The collapse of diaphragm walls retaining clay

Abstract

Five centrifugal model tests are reported which illustrate aspects of the collapse of stiff cantilever retaining walls embedded in overconsolidated clay. The drainage of a heavy fluid in flight was used to simulate the effects of excavation, following the establishment of a high initial groundwater level. Two modes of collapse were observed with unpropped walls. The temporary stability of walls with small penetration was interrupted by the hydraulic action of a water-filled crack opening on the retained side of the wall. The long-term rotational failure of walls of deeper penetration was also observed, involving distributed strains in ‘active’ and ‘passive’ zones which could lead ultimately to sliding on shear rupture surfaces. An analysis was developed based on admissible stress fields, with active and passive zones switching about a pivot point, so that the unpropped wall could satisfy the conditions of both moment and force equilibrium. A back analysis of the two sudden failures using an undrained strength based on the overconsolidation ratio was successful in matching the critical penetration ratio and pivot position observed in the tests. A drained analysis using ϕ′ derived from triaxial and plane strain tests was equally successful in comparison with the data of long-term failure. A similar stress analysis for a wall propped at the top was shown to be conservative. This was thought to be due to the kinematic restraint of the prop which produced a rupture surface on the active side which was much steeper than those observed before. A back analysis of the observed failure mechanism generated a credible value of mobilized soil friction close to the peak observed in soil tests. This value also gave a consistent match for the bending moments and propping force measured in the test. Care must be taken to account for the possible effects of progressive failure. Critical state soil angles, with fully mobilized wall friction, can be anticipated to relate to the gross long-term deformation of walls. Loss of retained height and heave in the exacavation lead eventually to self-stabilization. © 1987, Thomas Telford Ltd. All rights reserved.