Three-dimensional Structure Analysis For Rock Masses At Right Abutment In Maji High Arch Dam

The proposed Nujiang Maji high arch dam is a world-class high dam, with thedesigned dam height of300m and4200MW equipped capacitor. The stability of archdam mainly depends on the abutment rock masses, of which the uniformity, loadcapacity and stability directly related to the safety of arch dam. The structuralcharacteristics of abutment rock masses controls the deformation, failure mechanismand mechanical properties of dam abutment rock mass, as well as the formation ofdam abutment resisting force body’s boundary, and then determines the stability ofthe arch dam.Therefore the study on abutment rock mass structure characteristics ofMaji high arch dam has great significance for the feasibility, safety and economy ofthe whole project.Using “controlling theory of rock mass structure” as the guiding ideology,based on one survey line of right bank, a research on the structural characteristics ofright abutment rock masses, which is the focus of the safety and stability of archdam, is conducted, and a new method using commercial software to present rockmass structure in three dimension is proposed. Through further research in thisarticle, The conclusions were drawn as follows:(1) By study the geological engineering conditions for Mujiajia dam site ofMaji hydropower station, it is found that this site belongs to Bingzhongluo virgationin the Gongshan to Fugong arc-edge fold belt, and regional tectonic does notdevelop for no deep fault and active fault. This area does not have the tectonicconditions for a6.5and more magnitude earthquake to happen. Neotectonicmovement at this area shows as regional ascending motion. Its landscape is alpinevalleys, and the bank slopes are almost symmetrical. The lithology is mainly mixedgranite and compound gneiss. The rock is hard. The rock mass is integral. Thestratum is steep. So this site is suitable for construction of dam.(2) In the dam area, structural plane is developed, mainly including fault, compressed belts, shear zones and joints. On right bank, there is no structural planesof I、II grade. Faults of III and IV grade are mainly divided into three groups,whose mean strikes are SE150°, SW229°and SW252°. Most of these faults extend200m or so, with the broken belt width0.25m which consists of debris and rockblock with a little mud, dip angle high. The natures of faults are mainly tension withtwist and press with twist. V grade structural planes consist of stochastic fractures,of which high dip is the most; medium takes second place and low-angle dip alsodevelops. The fractures are divided into four groups according to the advantagedirection. Fractures’ character is well, and they are mostly rigid or less hardstructural planes.(3) According to the standard with the introduction of three quantitative indexesof seismic p-wave velocity, RQD, and joint spacing, divide the weathered band ofrock mass in Maji right dam abutment, then use Monte Carlo method to conductthree-dimensional fissure network simulation by weathered band.(4) Establish a3D geological model by the ratio of1:1000, which includestopographic surface, rock formations, dike inclusion, weathered band, faults andsimulated fractures. In the process, a new method using commercial software toestablish rock mass structure model is first proposed, and this method has easepopularization.(5) Based on three-dimensional representations of rock mass structural, thestructural plane and body which form the structure of rock mass are studied and theresults are as follows. The fractures are steep in upper part and gentle in lower part,and the fracture spacing increases with adit depth and elevation, the trace length offractures can be adequately modeled by a gamma distribution. There are only asmaller number of isolated blocks formed by fractures in the rock mass and a rockmass cube element with edge length of2m was extracted and studied, the resultsshows that the block element types contains plate, column, pyramid, prismatic andcomplicated shape，and the pyramid type is the most, plate and prismatic types takesecond place. The strong weathered rock mass has cataclastic structure to block crack structure, and the moderately weathered upper zone has block crack structureto interlocked structure, and the moderately weathered lower zone has interbeddedstructure to middle to thick layer structure, slightly weathered rock mass has thicklayer structure to block structure, fresh rock mass has thick layered structure andwhole block structure. The scale of rock mass structure formed by fractures is small,and it can only control the mechanical characteristics of rock mass, thus it’s of smallpossibility to form the boundary conditions which can influence the stability of thedam abutment.(6) Viewing through the topographic surface, the research on macro rockstructure formed by the fault system is conducted. The rock in the dam abutment iscut into huge column and layers by faults, and they control the stability of the damabutment with the low-angle fractures dipping outside slope of which the highestconnectivity rate is55.8%. The resisting force body’s boundary conditions areanalyzed in combination with the direction of thrust of arch dam, and the bottom slipand sideslip surfaces are extracted. Six potential sliding blocks are sought out anddisplayed by using three-dimensional structure model, which establishes thefoundation for the stability evaluation of the dam abutment.