The Effect Of Fractures On Rock Stresses And Its Significance In Geological Engineering

Knowledge of the in situ stress in the Earth's crust is very important for many problems in civil, mining and petroleum engineering as well as in geology and geophysics. Despite the importance of in situ stress and the development of a variety of methods to determine in situ stresses in rock masses, rock stress has not been well understood. Fracture is one of the most important factors affecting rock stresses and results of site measurements indicate that there exist reorientation and magnitude variation of stresses in the vicinity of fractures. This thesis is intended to study the effect of fractures by site measurement analysis, field geological survey and numerical modeling. It consists of five parts: (1) By studying current results in rock stress and its measurement, the factors in generating and affecting rock stresses is analyzed and the role of fractures in disturbing in situ rock stresses is documented; (2) By carefully studying the in situ stress measurement results over the world, the general features of the effect of both active faults and inactive faults on rock stresses are concluded; (3) The geological structural characteristics and the modern activities of major active faults in Lijiang region of Yunnan Province and upper stream of Minjiang River in Sichuan Province of West China and their effect on rock stresses are studied through site investigation and laboratory tests. (4) By systematic numerical modeling by the distinct element method with UDEC code, the features of rock stress perturbation caused by the existence of fractures ( both single fracture and compound fractures) and the mechanism are analyzed; (5) The significance of rock stress perturbation produced by fractures in geological engineering is discussed on the basis of engineering practice both in regional crustal stability and rock mass stability. The main conclusions drawn from this study are as follows: (1)Gravitation and tectonic movement are the fundamental causes for producing rock stresses. While factors that may influence rock stresses are various, such as geological structures, topography, rock types and the so on. However, the effect of fractures on rock stresses is very common, and the existence of fracture is one reason of the complicated variations of rock stresses in the Earth's crust. (2)Both a single active fault and compound active faults can perturb stresses in rock masses, Compared with regional stress, the orientation and magnitude of stresses around active faults changes in different extent, and most of this kind of change occurs in a certain range around the fault. However, the orientation of principal stresses trends to be in line with regional stress far from the fault. The variation of stress magnitude around active faults is a little bit complex, that is, stresses may be increased or be decreased. The decrease in stress magnitude is manifested by being lowered near the fault, increasing with the distance from the fault and becoming constant at certain distance from the fault. Increase or decrease of stress magnitude is determined by the geometry of the fault and the angle relations between the orientation of regional stress and that of fault, and the magnitude of variation largely depends on the scale of faults concerned. There are different stress regime in different sections along a fault, both in the orientation and magnitude of principal stresses. Stress regime in the vicinity of active faults changes with time, especially in the seismically active region. Compound active faults can lead to local stress concentration near the fault, generating earthquakes and high-stress phenomena. The effect of compound fault on stresses is strongly controlled by the geometry of the fault. The geometry of any single fault among the compound faults may influence the stress regime. (3) Non-active faults also have an effect on the stress field near fractures. Near fractures, the stress may reorients in an angle as large as 90°. Fractures make up local interface of stress regime, and both magnitude and orientation may be different at the two sides of a fault. The density and intersection of fractures determine the magnitude of stress variation. (4)Numerical modeling by the distinct element method demonstrate much information about stress variation caused by the existence of fractures. It is found from modeling that the variation stresses in the vicinity of fractures depends on the mechanical and strength properties of both fractures and rock masses as well as boundary conditions. The range of stress variation is strongly determined by the friction angle (φ) of the fault, the boundary stress ratio (Kb=σ1/σ2) and the angle (α) between the fault and the direction of regional maximum principal stress. Among the mechanical properties of faults influencing the stresses around a fault, the friction angle is the most important of those studied. (5) By employing the technique that uses fictitious joints behaving as intact rock, the stress state at fault ends is successfully modeled. (6) Site investigation and numerical modeling indicate that the stress field and seismic activity in Lijiang region of Yunnan Province and the upper stream of Minjiang River are controlled by the compounding of active faults. (7) The stress field in ?sp? area of Sweden is much dependent on the development of fractures and their combination. (8) The effect of fractures on rock stresses may be successfully applied to solve the problems concerned with geological engineering like regional crustal stability and rock mass stability. (9) The study of this thesis is a contribution to understand rock stresses in the Earth's crust and provides theoretical evidence for the evaluation of crustal stability and design of rock engineering.