In order to study the confining pressure effect and particle size effect of shear properties of calcareous sand

triaxial shear tests were carried out under different particle sizes

relative densities and confining pressures. The stress relative softening coefficient and dilatancy coefficient were introduced to quantitatively characterize the strain softening and dilatancy characteristics. The experimental results show that with the increase of confining pressure

the strain softening and dilatancy characteristics of calcareous sand samples with different particle sizes decrease gradually

and the confining pressure is semi-logarithmic linear with the relative stress softening coefficient and dilatancy coefficient. The strain softening and dilatancy characteristics of calcareous sand samples with different particle sizes disappear due to the existence of critical confining pressure of strength and critical confining pressure of volume variation. In the range of particle size 5-0.075 mm

confining pressure has a significant effect on softening and dilatancy characteristics of loose samples

but there is no significant correlation with particle size. For dense samples

the effect of confining pressure on softening characteristics of samples increases with decreasing particle size

while the effect on dilatancy characteristics of samples decreases. Under a low confining pressure (50 kPa)

the breakage of 0.5-0.25 mm particle size group is the most significant.