to Arlington St.

beyond Mass. Ave. & Copley SquareHuntington Beneath Exeter St. 1937-1940 el. -20.0 Lowest under Avenue Subway & Huntington Ave Mass. Turnpike

to beyond Mass. Ave. & railroad

Huntington Huntington Ave. near 1969-1970 el. -15.0Avenue Subway West Newton St.Prudential Entrance

Southwest Mass. Ave. to 1981-1985 el. -15.0 None required Corridor Harcourt St. to el. -28.0Project

Harcourt St. 1982-1985 el. -28.0 Sumps in to Berkeley St. el -4.0 excavation

Buildings

Christian Norway St. 1931-1934 el. -3.0 Wellpoints Science to el. -6.0 in SandPubl. HouseNew England Clarendon St. 1939-1940 el. -21.0 Sumps in Mutual Life between Newbury excavationIns. Co. Bldg. & Berkeley Sts.John Hancock Berkeley St. between 1946-1947 el. -25.0 Sumps in Berkeley Bldg. St. James & Stuart Sts. excavationBoston Herald Harrison Ave. between 1957 Deep in sand Traveler Bldg. Herald & Traveler Sts. above bedrockChristian Science Norway St. 1958 el. -3.0 Wellpoints Publ. House adjacent to in SandUnderground Mother ChurchEquip. Rm.Prudential Center of Prudential 1959-1960 el. -12.0 Wellpoints Center Tower Center Complex in SandSheraton Hotel Dalton & 1962-1963 el. -4.0 Wellpoints at Prudential CenterBelvidere Sts. in Sand180 Beacon St. Beacon & 1964-1966 el. -14.0 UnknownApart. Bldg. Clarendon Sts.Christian Huntington Ave. 1967-1968 el. -5.0 Open pumping Science between Belvidere & in excavationAdmin. Bldg. Cumberland Sts.Christian Science Clearway & 1968-1969 el. 0.0 Shallow, Colonnade Bldg. Belvidere Sts. probably by sumps

John Hancock Clarendon, between 1968-1974 el. -28.0 Sumps Tower St. James & within site

Stuart Sts.Christian West Side of 1973 el. -4.0 Wellpoints Science Church Mother Church in SandNew PorticoSymphony Plaza Mass. Ave. at 1977 el. +4.0 Sumps in pile Apartments St. Botolph St. to el. -3.0 cap pitsGreenhouse Huntington Ave. 1981 el. -2.0 Sumps in pile cap Apartments at W. Newton St. & elevator pitsCopley Huntington Ave 1981-1982 el. -4.0 Sumps in pile Place between Harcourt cap excavations

& Dartmouth Sts.One Exeter Place Boylston & Exeter Sts. 1982-1983 el. 0.0 Sumps in excavation


Lateral Earth Effect of Pumping on Support System Groundwater Levels

Lowest Stratum Distance from

Type Penetrated Excavation Drawdown Other Remarks

Probably wood Organic Silt No records available Significant drawdown in & Sand adjacent areas probably

occurred. See Figure 4.No records available Significant drawdown in

adjacent areas probably occurred.Steel sheet Clay 100 ft. 8 to 10 ft. in Sand Recharge around Christian piles 700 ft. none in Sand Science Church eliminated

100 ft. 3 ft. in Fill drawdown in Fill.

Steel sheet Clay No reports available Siphon pipes installed to allow structure. pile groundwater movement across

See Table 2.Steel sheet Clay 200 ft. 13 ft. in Sand Dewatered for 3 yrs. WPApiles 800 ft. 12 ft. (after 2 yrs.) data indicated large area

1100 ft. 8 ft. in Sand affected (Mass. Ave. to 0.4 mi. 7 ft. in Sand Dartmouth St.). See Table 3.

Steel sheet 300 ft. 18 ft. in Sand Dewatering for 5 months. piles 400 ft. 4 ft. in Sand No drawdown observed in

800 ft. 6 ft. in Sand Fill.1000 ft. 8 ft. in Sand 1200 ft. 6 ft. in Sand 1400 ft. 4 ft. in Sand

Concrete Top of Clay None observed that was Excavation open 1-2 yrs. diaphragm walls attributable to corridor Observ. wells at each street.

tunnel construction. See Table 4. Sheet piles, soldier Organic Silt Rises to el. -4.0 beams & lagging east of Dartmouth St.

Unknown Sand 300 ft. 8 ft. in Sand 300 ft. 2.5 ft. in Fill 1000 ft. 4-5 ft. (in Sand?)

Soldier piles Clay Nearby 4-6 ft. in Fill No Sand at site.& lagging

Soldier piles 90 ft. 10 ft. No Sand at site.& lagging 140 ft. 7 ft. in Fill

Unknown 1 mi. 30 ft. in Dewatering for deep sand caissons to rock.

Steel sheet Sand, At excavation 11 ft. in Sand Little drawdown piles halfway 500 ft. 8 ft. in Sand in Fill.

through 1200 ft. 4 ft. in Sand

Steel sheet Clay Nearby 16 ft. in Sand Recharged Fill & Sand piles 1-2 ft. in Fill outside sheeting.Steel sheet Clay 400 ft. Initially, Recharge system piles 3.5 ft. in Sand correction eliminated drawdown.Concrete Unknown Nearby 12-15 ft. in Sand Slurry wall leaked. slurry walls Recharged Sand outwash unsuccessful.

Unknown 200 ft. “Some” in Fill 500 ft. 0 ft. in Fill

Unknown “Minor” in Fill No data available on off-site drawdown.

Steel sheet Clay Near Negligible in Fill No Sand at site.piles

Partially Sand 40 ft. 8 ft. in Sand No drawdown observed sheeted 90 ft. 4.5 ft. in Sand in Fill.

