| Prestressed concrete lining with un-bonded circumferential tendons(PCLUCT)is a new type of prestressed lining structure.It is mainly applied to the hydraulic tunnel structure which has high pressure,thin surrounding rock,poor geological conditions of surrounding rocks,and water seepage after lining cracked may cause the instability of the adjacent buildings or slopes.Compared with the prestressed concrete lining with bonded circumferential tendons,the PCLUCT has some advantadges like that small loss of prestressed along the tendons,uniform distribution of compressive stress in the lining,relatively small thickness of the lining,and a small number of anchorage slots,low project cost and short construction period etc.Therefore,the structure is established as the preferred lining structure type by the current specification,and it has a broad application prospect in the future construction of water conservancy projects.At present,this lining is only used in the three tunnels of the Xiaolangdi Project and the Dahuofang Reservoir Water Transfer Tunnel.There are few engineering examples,and most of the existing research has focused on the design,construction organization,testing and results analysis.On the other hand,the technology of un-bonded prestressed lining has some problems to be solved urgently in the calculation methods of key parameters,anchorage slot areas optimization and anchorage reliability demonstration.The Xiaolangdi sediments were regarded as the main research object,and the mechanical and numerical finite element models were analyzed as the main research methods.The PCLUCT was analyzed and studied through many years of actual observation data of Xiaolangdi Project.The results of the study are summarized as follows:Through the study of the elastic mechanical model of the annular lining structure,the formula of the adjacent anchor effect zone of the PCLUCT can be obtained,the superposition formula of the maximum anchor spacing can be determined,and the new algorithm for lining thickness and the number of anchor cable can also be obtained.After verification,the theoretical calculation results have a high degree of fitting with the actual observation data,and are suitable for practical projects.Based on the finite element modeling of the PCLUCT,the maximum tensile stress which generated at the weakest position at the highest operating water level(120m)is analyzed in conjunction with the orthogonal test theory,and the best combination of key design parameters for this operating water level has been found.At the same time,in the study of the effect of surrounding rock and grouting circle in the operation of the PCLUCT,it is found that the greater the elastic modulus of the surrounding rock,the more pronounced the sharing of internal water pressure.The elastic modulus of the surrounding rock has been strengthened by grouting circle,but its role in sharing internal water pressure is not significant.After analyzing the various problems in the anchorage slot area of the existing project,a new design idea of “emphasizing the compactness of backfill concrete,weakening the role of cementation with lining” as well as an optimization scheme for the layout of the anchorage slot area are proposed.From the results of finite element analysis and the actual observations of the lining during operation,the results show that the optimized structure has uniform stress in the entire lining under the same internal water pressure,and the minimum circumferential stress is still compressive,which satisfies the full prestressed requirement of lining.This analysis result has certain reference value for similar engineering design in the future.After analyzing the influencing factors of anchorage reliability of PCLUCT,it can be seen that the temperature change has a significant effect on the stress state of the prestressed anchorage cables,and the other factors have little influence.During the lining operation,the most unfavorable condition is assumed to be the failure of the first anchor cable at the end. |
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