Micro-textural variations are essential for understanding variation in strength and failure behavior of rock material. Therefore the effects of petrography and mineralogy on geomechanical properties have been widely investigated. In this study andesitic rock samples with different micro-textural characteristics from three different locations inTurkeywere selected. Unconfined compressive strength tests revealed substantial variations in Unconfined Compressive Strength (UCS) and the micro-crack initiation (σCI) threshold. Detailed petrophysical, petrographic and mineralogical analysis (XRD) were used to understand these strength variations. The synthesis of strength, petrographic, micro-structural and mineralogical data suggest, that both, the peak strength and crack initiation threshold are strongly influenced by the distribution of plagioclase, amphibole and biotite phenocrystals, and the mineral composition of the fine-grained matrix.

1 Introduction

Petrographic, mineralogical and microstructural features constitute the heterogeneous and anisotropic rock fabric. Basically strength and deformation characteristics of rocks (e.g. unconfined compressive strength (UCS) and micro-crack initiation (σCI)) rely on these constituents. The effect of micro-texture (including mineralogical composition, grain sizes, microstructure etc.) on the UCS and micro-cracking was investigated by Rigopoulos et al. (2011), Nicksiar & Martin (2013) and others utilizing modal analysis. Additional to previous studies, detailed quantitative mineralogical and petrographic analyses were performed before and after unconfined compression tests to improve our understanding of micro-crack initiation and strength variations in andesitic rocks.

2 Methods
2.1 Sample description

The andesitic rock samples for this studywere obtained from 3 different locations in NW of Turkey. These rocks belong to Miocene aged volcanic sequence. The samples were in fresh or slightly weathered state according toANON (1995). Unconfined compression tests were performed on 43mm diameter samples. These rocks were generally composed of plagioclase, amphibole, biotite phenocrystals and fine grained groundmass (Table 1).

2.2 Unconfined compression tests

For the unconfined compressive strength tests a 2000 kNWalter and Bai servo-hydraulic rock testing device with digital feedback control was utilized. Axial and circumferential strain gages were mounted onto the specimen at half of the specimen height to eliminate the influence of end effects on the strain measurements (Fig. 1). Two axial strain gages (Type BD 25/50, DD1) were firmly attached on opposite sides of the specimens. The measurement base-length was 50 mm. The radial strain was calculated from the displacement measured by a single gage (Type 3544- 150M-120m-ST) attached to a chain wrapped tightly around the specimen (Fig. 1). The radial displacement rate was utilized as the controlling feedback signal. The selected rate was 0.03 mm/min. Before testing of the rock specimens strain gages and the load cell were calibrated.

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