Researches On The Failure Law Of Columnar Jointed Basalt Under Unloaded Conditions

Different stress paths resulting in failure process of rock is different becauserock damage is closely related to the stress environment, and rock burst is acomplex evolutive processes which is non-linear and non-equilibrium. At present,the failure law of columnar jointed basalt is an urgent issue in the area ofhydropower project because its mechanical characteristics and rock massesstructure are very unique. The research result of columnar jointed basalt’s failurelaw is of great theoretical significance and practical value for many problems suchas revealing macroscopic nonlinear mechanical behavior of columnar jointedbasalt’s rock failure process, evaluating the security status of columnar jointedbasalt, analyzing structure stability of columnar jointed basalt, taking a reasonableamount of support measures and so on.This paper researches on the failure law of columnar jointed basalt inBaihetan Hydroppower Station based on primary methods of in-situ true triaxialunloading test under different stress paths. Based on the method and theorymentioned above, such aspects of the research are included in this paper. First,Comprehensive analysis of columnar jointed basalt structure of dam site inBaihetan Hydroppower Station. Second, through the comprehensive analysis of theresult of the in-situ true triaxial test, the destruct characteristics of columnar jointedrock masses under unloading conditions were summarized. Third, the failuremodes of columnar jointed basalt were analyzed through positioning technology ofAE and the sketch map of geological fissures. Fourth, rock masses’ strengthparameters were estimated by applying Mohr-Coulomb strength criterion.Based on the researches made above, a deeper understanding to deformationanisotropy of columnar basalt was obtained. The major conclusions show asfollows:a. With a high strength of rock block and a low strength of columnar joints,the strength of columnar joints basalt was mainly decided by the combination ofthe strength of columnar joints.b. The columnar jointed basalt was demonstrated by the apparent expansionbecause its lateral strain was significantly larger than its axial strain. At the peak, inthe process of rapid destruction of rocks, it occurred with crisp pop sound when thelarge drop of rock stress with the small rock strain. It was showed that rockoccourred brittle fracture in loaded conditions.c. Some residual strength still existed in the body of columnar jointed basaltafter it had been damaged, which showed that rock masses would still have someload capacity through crack friction after being weakened.d. The destruction of columnar jointed basalt was a process of further evolution, coalescence and breach based on the original joints inside rock masses.e. The cohesive force of rock which was getted by true triaxial uninstallfailure test was higher than its direct shear test cohesive force. By the formermethod, the cohesive force of unloading of columnar jointed basalt were greaterthan4MPa.