OBSERVED PERFORMANCE OF A D E E P

EXCAVATION IN CLAY

By Richard J. Finno,1 Member, ASCE, Dimitrios K. Atmatzidis,2

and Scott B. Perkins,3 Associate Member, ASCE

ABSTRACT: A test section was established next to a 40-ft-deep, braced excavation in soft to medium-stiff, saturated clays in Chicago, 111. Surface and subsurface three-dimensional ground movements, pore water pressures, sheet-pile deforma­tions and strut loads were measured. Results of these observations were correlated with construction activities at the test section. Larger than expected ground-surface settlements adjacent to the excavation were observed. The large movements oc­curred as a result of overexcavation during construction. The largest incremental ground movements occurred when the excavation was approximately half com­pleted. Base stability computations indicated that the factor of safety against basal heave at that point in construction was 1.1. Two distinct shear zones developed in the soil mass; their initiation corresponded to the times when the largest incre­mental movements occurred. Soil displacements were always directed towards the excavation with magnitudes, at later stages of construction, larger than those mea­sured on the sheet pile. Pore water response was markedly influenced by sheet-pile installation and strut preloading; as a result, little net change in pore pressures was observed at the end of construction. Magnitudes of measured strut loads were within the levels expected, based on standard design procedures.

INTRODUCTION

The performance of braced excavations has been extensively studied dur­ing the past 20 years (Peck 1969; Bjerrum et al. 1972; O'Rourke 1981). Soil response during excavation has been measured in reasonable detail, partic­ularly in studies by the Norwegian Geotechnical Institute in the 1960s (Mea­surements 1962a, b, c). This paper expands on previous studies by providing additional detail through observations made with a dense array of inclino­meters, extensometers and piezometers.

In 1987, the Department of Public Works of the city of Chicago con­structed a connection between two existing transit lines. Part of the work, known as the HDR-4 Contract, included constructing a subway tunnel using cut-and-cover techniques. The project was located one mile south of down­town Chicago. Conditions at the site during construction are shown in Fig. 1. Note that because of the lack of nearby structures, the contractor was not overly concerned about ground movements associated with excavation.

Northwestern University, in conjunction with the Department of Public Works, established a test section—adjacent to the 40-ft-deep excavation in saturated clays—to record surface and subsurface three-dimensional ground movements, pore water pressures, sheet-pile deformations and strut loads. Soil responses were measured daily as the cut was excavated at the test

'Assoc. Prof. Civ. Engrg., Northwestern Univ., Evanston, IL 60208. 2Prof. Civ. Engrg., Univ. of Patra GR 26110, Patra, Greece. 3Proj. Engr., Ardaman & Associates, Inc., Sarasota, FL 34239; formerly grad.

stud., Northwestern Univ., Evanston. Note. Discussion open until January 1, 1990. Separate discussions should be sub­

mitted for the individual papers in this symposium. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on Au­gust 22, 1988. This paper is part of the Journal of Geotechnical Engineering, Vol. 115, No. 8, August, 1989. ©ASCE, ISSN 0733-9410/89/0008-1045/$1.00 + $.15 per page. Paper No. 23729.

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, « • • -

*0C~

.^Wi

4SF

FIG. 1. Conditions at HDR-4 Test Section

section location. Presented herein are subsurface conditions and instrumen­tation at the test section, pertinent construction activities, and results of the field observations made before, during and after excavation. Conclusions are drawn concerning the overall performance of the excavation. A companion

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paper evaluates measured ground response and discusses effects of soil be­havior on observed response (Finno and Nerby 1988).

SUBSURFACE CONDITIONS

The subsurface conditions at the test section consist of a rubble-fill deposit overlying a sequence of glacial clay tills laid down during the Wisconsin stage of the Pleistocene period. Soil strata are shown in Fig. 2 in terms of Chicago City Datum (CCD) elevation. The rubble fill contains concrete blocks, bricks, and railroad ties within a primarily granular soil matrix. Beneath the fill lie four distinguishable tills; in descending order, they are the Blodgett, Deerfield, Park Ridge, and Tinley tills (Peck and Reed 1954). Beneath the tills, a 5-ft-thick, dense granular deposit rests atop Niagaran limestone.

The Tinley till at the site was deposited subglacially and is well drained; as a result, it is overconsolidated from the ice load and is very stiff to hard. The upper three tills were deposited beneath the waters of the progenitor to Lake Michigan as the Wisconsin glacier retreated northward and eastward from the Tinley morraine. Thus, these till strata initially were normally con­solidated until lake water levels dropped as much as 80 ft below that of the present level of Lake Michigan (Peck and Reed 1954).

