Altered-Rock Characteristics And Its Engineering Respondences Studying

Dam abutment is the directly loading geology body of arch dam, so its stability is important to insure the safety of water and electricity engineering, and the soft structural plane of it is the key factor to insure long time stability of the dam abutment. Among these arch dams under construction, Xiao Wan hydropower Station arch dam is the second highest in our country. There will form tremendous additional stress in the stressed body behind the dam because of the huge engineering load. Furthermore, some altered-rocks in this part have the characteristics of porosity and incompactness. Obviously, the distribution and characteristics of the altered-rocks, especially after the environment changed, will affect dam abutment deformation and stability, even the whole dam safety. Therefore, the physical-mechanical characteristics and engineering respondences of altered-rocks is the key geology problem to insure the power station safe operation. In this paper, the author has systematically studied this question, and acquired the main achievements as follows:(1) The academic thought that geological process mechanism analysis and quantitative evaluation has been used to study the altered-rocks of key part in the dam abutment. On the basis of the in situ foundation data, the morphogenesis, developed characteristics, and the distribution have been analyzed. According to the rock and mineral test results, the cause of formation types and mineral characteristic of the altered-rocks have been confirmed. By means of conventional rock mass mechanical test, the altered-rock strength, and deformation character have been acquired. Using the above data, the integral stability of the dam containing altered-rocks during excavation and reservoir impounding has been studied with numerical simulation, and the altered-rocks mechanical respondences in different phases have been studied. At last, the long time stability of stressed body behind the dam has been evaluated. The research results established the especial rock mass and its engineering respondences studying methods in important water and electricity engineering, and the research results can be directly used in the practice engineering. Combined with the in situ monitoring data, it can be used as the evaluation rules for the dam long time stability.(2) According to its cause of formation and structure characteristics, the altered-rocks of Xiao Wan Hydropower Station belongs to hydrothermal alteration, and the rupture conformation becomes the ascending channels of the heat liquid . The distribution of altered-rocks just is located in the rupture zone and its adjacent zone. The structure of these altered-rocks is relatively homogeneous, but the structure of its periphery rock mass is relatively fracture because the ascending heat liquid compacts and presses. According to its alteration and weathering degree, the altered-rocks can be divided five types, that is, type I , which is strong alteration and strong weathering; type II, strong alteration and medium weathering; type III, strong alteration and feeble weathering; type IV, medium alteration and feeble weathering; type V, feeble alteration and feeble weathering. According to the characteristics of rock and mineral, the altered-rocks can be divided six types, that is, un-isogranular albite rock (A), un-isogranular quartz albite rock (B), clay un-isogranular albite rock (C), carbonate un-isogranular albite rock (D), altered biotite granite gneiss rock (E), fine-grained albite rock(F). According to the porosity, the type of A can be divided into the porosity more than 20% (A1), porosity between 10% and 20% (A2), porosity between 5% and 10% (A3), and porosity less than 5% (A4). Similarly, the type of D can also be divided four sub-types: D1, D2, D3 and D4. Therefore, the altered-rock can be divided six types and twelve sub-types according to its rock and mineral characteristics.(3) The engineering characteristics of altered-rocks have been completed cognized. The hydro-physical characteristics test illustrated that the altered-rocks was low water absorption and feeble expandability rock. And the results have been acquired that the strength and deformation characteristics of altered-rocks are affected by the alteration degree, weathering degree and rock quality. The exhibition is that along with the alteration degree increase, the compressive strength, shearing strength and module values are decreased, but the peak value strain and Poisson's ratio is increased. When the alteration degree is same, the strength and module value are controlled by the weathering degree, the stronger weathering degree, the lower strength and module value. When the alteration and weathering degree is same, the strength and module value are affected by altered-rocks quality. The alteration mainly reduces the rock cohesive strength, and the water has the same function. Along with the alteration increase, the weathering affect of water is increased. (4) The porosity is firstly presented as a new criterion to classify the altered-rocks soft and hard degree. On the basis of soft and hard rock classification criterion and the altered-rocks pore growth character, after evaluation and calculation, the soft and hard degree porosity classification criterion of Xiao Wan Hydropower Station has been presented as: porosity more than 30%, the altered-rock is deep soft rock; porosity between 18% and 30%, soft rock; porosity between 12% and 18%, sub-soft rock; porosity less than 12%, hard rock. On the same time, the secant modulus has also been supplied as the soft and hard degree classification criterion. The results have been presented as that when secant modulus≤0.5GPa, the altered-rock is deep soft rock; secant modulus between 0.5 and 2GPa, soft rock; secant modulus between 2 and 5GPa, sub-soft rock; secant modulus more than 5GPa, hard rock.(5) The several criterions have been used to classify the altered-rock soft and hard degree. The new and old criterion classification results are approximately same, which proved that the porosity, saturation uniaxial compressive strength and secant modulus together can be took as the altered-rock soft and hard degree classification criterion. The synthetical classification result of altered-rocks soft and hard degree is that IV,V type altered-rock is hard rock; among III type altered-rock, A3 is sub-soft rock, D3 is hard rock; II type altered-rock is sub-soft rock; and among I type altered-rock, C,F is deep soft rock, A1,D1 is soft rock.(6) The destroyed types of altered-rock have been well cognized: the lower alteration (higher strength), the higher brittle and ductile transition stress, contrarily, the lower brittle and ductile transition stress. The rock brittle and ductile failure classification parameter is complex. When the porosity is lower (less than 16%), there will be the trend that the ductile failure degree increases along with the increasing porosity. But it can not be more than 3% under the indoor test confining press scope, that is, it belongs to brittle destroy. Only when the porosity is more than 16% and confining pressure is more than 4MPa, can the ductile failure degree be more than 3%, which is the transition state of brittle and ductile.(7) The subsection fitting creep test curve method has been used to get rheologic parameter, which is the foundation of numerical calculation parameter selection. The long-time strength was acquired by two representative methods. The test result shows that the long-time strength of altered-rock decrease along with the porosity increase.(8) The stress-strain and plastic zone respondences of altered-rocks during the dam abutment excavation and dam body construction process has been completely and systemically analyzed by 3D numerical simulation technique. The result illustrated that the change value of altered-rock body stress and strain field was low under excavation and construction process, and the altered-rocks body mechanical property could not been changed.(9) The time effect has been firstly took into account in huge water and electricity engineering stability 3D numerical simulation. The stability of stressed body containing altered-rock behind the dam has been simulated and evaluated. The result have been acquired that the stress and strain respondences have been stopped after six month, and the dam body and stressed body behind the dam integral stability can not be influenced. But part of unstable rock mass must be treated with engineering method. The triangle zone of altered-rock body E1, E9 lower river part along with F5 fault might appear partly shear failure phenomenon in right bank.The high elevation south of altered-rock body E8 which exposed on surface might appear breakage phenomenon in left bank.(10) After the altered-rock zone has been disposed (actually, the calculation parameters have been improved), the calculation result shows that the main stress in stressed body behind the dam has the characteristics as follows: with theσ1 andσ3 increase, the scope of tension stress decrease and its value reduce; The total displacement decrease along with the parameters improve. The position of max total displacement has been changed to the downstream side of the arc dam, and the total displacement D-value of left and right bank decreases; The plastic zone area of the stressed body behind the dam and altered-rock zone have been greatly decrease. These phenomena shows that the whole and local stability of the stressed body behind the dam are all improved after the altered-rocks zone been engineering disposed.