Engineering behaviour of in situ cored deep cement mixed marine deposits subjected to undrained and drained shearing

The deep cement mixing(DCM)is used to improve the capacity and reduce the settlement of the soft ground by forming cemented clay columns.The investigation on the mechanical behaviour of the DCM samples is limited to either laboratory-prepared samples or in-situ samples under unconfined compression.In this study,a series of drained and undrained triaxial shearing tests was performed on the in-situ cored DCM samples with high cement content to assess their mechanical behaviours.It is found that the drainage condition affects significantly the stiffness,peak and residual strengths of the DCM samples,which is mainly due to the state of excess pore water pressure at different strain levels,i.e.being positive before the peak deviatoric stress and negative after the peak deviatoric stress,in the undrained tests.The slope of the failure envelope changes obviously with the confining pressures,being steeper at lower stress levels and flatter at higher stress levels.The strength parameters,effective cohesion and friction angle obtained from lower stress levels(c′0 andφ′0)are 400 kPa and 58°,respectively,which are deemed to be true for design in most DCM applications where the in-situ stress levels are normally at lower values of 50-200 kPa.Additionally,the computed tomography(CT)scanning system was adopted to visualize the internal structures of DCM samples.It is found that the clay pockets existing inside the DCM samples due to uneven mixing affect markedly their stress-strain behaviour,which is one of the main reasons for the high variability of the DCM samples.

Comparison of in situ Shear Test and In-Lab Triaxial Shear Test of Compacted Rockfill

The in situ shear test and in-lab triaxial shear test on compacted rockfill of Fuling safety embankment were carried out and their advantages and disadvantages were compared. The discreteness of cohesive force C and internal frictional angle by in situ shear test is much severer than that by in-lab triaxial shear test. The consolidation of in-lab triaxial shear test is bigger than actual consolidation in rockfill engineerings, so the confining pressure should be reduced to a low level in-lab triaxial shear test.

Shear behavior of coarse aggregates for dam construction under varied stress paths

Coarse aggregates are the major infrastructure materials of concrete-faced rock-fill dams and are consolidated to bear upper and lateral loads. With the increase of dam height, high confining pressure and complex stress states complicate the shear behavfor of coarse aggregates, and thus impede the high dam＇s proper construction, operation and maintenance. An experimental program was conducted to study the shear behavior of dam coarse aggregates using a large-scale triaxial shear apparatus. Through triaxial shear tests, the strain-stress behaviors of aggregates were observed under constant confining pressures： 300 kPa, 600 kPa 900 kPa and 1200 kPa. Shear strengths and aggregate breakage characteristics associated with high pressure shear processes are discussed. Stress path tests were conducted to observe and analyze coarse aggregate response under complex stress states. In triaxial shear tests, it was found that peak deviator stresses increase along with confining pressures, whereas the peak principal stress ratios decrease as confining pressures increase With increasing confining pressures, the dilation decreases and the contraction eventually prevails. Initial strength parameters （Poisson＇s ratio and tangent modulus） show a nonlinear relationship with confining pressures when the pressures are relatively low. Shear strength parameters decrease with increasing confining pressures. The failure envelope lines are convex curves, with clear curvature under low confining pressures. Under moderate confining pressures, dilation is offset by particle breakage. Under high confining pressures, dilation disappears.

Discrete element simulation of crushable rockfill materials

A discrete element method was used to study the evolution of particle crushing in a rockfill sample subjected to triaxial shear. A simple procedure was developed to generate clusters with arbitrary shapes, which resembled real rockfill particles. A theoretical method was developed to define the failure criterion for an individual particle subjected to an arbitrary set of contact forces. Then, a series of numerical tests of large-scale drained triaxial tests were conducted to simulate the behaviors of the rockfill sample. Finally, we examined the development of micro-characteristics such as particle crushing, contact characteristics, porosity, deformation, movement, and energy dissipation. The simulation results were partially compared with the laboratory experiments, and good agreement was achieved, demonstrating that the particle crushing model proposed can be used to simulate the drained triaxial test ofrockfill materials. Based on a comparison of macro behaviors of the roekfill sample and micro structures of the particles, the microscopic mechanism of the rockfill materials subjected to triaxial shear was determined qualitatively. It is shown that the crushing rate, rather than the number of crushed particles, can be used to reflect the relationship between macro- and micro-mechanical characteristics of rockfill materials. These research results further develop our understanding of the deformation mechanism of rockfill materials.

A method to experimentally investigate injection-induced activation of fractures

Natural fractures are generally well developed in most hydrocarbon and geothermal reservoirs,which can produce complex fracture networks due to the activation of fractures during hydraulic stimulation.The present paper is devoted to developing a method to investigate the activation characteristics of fracture under injection-shearing coupled condition at laboratory scale.The fluid is injected into the single-fractured granite until the fracture is activated based on the triaxial direct shear tests.The results show that injection process can significantly influence the shear stress distribution field,resulting in release of shear stress and relative slip between the opposite sides of the fractured surface.The injectioninduced activation of fracture is strongly dependent on the stress states.When the normal stress increases,the injection-induced activation pressure increases,and the comparatively high normal stress can restrain the fracture activation.The fracture deformation mechanisms during fluid injection are also discussed preliminarily with the experimental data.The sensitivity of shear stress to fluid injection increases with increase of shear stress level,while it decreases under high normal stress.The results can facilitate our understanding of the natural fracture activation behavior during fluid pressure stimulation.