Underground Singapore 2011 2 SPECIFICATION OF FINITE ELEMENT MODEL The deep braced excavation modelled is about 30m wide and 21m deep and comprises 5 levels of steel struts. The retaining wall comprises steel soldier-piles placed at 1.60m spacing with sheet-piles as lagging that terminate at the final excavation level. The ground comprises Fill material overlying residual soil of Jurong Formation in horizontal layers. The benchmark problem is therefore simulated as a plane strain problem by considering half of the geometry using a mesh with a width of 125m and a depth of 50m. A cross section of the problem is depicted in Figure 1. A surcharge of 20kPa is assumed to extend 20m behind the wall. The boundaries are horizontally restrained at the lateral boundaries and fixed in both directions at the bottom boundary. Initial stresses have been taken as σ‟v = γ‟.h and σ‟h = Ko.γ‟.h where h is the depth below ground surface. The vertical and horizontal boundaries are assumed closed for consolidation while the water table is assumed „perched‟ at 1.5m below the ground surface i.e. not allowing drawdown to occur. Only a drained analysis is performed where the assumption regarding steady ground water drainage will be performed at every construction/excavation stage of the analysis. This is unrealistic in a real problem but greatly simplifies the problem for this exercise. Unit weight of the wall is assumed the same as the soil replaced. Material parameters of the underlying subsoil and the structural support system are listed in Tables 1 and 2 respectively. Computational steps are listed in Table 3. 5 models with various assumptions are carried out with the 2 mentioned software and they are summarised in Table 4. In Table 4, the strength of the interface between the retaining wall and the adjacent soil is represented by R, a factor of the strength of the adjacent soil. The constitutive model to be adopted is the elastic-perfectly plastic model with the Mohr Coloumb criteria.
Figure 1. Geometry of FE model
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