1 INTRODUCTION For deep excavations in soft marine clay, the temporary retaining wall system is commonly designed using the finite element method. Of all the soil models, the Mohr-Coulomb model is frequently adopted in the design because of its simplicity. There are two options to conduct an undrained analysis to simulate the end-of-construction condition using this model. The first option is to adopt the total stress approach using total stress parameters (cu, φu, Eu, and ν = 0.495). The second option is to con-duct an effective stress analysis using effective stress parameters (c′, φ′, E′ and ν′). For undrained analysis involving soft clay, the first option can produce reasonable results whereas the second option will produce erroneous results (Wong, 2003 & 2004). There are three major shortcomings associated with Option one of the Mohr-Coulomb model. Firstly, the nonlinear behaviour prior to failure is not modeled. Secondly, the option is unable to generate reli-able pore pressure if any. Thirdly, it cannot estimate the post-excavation settlement. Even though Op-tion 2 can generate pore pressures and can estimate the post-excavation settlement, the results are unre-liable. One way to overcome these shortcomings is to adopt the modified Cam Clay model in the analysis. This is an elasto-plastic soil model developed specifically to simulate the soft clay behaviour. The theory behind it is complex but the soil parameters involved are relatively simple. Many of them can be estimated using published correlations based on Atterberg’s limits. The objective of this paper is to validate the reliability of results generated by this soil model together with the soil parameters obtained from published correlations. Six deep excavation projects were back-analyzed with the aid of the computer program SAGE CRISP. They are Lavender MRT Station, MOE building, Syed Alwi project, Rochor complex, cut-and-cover tunnel between Farrer Park and Kandang Kerbau, and an office building in Taipei. In this study, the Modified Cam Clay model was used to simulate the marine clay behaviour and the Mohr-Coulomb model was used for other soils. Only undrained analyses were carried out. The water table behind the wall was assumed to be unchanged. The water table within the excavated area was assumed to follow the excavation depth.

Evaluation of Modified Cam Clay Parameters for Deep Excavation Analysis D. Halim & K. S. Wong Nanyang Technological University, Singapore

ABSTRACT: In Singapore, many deep excavation projects are in soft clay. The Mohr-Coulomb model is commonly adopted in the finite element analysis of such projects. This model works well for the undrained analysis of saturated clay based on total stress approach. However, it may not be appro-priate to use it for consolidation analysis. A better alternative is to use the modified Cam Clay model. The required soil parameters can be readily estimated from published correlations using the Atter-berg’s limits. The validity of this soil model together with the soil parameters from simple correlations has been checked against a number of case studies through back-analysis with the aid of the computer program SAGE CRISP. The agreements between measured and computed maximum wall deflections are approximately within 25%.

2 MODIFIED CAM CLAY PARAMETERS The basic parameters are: κ = slope of swelling and recompression line λ = slope of normally consolidated line eΓ = void ratio on the critical state line at p’ equals to 1 kPa M = slope of critical state line projected to q’ versus p’ plane ν = Poisson’s ratio pc’ = maximum past pressure in terms of mean effective stress Parameter M and pc’ can be calculated as follows:

(1)

(2) (3)

(4) where: φ′cs = critical state friction angle Knc = coefficient of in situ earth pressure in normally consolidated state OCR = Over-consolidation ratio σv’ = vertical effective stress Mitchell (1976) proposed a correlation between the critical state friction angle (φ’cs) and plasticity in-dex PI (%) as shown in Figure 1. The correlation is as follows:

PIcs ln094.08.0sin ' −=φ (5)

Figure 1. Friction angle at critical state φ′cs or φ′cv versus Plasticity Index (Mitchell, 1976)

cs

csM'sin3

'sin6φ

φ−

=

'max'

max

2max

' ppM

q

pc +

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛⎟⎠⎞

⎜⎝⎛

=

( )( )OCRKq vnc'

max 1 σ−=

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛+=

312

''max

OCRKp v

ncσ

Kulhawy & Mayne (1990) proposed correlations between compression index Cc and unloading-reloading index Cur with the plasticity index PI (%) as shown in Figure 2. The correlations can be ex-pressed as follows:

