The Impact To Bridge Pile Foundation From Drilling And Blasting In Metro

Railway and subway are rapidly being developed in China, and in the construction process of railway and subway, hard rock geology is always encountered. Compared with TBM, drill and blast method is particularly suitable for construction of hard rock tunnel, broken rock tunnel and lots of short tunnel due to its good adaptation for geological conditions and low cost. As the city roads, bridges and buildings get increasingly intensive, the probability of subway beneath is increasing. This paper takes the Chongqing Metro line 6 excavated under Hangnipang Overpass as background, mainly study the technical of drilling and blasting subway under the pile foundation of bridge.During the construction process of drilling and blasting, how to reduce the disturbance to the surrounding rock is a key technology. So this paper study the disturbance from the perspective of loosen zone. Based on the cause, loosen zone can be divided into two parts:cavity size effect loosen zone and blasting dynamic loosen zone. This paper firstly summarizes the existing theory of loosen zone, including describing the production, main points and advantages and disadvantages of these theories. And then focus on the elastic-plastic mechanics which has a clear understanding of the formation mechanism of loosen zone and also provide a more complete mechanical model. This method is used to derive the range of cavity size effect loosen zone in the ideal natural rock, and the main factors which affect the loosen zone is also discussed.Base on the study of blasting mechanism in rock, and some data from the experimental and construction site, this paper summarizes the propagation of the loosen zone caused by drilling and blasting, and also get the approximate ratio of blasting dynamic loosen zone and cavity size effect loosen zone. In a risk point of Chongqing Metro line 6, the size of loosen zone is calculated by using the theoretical formula and blasting correction. And the stress of rock between tunnel and bridge pile is solved by analytical methods.Finite element method is used to simulate the construction process, which focus on the development law and scope of loosen zone during the excavation of tunnel beneath the bridge pile. Three conditions are study and compared, including single tunnel excavation, right tunnel excavation after the left one is finished, and two hole excavation at the same time. Meanwhile, stress redistribution and settlement of the rock and internal forces of piles were studied.