230 ft. 5.5 ft. in SandUnknown Nearby 3 ft. in Fill

Soldier pile Organic 40 ft. 3 ft. in Fill& lagging SiltSoldier pile Organic Near Minor in Fill Only pile cap & lagging Silt excavations went

below groundwater.Steel sheet piles Clay Nearby Negligible in Fill


pre-construction levels. Construction of the John Hancock To w e r,

begun in late 1968, required excavation to el. -28.0 (45 ft. below ground surface).(11 )Interlocking steel sheetpiling was drivenapproximately 5 ft. into the clay to form a cof-ferdam around the site. The sheeting extended toground surface, unlike excavations at the NewEngland Mutual and the Hancock Berkeley

buildings where the fill was sloped down to thelaterally-supported organic silt. Prior to con-struction, during October and November 1967,water levels at the site varied between el. 4.5and 6.0, with an average of el. 5.0.

Contrary to experience at the PrudentialC e n t e r, there was no significant drawdown ofthe water table beneath the streets surround-ing the site. Water levels measured in the fill

FIGURE 12. The locations and elevations of major deep excavations in Back Bay.


during construction were generally from el. 4.0to 5.0. Immediately behind the steel sheeting, alocal drawdown exceeding 10 ft. was measuredin a piezometer installed in the organic silt. Ve r ylittle pumping was required inside the sheetingand no recharging was performed. Drawdown atthe John Hancock Tower was insignificantbecause steel sheet piling was used and the sandoutwash stratum was absent.

Foundation construction for Copley Placebegan in 1981 on a 10-acre site bounded byHuntington Avenue, the Southwest Corridor,Harcourt and Dartmouth Streets. The lowestfloor level was established at el. 6.6, somewhatabove the pre-construction groundwater table.Dewatering was required only for the construc-tion of pile caps and a deep water main.Excavation and dewatering for one large pile capbelow the Westin Hotel was carried to approxi-mately el. -5.0, using steel sheeting for lateralsupport. Elsewhere, the sides of the excavationwere either unsupported slopes or supported withsoldier piles and wood lagging.

The impact of dewatering on groundwaterlevels adjacent to the site was minor. An obser-vation well on Blagden Street adjacent to theBoston Public Library dropped temporarilyabout 3 ft. However, there was no observabledrawdown at Trinity Church.

A q u a r t e r-mile north of Copley Square, atthe corner of Beacon and Clarendon Streets, sig-nificant construction was required for a 17-storyapartment building constructed in 1964-1966 at180 Beacon Street. Here, soil conditions weremuch like those in Copley Square. Concretediaphragm walls installed in slurry trenches wereused both for lateral support of the sides of exca-vation and as permanent basement walls, the firstsuch use in Boston. The 2-ft. thick reinforcedconcrete walls were internally braced and sur-rounded the site. These walls penetrated aboutseven feet into the clay stratum, which is over-lain at this location by 5 ft. of sand outwash. Thesite was excavated down to about el. -20.0 forthe 3-1/2 basement levels required (12 to 14 ft.above the outwash stratum).

Leakage through the concrete wall wasapparently the cause of a 12 to 15-ft. draw

down in observation wells installed in the out-wash on adjacent property. Water from citymains was pumped into the outwash throughfive 2-inch diameter recharge wells, but withonly moderate success in raising the piezomet-ric head. The extent to which the perched waterlevel in the fill was affected is not known.Some minor settlement of an adjacent woodpile-supported 10-story apartment building wasattributed to the construction, but the cause wasnever clearly established.

The Christian Science Church Center issouthwest of Copley Square. In this area, thepervious sand outwash stratum is fully devel-oped to a thickness of 12 to 20 ft. Most of thestructures built in the Christian Science com-plex have been founded, in one way or another,on this outwash. The Mother Church wasfounded on untreated wood piles. Dewateringof the confined outwash aquifer had beenrequired during construction of several build-ings constructed in the last 55 years in thiscomplex.

In 1932, excavation for the 100 by 630 ft.Christian Science Publishing House building onthe former Norway Street was carried as low asel. -6.0 for spread footing construction on thesand outwash. Dewatering by wellpoints in theoutwash dropped the piezometric head an esti-mated 10 to 15 ft. for approximately eightmonths. The lateral earth support system usedfor this excavation is not known.

Water levels in the confined outwashaquifer responded quickly to the dewatering overa large area. The water level in an observationwell located 1,200 ft. Southwest of the sitedropped 4 to 5 ft. in two weeks, while at theMother Church, the water level was lowered 8 ft.The effects of construction dewatering in the fillwere much less, with the water table droppingonly 2.5 ft. at the Mother Church.

Drawdown in 1958 for the excavation ofthe Chris t ian Science Publ ishing HouseU n d e rground Equipment Room adjacent tothe Mother Church was similar to that experi-enced in 1932 during Publishing House con-struction. Excavation extended to el -3.0 inthe sand outwash. Lateral earth support was


provided by steel sheet piling that penetratedpart way through the sand. Wellpoints wereused inside the sheeting to dewater the out-wash to approximately el. -4.0. Water levelsin observation wells in the outwash 500 and1,200 ft. from the site were lowered to el. -1.0and 3.0, respectively.

As part of a major construction program atthe Christian Science Church between 1969 and1972, an approximately 1,000-ft. long section ofthe West Side Interceptor was relocated into ag a l l e r y. Construction was carried out betweentwo rows of steel sheet piling, 16 ft. apart, anddriven 5 ft. into the clay stratum. The bottom ofthe foundation for the gallery was el. -7.5 in thesand outwash. Dewatering was accomplished bywellpoints installed inside the cof f e r d a m .Drawdown in the sand stratum at the nearbyMother Church was initially to el. -2.0. Sixl a rge-diameter recharge wells installed aroundthe Mother Church raised piezometric levels toabout el. 3.0. Later, as recharge became ineffec-tive, outwash water levels fell back to el. -2.0and -3.0. In the fill stratum, the initial drawdownto el. 3.0 was successfully recharged to el. 6.0.

