STRESS PATHS IN RELATION TO DEEP EXCAVATIONS

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F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .
FIG. 1.Idealized Stress Path Associated with Stress Relief:(a)Effective-Stress Path;(b)T otal Stress
Path
FIG. 2.Instrumented Panel
In contrast to element P ,the total stress change experienced by element A results from a reduction in horizontal stress.Changes in vertical stress are comparatively small.The total stress path A 2A 3will move toward the K f compression line with a gradient of 45Њin s Ϫt space,i.e.,⌬t /⌬s =Ϫ1.If the soil is unloaded rapidly so that undrained conditions are main-tained,the corresponding effective-stress path A 2ЈA 3Јwill be vertical.The induced pore-water pressure is negative.As the excess pore-water pressure disspates,the effective-stress path will shift toward the K f compression line.TOP-DOWN CONSTRUCTION AT LION Y ARD
The Lion Yard site is located in the city center of Cam-bridge,U.K.and it is approximately 65ϫ45m on plan.The site was developed as a three-level underground car park be-neath a five-story hotel above ground.The 10-m-deep under-ground car park is retained by a 17-m-deep perimeter rein-forced concrete diaphragm wall that is 0.6m thick (Fig.2).The initial stiffness of the uncracked concrete wall is estimated to be 580MN иm 2/m run.The car park floors,which prop the diaphragm wall,are supported by steel columns connected to the tops of bored piles.The design of the supporting system was fairly conservative and a K 0value of 3was adopted to calculate the design earth pressures for gault clay.
The ground conditions comprise approximately 3m of fill and gravel over 38m of gault clay.The gault clay is over-consolidated heavily and consists of stiff-to-hard silty grey fis-sured clay of high plasticity.The ground-water table was at approximately 3m below ground surface before construction.To speed up the construction and to minimize ground de-formations during excavation,the top-down construction method was adopted.For providing a working platform,the ground floor slab (level 4)was cast as soon as the diaphragm wall and bored piles and associated steel columns had been completed.Soil then was excavated from beneath it by me-chanical plants down to the next level (level 3)and removed
through an opening left in the slab.Similar operations were repeated for subsequent stages of construction until the bottom level (level 1)was reached.Erection of the superstructure was carried out simultaneously.Other details of the construction are given by Ng (1998).
For providing vehicle access to each underground parking level after construction,there was a 3.5-m-wide and 19-m-long rectangular opening left in the slabs,adjacent to an instru-mented panel shown in Fig.2.The selected instrumented panel was located near the center of one long side of the site.Tem-porary steel props (152ϫ152ϫ23universal steel columns
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TABLE
1.
Key Construction Stages
Stage
(1)Construction operation
(2)Week number
(3)
I Construction of diaphragm wall 1–7II Excavation to level 318–20III Excavation to level 223–25IV Excavation to level 1
29–32V 9months after casting level 1slab 78VI
25months after casting level 1slab
150
FIG. teral Displacement of Diaphragm Wall
FIG. 3.T otal Lateral Pressures during and after Construction
at level 4and 203ϫ203ϫ60universal steel columns at levels 2and 3)were installed at 1.7m spacing (on plan)across these 3.5-m-wide openings at each level to support the dia-phragm wall (including the instrumented panel)during exca-vation.The props were not prestressed,but the ends were grouted after installation.After completion of the level 1slab,the vehicle access ramps were constructed from the lowest level upward and the temporary props were removed once all the concrete ramps had been completed.Key excavation stages,which are relevant to this paper,are summarized in Table 1.