Index properties, stress history and undrained shear strength of the glacial clays also are shown in Fig. 2. Maximum past pressures (Up) have been evaluated from results of standard oedometer tests. With the exception of the dessicated crust in the top few feet, the compressible clays of the upper three till strata are lightly overconsolidated, as a result of lowered ground­water levels after deposition and/or aging.

ELEV FT(CCD)

TIL

L U

NIT

- H

- t i o o

1 m

_i

cn

o

3ARK

RID

GE

TIN

LE

Y

1

1 1 I

V

1

hard

CL

AY

1

WATER C0NTENT(%) 0 30 60 0

EFFECTIVE VERTICAL STRESS (KSF)

20 40 6.0 8.0 0

UNDRAINED SHEAR STRENGTH (KSF)

0.5 1.0 1.5 2.0

J&

S' " \ _ V

Ar

V

Range of a-,0 — - based on pore

pressure data

1 ' I •

*~~* 9 .0 -

-

j— 24 hour op' J *— end of primary q,'

\ —

I ,1

1 1 1

-

• u \ . '

. \ V

1

A

-

A

= 0.28 "

+ Field vane -SBPM a Menard PM • U Shelby tube A UU Fixed Piston

FIG. 2. Geotechnlcal Characterization of Soils at Test Section

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Undrained shear strengths obtained from both field and laboratory tests are shown in Fig. 2. All strength testing was performed on 2.8-in.-diameter samples. No trimming was required after the specimen was extruded from its tube. This eliminated the potential sample-preparation problems caused by the presence of gravel within the clays. The city of Chicago performed 20 unconfined compression (U) tests, on specimens extruded from 3-in.-diameter shelby tubes. Northwestern performed 10 unconsolidated-undrained triaxial compression (ULT) tests on specimens extruded from 3-in.-diameter fixed-piston samplers. A rather wide scatter in undrained shear strength was observed when tests were made on specimens obtained near contacts be­tween till strata. This trend is consistent with the observation that natural water contents are typically quite variable at these till contacts in the Chicago area (Peck and Reed 1954). Results of field vane, self-boring and Menard pressuremeter tests are also shown in Fig. 2. Average values of undrained shear strength normalized by effective vertical stress (Su/a'vo) found from the field vane and self-boring pressuremeter tests were 0.23 ± 0.04 standard deviation and 0.30 ± 0.09 standard deviation, respectively.

INSTRUMENTATION

A plan and a section of the instrumentation are presented in Fig. 3. With the exception of the two inclinometers grouted into angles welded to the sheeting, all of the ground instruments were located at least 8 ft west of the western sheet pile. It was not possible to place any instruments closer be­cause of a pilot trench that was excavated through the fill layer prior to sheet-pile installation. This trench extended outward nearly 8 ft from the location of the sheet pile. Therefore, the instruments were placed as close as possible to the sheet pile, and yet far enough away so they would not be damaged while the pilot trench was excavated.

The main line of instrumentation consisted of three clusters of instruments placed along a line perpendicular to the sheet pile. Each cluster consisted of three pneumatic piezometers, a 65-ft-deep inclinometer casing, and a five-point, mechanical extensometer. It was not possible to perfectly align the instruments on this main line due to obstructions encountered in the fill layer. ITierefore, these three clusters were laid as closely perpendicular to the sheeting as the fill permitted. In addition to these clusters, an inclinometer and ex­tensometer pair, and two single inclinometers were offset from the main line. Instruments were not located any farther from the sheet-pile wall because of the presence of a road immediately behind the last instrument cluster.

The purpose of the main clustered instrument line was to provide subsur­face displacements and pore pressures within the zone of significant move­ment near the excavation. The purpose of other instrumentation was to pro­vide redundancy in the data. To complete the ground-response picture, a surface survey net was established (Fig. 3). A complete set of instrument readings was obtained every day excavation occurred at the test section. The response of the bracing system itself was monitored by measuring sheet-pile deflections with an inclinometer and measuring strut loads with vibrating wire strain gauges. Only one strut at each level at the test section was in­strumented.

All instrumentation was installed at least one month prior to any construc­tion activity at the site. This quiet period permitted collection of sufficient

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tN Settlement points-

Piezometers

Extensometers y

oE-4 \ +' +SI-4

Sl-5

E-3 t>. a

a t* •

SI-3 -4: -

A * SI-I qT.p fc» ti b i d P2 PI

SI-6

PLAN

a -10 o o

\

\*f

Inoperative

West - pilot trench

SCALES (ft)

SECTION A-A'

FIG. 3. Instrumentation Plan and Section

baseline data to establish the precision of inclinometer and extensometer data (equipment and operator variability) and in situ groundwater conditions. The orientations of the four alignment grooves of the inclinometer casings rel­ative to the sheet pile were measured after the sheet piles were installed so that the direction of the resulting movement of each axis could be accurately defined. Accuracy of the inclinometer data was periodically checked during construction by hand taping horizontal movements between surface survey pins and comparing those results with differences in incremental horizontal ground-surface movements computed from inclinometer data. Comparison of these data indicated consistencies with those of the manufacturer.

