Improvement of borehole shear test device and test validation on a loess site
摘要
A fully automated modified borehole shear test (MBST) device in which, the shear failure mechanism is revealed using computed tomography scanning technology is presented. The equipment was evaluated as an accurate method for determining normal stress, consolidation time, and shear strength characteristic values. A comparative analysis of the MBST and Handy borehole shear test (HBST) results for investigating a slope was performed. This showed that failure occurs when the soil sheared along a weak surface, and the normal stress-displacement curve for a typical test in loess could be divided into initial, quasi-elastic, and plastic stages. The shear plate and soil could form a stable relationship when the normal stress is in the quasi-elastic stage. The initial σ1 and maximum σn normal stresses were approximately equal to the σ0 and plastic σf normal stresses. Based on the increment of normal displacement S0, the initial normal displacement S0 and the plastic normal displacement Sf satisfied the relationship: 0.16≤ ∆S ≤ (Sr-S0)/4 or 0.16 ≤ ∆S ≤ (Sr-S0)/3. The remaining normal stress was determined by linear interpolation. Moreover, the consolidation time under the initial normal stress in a typical loess site was 13 min, while that under the remaining normal stresses was 10 min. The proportional limit, peak, and residual strengths were easily determined from the shear stress-displacement curve. The cohesion c value measured by the MBST device did not have a negative value as can occur with the HBST, which could more accurately reflect the shear strength parameters of the soil.