DEFORMATION CHARACTERISTICS AT SMALL STRAINS OF SEDIMENTARY SOFT ROCKS BY TRIAXIAL COMPRESSION TESTS

It is shown that in triaxial compression tests of sedimentary soft rocks, axial strains from the axial displacement of the loading piston or specimen cap could be totally unreliable due to bedding errors at the top and bottom ends of a specimen. A local deformation transducer was used to measure axial strains free from the bedding error continuously from less than 0.001% to about 1%. At strains less than about 0.001% the deformations are virtually linear, recoverable and strain rate-independent and their values of Young's modulus E(max) are virtually the same as those obtained from field shear wave velocities. The deformations become non-linear before strain reaches 0.01%. At a strain of 0.1% and at the stress level of half the peak strength, which are the typical largest strain and stress levels in soft rock at working load levels, the secant Young's moduli are still about half of E(max). The ratio of E(max) to the compressive strength ranges from about 400 to 1000; about 100 is suggested by many previous studies.