Shear dilatancy is one of the most common phenomena of geological materials. A series of experiments were conducted in a newly manufactured large-scale direct shear apparatus to study the dilatant behavior of intensely weathered coarse-grained soils

including that 3 groups of various moisture contents

4 groups of various shearing rates and 5 groups of various gravel contents samples were tested

respectively. Dependence of vertical and horizontal displacements on the applied normal stress was analyzed. The results show that when other controlling conditions are fixed

the shear shrinkage increases with the applied normal stress. The maximum shrinkage increases with the increase of moisture content or the decrease of shearing rate; and the shrinkage of samples with gravel content less than 30% is significantly larger than that with higher gravel content. Our results suggested that for those samples with gravel content less than 50%

the effect of fine content became significant

so that the samples show a strain-softening behavior; and the dilation tends to a fixed value. It is shown that the two-dimensional dilation formulation proposed by Matsuoka is not applicable to the coarse-grained materials. Instead of the formulation

a quadratic polynomial can be adopted to represent the relationship between the stress ratio and the strain increment at the peak strength

the value range of experience parameters μ in this formula is given.