Estimation Criteria for Static Rock Mass Deformability Modulus for Rock-Socket Design in Metamorphic Rock Masses

This study investigates the most appropriate empirical criteria to estimate the static rock mass deformability modulus (𝐸_𝑚) in the design of rock-sockets in cast-in-situ bored pile construction. The in-situ 𝐸_𝑚 values are initially estimated through back analysis of static pile load test data. Secondly, the rock mass deformability estimated from back analysis (𝐸_𝑚b) are tested statistically against selected established empirical equations to determine whether the latter are appropriate for use in metamorphic rock terrain of Sri Lanka. It is found that the existing empirical criterion based on the square root of intact unconfined compressive strength (𝜎_𝑐) derived from back analysis of pile load test results is appropriate for weak-poor rock masses. For strong-poor rocks, it is recommended to employ the equation based on 𝜎_𝑐', and in general the two equations generate lower and upper bound solutions. The equation based on intact deformability modulus (𝐸_𝑖) performs well in strong-excellent quality rock masses, while the equations based on 𝐸_𝑖 and rock quality designation (𝑅QD) are found to be appropriate for weak-fair to excellent rock masses. Finally, a new set of equations appropriate for different rock mass types have been proposed through regression analysis along with appropriate design measures to be adopted.