ABSTRACT
Different explanations exist for the change from hydrostatic to non-hydrostatic increase of the concrete pressure. Rodatz et al. (1994) explained the change by shear forces acting between the concrete and the soil walls within the trench (silo effect). This implies settlements of the concrete to activate shear forces either as friction forces, which postulate effective stresses within the concrete, or as adhesion between concrete and soil. Lings et al. (1994) stated a reduction of the ratio between horizontal and vertical stresses due to an increasing internal stability of the concrete activated by consolidation and dissipation of pore water pressure. Plank (1971) and De Schutter & Taerwe (1996) observed a significant reduction of the Poisson
1 INTRODUCTION
Numerical studies on the influence of the installation of a diaphragm wall have been performed based on 2D FEM analyses by Gunn et al. (1993), Schweiger & Freiseder (1994), de Moor (1994) and Ng et al. (1995). Hydrostatic distribution of the fresh concrete pressure or a bilinear pressure distribution with a hydrostatic increase until a certain depth and from that depth with a constant value was assumed. Ng & Yan (1998) simulated the excavation and the concreting of one trench by a 3D FEM analyses using the shape of the fresh concrete pressure distribution pb proposed by Lings et al. (1994) based on in-situ measurements described by equation 1:
(1)
with γ1 the unit weight of the fresh concrete, γ2 the unit weight of the bentonite slurry, z the depth from ground level and hkrit the critical depth, where the increase of the pressure is changing from hydrostatic conditions characterized by γ1 to hydrostatic conditions characterized by γ2. Ng et al. suggest hkrit = H/3, with H total depth of the trench.
