ABSTRACT:

When rock samples are loaded, Acoustic Emission (AE) occurs when stress reaches a level greater than that which the rock has previously experienced. This phenomenon, known as Kaiser Effect, which has been used as an indirect method of in-situ stress measurement in rock mass that can be one of the most important parameters in designing of underground structures. Several physical parameters have effect on Kaiser Effect such as porosity, sample size, moisture content and so on. In this research, effect of porosity as a physical property of rock on in-situ stress measurement using AE method is studied. Porosity, creating a heterogeneous environment in the rock sample and causes effects on propagation of acoustic signals. In order to omit interfering with other parameters artificial rock samples (Ferro Cement Mortar), with fixed grading concrete and no memory of previous stress are used so the only variable factor is air cavities in the samples (porosity). Several samples with different porosity percentages ranging %14 to %20 were compressed uniaxialy. The results shown that porosity have effect on Kaiser Effect and in-situ stress amount perdition. Ultimately, amongst various AE parameters, energy and count parameters are better for determination of pre stress.

1 INTRODUCTION

Knowledge of an amount and direction of in-situ stress in many rock structures such as; an underground space, rock slopes is essential. There are various methods to estimate the in situ stress in field and laboratory. Most of these methods are time consuming and require high costs. In the laboratory, several methods have been proposed. All experimental methods are based on the drilled cores. When the depth is high, the measurement will be based on core drilling. Since core-based methods are very easy, cheap and less time. So this method is interested by rock mechanician. One of the core-based techniques is using of Acoustic Emission (AE) method. This method is based on Kaiser Effect. Kaiser Effect is emission of acoustic when rock undergoes stress. In this method, the maximum stress pre applied on the rock sample is equal to the starting point in AE activity. This phenomenon is due to the lack of acoustic signals at levels lower than the maximum stress which rock has experienced before (Seto et al. (1999), Tuncay & Ulusay (2008), and Tuncay & Obara (2012)). To verify the accuracy of Kaiser Effect, Felicity ratio is used. Felicity ratio takes different values for different stress levels and at different behavior stages of rock.

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