During the course of construction,comprehensive instru-ments were installed to monitor the performance of the mul-tipropped excavation.At the instrumented panel shown in Fig.2,seven total earth pressure cells (EPCs)(Glo ¨tzl)and seven pneumatic piezometers were installed on both faces of the di-aphragm wall panel.Each total pressure cell consisted of a sensitive pad (150ϫ250mm),formed by joining two thin plates of steel at their edges.The space between the plates was filled with mercury.The pressure applied by the surrounding ground was transmitted through the mercury to an integral pneumatic diaphragm similar to those used in pneumatic pi-ezometers.The mercury pressure was measured with a pneu-matic readout unit as used for reading piezometers.The cells were designed to operate to a maximum pressure of 1,500kPa.The seven pneumatic piezometers were positioned 150mm below the EPCs to measure the pore-water pressures on the faces of the wall.Hence the effective horizontal stresses can be determined.The surface contact tips of the piezometers were fitted with flat flush stones,which were pushed approx-imately 5mm into the excavated clay surface.The pneumatic piezometer system selected had been developed to measure positive pore pressures only.This imposed a limitation on measuring negative pore-water pressures caused by stress re-lief.In fact,one of the piezometers (PP 1)went to zero during the final stage of excavation (level 1)and so changes of ef-fective stress were not known at that location.Details of other instrumentation and interpreted results including wall deflec-tions,prop forces,and ground deformations are given by Ng (1998).
FIELD STRESS P ATHS
T otal Earth Pressures at Soil-Wall Interface
Initial stresses in the ground are very difficult to measure and determine accurately.At Lion Yard,initial stresses in the ground were estimated using a self-boring pressuremeter,Schmidt’s (1966)semiempirical rules allowing for reloading,and numerical simulations of the geological processes.Simp-son’s (1992)nonlinear Brick model was used for the numerical simulations.Based on results obtained from these methods,it was concluded that the initial K 0values varied somewhere be-tween 1.0and 2.0for the top 10m of the clay (Ng 1998).The measured lateral pressures on both sides of the wall are shown in Fig.3.A constant initial K 0=1.5with depth is provided for reference.Reduction in lateral stresses at the soil-wall interface following the construction of the diaphragm wall was observed.The reduction in total horizontal stress was
substantially larger during wall installation than during any subsequent stage of construction.During the subsequent ex-cavation stages,all EPCs excluding EPC7,showed consistent reduction in lateral stresses until the deepest basement level was reached.This observed reduction in lateral stresses was consistent with gradual inward deflections of the wall (Fig.4).In the long-term,there was a general trend of increasing in lateral stresses with time following excavation,except at EPC3.At stage VI (25months after casting level 1slab),four EPCs,EPC4–7(those still functioning properly),recorded an increase in total earth pressure,compared with measurements made before the main excavation.The increase in pressure probably resulted from general swelling of the clay.More de-tails are discussed later in this paper.
When considering the measured earth pressures in the ground,it is important to check that equilibrium requirements are satisfied.From beam bending theory,it is well known that there are relationships between applied net pressure,shear force,bending moment,wall curvature,rotation,and deflec-tion.At the instrumented diaphragm wall panel (Fig.2),the field data recorded including measured prop forces (Table 2)and wall rotations measured by electrolevels,enabled an in-ternal check to be carried out on the measured earth pressures if some boundary conditions and wall stiffness were known.This was done by constructing net earth pressure diagrams,which in conjunction with measured prop forces,satisfied the requirements of horizontal force and moment equilibrium,and after triple integration showed agreement with measured wall
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TABLE
2.Summary of Measured Prop Forces
Prop force
(1)
Measured
(2)
Average L4prop forces (kN/m)14Average L3prop forces (kN/m)119Average L2prop forces (kN/m)
144
FIG. 5.Interpreted Field Effective-Stress Path
rotations.These deduced earth pressure diagrams then were compared with the measured values of earth pressure (Ling et al.1993).Reasonable consistency was obtained,with an ex-ception at EPC3,which underrecorded lateral stress by ap-proximately 50kPa at the final stage of excavation based on the equilibrium analysis.The continuous decrease of total earth pressure at EPC3probably was caused by the softening of the clay.The EPC1recorded abnormal high stress at stage V (9months after casting level 1slab),and subsequently both EPC1and EPC2ceased functioning.