Accuracy of the piezometer data was checked during installation. Pore pressure readings were taken after the transducers were set at their final el­evation in the borehole and packed with sand prior to the placing of a ben-tonite seal. Readings were compared to the height of water in the borehole; accuracy was generally found to be within the manufacturer's specifications.

Movements of the top of each extensometer rod were measured from a reference plate using a micrometer with a precision of 0.001 in. The ele­vation of the reference plate was surveyed each time extensometer data were

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obtained. Accuracy of the leveling procedures was judged to be ±0.01 ft. Because of the relatively large vertical movements that occurred during con­struction, vertical movements computed on the basis of reference-plate el­evations were checked against movements computed upon relative move­ments between extensometer rods, assuming that the deepest anchor of each extensometer did not move. The lowest anchor for each instrument was set approximately 20 ft below the bottom of the excavation in the Park Ridge till. This latter procedure gave similar but more consistent results than the former, and was used to compute vertical movements presented in this paper.

CONSTRUCTION SEQUENCE AND PROCEDURES

The construction activities at the test section are summarized in Table 1. Work began with the digging of two, 13-ft-deep pilot trenches along the axes of the sheet-pile walls. The purpose of these trenches was to clear out the random fill material from the path of the driven sheet piles because ob­jects within the fill could damage and bend the sheet-pile tips. Excavation for the east pilot trench began December 15, 1986, which is referred to as day 1 of activity. Sheet pile was installed in both trenches in two stages. Sheet pile was driven to an elevation of —28.0 ft CCD using a Vulcan 01 hammer; in a second pass, it was seated at a final elevation of —50.0 ft CCD. A vibratory hammer was used to seat the piles along the west side. On day 101, the top 2-3 ft of fill material between the two sheet-pile walls was excavated and dumped into the pilot trench on the outside of the west row of sheet piles. The outside of the east sheet-pile trench was never com­pletely backfilled near the test section. Excavation then proceeded as detailed in Table 1 and illustrated in Fig. 4. Excavation generally began to the south of the instrumented site first, then proceeded north. The excavation reached a maximum depth of 40 ft (elevation -27.0 ft CCD).

During the excavation process, excavated material often was unloaded on the ground to the west, prior to being loaded into trucks and carried away. This soil, together with the presence of heavy construction equipment, cre­ated a transient surcharge applied to the surface, but only on the west side of the excavation. No equipment or excavated material was placed on the east side of the excavation due to its proximity with State Street, which was kept open to traffic.

As the excavation proceeded, four levels of wales and struts were installed to brace the PZ-40 sheet piles (Fig. 3). Struts were placed horizontally every 12 ft. The first level of bracing consisted of HP 14-by-109 sections that abutted W14X90 wales. The second level of bracing consisted of 3/8-in. wall, 22-in.-diameter pipe piles that abutted W24X117 wales. The third and fourth strut levels consisted of 1/2-in. wall, 24-in.-diameter pipe piles that abutted W24X117 wales. The excavated width of the cut was nominally 40 ft. Note that the third level of struts was installed prior to the second level. The second strut level did not support the sheeting during excavation; these struts picked up load only after the two lower strut levels had been pulled while the tunnel was being poured.

Struts at the first, third and fourth level were preloaded with approximately 50% of the design strut load. Preloading was not done on a strut-by-strut basis; rather, preloading was accomplished by forcing the wales apart with two box beams, each with an internal hydraulic jack. The struts were cut to

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TABLE I. Activities at Test-Section Location

Construction day number

(D 1 4

17 52 64

101

108

127

141 143

144 148

152 156,157

163

164 165

166 170-176

201 239 282

Period (2)

1

2

3

4

5

6

7

Event (3)

Excavate east trench through fill on Dec. 15, 1986 Drive sheeting in east trench Excavate west trench through fill Drive sheeting to el.—28 ft CCD in west trench Vibrate sheeting to e l . -50 ft CCD in west trench Excavate to el. +10 ft CCD and backfill west trench

outside of sheet pile Excavate to el.O ft CCD from utility crossing to test

section Excavate to el.—7 ft CCD from utility crossing to

test section; crack observed at ground surface south of main line of instruments

Excavate to el.O ft CCD at test section Excavate to el.—2.5 ft CCD at test section;

maximum cantilever conditions at test section Install W14X109 sections at first strut level Excavate to e l . - 7 ft CCD at test section; crack

observed at ground surface through main line of instruments

Excavate to el. —13 ft CCD at test section Install and preload 24-in. diamxl/2-in. pipes at

third strut level Install with no preload 22-in. diamx3/8-in. pipe at

second strut level Excavate to el. —24 ft CCD at test section Install and preload 24-in. diamXl/2-in. pipes at

fourth strut level Excavate to e l . -27 ft CCD at test section Pour mud slab at test section Remove fourth level of struts Remove third level of struts Remove first and second levels of struts

