Analysis on Deep Excavation in Soft-Soil Located on Sloped Bedrock

Shong-Loong Chen1,a, Cheng-Tao Ho2,b 1Graduate Institute of Civil and Disaster Prevention Engineering, National Taipei University of

Technology, Taipei 10608, Taiwan.

2Graduate Institute of Engineering Technology, National Taipei University of Technology, Taipei

10608, Taiwan.

a f10391@ntut.edu.tw,

bs4679005@ntut.org.tw

Keywords: Sloped Bedrock, Soft-Clay, Deep Excavation, Finite Element

Abstract. Deep excavations in soft-clay layer on sloped bedrock often leads to lateral displacement

on retaining structures and uneven settlement due to unbalanced pressure generated from excavation.

A construction project for which an excavation was complete in soft clay layer on sloped bedrock in

Taipei City was adopted in the study. It is learned from the observation logs of the studied case that a

significant difference exists in the lateral displacement of diaphragm wall and settlement between up

and down-slope sides of sloped bedrock. Deep excavation is in fact profoundly complicated

interaction between excavation strutting and soil. In general practice, the design of excavation is

frequently simplified as a 2D strain behavior. However, the actual excavation on sloped bedrock is

quite different from 1D or 2D simulation in a symmetric manner. Therefore, 2D finite element

analysis program, PLAXIS, is introduced for the analysis on the behaviors of soil clay layer on sloped

bedrock in excavation. The result is compared with onsite observation data, including displacement of

retaining wall, settlement, axial loads of struts and others. The result of retaining wall displacement

analysis is found consistent with the trend derived from onsite observation, which is possible for

reference of similar engineering analyses and designs in the future.

Introduction

Deep excavation has become an indispensable link of urban development as the urban economy

advances, and the excavation is becoming deeper and deeper. The safety of construction and the

minimization of impact on surrounding environment are some of the purposes for studies in this field.

Taipei Basin is covered in soft and extremely soft soil at uneven depths. Therefore, sloped bedrock is

one of the issues facing basement excavation. If the excavation behaviors are analyzed only with 1D

or 2D symmetric cross-sections, there is a risk that the actual excavation will be different from the

analysis. The subject of this study is the deep excavation of a building project in Taipei City which

was recently completed. Different approaches were adopted for analysis. For clay, the Soft Soil (SS)

and Soft Soil Creep (SSC) models provided in PLAXIS were used for comparison. The creep in the

excavated soft-clay was investigated and compared with the monitoring data obtained on site,

including retaining wall displacement, ground settlement and axial force in struts.

Project Brief and Ground Conditions

Project Background. Located at northern Taiwan, the Taipei Basin is roughly in triangular shape.

The basin extends to Nangang of Keelung River Valley to the east, Xindian of Xindian River Valley

to the south, Dahan River Valley to the southwest, and Guandu of Danshui River Valley to the north.

Y.J. Fu et al. [1] indicated that the basin has a circumference of nearly 70 km and a size of 243 km2

above EL. +20m, only next to Taichung Basin. The case is located at Nangang, Taipei. In general, the

soil formation is mostly homogeneous clay of low plasticity -8m~21m below ground, with SPT-N of

mostly <1~2, thus very soft soil.

Applied Mechanics and Materials Vols. 170-173 (2012) pp 13-19Online available since 2012/May/14 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.170-173.13

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 141.213.236.110, University of Michigan Library, Media Union Library, Ann Arbor, USA-21/11/14,02:22:13)

The 70 m long by 20~35 m wide excavation site, which was required for the construction of a

two-story basement, was located in Nangang District of Taipei. There was a vacant lot on its west

side, while its north, east, and south sides were abutted to a 12 m wide road and buildings of 4 to 5

storeys. The bottom-up construction was supported by a 0.6 m thick diaphragm wall that penetrated 1

to 3 m deep into the sandstone bedrock. The basement was excavated in five stages and braced with

four levels of strutting (see Table 1). The excavation safety monitoring system consisted of

inclinometer installed in the diaphragm walls and outside of the walls, settlement points, and

inclinometers in adjacent buildings, piezometers and observation wells for groundwater table.

