| As the fifth cascade station in the middle reach of Yalong River, Yangfanggou hydropower station is located in Muli County, Liangshan Prefecture in Sichuan. According to the pre-feasibility study results, the dam in Yangfanggou hydropower station is recommended as concrete double-curvature arch dam, with the maximum height of 158 meters, normal water level of 2092 meters, and total capacity of 444.2 million cubic meters. Dam site belongs to landforms of highland and ravine. Yalong River flows in the direction of S30 ~ 40°E with water level elevation of 1984 ~1985m, and is usually 50~100m wide in dry season. Terrain on two banks is steep with exposed rock. Emergence stratum in the dam area is mainly hard and intact Yanshanian granodiorite. There is metamorphic siltstone with carbonaceous slale interbeds distributed only in derivation tunnel and underground powerhouse on left bank. Terrain beside the narrow valley basically assumes symmetrical type "V", and the covering layer on riverbed is 24-30m thick. Engineering geological condition in dam site is quite good overall with no regional faults, so it is suitable for building double-curvature arch dam.For the plant of 1000mw installed capacity, the dam construction often involves deep valley area. The current level of economic development and social investment determines the engineering reconstruction of rock and soil is confined to the near-surface tens to hundreds meters. As the foundation of dam project, surface rock in dam area is mainly reformed by two important exogenic forces of weathering and unloading effects. So the study of weathering and unloading rock mass has extremely vital significance for engineering design, rock excavation, rock quality grading and bearing surface selection etc. Research on rock mass (or rock) weathering and unloading has been an old task discussed for decades. Although progress and achievements on rock weathering and unloading research have been made in recent years through a large number of projects, translation of these new achievements to the guidance of engineering practice is quite lagging behind. According to this, quantitative characteristics of rock weathering and unloading in Yangfanggou hydropower station granodiorites area were researched. Besides, some tentative exploration of relationship between weathering and unloading was tried to enrich the refined research of weathering and unloading research in different lithology.On the analysis of rock weathering mechanisms, rock weathering features in Yangfanggou dam area were preliminary investigated, and three kinds of weathering patterns were summarized. Then rock weathering in quantity was studied. Rock weathering degree can be characterized through many factors, the main indexes were selected as follows in Yangfanggou station according to the rock engineering geology and site conditions: rock point load strength (PLS), rock deformation performance rebound value (Re), rock quality designation (RQD), p-wave velocity of rock mass elastic waves and sound wave (Vp), integrity index (Kv). These indicators can be obtained in exploration adits, surface occurrences by field measurement. Rock quality designation (RQD), borehole p-wave velocity (Vp) and integrity index (Kv) can be easily obtained from boreholes. Based on the analysis of correlation and reliability of feature indexes, the progressive rock weathering was then analyzed. Rock weathering quantitative criteria of Yangfanggou dam granodiorites was established, considering comprehensively of the quantitative selection of weathered zone parameters in multiple magmatic-foundation dams, and progressive feature of representative index in Yangfanggou dam granodiorites. The dam rock was classified according to the established quantitative indexes, and rock mass structure type was added as an index in final integrated judgment of rock weathering zones. Finally spatial distribution features of rock weathering in Yangfanggou station were shown out.The rock mass unloading mechanism was analyzed similarly before the study of rock mass unloading in dam area. The field investigation indicates that bank slope unloading is mainly Overall Relaxation, and 2100m elevation is a boundary of different unloading characteristics. Mass unloading zone is partitioned using qualitative and quantitative method. Qualitative investigation includes the surface survey and adit survey; while quantitative division are mainly obtained from the number increase and open width change of open fractures, rock structure relaxation, etc. Fissure rate (number of open fractures of unit length) and fracture apertures (accumulative total gap width of unit length) got from field adits measurement were chosen as major indicators for quantitative study of rock mass unloading in Youngfanggou station. Speed of longitudinal wave from drilling was used to verify the unloading zones dividing. Rock weathering quantitative criteria of Yangfanggou dam granodiorites was established, considering comprehensively of the quantitative selection of weathered zone parameters in multiple magmatic-foundation dams, and progressive feature of representative index in Yangfanggou dam granodiorites. The dam rock was classified according to the established quantitative indexes. Finally spatial distribution features of rock weathering in Yangfanggou station was shown out.Rock's weathering and unloading were studied quantitatively in the same pattern. However, there is an inseparable connection between weathering and unloading actually. The comparison of weak unloading maximum depth and weak weathering maximum depth of rock mass shows a coupling relationship between them. In order to further explore the connection between unloading and weathering, numerical simulation was used to converse stress distribution of the whole valley on the basis of the field observation. Two different kinds of generalized model were established respectively, the first of which considered the influence of different-degree weathering rock mass on the valley stress distribution, while the other treated rock fresh without considering the weathering. Depth of weak unloading was divided according to the valley stress distribution obtained from two generalized models. After comparing the two models, conclusion was drawn that rock weathering can affect the rock mass unloading depth, especially the weak unloading depth. |
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