Soil stiffness in stress paths relevant to diaphragm walls in clay

A series of triaxial tests was carried out on samples of speswhite kaolin, to investigate the stress-strain relations appropriate to diaphragm walls in clay. The results of the tests highlight the influence of the recent stress history on the behaviour of the soil. In particular, the recent stress history imposed during wall installation was found to have significant effect on the stiffness of the soil during the subsequent excavation stage. Although the pre-excavation stress state of the soil may be closer to the passive than the active condition, the reversal in the direction of the stress path at the start of the excavation stage means that the response of the soil behind the wall will probably be very stiff. The high in situ earth pressure coefficient, which has already been reduced during wall installation, thus falls quite rapidly towards the active limit with comparatively small wall movement. In front of the wall, the normalized soil stiffness G/p' is rather smaller; in undrained conditions, fully passive pressures do not seem to be mobilized at small strains. This difference in soil stiffness, as well as the pre-excavation stress state of the soil, should be taken into account in determining the factors of safety on soil strength (or strength mobilization factors) for use in a limit-equilibrium based design calculation.