Table 1 Sequence of excavation for each excavation stage

Sequence of excavation Activities

1 1

st excavation stage to EL -2.2 m, and installed 1 st level

strutting ST1 at EL -1.5 m.

2 2

nd excavation stage to EL -5.1 m, and installed 2 nd level

strutting ST2 at EL -4.2 m.

3 3

rd excavation stage to EL -7.5 m, and installed 3 rd level

strutting ST3 at EL -6.6 m.

4 4

th excavation stage to EL -9.0 m, and installed 4 th level

strutting ST4 at EL -8.1 m.

5 Final excavation stage to EL -11.6 m.

Site Geology and Groundwater Table. 4 distinctive soil layers were observed according to the

report of geotechnical investigation. As shown in Fig.1, the site consists of a 1.8 to 2.8 m thick fill

material, followed by a 6 to 18.6 m thick very-soft (SPT “N” value of about 1 to 2) clay layer that

overlies a 4.2 to 8 m thick sand layer. Beneath the sand layer was a sandstone bedrock that was

inclined towards the west. Table 2 shows the depth of the soil layers and some of the soil properties

obtained from a series of field and laboratory tests for the study site. The water table was very close to

the ground surface.

Fig. 1 Layout of retaining structures and profile of soil layer.

Table 2 Soil properties of the study site

Soil Depth SPT γt Su Effective stress Compressibility

Layer “N” C ' φ' Ｃc Ｃr

(m) Value kN/m3 (kPa) (kPa) (°)

Backfill (SF) 0~2.7 10 19.33 － 0 28 － －

Clay (CL) 1.8~21.5 1~2 17.07 19.82 0 26 0.45 0.05

Fine sand (SM) 4.4~25.8 12 19.82 － 0 30 － －

Bedrock (SS) 4.0~>25.8 >50 22.56 － 34.34 31 － －

14 Progress in Civil Engineering

Numerical Modeling

Materials Parameters. The 2D FEM program PLAXIS was used for analysis. SS and SSC models

were introduced for clay and Mohr-Coulomb (MC) model for sand and rock. Soil parameters γt, C '

and φ ' of 1D analysis were obtained in the lab soil tests as shown in the report of geotechnical

investigation. No test was conducted to determine the Young’s modulus E, and therefore the triaxial

compression tests and triaxial extension tests conducted by Kung et al. [2] on silty-clay of Taipei were

studied to develop the normalized relationship between secant modulus and undrained shear strength

for the clay of Taipei. The E/Su ratio was approximately 400~600 when the axial strain was

0.05%~0.10%, where Su is the undrained shear strength. Wang et al. [3] pointed out that a number of

previous tests conducted for Taipei Basin revealed that the normalized undrained initial stiffness

modulus of clay was 500 when the strain was below 0.05%. Thus, E/Su ratio was between 400 and

600 for typical numeric analysis. The Young’s modulus of the silty-clay was estimated from the

equation E=500 Su. The Young’s modulus of the silty-sand was estimated from the equation

E=1950N, where N is the average SPT ‘N’ value. Plane strain model requires shear modulus G and

bulk modulus B, which can be determined using Eqs. (1) and (2) below:

)21(3 ν−=

EB (1)

)1(2 ν+=

EG (2)

No test was conducted to determine the Poisson’s ratio (ν) in this case, and the relations of ν and φ' was determined using the static soil pressure coefficient K0 for normal consolidated soil proposed by

Jaky [4] and that proposed by Ishihara [5] for the MC model, as shown in Eq. (3):

)'sin2(

)'sin1(

φ

φν

−

−= (3)

The Poisson’s ratio ν for the SS and SSC models for normal consolidated clay is completely an

electric constant under unloading/reloading condition. It is different from that of the MC model,

which is usually 0.10~0.20. ν=0.15 was used for the analysis of this study. The compression index (λ)

and unloading/reloading index (or recompression index) (κ) were determined using Eqs. (4) and (5)

below after the unidirectional consolidated compression coefficient Cc and recompression coefficient

Cr were determined:

10ln

Cc=λ (4)

10ln

Cr=κ (5)

The relationships between the corrected compression index (λ*) and corrected recompression

index (κ*) of the SS and SSC models and the void ratio (e) were shown in Eqs. (6) and (7):

)1(*

e+=

λλ (6)

)1(

2*

e+=

κκ (7)

The time-dependent creep effect of soil was considered for the SSC model. The relationship

between the corrected creep index (µ*) and secondary compression index (Cα) is shown in Eq. (8).

