The mechanical properties of preconsolidated clays are studied at high pressures under undrained, triaxial conditions. At confining pressures equivalent to those at midcrust, different clays have the following common characteristics which differ significantly from those at low pressures: (1) the clays possess significant strength of several hundred bars (10 bars = 1 MPa); (2) the constitutive relations are marked by ductile yielding and strain hardening, followed by a broad peak strength and a gradual decrease in strength at greater deformation; and (3) the change in volume during shearing is small. On the other hand, the behavior of montmorillonite differs from that of the other clays (illite, chlorite, and kaolinite) in the following ways: (l) its peak strength at a given confining pressure is about half of the strengths of the other clays; (2) its peak strength occurs at a shortening of about 10%, while for the illite and chlorite the peak strength occurs at a shortening of 20–25% (for kaolinite, strain hardening continues even at 30% shortening); and (3) the fracture surfaces of some montmorillonite samples deformed at relatively low confining pressures show features resembling those in natural clayey fault gouge, whereas at higher confining pressures, montmorillonite and the other clays remain unfractured at shortening up to 30–40%.

Saturated clays are sheared between rock joints at various pore water pressures and at confining pressures up to 3 kb (300 Mpa). Sliding on these joints is stable. For a given claythe shear stress required to initiate sliding increases linearly with the effective normal stress across the sliding surfacewith a slope of 0.08 ± 0.01 for joints filled with saturated montmorillonite0.12 ± 0.01 with saturated chlorite0.15 ± 0.01 with saturated kaoliniteand 0.22 ± 0.02 with saturated silty illite. Thus at high confining pressures the shear stress required to initiate sliding on joints filled with saturated clays are very much smaller than that required to initiate sliding on clean rock joints or on joints filled with dry gouge materials. In the crustsaturation of gouge materials along active faults would greatly lower the frictional resistance to faulting and would stabilize fault movement. Different fault behaviors such as stable creep along some faults and intermittent but sudden slip along others may reflect in part different degrees of saturation of fault zones at depth.

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Preconsolidated montmorillonite clay was deformed up to 30% strain at confining pressures up to 4 kb (400 Mpa) under undrained conditions. The deformation was characterized by plastic yield, strain hardening, a broad peak strength at large strain, and a gradual decline in strength thereafter. In some cases the sample was ruptured, with stress drop of the order of 0.1 kb. At confining pressures greater than 2 kb, the peak strength was about 0.35 kb and did not depend markedly upon confining pressure. These results may have important implications on the stability and the strength of faults.

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