surface settlements at a soft soil site due to bedrock dewatering

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Page 1: Surface settlements at a soft soil site due to bedrock dewatering

Engineering Geology 107 (2009) 109–117

Contents lists available at ScienceDirect

Engineering Geology

j ourna l homepage: www.e lsev ie r.com/ locate /enggeo

Surface settlements at a soft soil site due to bedrock dewatering

Debasis Roy a,⁎, Keith E. Robinson b

a Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, WB, Indiab EBA Consulting Engineers Ltd., Oceanic Plaza, 9th Floor, 1066 W Hastings Street, Vancouver, BC Canada V6E 3X2

⁎ Corresponding author. Tel.: +91 3222 283456 (Offic+91 9333 451843 (cell); fax: +1 208 361 6451.

E-mail addresses: [email protected] (D. Ro(K.E. Robinson).

0013-7952/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.enggeo.2009.05.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 July 2008Received in revised form 29 April 2009Accepted 16 May 2009Available online 27 May 2009

Keywords:DepressurizationExcavationArtesianConfined aquiferSettlementPeatSiltClay

Construction of a 16-m deep, 55-mwide, almost square, underground structure through 8-m of soft soils and8-m of sandstone and siltstone led to the development of widespread settlements. The affected areaextended to distances of over 200 m from the perimeter of the structure. The floor slab of the undergroundstructure was not designed for water pressure. Thus, operation of the structure requires continuous pumpingof seepage water collected at a sump located at the lowermost elevation within the structure. Subsurfaceinvestigation and monitoring data obtained over 5.75-year period following the construction of the structureindicated that the settlement resulted from consolidation of soft soils due to depressurization of an aquiferwithin the underlying bedrock caused by continuous dewatering needed for the operation of the structure.An analytical study was undertaken to project the long term settlement. A simple analytical model could beused to simulate the complex hydrogeological problem reasonably. The details of hydrogeologic setting,subsurface investigation and monitoring activities, and the analytical model for projecting long-termsettlements are presented in this paper.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Construction of an underground structure through 8-m of soft soilsand 8-m of sandstone and siltstone bedrock in Vancouver, BC, Canadaled to the development of settlements of up to 360 mm within56 months of completion of construction. The area affected bysettlements as large as 200 mm extended as far as 150 m to the northof the perimeter of the structure (Fig. 1). Although these settlementsare much smaller than those observed at many other sites underlainby soft and sensitive fine grained soils, the distress led to severeoperational problems for the commercial properties around theunderground structure. The involvement of these writers with theproject began with the investigation into the possible causes ofsettlements and identification of possible remedial measures.

The site is underlain extensively by soft and compressible peat andclayey silt or silty clay. There are two aquifers at the site: a shallowunconfined aquifer and a deeper confined aquifer within bedrock. Anupward groundwater gradient prevailed at the site before theconstruction of the underground structure. The bottom slab of theunderground structure was not designed for uplift. To prevent upliftpressure from developing below the floor slab, a sump is in operationat the lower-most elevation within the structure. The groundwaterseepage collected in the sump is pumped out continually.

e), +91 3222 283457 (Home),

y), [email protected]

ll rights reserved.

To prevent triggering of settlement at the study site, a shallowrecharge trench terminating at a depth of 1.5 m below ground sur-face was constructed during the excavation. Although, the operationof this trench by and large maintained the piezometric elevations ofthe upper unconfined aquifer it had no impact on the lower confinedaquifer. Consequently, the recharge trench was unable to preventacceleration of settlement rates around the underground structurefollowing excavation. The impact of pressure loss within the deeper,confined aquifer was not examined during the original design.

Groundwater recharge has been used to maintain piezometricelevations and minimize settlement at soft soil sites (see, e.g.,Zeevaert, 1983; Galeati and Gambolati, 1988). Deep water injectionsystem together with barriers to prevent lateral groundwater move-ment have also been used to preclude depressurization of confinedaquifers and triggering of settlements within soft soil aquitardscapping confined aquifers (Ilsley et al., 1991; Ervin and Morgan, 2001;Ervin et al., 2004). Development of settlements over a large area hasbeen reported under similar circumstances due to the depressuriza-tion of a confined aquifer underneath a compressible and relativelyimpermeable stratum (Brinkhorst, 1936) unless low permeabilitycurtains are installed to confine the impact of dewatering spatially(Galeati and Gambolati, 1988).

