Tensile strength of rocks is an important parameter in the design of underground structures and for various engineering applications. Lack of literature and rapid development of infrastructure in central region of India calls for a need to characterize the mechanical behaviour of rocks in Deccan trap region which is mainly composed of Basalt. In this paper, characterization of tensile behaviour of Basalt rocks from Deccan trap is done at different loading conditions. Indirect test method namely Brazilian test is used to calculate the static indirect tensile strength at a loading rate of 0.2 kN/s. Tests are performed on different diameters samples and desirable failure modes are recorded. Tensile strength values recorded at slow loading are obtained to be different for specimen having different diameters. However, strength of rock in engineering problems like blasting, mining, rock bursting and breakage are affected by high loading rates. To consider the dynamic loading effect, a modified split Hopkinson pressure bar technique is used. For dynamic tensile experimentation, Brazilian disc specimen are used which are slightly flattened before testing. The flat ended Brazilian discs (FEBD) are placed diametrically in SHPB setup. Dynamic force equilibrium is successfully achieved for the initial stages of fracture propagation till complete failure of sample. The factor for increase in dynamic tensile strength of Deccan Basalt has been evaluated in this work. Scope of further research has been identified for experimentations on soft rocks to determine their dynamic tensile strength characteristics.

1. Introduction

Strength characteristics of rocks are one of the most important parameters required for mechanical analysis related to various applications in rock engineering field. This work has been progressed on the idea of extension of various experimental methods available in Rock engineering field. The Deccan trap region in central India, which is mainly composed of Basalt [1], is a type of igneous rock formed from volcanic lava flow. This region is becoming an attractive site for large construction projects like dedicated freight corridor project (DFC), multistoried buildings, large hydropower projects like Koyna Dam, underground tunnels (nearly 20 tunnels exist on the newly built Mumbai-Pune expressway as observed during site visits for this work). It becomes imperative to design such important structures with good understanding of behavior of the rocks under low as well as high loading conditions, as mechanical behavior of rocks is dependent on the loading rate [2, 3, 4, and 5]. Determination of tensile strength of rocks using direct methods like pull test is a difficult task. From various indirect test methods, Brazilian test is widely used and the mechanism behind it has been considered in this work. Split Hopkinson Pressure Bar (SHPB) which is widely used for determination of compressive strength of rocks and other materials [6, 8, and 9], has been extended for tensile testing at high loading.

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