For the 1973 construction of a new porticofor the Mother Church, the last major project atthe Christian Science complex, foundations atthe front of the Mother Church were under-pinned with six concrete piers bearing on theoutwash at el. -3.5. Dewatering by wellpointslowered water levels in the outwash by 8 and -5.5 ft. at distances of 40 and 230 ft. from theexcavation, respectively. There was no observ-able drawdown in the fill.

The Prudential Center is immediately westof Copley Square. Construction began in 1959with a 52-story tower. The entire development,the largest in Boston at the time, was enclosedwithin a wall of interlocking steel sheeting thatwas reported to have been driven 5 ft. into theclay stratum to form a relatively impermeablec o fferdam. Internally, the area was divided bysheet pile walls into several cells.

A parking garage was constructed beneathmost of the Prudential Center, with the lowestfloor level at el. 3.0. A portion of the slab wassupported on compacted sand and gravel fill

that was placed after the organic soils wereexcavated. This excavation and backfill opera-tion, and other excavations requiring dewater-ing, were accomplished with wellpoints.

Drawdown in the outwash sand just outsidethe sheet piles was reported to have been to aslow as el. -12.0. Construction specificationsrequired recharging outside the sheeting to main-tain groundwater levels at or above el. 5.0. Therewere considerable problems with recharging thesand outwash, and it was only moderately suc-cessful. However, there were no significant prob-lems in maintaining water levels outside thesheeting in the fill.

In 1969 and 1970, an area at the edge of thePrudential Center was dewatered for the con-struction of a new entrance to the HuntingtonAvenue subway. Excavation and dewateringwere carried out to el. -15.0 in the sand outwash.Dewatering lowered water levels in wells at theMother Church and Massachusetts Avenue to el.-7.0 and 0.0 (1,000 and 1,200 ft. away, respec-tively). Drawdown in the sand outwash wasreported to have caused the Prudential Centergarage to settle 0.5 in. Here again, most of thesubsidence was recovered after dewatering whenwater levels returned to pre-construction lev-els.(9,12)

E ffects from Outside Back Bay. In 1957,the Boston Herald Traveler Corporation startedconstruction of a new building at 300 HarrisonAvenue, well outside of the fill area to the east ofCopley Square. Dewatering of the glacialtill/outwash strata occurring below the clay wasrequired for construction of deep caissons.Because these materials are relatively pervious,piezometric levels in the till and outwash werelowered significantly in Back Bay. Water levelsin deep observation wells at the PrudentialC e n t e r, approximately one mile away, droppedas much as 30 ft. within a month of the start ofpumping.(9) This drawdown over a period offive months caused about 0.1 and 0.3 in. of set-tlement at the New England Mutual Buildingand the Liberty Mutual Building, respectively,located about 0.8 and 0.6 miles from the HeraldTraveler Building.(10) Most of the subsidencewas recovered by rebound after dewatering.


Overall, for sites where the sand outwashstratum was absent, for example in the CopleySquare area, deep excavations have been success-fully accomplished within steel sheet pile coff e r-dams without significant groundwater drawdownin the fill and without having to install recharg esystems. Where the outwash occurred to the west,even the use of steel sheeting had not preventeddrawdown in the pervious sand stratum becauseof leakage through untensioned interlocks.

Drawdown in the sand outwash stratumgenerally extended 5 to 10 times further from adeep excavation than groundwater drawdown inthe fill. Water levels in the outwash were loweredsignificantly at distances of 1,000 ft. or morefrom an excavation. In the fill, however, draw-down greater than one foot did not usually occurat distances beyond 400 ft. Drawdown in the fillwas limited by the depth to the organic silt stra-tum. Groundwater recharge systems have beensuccessfully used to limit lowered water levels inthe fill. Similar systems have not been eff e c t i v ein the outwash stratum.

Historical Gro u n d w a t e rLevels in Back Bay

Groundwater levels in Back Bay are influ-enced by the natural process of precipitation andinfiltration, and by water levels in adjacent bod-ies of water. If there were no man-made struc-tures, the water table would be relatively uni-form across Back Bay and would vary little withtime, being affected only by the amount of pre-cipitation and local infiltration.

Construction over the past 100 years - sew-ers and drains, dams, transportation corridorsand building foundations described above - havediverted or withdrawn groundwater, have imped-ed its flow and, in other respects, have influ-enced the water table. Groundwater levels arenon-uniform, complex and have varied substan-tially in localized areas over time. Therefore, theinterpretation of groundwater data can be con-fusing, frustrating and misleading.

Concern for groundwater levels in Back Bayhas prompted sporadic action during the past 100years. Area-wide studies were made before andafter construction of the Boston Main Drainage

System in the late 1800s, during the 1890s for theCharles River Dam, in the late 1930s under a W PAprogram, by the USGS in 1967 and 1968, and in1985 for the BRA." In the past 25 years, numerousstudies have been undertaken in local areas forbuilding construction, most recently in 1985. 18H o w e v e r, there has been no long-term study.

1880s Study. Stearns reported that groundwa-ter levels in wells installed in 1878 before construc-tion of the main drainage system were practicallythe same in 1885, one year after construction, withwater levels "nearly level at Grade 7.7 over thewhole district" (P. 26)."(3) The data indicated levelsbetween el. 6.7 and 8.5. Engineers of that time real-ized the importance of maintaining groundwaterlevels and were concerned about the effects of thenew main drainage works on the water table.