These writers are unaware of any prior study on the impact ofdepressurization of an artesian confined aquifer on the settlements ofoverlying soft soils and the control of such settlements. The objectiveof this article is to share the experience of simulating the develop-ment of settlements over a period of 25 years at a site in such ahydrogeological setting with a simple numerical model and the

Page 2: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 1. Site layout and settlement contours.

110 D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

subsurface investigation undertaken for the calibration and use of themodel.

2. Topography and geology

The site is located near the northwest corner of a naturaldepression surrounded by uplands on all sides except an openingthat exits the area towards the southeast (Fig. 2). According to Turneret al. (1997) the lower portions of the slopes overlooking the studyarea to the north, northeast, west and south are underlain by welldrained sand and gravel of Holocene age, while the upper reaches ofthese slopes are underlain by poorly drained Pleistocene till. The studyarea itself is mainly underlain by peat and clayey silt or silty clayexcept near the north boundary, where the presence of Holocene sandand gravel is indicated.

After the development of the settlement over a widespread areaaround the underground structure became noticeable, a geotechnicalinvestigation program was undertaken to assess the influence of theunderground structure on the settlements and estimate the long-termimpact of the structure. The investigation included drilling of mudrotary boreholes to depths of up to 18.3 m and piezocone penetrationtesting (CPTu) (Fig. 1). These investigations were conducted approxi-mately 2.25 years after the completion of construction of theunderground structure. The boreholes completely penetrated thesoil layers and the underlying highly weathered bedrock, andterminated within sound bedrock. Standard penetration tests (SPTs)

were conducted within soil layers and representative split spoon andthin-walled tube samples were extracted. The soil samples were sub-jected to routine index tests and incrementally-loaded, one-dimen-sional consolidation tests. Continuous, triple barrel core samples wereobtained from bedrock for visual identification and index testing.Packer tests were also conducted within the boreholes. Single or multipoint standpipe piezometers were subsequently installed withinboreholes for permeability testing and long term monitoring ofgroundwater conditions. After installation of the piezometer pipes,the boreholes were filled with bentonite except around the slottedsections. The slotted sections were encased in silica sand. Shut-offvalves were installed at the top of the standpipes. Extensive bentoniteseal and shut-off valves were used tominimize depressurization of theaquifer within the upper portion of bedrock. Also, to facilitategroundwater monitoring, mud additives that degrade within threedays were used during drilling. The piezocone penetration tests(CPTu) were conducted for obtaining the shear strength, hydraulicconductivity, coefficient of consolidation of the soil layers and theprevailing groundwater gradients. Typical CPTu data from the site arepresented in Fig. 3. The presence of upward hydraulic gradient isevident from the equilibrium pore water pressure measurementscarried out as part of the CPTu investigation.

The geotechnical investigation indicated that the site is underlain bya layer of granular fill at surface over a sequence of peat and relativelyimpervious, very soft and sensitive, normally consolidated clayey silt orsilty clay and glacially overridden, stony sandy silt (till). The normallyconsolidated deposit is 5000 to 10000 years old (Turner et al.,1998). Thetill layer is, in turn, underlain by pervious sandstone and siltstone or, atisolated locations, by a thin veneer of clean, well-rounded gravel thatlocally capped the bedrock. To illustrate the stratigraphy, geologicsections along lines AA and BB of Fig. 1 are presented in Fig. 4.

The near-surface granular fill has a thickness of between 1 and 2m.The underlying, saturated, brown, fibrous peat has of thickness of 1.5to 2.8 m and a natural gravimetric moisture content of about 300%.The estimated peak undrained shear strength of this layer was about25 kPa. Borehole permeability tests within this layer indicated ahorizontal hydraulic conductivity of about 1.5×10−5 cm/s.

The underlying clayey silt or silty clay unitwas very soft and sensitiveat shallow depths and became firm with increasing depth. This layer isnormally consolidatedwith peak undrained shear strengths between 10and 25 kPa, and residual undrained shear strengths between 2.5 and5 kPa. The layer was 1.2 to 4.4-m thick with horizontal hydraulicconductivity between 5×10−7 and 1×10−6 cm/s.

