ABSTRACT:

Hydro-fracturing in-situ stress measurements were conducted in the Jijicao rock block of the Beishan area in Gansu Province, China. A total of 40 hydro-fracturing tests and 15 fracture impression tests were performed in three sub-vertical boreholes at depths ranging between 30 and 640 m below the ground surface. The measurement results indicate that the maximum horizontal principal stress is generally larger than the vertical stress, indicating that within the range of tested depth, the stress filed is dominated by tectonic horizontal stress. Linear regression analysis on the magnitude of principal stress components with depth shows that the in-situ stress field in this rock block can be characterized by a piecewise distribution in three zones. The measured maximum horizontal principal stresses are mainly oriented in the NNE-NEE direction, approximately parallel to the maximum regional tectonic stress inferred from compressive stress origin due to plate tectonics. This agreement supports the accuracy of the in-situ stress measurement results. It is also found that local fault structures can significantly modify the regional stress field, leading to the rotation of the maximum horizontal stress.

I INTRODUCTION

Safe disposal of High-Level radioactive Waste (HLW) is one of the most challenging rock engineering issues. Over the past few decades, extensive research related to deep geological disposal of HLW has been conducted by the international nuclear community (Wang 2010). In China, work on site selection and characterization for the HLW repository has been carried out since 1985. The efforts have been focused on the Beishan area in Gansu Province of northwestern China, which contains many granite intrusions. During the site selection and characterization in the Beishan area, a series of site investigations and tests have been conducted. The in-situ stress at a site is required for the design of underground engineering structures employing, in particular, numerical tools. Over the last three decades, advances in the use of numerical methods in rock mechanics have been impressive, and nowadays almost all rock engineering design utilized numerical tools. Hence, to provide reasonable modeling parameters of rock stresses and subsequently to optimize design and construction of the HLW repository, reliable assessment of the in-situ stress state at the sites is a prerequisite.

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