Simulation of excavation in a poro‐elastic material

Abstract

A method is presented which allows the computation of the displacements and pore pressures which are generated in an elastic soil when excavation is carried out. The formulation is based on Biot's theory and is fully coupled, with consideration also given to the effects of the lowering of the water table which often accompanies the excavation of soil. Example problems are solved to illustrate the theory which has been presented. cr; = a;-' + Aa; and hj = hj-' + Ahj, equation (11) can be written as a. = a'. + q . J J JSubstituting equation (12) into equation (10) results in the finite element equation where the incremental force vector due to excavation Ag is given by c c c Ag = J BTa;-'dV+ yWLT(hj-, -hEL) -J NTydV-J NTtdS V i Vi S ja.nd N = shape function matrix. Note that Ag is a vector containing the changes in nodal forces due to the stresses and total water heads at the last stage ( j -1) of excavation. This Ag can be imposed at the excavation boundary to simulate the overburden removal. For the jth stage of excavation, the calculation of Ag is carried out using the stress and water pressure levels at the previous stage ( j -1) and performing the integration on the remaining domain of stage ( j ) .

Importance of the Excavation Level on the Prediction of the Settlement Pattern from Piled Raft Analyses

“…The new effective pressures were estimated by simple hand calculations where the effective vertical pressures below the raft were determined with the excavation effect on the groundwater pressure. That means, immediately after 2.5 m (1D) excavation the positive water pressure (with water table at ground surface) became negative in accordance to Hsi & Small (1992) advocated technique. Once the raft load is applied, by casting of the foundation, the water pressure changes again to a positive value.…”

“…A simplified approach (hand calculation) was carried out to evaluate the net final effective pressure at raft level (as stated before, around 30 to 35 kPa). This approach adopted the same qualitative soil behavior as numerically found by Hsi & Small (1992) when simulating a 1-D excavation in a poro-elastic material. It was considered that the raft load was applied immediately after the soil excavation, i.e., before any pore pressure change from negative to positive values -which is obviously another simplified assumption.…”

“…This excavation was not considered in the initial numerical analyses with Cases 1, 4 and 6, given the lack of detailed information about this particular case history. Indeed, the excavation process prior to the raft placement may have a large effect on the final settlement results, since it considerably changes the original stress state (see for instance Hsi & Small, 1992). According to Sales et al (2010), the stress relief and preloading process that takes place during the excavation and casting of the foundation raft must be taken into account to properly simulate the loading pattern, and hence to accurately forecast the settlements at working load.…”

Simulation of Conventional and Inverted Braced Excavations Using Subloading tij Model

“…Each line of element is 0.75 cm in height and overall 30 lines (22.5 cm) of elements are removed. When the elements are deactivated the out-of-balance forces are computed and applied to the boundary between excavated and remaining elements (Hsi and Small, 1992;Nakai et al, 1999). The joint elements in the symmetry boundary of the right hand side are also deactivated.…”

Computational Geomechanics Including Consolidation

“…Any out of balance forces are applied to the sides of the excavation to simulate the removal of material. The process of excavation in poro-elastic materials has been examined in Hsi and Small (1992a) where allowance was also made for the drawdown of the water table as water flows into the excavation. Excavation in plastic soils has been presented by Hsi and Small (1992b), Borja, Lee and Seed (1989), and Borja (1990) for two and threedimensional cases.…”