Estimation of the Mechanical Properties of Igneous Rocks in Consideration of Seismic Effects

Abstract

It is not widely accepted that the dynamic strain rate experienced by a rock mass during an earthquake is lower than that during a high strain rate scenario such as an explosion or a rock burst. The present study investigated the problem of determining the mechanical properties of rock considering earthquake effects using experimental tests and empirical formulas. First, to obtain the most concentrated frequency band of earthquake energy, a statistical analysis was conducted to obtain the response spectra of ground motion records near the example projects. The mean response spectrum was then fitted with a design response spectrum formula. The most concentrated frequency band of earthquake energy was then determined with the flat segment of the design response spectrum. Based on this frequency range, cyclic loading-unloading testing that simulated the earthquake effect was conducted on rock samples. The strain rate range experienced by rocks during an earthquake was then obtained. This strain rate range is between the quasi-static and intermediate strain rate ranges and is significantly lower than the high strain rate range. Based on this strain rate range, dynamic triaxial compression tests were conducted on igneous rock specimens from the case study areas. It is confirmed that the fit of the dynamic compression test results with the Hoek-Brown (HB) criterion is more accurate than that with the Mohr-Coulomb criterion. The HB strength envelope under static conditions can be used to predict the seismic strength behaviors, requiring an update of only the intercept of the envelope. Thus, the HB strength criterion considering the seismic effect was obtained. Moreover, the seismic modulus of the intact rock can be estimated with the values of the uniaxial compressive strength (UCS) under different strain rates and the modulus ratio, which was found to be independent of the strain rate. Hence, it is practical to estimate the mechanical properties of rock masses in consideration of seismic effects with the seismic HB strength criterion and the Hoek and Diederichs equation based on the UCSs under various strain rates. Ultimately, the rock mechanical properties in consideration of seismic effects were estimated via the proposed approach for the Baihetan hydropower plant project as an example.