ABSTRACT
Murray and Sivakumar (2010a and b) developed an analysis for the critical state strength of unsaturated fine-grained soils. Such soils are shown to form aggregates comprising the soil particles and water which influence soil strength. The analysis is based on the following controlling stress equation determined from thermodynamic equilibrium considerations:
p p s v vc w′
= +
(1)
where, pc ′ is defined as the coupling stress; p is the
mean net stress (p − ua); s is the matric soil suction (ua − uw); ua is the pore air pressure (generally the datum of atmospheric pressure in practice and equated to zero); uw is the pore water pressure (negative in unsaturated soils); vw is the specific water volume (vw = 1 + Sre); v is the specific volume (v = 1 + e); e is void ratio; Sr is degree of saturation; p is the mean stress (under triaxial conditions p = (σ1 + 2σ3)/3)); σ1 is the axial stress under triaxial conditions; σ3 is the cell pressure or lateral stress under triaxial conditions
Figure 1 shows a plot of q/s (where the deviator stress q is given by (σ1 – σ3)) against p sc
′ for kaolin from the results of Sivakumar (2005) and Sivakumar et al. (2009). The results show a consistent trend with a slope of Ma and an intercept Ω = 0.6 on the q/s axis at p sc
′ = 1. Similar rela-
tions have been show for a range of soil types and test protocols where aggregation of particles
1 INTRODUCTION
Determination of the strength of unsaturated soils requires complex laboratory testing which is time consuming and may in practice be deemed prohibitively expensive. Such testing is at present limited mainly to research facilities. Commercial laboratories have been slow to expand their testing capabilities into unsaturated soil testing and practicing engineers are not in a position to utilise unsaturated soil mechanics principles on a routine basis.
