An Empirical Correlation of Uniaxial Compressive Strength with P-wave Velocity and Point Load Strength Index on Marly Rocks Using Statistical Method

Excerpt

Estimation of rock mechanical properties is considered among the most important components in any engineering project. In this regard, one of the most commonly used and fundamental mechanical parameter is uniaxial compressive strength (UCS) (Bruno et al. 2012; Minaeian and Ahangari 2011). UCS (σ_c) is also one the key mechanical properties of rocks as its assessment is necessary in various rock engineering projects. It has a large number of applications including assessment of rock mass rating (RMR), assessment of failure criterion of intact rock and rock mass under triaxial stresses, estimation of the pillar strength in underground mines, assessment of rock fragmentation in blasting operations, estimating the penetration rate of tunnel boring machine (TBM), determination of drillability of rock, and assessment of bearing capacity of rock foundations (Moomivand 2011). There are basically two methods for assessing the UCS of rocks. One, known as the direct method, is to test the specimens in the laboratory, the other, known as the indirect method, is to use previously derived empirical equations from the literature (Baykasoglu et al. 2008). Testing procedures for the direct method have been standardized by both the American Society for Testing and Materials (ASTM) and International Society for Rock Mechanics (ISRM). High-quality core specimens are needed for direct determination of UCS in a laboratory (Ceryan et al. 2012). However, high quality cores in sufficient quantities cannot always be extracted from weak, highly fractured, weathered, and thinly bedded rocks. Besides, the careful execution of this test is very difficult, time consuming, and expensive and involves destructive tests (Gokceoglu and Zorlu 2004; Baykasoglu et al. 2008). To overcome these difficulties, indirect tests such as the determination of the point load strength index (PLSI) and P-wave velocity are widely used and accepted for estimation of the UCS value. The index-to-strength conversion factors have been proposed by a number of researchers and have been found to be rock-dependent (Akram and Bakar 2007). However, there are few equations in this regard for Eocene Marly rocks. Considering the vast distribution of Marly formations around the world and especially in Iran, there is a shortage in knowledge concerning the behavior of this type of material. The aim of this technical note is to evaluate the indirect methods such as P-wave velocity and PLSI to estimate the UCS by using statistical method. The detailed petrographic study of marly rock used in present study has been carried out. Mineralogy of the analyzed samples was determined by X-ray diffraction (XRD) method. According to the XRD results, the marl samples mainly consist of clay minerals (illite, chlorite), quartz, calcite, and dolomite. The studied rock samples were collected from the southwestern part of Iran. Regression analysis was applied to define the relation among the UCS with P-wave velocities and PLSI. The determination coefficients (R²) and the equations of the fitted lines were calculated. …