Probabilistic assessment of soil shear strength parameters using triaxial test results

According to the currently valid standards, bearing resistance can be calculated applying direct informational-statistic method which includes probabilistic models of margin of resistance Z = R - E. Probabilistic methods allow to determine the influence of which design condition argument on the uncertainty of margin of bearing resistance is the greatest. That enables to project the directions of further investigations. The greatest influence on the uncertainty of margin of bearing resistance is made by the tangent of angle of internal friction and cohesion. Therefore it is required that these strength parameters should be determined more exactly. This would enable to achieve the economy of materials and labour expenditures for the construction of foundation without decreasing reliability of ground. In order to estimate the soil strength parameters more exactly, some changes in the triaxial test apparatus have been made so that the sample base has been allowed to move freely in the horizontal direction. Several experiments have been carried out by improved triaxial test apparatus. It has been determined that the values of the tangent of angle of internal friction and cohesion have been less by 10,8% and 43% respectively for dense sand samples with free horizontal base movement than those determined for samples with restricted horizontal movements of ends (regular ends). Foundation width B calculated according to the parameters of residual shearing strength, determined by usual triaxial test apparatus, is smaller by 23% than that calculated according to the data obtained in the improved triaxial test apparatus. Reliability index of bearing resistance for a sample with regular ends calculated according to the residual values of soil shearing strength parameters by means of first order probabilistic methods for design approach 3 is beta = 4,4.