Note: Elevations shown are average elevations within test-section area. Construction day numbers correspond to times when various activities began or ended. Actual exca­vation progress is shown on Fig. 4. Utility crossing is located 54 ft south of the location of inclinometer SI-1.

length in the field and forced between steel plates prewelded to the wales. Three struts were attached to each 40-ft wale. After the ends of the three struts were welded to the plates, the hydraulic jacks were retracted. The preload was computed by assuming that the observed forces in the two jacks were equally distributed among the three struts on each wale.

OBSERVED RESPONSE

The entire construction process has been divided into seven distinct pe­riods of activity, as noted in Table 1. The overall ground response through-

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• Test Section • ^ Test Section •

PERIODS 2 and 3

L Test Section

"143 143 I i

\ (151)

L

143 143 144 144^- , I I I I /

\(i50>T«45)Xr7i45ri

1 1150)

(1525s

PERIOD 4

Test Section

r

- I

-0163

OI57

L

I

OI63

OI57

I I

0163 OI63

0157 OI56

H PERIOD 5

i

OI63

0156

JUgT^ Qj |§r

i

OI63

OI56

^

_"""""'' . I

- 0

O

0169

L

Strut location ( t y p . ) ^

I

o

o

0169

(166)

I . . ^

o o

0 O

\OI65 0165

1 (164)

PERIOD 6

I

'o

o

OI65

I

0

0

0165

J SCALE

(ft)

FIG. 4. Excavation and Strut Installation Sequence

out construction is described herein by examining sheet-pile and ground-surface displacements (data that are commonly observed in excavation mon­itoring programs), horizontal ground displacements, vertical and horizontal ground displacements in a plane perpendicular to the excavation axis (dis­placements in this plane are herein called "transverse"), and pore pressures. Emphasis is placed on response during excavation. Response during the first period (pilot-trench excavation and sheet-pile driving) has been described by Finno et al. (1988a). Detailed response is illustrated for period 4, when a shear zone formed within the clay mass and the largest incremental ground movements occurred. Finally, observed strut loads are presented.

Overall Response at Main Line of Ground Instrumentation Sheet-pile and transverse horizontal ground deformations, and ground-sur­

face settlements measured along the main line of instrumentation throughout construction are summarized in Fig. 5. Also shown are the excavation levels corresponding to the days when the particular inclinometer and survey data were obtained. For clarity, only ground inclinometer SI-2, located 20.6 ft behind the sheet-pile wall, is shown.

Several trends can be seen in Fig. 5. Prior to installing the first level of struts, cantilever conditions existed on day 143 on both sheet-pile walls and in the ground adjacent to the wall. Magnitude of movements at each of the sheet-pile walls were different at this time and throughout construction. Ini­tially, these differences were likely caused by the differences in support along the sheet-pile walls caused by the incomplete backfilling of the eastern pilot

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r

•

-

. _». '.'

FIL

\ ^ , \ ^ ^ - IN

CO

\ \ ^ ^Ul\

^ N

Q

\ N

V U

<ru ^ .

Tension crack7 --_ (day 146) /

Inclinometer-number

KEY

Day

•77

ii

i !l /

Inclinometer data

Initial 143 152 163 169

SO «.

III 1 0 " ' '

V K

i \

7/

4H (169)

'/I Inclinometer Displacements

(in.)

SCALES (ft)

FIG. 5. Transverse Horizontal Displacements and Surface Settlements

trench. The pattern of movements associated with subsequent construction was similar for each wall, but the magnitudes were again dissimilar. This could perhaps be attributed to the facts that construction equipment operated solely atop the west side of the excavation and that larger cantilever move­ments on the west side of the excavation induced larger strains, which mo­bilized significantly more of the shear strength of the soil on that side. As shearing stresses increased during excavation, the soil on the west side of the cut had less available resistance than the soil on the east side, and thus more load had to be carried by the structural support.

After the first strut was installed, the contractor excavated 19.5 ft below the first strut level (by day 152) before installing the third strut. Deep-seated movements developed on the sheet pile while this excavation occurred. The large increment of sheet-pile movement is also reflected in both the inward soil movements and ground-surface settlements. These movements were large enough to cause a tension crack that was the surface expression of a shear zone observed in all inclinometers. The magnitude of the maximum inward movement of the sheet pile did not significantly change after this time, but essentially moved deeper as excavation proceeded. However, the soil pro­gressively moved toward the excavation while the ground surface continued to settle.

