Determination of strength properties of bischofite in triaxial stress state

Construction of underground gas storages in rock mass containing bischofite faces some problems connected with formation of large caverns and instability of casings. Bischofite possesses high hygroscopicity and low strength. Bischofite strata cause, therefore, complications in well constructions, and the most common trouble is the tight hole due to high tendency of bischofite to creep. When assessing suitability of salt rock masses for the construction of underground gas storages, peculiar place belongs to the research of the mechanical properties of rock salt since they have a substantial influence on dimension and performance characteristics of a storage. Since origin of rock salt can differ even within a single deposit, the mechanical properties of rock salts are also greatly different. The strength properties of bischofite are estimated by triaxial stress testing. The stabilometry test procedure is described, the test data of bischofite samples are presented, the comparative tests of rock salt samples are carried out, and the failure curves are plotted. The cohesion and internal friction angles of bischofite are determined. It is shown how the triaxial compressive strength changes as function of lateral pressure value. It is proved that at the same lateral pressure, the ultimate strengths of bischofite are lower than in rock salt not less than by two times, and the internal friction angles in these types of rocks are similar.