However, Binkgreve [6] suggested that the ratio of λ*/µ* generally ranged between 15 and 25, a value

of 25 had been used in this study.

)1(3.2*

e

C

+=

αµ (8)

Applied Mechanics and Materials Vols. 170-173 15

The range was considered up to 4 times of the excavation depth. 9.81 kPa of uniform load

distributed along the east and west sides was assumed. The time-dependent creep effect of clay was

analyzed based on the actual construction schedule. Plate elements were used for the simulation of

diaphragm walls, 2-Node anchor elements for struts, and plane strain triangular 6-Node elements for

soil. The deformation of numeric grid after excavation is shown in Fig. 2.

(West) (East)

50m

125m

Fig. 2 Exaggerated deformed mesh after excavation

Structure Materials and Soil Parameters

Retaining Structure Parameters. To consider the general construction quality and the load-transfer

efficiency between the diaphragm wall and the strutting system, an efficiency ratio has been applied to

Ec. Through a series of back analysis of diaphragm walls displacements in soft-clay excavations, Ho

and Chen [7] suggested a value of 0.7 for the efficiency ratio. Thus, the 0.6 m thick diaphragm wall

has been assigned with a flexural stiffness, EcI, and normal stiffness, EcA, of 290 MPa and 9,700

MN/m, respectively. The Poisson’s ratio of the concrete used was 0.15. The Young’s modulus, Es,

and Poisson’s ratio for the steel strutting system was 2.06×105 MPa and 0.3, respectively. The

strutting sizes and parameters used in the FE simulation have been listed in Table 3.

Table 3 Strutting parameters

Strut level

Installation depth

ST1

GL-1.5m

ST2

GL-4.2m

ST3

GL-6.6m

ST4

GL-8.1m

Dimensions (mm) H-350×350×12×19 2H-350×350×12×19 2H-350×350×12×19 2H-400×400×13×21

Axial stiffness, EA (kN) 2.51E+06 5.01E+04 5.01E+04 6.32E+04

Preload (kN/m) 89.2 285.4 356.7 356.7

Area (cm2) 173.9 347.8 347.8 437.4

Soil Parameters. In summary, four cases of analysis (Table 4) have been performed, and Table 5

provides the parameters of each layer of soil.

Table 4 Combinations of analysis with different soil models

Cases Fill Soft-clay Sand Sandstone

Method A Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb (inclined)

Method B Mohr-Coulomb Soft Soil Mohr-Coulomb (inclined)

Method C Mohr-Coulomb Soft Soil Creep Mohr-Coulomb (inclined)

Method D Mohr-Coulomb Soft Soil Creep Mohr-Coulomb ((horizontal)

Table 5 Soil Parameters

Layer

C'

(kPa)

φ '

(°)

E

(kPa)

ν

λ*

κ*

µ*

SF 0.0 28 19620 0.35 － － －

CL 0.0 26 － 0.15 0.087 0.019 0.03

SM 0.0 30 23544 0.33 － － －

SS 34.34 31 98100 0.33 － － －

16 Progress in Civil Engineering

0

5

10

15

20

25

30

-5 0 5 10 15 20

Dep

th (m

)

Wall displacement (cm)

Mehtod AMehtod BMehtod CMehtod DField data

0

5

10

15

20

25

30

-5 0 5 10 15 20

Dep

th (m

)

Wall displacement (cm)

Mehtod A

Mehtod B

Mehtod CField data

Results and Discussion

The maximum values and field data after excavation are summarized in Tables 6. After excavation,

the maximum wall displacements on the west-side were 8.82cm~10.99cm (as Fig. 3a) for Methods

A~C. Compared to the maximum diaphragm displacement of 11.10 cm, they fell between 80%~99%,

where the displacement of Method C reached 99% of the monitoring data. Fig.5 shows the FE

simulated and field monitored axial forces of the strutting system. For the first three levels of strutting

forces, the back-analyzed strutting forces for Case C were almost similar to that observed in the field

while Cases A, B and D showed that they were within ±15% of the field monitored value. The

diaphragm wall displacements of these two were extremely consistent, indicating that the

consideration of creep effect in soil is suitable for deep excavation in soft clay and likely to provide a

result close to the field data.

