Strain softening and state parameter for sand modelling

INTRODUCTION Triaxial compression tests on sand typically show density-dependent and pressure-dependent response of the form shown in Fig. I. Strain softening from a peak stress ratio is a familiar feature of the observed stress-strain relations, and it is important that this should be incorporated into constitutive models of sand behaviour. This Technical Note describes a method whereby the type of response shown in Fig. 1 can be repro- duced by combining a simple monotonic hard- ening relationship with a link between current peak strength and a state parameter (a com- bination of volumetric and mean effective stress information). PEAK STRENGTHS OF SANDS Peak strengths of sands are known to depend on initial density of packing and on the stress level at which the test is performed. Extensive studies of the combined influence of stress level and density on strength are reported by Bolton (1986) and Been & Jefferies (1985). Bolton sug- gests that the peak angle of shearing resistance can be estimated from 4p’ - 4,,’ = 31,(10 - In p’) - l (1) where &,’ and 4,,’ are peak and critical state angles of shearing resistance respectively mea- sured in degrees, I, is initial relative density and