| With the improvement of urbanization and motorization,urban traffic demand is growing rapidly.The rational development of underground space resources for traffic has become one of the important means for cities at home and abroad to alleviate traffic congestion.As one of the important means to solve the problem of urban traffic congestion,the subway has facilitated the travel of residents,eased the pressure of urban ground traffic,and has broad prospects in the future urban development and construction.However,with the rapid development of urban rail transit,and the early planning of most cities has not taken such projects into account,a large number of building facility conflicts,space intersections,cultural relics and environmental resources need to be protected in the subway project.The newly-built,reconstructed and expanded subway projects interfere with the original building foundation and adjacent projects,thus affecting the use safety of the original buildings,In order to solve the problem that the subway construction affects the existing building foundation,it is imperative to adopt the pile foundation underpinning technology in the subway tunnel construction.The pile foundation underpinning process of domestic tunnels is mostly carried out on the ground,but with the increase of subway tunnel construction,due to the limitations of the surrounding environment,many projects cannot be underpinned on the ground,so the pile foundation underpinning scheme in the tunnel must be adopted.At present,the research on underpinning construction technology of pile foundation in subway tunnel is mostly concentrated on the shield tunnel crossing project,and few scholars study the construction of pile foundation underpinning technology of large section mine tunnel,which cannot provide reasonable technical guidance for the selection of underpinning scheme.There is little research on the construction of pile foundation underpinning structure under the completely/strongly weathered mixed granite stratum,and the existing pile foundation settlement change rules and key influencing factors are still unclear,which cannot provide specific suggestions for the selection of safety control measures for pile foundation underpinning technology.The research objects of pile foundation underpinning technology mainly focus on construction technology,prediction and control of deformation of existing pile foundation and underpinning structure.Numerical analysis and on-site settlement monitoring methods are mostly used,and few are combined with on-site mechanical tests.There is still a lack of in-depth understanding of the actual stress deformation characteristics and safety performance of the new tunnel pile foundation underpinning structure under large axial force.Shallow buried underground excavation method is a method of tunnel excavation in the ground near the surface.Due to its low cost,less demolition,flexibility,no need for too many professional equipment and no interference with ground traffic and the surrounding environment,it has been widely used in various underground tunnel projects across the country.The subway tunnel project of the newly-built Pan-Nan section of Guangzhou Metro Line 18 studied in this study was constructed by shallow buried excavation method.The construction section of the tunnel under the tunnel was large and varied.Moreover,the right line tunnel goes through Wanbo CBD,and the construction of the right line tunnel conflicts with the five pile foundations in the underground space of Wanbo CBD.When the tunnel goes through the underground space buildings,ground underpinning cannot be carried out,but pile foundation underpinning technology in the tunnel is required.Therefore,in order to achieve the safety of tunnel construction and the stability of the superstructure,it is necessary to study the safety control technology of underpinning of pile foundation in the tunnel under the existing buildings,analyze the mechanical properties of underpinning structure and optimize the design.Based on the research background of Guangzhou Metro Line 18 Pan-Nan Section Tunnel Project,this paper first introduces the general situation,geological and hydrological conditions,tunnel construction procedures and pile foundation underpinning construction process of the large section mine method tunnel,explains the design of the entire support structure and related auxiliary methods,and finally reviews the construction effect of the entire project through the onsite construction records.It is concluded that the best choice of the construction scheme of the large section tunnel in soft soil layer is to use CRD method to divide the tunnel into several small sections for excavation and provide the supporting structure in time.At the same time,because the underground construction space is limited,and the tunnel diameter(minimum span)here exceeds 11 m,the implementation of pile beam underpinning is difficult,so the passive underpinning form of arch structure in the tunnel is adopted.The excavation contour of the section where the existing pile foundation intrudes into the tunnel is widened.The length of each underpinning section is 5.2m,and the widening amount is1.7m.The pile foundation underpinning construction is divided into expanding the tunnel excavation section,pouring the underpinning beam base at the lower pilot tunnel,extending the reinforcement to the upper underpinning structure of the tunnel,and pouring C35 concrete.Then,the internal force and deformation of pile foundation underpinning structure are tracked and tested through field tests,relevant data under each process are collected,various rules of structural deformation and stress are analyzed,and the structural safety is evaluated.Then combined with literature research and