PIPICc 00586.07410ln

110ln

=⎟⎠⎞

⎜⎝⎛==λ (6)

PIPICur 00117.037010ln

110ln

=⎟⎟⎠

⎞⎜⎜⎝

⎛==κ (7)

Figure 2. Correlations between Cc and Cur with Plasticity Index (Kulhawy and Mayne, 1990)

Wood (1990) proposed an equation that relates eΓ as a function of specific gravity (Gs), liquid limit LL (%), and plasticity index PI (%). The correlation is as follows:

( )

1003.0 PILLG

e s +=Γ (8)

OCR value can be approximated by Equation 9 (Jamiolkowski, et al, 1985).

( ) 8.004.023.0'

OCRc

v

u ±=⎟⎟⎠

⎞⎜⎜⎝

⎛σ

(9)

where cu = undrained cohesion; σv’ = vertical effective stress; and OCR = over-consolidation ratio. 3 CASE STUDY Six deep excavation cases were back-analyzed. The first case is an office building in Taipei Basin in Taiwan. The remaining five cases are from Singapore. They are Lavender MRT Station, MOE build-ing, Syed Alwi project, Rochor complex, and cut-and-cover tunnel between Farrer Park and Kandang Kerbau MRT stations. 3.1 Case 1: Taipei Basin

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

Top Clay 16.5 25 50 2.65 0.0293 0.1465 1.52 29.8 1.19 0.33 2.4 to 1

Soil Type Gs κ λ eΓ M ν'

γsat E50 c φ

(kN/m3) (kPa) (kPa) (o)Back Fill (Sandy Clay) (PI = 40) 17 0.495 11250 25 0 Total StressIntermediate Clay (PI = 25) 17 0.495 80000 100 0 Total StressSand (N > 40) 20 0.33 130000 0 42 Effective Stress

ApproachνSoil Type

Fang (1987) reported a 14.1 m deep excavation in the eastern part of Taipei Basin. The 700 mm thick diaphragm wall penetrated into the intermediate clay layer where the SPT N-value was about 22. The wall length was 29.8 m. The excavation width was about 68 m. The cross-section and soil profile are shown in Figure 3. The soil parameters adopted in the analysis are shown in Table 1. The ground water table was about 2 m below surface. Preloading of struts was not included in the analysis. The com-puted and measured wall deflection profiles are shown in Figure 4. The maximum measured and pre-dicted wall deflections are 180 and 160 mm, respectively. The difference is about -11%.

Table 1. Soil properties of Case 1 – an office building at Taipei Basin in Taiwan

(a) Soil properties of soft clay

(b) Soil properties of other soils

Figure 3. Typical soil profile and cross-section of the excavation – Case 1 (Fang, 1987) 3.2 Case 2: Cut-and-Cover Tunnel between Farrer Park and Kandang Kerbau (CH 31+895) This cut-and-cover tunnel was part of the MRT North East Line construction in Singapore. The sec-tional view is shown in Figure 5. The 800 mm thick diaphragm wall was constructed to a depth of 26 to 28.4 m. The excavation width was 21 m. The original ground water level was about 1 m below sur-face. The soil parameters adopted in the analysis are shown in Table 2. Preloading was not included as the preload values were not available to the authors. The computed and measured wall deflection pro-files are shown in Figure 6. The maximum measured and predicted wall deflections are 60 and 80 mm, respectively. The difference is about +33%.

Fill102.0 m

99.5 m

97.0 m

94.5 m

91 m

88.5 m

85.5 m

81.0 m

74.6 m

65.0 m

103.0 m

99.5 m

94.5 m

91.0 m

88.5 m

81.0 m

74.6 m

65.0 m

Fluvial Sand (F1)

Upper Marine Clay

Fluvial Clay (F2)

Lower Marine Clay

OA 1

OA 2

OA 1

800 mm Diaphragm Wall

101.0 m

10.5 m

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

UMC 16 55 85 2.65 0.0644 0.3223 2.69 25.0 0.984 0.33 1LMC 16 45 75 2.65 0.0527 0.2637 2.34 26.2 1.04 0.33 1

Soil Type Gs κ λ eΓ M ν'