1894 Charles River Dam Study. Water lev-els were measured in wells installed for an 1890sstudy of the proposed Charles River Dam asreported by Stearns.(3) Generally, groundwaterlevels were similar to those measured between1878 and 1885. Stearns blamed leaky sewers forsome levels below el. 5.0, but considered theseinstances to be local and isolated. He recom-mended that el. 8.0 be established as the waterlevel for the new Charles River Basin. In discus-sions to Worcester's 1914 paper, Gow andStearns cited leaky sewers as a cause of localgroundwater depressions.(8)

1930s Copley Square Study. In 1929, publico fficials and residents noticed several alarmingcracks in the Boston Public Library Building, andsettlement of the stone platform in front of thelibrary facing Copley Square on DartmouthStreet. Investigations by the Building Departmentand consulting engineers found that the tops ofmany wood piles supporting the building werecompletely rotted away or badly decayed.(14,15)Piles were originally cut off at approximately el.5.0 and groundwater was found to average el. 4.0at the time of underpinning.

Rotted piles below approximately 40 per-cent of the building area were cut off to soundwood and were posted with 6-in. steel H-sec-tions bearing on steel plates and wedgedagainst the underside of the stone foundation.The cost of this underpinning in 1929-30 was


reported to be "nearly $200,000." The discovery of rotted wood piles under the

Library sparked renewed interest in groundwaterlevels, especially among the Trustees of the nearbyTrinity Church that was constructed in 1876 on4,500 wood piles. Numerous observation wellswere installed in the Copley Square area, showingwater levels as low as el. 2.0.

When contours of equal water level wereanalyzed (see Figure 13), the loss of groundwaterwas traced to leakage into a 30-in. diameter seweron St. James Avenue (see Figure 9). Constructionof a partial dam in the sewer on Dartmouth Street,where it joins the Boylston Street sewer in front ofthe Public Library, caused observation wells to risei m m e d i a t e l y, proving without a doubt the sourceof the lost groundwater.

F o r t u n a t e l y, Trinity Church was spared.Excavations to examine the condition of woodpiles, originally cut off from el. 5.0 to 5.5, showedno significant deterioration. The structure had set-tled nearly one foot in 50 years and pile butts werenow lower. This case history was documented byRobert Treat Paine in 1935.11, In addition, Snowtraced the loss of groundwater in the area.(14)

1936-1940 W PA Surveys. The city of Bostonmeasured groundwater levels throughout theBoston Peninsula between 1936 and 1940. T h eproject was funded by the Works ProgressAdministration under projects No. 5325 and No.188868. The impetus for this study was the grow-ing concern about groundwater levels in the cityduring the 1920s and 1930s, heightened by the dis-covery of the rotted wood piles at the BostonPublic Library.

Observation wells installed for the W PA p r o-ject and wells previously installed by the BostonSewer Department were monitored. T h r o u g h o u tthe Boston Peninsula, a total of approximately 700observation wells were used in the W PA s u r v e y.Approximately 300 of these wells were located inBack Bay. A report prepared for the BostonRedevelopment Authority (BRA) contains tablesand plans that describe the location of each well,and the highest and lowest water levels recordedduring the four-year monitoring period."U n f o r t u n a t e l y, complete records of all water levelsrecorded during the program are not available,

since they were destroyed in a fire at Boston CityHall. Figure 14, on page 56, prepared from contourplans by Cotton and Delaney, shows areas in BackBay having water levels below el. 5.0 during that4-year period.(4)

Most wells in Back Bay experienced a waterlevel below el. 5.0, and, in seven wells, the highestwater level measured was also below el 5.0. Localdrawdowns from leaking sewers are the mostprobable cause for the low water levels in theseven wells. Precipitation during the period wasabout average for Boston, with yearly deviationsup to 6 in.

Significant construction projects during1936-1940 included the Huntington Avenue sub-way that dewatered the sand outwash to el. -20.0or below; the Liberty Mutual building, with exca-vation below el. 0; and the New England Mutualbuilding where excavation extended to el. -21.0.These projects cannot, however, account for thebroad extent of low water levels in Back Bay.Leaking sewers and pumping from sumps in base-ments of buildings undoubtedly were major con-tributors.

Dewatering of the pervious outwash stratumfor subway construction was probably responsiblefor the lowered groundwater levels north of theSouthwest Corridor alignment from MassachusettsAvenue to Clarendon Street. Groundwater draw-down immediately adjacent to the HuntingtonAvenue excavation is not known because data arenot available for wells there during the construc-tion period.(4)

Throughout much of this area, the outwashstratum is particularly well developed and is sepa-rated from the fill by a relatively thin layer (as lit-tle as 3 ft.) of organic silt and/or peat. However, inmany locations, where trenches and holes havebeen excavated, the outwash and fill strata are con-nected and lowered water levels in the outwashcan directly affect water level in the fill. Some ofthe W PA observation wells may have beeninstalled into the outwash stratum. Water levelsobserved in some wells may, therefore, have beenlower than groundwater levels in the fill.

In the Copley Square area, south ofBoylston Street between Dartmouth andBerkeley Streets, the low groundwater levels


may have been caused by leakage into sewers anddrains and pumping from sumps in building base-ments. Because the outwash stratum generallydoes not extend into this area and the BoylstonStreet subway structure forms a barrier to ground-water seepage from across the street, low ground-water levels in this area were probably not relatedto construction dewatering and drawdown.

The St. James Avenue sewer has a historyof causing local groundwater lowering.Groundwater levels have also been lowered bydrainage in a crawl space along the easterly sideof the John Hancock Clarendon building thatreduces hydrostatic pressures on basement floorsand walls. Until 1984, the water level had beenheld at el. -0.5. It has been raised somewhat withrecent renovations to the structure. Sump pump-ing has also been performed at the YWCA build-ing at Stuart and Clarendon Streets.

In other areas, low groundwater levels were

probably due to leakage into sewers. The We s tSide Interceptor beneath Beacon and CharlesStreets may have been responsible for low ground-water levels in that area. Low groundwater levelsalong Tremont Street in the South End were proba-bly caused by leakage into major sewers that joinin that area. Some water levels there were as lowas el. 0 to -3.0. In this area, there were also severalgroundwater mounds, probably due to water mainleaks. These local recharges intermittently inter-rupt the drawdown pattern toward Tremont Street.