Effective-Stress Paths at Soil-Wall Interface
Changes in effective stress at the soil-wall interface are pre-sented in Fig.5in the form of stress paths.Four limiting pressure lines also are plotted,which represent Rankine’s ac-tive and passive pressures with and without wall friction.The limiting pressures have been calculated using the results of laboratory tests on intact specimens trimmed from block sam-ples obtained from the site.Details of the laboratory tests on gault clay are given in a subsequent section in the paper later on.
Horizontal effective stresses shown in the figure were cal-culated from measured total earth pressures and pore-water pressures at the seven locations of a vertical section at the soil-wall interface.The measured total earth and pore-water pres-sures were very likely to represent the localized behavior at the soil-wall interface only.Vertical effective stresses were cal-culated by assuming that total vertical stresses were equivalent to overburden pressures remote from the wall.The calculated horizontal and vertical effective stresses did not take wall fric-tion into account.The presence of wall friction (or shear stress)at the soil-wall interface would reduce the actual mag-nitude of the vertical stress.However,this reduction is likely to be small and will not affect any conclusions drawn in this paper.As the total vertical stress is assumed to be constant,
any change of pore-water pressure thus is reflected by a change of vertical effective stress.
The initial state of stress corresponding to the location of each EPC has been determined using the varying K 0profile with depth predicted by the nonlinear brick model.Reloading from gravel on the site has been taken into account (Ng 1992).Immediate after the installation of the instrumented panel,there were substantial reductions in horizontal effective stress at the soil-wall interface at all seven EPCs.Pore-water pres-sures at the soil-wall interface returned to their initial condi-tions within 10days later (Lings et al.1991;Ng 1992).If similar total stress and pore pressure reductions occurred in the soil farther away from the wall,then it would be expected that subsequent equalization of pore pressures would have been accompanied by swelling of the clay during the 3-month period after the construction of the wall.This would be ex-pected to result in an increase in total horizontal stress.On the contrary,all the earth pressure cells registered a decrease rather than an increase in total earth pressures (Fig.3).The recorded decrease in the total earth pressures could be caused by the gradual contraction of the EPCs as the temperature in the ground had decreased continuously after concrete curing.During the main excavation,three effective-stress paths (EPC2,4,and 6)behind the wall moved almost vertically up-ward,as a result of reducing pore pressures accompanied by a small increase in horizontal effective stress.The states of stress behind the wall at the soil-wall interface reached,or were close to,the assumed active condition.After stage V (9months after casting level 1slab),there was a continuous in-crease in horizontal effective stress accompanied by a decrease in vertical effective stress behind the wall.The increase in horizontal effective stress in the long-term is a result of an increase of total earth pressure,which is greater than the rise of pore-water pressure.The increase in the total earth pressure might be caused by long-term swelling of the clay behind the wall.
In front of the wall,all three stress paths show an increase in horizontal effective stress with a decrease in vertical effec-tive stress during the period of the main excavation.The soil at EPC3appears to have reached passive failure at stage V as illustrated by the abrupt change of direction of the stress path toward the origin.In contrast,the lowest two EPCs show a continuation of the same stress paths and do not appear to have reached passive failure.
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The field-observed effective-stress paths behind the wall clearly are very different from the idealized conceptual model shown in Fig.2.The idealization of constant mean effective stress (s Ј)during each excavation stage does not hold (not an undrained behavior)for the stiff fissured clay at the soil-wall interface.This discrepancy could be caused by a number of factors such as observed rapid equilibrium of pore-water pres-sures at the interface during and after each stage of excavation as reported by Lings et al.(1991).‘‘Opening up’’fissures in the clay caused by lateral stress relief or the formation of a ‘‘bleed channel’’at the soil-wall interface during diaphragm wall construction could result in the observed rapid rise of pore-water pressure.On the contrary,the field-observed effec-tive-stress paths in front of the wall can be represented rea-sonably by the idealized conceptual model during the main excavation stages.It can be deduced that the mean effective stress holds approximately constant (an undrained behavior)probably as a result of inward movement of the diaphragm wall during the main excavation.This inward movement could suppress the opening up of fissures in the clay caused by ver-tical stress relief and closing up the bleed channel at the in-terface.
