Underground Singapore 2011 surcharge due to heavy lorries and lifting cranes may be the main reason for the sway type movement. In order to produce similar wall deflection, unbalanced surcharge is placed in the Plaxis model. The surcharge on the left side was varied from 20 to 60 kPa, and right side surcharge is zero. The effect of different surcharge on the wall deflection is shown in Figure 7 which indicates that unbalanced surcharge of 50 kPa matches the monitored wall deflection well. The soil parameters used in the analyses to generate Figure 7 are from Table 1B. Right Wall Deflection Due to Unbalanced Surcharge Effect
Left Wall Deflection Due to Unbalanced Surcharge Effect
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50 -20
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Surcharge = 50 kPa Surcharge = 60 kPa IW204 Monitored Surcharge = 35 kPa Surcharge = 20 kPa
-20
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Surcharge = 50 kPa Surcharge = 60 kPa IW203 Monitored Deflection (mm) Surcharge = 35 kPa Surcharge = 20 kPa
Figure 7 Effect of unbalanced surcharge on the retaining wall deflection
9 COMPARISON OF MC AND HS SOIL MODEL The MC model is commonly used in practice in Singapore although advanced models have been in existence and available for many years. In this study, four analyses were conducted. The first one uses the MC model for all soils with parameters shown in Table 1A. The second one uses the HS model for the fill and MC for all other soils with parameters shown in Table 1B. The third one uses the HS model for all soils with parameters shown in Table 1C. The fourth one is similar to the third one except the DCM modulus was reduced from 1000 to 500 MPa. The back-analyzed wall deflection profiles are shown in Figure 8. It can be seen that the all layers using HS model with DCM E50, ref = 1000MPa produces better agreement with the field measured than those with the MC model. 10 COMPARISON OF UNDRAINED AND DRAINED MODELLING FOR OA LAYER The underlying old alluvium material in this project shows variable sand and clay contents resulting in highly variable permeability. Some limited permeability tests show that OA permeability varies from 1.0E-4 m/sec to 1.0 E-8 m/sec depending on the and silt and clay content. It is a common practice to run both drained and undrained analysis for this material. The OA parameters for the undrained case are given in Table 1B and the drained case in Table 2. Parameters for all other soils are given in Table 1B. Results of both analyses are shown in Figure 9. The differences between both cases are negligible mainly because the OA soils are located far below the final excavation level.
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