Below the clayey silt/silty clay, overconsolidated, stony sandy siltwith clay was encountered at most locations across the site. The stonysandy silt unit reached a thickness of 8 m thick 80 m east of theeastern edge of the underground structure and pinched out 40mwestof the western edge of the structure. Permeability tests within stonysandy silt indicated a horizontal hydraulic conductivity of between9×10−6 and 8×10−5 cm/s. The estimated peak undrained shearstrength of the layer is about 125 kPa.

The thin veneer of clean, well rounded gravel layer found at somelocations above the bedrockwaswater bearing. Thehorizontal hydraulicconductivity of this layer was between 1×10−4 and 6×10−4 cm/s.

The underlying bedrock, encountered at a depth of about 8m at thelocation of the underground structure, was highly weathered andjointed with an RQD of less than 45% within the top 5 to 9 m frombedrock surface. At greater depths, RQD generally exceeded 90%except at lignite seams and shear zones that reached thicknesses of upto 400 mm. The unconfined compressive strength (UCS) of intact corespecimens for the upper highly weathered bedrock was generally lessthan 500 kPa. The corresponding value for the lower portion ofbedrock generally exceeded 1000 kPa. The horizontal hydraulicconductivity ranged between 1×10−5 and 1.2×10−3 cm/s for thetop highly weathered portion of bedrock and between 8×10−7 and2.5×10−6 cm/s for the lower portion. Literature review indicates that

Page 3: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 2. Topography and surface geology.

Fig. 3. Typical piezocone penetration test data.

111D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

the bedrock is of upper Cretaceous to lower Tertiary origin (Sketchleyand Clowes, 1976).

Standpipe piezometers were installed in the bedrock as part of anattempt to project future settlements. At the time of installation of thepiezometers, significant settlements along the perimeter of thestructure were already observed. A piezometer installed at distanceof 235 m north of the underground structure indicated that thegroundwater pressure within the bedrock would rise to about 1.4 mabove the relatively flat existing ground surface at that location whenmonitoring of these instruments began about 2.25 years after the endof construction of the underground structure.

Information obtained from the geotechnical investigation andlaboratory testing program described earlier indicated that consolida-tion settlements were originating primarily because of the stressincreases in the peat and clayey silt/silty clay layers due to depressur-ization of the artesian conditions in the bedrock. Laboratory tests onsamples obtained from these soil layers indicated thatCc/(1+e0) for thepeat and silty clay/clayey silt units were 0.35 and 0.20, respectively. TheCr/(1+e0) value for the deeper, overconsolidated, gravel bearing sandysilt unit was estimated to be about 0.02. The estimates for Cα/Cc for thepeat and silty clay/clayey silt units were 0.05 and 0.04, respectively, andCα/Cr for the sandy silt unit was 0.02. Symbols, Cc, Cr, e0, and Cα, denotethe compression index, recompression index, pre-consolidation voidratio and coefficient of secondary consolidation, respectively. Inter-pretation of piezocone data according to Robertson and Campanella(1986) indicated that the coefficient of consolidation for drainage in thehorizontal direction (i.e., perpendicular to the direction of deposition),cvh (in m2/s), for the peat, the silty clay/clayey silt, and sandy silt unitswere between 1.0×10−4 and 3.2×10−4, 3.5×10−5 and 3.5×10−4, and2.3×10−3 and4.1×10−4, respectively. For vertically deposited,massive,mineral soil formations such as those encountered at the site, thecoefficient of consolidation for vertical drainage, cvv, is expected to beabout 0.5×cvh.

Page 4: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 5. Typical settlement monitoring records.

112 D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

3. Development history

Development of the study area began in early 1980s, when 1 to 2mof site grading fill was placed over the native peat and soft clayey silt/silty clay. Subsequently, several commercial structures were con-structed in this area mainly supported on driven piles. The site gradingfill had resulted in significant settlement of the compressible soilsthroughout the area in the 15 years prior to start of construction of thestructure.