design scheme,Using 3D numerical simulation software(FLAC3D0 simulates the whole underpinning construction process in combination with the actual situation,analyzes the deformation and stress characteristics of the pile foundation underpinning structure and secondary lining before and after the underpinning construction,proves the influence characteristics of the mine method tunnel excavation and pile foundation underpinning construction on the existing pile foundation structure and pile foundation underpinning structure,and compares with the field measured data to verify the feasibility of the pile foundation underpinning scheme in the tunnel tunnel.In addition,the numerical simulation method is also used to support the pile foundation The process was optimized.The construction feasibility of arch underpinning beam under the condition of bearing large axial force in the proposed tunnel and the basic provisions for the size of pile foundation underpinning structure were inspected,compared and optimized,providing technical support and suggestions for similar processes in the future.The specific main research contents and achievements are as follows:(1)The new mine method tunnel is constructed by CRD method.In order to protect the pile foundation and superstructure in the excavation clearance,the pile foundation underpinning method in the tunnel is adopted.Because the upper bearing capacity of the existing pile foundation is large,the tunnel excavation and pile foundation underpinning construction are accompanied by the redistribution of surrounding rock and structural stress,and the mechanical changes in the whole process are very complex.Therefore,it is of great significance to systematically track and monitor the stress and deformation of pile underpinning structure during tunnel excavation and pile foundation underpinning construction.By monitoring the stress and deformation of pile foundation underpinning structure,it is helpful to better understand the internal stress and deformation law in the process of pile foundation underpinning technology,and guide the design and construction of similar subway projects in the future.Through field monitoring,the stress and strain of the primary and secondary lining support structure in each process of pile foundation underpinning replacement are tracked and recorded.Through the statistics,sorting and analysis of the test monitoring results,the change rule of the structure stress in the pile foundation underpinning construction is obtained.The results show that with the removal of formwork and pile cutting of the underpinning beam,the upper load of the pile foundation gradually shifts to the underpinning beam,making its stress begin to increase sharply,and then the stress increases slightly and tends to be stable with the application of secondary lining and the removal of temporary support,indicating that the application of secondary lining has little impact on the stress of the underpinning beam,and most of its stress comes from the upper load after connecting with the pile foundation.The weak and dangerous parts of the underpinning beam are mainly the arch crown,the arch waist around the pile foundation and the arch foot.After the pile cutting and secondary lining are constructed,the internal and external stress of these parts will change greatly and produce a certain amount of bending deformation.After the pile cutting,the downward settlement of the pile foundation often causes the surrounding underpinning beam to bend upward,and then a large tensile stress will be generated outside the underpinning beam to crack the concrete.At the same time,due to the influence of the excavation of the left line tunnel,the tensile and compressive stress on the left side of the underpinning beam is generally greater than that on the right side of the structure,except for the pile foundation location.Therefore,attention should be paid to safety control in these places and reinforcement of the underpinning beam should be strengthened.During the construction of the secondary lining of the tunnel after the construction of the underpinning beam,the secondary lining shared some of the stress of the underpinning beam structure.At the same time,the removal of the temporary support structure and the grouting behind the lining increased the stress of the secondary lining.The compressive stress at the left and right arch waists increased significantly,while the tensile part of the structure was mainly concentrated inside the arch crown and around the pile foundation.In addition,the uplift of the surrounding rock at the bottom of the second lining after the removal of the temporary support will cause the tension at the inner side of the arch bottom.Therefore,for the secondary lining at the underpinning beam,the reinforcement around the pile foundation and the inside of the arch crown and arch bottom shall be emphasized.For the internal force of the underpinning beam,except for the arch crown,the negative moment of tension at the right arch waist of the pile foundation and the inside of the left arch foot of the underpinning beam is large,and the underpinning beam is weak in these places.The connection between the pile foundation and the underpinning beam changes the deformation direction of the original underpinning beam,making the internal tension of the arch crown become the external tension,and the resulting bending moment is also large.And due to the excavation of the right line tunnel,the stress difference of the support structures on both sides of the left line tunnel is large,and the bending moments on both sides of the underpinning beam and the secondary lining are asymmetric.According to the internal force results of the underpinning structure and the secondary lining,the safety factor of the weak area of the underpinning structure at the top of the arch crown is 4.34.The left upper part of the lining ring of the secondary lining is the weak area,and the minimum safety factor is 6.10.Compared with the minimum safety factor specified in the Code for Design of Railway Tunnels,the entire support system is within the safety range.