Use of down-hole triaxial apparatus to estimate the mechanical properties of heterogeneous mudstone

A down-hole triaxial testing (DHTT) method is used to measure the shear strength and deformability of a rock mass. The apparatus consists of a triaxial cell and an axial loading device that permits testing a columnar specimen with a diameter of 90 mm and length of 285 mm at the bottom of a drill hole at any depth. A series of tests were conducted at a 50 m deep experimental cavern contained in mudstone with some inter-bedded thin sand layers. The tests were done in three different depths by a multiple-step loading method. The obtained results were compared with those by traditional laboratory tests and field measurements. Although, DHTT was able to measure the stress–strain relationship of the rock mass, different results for shear strength and stiffness were observed in the specimens containing inter-bedded sand layers. The effect of inter-bedded sand layers was a reduction of shear strength and deformability properties. Stiffness was influenced by the level of confining pressure in each loading cycle, and by accumulated damage in the specimen from repeated steps of loading–unloading. Because of loosening and disturbing and some inherent errors in measuring the deformation of rock mass, the stiffness calculated by pressuremeter tests and plate loading tests is underestimated. Sampling disturbance reduced both stiffness and strength of rock material measured in the laboratory. It was concluded that the quality of drill-core in DHTT is reasonable, and its results are reliable compared with those obtained by traditional experimental methods.