1967-1968 USGS Measurements. On twooccasions, in September 1967 and March 1968,the United States Geological Survey (USGS)measured groundwater levels throughout theBoston Peninsula. This study was made inresponse to a request by the MassachusettsDepartment of Public Works which was con-cerned about the potential adverse effects of con-struction for the then proposed Inner

FIGURE 13. Water level contours along St. James Avenue in 1932. (16)


FIGURE 14. Areas of Back Bay having groundwater levels below el. 5.0 during 1936-1940.


Belt expressway on groundwater levels. T h eUSGS used observation wells extant from theW PA survey completed in 1940. Less than halfof the original wells were found to be usable.Results of the USGS study were published in1975 as Hydrogeologic Investigation Atlas HA-513.(4) Figure 15 has been prepared from thegroundwater contours presented therein.

Areas in Back Bay where the September20-21, 1967, water levels were below el. 5.0 areshown in Figure 15. Areas where both readings,the second on March 20-22, 1968, were belowel. 5.0 are also indicated. In addition to the areasshown, low levels were observed in wells aroundthe Christian Science Center; however, thosedata may not have been available to the USGS.

It would appear, by comparison of Figure15 with Figure 14, that water levels throughoutBack Bay were higher in the 1960s than in the1930s. Note, however, that Figure 15 was basedon two isolated readings while the 1930s datarepresent extreme lows from numerous readingsover a four-year period. Furthermore, the 1960sreadings were taken during a wet period; precipi-tation in 1967 was 6 in. above normal and 5 in.of rain fell on March 17-18, 1968. On the otherhand, looking at the high water level readings,two of the seven wells that were never above el.5.0 in the 1930s, were above that level in the1960s. No significant construction dewateringduring the 1967-68 period has been reported.Groundwater levels were again below el. 5.0 inthe John Hancock area and along Tremont Streetin the South End.

Construction within the study area between1940 and 1967, which includes the PrudentialC e n t e r, does not appear to have permanently low-ered groundwater levels below el. 5.0 by 1967.

1970-1985 Groundwater Levels. The BRAreport summarized available groundwater datafrom numerous building projects.(13) Wa t e rlevel observations were generally made at thebuilding sites and at immediately adjacent areasbefore, during and shortly after construction.Table 5 lists major projects constructed bothbefore and during the period.

In addition, data were collected fromother sources where monitoring is ongoing, for

example, at the Christian Science Church,Prudential Center, Boston Public Library,Trinity Church, Massachusetts T u r n p i k eExtension and Church of the Advent. Primarily,data were available for the area along the cen-tral spine across Back Bay. Essentially, norecent groundwater data are available for theBack Bay Historic District and other importantareas having buildings founded on wood piles.

Figure 16 shows the location of most of theobservation wells monitored at some point dur-ing the 15-year period from 1970 to 1985 andthe area where the lowest water level observedwas below el. 5.0. The monitoring period oftenlasted less than a year, since the purpose of themonitoring was to monitor the effects of con-struction and dewatering that caused the tempo-rary lowering of water levels in areas adjacent tosites. Some data in Figure 16 were affected byconstruction dewatering while the two readingsin 1967-68 (see Figure 15) were not affected.

Groundwater levels substantially below el.5.0 in the area of the Christian Science Centerand in the Park Square area south of the PublicGarden were due to construction-related dewa-tering. Observed low groundwater levels aroundHadassah Way were due to sump pumping froma basement in that area. Groundwater levels ofapproximately el. 3.0 were observed within thePrudential Center, probably caused by leakageinto the underground parking garage. The effectof these low levels on groundwater in adjacentareas is mitigated by a wall of steel sheetpilingthat encloses the Prudential Center site. (Notethat the USGS observations shown in Figure 15did not include the Prudential Center.)

Other areas of low groundwater include thearea bounded by the Prudential Center, DartmouthStreet, Boylston Street, and the MassachusettsTurnpike, and the block occupied by the JohnHancock Clarendon and Berkeley buildings. T h edrain between the two older Hancock buildingscontinues to cause lowered groundwater levels inthat area. During subsurface investigations for theCopley Place project, low groundwater levelsbetween the Boston Public Library and theMassachusetts Turnpike were concluded to havel a rgely been due to leakage into the


St. James Avenue sewer. The Prudential Center,the subway tunnel beneath Exeter Street and theConrail alignment may also be lowering ground-water levels in this area.

Fast of the Back Bay railroad station,groundwater levels 1 to 2 ft. below el. 5.0 havebeen observed on both sides of the right-of-wayoccupied by the Massachusetts Turnpike and theSouthwest Corridor Project. Groundwater levelsin this area were observed to be below el. 5.0 forseveral years before Southwest Corridor con-struction began and therefore do not reflect con-struction-related groundwater lowering. Drainsin the former railroad right-of-way were proba-bly responsible for the lowered water levels.Massachusetts Turnpike drains were probablynot the cause since they are located above theobserved low water levels.

Along Tremont Street in the South End,where groundwater levels had been below el. 5.0in the two previous monitoring periods, data wereavailable for only one observation well. Low waterlevels in this well, near the intersection ofBerkeley and Tremont Streets, were below el. 5.0.These findings could indicate that lowered ground-water levels along Tremont Street still exist.

Recent Lower Beacon Hill Study In early1984, attention was focused once again on foun-dation problems caused by lowered groundwaterlevels, on this occasion in the lower Beacon Hillarea from Charles Street to Embankment Road,bounded to the south by Beacon Street.Residents along the waterside of Brimmer Street,between Pinckney and Mt. Vernon Streets,became alarmed when cracks developed in inte-rior and exterior walls and when other evidenceof differential settlement appeared. Test pitswere excavated to enable visual examination ofthe wood piles. In most cases, the wood in thetop 1 to 3 ft. of the piles was severely decayed.Groundwater levels were found to be several feetbelow the pile tops and as much as 6 ft. belowthe water level in the Charles River Basin.