(2)The numerical simulation method is used to reproduce the whole construction process,analyze the pile foundation deformation law and the stress change law of underpinning structure and secondary lining,and evaluate the safety of pile foundation underpinning construction by comparing and verifying the on-site monitoring data.During tunnel excavation,monitor the vertical displacement change of existing pile foundation to provide early warning and ensure the safety of pile foundation superstructure.The results show that the maximum vertical displacement of the existing pile foundation occurs at the stage of tunnel excavation crossing the existing pile foundation.Therefore,during tunnel excavation,it is necessary to monitor the change of pile top settlement of the existing pile foundation at all times,and take safety control measures to reduce the pile foundation settlement;In each construction step,the overall change trend of bending moment and axial force of pile foundation underpinning structure is symmetrical,which mainly bears the load brought by the existing pile foundation with large upper part.The secondary lining pouring can effectively reduce the stress on the joist.For three underpinning structures,the maximum positive bending moment calculated by the first underpinning structure is 305 k N·m at the connection between the left arch and the existing pile foundation(the bending moment is located at the tensile side of the structure).The change trend of axial force and bending moment on the whole section of the second underpinning structure is approximately symmetrical,and the axial force on the section is in compression.The maximum positive bending moment is 252 k N·m at the vault.The change trend of axial force and bending moment of the whole secondary lining of the third underpinning structure is roughly symmetrical,and the axial force of the whole structure is in compression.The maximum positive bending moment is 459 k N·m at the vault.It is worth noting that for all underpinning sections,the maximum bending moment and axial force of the primary lining mainly appear at the arch crown,which is caused by the large load of the upper pile foundation borne by the underpinning beam.In addition,the maximum tensile stress(maximum principal stress)and maximum compressive stress(minimum principal stress)of the underpinning structure are 2.35 MPa and 8.12 MPa respectively,meeting the requirements of relevant specifications.The internal force distribution law and numerical calculation results of underpinning structures measured on site are basically consistent at each construction step,but the field test results are relatively large compared with the numerical calculation results,which may be due to many inaccurate factors in the field construction and monitoring process.In order to ensure the safety of lining structure,the safety factors of underpinning beam and secondary lining structure are calculated.The minimum safety factor of the first underpinning beam structure is located at the vault,and its value is 1.89,greater than 1.7,indicating that the structure is in a safe state;The minimum safety factor of the second joist structure is located at the vault,and its value is 1.81,greater than 1.7,indicating that the structure is in a safe state;The minimum safety factor of the third underpinning beam structure is located at the vault,and its value is 1.90,greater than 1.7,indicating that the structure is in a safe state.It can be seen that the weak section of the underpinning structure is located at the vault and the connection with the existing pile foundation,and all the safety factors of the underpinning structure meet the requirements.In addition,the calculated safety factor of the secondary lining also conforms to the specifications and standards.(3)The design of underpinning structure of pile foundation is discussed by using numerical simulation method.The proposed optimization technology of arch underpinning of pile foundation is analyzed and compared with the original design of underpinning beam structure.The improved underpinning beam can be used when the axial force of the pile foundation is large,and its size is smaller,which can solve the problems such as the tunnel can not be widened,the cost of concrete pouring is large,and the construction time is long.The results show that the internal forces of the improved underpinning beam and the designed underpinning beam follow similar distribution patterns.The maximum tensile stress(maximum principal stress)of the first section of improved underpinning beam is 2.783 MPa.The ultimate tensile strength of C35 concrete used for underpinning beam is 2.4MPa.The stress of the improved underpinning beam in this section is greater than the specification value,and the structure is in tension.Therefore,it is recommended to add more reinforcement or use upgraded concrete grade to meet the requirements.The maximum tensile stress values of the improved underpinning beam in the second and third sections are 1.32 Mpa and 2.18 Mpa respectively,which meet the specification requirements.The maximum compressive stress(minimum principal stress)of the improved underpinning beam in the first section is 5.70 MPa,the maximum compressive stress of the improved underpinning beam in the second section is 6.8MPa,and the maximum compressive stress of the improved underpinning beam in the third section is 9.2MPa.The ultimate compressive strength