A new experimental method for tensile property study of quartz sandstone under confining pressure

Tensile strength of rocks is an important parameter involved in the design and stability analysis of rock structures. The present paper is devoted to develop a new experimental method to investigate tensile strength, and the effect of confining pressure (P-c) on tensile behavior of rocks in underground rock engineering. A triaxial direct tension (noted as TDT) apparatus was developed based on the conventional triaxial compression (noted as CTC) device with the inspiration of the glue on steel cap type of method. TDT tests were carried out on the Fontainebleau quartz sandstone specimens under four different confining pressures. CTC tests under nine confining pressures were also carried out in order to compare the tension and compression behavior of the tested quartz sandstone. Tensile stress-strain curves were obtained during the CTC and TDT tests. Post-failure specimen analysis was also carried out to identify the failure mode in TDT test. The triaxial tension failed surfaces was sampled and scanned with the optical microscopy to obtain three dimensional images to show the effect of the confining stress on the failed surface roughness. The relationship between tensile and compressive strength and the strength criteria were analyzed with the experimental data. The results show the failure mechanism under TDT loading is different from that under CTC. The inter-grain cement is the dominant factor that contributes to the triaxial tensile strength of the quartz sandstone although the increase of confining pressure can induce a decrease of the triaxial direct tensile strength. It is suggested that the direct tensile strength of rocks should be tested directly for underground structures that confining pressure exists since uniaxial tensile strength can induce overestimate of the triaxial tensile strength.