Zum Kontaktverhalten zwischen suspensionsgestützten Ortbetonwänden und dem anstehenden Boden

Skin friction of cast-in-place walls.

Analytical and numerical calculations of retaining structures require the wall friction angle as an input parameter. It is specified as the maximal shear strength of the concrete-soil interface due to normal effective load. For the design of diaphragm walls the national engineering standards recommend an angle of wall skin friction of |δ| ≤ ϕ/2. In the framework of present design numerical calculations are performed to determine the deformation behaviour of structures, so that the contact formulation becomes fundamental. Bentonite suspensions are used to support the sides of excavation for diaphragm walls and uncased cast-in-place piles. When concrete is cast by tremie methods the filter cake remains adhering on side walls and becomes part of the concrete-soil interface and influences the characteristics of wall skin friction. In-situ specimens of the filter cake were taken from a diaphragm wall and examinations reveal that the filter cake consists of bentonite and fine soil particles. Due to the excavation process fine particles from the soil are suspended into the supporting fluid due to the liquid limit of the bentonite slurry. Thus, the suspension, in a process of filtration into the surrounding soil, forms a filter cake with a certain shear strength caused by the fine soil particles. This paper presents the results of field and laboratory tests for the investigation of the effective contact behaviour between cast-in-place walls and the surrounding soil.