of C35 concrete used in the underpinning beam structure is 26 MPa,so the maximum compressive stress of the improved underpinning beam meets the requirements,and the structure is safe.From the results,it can be concluded that the axial force of the three underpinning structure sections is in compression,and the maximum bending moment occurs at the connection between the arch crown of the underpinning beam and the existing pile foundation.The safety evolution results show that the minimum safety strength value of the structure is 1.32,which is less than the standard value,indicating that the safety strength of all structures is within the safety range after secondary lining drilling for all underpinning sections.Therefore,the improved underpinning structure and secondary lining can bear the axial load of existing pile foundation.The numerical calculation of the improved pile foundation underpinning structure is consistent with the designed underpinning structure,and the change trend of the internal force distribution of the improved supporting structure(underpinning beam and secondary lining)obtained through the numerical calculation is consistent with the corresponding design underpinning structure,indicating the rationality of the calculation model.The proposed arch underpinning beam structure of large axial force pile foundation effectively solves the problems of tunnel widening and concrete pouring by optimizing the size of the underpinning beam structure,and meets the requirements of construction cost and duration.Finally,according to the simulated pile foundation settlement change and the mechanical evolution law of underpinning beam,some safety control measures for pile foundation underpinning construction are summarized and discussed according to its various safety risks:(1)Before the excavation and construction of the tunnel at the section where the pile foundation is located,certain advance ground reinforcement measures can be taken.In addition to WSS grouting and double-layer pipe shed in the tunnel,sleeve valve pipe grouting can also be used on both sides of the new large section tunnel to carry out surface zonal pre grouting reinforcement,The grout stop and water separation curtain can be formed from the top of the tunnel structure to the bottom of the pile foundation superstructure in the pile foundation underpinning section to minimize the settlement of the pile foundation.(2)In order to ensure that the axial force of the underpinned pile can be transmitted to the stratum through the pile end before the force conversion,the rock column at the pile base shall be retained during tunnel excavation,and the rock column shall be fixed with a steel ring to ensure the stability of the upper building.The initial support must be closed as soon as possible.The rapid closure of the initial support can reduce the settlement of the pile foundation to a certain extent.After each pilot tunnel is completed and the initial support of the underpinning section is completed,the internal support of the pilot tunnel can be carried out by setting the internal support of the profile steel frame,so as to increase the overall stability of the tunnel support structure,reduce the settlement deformation of the initial support structure in the tunnel,and facilitate the subsequent underpinning beam construction.Reinforce and grout the weak positions around the pile foundation,left upper arch waist and two arch feet of the initial support,strengthen the reinforcement around the pile foundation,arch crown and left arch waist of the underpinning beam where the tensile stress is high,and reinforce the grouting behind the underpinning beam to strengthen the integrity of the underpinning beam and the initial support.(3)When removing the support,it shall be broken by sections,and the method of "separating everything" shall be used to break the temporary partition support that needs to be poured.When the inverted arch part of the secondary lining and the contact part between the arch crown and the pile foundation are constructed,the reinforcement of the tensile side shall be strengthened,and the deformation and cracking shall be monitored in real time after the pouring.If problems are found,local reinforcement and crack repair shall be carried out at these locations in a timely manner.After the secondary lining concrete reaches the design strength,the temporary support structure shall be mechanically chiseled from top to bottom,so that the surrounding rock load it bears can be transferred to the primary support,underpinning beam and secondary lining structure,so as to complete the transformation of the stress system and ensure the stability of the superstructure.As a control measure of engineering safety structure,pile foundation underpinning structure in tunnel can better control the settlement of existing pile foundation and bear large axial force.This study has important theoretical significance and practical value,and can provide practice and reference basis for design optimization and numerical modeling of other similar pile foundation underpinning projects.In addition,this paper focuses on the underpinning construction stage and tunnel excavation stage,and does not study the underpinning construction and structural durability after the completion of tunnel construction.As time goes by,the safety and service life of underpinning structures and tunnel structures will become an important focus in the future.Therefore,the long-term deformation and stress evolution of underpinning structure and the performance of existing pile foundation can be further observed and studied during the operation of the new Metro Line 18.This thesis includes 83 figures,10 tables,and 67 references. |
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