γsat E50 c φ

(kN/m3) (kPa) (kPa) (o)Fill (Sandy Soil, N = 6) 19 0.33 15000 0 36 Effective StressFluvial Sand (F1) (N = 15) 19 0.33 30000 0 39 Effective StressFluvial Clay (F2) (N = 5, PI = 30) 17 0.495 15000 25 0 Total StressOA 1 (N = 30, PI = 20) 20 0.495 375000 150 0 Total StressOA 2 (N = 100, PI = 20) 20 0.495 450000 500 0 Total Stress

ApproachνSoil Type

-30

-28

-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00 40 80 120 160 200 240 280 320 360

Wall Deflection (mm)

Dep

th (m

)

Measured

Predicted (10 kPa surcharge extendingto 10 m from wall)

Figure 4. Computed and measured wall deflections of Case 1 - office building at Taipei Basin

Figure 5. Cross-section of Case 2 - cut-and-cover tunnel at Farrer Park – Kandang Kerbau (CH 31+895)

Table 2. Soil properties of Case 2 - Farrer Park – Kandang Kerbau (CH 31+895)

(a) Soil properties of soft clay

(b) Soil properties of other soils

73

75

77

79

81

83

85

87

89

91

93

95

97

99

101

103-10 10 30 50 70 90 110 130 150

Wall Deflection (mm)D

epth

(m)

Measured at 2.5 m behind the wall

Predicted (10 kPa surcharge extendingto 10 m from wall)

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

UMC 15 65 93 2.65 0.0761 0.3809 2.98 24.1 0.943 0.33 3 to 1LMC 16 40 68 2.65 0.0468 0.2344 2.12 27.0 1.07 0.33 1

Soft Silty Clay 17 40 60 2.65 0.0468 0.2344 1.9 27.0 1.07 0.33 1

Soil Type Gs κ λ eΓ M ν'

γsat E50 c φ

(kN/m3) (kPa) (kPa) (o)Fill (Silty Clay) (PI = 40) 16 0.495 8100 18 0 Total StressSoft Sandy Clay (PI = 20) 17 0.495 11250 15 0 Total StressLoose Sand 19 0.33 30000 0 35 Effective StressDense Silt 19 0.495 90000 150 0 Total Stress

ApproachνSoil Type

Figure 6. Computed and measured wall deflections of Case 2 - cut-and-cover tunnel at Farrer Park – Kandang Kerbau (CH 31+895) 3.3 Case 3: Ministry of Environment (MOE) Building

Figure 7 shows the cross-section and soil properties of this deep excavation. The excavation width was 70 m which was supported by YSPIV sheet piles penetrated to a depth of 24 m. The original ground water table was about 0.5 m below surface. The soil parameters adopted in the analysis are shown in Table 3. The preloads for all struts were 66.2 kN/m. The computed and measured wall deflection pro-files are shown in Figure 8. The maximum measured and predicted wall deflections are 310 and 240 mm, respectively. The difference is about -23%.

Table 3. Soil properties of Case 3 - MOE Building Project (CH 31+895)

(a) Soil properties of soft clay

(b) Soil properties of other soils

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00 100 200 300 400 500 600

Wall Deflection (mm)

Dep

th (m

)

Measured

Predicted (10 kPa surchargeextending to 10 m from wall)

Figure 7. Cross-section of Case 3 - MOE Building (Kok, 1985)

Figure 8. Computed and measured wall deflections of Case 3 - MOE Building

3.4 Case 4: Rochor Complex Figure 9 shows a typical cross-section of this project. The width of the excavation was about 95 m. It was supported by 24 m long FSPIIIA sheet pile braced at three levels. The original ground water table was about 1.5 m below surface. The soil parameters adopted in the finite element analysis are shown in Table 4. The preloads of strut from top to bottom were 28, 104.3, and 175.1 kN/m, respectively. The computed and measured wall deflection profiles are as shown in Figure 10. The maximum measured and predicted wall deflections are 150 and 188 mm, respectively. The difference is about +25%.

