Undrained Shear Characteristics of Normally Consolidated Peat Under Triaxial Compression and Extension Conditions

Abstract

Both vertical and horizontal samples of undisturbed fibrous peat, which were obtained by maintaining the axes of thin-walled tubes parallel to the vertical and horizontal directions in peat ground, were used in this investigation. These samples contained organic matter in the amount of 10%-80%. After normal isotropical consolidation of the samples in the triaxial cell, undrained compression and extension tests with pore water pressure measurement were performed, and the influences of confining pressure, loading path, amount of organic matter and fabric anisotropy on the undrained shear behavior of peat were investigated. Test results indicate that the undrained shear behavior of saturated peat can be discussed based on the principle of effective stress, in the same manner as inorganic soils. However, as the anisotropic fabric of fibrous peat which had formed during accumulation still remained after the isotropic consolidation, anisotropic shear behavior was observed ; the undrained deformation- strength properties observed in the compression test were considerably distinct from those obtained in the extension test. Moreover, it was also found that the normally consolidated fibrous peat had a cohesion intercept due to the effect of tension in the fibers, and undrained- strength parameters were found to be greater than those of inorganic soils. Furthermore, based on the test results, the authors proposed a new method of predicting stress-strain behavior of peat under triaxial compression and extension conditions.