Table 6 Comparison of FEM simulated maximum wall displacement and maximum ground

settlement with field monitored data

Cases West-side East-side

δhmax (cm) δvmax (cm) δvmax/δhmax δhmax (cm) δvmax (cm) δvmax/δhmax

Method A 9.41 5.06 0.54 -2.03 2.51 1.23

Method B 8.82 5.62 0.64 -1.36 3.24 2.38

Method C 10.99 5.87 0.53 1.50 1.96 1.31

Method D 9.13 5.62 0.62 9.13 5.62 0.62

Field Data 11.10 8.59 0.77 0.58 1.78 3.07

Fig. 3b shows that the maximum wall displacement on the east-side was 1.50cm~-2.03cm for

Methods A~C after excavation, somewhat different from the maximum monitoring data of 0.58 cm.

The displacement from analysis was minimal, and displayed negligible difference compared to the

monitoring data. If Method D was taken without considering the sloped bedrock, the displacement

obtained was 9.13cm, indicating significantly overestimated displacement.

The maximum ground settlement on the west-side obtained from analysis was 5.06cm~5.87cm,

approximately 60%~70% of the maximum ground settlement monitored, 8.39cm. However, the

location where the maximum analytic settlement occurred was different from that of field monitoring

data (Fig. 4a). The maximum ground settlement on the east-side obtained from analysis was

1.96cm~3.24cm, approximately 110%~182% of the maximum ground settlement monitored, 1.78cm.

The settlement troughs from analysis were relatively consistent with those from monitoring data (Fig.

4b).

(a)

(b)

Fig. 3 Comparison of (a) west- side; and (b) east-wall displacement profiles with field observed data.

Applied Mechanics and Materials Vols. 170-173 17

-15

-10

-5

0

010203040

Gro

und Set

tlem

ent (c

m)

Distance from west diaphram wall (m)

Mehtod A

Mehtod B

Mehtod C

Mehtod D

Field data

(a)

-15

-10

-5

0

0 10 20 30 40

Gro

und S

ettlem

ent(cm

)

Distance from east diaphram wall (m)

Mehtod A

Mehtod B

Mehtod C

Mehtod D

Field data

(b)

Fig. 4 Comparison of (a) west-side; and (b) east-side ground settlement trough with field observed data.

The ratio between the maximum ground settlement and maximum wall displacement, δvmax /δhmax

ratio (see Table 6), fell between 0.53 and 0.64, slightly smaller than the monitoring data δvmax /δhmax

ratio. Looking at Ou et al. [8], the analysis results were close to the lower limit of the empirical value

for Taipei Basin, which is δvmax /δhmax =0.5~1.0.

The creep ratio δChmax/δhmax was the ratio of the maximum time-dependent creep of the wall δChmax

to the maximum displacement. Fig. 6 shows that the creep ratio increased linearly with the depth, and

the largest creep ratio was approximately 40%, whereas that near the excavation level was 30%,

consistent with 23%~30% proposed by Lin and Wang [9]. Therefore, it is suggested to consider

time-dependent creep effect for deep excavation in extremely soft-clay for an extended excavation

schedule, as to avoid underestimating the displacement.

0

500

1000

1500

2000

2500

3000

3500

0 500 1000 1500 2000 2500 3000 3500

Obsevations (kN)

FEM

anal

ysis

(kN

)

Method AMethod BMethod CMethod D

+15%

-15%

Fig. 5 Comparison of FEM simulated and field

monitored strutting axial force.

