Mechanisms of failure in saturated and unsaturated clayey geomaterials subjected to (total) tensile stress

Abstract

The paper presents an experimental investigation into the mechanisms of tensile failure in clayey geomaterials under saturated and unsaturated conditions. An experimental apparatus was developed to test specimens in uniaxial tension with the facility to monitor suction (porewater tension) using high-capacity tensiometers. This allowed interpretation of failure data in terms of effective stress and average skeleton stress for saturated and unsaturated specimens respectively. Experimental data from normally consolidated samples showed that failure under uniaxial tension occurs in shear and tensile cracks form as a combination of Mode I and Mode II fracture. In the saturated range when samples were prepared with de-aired water, tensile failure occurred at deviatoric stresses corresponding to the critical state line derived from triaxial and uniaxial compression tests. When using non-de-aired water and at suction levels approaching the air-entry value, failure occurred at deviatoric stresses lower than the ones corresponding to the critical state line derived from compression tests. It has been suggested that water cavitation may be one of the mechanisms that control premature rupture of saturated clay when subjected to a (total) tensile stress state. Finally, tensile failure data from unsaturated samples showed that there is continuity between saturated and unsaturated states. I. Murray and A. Tarantino 'Mechanisms of failure in saturated and unsaturated clayey geomaterials subjected to (total) tensile stress'

Cracking behaviour of fine-grained soils: from laboratory testing to numerical modelling

“…assuming that crack initiates by shearing (Mode II) under tensile total stress states. This finding supports therefore recent conclusions in the literature [8,9] around this issue of the failure mechanisms under total tensile stress.…”

“…The combination of the effective stress states in the extreme tension fibre of the different bars at the initiation of failure allows defining a shear failure envelope similar to the one observed under triaxial compression. More recently Murray et al [9] extended those conclusions to unsaturated specimen. They performed direct tensile tests on a series of initially saturated and unsaturated clay samples prepared at different initial suctions.…”

“…The failure criterion was derived from direct tensile tests on samples with different initial suctions where suction was monitored using high-capacity tensiometers [9]. The failure data could be satisfactorily reproduced by a failure envelope defined by a friction angle ϕ'=23,7° and an effective stress parameter ζ=2 (Fig.…”

“…where Sr is the degree of saturation, Sres is the residual degree of saturation, Sre is the effective degree of saturation, s is the suction, Pr and n are soil parameters. The soil parameters have been fitted on the water retention data from the Vitreous China mix [9], and the fitting provided Pr=1430 kPa; n=2.4 and Sres=0.08. The unsaturated hydraulic conductivity K is based on a modified Kozeny-Karman relationship.…”

“…Calibration of van Genuchten water retention curve model on retention data on Vitreous China mix and obtained by[9].…”