LABORATORY STRESS P ATHS
During the last two stages of excavation at Lion Yard,300ϫ300mm intact block samples were taken using an electric chain saw.They were covered immediately with cling film and nonshrinkable wax and stored in a humidity-controlled room.All the samples used in the tests were from the elevation of ϩ4.8m above ordinary datum (OD)outside diameter,i.e.,approximately 5m below the original ground surface and 2–3m below the clay surface.At that depth,the estimated over-consolidation ration was 25–50.
Very few laboratory tests on gault clay have been performed using stress paths and stress levels that are relevant to deep excavations,during which the soil is subjected to unloading either vertically or horizontally at relatively low stress levels.Therefore,a laboratory program was carried out to investigate the response of gault clay related to the excavation at Lion Yard.
T est Program
Natural specimens were reconsolidated isotropically to var-ious preshearing pressures,which were relevant to the stress conditions around the excavation,and then loaded or unloaded along specified stress paths.The K 0reconsolidation was not carried out because zero lateral strains were difficult to achieve even in a computer-controlled triaxial stress path apparatus.In the following,the first and second letters of each set of ab-breviations are used to denote the drainage condition and the shearing mode.
1.Drained compression (DC)tests in triaxial cell •DC1:undisturbed,constant p Ј
•DC2:undisturbed,constant axial stress but with de-creasing radial stress
•DC3:undisturbed,⌬q /⌬p Ј=Ϫ1
2.Drained extension (DE)tests in triaxial cell •DE1:undisturbed,constant p Ј•DE2:undisturbed,⌬q /⌬p Ј=1
3.Undrained compression (UC)tests in triaxial cell
•UC1:undisturbed,constant radial total stress but with increasing total axial stress
•UC2:undisturbed,constant axial total stress but with decreasing total radial stress
4.Undrained extension (UE)tests in triaxial cell
•UE1:undisturbed,constant radial total stress but with
decreasing total axial stress
•UE2:undisturbed,constant axial total stress but with increasing total radial stress
All the tests were conducted under a constant room tem-perature of 24ЊC using a computer-controlled triaxial stress path apparatus.
Sample Preparation and T est Procedure
For the triaxial tests,each block sample was divided into four specimens using a thin wire.Each specimen then was mounted in a soil lathe and trimmed carefully to the required size (150mm long and 75mm in diameter)using various sharp blades.Great care was taken because the clay was fis-sured highly and very apt to fall apart.Small hard nodules frequently were found during the preparation process.The nodules were removed carefully and the cavities were filled with fine material from the parings.
For all triaxial tests,the specimens initially were consoli-dated isotropically unless stated otherwise to various pressures before shearing.No side drains were used.The consolidation was carried out against an elevated back-pressure of 200kPa (Atkinson et al.1989)to ensure complete saturation of the specimen.During the shearing stage,the loading or unloading rate was applied slowly at 1.0–1.5kPa/h in drained tests and 5–10kPa/h in undrained tests.
On completion of a test,the specimen was removed from the cell.It then was weighed and measured with venier cali-pers,and its water content was determined.
Observed Stress Paths and Measured Shear Strength The applied or observed stress paths for the natural speci-mens are shown in Fig.6.The stress paths are expressed in the (s Ј,t )stress space to compare field observations that are likely to be under plane strain conditions.For clarity,only a single stress path is used to denote each series of tests.The critical state line shown in the figure was obtained from tests on reconstituted samples,which possess a critical state friction angle of 26Њ.Details of the tests are given by Ng (1992).All the test results for both reconstituted and natural specimens are summarized in Fig.7,which shows the end points of the stress paths based on the maximum deviator stress failure cri-terion.Although the specimens were subjected to various stress paths,the failure points lie on two common failure en-velopes for each type of material.It is evident that the failure envelopes are different in compression and in extension for both reconstituted and natural samples.