0

5

10

15

20

25

30

0 20 40 60 80 100

Dep

th (m

)

Creep ratio (%)

Fig.6 Creep Ratio vs. Depth

Conclusions

The maximum displacement observed in the diaphragm walls on the side where the clay was

thicker was 11.10cm, which is close to the results from the analysis methods (8.82cm~10.99cm). This

indicates that FEM is a feasible means for analysis of deep excavations in soft-soil above sloped

bedrock. However, the actual displacement would be overestimated on the side where clay is thinner

if the analysis is performed with a simplified 1D or 2D symmetric model.

Time-dependent analysis of consolidation theory was introduced in this study. The result obtained

from the side where the soft-clay was deeper was approximately 99% of the monitoring data, and the

diaphragm wall displacement curves of both cases were very close, suggesting that proper

consideration of the time-dependent creep effect for deep excavation in soft-clay provides results

closer to the monitoring data.

18 Progress in Civil Engineering

The creep rate of diaphragm walls in deep excavation in soft-clay increased linearly with the depth

with the maximum creep rate of 40% and 30% close to the excavation level. It is suggested to prevent

underestimation of diaphragm wall displacement by considering time-dependent effects in deep

excavation in weal clay for an extended period of time.

The influence of sloped bedrock and clay’s time-dependent creep effect to deep excavation was

investigated, and the diaphragm wall displacement and deformation curve extremely matching the

monitoring data and reasonable axial force in strutting were obtained. However, the ground

settlement at backfill side was still contradictive to the monitoring data. A few studies are being

conducted on the settlement on backfill side using small soil strain behavior, and ground settlement

data have been obtained that are close to monitoring data. This is a topic worth further investigation.

References

[1] Y. J. Fu, C. T. Chin, Y. L. Wang and M. S. Chen: J. of Civil and Hydraulic Engineering. Vol. 16(4)

(1990), p. 59 (in Chinese)

[2] G. T. C. Kung, C. Y. Ou, and C. H. Juang: Computers and Geotechnics. Vol. 36 (2010), p. 304

[3] C. C. Wang, H. D. Lin and M. F.Wu: Sino-Geotechnicals. Vol. 76 (1999), p. 51 (in Chinese)

[4] J. Jaky: Preceeding of the 2nd Interational conference on soil mechanics and foundation

engineering, Rotterdam, Vol. 1 (1948).

[5] K. Ishihara: Soil Behaviour In Earthquake Geotechnics, Oxford University Press Inc., NY (1996).

[6] R. B. J. Brinkgreve: PLAXIS-2D — user’s manual and scientific manual. Version 8 [computer

program]. A. A. Balkema, Rotterdam, the Netherlands (2004).

[7] C. T. Ho and S. L. Chen: Chinese J. of Geotechnical Engineering. Vol. 30(2) (2008), p. 185 (in

Chinese)

[8] C. Y. Ou, P. G. Hsieh and D. C. Chiou: Can. Geotech. J., Vol. 30(5) (1993), p. 758.

[9] H. D. Lin, C. C. Wang and C. Y. Ou: J. of the Chinese Institute of Engineers, Vol. 25(2) (2002), p.

223

Applied Mechanics and Materials Vols. 170-173 19

Progress in Civil Engineering 10.4028/www.scientific.net/AMM.170-173 Analysis on Deep Excavation in Soft Soil Located on Sloped Bedrock 10.4028/www.scientific.net/AMM.170-173.13

DOI References

[2] G. T. C. Kung, C. Y. Ou, and C. H. Juang: Computers and Geotechnics. Vol. 36 (2010), p.304.

http://dx.doi.org/10.1016/j.compgeo.2008.01.007 [8] C. Y. Ou, P. G. Hsieh and D. C. Chiou: Can. Geotech. J., Vol. 30(5) (1993), p.758.

http://dx.doi.org/10.1139/t93-068 [9] H. D. Lin, C. C. Wang and C. Y. Ou: J. of the Chinese Institute of Engineers, Vol. 25(2) (2002), p.223.

http://dx.doi.org/10.1080/02533839.2002.9670697