Title

Stress-strain-strength and stiffness characteristics of rock samples from Maddhapara, Bangladesh

Abstract

Bangladesh has an underground hard rock deposit of considerable size in Maddhapara. The Maddhapara deposit was discovered in 1963 and it is notably important for its shallowest occurrence of crystalline rock. Recently, five boreholes were drilled in Maddhapara and samples of rock were retrieved by continuous sampling using rotary driller. A comprehensive laboratory investigation was carried out to evaluate the strength, deformation, stiffness and flexural properties of rock samples collected from two boreholes from a depth of 149 m to 324 m. Unconfined compressive strength tests, flexural strength test using three point loading system and split cylinder tests were carried out on rock samples. Unconfined compressive strength tests were performed on both prismatic and cylindrical samples in order to assess the scale effects (shape and size) on measured strength and initial tangent modulus (i.e., Young's modulus). Flexural strength and split tensile strength of the rock samples were found to be in the range of 4.8 to 21.7 MPa and 5.5 to 13.8 MPa respectively. Unconfined compressive strength and Young's modulus of the prismatic samples were found to be higher than those for the cylindrical samples. Unconfined compressive strength for cylindrical and prismatic samples were found to vary from 44 MPa to 86 MPa and 72 MPa to 139 MPa respectively while Young's Moduli of prismatic samples varied from 26000 MPa to 500000 MPa, compared with only 8000 MPa to 58000 MPa for the cylindrical samples. Poisson's ratio of the prismatic and cylindrical samples ranged between 0.10 and 0.33. Petrographic study was also carried out on two rock samples of 60 mm diameter. Both the samples were dark coloured, very compact and hard and show no weathering stain. The sample feel rough, subconchoidally fractured and massive in structure. Under microscope, one sample showed holocrystalline, phaneric, inequigranular medium to coarse grained and porphyritic in texture while the other sample was hemicrystalline, aphenitic, inequigranular fine grained in texture.

Publication Title

The geotechnics of hard soils - soft rocks. Proceedings of the second international symopsium on hard soils-soft rocks, Naples, October 1998. (Two volumes).

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