The principal cause of lowered groundwaterhas been determined to be leakage through cracksand joints in combined sewer overflows, wherethey join the Boston Marginal Conduit at the footof Pinckney and Mt. Vernon Streets. T h e

Metropolitan District Commission (MDC),Massachusetts Water Resources A u t h o r i t y(MWRA) and Boston Water and SewerCommission have investigated the problem andare taking steps to correct the loss of groundwater.

O c c u r rences of Rotted Wood PilesExcept for well-publicized cases, records of

wood pile deterioration are buried in BostonBuilding Department files, in the files of build-ing owners, their architects, engineers and con-tractors, or they do not exist. Owners are under-standingly reluctant to talk about the problem.

Six thousand applications for building per-mits filed with the building department between1979 and 1984, and representative samples ofpermit data from 1967-1972 and 1976-1979were examined in the preparation of the 1985BRA report.(13), Only two of the permits issuedwere for repairs to wood piles, suggesting thatthe problem in recent years (to 1984) has beenminor or not reported.

With the exception of the lower BeaconHill area, other cases throughout Back Bayappear to have been isolated and infrequent. Inaddition to the Boston Public Library problem in1929-30, J.R. Worcester identified two occur-rences of rotted wood piles:(15)

"Such extensive repair work has beennecessary in other localities in the Back Bay;e.g., the Fire Insurance ProtectiveHeadquarters at 4 Appleton Street (South End)in July 1921 had to be underpinned where pilescut as low as elev. 3.96 were rotted off aboveground water level found at the time at elev.3.30; again at 12 Hereford Street, corner ofBeacon Street, in June 1933, piles cut at elev.8.13 were rotted to within 3" of ground waterlevel found to be at elev. 6.50."

A d d i t i o n a l l y, four buildings between Boylstonand Beacon Streets are known to have had dete-riorated wood piles that required repair.

The lower Beacon Hill area from CharlesStreet to Embankment Road has had a historyof problems related to rotted wood piles datingback to 1927. The Boston Inspectional


Services Department (formerly the BuildingDepartment) reported that repairs to wood pileshad been made at 38 of the 188 houses and build-ings in this 10-block area, some having beenmade in each decade since the 1920s.(17) T h eBrimmer Street problem described earlier is themost recent example.

Problems in the lower Beacon Hill areaappear to be related to both a lowered water tablecaused primarily by leakage into sewers anddrains, and an original wood pile cutoff level at el.7.0, 2 ft. above the el. 5.0 that was commonlyused throughout Back Bay. The area is outside ofthe Mill Dam and West Side Interceptor and, until1910, groundwater was readily recharged by thenearby and then tidal Charles River. Constructionof the Boston Marginal Conduit and embankmentappeared to have impeded groundwater recharge.

E ffect of ContemporaryBuildings on the Wa t e r Ta b l e

From the available groundwater data, it isd i fficult to assess long-term changes in ground-water levels that may have resulted from the con-struction of buildings in Back Bay. While tempo-rary drawdown has occurred during construction,groundwater has shown that it will return to pre-construction levels unless there is continuedpumping from foundation drains or leakage intobasements. There is no evidence that buildingsconstructed in Back Bay within the last 50 yearshave caused significant permanent adversee ffects to groundwater levels. However, olderbuildings are known to have foundation walls andfloors that leak, requiring sump pumping.

Recent heightened public interest in Back Baygroundwater levels prompted an extensive study ofexisting and probable post-construction groundwa-ter levels around the proposed Hines/New EnglandMutual Life development at 500 BoylstonStreet.(18) The study concluded that the building'sproposed deep basements would have little impacton long-term off-site groundwater levels.

P reserving Gro u n d w a t e r L e v e l sThe importance of maintaining groundwa-

ter levels in Back Bay has been recognized since

the late 1800s. Pipes were placed to act assiphons beneath an early sewer and subway tun-nel to mitigate their impact on groundwatermovement and levels. In several areas, perma-nent recharge systems have been installed toreplenish groundwater, particularly around his-toric structures founded on wood piles.Temporary recharging around excavations forbuilding construction projects has also beenused.

Siphons. In order to lessen the dammingeffect of the 8-ft. high Boston Marginal Conduitand the wood sheeting that was left in place,Worcester reported that siphon pipes were"placed under the conduit from the Basin to theBack Bay intended to carry groundwater fromone side to the other."(15) Worcester questionedthe long-term effectiveness of these siphonsbecause they would probably have filled with siltand would have been only locally effective.

Four 12-inch diameter tile siphon pipeswere placed under the Boylston Street subwaytunnel in the vicinity of Copley Square to trans-port groundwater from one side to the other.These pipes were probably considered necessarybecause the bottom of the tunnel was in the clayand its top was at about el. 6.0, thus forming avirtual dam along Boylston Street. The distinctd i fference in groundwater levels observed onopposite sides of Boylston Street since 1930,confirmed by studies for several projects inrecent years, raises doubts about the eff e c t i v e-ness of these siphons.

Siphon pipes were also used in the ground-water equalization system of the recently con-structed Southwest Corridor structure to connectperforated header pipes placed on either side ofthe tunnel.

R e c h a rging. Early inadvertent recharg i n gwas undoubtedly performed at many locationsby drywells that were used to dispose of precipi-tation from roofs. These systems were probablynot installed frequently enough to have a signifi-cant impact on groundwater levels.

In 1930, the first reported recharge systemintended to raise groundwater levels in order toprotect wood piles was installed at Tr i n i t yChurch. Only a year before, severely


rotted wood piles were found at the nearbyBoston Public Library. Large conductors fromthe Church's roof gutters were connected intolong, stone-filled drywells outside the Churchand into a brick-lined pit in the basement.Although dry weather groundwater levels aroundTrinity Church were below el. 4.0 to 5.0, theintermittent rise in water level and wetting ofwood piles due to the recharge system wereprobably responsible for the preservation of theChurch's foundations.