For all tests on natural samples,failure took place abruptly along a single slip surface.It was not possible to ascertain whether failure occurred along a preexisting discontinuity,al-though all specimens contained numerous fissures.A higher shear strength was mobilized in extension than in compression.Similar results were found by Tedd and Charles (1985)from tests on natural London clay from Bell Common.Slightly dif-ferent Mohr-Coulomb parameters were found in compression (c Ј=3kPa and ␾Ј=32Њ)and in extension (c Ј=2kPa and ␾Ј=34Њ).
COMP ARISON OF FIELD AND LABORATORY STRESS P ATHS
Four representative field stress paths monitored at the soil-wall interface are compared with four corresponding stress paths measured in the laboratory in Fig.8.The stress path of UE1terminated a long way from the average passive pressure line probably because the specimen was noted to be extremely fissured during sample preparation and failure took place along a preexisting fissure.
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FIG. 6.Effective-Stress Path for Natural
Specimens
FIG.7.Failure Points of All Specimens
The stress paths followed in the UE tests are similar to the field stress paths in front of the wall.There is no obvious difference between a test with constant radial total stress but with decreasing total axial stress (UE1)and an experiment with constant axial total stress but with increasing total radial stress (UE2).This seems to suggest that the stress changes in front of the wall can be represented reasonably by UE tests in the laboratory.In contrast,neither type of compression tests (UC1and UC2)modeled the field conditions behind the wall particularly well,except when the stress state approached ac-tive failure.This is not surprising because the soil at the soil-wall interface had already been reached or was close to the active condition after wall installation,resulting from a sub-stantial horizontal stress relief during the wall construction.Details of a three-dimensional numerical analysis of the dia-phragm wall panel construction at Lion Yard are given by Ng and Yan (1998).Thus it would be consistent to compare the field and laboratory stress paths only in the region near the active condition.It can be seen that the field and laboratory stress paths compare favorably during the main excavation stages.
Based on the comparisons of field and laboratory stress paths,practicing engineers may realize that stiff clays located
behind a diaphragm wall may easily reach the active condi-tions locally after wall installation.Experimental results from undrained triaxial compression tests (i.e.,reducing radial but keeping axial stress constant)and an undrained assumption only may be used with caution when analyzing and design-ing retaining-wall systems in stiff clays.On the contrary,an undrained assumption inside excavations and the use of un-drained triaxial extension stress path tests (i.e.,reducing axial but keeping radial stress constant)seem to be relevant for en-gineers to derive design parameters for excavations in stiff clays.
CONCLUSIONS
Horizontal earth and pore-water pressures at the soil-wall interface during the construction of a deep excavation in the stiff fissured clay (gault clay)at Lion Yard,Cambridge,U.K.were measured locally using seven total EPCs and seven pneu-matic piezometers,respectively.Effective-stress changes as-sociated with the vertical and horizontal stress relief during the excavation were obtained from these measurements and expressed in terms of stress paths.The field stress paths were compared with an idealized conceptual model and laboratory measured stress paths.Because the field measurements were
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parison of Laboratory and Field Stress Paths
taken locally behind the wall,the conclusions drawn in this paper should be considered carefully for general applications.Based on the local field-observed effective-stress paths be-hind the wall,the theoretical concept of constant mean effec-tive stress during each excavation stage does not hold in the stiff fissured clay,i.e.,the response of the clay was not un-drained at the interface.On the contrary,the field effective-stress paths in front of the wall seem to support the conven-tional assumption about undrained behavior of the clay (constant mean effective stress)inside the excavation.
