Research On The Construction Mechanical Characteristics Of JPCCP Under The Action Of Jacking Force In Rock Strata

As a huge mountainous city with a vast area,Chongqing has a changeable topography,there are many rivers interspersed in the region.The total amount of water resources is large,but the distribution is scattered.In order to improve the utilization process of water resources of Chongqing,and improve the water supply capacity,it is still necessary to plan and construct a large number of urban diversion tunnels.while the underground in Chongqing is dominated by soft and hard rocks such as mudstone and sandstone or weak interlayer surrounding rocks.In some areas,there are also underground water or underground caverns.The pipe jacking construction process,the pipes,under the combined action of groundwater pressure,surrounding rock pressure and jacking force,are prone to cracking on the inner and outer surfaces,which increases hidden dangers in water delivery.In response to this problem,PCCP pipes(Prestressed concrete cylinder pipe),usually used as water pipelines in diversion,was improved to form a new type of JPCCP pipe(Jacking Prestressed concrete cylinder pipe).Due to the lack of research in axial bearing properties and construction mechanical effects about this new type pipe,it is urgent to carry out the study about mechanical effect of the concrete JPCCP pipe under the action of the jacking force.This paper relies on the water delivery pipeline project of Chongqing Guanjingkou Water Conservancy Project,the JPCCP pipe jacked in pressured tunnel is the research object,and the research method of theoretical analysis,field test,and numerical simulation is adopted.The internal force state of the JPCCP pipe,the stress transfer characteristics of the pipe,the bearing characteristics of the pipe and the friction properties between the pipe and the rock have been comprehensively studied.The main contents and research results are as follows:(1)The mechanical analysis about the stress state of the structural layers of the JPCCP under the axial jacking force is carried out,and the JPCCP composed of the five-layer structure is simplified into three layers.Introducing the calculation theory of composite beams and combining the calculation theories of eccentric compressed structure in the reinforced concrete standard,the internal force calculation model of JPCCP pipes under eccentric and axial compression is established.(2)The on-site axial loading prototype test of the JPCCP pipe was carried out,and the changing law of the stress state of the pipe with the jacking force was uncovered.The field test data was compared with the theoretical calculation data of the eccentric compression analytical method.On the basis of field test,numerical simulation is adopted to evaluate the failure of the JPCCP pipe,and the crack propagation mode and stress transfer mode of the pipe are obtained.(3)The axial loading tests under different contact area between JPCCP and surrounding rock are carried out,the axial stress distribution,stress transmission mode,and the change of load-bearing ratio is analyzed.According to the appearance of microcracks on the inner and outer surfaces of the pipe and corresponding load,the most unfavorable situation among the three contact conditions is judged.Under the calculation of average friction between the pipe and the rock.the maximum jacking force and the maximum number of pipe sections under different contact conditions is obtained,(4)Axial loading tests of JPCCP under different structural sizes,reinforcement ratios,tunnel depths and surrounding rock level are carried out.A comprehensive analysis with various influencing parameters is conducted,and the optimal solutions of the bearing ratio,the development of cracks,the failure load and the pipe-rock friction under different influencing factors are obtained.