Stress-Strain Behaviour of Completely Decomposed Granite in Both Triaxial and Plane Strain Conditions

Most of the field problems in geotechnical engineering are in three dimensional state or close to a plane strain condition. Strength and deformation properties of soils in plane strain condition are considerably different from those in an axisymmetric condition. Many researchers have investigated the behaviour of soils under a plane strain condition. However, most of the previous studies have concentrated on sedimentary type of soils like sand and clay. Our understanding on the plane strain behaviour for residual soils is less than that for sedimentary soils. A true triaxial system with four sliding rigid-plates and real time feedback control has been used to test specimens of a completely decomposed granite (CDG) soil (a residual soil) under plane strain condition. The setup of the true-triaxial rigid plates is briefly introduced first. The preparation of soil specimens and testing procedures are described. The basic properties of the CDG are presented. The stressstrain and strength behaviour of the soil obtained under plane strain condition was investigated and compared to the behaviour obtained under axisymmetric loading conditions. The results revealed that the critical state line in q-p′ space obtained under plane strain condition is the same as that obtained under axisymmetric condition. However, the critical state line in e- ln p′ space obtained under undrained plane strain condition is different from that obtained under axisymmetric condition. The peak friction angle for plane strain tests is higher than that from axisymmetric loadings. It is also found that shear bands occur only in drained plane strain compression. Defuse bulging is the mode of failure for undrained plane strain as well as triaxial loading

A laboratory study of anisotropic geomaterials incorporating recent micromechanical understanding

This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stress–strain–strength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stress–force–fabric relationship. It was observed that intermediate principal stress parameter b(b = (σ2 − σ3)/(σ1 − σ3)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane

Influence of principal stress direction on the stress-strain-strength behaviour of completely decomposed granite

The measurement and study of the stress-strain-strength behavior of soils in general stress states involving principal stress rotation are necessary and valuable. To investigate the strength behavior under principal stress rotation, a series of undrained tests on compacted hollow cylinder specimens of completely decomposed granite (CDG) was carried in hollow cylinder apparatus. Tests were conducted using constant inside and outside pressures and maintained a fixed angle of rotation of principal stress with the vertical (α). Seven different angles of major principal stress orientations were used to cover the entire range of major principal stress directions from vertical to the horizontal. Two different confining stresses were used to find out the variations of the experimental results. It is observed that the deviator stresses as well as excess pore pressures decrease with the angle α. It is also observed that specimens were getting softer with the increase of α. The results also show a significant influence of principal stress direction angle on the strength parameters. It is found that the angle α is related to the occurrence of cross-anisotropy and the localization which resulted in a pronounced influence on the strength parameters of the CDG specimens.Department of Civil and Environmental Engineerin

INFLUENCE OF PRINCIPAL STRESS DIRECTION ON THE STRESS-STRAIN-STRENGTH BEHAVIOUR OF COMPLETELY DECOMPOSED GRANITE UDC 691.212:531.47+539.211+539.42(045)=111

Abstract. The measurement and study of the stress-strain-strength behavior of soils in general stress states involving principal stress rotation are necessary and valuable. To investigate the strength behavior under principal stress rotation, a series of undrained tests on compacted hollow cylinder specimens of completely decomposed granite (CDG) was carried in hollow cylinder apparatus. Tests were conducted using constant inside and outside pressures and maintained a fixed angle of rotation of principal stress with the vertical (α). Seven different angles of major principal stress orientations were used to cover the entire range of major principal stress directions from vertical to the horizontal. Two different confining stresses were used to find out the variations of the experimental results. It is observed that the deviator stresses as well as excess pore pressures decrease with the angle α. It is also observed that specimens were getting softer with the increase of α. The results also show a significant influence of principal stress direction angle on the strength parameters. It is found that the angle α is related to the occurrence of cross-anisotropy and the localization which resulted in a pronounced influence on the strength parameters of the CDG specimens. Key words: Hollow cylinder, principal stress rotation, friction angle, failure surface, cross anisotropy

Behavior of EPS geofoam in true triaxial compression tests

This paper investigates the behavior of EPS geofoam in a true triaxial apparatus using 70 mmÃ70 mmÃ140 mm prismatic brick-shaped specimens. The specimens are subjected to different stress paths in the deviator (π) plane by means of stress-controlled loading, in which the axial stress is imposed at a rate of 75 kPa/min in the major principal direction. Stress-strain characteristics and volume change behavior have been recorded, and the yield surface has been deduced from the experimental data. The following observations have also been made for the geofoam: (a) it is an elastoplastic hardening material with plastic contractive volume change under compressive loading, (b) it softens stiffness-wise under confining stress, (c) the onset of contractive volume change corresponds quite well to the proportional limit, and (d) yielding is a slightly decreasing function of the intermediate principal stress. The study found that yielding can be represented reasonably well by a Drucker-Prager yield surface