The structure in essence is a five-storey deep reinforced concreteframe being used as an underground parking facility. Along itsperimeter is a reinforced concrete wall designed to withstand the soiland water pressures. In contrast, the floor slab of the structure was notdesigned for water pressure. During construction and prior to placingthe floor slab, seepage behind the perimeter walls had to be drained toan under floor drainage system comprised of a layer of granular, free-draining soil and a network of perforated pipes connected to apumped sump for continuous removal of collected water. The sumpcontinues to be dewatered to prevent water pressure from developingunderneath the floor slab. Above the parking facility there is amultistoried reinforced concrete frame office building. The under-ground parking structure penetrates about 8-m of soil and 8-m ofbedrock. During construction, the excavation through the soil layerswas supported by an anchored sheetpile wall. The sheetpiles weredriven to refusal into the upper, highly-weathered portion of bedrock.The deeper portion of excavation that penetrated the bedrock wassupported by shotcrete and tieback anchors.

In an attempt tominimize the adverse impact of dewatering on theunconfined upper groundwater, a recharge system consisting of atrench filled with free-draining granular soils was installed around theperimeter of the underground structure. The trench was terminatedbetween 1 and 1.5 m below grade. The recharge system was inoperation with a constant recharge rate of 300 l/m approximatelyfrom the beginning of dewatering to about 2.5 year after completion ofconstruction of the underground structure.

To check the effectiveness of the recharge system, nine shallowstandpipe piezometers were installed during the construction of theunderground structure with their base within the peat layer (notshown in Fig. 1). Although these shallow piezometers indicated thatthe groundwater table remained within 1.0 and 1.4 m depth of groundsurface while the recharge system remained operational, the injectionsystem was not effective in arresting depressurization of the deeperartesian aquifer. This resulted in increasing magnitudes of settlementsdeveloping over increasingly larger areas around the underground

Fig. 4. Geologi

structure. Since the recharge system had no effect in arresting thesettlement, which was developing because of depressurization of thedeeper aquifer, its operation was discontinued after about 2.5 yearsfrom completion of construction of the underground structure.

Commercial structures and underground utilities exist around theunderground structure. Most of these buildings and undergroundutilities are supported on piles. As a result, they were not damagedbecause of settlements except that the yard areas around them settledaway affecting the services and access to the buildings. Only onebuilding not supported on piles suffered distress.

The study area had undergone significant settlements prior to theconstruction of the underground structure. Building construction byand large ended in the area approximately in the mid 1980s — about15 years before the start of excavation for the construction of theunderground structure. At least 300 to 400 mm of settlement hadoccurred over the period between the beginning of general develop-ment of the area and the beginning of excavation.

4. Monitoring

Settlements and groundwater levels were monitored within thestudy area for a number of years after the end of construction on a

c sections.

Page 5: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 6. Depths of piezometric surface in the bedrock.

Fig. 7. Typical piezometer monitoring records.

113D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

monthly basis. Inflow into the sump below the underground structurewas also monitored over several weeks approximately 2.5 years afterthe end of construction.

Settlement data obtained after 3 and 5.75 years from the start ofdewatering are shown in Fig. 1. Typical settlement plots are shown inFig. 5.

The Year-3 and Year-5.75 piezometer data and contours forpiezometric depths for the deeper aquifer are presented in Fig. 6.Typical piezometer monitoring records are presented in Fig. 7.Groundwater data are not available from all instrument locations.Piezometer monitoring records indicate that the upward hydraulicgradient has all but dissipated in the immediate vicinity of theunderground structure, while the presence of an upward groundwatergradient was still noticeable in the northern portion of the study area(Figs. 6 and 7). It also appears that the groundwater pressurerepresenting the deeper aquifer has decreased since the constructionof the underground structure. This resulted in a “cone of depression”for the piezometric surface representing the deeper aquifer (Fig. 6).The 3 and 5.75 years shallow piezometer records indicate that thedepth of the shallow water table remained at a depth between 1.14and 1.53 m from ground surface beyond a distance of 15 m from theperimeter of the underground structure. In the immediate vicinity ofthe underground structure the shallow piezometers indicated low-ering of the piezometric elevations immediately after decommission-ing of the injection system. No such immediate impact ofdecommissioning of the recharge system was observed on thepiezometer levels for the deeper aquifer. The data presented inFig. 7 also indicate that there were noticeable changes in groundwaterpressure conditions before Year-3 monitoring. Subsequent changes ingroundwater condition have been relatively minor. However, thesettlements are continuing to develop across the area (Fig. 5).

