THE ROLE OF WATER ON DEVELOPMENT OF SCALY CLEAVAGE AND GEOMECHANICAL BEHAVIOUR. RESULTS FROM EXPERIMENTAL SHEAR DEFORMATION

In order to outline the role of lattice ordering and water content of Ca-smectite on the development of scaly cleavage and geomechanical behaviour, a cycle of controlled experimental shear deformations on natural and structurally modified Ca-smectites have been performed and correlated to structural and morphological analysis. Experimental shear tests were performed on five types or Ca-smectites: (1) a natural smectite; (2) the same smectite heated up to 80°C; (3) up to 250°C; (4) up te 335°C and (5) octahedral defective smectites. The materials have been deformed by a shear box apparatus under 19.6 MPa condition of normal pressure. Ali samples, before and after the shear deformation, have been analysed by SEM and XRD. The shear deformed samples or natural smectite are cohesive and present striated and polished shear planes along the displacement surfaces imposed by the experimental apparatus. The shear planes are arranged in sets disposed at low angle with respect to the principal shear surface, which correspond to R, P and D planes, (Skempton, 1966; Tchalenko, 1968). The disposition of planes and the lustrous aspect of the surfaces are quite similar to the ones observed in natural examples, In our experiments striations are not related to ploughing by coarse grains or asperities because ave have deliberately used smectite with homogeneous composition and granulometry. XRD and SEM analyses indicate that in the vicinity of the shear planes the day platelets appear preferentially orientated and with a "face te face" arrangement, whereas in zones far from the shear planes the platelets present a lower degree of orientation and an "edge-face" disposition. The shear strain also produces a reduction of structural ordering and an increase or both lattice defects and microstrain in the smectite. Moreover, natural smectite presents a typical strain softening behaviour and the geotechnical parameters (cohesion and internal friction angle) of overconsolidated clayey rocks. The cohesion and compaction of the material are essentially related to the capability to release the so called "absorption water", whereas the development of striated shear planes has to be attributed to the role of interlayer water. The last hypothesis is supported from the experiments carried on the same smectites heated up to 8O°C (to desorb completely the water), which show only scaly cleavage and striations. Samples heated up to 250 and 335°C, which have almost completely lost the interlayer water molecules, do not show neither development of scaly cleavage nor striations on the principal shear surface, moreover they show a very moderate strain hardening behaviour. Defective smectite with a disordered structure that inhibits absorption and release of water (Dellisanti & Valdrè, 2005), presents a strain hardening behaviour and does not show any striations and scaly cleavage. The general strain softening behaviour of natural smectite can be explained by to the presence of both absorption and interlayer water that allow sliding of clay platelets along the crystallographic layers. However, the defective smectite presenting a lack of structural ordering and a partial destruction of the ab planes, is hindered to a deformation by sliding mechanisms and, therefore does not develop shear planes and alignment of day platelets. The above mentioned results obtained in monomineralic homogeneous smectite rock further stress the importance of water on the development of scaly cleavage. In particular, this research introduces new insights on the dependence of the geomechanical behaviour from the different kind of water present in the material.