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

Rock mass deformation modulus (Erm) has been considered as the most typical parameter illustrating the pre-failure mechanical behavior of rock mass. Erm is a basic input parameter for most rock engineering projects including tunneling, support designing, foundation designing, etc. For these purposes, it is necessary to obtain this parameter before starting numerical modeling. Intact rock elastic modulus is measured by uniaxial compression strength test in laboratories. But, the existence of discontinuities, makes a great difference between the mechanical properties of rock mass and intact rock. Erm can be estimited by emprical equations or determined by in-situ tests such as plate loading, dilatometer, flat jacking, and radial jacking tests. The plate loading test is a common test to determaining rock mass deformation modulus of rock mass that has been used in several project 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 Dorud city (Lorestan province). In recent paper, it has been compared the obtained Erm from plate loading tests with different stress values and different plate sizes 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 tests, the rock displacements are measured using borehole extensometers that fixed in several positions inside the boreholes drilled 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.
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 result in significant differences between results. For high quality rock masses, the larger the plate diameter is, the greater the evaluated Erm is. This difference is recognized related to the ratio of gallery dimension proportional to loading plate size.