Ground displacements were essentially oriented toward the area where ex­cavation occurred. At the test section, excavation generally proceeded south to north. The effect of this construction detail on the observed ground move­ments is shown in Fig. 6, which gives a plan view of horizontal displace­ments at elevation —14 ft CCD in the clay based on inclinometer data. Also shown in insets are displacements measured at SI-1 for other elevations and

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FIG. 6. Horizontal Soil Displacements

more frequent readings. These deformations are generally planar and ori­ented about 15° to the south of perpendicular to the sheet-pile alignment. Note that deformation of the sheet piles was also oriented to the south. This is thought to be the result of placement of the inclinometer casing in an angle located in the corner of the sheet pile where rotation about the joints would permit locally significant nonplanar movements.

As noted in Table 1, the first ground-surface crack was observed in an area just south of the main line of instruments. As shown in Fig. 7, this first crack encompassed ground inclinometer SI-6 and sheet-pile inclinometer SI-48. This crack appeared on day 127 when the top 20 ft of sheet pile were completely unsupported along an approximately 50-ft-long section of wall between the test section and the utility crossing. Fig. 8 shows plots of trans­verse horizontal displacements of the sheet pile and soil both inside (incli­nometers SI-6 and SI-48) and outside (inclinometers SI-1 and SI-3) of this first zone of large movement. These data indicate that most of the difference

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Observed surface crack on day 148

Inclinometer number

yw- vu-

Soldier pile and lagging section

I - - IT1

t_ Utility crossing

FIG. 7. Surface Expression of Tension Cracks

SCALE (ft)

in ground movements between soil inside and outside this zone was restricted to the fill and the upper few feet of the clay. Within the clays below ele­vation —5 ft CCD, little difference is observed between the movements.

Vectors showing transverse displacements along the main line of instru­ments throughout construction are shown in Fig. 9. With the exception of the vectors at the ground surface, these vectors are based on extensometer and inclinometer data from each of the main line clusters. At the ground surface, the vertical components are based on settlement point data. The movements, particularly within the clays, were oriented at approximately 45° and indicated progressive movements toward the sheet pile. Based on survey measurements of the top of the sheet pile, the sheeting heaved 0.03 ft when the excavation reached its full depth.

Pore pressure response throughout construction is illustrated in Fig. 10 with the piezometric levels of piezometer group 2, which was located 14 ft away from the sheeting. While driving the sheet pile in the west pilot trench, pore water pressures rose sharply at both stages of driving. At piezometer P 2-3, the excess pore pressure exceeded the vertical effective stress. These excess pore pressures partially dissipated prior to the start of excavation at the test section on day 101. Thereafter, the rate of dissipation increased slightly as a result of gradual unloading. However, two periods of rapid change in response were observed. Both periods are associated with times when large elevation differences existed between the lowest in-place strut and the bot­tom of the excavation. The first, and largest, drop in pore pressure was associated with the formation of the shear zone (between days 144 and 152). The effect was most pronounced for the deepest piezometer. After reaching minimum values on day 152, pore pressures increased. Similar behavior was noted starting on day 163, when the excavation approached final grade and the unsupported sheet-pile length reached 18 ft. After the mud mat was poured on day 170, pore pressures gradually decreased.

To further illustrate ground response during these periods when the rather

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Q O O

10

o -

-10

p - 2 0

-30

-40 -

- 5 0 L

(Projected)

/st3>

' o •5dN

S U ­IT

\LiJ-

(Projected)

SCALES (ft)

DISPLACEMENT SCALE (in.)

I I

KEY

Day Inclinometer data

Initial 127 • 138 143

FIG. 8. Transverse Horizontal Displacements During Development of First Shear Zone

unexpected pore water response was noted, and where the largest incre­mental movements occurred, incremental transverse displacements and de­tailed changes in pore water pressure are presented for period 4.

Large incremental displacements occurred between day 143 and 152, when the unsupported span of the sheet-pile wall was as much as 19.5 ft over a horizontal length of approximately 80 ft. Incremental transverse displace­ment vectors for this period are plotted in Fig. 11. These incremental move­ments are indicative of a block-type movement that is associated with the development of a shearing surface. Similar response was observed when in­cremental transverse displacements were plotted for days 163 to 169, when the second unexpected pore water response was observed.

Changes in pore water pressures during this time are presented in Fig. 12. Daily observations are shown in the figure. The change in pore pressure

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N

Ground surface —,

X \

H

%

.

.

\

\

X «*

e

\ Q.IOI

NJ52 Nl63-

S» 240

x •v

0

Sirut installed on day no.

-Sheet I Pile *

< LH3

< on w(l43) < Q56]

-fl65l

Excavated surface on day no.

DISPLACEMENT SCALE (in.)

