ABSTRACT:

The geomechanical properties of limestones often show substantial variations which are associated with differences in the constituents and structure of the limestone. In this study variations of geomechanical properties of limestones with varying micro-textural properties are investigated. Unconfined compressive strength tests (UCS) on limestone specimens were performed to determine the peak strength and crack initiation threshold. Additionally, the deformation characteristics were determined. Mineralogical analyses including X-Ray Diffraction were performed on each specimen. Micro-textural variations were investigated with thin sections of each tested specimen before and after mechanical perturbation. The thin section analysis was focused on determining the orientation and distribution of micro- and macro-cracks that formed during compressive loading.. Data obtained from mechanical tests reveal that the UCS values for the studied limestones varies between 38–140 MPa, and cracks are initiated at stress levels between 0.30–0.50 of the UCS. It is exposed that the peak strength, crack initiation and elastic constants are strongly influenced by the heterogeneous micro-fabric of the specimens. The existence of graphite veins, fossil fragments and calcite veins makes substantial changes in UCS, crack initiation stress levels, elastic constants and substantially affects crack propagation during compressive loading.

1 INTRODUCTION

Strength, deformation and cracking characteristics of rocks are dependent on the petrography, mineralogy and microstructure of rocks (Cogganetal. 2013, Ündül et al. 2015). Due to the heterogeneous and anistropic characteristics of most rocks, establishing relations between micro-texture and strength, as well as deformation and cracking characteristics is challenging and a time consuming task to accomplish (Prikryl 2001, 2006). This research aims to identify the variations in strength and deformation and to discuss the cracking characteristics of limestones selected from the Paleozoic sequence in Istanbul, Turkey and to compare these results to limestones from Switzerland and Canada.

2 TESTING AND ANALYSIS METHODS
2.1 Sample description

The main limestones used for this study were obtained from three different locations in Istanbul, Turkey. Two of these selected rock groups are of Ordovician age (Type A and C) and the other is from the Devonian sequence (Type B). Type A and C rocks are biomicritic limestones with a fine grained matrix. Occasionally layering can be identified. Type B rocks are nodular limestones with bedding. For all rock types calcite veins of random orientation are characteristic (Fig. 1).

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