5. Settlement estimation

To assess the long-term impact of the underground structure on thesurrounding area, a numerical model was developed for estimating thelong-term depressurization of the deeper aquifer. While the complexhydrogeology of the site can only be modeled with a three dimensionalfully coupled formulation (Biot, 1941), practical difficulties in assessingthe appropriate input parameters for such a model and lack ofavailability of relevant computational resources prompted thesewritersto develop a simpler alternative. The model assumes the axiallysymmetric seepage directed towards the underground structure forestimating the decrease in the porewater pressure for the lower aquiferas a function of time. The assumption of axially symmetric seepageregime is in approximate agreement with the local topography andsubsurface stratigraphy. The increase in effective stress is thenusedwithone dimensional consolidation (vonTerzaghi and Fröhlich,1936) theoryscaled to account for the secondary consolidation for estimating thevertical settlement. As shown in the following subsections, the simpleapproach appears to capture the observed settlement pattern approxi-mately. Using the results, settlements within the study area wereestimated.

5.1. Seepage model

Computer program SEEP/W version 4.23 (Geo-slope International,1998) was used in the analyses. In the axisymmetric geometry, thesoftware package solves the following partial differential equationwith an implicit finite element formulation

@

@rk@h@r

� �+

kr×

@h@r

+@

@yk@h@y

� �= − γwm

@h@t

ð1Þ

where r and y are the radial and vertical directions, respectively, k isthe radial (horizontal) hydraulic conductivity (which depends onmatric suction), h is the hydraulic head, m is the derivative of the soil

Page 6: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 8. Seepage model for finite element simulation.

Fig. 9. Hydraulic conductivity functions.

114 D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

water characteristic curve with respect to the matric suction, γw is theunit weight of water, and t is time. The right hand side of Eq. (1) isapproximated with backward finite difference. The hydraulic con-ductivity corresponding to a suction, which is the arithmetic averageof that in the last time step and that in the present iteration of thecurrent time step, is used in the analysis. The influence of vapor phaseof pore fluid was not considered in the analysis.

In the model, the stratigraphy around the underground structurewas approximated as shown in Fig. 8. The geologic section used in thesimulation approximates the subsurface conditions along line A–A ofFig. 1. The analyses were carried out assuming an axially symmetricgeometric condition with the underground structure at the center ofthe domain. Eight-node quadrilateral and six-node triangular ele-ments were used in these analyses. Compatible infinite elements wereused at far-field boundaries. Constant-head boundary condition wasused at the external nodes of the infinite elements. The boundaryadjacent to the underground structure was assumed as free-draining.Surface infiltration was not accounted for in the numerical model asprecipitation is diverted to storm drain systems. The input parametersfor seepage analyses included hydraulic conductivity and moisturecontent of all soil and rock units. The hydraulic conductivities wereassumed to be isotropic with input values as estimated from boreholepermeability testing. The moisture contents were obtained fromlaboratory tests on samples recovered from the site. The hydraulicconductivities for partly saturated soil and rock were made to varywith matric suction using functions shown in Fig. 9. Soil units abovethe soft clayey silt/silty clay layer were assumed not to lose saturationover the entire duration of simulation.

5.2. Modeling sequence

The seepage model incorporated the following chronology tosimulate the actual construction sequence:

• Day 0: Completion of the construction of the underground structureincluding construction of sand and prefabricated drains around theunderground structure and beginning of operation of the rechargesystem with a constant injection rate of 300 l/m.

• Day 728: Decommissioning of recharge system.• Day 4732: End of simulation.

5.3. Model calibration

The numerical model was calibrated against the followingmonitoring data:

• Observed piezometric elevations for the deeper aquifer 3 years aftercompletion of construction.

• Groundwater infiltration data at the dewatering sump at thelowermost elevation inside the underground structure.

Page 7: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 10. Computed piezometric surfaces for the deeper aquifer — 3 to 12 years after construction.

