Study On Rock Mass Quality Evaluation And Foundation Surface Selection Of Riverbed Dam Foundation

The quality classification of the rock mass of the dam foundation can comprehensively reflect the quality of the engineering characteristics of the rock mass,and is the basis for the stability analysis of the engineering rock mass and the selection of the foundation surface.This paper summarizes the rock mass quality classification methods widely used at home and abroad,and introduces the correlation between various rock mass quality classification methods and each classification index in detail.In this paper,the dam foundation rock mass of the Maerdang Hydropower Station is taken as an example for research.The lithology of the dam site of the power station mainly includes metamorphic sandstone of the Triassic system,mesozoic monzonite,sandstone of the Tertiary system and loose deposits of the Quaternary system.The lithology of the rock body at the base of the river bed dam studied in this paper is monzonite.The thesis is based on various exploration and measured data in the monzonite in the dam site area.First,the relationship between the indexes of rock mass quality classification is studied,and then a new rock mass quality classification scheme is proposed to provide a reference basis for the selection of the river bed foundation.The following research results have been achieved:（1）According to the engineering geological conditions of the riverbed dam section and the measured data of the borehole of the riverbed dam foundation,the weathering and unloading depth of the riverbed dam foundation are divided.（2）Using data processing software,the variation law of rock quality designation（RQD）,acoustic longitudinal wave velocity（V_p）and Lugeon value（ω）with buried depth（h）was studied.The result shows:RQD increases linearly with depth.The velocity of a single-hole acoustic wave increases first with depth and then gradually stabilizes.The regularity of wave velocity with depth is not obvious in very few drill holes due to the development of joint fissures.The Lugeon value decreases with the increase of depth as a power function.The two have a good correlation,with a correlation coefficient of 0.6.（3）Further study the relationship between the composite parameters RQD/ω,K_v/ωand the burial depth of the rock mass,and the results show that there is a good correlation between them.On this basis,an improved rock mass quality classification scheme（BQ₁）based on the Lugeon value,buried depth,and uniaxial saturated compressive strength（Rc）is proposed.At the same time,an improved rock mass quality classification scheme（BQ₂）based on the Lugeon value,RQD,Rc is also proposed.Through comparative analysis,it is found that the BQ₂ rock mass quality classification scheme is basically consistent with the national standard BQ and RMR₈₉ classification.（4）The rock mass quality classification scheme of compound parameter RQD/ωis proposed,and the RQD classification,RQD/ωclassification and BQ2 classification of 11river bed dam foundations at the same elevation are compared and studied.The result shows:The single RQD classification lacks regularity and cannot reflect the quality of the dam foundation rock well.The results of RQD/ωclassification and BQ₂ classification are basically the same.In general,the rock mass is getting better and better from the shallow surface of the dam foundation and the regularity is obvious.It is feasible to use RQD/ωclassification and BQ₂ classification.（5）In order to select a reasonable foundation surface,make contour maps of rock mass quality for every 5m section under the three classification methods of RQD classification,RQD/ωclassification and BQ₂ classification.It is concluded that the good foundation elevation selected by the RQD/ωclassification is 3065m～3060m,while the foundation elevation selected by the BQ₂ classification is 3060m～3050m.Compared with the RQD/ωclassification,the BQ₂ classification takes into account the uniaxial saturated compressive strength of the rock block.Therefore,in the shallow part of the dam foundation,the results of the classification are locally lower than the RQD/ωclassification,and then the foundation surface selected by the BQ₂ classification will be more safer and reliable.Use BQ₂classification to further refine the selection of the river bed foundation,and make a BQ₂contour map of every 2m in the 3060m～3050m elevation section.It is recommended that the elevation of the foundation of the riverbed dam in Maerang be 3056m,but the rock mass with poor local quality still needs further treatment.