Experimental investigations of fractured rock deformation: A direct measurement method using strain gauges

Measuring fractured rock deformation under changing loading conditions is important in rock engineering projects to estimate rock strength and permeability. Measurement of deformations at less than micrometer level for rock fractures is difficult. To investigate the feasibility of the resistance strain gauge method, an experimental system was built to measure the fractured rock deformation due to cycling of confining pressure. The test results of different positions were compared and relatively reasonable test locations were selected based on the test data and regression analysis. The measurement tests were conducted on cylindrical samples with diameters of 50 mm and lengths of 100 mm of shale, granite, limestone, and marble containing a single fracture or two parallel fractures. The experimental results show that the strain gauge method and the proposed test point pattern perform well for measuring fractured rock deformation under changing loading conditions. The experimental data for the loading progress for a single fracture fits the existing fractured rock deformation models well (i.e., the Goodman model, Bandis model, and Sun model). For cyclic loading, observed cycles compared with the existing models and the hysteresis loop during deformation under cycling of the confining pressure, the strain- pressure curves of the loading and unloading processes are consistent with existing models. The strain gauge method provides a deformation measurement method of two parallel fractures. Under the pressure 15 MPa for plane alpha the deformation is in a state of opening with strain value more than 1003 mu epsilon; but plane beta is in a state of closure with strain value near-115 mu epsilon.