SCALES (ft)

FIG. 9. Transverse Soil Displacements

refers to the first day of the period. Pore pressures initially dropped for all piezometers during period 4. However, after reaching a minimum value be­tween days 148 and 151, pore pressures increased. Note that the third strut level was jacked into place on days 156 and 157. This response is somewhat surprising because of the general unloading caused by the excavation process and the rather large distances between strut level 3 and some of the piezom­eters.

Excavation depth ( f t ) -

£ 2000

100 150 Construction Day Number

FIG. 10. Pore Water Pressures from Piezometer Group 2

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ELEV. (FTCCD)

20

- 2 0

- 3 0 -

- 4 0

-50 L

Surface crack forms on day 148

s in terpreted " ^f~ shear zone

144,148,152

-Sheet pile

Excavation

144

(144)

(148) 77TTT7TT7-7irTVTTTT"

(152) WAW; n / n m y / J 1

DISPLACEMENT SCALEdN.)

J Zones of maximum change of inclinometer readings

10

SCALES

(FT)

HO

FIG. 11. Incremental Transverse Displacements, Period 4 (after Finno et al. 1988b)

Strut Loads Loads in the struts at the' test section were computed from vibrating wire

strain gauge data. Strains were measured near each end of a strut. For the W14x 109 section at the first level, a gauge was placed on each side of the web along the neutral axis. For the pipe sections used for strut levels 2 through 4, a gauge was placed at the top, the bottom, and both springline locations. Measurements were taken prior to installation to obtain initial val­ues, and strut loads were computed on the basis of changes in strain from these initial values. Corrections were made to account for the difference of the thermal expansion of the structural member and the gauge wire.

Strut loads measured throughout construction are given in Fig. 13. Only one strut at each level was instrumented. The variation of loads along any horizontal level could not be evaluated. Recall that the second strut level was not preloaded. As a result, gaps as large as 2-3/4 in. were left between the sheet pile and wales after the struts were placed at the test section. Strut loads did not increase in the second level of struts until enough lateral dis­placement of the sheeting occurred to close these gaps. In this case, the gap

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ELEV (ft. CCD)

10

0

10

20

30

40

•=,n

•

~

LL\

; 'F

\ I.

t - '

\

GETT

BLOD

v '

DEERFIELD

.UJ CD

RKR

m

Sheet pile

-• /*VWxV //VSWV/

/Piezometer location and no.

• 3-1B

3-2 B

3-3 B ^r

2-1H

2-21

2-3 B

Grade on day no.

Strut installed (144) o n d°V n a

(152) / ///////// //// /

lZ 500 SCALES (ft)

FIG.

|i..M?.ilf,? Construction day

12. Incremental Pore Water Pressures, Periods 4 and 5

did not close until the two lower strut levels had been pulled out on day 239. Then the gap between the wales and the sheeting closed and allowed full load transfer from the soil to the strut.

Discussion of Observations The overall performance of this test section in relation to other field data

can be evaluated by examining Fig. 14, a plot of normalized surface settle­ment versus distance from edge of the excavation (Peck 1969). The figure indicates the boundaries of zones that mark typical performance for exca­vations made through different subsurface conditions. Normalized settlement profiles along the main line of instraments are shown for the end of period 3, the time when maximum cantilever conditions prevailed; period 4, when excavation had proceeded to - 1 3 ft CCD and the first shear zone was ob-

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a o

9 U U

5 0 0

U

Excavation Complete

J

A

••' fr—--

,G i

-° \ i i : °i i : 1 | : :

, i l ! i ! i .

Level 4 Removed

- - -«_._, ,

A - " A- A ; . .-•• '

: A - ' '

• Strut installed

Level 3 Removed .

_

A..-

1

150 200 250

Construction Day Number

KEY o o Strut loads for level I

Strut loads for level 2 Strut loads for level 3 Strut loads for level 4

FIG. 13. Measured Strut Loads at Test Section

served along the main line; and period 6, when the excavation had reached full depth. It can be seen from the figure that control of ground movement was lost by the end of period 4. At that time, approximately 19.5 ft of sheet pile was unsupported between the first strut level and excavated grade. Pre­vious experience with excavations in Chicago indicates that large surface settlements occur when unsupported spans of sheet pile exceed 10 to 15 ft (Peck 1942).

It is important to note that control of ground movement was suddenly lost at an intermediate stage of construction (period 4), not gradually as exca­vation proceeded to full depth. At the end of period 4, when the unsupported sheet pile spanned about 19 ft, the bottom of the excavation was located in the soft Blodgett till with 27 ft of soft to medium clay between the bottom of the excavation and the stiffer Park Ridge till. At the end of period 6, there were 18 ft of unsupported sheet pile, but only 16 ft of medium clay between the bottom of the excavation and the stiffer soils. As shown in both Figs. 5 and 9, more incremental movements were associated with period 4 than with period 6. This is most likely a result of the lack of a stiff soil stratum near the bottom of the excavation. Mana and Clough (1981) showed that the rate and magnitude of movement increases rapidly as the factor of safety against basal heave approaches 1.

