Numerical Simulation of Shear Behaviour of Non- Persistent Joints under Low and High Normal Loads

In this paper, the effect of rock bridge surface on the shear behavior of planar non-persistent joints under low and high normal loads has been investigated using particle flow code in 2 Dimensions. PFC2d was calibrated with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. The models consisting non-persistent joint were simulated and tested by shear loading under low and high normal loads. The discrete element simulations demonstrated that the failure pattern was mostly influenced by normal load, while the shear strength was linked to the failure pattern and failure mechanism. The failure patterns were found reasonably similar to the experimentally observed trends

Proposed constants for Bieniawski’s strength criterion for rocks and coal

Bieniawski’s strength criterion is one of the most widely used criteria for strength estimation of intact rocks. This criterion, however, only considers compression loading. In Bieniawski’s criterion rocks are distinguished in their properties using two parameters В and a. Selecting these parameters, through lab experiments, as representative as possible for a certain type of rock is significantly important. The quality of lab tests, the number of tests and statistical approaches used to estimate these parameters are some of the important factors, which can influence the accuracy of the estimation. Several attempts have been made by different researchers to propose these parameters for different rock types in different regions. In this paper a similar attempt was made to determine more representative constants for Bieniawski’s criterion. This work is different from past studies in that we have based our analysis on a very large number of lab experimental data gathered from the literature and some carried out for the purpose of this study. The studied data includes a wide range of rock types from soft to hard including sandstones, shales and coals. Both linear conversion and nonlinear regression models were applied to the lab data and as a result Bieniawski’s constants were proposed for each rock type. For coal, the results are presented as a function of the loading angle with respect to the coal’s fractures. The results of nonlinear models were found to be associated with higher correlation coefficients. Also a correlation between parameter В and unconfined compressive strength was proposed. The results of this study were also compared with similar work presented in the past

A New Approach for Measurement of Tensile Strength of Concrete

Tensile strength is one of the important mechanical properties of concrete. Indirect methods have been used up till now for its measurement. These methods though widely accepted, do not furnish the true tensile strength of concrete. In this paper, a compression to tensile load transformer device is developed to determine direct tensile strength of concrete. The cubic concrete samples with dimension of 15*19*6 cm containing internal hole with dimension of 75mm is prepared using the mixture of cement, fine sand and water. This model is subjected to direct tensile loading by special loading device. A series of finite element analysis were performed using FRANC2D to analyse the effect of hole diameter on the stress concentration around the hole and to render a suitable criteria for determination of real tensile strength of concrete. Concurrent with direct tensile test, Brazilian test is performed to compare the results of two methods. Results obtained by this test are quite encouraging and shows that the direct tensile strength is clearly lower than the Brazilian test. The dierence between the Brazilian and direct tensile strengths is about 33%

New Development to Measure Mode I. Fracture Toughness in Rock

Tensile fracture toughness is one of the dominant characteristics of rocks that play an important role in fracture mechanics of rock structures. In spite of the substantial amount of work that has been conducted as suggested methods to determine the fracture toughness of rock in Mode I., research on new methods are still demanded. The compact tension (CT) specimen is widely used to determine the fracture toughness of metals and is a standard method in accordance with ASTM standard. To conduct similar direct tests in rock, preparation of rock specimen a difficult task and needs some special tools to create notched sample. To address these issues, experimental techniques based direct tensile apparatus have been developed. Developed experimental procedure involves the use of the specimens having a central hole and two notches or cracks at both inner side of proposed specimen. The stress intensity factor formula at the crack tip of a CT specimen can be availed through the literature. However a new expression of stress intensity factor is needed for the developed test in order to account geometry and loading configuration. To develop a new stress intensity factor expression, the numerical models of suggested method were worked out. The focus of the current work is on development of the experimental technique, which involves determining the optimum sample size and shape, loading procedure, fracture toughness for different rock types, and verification of suggested method based on CT test which has been carefully considered in this investigations