Effect Of Fractures On In Situ Rock Stresses Studied By The Distinct Element Method

S. Su
International Journal of Rock Mechanics and Mining Sciences
Civil, Discrete element method, mining, Petroleum Engineering

The knowledge of the in situ state of stresses in the Earth’s crust is very important for many problems in civil, mining and petroleum engineering, as well as in geology and geophysics [1]. It is also critical for the storage and/or disposal of toxic and radioactive waste in rock.

Despite the importance of in situ stresses and the development of a variety of methods to determine in situ stress states in rock mass, rock stresses have not been fully understood. A wide scattering in stress orientations and magnitudes shows this and indicates that stresses are influenced by several factors. The presence of geological fractures is one of the most important factors [2] and is also one reason for high horizontal stresses [3]. Results of in situ measurements indicate that stresses reorient and their magnitudes vary in the vicinity of fractures. One example has been reported at the Underground Research Laboratory (URL) located in the Canadian Shield, where a shaft intersected two major thrust faults with splays. Characterisation of in situ stresses there indicates that there are two distinct stress domains separated by a major fracture zone [4] and [5]. The stress orientation may rotate as much as 90° with depth and the magnitude can also change when the fracture zone and its splay are intersected.

The author has studied the effect of a single fault on in situ stresses by using the distinct element method. It was concluded from that study that in situ stresses are reoriented and their magnitudes vary near faults. The main factors influencing the perturbation of in situ stresses in the vicinity of a single fault were identified [6]. However, rock masses have usually been subjected to several epochs of loading throughout geological time, producing fractures and faults of different orientations in space, resulting in a three-dimensional network of interconnected blocks [7]. This paper intends to study the effect of fracture sets and intersecting fractures on in situ stresses by numerical modelling using the two-dimensional distinct element method and the code UDEC [8]. This method is applied to study the changes of stresses with depth at Äspö Hard Rock Laboratory, Sweden.

Keywords: Discrete Element Method, Civil, Mining, Petroleum Engineering

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