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

UMC 16 45 75 2.65 0.0527 0.2637 2.34 26.2 1.04 0.33 1LMC 16 40 70 2.65 0.0468 0.2344 2.173 27.0 1.07 0.33 1

Soil Type Gs κ λ eΓ M ν'

γsat E50 c φ

(kN/m3) (kPa) (kPa) (o)Sand 20 0.330 15000 0 30 Effective StressFirm Clay 17 0.495 75000 100 0 Total StressVery Stiff Silty Clay 18 0.495 150000 200 0 Total Stress

ApproachνSoil Type

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0-20 20 60 100 140 180 220 260

Wall Deflection (mm)

Dep

th (m

)

Measured

Predicted (10 kPa surchargeextending 10 m from wall)

Figure 9. Cross-section of Case 4 - Rochor Complex (Lim et al., 2003)

Table 4. Soil properties of Case 4 - Rochor Complex Project (CH 31+895)

(a) Soil properties of soft clay

(b) Soil properties of other soils

Figure 10. Computed and measured wall deflec-tions of Case 4 – Rochor Complex

γsat E50 c φ

(kN/m3) (kPa) (kPa) (o)Clay Silt with Hardcore (PI = 30) 20 0.495 60000 100 0 Total StressSand 20 0.330 20000 0 37.5 Efective StressDense Silty Sand (PI = 20) 21 0.490 262500 350 0 Total StressVery Dense Silty Sand (PI = 20) 21 0.490 375000 500 0 Total Stress

ApproachνSoil Type

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

Soft Marine Clay 15 55 85 2.65 0.0644 0.3223 2.69 25.0 0.985 0.33 1.4 to 1Soft Clay 17 45 75 2.65 0.0527 0.2637 2.34 26.2 1.04 0.33 1

Soil Type Gs κ λ eΓ M ν'

3.5 Case 5: Syed Alwi Project

A typical cross section for this project is shown in Figure 11. The excavation width was about 28 m. There were only two levels of struts at 2 m and 5 m below the ground surface. The original ground wa-ter level was about 1.0 m below surface. The soil parameters adopted in the analysis are shown in Ta-ble 5. The preloads of the top and bottom struts were 25 and 100 kN/m, respectively. The computed and measured wall deflection profiles are as shown in Figure 12. The maximum measured and pre-dicted wall deflections are 49 and 59 mm, respectively. The difference is about +20%.

Figure 11. Cross Section of Case 5 - Syed Alwi Project (Lim et al., 2003)

Table 5. Soil properties of Case 5 - Syed Alwi Project (CH 31+895)

(a) Soil properties of soft clay

(b) Soil properties of other soils

3.6 Case 6: Lavender MRT Station Figure 13 shows the sectional view of excavation. It had six levels of struts and was supported by 1000 mm thick diaphragm walls. The depth and width of excavation were 15.7 and 23 m, respectively. The ground water table was about 1.5 m below surface. The soil parameters adopted in the analysis are shown in Table 6. The preloadings of struts level 1 to 6 were 190, 390, 327, 260, 233, and 220 kN/m, respectively. The computed and measured wall deflection profiles are as shown in Figure 14. The maximum measured and predicted wall deflections are 38 and 39 mm respectively. The difference is only +3%.

1000 mm Diaphragm W all

11.5 m

Fill

Very Dense Clayey Silt (N > 100)

Dense Silty Coarse Sand (N = 83)

M edium Dense Silty Course Sand (N = 27)

Lower Marine C lay

Upper Marine C lay

-1.5 m

-40 m

-26.6 m

-22.6 m

-17.5 m

-13 m

-3.6 m

0 m

-40 m

-26.6 m

-22.6 m

-15.66 m-13.21 m-11.31 m-9.4 m

-6.53 m

-3.6 m

-0.5 m

γsat PI LL φcs Estimated(kN/m3) (%) (%) (o) OCR

UMC 16 55 85 2.65 0.0644 0.3223 2.69 25.0 0.985 0.33 3.2 to 1.1LMC 16 45 75 2.65 0.0527 0.2637 2.34 26.2 1.04 0.33 1.7 to 1.3

λ eΓSoil Type Gs κ M ν'

γsat E c φ

(kN/m3) (kPa) (kPa) (o)Fill 18 0.495 24000 80 0 Total StressLoose coarse sand 20 0.330 12000 0 30 Effective StressMedium dense silty course sand (N = 27) 20 0.330 60000 0 43 Efective StressDense silty coarse sand (N = 83) 20 0.330 88000 0 45 Efective StressVery dense clayey silt 19 0.495 24000 800 0 Total Stress