115D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

Fig. 10 shows the location of the computed piezometric surface atthe end of 3 years from completion of construction and thecorresponding observed piezometric levels at the monitoring loca-tions within the study area. While the comparison indicates areasonable agreement, near the excavation the drawdown is overestimated and farther away the drawdown is underestimated. Theapparent mismatch appears to be due to: (a) the axisymmetric

Fig. 11. Estimated stress increments.

idealization of the seepage model, (b) uncertainties in the stratigra-phiy and hydraulic conductivity functions, and (c) the isotropicmaterial behavior assumed in the finite element model.

5.4. Settlements

Yearly estimates of drawdown of the artesian aquifer obtainedfrom seepage analyses between 3 and 12 years after completion ofconstruction are presented on Fig. 10. These results indicate that thepiezometric surface has approximately attained a steady state after

Table 1Computed pore water pressures.

Distancefrom centerof excav. (m)

Top/bottom elevationheads (m)

Time afterstart (year)

Top/bottom pore waterpressures (m of water)

Peat Clayeysilt

Sandysilt

Peat Clayeysilt

Sandysilt

20 2.0/3.6 3.6/6.2 6.2/8.1 0 1.0/3.9 2.6/6.5 5.2/8.43.0 1.0/0.0 2.3/0.0 5.4/0.05.75 1.0/0.0 2.3/0.0 5.4/0.0

50 2.0/3.6 3.6/6.9 6.9/9.7 0 1.0/4.1 2.6/7.4 5.9/10.23.0 1.0/0.0 2.5/2.3 6.0/5.55.75 1.0/0.0 2.5/0.9 6.0/3.9

75 2.0/3.6 3.6/7.5 7.5/12.0 0 1.0/4.3 2.6/8.2 6.5/12.73.0 1.0/1.0 2.6/4.7 6.6/8.05.75 1.0/0.0 2.6/3.4 6.6/6.8

100 2.0/3.6 3.6/8.2 8.2/12.5 0 1.0/4.4 2.6/9.0 7.2/13.33.0 1.0/2.0 2.6/6.4 7.2/10.85.75 1.0/0.8 2.6/5.4 7.2/9.6

125 2.0/3.6 3.6/8.7 8.7/13.7 0 1.0/4.5 2.6/9.6 7.7/14.63.0 1.0/2.6 2.6/7.9 7.7/12.55.75 1.0/2.0 2.6/7.2 7.7/12.0

150 2.0/3.6 3.6/8.7 8.7/13.7 0 1.0/4.7 2.6/9.8 7.7/14.83.0 1.0/3.4 2.6/8.5 7.7/13.55.75 1.0/3.0 2.6/8.0 7.7/13.0

Page 8: Surface settlements at a soft soil site due to bedrock dewatering

Fig. 12. Computed and observed settlements along line A–A, Fig. 1.

116 D. Roy, K.E. Robinson / Engineering Geology 107 (2009) 109–117

about three years from end of construction. Nevertheless, the verticaldeformation is expected to increase because of secondary settlement.

The stress increments due to depressurization of the deeperaquifer are plotted on Fig. 11. The pore water pressure estimates at theend of 3, 5.75 and 25 years for various soil layers are summarized inTable 1 and the corresponding stress increments are summarized inTable 2. Using these results, the one dimensional vertical consolida-tion settlements were estimated. The time varying nature of stressincrease is approximately accounted for following Taylor (1948).

These calculations indicate that the maximum settlementsestimated from numerical modeling after 3, 5.75 and 25 years fromthe end of construction were 320 mm, 390 mm and 470 mm,respectively, inclusive of secondary settlements. The maximumcomputed settlement at 5.75-year develops at a distance of about55 m towards north from the northern face of underground structure.In comparison, the maximum observed settlements after 3 and5.75 year from end of construction were 300 mm and 360 mm,respectively. The maximum settlement was observed at a distance ofabout 45 m north of the northern face of the underground structure.Such a deformation pattern should not be construed as an indicator oftrue three dimensional nature of the seepage problem at hand: it is anoutcome of the stratigraphic peculiarities at the site.