The field effective-stress paths in front of the wall were similar to those stress paths observed in the UE tests in the laboratory.There is no obvious difference between a test with constant radial total stress but with decreasing total axial stress and an experiment with constant axial total stress but with increasing total radial stress in terms of effective-stress path and shear strength.However,for the stress paths behind the wall the field observations did not correspond particularly well with those from laboratory undrained tests on natural speci-mens in compression,except when the stress state approached active failure.
ACKNOWLEDGMENTS
The writer would like to thank the financial support from JT Design Build and the Science and Engineering Research Council,and the co-operation of site staff of JT Design Build.The writer also thanks Dr.B.Simpson of Arup Geotechnics and M.Lings and D.Nash of Bristol Uni-versity,U.K.for their useful discussions and comments on the field mon-itoring results.
APPENDIX I.REFERENCES
Atkinson,J.H.,Lau,W.H.W.,Powell,J.J.M.(1989).‘‘Determination of soil stiffness parameters in stress path probing tests.’’Proc.,12th Int.Conf.on Soil Mech.and Found.Engrg.,Rio de Janeiro ,V ol.1,Balkema,Rotterdam,The Netherlands,7–10.
Lambe,T.W.(1967).‘‘Stress path method.’’J.Geotech.Engrg.Div.,ASCE,93(6),309–331.
Lings,M.L.,Nash,D.F.T.,and Ng,C.W.W.(1993).‘‘Reliability of earth pressure measurements adjacent to a multi-propped diaphragm wall.’’Retaining structures ,Thomas Telford,London,258–269.
Lings,M.L.,Nash,D.F.T.,Ng,C.W.W.,and Boyce,M.D.(1991).‘‘Observed behavior of a deep excavation in Gault clay:A preliminary appraisal.’’Proc.,10th Eur.Conf.on Soil Mech.and Found.Engrg.,Florence ,V ol.2,Balkema,Rotterdam,The Netherlands,467–470.Ng,C.W.W.(1992).‘‘An evaluation of soil-structure interaction asso-ciated with a multi-propped excavation,’’PhD thesis,University of Bristol,U.K.
Ng,C.W.W.(1998).‘‘Observed performance of multipropped excavation in stiff clay.’’J.Geotech.and Geoenvir.Engrg.,ASCE,124(9),889–905.
Ng,C.W.W.,and Yan,W.M.(1998).‘‘Stress transfer and deformation mechanisms around a diaphragm wall panel.’’J.Geotech.and Geoen-vir.Engrg.,ASCE,124(7),638–648.
Schmidt,R.(1966).‘‘Discussions.’’Can.Geo.J.,Ottawa,3(4),239–242.
Simpson,B.(1992).‘‘Thirty-second Rankine lecture:Retaining struc-tures:Displacement and design.’’Ge ´otechnique ,42(4),541–576.Tedd,P.,and Charles,J.A.(1985).‘‘The strength of London Clay in relation to the design of embedded retaining walls.’’Ge ´otechnique ,35,199–204.
APPENDIX II.NOTATION
The following symbols are used in this paper:c Ј=effective cohesion;
h =subscript h means horizontal direction;K 0
=coefficient of earth pressure at rest;K f -line
=failure line in s ЈϪt stress space;p Ј=ϩ;(␴Ј2␴Ј)/313q =Ϫ;(␴Ј␴Ј)13s =(␴v ϩ␴h )/2;s Ј=ϩ(␴Ј␴Ј)/2;v h t =t Ј
=(␴v Ϫ␴h )/2;
v =subscript v means vertical direction;u =pore pressure;⌬=increment;
␴1,␴3=major and minor principal total stresses;␴Ј,␴Ј13
=major and minor principal effective stresses;␴h =total horizontal stress;␴v =total vertical stress;
␴Јh =horizontal effective stress;␴Јv =vertical effective stress;and ␾Ј
=
effective angle of friction.
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