The influence of the stiff stratum can be evaluated by using the factor of

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Distance from Excavation Max. Depth of Excavation

I 2 3

(%)

Sand and soft to hard clay average workmanship

Very soft to soft clay construction difficulties

I I I Very soft to soft clay to significant depth below bottom of excavation

KEY

o End of period 3:15.5 ft deep excavation

A End of period 4 : 2 6 ft deep excavation

a End of period 6 : 4 0 ft deep excavation

FIG. 14. Ground-Surface Settlements During Construction

safety against bottom heave using Terzaghi's method at various stages of construction (Terzaghi 1943). Strength anisotropy was accounted as sug­gested by Clough and Hansen (1981). The factors of safety against basal heave for the two situations were computed using undrained shear strengths that were back-calculated using a limit analysis based upon the geometry of the observed locations of shear zones on the active size of the wall and assumed locations of shear zones on the passive side (Finno et al. 1988b). These strengths correspond to Sja'vo ratios of 0.28 for active loadings and 0.12 for passive loadings. A series of ^-consolidated undrained triaxial compression and extension tests performed using SHANSEP procedures in­dicate that these strengths for normally consolidated, test-section clays are 0.24 and 0.16 for the active and passive loadings, respectively. Results of the computations indicate the factors of safety against basal heave are 1.1 and 1.3 for days 152 and 164, respectively. These results indicate that larger incremental movements would occur when the excavation was shallower. Thus, overexcavation and the lack of a stiff stratum below the bottom of

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TABLE 2. Comparison Between Design and Maximum Measured Strut Loads

Level number

(1)

1 2 3 4

Design strut load (kips) (2)

304 380 504 441

Maximum measured strut load (kips) (3)

298 160 529 422

Note: Design load for level 2 was computed from condition that existed after strut levels 3 and 4 had been pulled.

the excavation most likely caused the large ground movements at the test section.

It should be noted that ground movements were not a major concern to the contractor at the test-section location. No structures or unsupported util­ities were located near the test section. The contractor offset the sheeting from its design location to allow for inward movements and to maintain the pay line for the concrete of the subway tunnels.

The earth pressures for the structural support system were selected using maximum apparent earth pressure diagrams recommended by Peck (1969) and assuming the undrained shear strength of the clay was 0.4 ksf. A uni­form surcharge of 600 psf was applied at the ground surface to account for temporary construction loading. This design value of surcharge is conser­vative and large relative to practices elsewhere. Design strut loads computed using these loadings are compared with maximum measured strut loads in Table 2. These design strut loads are approximately equal to the maximum loads measured during construction. This adequate strut load design did not guarantee that ground movements were kept to expected levels. Also, note that the large ground movements did not induce larger than expected strut loads, as has been reported when movements associated with braced exca­vations fall within Peck's zone III. This may be, in part, due to the con­servative value of undrained strength used to compute the design strut loads (compare design value of 0.40 ksf with values shown in Fig. 2) and to the presence of a stiff stratum near the bottom of the excavation.

CONCLUSIONS

Construction activities and results of field observations made before, dur­ing and after construction of a 40-ft-deep braced excavation through satu­rated clays have been described. Overall ground and structural response in­cluding surface and subsurface three-dimensional ground movement, pore water pressures, sheet-pile deformations and strut loads have been presented. Based on the results of these observations, the following conclusions can be drawn.

1. Large ground surface settlements occurred at the HDR-4 test section. The settlement/depth of excavation ratio indicates the movements that occurred were larger than normally expected for the HDR-4 ground conditions. These large movements occurred when the contractor excavated below strut levels prior to placing the supports.

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2. The largest incremental movements occurred at a depth of excavation be­tween 20 and 26 ft. Base stability computations showed that this intermediate depth had a factor of safety of 1.1, as compared to a factor of safety of 1.3 in the fully excavated depth. The higher factor of safety arises primarily from the presence of stronger, less compressible clay below the bottom of the excavation.

3. The largest incremental ground-surface settlements were associated with the development of two distinct shear zones; the soil within the shear zones incrementally moved in a block-like fashion.

4. The horizontal soil movements were oriented towards the area where ex­cavation had first been made. At the HDR-4 test section, excavation proceeded from south to north and movements were oriented at approximately 15° south of the perpendicular to the sheeting. Thus, the soil displacements were generally planar, but not in a direction perpendicular to the sheeting.

5. Inclinometer data indicated that the soil outside the sheet piles always moved towards the excavation. The maximum horizontal ground displacements were approximately 10 in.; these were greater than the 7-in. maximum horizontal dis­placement of the sheet pile.