The computed results are plotted on Fig. 12 together with theobserved settlement profiles after the passage of 3 and 5.75 years fromend of construction. Comparison of computed settlement profilesfrom numerical modeling with observations indicates that thecomputed settlements are in reasonable agreement with 3-yearobservations, while the computed settlements are somewhat largerthan 5.75-year observations. The computed and observed deforma-tion patterns are in approximate agreement in spite of the simplifiednature of the numerical model used in the computation.

Because of the preponderance of secondary consolidation at thesite under consideration, methods commonly applied to extrapolatesettlements observed in the field due to a surcharge fill for estimatinglong-term settlements, e.g., Asaoka (1978), is not directly applicablefor the field settlement data obtained in this study (Matyas andRothenburg, 1996). Consequently, the simple alternative of extra-polating the observed trend of evolution of settlement againstlogarithm of time has been adopted. In the experience of the writers,this approach works reasonably at sites underlain by soft fine grainedsoils that exhibit significant secondary consolidation in the vicinity ofthe site under study. The maximum settlement after 25 yearsestimated in this manner is about 470 mm (as shown in Fig. 5 using

Table 2Computed vertical effective stress.

Distance fromcenter ofexcav. (m)

Time afterstart (year)

Top/bottom pore water pressures(m of water)

Peat Clayey silt Sandy silt

20 0 28.2/20.5 33.3/37.9 50.7/51.63.0 28.2/58.8 36.7/101.7 48.7/134.05.75 28.2/58.8 36.7/101.7 48.7/134.0

50 0 28.2/18.6 33.3/40.7 55.4/60.83.0 28.2/58.8 34.3/90.7 54.4/106.95.75 28.2/58.8 34.3/104.4 54.4/122.6

75 0 28.2/16.6 33.3/42.7 59.4/75.13.0 28.2/49.0 33.3/77.0 58.4/121.25.75 28.2/58.8 33.3/89.8 58.4/132.9

100 0 28.2/15.6 33.3/46.4 64.1/77.33.0 28.2/39.2 33.3/71.9 64.1/101.95.75 28.2/51.0 33.3/81.7 64.1/113.6

125 0 28.2/14.7 33.3/48.8 67.4/84.73.0 28.2/33.3 33.3/65.5 67.4/105.35.75 28.2/39.2 33.3/72.3 67.4/110.2

150 0 28.2/12.7 33.3/46.8 67.4/82.83.0 28.2/25.4 33.3/59.6 67.4/95.55.75 28.2/29.4 33.3/64.5 67.4/100.4

thick dark arrows). The analytical results therefore compare reason-ably with these extrapolated estimates.

6. Lessons learnt

A case study has been presented documenting the geotechnicalimpacts of construction of a 16-m deep underground structure at asoft soil site. The structure intersects an aquifer in bedrock underartesian pressure. Permanent dewatering for the undergroundstructure resulted in settlements as large as 360 mm within5.75 years of construction. The affected area extended to distancesas far as a few hundred meters beyond the perimeter of the structureskewed to the north. Monitoring records indicate that while thegroundwater condition (depressurization of the deeper aquifer due tothe construction) appears to have stabilized within an area extendingto a distance of about two times the width of the undergroundstructure from its perimeter, settlements continue to develop.

A simple axisymmetric seepage model was developed in this studyto estimate the long term change of effective stress. Using theseresults, the long term settlements were estimated using conventionaluncoupled one dimensional consolidation theory. While the analyticalapproach is based on a number of simplifying assumptions, it appearsto calibrate reasonably with the observations from the site.

Monitoring data obtained over 5.75 years from completion ofconstruction combined with the results from a series of numericalanalyses confirm that depressurization of the artesian aquifer withinbedrock is mainly responsible for the increasing settlement rates.

At soft soil sites underlain by multiple aquifers settlements areunavoidable unless depressurization of all the aquifers is avoided. Asthe observational evidence and analytical results presented in thispaper suggest, the consequences of depressurization could affect anarea extending to distances of several times the dimension of areawithin which actual dewatering is taking place.

References

Asaoka, A., 1978. Observational procedure of settlement prediction. Soils andFoundations 18 (4), 87–101.

Biot, M.A., 1941. General theory of three dimensional consolidation. Journal of AppliedPhysics 12, 155–165.

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