ABSTRACT

: Thermal loading can impact the mechanical properties of rock. In deep excavations, for example, ventilation can result in a significant rapid cooling of the rock. In this study sandstone samples were subjected to slow heating followed by rapid cooling, referred to as thermal shock. An initial suite of tests were conducted at temperatures of 100°C, 200°C, and 300°C. In these tests, samples were subjected to a single cycle of heating and cooling and then tested. Measurements included P and S wave velocity, fracture toughness and tensile strength. Even though only small changes were seen at 100°C, further studies were conducted at this temperature because of the practical importance of this temperature range in mining and civil design. Cyclic heating and cooling was conducted at 100°C, with measurements of fracture toughness and tensile strength at 10, 15 and 20 cycles. Even though the overall results from the tree types of measurements (seismic velocity, fracture toughness, and tensile strength) are quite complicated, they can be at least partially explained by considering three types of crack density changes: a small decrease in crack density (crack healing), a small increase in crack density (blunting of macrocracks), and a large increase in crack density (rock damage).

1. INTRODUCTION

Thermal stresses are generated in rock from changes in temperature due to two primary causes, steady state heat flow and transient heat flow. Under steady state heat flow, thermal stresses are generated in the rock microstructure due to elastic mismatch between grains and pre-existing cracks and pores. Under transient flow, rapid heating can result in large compressive stresses, and rapid cooling can result in large tensile stresses [1, 2, 3]. Deep underground excavations are subjected to cyclic thermal stresses due to rapid ventilation cooling and intermittent stoppages. For example, for an underground excavation at a depth of 2000 meters, rock temperatures as high as 80°C could occur and rapid cooling by as much as 60°C could occur due to ventilation. This could result in increased drift degradation due to micro- and macro-cracking [4]. Laboratory tests have been conducted to evaluate the effects of thermal loading on the mode I fracture toughness, tensile strength, and seismic wave velocity. To determine the mode I fracture toughness, the Edge Notched Disc (END) method was used [5]. To determine tensile strength, Brazillian disc tests have been conducted. A seismic test has been conducted to investigate the compressional wave velocity and crack density after the thermal loading. Coconino sandstone has been used to investigate the change of rock properties after the thermal loading. 50.8 · 25.4 mm, 50.8 · 25.4 mm and 50.8 · 50.8 mm (diameter · thickness) samples are used for the END test, the Brazillian test, and the seismic test respectively. In this study, thermal loading is modified compared to most previous investigators [3,6,7,8]. To simulate the ventilation in the deep underground mines, samples are slowly heated up to a specific temperature, and then cooled rapidly using a fan. This kind of thermal loading will be referred to as "thermal shock."

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