6. Pore pressure response was somewhat unexpected. Relatively high excess pore pressures developed during sheet-pile installation; but these initial pore pres­sures dissipated rapidly. However, pore pressure reductions due to excavation unloading were rather small. These small net reductions during excavation were a result of pore pressure increases that occurred immediately after each shear zone developed.

7. In spite of the large movements at the test section, the performance of the system was satisfactory. No structures or unsupported utilities were near the test section. The contractor had set the sheet-pile line back several inches from its design grade so that inward movement could be accommodated. All concrete pay lines were maintained and adjacent structures were undamaged.

8. The maximum measured loads in each strut were approximately equal to the magnitudes specified by the design earth pressure envelope. At this site, adequate strut load design did not necessarily guarantee that ground movements were kept to expected levels.

ACKNOWLEDGMENTS

Many people and several organizations contributed to the success of the field instrumentation effort of this project. Special thanks are due to Mr. Thomas Wagner, Kenny Construction Co. site superintendent, who made outstanding efforts to ensure that construction equipment did not inadvert­ently damage instruments at the test section. Kenny's cooperation was above and beyond the call of duty, especially considering that the ground instru­mentation was not part of the contract documents. Messrs. Ted Maynard, Jim Davis and Zenon Stuck of the Soils Section, Department of Public Works of the city of Chicago, coordinated their structural instrumentation with the ground instruments at the test section and helped in many of the planning aspects of the work. Their encouragement throughout the project is greatly appreciated. Mr. Richard D'Ambrosia, Chicago Metropolitan Sanitary Dis­trict, assisted in the early planning stages of the work. Messrs. Marc-Andre Kamel, Steven Nerby, Yori Sofrin, and Choong-Ki Chung, graduate stu­dents at Northwestern University, aided in the construction-monitoring ef­fort. Prof. Jean Benoit of the University of New Hampshire performed the

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SBPM tests. STS Consultants of Northbrook, 111., performed the field vane and Menard pressuremeter tests.

This material is based upon work supported by the National Science Foun­dation under Grant No. MSM-8796169.

The writers thank the ASCE reviewers for their thoughtful comments.

APPENDIX I. UNITS

To convert To Multiply by

in. mm 25.4 ft m 0.305 kips kN 4.450 kip/sq ft kPa 47.9

APPENDIX II. REFERENCES

Bjerrum, L., Clausen, C. J., and Duncan, J. M. (1972). "Earth pressures on flexible structures (a state-of-the-art report)." Proc. 5 th European Conf. on Soil Mechanics and Foundation Engineering, 2, Madrid, Spain.

Clough, G. W., and Hansen, L. A. (1981). "Clay anisotropy and braced wall be­havior." J. Geotech. Engrg. Div., ASCE, 107(7), 893-914.

Finno, R. J., Atmatzidis, D. K., and Nerby, S. M. (1988a). "Ground response to sheet pile installation in clay." Proc. 2nd Int. Conf. on Case Histories in Geo-technical Engineering, St. Louis, Mo.

Finno, R. J., Nerby, S. M., and Perkins, S. B. (1988b). "Soil parameters implied by braced cut observations." Proc. Symp. on Soil Properties Evaluation from Cen­trifugal Models and Field Performance, ASCE, Nashville, Tenn., 71-87.

"Measurements at a strutted excavation, Oslo subway, Gronland 1, Km 1.559." (1962a). Technical Report No. 1, Norwegian Geotechnical Institute, Oslo, Norway.

"Measurements at a strutted excavation, Oslo subway, Everhaugen South, Km. 1.982." (1962b) Technical Report No. 3, Norwegian Geotechnical Institute, Oslo, Norway.

"Measurements at a strutted excavation, Oslo subway, Vaterland 1, km. 1373." (1962c). Technical Report 6, Norwegian Geotechnical Institute, Oslo, Norway.

Mana, A. I., and Clough, G. W. (1981). "Prediction of movements for braced cuts in clay." J. Geotech. Engrg. Div., ASCE, 107(6), 759-778.

O'Rourke, T. D. (1981). "Ground movements caused by braced excavations." J. Geotech. Engrg. Div., ASCE, 107(9), 1159-1178.

Peck, R. B. (1942). "Earth pressure measurements in open cuts, Chicago (111.) sub­way." Proc, ASCE, 68(6), 900-928.

Peck, R. B. (1969). "Deep excavations and tunneling in soft ground." State-of-the-art report, 7th Int. Conf. on Soil Mechanics and Foundation Engineering, Mexico City, Mexico, 225-281.

Peck, R. B., and Reed, W. C. (1954). "Engineering properties of Chicago subsoils." Bull. No. 423, Engineering Experiment Station, Univ. of Illinois.

Terzaghi, K. (1943). Theoretical soil mechanics, John Wiley and Sons, Inc., New York, N.Y.

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