The Prediction And Evaluation Of Riverbed Foundation Rockmass Quality Of A High Arch Dam In Jinsha River

Because of the special position of the riverbed in the hydropower project,the exploration difficulty and density are limited,therefore,there may be a weaker part in it.And the rockmass quality in dam foundation excavation is a dynamic process.The surface rockmass will leader to greater unloading relaxtion caused by the geo stress adjustment for a generous amount of rock excavation,and deteriorate the rockmass quality.According to previous exploration data,the rock mass quality of riverbed foundation rock is III₁ level.After excavating the overlying load,The rock mass quality of dam foundation may be decrease from III₁ level to III₂ level,and even to VI level.Especially for the locally developed poor quality rockmass,it is easier to be further relaxed and deteriorated,and ultimately resulting in failure to use.In view of the problems of the excavation process for high arch dam riverbed foundation rockmass,on the basis of a large number of pre grouting test holes arranged on the 5m thick protective layer covered on the riverbed dam foundation,predicting and evaluating the riverbed rockmass quality.The main achievements are as follows:?1?Advanced information analysis of the geological conditions in the riverbed dam foundation:According to the thickness of the breccia layer and the relation between its top surface and the designed base surface,it is suggested that the thinner area of the diagonal pebble lava layer be retained beyond the base surface and the first column type joint basalt remaining on the upper and lower sides should be removed.The development characteristics of the riverbed geological structure are obtained.The location and depth of the moderately weathered zone developed in dam foundation are determined.?2?By analyzing the characteristics of relaxation zoning on the protective layer of riverbed dam foundation show:The relaxation degree of rock mass in relaxation zone is basically the same on left bank,right bank and channel of riverbed protective layer.The relaxation depth of rock mass of left bank and channel is close,the right bank should be slightly larger.The regional characteristics of the relaxation depth of riverbed protection layer are summarized:Compared to the footwall of weak band,the upper wall rockmass relaxation is deeper;The relaxation depth of the rock mass which is Clamped by similar occurrence structure is greater than the structure alone;The wedge formed by the steep dip fault and the slow inclination layer shear zones will promote the rockmass relaxation;Different rock mass structure has different influence;The scale of structure has some influence on the relaxation of rock mass;the thin plate block formed by the slow inclination layer shear zones developed along the slope direction and the excavation surface is easy to relax;The relaxation extreme region forecast of excavating protection layer of riverbed foundation rock.?3?According to the foundation rock quality evaluation standard,obtaining the rockmass quality distribution characteristics of the foundation:the mainly rockmass quality grade is?,the proportion of left bank near to 70%,right bank is 57%,the channel is 75%;The quality of the left bank rock mass is greatly affected by the relaxation and the right bank is maily influenced by the geological structure.?4?Poor quality rock mass distribution c0haracteristics of riverbed foundation rock:Based on the analysis of the relaxation zone of protective layer,considering the excavation unloading relaxation effect on rock mass quality weakening,to predict the distribution of poor quality rock mass of riverbed foundation,According to the predicted results,It is divided into different types of poor quality rockmass for its genetic types,and then provides suggestion for later excavation.