Review On Engineering Characteristics Of Weak Structural Plane Of Riverbed Dam Foundation Of A High Arch Dam In Jinsha River

The excavation of the foundation of a high arch dam in Jinsha River has a large number of structural planes,wide distribution and complex types.All kinds of structural planes are interlaced in the foundation rock mass of the river bed,and some weak structural surfaces become the main boundary of the dam foundation block,which threatens the stability of the local geological bodies.In this paper,the soft structural surface exposed by the foundation of the river bed dam is studied.The characteristics of the development distribution and the mechanical parameters of the weak structural surface are studied.At the same time,it is checked with the results of the previous exploration,and the changes of the engineering characteristics and the force parameters of the soft structure surface of the river bed dam foundation before and after the excavation are compared and analyzed.On the basis of field geological survey,through field sampling,laboratory test and collection and analysis of previous achievements,this paper reviewed the distribution and development characteristics of soft structure surface,material composition,structure surface type,shear strength and other engineering characteristics revealed by the foundation excavation of the river bed dam foundation,and the following conclusions and understanding were obtained.(1)The structural plane exposed by the excavation of the riverbed dam foundation is divided into grade II to V grade according to the scale.Grade II and III are weak structural planes with a certain thickness interlayer.The weak structural surfaces exposed by excavation are mainly divided into 3 types,N25°～65°E,SE∠10°～42°,N50°～75°E,SW(NE)∠75°～90°,N65°～75°W,NW∠72°～90°.The fault mainly concentrates on the right bank of the river bed and the slope of the river valley,and the fault zone is developed in the rock flow or in the rock layer,which coincides with the characteristics of the development distribution in the early exploration stage.(2)The soft interlayer is mainly composed of rock fragments,rock pieces and cuttings after the rock is broken,and the content of mud is very small.The grade II 2and grade III 2 steep dip faults are all rock block type,and the grade III 1 weak structural plane are mostly rock block type,and some are rock debris type and local is strongly weathered.(3)The physical and mechanical simulation tests are carried out on the samples taken on the site.The strength parameters of the weak structural surface of the excavation exposed parts are basically consistent with the previous investigation results.The strength parameters of the soft structural surface in the river bed under the initial state of the excavation are basically consistent with the results of the earlier exploration.(4)By comparing the wave velocity with the long term observation hole after the preliminary exploration drilling and the excavation,the disturbance depth of the foundation rock mass is obtained between the dam foundation excavation and the10.2～14.6m.The maximum disturbance depth of the excavation unloading to the foundation rock is about 15 m after the comprehensive analysis.(5)The dam foundation excavation has little influence on the mechanical parameters of the weak structural plane under the riverbed construction surface.The reduction part of the shear strength parameters is mainly in the depth range of 0～15m buried under the built base surface,which is the same as that of the unloading damage depth defined by the wave velocity contrast.The strength parameters of the weak structural surface under the base surface 0～5m are obviously reduced,and the gentle dip angle is soft.The decreasing extent of weak structural plane is larger than that of steep dip fault.The strength parameters of weak structural planes below 15 m are basically the same as those before excavation.The strength parameters of weak structural planes at different levels of riverbed dam foundation after excavation are still high.(6)The soft intercalation is mainly composed of rock fragments with coarse particles,and the strength parameters of the rock block type weak structural surface are obviously larger than the strength parameters of the cuttings,and the increase of the clay content reduces the strength parameters of the weak structural surface to a certain extent.