Other recharge systems have since beeninstalled in Copley Square. In the mid-1950s, ar e c h a rge system was constructed at a triangulargrass plot across Dartmouth Street from theBoston Public Library. In 1968, when CopleySquare was redeveloped with its current sunkenplaza and fountain, another recharge system wasinstalled below the plaza. Both systems con-veyed surface water runoff into the fill throughperforated pipes laid in 3 to 5-ft. thick beds ofgravel or screened stone.

An underdrain system was provided belowthe slab-on-grade floor of the Christian ScienceChurch Center parking garage. This system wasdesigned to function only when water levels roseabove approximately el. 6.7. The underdrainscould be reversed to recharge groundwater shouldwater levels in the fill fall to levels that wouldthreaten wood piles that support the MotherChurch. Again, a system of perforated pipes in athick granular drainage blanket were used.

R e c h a rging to minimize temporary draw-down outside of construction sites had beenundertaken for several building projects wherethere was particular concern for wood piles sup-porting nearby structures. In these instances,r e c h a rging usually involved injecting water,under pressure, into the fill or sand outwash stra-tum through wellpoints. In some cases, waterwas pumped into open ditches and large diame-ter recharge wells and then allowed to percolateinto the ground.

Sewer Dams. In the early 1930s, the St.James Avenue sewer was found to be the causeof lowered groundwater levels along most of itslength. It was found that when sewage wasbacked up behind a dam installed in the sewer,

groundwater levels rose to "normal" levels,thereby mitigating the effects of leakage into thes e w e r. Over the years, the original butterflyvalves deteriorated and were replaced by a sandbag dam that requires periodic repair.

Since 1985 the MWRA has maintainedraised water levels in the Boston Marg i n a lConduit in order to minimize the impact of localleaking sewers that lower groundwater levels. Apermanent solution is still being sought.

S u m m a ryPumping from water supply wells, accompa-

nied by lowered groundwater levels, has causedsubsidence of major cities around the world -including Mexico City, Venice, Taipei andBangkok. While Boston has not experienced acomparable problem, areas of the Back Bay haves u ffered damage from lowered groundwater lev-els. The groundwater table should be restored tolevels that preserve the integrity of foundationsfor the city's historic nineteenth century buildings.

If there were no loss of groundwater bypumping and by leakage into sewers, drains andfoundations, and no additions to groundwaterfrom leaking water mains and other man-madesources, the probable groundwater table through-out Back Bay would be expected to vary from el.8.0 to 10.0. Actual groundwater levels in the fillare lower, except for local groundwater moundsprobably caused by leaking water mains. In someareas, the water table is below el. 5.0, a commonlevel at which wood piles were cut off in the nine-teenth century.

The principal cause of lowered water levelsis leakage into sewers and drains. Groundwaterloss through the walls and floors of the StorrowDrive underpass, into subways and the basementsof older buildings below the water table alsooccurs.

With the available data, it is virtuallyimpossible to determine if "permanent" waterlevels have changed significantly during thepast 50 years, except in one or two local areas- for example, the Brimmer Street area andwestward along the Boston Marginal Conduitwhere low water levels have been discoveredin the past two years. Little or no water level


data have been available over the past 20 yearsfor major sections of Back Bay, notably theBack Bay Historic District where most build-ings are supported on wood piles. A l o n g - t e r mgroundwater monitoring program should beestablished in this and other areas.

The Charles River Basin cannot effectivelyrecharge the groundwater table in the fill becausethe Mill Dam and Boston Marginal Conduit actas dams. However, some recharging to the sandoutwash may occur, but the overall ef f e c tthroughout the entire Back Bay area is not verysignificant.

Of the three principal adverse effects oflowered groundwater levels, temporary or per-manent, the major future concern in Back Bay isfor the deterioration, or rotting, of untreatedwood piles. Numerous buildings in Back Bayhave suffered damage during the past 60 yearsfrom differential settlement caused by rottedwood piles. Problems have been reported in thelower Beacon Hill area in the past two years.Underpinning is currently underway to restorefoundations. Future ground subsidence and nega-tive friction on pile foundations are not likely tobe significant because of extensive and pro-longed dewatering for construction projects overthe past 100 years.

There is no evidence that buildings con-structed during the past 50 years have causedpermanently lowered or significant changes ingroundwater levels. Future development in BackBay would similarly not be expected to causepermanent adverse effects on water levels, pro-vided foundation walls and basement floors arewatertight.

Dewatering for the construction of sewersand drains, subways and other transportationcorridors, and many buildings has temporarilylowered the groundwater table in the fill and, inparticular, the piezometric head in the sand out-wash over a large area of Back Bay, in somecases for a period of several years.

Temporary drawdown of piezometric levelsin the sand outwash are not likely to cause dete-rioration of wood piles. There is no evidence offailure or settlement attributed to rotting at thetips of wood piles that commonly bear on the

outwash stratum a few feet below the org a n i csilt. Nevertheless, it must be assumed that thesand outwash and fill are connected where con-struction has penetrated the organic soils.Therefore, dewatering in the sand stratum mayaffect the groundwater levels in the fill at local-ized areas that lie a considerable distance fromthe source of pumping.

R e c o m m e n d a t i o n sIn situations where buildings have been

constructed with groundwater level sensitivefoundations, special emphasis must be given to:

1. An extensive and continuing water tablemonitoring plan.

2. A thorough review of the temporary andpermanent effects of all construction projects onthe water table, with construction plans and/ormethods altered to minimize their effects on, orreplenish, the water table.