νSoil Type Approach

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00 10 20 30 40 50 60 70 80 90 100

Wall Deflection (mm)D

epth

(m)

Measured

Predicted (10 kPa surchargeextending to 10 m from wall)

Figure 12. Computed and measured wall deflections of Case 5 - Syed Alwi

Table 6. Soil properties of Case 6 - Lavender MRT Station

(a) Soil properties of soft clay

(b) Soil properties of other soils

Figure 13. Cross-section of Case 6 - Lavender MRT Station (Lim et al., 2003)

-30

-28

-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0-20 0 20 40 60 80 100 120 140

Wall Deflection (mm)D

epth

(m)

Measured

Predicted (10 kPa surchargeextending 10 m from wall)

Figure 14. Computed and measured wall deflections of Case 6 – Lavender MRT Station 4 CONCLUSIONS Back-analyses were carried out on six case records with the aid of the computer program Sage Crisp. The modified Cam Clay model was used to simulate the soft clay behaviour. The Mohr-Coulomb model was used for other soil types. The soil parameters for the modified Cam Clay model were de-termined using published correlations based on Atterberg limits. Reasonable results can be obtained using this simplistic approach. The computed maximum wall deflections fall within ±25% of the measured values for all except one case. Three cases over-predicted the maximum wall deflection by 20, 25 and 33%. Two cases under-predicted the maximum wall deflection by 11 and 23%. One case yielded essentially the same maximum wall deflection. This simplistic approach in determining the soil parameters can be adopted in the absence of test data. However, it is preferable to determine the soil parameters from high quality test data whenever available. REFERENCES Fang, M. L. 1987. A deep excavation in Taipei Basin. Ninth Southeast Asian Geotechnical Conference (1): 35-

42. Bangkok. Jamiolkowski, M., Ladd, C. C., Germaine, J. T., and Lancellotta, R. 1985. New developments in field and labo-

ratory testing of soils. Proceedings of 11th International Conference on Soil Mechanics and Foundation En-gineering (1):57-153. San Fransisco.

Kado, Y., Ishii, T., Shirlaw, J. N., & Lim, K. 1987. Chemico lime pile soil improvement. Case Histories in Soft Clay; Proceedings of the 5th International Geotechnical Seminar: 207-218. Singapore.

Kok, C. Y. 1985. A brace sheet pile excavation in very soft Singapore marine clay, the MOE building at Scotts Road. Geotechnical Publications by Public Works Department (3):1-45. Singapore.

Kulhawy, F. H. & Mayne, P. W. 1990. Manual on estimating soil properties for foundation design. Report EL-6800, Electric Power Research Institute, Palo Alto, CA.

Lim, K., Hosoi, T. & Ishii, T. 1991. Behavior of diaphragm walls and settlements from deep excavations in a marine clay. Proceeding of the 9th Asian Regional Conference on Soil Mechanics and Foundation Engineer-ing (1):331-334, Singapore.

Lim, K. W., Wong, K. S., Orihara, K. & Ng, P. B. 2003. Comparison of results of excavation analysis using WALLAP, SAGE CRISP, and EXCAV97. Underground Singapore 2003: 83-94, Singapore.

Mitchell, J. K. 1976. Fundamentals of Soil Behavior. New York: John Wiley and Sons. SAGE Engineering Limited. 1995. SAGE CRISP User Guide. United Kingdom. SAGE Engineering Limited. 1995. SAGE CRISP Technical Reference Guide. United Kingdom. Wood, D. M. 1990. Soil Behavior and Critical State Soil Mechanics. Cambridge: Cambridge University Press. Wong K. S. 2003. Observational Approach to Avoid Failures in Temporary Works, Seminar on Avoiding Fail-

ures in Excavation Works organized by Building & Construction Authority, Singapore, 11 July 2003. Wong, K. S. 2004. How to Avoid Failures in Deep Excavation. Proc. Int’l Conf. on Structural & Foundation

Failures, Singapore, pp. 384-397.