Effects of grain size properties on the shear strength of a pyroclastic crushable soil

The paper presents some experimental results of a standard laboratory investigation carried out on reconstituted samples of a pyroclastic weak rock of the area of Roma, Italy. In its natural state the material may be assimilated to a bonded coarse-grained material; interparticle bonds are made of the same chemical and physical constituents of grains. Therefore bond degradation and grain crushing, which occur at relatively low stress levels, of the order of 50 kPa, can be regarded as indistinguishable features of the mechanical behaviour. The analysis of data obtained from conventional direct shear tests carried out on samples with different initial grain size distributions reveals that D50 is the main factor controlling the peak strength and the rate of dilation of the material; on the other hand the "ultimate" shear strength, evaluated at the end of test, is found to be mainly affected by both grading and grain crushing.