Direct shear tests on an uncemented, and a very slightly cemented, locked sand

Direct shear tests up to 10 MPa normal stress have been carried out on two sets of intact block samples taken from the Reigate Silver Sand (part of the Cretaceous Folkestone Sand formation). One set (called A2) is completely free from any trace of authigenic cementation but its well interlocked fabric gives it a small, measurable cohesive strength. The other set (A3) possesses a trace of iron oxide and clay cementing and is noticeably more cohesive and easier to sample. Both sets of samples show a power law form of shear strength envelope but vary in their dilatancy characteristics. Observations have been made of the macro- and microfabric features produced during and at the end of the tests. These indicate that stress rotation is accompanied by microscopic rotation of blocks of intact sand during shear with buckling in the shear zone. Study of the literature on dense sands and sandstones suggests that a tendency for such block behaviour is a common feature of arenaceous materials but that it is only with locked sands that this mechanism is displayed to best advantage. Cementation increases the resistance of the blocks to rotation. Saturated samples of A3 have a slightly lower shear strength and it is inferred that the presence of moisture reduces the buckling resistance of the blocks.