Systematic Laboratory and Field Investigations on the Soil-Concrete Interface of Diaphragm Walls and Bored Piles with Focus on the Influence of Wall Roughness

The construction of diaphragm walls and bored piles requires temporary fluid support to stabilise the open trench or borehole before concrete placement. During excavation, a filter cake may develop remaining between hardened wall and soil. The effect of formed filter cake and influencing factors are currently controversially discussed, especially with respect to faults and economic efficiency. First results from an R & D project on this soil -cake – concrete interface have been published (last DFI/EFFC conference) covering the evaluation of the shear resistance of the interface between diaphragm walls and soils. This paper summarises the remaining findings of this project considering laboratory and field results on the influence of wall roughness, long fluid exposure times and a detailed investigation of penetrated soil characteristics depending on the support fluid constitution. Wall roughness was found to increase the shear strength of the interface. This was proven even for the case of sandy soils and a filter cake thickness larger than the roughness depth of the concrete surface by direct shear tests. For fine-grained soils, the wall roughness depth exceeds the filter cake thickness, as shown by site investigations, and thus dominates the shear strength of the interface. Field observations on diaphragm walls in gravel confirmed a very rough concrete surface. Wall roughness and coarse grains in the soil lead to direct contact between concrete and soil. However, with longer opening times and effective regeneration plant, filter  cake thickness may continuously increase as well as the bentonite content of the filter cake and direct concrete soil contact may decrease. Measures are discussed which positively affect interface characteristics and preserve higher shear stress transfer even for longer periods of fluid exposure.