Cap-hardening parameters of Cam-clay model variations with soil moisture content and shape-restricted regression model

Abstract

Modeling of soil elastic and permanent plastic volumetric strains (compaction) caused by loading from machinery vehicles using the modified Cam-clay soil constitutive model requires understanding the behaviors of compression and rebound parameters under unsaturated soil conditions.  Oedometer tests were conducted on a sandy loam, a loam, and a clay loam soil, all tropical soils, at three initial soil moisture contents and five maximum stress levels (50, 100, 200, 300 and 400 kPa).  The objectives were to investigate the effects of soil moisture content and maximum applied stress on the modified compression index (l*) and modified rebound index (k*) parameters of a modified Cam-clay soil model on the three soils and predict the compressibility indices using the shape-restricted modeling technique.  The clay loam soil showed higher compressibility at lower maximum stress levels and wet moisture conditions (-10 kPa soil moisture potential) but as the maximum applied stress increased (> 200 kPa), the modified compression index (l*) variations with soil moisture content were insignificant (p > 0.05).  A loam soil exhibited similar compression characteristics to a clay loam soil at 26.12% d.b. and 23.67% d.b., respectively.  For a sandy loam soil, both critical state parameters were less sensitive to the variations in soil moisture content.  The loam soil, which had an organic matter content of 6.33%, rebounded more than clay loam and sandy loam soils especially at higher applied stress values.  On average, the modified compression index (l^*) was about 23 to 36 times the modified rebound index (k^*).  Shape-restricted and quadratic model fittings are presented to explain the relationship between the critical state parameters and maximum applied stresses for each soil moisture content.  The model fitting results indicated that shape-restricted regression predicted the modified Cam-clay model parameters as a function of maximum applied stress (or pre-compression stress) at very low Average Squared Error Loss (ASEL) and did so better than parametric quadratic equations.

Keywords: modified compression index (l^*), modified rebound index (k^*), axial stress, uniaxial compression cyclic test, soil moisture, soil types