Effects of Stress Value and Loading Plate Dimensions on In-situ Deformation Modulus of Rock Mass in Plate Loading Test

Abstract

Summary The plate loading test is one of the most common in-situ tests used for evaluation of rock mass deformation modulus. In this test, during several cycles of loading and unloading, corresponding displacements are measured. These tests are carried out under different values of stress and by different plate dimensions. In this study, a comparison has been made between the results of this test considering two different stress values and two different plate sizes. Introduction Rock mass deformation modulus (Erm) is a basic input parameter for most rock engineering projects. For these purposes, it is necessary to obtain this parameter before starting numerical modeling. The plate loading test is a common test for determination of rock mass deformation modulus. This test has been used in several projects around the world. Erm of rock masses in Bakhtiari dam site is determined by Plate loading tests using different loading plate sizes and different stress values. Bakhtiari dam site is located in southwest part of Iran, almost 70 km northeast of Andimeshk city (Khuzestan province), and 65 km southwest of Doroud city (Lorestan province). In this paper, the obtained Erm from plate loading tests with different stress values and different plate sizes has been compared. Methodology and Approaches Plate loading tests at Bakhtiari Dam site are carried out using rigid loading plates with diameters of 650 and 915 mm and under maximum stresses equal to 20 and 40 MPa. In this test, two opposite areas in test galleries are loaded using rigid loading plates by means of hydraulic pump system. During the tests, the rock displacements are measured using borehole extensometers that fixed in several positions inside the drilled boreholes in the center of each loading area. The deeper extensometer in each borehole (used as a fixed point) is positioned at the end of borehole at depths more than 6 m. Erm is calculated according to ASTM D 4394 – 08. Results and Conclusions In the Bakhtiari dam site, plate loading tests carried out to determine deformation modulus of rock mass with different loading plate size and different stresses. In case of two maximum stress values of 20 and 40 MPa, no significant difference is evaluated. Conversely, using two different diameter plates in same position resulted in significant different results. For high quality rock masses, the larger the plate diameter is, the greater the evaluated Erm is. This difference is recognized to be related to the ratio of gallery dimension to loading plate size.