Estimating critical state soil mechanics parameters from shear box tests

Abstract

Summary Prediction of compaction, tillage, root growth or other soil deformation events requires a description of the stress–strain properties of the soil such as the critical state model, but estimating the parameters is time consuming and expensive. I have developed a method of estimating critical state properties from a single shear box test, both saving much labour and providing more information than traditional analyses. The method is based on critical state analyses of the constant stress and constant volume shear box tests using the total stresses applied at the boundary. It derives the critical state property parameters from test data by minimizing the difference between test data and the simulated soil deformation (and hence the properties used in that simulation). The minimization is a form of regression analysis. The analyses resulted in good simulations of the history of states in space defined by τxyγxy and e space (constant stress test) or τxyγxy and σy space (constant volume test). For normally consolidated samples, the analysis of a single constant stress test provided estimates of the slope of the critical state line (M), the slope of the virgin compression line (λ), the slope of the rebound line (K), and the elastic modulus (E). The standard deviation of the estimate of kcould not be found. By contrast, the analysis of the constant volume test resulted in poor estimates, particularly of λ. This is because the test yields no information on changes of volume during deformation: hence volume change parameters are not successfully estimated. The constant volume test is therefore not suitable for back analysis.

Influence of soil strength on root growth: experiments and analysis using a critical‐state model

“…The critical‐state model, in particular a variant called Modified Cam Clay, provided a good description of the stress–strain behaviour of these soils for a wide range of soil moisture contents (Kirby, 1997; Kirby & O'Sullivan, 1997; Kirby et al ., 1998). The values of the model parameters (slope of the critical‐state line, M , slope of the virgin compression line, λ , slope of the rebound line, κ , and elastic modulus, E ) were determined using a constant cell volume triaxial test.…”

“…The values of the model parameters (slope of the critical‐state line, M , slope of the virgin compression line, λ , slope of the rebound line, κ , and elastic modulus, E ) were determined using a constant cell volume triaxial test. The measurements and their interpretation have been described in detail elsewhere (Kirby, 1997; Kirby & O'Sullivan, 1997; Kirby et al ., 1998).…”

“…where e r and P cr are the void ratio and precompression stress at some reference state. The reference state was chosen as the initial state in the triaxial tests (Kirby, 1997; Kirby & O'Sullivan, 1997; Kirby et al ., 1998). Estimated values of the precompression stress are shown in Table 1.…”

Triaxial Testing to Determine the Effect of Soil Type and Organic Carbon Content on Soil Consolidation and Shear Deformation Characteristics

“…While the determination of the CSL is more complex than most soil measurements, in sandy soils there is a rapid procedure (Santamarina and Cho, 2001). The critical state parameters may also be determined more rapidly from shear box testing than triaxial tests (Kirby, 1998).…”

Soil Stress Measurement: Part I. Transducer in a Uniform Stress Field

“…5c). Figure 5c also shows the applied stresses calculated using the theoretical procedure described by Kirby 20 for an elasto-plastic soil and using Eqn (2) to calculate the applied stress in the inclined direction from the theoretical vertical and horizontal stresses. The over-and underestimates of stress with changing stiffness of the disturbed zone were less than in the linear elastic case (compare Figs 5c and 5a).…”