Prediction of Strength and Modulus of Jointed Rocks Using P-wave Velocity

Strength and deformation characteristics of jointed rocks are important parameters for the design of many civil and mining engineering structures. These parameters are difficult to obtain from direct testing of jointed rocks, hence it is usual practice to derive them through indirect approaches. In the present study, an attempt has been made to obtain rough estimates of strength and modulus of jointed rocks through P wave velocity. Jointed specimens of a model rock were prepared for laboratory testing. Joints at different orientations and comprising of different joint roughness coefficients were used (JRC=2-4, 12-14 and 18-20). Ultrasonic P-wave velocity and UCS tests were conducted on the specimens. The effect of joint orientation and joint wall roughness on P-wave velocity was studied. An index, Joint Factor was used to quantify the effect of joint attributes on strength and modulus of jointed rock. It was observed that P-wave velocity is closely linked with Joint Factor. This study suggests a correlation to obtain Joint Factor Jf from P-wave velocity. It is recommended that this correlation may be used in the field to assess the Jf. The computed Jf may then be used to get the strength and modulus values of jointed rocks in the field. Charts are also suggested to roughly assess the shear strength parameters c and ϕ of the jointed rock.