In situ stress measurements of two hydropower projects in Iran by hydraulic fracturing method

Abstract

Hydraulic fracturing is one of the best-known methods of in situ stress measurement at great depths. In the last two decades, this method has been used as an important tool for pre-excavation designing of large projects, e.g., hydroelectric power plants, tunnels, mines, waste disposal galleries, etc. In this paper, some methods of stress calculation in fractured and intact rocks by means of hydraulic fracturing test are described, and the in situ stress states of Aras hydropower project and Roudbar Lorestan pumped-storage project are determined afterwards. The hydraulic fracturing (HF) and hydraulic tests on preexisting fractures (HTPF) tests were carried out in four vertical boreholes at shallow depth (100–190 m) at Aras and two vertical boreholes at almost great depth (440–520 m) at Roudbar Lorestan. The in situ stress magnitudes and orientations are calculated by classical and inversion methods in HF method (great depths) and by differential evolution and Gauss–Newton algorithms in HTPF method (shallow depths). The results of stress measurements illustrate that differential evolution provides better solutions than Gauss–Newton method. It is also demonstrated that the maximum horizontal stress orientation falls in southeast quarter which is in good agreement with the reports published from focal mechanism analysis and the direction of existing faults in Aras Project but deviates from world stress map data in the Rudbar site where it is influenced by the local joints and fractures in the test zone. Eventually, it should be mentioned that the ratio of horizontal to vertical stresses is significantly affected by topography and geology of the test areas. However, this ratio decreases as the depth increases.