3. Rapid response by appropriate agenciesto correct lowered groundwater levels found inthe monitoring program.

The consequences of repairing and improv-ing water distribution and the sewerage drainagesystems throughout Back Bay must be consid-ered. Unless the work is undertaken in the cor-rect sequence, these improvements may adverse-ly affect groundwater levels. For example, sinceleaking water mains recharge the water table andleaking sewers deplete it, it seems obvious thatsewers should be repaired before water mainsare fixed. Furthermore, improvements in thesewer system or changes in operations that facil-itate drainage and lower fluid levels in pipes willexacerbate the groundwater problem unless leak-ing pipes are repaired prior to the improvements.

ACKNOWLEDGEMENTS - This article wasoriginal ly presented at a meet ing of theBSCES Geotechnical Group on March 10,1986. The authors wish to thank the BostonRedevelopment Authori ty and Gerald D.Hines Interests for funding the Haley &Aldrich, Inc., study on groundwater in BackBay Boston, for which the authors were

principal investigators and upon which muchof this article is based.(13) Local org a n i z a -tions providing data on groundwater levelsand information on construction included:Christian Science Church, Trinity Churc h ,C h u rch of the Advent, Boston Public Library,P rudential Center, and the MassachusettsTurnpike A u t h o r i t y. The illustrations weredrawn by Ms. Acey Welch and the text wasp re p a red by Ms. Marion Keegan, whose assis -tance is gratefully acknowledged.

HARL P. ALDRICH is Chairman ofthe Board and a founding principalof Haley & Aldrich, Inc. Hereceived an Sc.D. in civil engineer -ing from MIT in 1951. Dr. A l d r i c h

has over 30 years of experience in solving foun -dation problems in Back Bay Boston, includingrecent work on the study of "Groundwater inBack Bay Boston "for the Boston RedevelopmentAuthority.

JAMES R. LAMBRECHTS is aSenior Engineer with Haley &Aldrich, Inc. He received hisB.S.C.E. from the University ofMaryland in 1973 and an M.S.C.E.

from Purdue University in 1976. His experiencewith the geotechnical problems in Back BayBoston have been principally associated with theMBTA Southwest Corridor Project. He was alsop r i m a ry investigator for the study of" G roundwater in Back Bay Boston" for theBoston Redevelopment Authority.

REFERENCES

1. Aldrich, H.P., "Back Bay Boston - Part I," Journal of theBoston Society of Civil Engineers, Vol. 57, No. 1, January1970, pp. 1-33.

2. All elevations used herein are referenced to the BostonCity Base (BCB) Datum which is 5.65 ft. below the NationalGeodetic Vertical Datum (NGVD), formerly called the U.S.Coast and Geodetic Survey Sea Level Datum of 1929. El. 0.0BCB datum is el. -5.65 NGVD.

3. Stearns, F. P., "Report of the Engineer," Report of the JointBoard Upon the Improvement of Charles River, House No.775, April 1894.

4. Cotton, J.E., and Delaney, D.F., "Groundwater Levels onBoston Peninsula, Massachusetts," Hydrologic InvestigationsAtlas HA-513, U.S. Geological Survey, Reston, VA, 1975, 4sheets.

5. Aldrich, H.P., "Preserving the Foundations of OlderBuildings," Technology & Conservation, Summer 1979.

6. Journal of Engineering Societies, The Boston Society ofCivil Engineers, Vol. 1, 1879-1882.

7. Camp, Dresser & McKee, Inc., "Report on Improvementsto the Boston Main Drainage System," HUD Project No. P-Mass-3306 for City of Boston, September 1967, 2 volumes.

8. Wo r c e s t e r, J.R., "Boston Foundations," Journal of theBoston Society of Civil Engineers, Vol. 1, No. 1, 1914, withdiscussions pp. 179-248 and 395-417.

9. Casagrande, A., and Casagrande, L., Investigation ofSettlement at Prudential Center, Report to Charles Luckman& Associates, September 1970.

10. Casagrande, A., and Av e r y, S.B., Jr., Investigation ofBuilding Settlements in Back Bay Area, Report to Metcalf &E d d y, March 1959.

11. Haley & Aldrich, Inc., Report on Investigation ofSettlement and Lateral Movement, Trinity Church, Boston,M AAugust 1982.

12. We b e r, R.P., "Foundation Response Caused byDisturbance of Clay," Journal of the GeotechnicalEngineering Division, ASCE, Vol. 104, No. GT5, May 1978.

13. Haley & Aldrich, Inc., Report on Groundwater in BackBay Boston, for Boston Redevelopment A u t h o r i t y, Boston,MA, March 1985.

14. Snow, B.F., "Tracing Loss of Groundwater," EngineeringNews-Record, July 2, 1936.

15. Wo r c e s t e r, J.R., & Co., Report on Pile Foundations andGround Water Levels at Trinity Church - Boston PublicLibrary - S.S. Pierce Bldgs., Copley Square, Boston, MADecember 31, 1945.

16. Paine, R.T., Trinity Church - The Church Endangered bythe Low Level of the Ground Water - How the Danger HasBeen Temporarily Averted, April 20, 1935.

17. Folkins, P.M., Report to the Commissioner ofInspectional Services Department - Structural Report - LowerBeacon Hill, WD 5, October 15, 1984.

18. Haley & Aldrich, Inc., Report on SubsurfaceInvestigations and Foundation Design Recommendations,Proposed Development 500 Boylston Street Boston,Massachusetts, for Gerald D. Hines Interests, March 1985.

19. Charles River Basin Commission, Boston Marg i n a lConduit: Section 2 - Details of Masonry, Record Plan SheetNo. 2, May 21, 1910.

20. Charles River Basin Commission, Boston Marg i n a lConduit - Section 3, Boston Embankment, Section 1, RecordPlan Sheet No. 8, May 21, 1910.

21. Boston Transit Commission, Plan No. 10478, forBoylston Street Subway, Section 4, December 1912.


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