Field Investigation on the Characteristic of Clayey Soil Close to Diaphragm Walls

During excavation of a diaphragm wall panel soil is supported by a support fluid, in many cases a bentonite suspension. Generally, the fluid pressure is smaller than the initial stress state in the soil. Due to this the soil tends to deform into the excavated panel. In water saturated clayey soil this is related to a decrease of pore water pressure. Simultaneously the bentonite suspension or only the water from the suspension may penetrate the pore space of the clay. Both effects may lead to a softening of the clay. After concreting the panel and finishing the construction of the diaphragm wall a zone of soften clay may remain close to the wall. Such zone with lower shear strength would influence the transfer of shear stress between the wall and the soil which has an impact on the magnitude of active and passive earth pressure as well as on the vertical bearing capacity of the diaphragm wall. Site investigations have been performed during the excavation phase of a construction pit formed by diaphragm walls in clayey soil. In different distances of the wall the soil strength has been measured by a pocket penetrometer and samples has been taken to determine water contents. The investigations have been performed at two panels, one with short opening time (time between start of excavation and beginning of concreting) and one with long opening time. Test results indicate a clear reduction of soil strength and an increase of water content with decreasing distance of the wall. This effect is more pronounced at the panel with long opening time. From the strength reduction adhesion factor is derived and compared to adhesion factors observed during installation of bored piles and barrettes, which are reported in the literature. During excavation of a diaphragm wall panel soil is supported by a support fluid, in many cases a bentonite suspension. Generally, the fluid pressure is smaller than the initial stress state in the soil. Due to this the soil tends to deform into the excavated panel. In water saturated clayey soil this is related to a decrease of pore water pressure. Simultaneously the bentonite suspension or only the water from the suspension may penetrate the pore space of the clay. Both effects may lead to a softening of the clay. After concreting the panel and finishing the construction of the diaphragm wall a zone of soften clay may remain close to the wall. Such zone with lower shear strength would influence the transfer of shear stress between the wall and the soil which has an impact on the magnitude of active and passive earth pressure as well as on the vertical bearing capacity of the diaphragm wall. Site investigations have been performed during the excavation phase of a construction pit formed by diaphragm walls in clayey soil. In different distances of the wall the soil strength has been measured by a pocket penetrometer and samples has been taken to determine water contents. The investigations have been performed at two panels, one with short opening time (time between start of excavation and beginning of concreting) and one with long opening time. Test results indicate a clear reduction of soil strength and an increase of water content with decreasing distance of the wall. This effect is more pronounced at the panel with long opening time. From the strength reduction adhesion factor is derived and compared to adhesion factors observed during installation of bored piles and barrettes, which are reported in the literature.