Analysis Of The Formation Mechanism About The Middle Lower Angle Joint In The Dam Area Of Songta Hydropower In The Nujiang River In Tibet

The middle lower angle joint is a common found structural surface whichis outcropped in gorge bank slope rock mass of most hydropower engineerin-gs.If this joint fully developed,it could be a controlling borders with rise steep-ly structural surface.Then this combination would make adverse impact on thestability of high slope and dam abutment sliding.So it’s necessity to figure outthe formation mechanism of the middle lower angle joint.This paper through the rock mass of Songta hydropower of Nujiang riverin Tibet,based on the fully aware of the research status at home and abroad a-nd the work of data collection and collation in field and laboratory,analysis ofthe middle lower angle joints’ characteristics of development and geology in t-he dam district,then classification the middle lower angle joints into3types bythe typical phenomenons description of these joints,according to the each typeof the joints analysis of the formation mechanism;in the end,use the strain so-ftening model and creep model to simulation the cutting valley in the differentstages,analysis the case of formation and spacedly distributed of the middle l-ower angle joint formation.The specific studies and achievements as follows:(1) the research of the middle lower angle joint general feature1,the middle lower angle joints are NNE-NEE trend SE fractures in the r-ight bank of valley in Songta dam,then in the left bank are NNE-NEE trend f-ractures,like reverse "八" distributed both of the banks.They appear slow downthe steep.2,the adits where the middle lower angle joints distributed can be clearlydevided into two section:multiple fractures and original rock section.Among th-e multiple fractures section,the also has been spaced distributed for the middlelower angle joints.3,the middle lower angle joints extends the length of the general in theadits less than10m,among those joints most are between1and3m.Calculatedfor each adit connectivity rate amost is between30to60presents,it turns out that there is no bottom sliding surface in the rock mass of the valley,also tellus that the middle lower angle joints still stay in the epigenetic reformationstage.4,through the slow cracks traits in engineering,we can find that most of t-he cracks are rigid structural surface with good property.(2) the analysis of the middle lower angle joint case of formation1,the are two middle lower angle joints which are original:1)the hidden jo-ints which are produced by dykes invaded bedrock appeared when the valleycutted;2)Xizhuang joints are retained by denudation unloading area.2,the are two middle lower angle joints which are structual:1)the low ang-le normal faults which perhaps produced by magmatism;2)the low angle crackswith high angle fractures called "Riedel Shear".3,the are three types epigenetic joints:1) Nearly horizontal shear cracks w-hich are produced by differences unloading rebound;2)Compression and shear f-ractures which are cased by stress concentration generated trough levels;3)she-et resistance, tensile-shear fissures which are generated lateral unloading slopeformed.(3) numerical simulation of the case of the joints formationBy numerical software,used zonal fracturing theory and creep theory,take t-he strain softening model and viscoelastic plasticity model,based on the historyof Nujiang river evolution,simulate the process of the valley cutting in the di-fferent period.In the end,I found that the stress field redistribution in the valleyslope rock mass lead to pressing tensile fissure in the valley midium area andshear stress focus in the base of slope are the main reasons to case the mid-dle lower angle joint.At the same time,this paper also explained the spacedlydistributed of the middle lower angle joint in the dam area case the shear stre-ss strain increment migrate with the valley cutting in different stages.