| In the alpine and gorge regions where large hydropower stations are built,the rock mass are hard featuring high magnitude of crustal stress,obvious unloading and deep weathering depth.Deep excavation of large scale for dam foundation and dam abutment at these regions will release and thereby lower the crustal stress contained in the rock mass in the dam foundation,causing the rock mass to rebound to the free face,thus reducing their modulus of deformation.Massive strong blasting,in addition,will speed up the shallow part of the dam foundation rock mass to become fragmented and slack,posing a major geological threat during construction of gravity dams that will affect deformation and stability of dam foundation rock mass.In consideration of this,it is of great engineering significance and economic value to carry out a new exploratory study,trying to understand how the deformation modulus of rock mass vary with unloading and loading of hard rock mass,and the engineering characteristics thereof,thereby addressing the issue of deformation modulus recovery of gravity dam foundations and improvement of foundation mass rock quality.This paper,after looking through geotechnical conditions and base surface conditions of foundations of several large-scale high concrete gravity dams,chose Guandi Hydropower Station in the Ya-lung River as its study object and carried a relatively deep study on recovery of deformation modulus via compaction.From mechanical aspect and relations between stress,strain and modulus,this paper analyzed variation of medium modulus with increase or decrease of stress and freshly applied it to the variation of deformation modulus of rock mass under unloading or loading conditions.By studying modulus variation caused by large scale excavation and unloading in foundation of Guandi Hydropower Station,as well as the study on compaction and deformation modulus recovery of loose rock mass in the excavated dam foundation under relatively large load and relatively high compressive stress in Guandi Station,the recovery of deformation modulus of dam foundation rock mass under relatively high compressive stress was derived.The measured data obtained in Jin'anqiao Hydropower Station and Guandi Hydropower Station was used in the ultrahigh compressive deformation test results for basalt fragments in Permian Mount Emei and in site inspection of buried ancient weathering granite in the dam site of Gongboxia Hydropower Station,which verified the variation of deformation modulus of dam foundation rock mass.The main study results of this paper are as follows:(1)Based on the regional geological structure and with reference to the measured value of regional crustal stress,a large-scale crustal stress inversion was performed;after values of the crustal stress such obtained were confirmed to be reliable,the geological structure of the area subject to study was inserted to invert the value and direction of principal stress in the dam site subject to study.(2)Using actual data about unloading belts in the gorges in the Guandi Dam site as well as data about unloading belts related with dam sites in the basalt mass,the characteristics of unloading belts in high and deep gorges were analyzed;based on the VII profile(profile of dam axis)of Guandi Dam site,a numerical analysis was performed to study changes of stress in the river valley grade and how the stress change(decrease or deloading)in such valley grade will be affected by unloading,so as to identify,from the view of mechanics,relations between “true unloading” of rock mass and unloading in the sense of current geotechnical structure.(3)The rock mass unloading,weathered degrees,correspondence between rock mass quality and rock mass stress change were studied for cross sections of river valleys used as dam sites for three hydropower stations,including Guandi,Jin'anqiao and Xiluodu,by which the effect of stress state change of rock mass was revealed.(4)Indoor high pressure test for fragmented basalt was performed to study variation of modulus under loading conditions and the correlation thereof;using modulus recovery of fossil weathering granite by co MPaction under the gravity of overlying strata in Gongboxia Hydropower Station,as checked analysis,the improvement(or recovery)of rock mass modulus by natural compression weight was proved.(5)Decrease of modulus caused by foundation excavation in high concrete gravity dams and recovery of modulus via compression weight was studied using stress changes of Guandi Dam foundation before and after excavation as well as stress state of high concrete gravity dams with compression weight.The actual data of the Jin'anqiao Hydropower Station in the Jinsha River was used as a case for demonstration,which supported the research results herein about rock mass modulus recovery of high concrete gravity dam foundations in a relatively comprehensive manner.The results in this paper have significant theoretical meaning and economic value in analyzing rock mass modulus of high concrete gravity dam foundations and in greatly reducing workload of consolidation grouting. |
PDF Full Text Request