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

Buried corrugated steel culvert failure mechanisms due to environmental deteriorations

Environmental factors and ageing influence deterioration mechanisms in buried metal culverts that may impair mechanical performance over the service life. Corrosion occurs, primarily at the culvert haunches, due to changes in the water level and repeated exposure to air circulation. This action provides a pathway for water to flow into the backfill that may cause erosion and the development of soil voids adjacent to the culvert. In this study, the behavior of corrugated metal culverts, with a circular cross section subject to overburden and surface loads, is simulated using calibrated finite element modelling procedures. A three-dimensional model of a deteriorated culvert-soil system was developed for different cover depths. The relationship between the culvert wall section loss, due to corrosion, on the load carrying capacity is investigated. Erosion voids are created around the buried culverts with distinct sizes and shapes. For the parameters examined, the soil erosion had a greater influence on pipe/soil interaction mechanisms, which ultimately influenced culvert performance and serviceability, than the effect of corrosion. The erosion void size and location were influential factors on the culvert performance that could result in local bucking and a decrease in the service life

Generalized Fragility Relationships with Local Site Conditions for Probabilistic Performance-based Seismic Risk Assessment of Bridge Inventories

The current practice of detailed seismic risk assessment cannot be easily applied to all the bridges in a large transportation networks due to limited resources. This paper presents a new approach for seismic risk assessment of large bridge inventories in a city or national bridge network based on the framework of probabilistic performance based seismic risk assessment. To account for the influences of local site effects, a procedure to generate site-specific hazard curves that includes seismic hazard microzonation information has been developed for seismic risk assessment of bridge inventories. Simulated ground motions compatible with the site specific seismic hazard are used as input excitations in nonlinear time history analysis of representative bridges for calibration. A normalizing procedure to obtain generalized fragility relationships in terms of structural characteristic parameters of bridge span and size and longitudinal and transverse reinforcement ratios is presented. The seismic risk of bridges in a large inventory can then be easily evaluated using the normalized fragility relationships without the requirement of carrying out detailed nonlinear time history analysis

Effects of Initial Direction and Subsequent Rotation of Principal Stresses on Liquefaction Potential of Loose Sand

The effects of the initial orientation of principal stress axes and subsequent rotation of principal stresses on liquefaction susceptibility of sands were investigated. Monotonic and cyclic hollow cylinder torsional shear tests were carried out on Fraser River sand specimens consolidated to different initial principal stress orientations and subjected to principal stress rotation during loading. Cyclic loading was applied with constant amplitude cyclic deviator stress, but along stress paths that impose different magnitudes of principal stress rotation. Test results demonstrate that the cyclic resistance ratio (CRR) is influenced by both the initial orientation of principal stresses and the magnitude of stress rotation during dynamic loading. These results suggest that the degree of stress rotation influences CRR more significantly than the initial principal stress orientation. Yet, the effects of the degree of stress rotation are not considered in current liquefaction assessment practice. The only available mechanism to account for principal stress directions is the use of the Kα factor, which focuses on the initial principal stress orientation only. Irrespective of the initial inclination of the major principal stress axis, the weakest cyclic resistance was noted in tests with a principal stress rotation of ±45°. The increased susceptibility to liquefaction is possibly due to factors such as the inclination of the plane of maximum shear stress with the bedding plane, inclination of major principal stress with the bedding plane, the presence of horizontal shear stress, and the nature of the variation of shear stress on the weak bedding plane

Shear and dewatering behaviour of densified gold tailings in a laboratory simulation of multi-layer deposition

Tailings may undergo desiccation stress history under varied climatic and depositional parameters. While tailings substantially dewatered prior to deposition may experience desiccation under the greatest range of climatic variation, even conventionally deposited tailings may desiccate in arid climates at lower rates of rise. Bench-scale research has shown that the stress history imparted by desiccation substantially improves strength in gold tailings. The present study further investigates this phenomenon by simulating multi-layer deposition of high-density tailings using a modular drying box, 0.7mby 1 m inplan. The box is instrumented for directly measuring evaporation, drainage, water content, vertical volume change, and matric suction. Additional measurements included total suction at the surface as well as observations of crack development. The dewatering behaviour conforms to that predicted by previously published generic modelling, specifically that the presence of partially desiccated tailings initially accelerates, but then decelerates dewatering of fresh tailings. The shear behaviour of samples obtained using buried tubes and by driving thin-wall tubes into the multi-layer simulation are compared with shear behaviour of samples from bench-scale experiments. Shear strength of samples from the multi-layer simulation is independent of the sampling method, and shows higher strength than the bench-scale samples. The higher strength may be due to the greater number of wet-dry cycles or other age-related processes

Confining stress and static shear effects in cyclic liquefaction

Liquefaction resistance of a sand under cyclic loading is assessed and the effects of the levels of confining pressure and static shear on resistance to liquefaction are investigated. Site-specific values of the resistance under specified levels of confining and static shear stresses are measured in the laboratory. The measured values are compared with those which would be predicted by the application of empirical multiplying factors K σ and Kα to the reference resistance at 100 kPa effective confining stress with no static shear. It is shown that Kσ and Kα are not independent, as assumed in current practice. The combined factor Kσ × Kα resulting from the empirical method is shown to underestimate the cyclic resistance ratio regardless of the initial density and confining and static shear levels. The degree of conservatism is most dramatic at looser density states