Stiffness of geomaterials at very small strains

Abstract

Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry±Pe Ârot laser interferometer. A displacement accuracy as high as 60 X 027 ìm was obtained, leading to an axial strain accuracy of the order of 610 À4 %. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress±strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0´002±0´003%) for all three materials, despite the maximum stiffness (E max ) ranging over two orders of magnitude. The initial in¯uence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0´01% strain being of the order of 80±95% of E max . Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using ®eld geophysical techniques, despite assumptions of isotropy, and the effects of sampling. The implications of these observations for practice are discussed.

The influence of particle characteristics on the behaviour of coarse grained soils

“…Over the past decade significant progress has been made; for example the relationship between particle strength and compression behaviour has been investigated in detail by McDowell & Bolton (1998) and Nakata et al (1999), while the discrete element method (DEM) analyses of Thornton (2000) and others have indicated a stronger correlation between the macro-scale angle of shearing resistance and inter-particle friction than the experimental work of Skinner (1969). In the small strain region, Clayton & Heymann (2001) demonstrated that particle shape can have a significant impact on stiffness. The current study adds to this recent research by exploiting new technologies to quantify accurately particle shape and roughness, and by using custom-built apparatuses to measure particle contact stiffness and inter-particle friction.…”

The influence of previous stress history and stress path direction on the surface settlement trough induced by tunnelling

“…However, Clayton & Heymann (2001), on the basis of similar experiments on natural Bothkennar Clay, have recently suggested that if, at the end of the approaching path, enough time is allowed for creep to take place, then recent stress history does not have an effect on soil stiffness, at least for natural soils undergoing modest excursions. Fig.…”

“…Fig. 2(a) shows the stress paths followed in a single sample of Bothkennar Clay (Clayton & Heymann, 2001). Three incoming stress paths BA, CA and DA, preceding a common undrained triaxial compression path initiating from A, were investigated.…”

Directional response of a reconstituted fine-grained soil—Part II: performance of different constitutive models

“…In such cases, the choice has been made to evaluate such material constants based on the experience gathered in previous experimental investigations on similar soils. Although such a choice necessarily introduces a certain degree of subjectivity in the comparative evaluation of the model responses, it can still be considered acceptable for our purposes, considering that the typical range of variation of such parameters for different soils is relatively limited, and the model response is not very sensitive to their variation, see e.g., Niemunis and Herle (1997); Stallebrass (1990);Ingram (2000); Clayton and Heymann (2001);Mašín (2004).…”

“…It is reasonable to expect that such difficulties would increase in the application of those models to natural clay deposits, with a more complicatedand possibly unknown -previous loading history. In such a case -based on some experimental observations of Jardine et al (1984) and Clayton and Heymann (2001), who report an increase in soil stiffness for all loading directions after a relatively short resting period - Niemunis and Herle (1997) suggest to assume δ = 0. This is equivalent to setting σ a = σ b = σ for the 3-SKH model.…”