Effect of strain rate on compressive and tensile strength of rock materials

The aim of this study was to determine and evaluate the time-dependent hardening behavior of selected model natural (marble) and artificial (concrete) rocks under different stresses and to develop a general approach. In line with this objective, a literature review was conducted. In this research, it has been seen that rock material and rock mass properties are primarily taken into consideration in the support design studies of rock engineering structures. There are recommendations put forward by ISRM and ASTM for rock mechanics tests required in the support design studies of rock engineering structures. However, the time effect is not taken into account in most of the test procedures for determining rock material and mass engineering properties that are used extensively. However, in field measurements, observations and investigations, it can be observed that even at low stress values below the known strength values of the rock material and mass, the rock material is defeated and unstable after the hardening process. Routine test results, which are taken into consideration especially in the design work of underground rock engineering structures, bring up the problem of not providing the expected stability of the excavation openings in the long term. Therefore, it would be more accurate to determine the strength values as a result of hardening and softening observed in a rock material under stress based on time and to take these values into account in support design analysis. In order to solve the above problem, a series of experiments were planned to be carried out in the laboratory environment. In this study, in order to examine the effect of time, 5 models of natural (marble) rock material (DKM) and 5 models of artificial (concrete) rock material (YKM) were selected in the strength ranges of 0-20 MPa, 20-40 MPa, 40-60 MPa, 60-80 MPa, 80-100 MPa to be used in the experiments. Strength-velocity-controlled (Group 1) and constant-strength-controlled-creep (Group 2) experiments were carried out on these models with the help of servo-controlled hydraulic press. Firstly, rock mechanics experiments recommended by ISRM (1981, 2007) were carried out on DKM and YKM model materials and a basic rock mechanics database was prepared. In the second stage, the changes in the strength properties of the model rock material based on hardening were investigated at different loading rate values (MPa/s) in stress-velocity-controlled experiments defined as Group 1 within the scope of exploratory rock mechanics studies. As a result of the experiments, it was observed that the failure values of the material decreased with decreasing loading rate values, while the failure times increased. This situation was considered as critical hardening and the critical limit failure values of the model materials exposed to different loading rates were determined based on hardening. Similarly, within the scope of exploratory rock mechanics studies, the change in strength behavior due to hardening in constant-stress-controlled-breakdown experiments, defined as Group 2, was investigated at different constant load (kN) values.As a result of the creep experiments, it was observed that the material did not fail as the constant load values decreased, and this was considered as critical hardening. The critical limit failure values of the model materials subjected to different constant loads based on hardening were determined. It was observed that the material properties based on hardening determined by the results of two different experiments overlapped with each other. Therefore, DKM and YKM test results were combined. As a result, the critical limit strength values of the rock materials selected as models in this thesis were determined using SPSS statistical program. In this thesis, mathematical models that give the values of uniaxial compressive strength (σc(t)), indirect tensile strength (σtB(t)), elastic modulus (Ei(t)), cohesion (c(t)), internal friction angle (Φ(t)), based on hardening, which are explained by strong R2 values, were presented. In addition, pre-failure elastic modulus (Ei) and post-failure elastic modulus (Ed) values were determined during the experiments, and a strong relationship between both quantities was determined and expressed by a mathematical model. Keywords: Mathematical modeling, rock material properties, rock mechanics, strength, strain, time-dependent hardening behavior.