Implication of end restraint in triaxial tests on the derivation of stress–dilatancy rule for soils having high compressibility

Abstract

Constitutive models for soils are developed and validated against laboratory tests assuming these give representative information on the true material behaviour. However, data from standard laboratory tests reflect the sample response rather than the true material behaviour, due to nonuniformities in stresses and strains generated over the experimental test. The work examines the implications of end restraint on the definition of the stress–dilatancy rule of highly compressible soils with a finite element numerical approach. The numerical model replicates a reconstituted peat, typically characterized by a combination of high compressibility and high friction angle, which increases the severity of end restraint effects. Simulated results show that the global measurements from standard triaxial tests with rough end platens would not give the proper stress–dilatancy rule, if they were interpreted as the response of a single soil element at the constitutive level. Both overestimation and underestimation of the true dilatancy compared to the material response can be observed, depending on the deformation mode. To support the validity of the numerical results, experimental findings from drained triaxial tests on reconstituted peat are presented. Practical indications are given on how the standard interpretation of drained triaxial tests data on peats can be improved.

Experimental determination of the shear strength of peat from standard undrained triaxial tests: correcting for the effects of end restraint

“…The high compressibility and the fibres content of peats are likely to magnify the severity of stress and strain non-uniformity coming from end-restraint, which can largely bias the interpretation of triaxial test data (Rowe et al, 1984;Muraro & Jommi, 2019). The role of end restraint on the shear strength evaluation of peat from undrained triaxial compression tests is systematically investigated here with a dedicated experimental study.…”

Pre-failure behaviour of reconstituted peats in triaxial compression

“…The volumetric and the deviatoric plastic strain increments, de p p and de p q , have been derived from the total ones by computing the elastic strains with a hypo-elastic law with constant Poisson's ratio, m = 0.3 and j = 0.3 [35].…”

“…A dedicated experimental and numerical investigation to derive the stress-dilatancy relationship for the tested peat was presented by Muraro and Jommi [35]. For the sake of clarity, only the main conclusions are briefly recalled.…”

“…Regardless of the different modelling approaches, the development of constitutive models for soils is typically supported and validated on data from laboratory tests interpreted as soil element tests. Muraro and Jommi [35] found significant differences between the material behaviour and the sample behaviour of peats caused by the adoption of conventional rough end platens in the triaxial apparatus. Shear stresses at the interface between the sample and the porous stone restrain the radial displacement of the sample to a large extent, in turn introducing an apparent contribution to anisotropy along compression paths and altering the computed dilatancy.…”

“…Stress-dilatancy rule derived from the K 0 consolidation path on sample S-K 0 (redrawn from[35]) Drained triaxial tests with multiple loading paths used to evaluate the model capabilities…”