In the present study, tests and empirical formulas were utilized to investigate the problem of determining the mechanical properties for igneous rocks considering seismic effect. The following points summarize the results obtained in the present note. The most concentrated frequency band of earthquake energy was determined by the platform segment of the design response spectra. With this frequency range, cyclic loading test was conducted for the rock sample, in which a strain rate range was determined for the rocks considering seismic effect. This strain rate range is lower than the traditional intermediate strain rate range. Dynamic triaxial compression tests were conducted for igneous rock specimens of two major underground rock engineering. A general trend of compression strength and the Young's modulus increase with the increasing strain rate can be observed. And it is found that, overall, fitting results with Hoek-Brown (HB) criteria are more accurate than those with the Mohr-Coulomb (MC) criterion for the dynamic compression test results. Finding in the test confirms that HB strength envelope under static conditions can be used to describe dynamic strength behaviors, and only need to change the intercept of the envelope, i.e. the uniaxial compressive strength (UCS). Thus, HB strength criteria considering seismic effect was obtained. It is practical to estimate the mechanical properties of rock mass considering seismic effect with the seismic HB strength criteria and Hoek & Diederichs equation based on the UCSs at various strain rates. The seismic modulus of the intact rock can be directly estimated with the UCS at different strain rates since the modulus ratio MR is strain rate dependent. The Baihetan (BHT) hydropower plant project was taken as a study case, its rock mass mechanical properties considering seismic effect were estimated with the proposed approach.
Various studies have been conducted to establish the well-known fact that the mechanical properties of rock materials are dependent on the strain rate (Green and Perkins 1968; Blanton 1981; Lajtai et al. 1991; Cai et al. 2006; Liang et al. 2011; Liu et al. 2012). Research investigations concerning the dynamic mechanical properties of rock materials have primarily involved laboratory tests, during which the rock mechanical properties at various strain rates have been explored. For example, the dynamic compression results of Li et al. (1999, 2000) and Zhao et al. (1999) concerning rock materials showed that the compressive strengths of rock materials generally increase with an increase in the strain rate. Similarly, the results of the experimental studies conducted by Zhao (1999) and Zhao and Li (2000) also showed that the tensile strength, shear strength and fracture toughness of rock materials all show increasing tendencies with increasing strain rates.