Determination of shear strength parameters of cohesionless soils by direct shear tests

Shear strength is the maximum shear stress that soil can withstand without any collapse. In the Mohr-Coulomb failure hypothesis shear strength is expressed as . Soil shear strength parameters can be determined by field and laboratory tests. In this study, a poor-graded coarse sand (SP) and a poor-graded gravel (GP) were used to perform standard and large scale (30cmx30cmx15cm) direct shear box tests. Test samples with loose (Dr = 30%), medium (Dr = 50%) and dense (Dr = 70%) were prepared. Tests were carried out under 20 kPa, 40 kPa and 60 kPa normal stress with 1mm/min deformation rate. Size effect of the box on the values of internal frictional angle was examined. Also monotonic triaxial tests with three different confining pressures (50 kPa, 100 kPa and 150 kPa) were conducted. The stress-strain relationship and strength parameters that obtained from shear box and triaxial tests were compared. Decrease in internal friction angle was observed due to box size increase. Increases in soil density and applied normal stress cause increases in shear strength. In triaxial compression tests, the ground is not forced to fail on a particular plane. For this reason internal frictional angle obtained from triaxial tests was lower than shear box value. By increasing the test system size an increase in system friction was observed. Therefore, the friction within larger systems should not be ignored.