Study of Deformation Control and Rule of Tunnel which Passes Underneath the Existing Tunnel in Underground Excavation of Subway Station

Transcription

1 2017 International Conference on Transportation Infrastructure and Materials (ICTIM 2017) ISBN: Study of Deformation Control and Rule of Tunnel which Passes Underneath the Existing Tunnel in Underground Excavation of Subway Station Jianchen Wang 1,2,3, Yunliang Liu 2, Dingli Zhang 3, Haiwei Bai 2, Xiaokai Niu 3,4 1 Beijing Urban Construction Group Co., Ltd,. Beijing , China 2 Beijing Urban Engineering Design and Research Institute Co., Ltd., Beijing , China 3 Key Laboratory for Urban Underground Engineering of Ministry of Education, Beijing Jiaotong University, Beijing , China 4 Beijing Municipal Engineering Research Institute, Beijing , China Abstract: Olympic Park Station of Beijing metro line 15 parallel closely crosses the existing large tunnel. Such a long distance to cross the existing tunnel, which is the first time in China. In order to control the deformation of existing tunnel, the monolayer pilot-tunneling pre-support pillar method is proposed. In this paper, the existing tunnel s deformations which induced by the new method and the traditional PBA method are compared by the numerical simulation method, and the existing tunnel s deformation low which is determined according to the measured value is analysed. Research shows that: (1) monolayer pilot-tunneling pre-support pillar method has advantages in controlling the deformation of the existing tunnel, decreasing the settlement of the ground and existing tunnel by dewatering and protecting groundwater resource. Compared with PBA method, the maximum deformation of existing tunnel is decreased by 36.5% after adoption of remove lower pilot tunnels and Forming a pile foundation underpinning system measures. (2) The deformation of the existing tunnel is mainly based on the cross section, and the measured maximum settlement is 42.2mm. Keywords: Crossing engineering; existing tunnel; control measure; deformation law 1 Introduction Due to the existing buildings, roads, bridges and other structures, it s necessary to apply boring excavation method in construction of urban subway stations. With large-scale construction of urban rail transit engineering it s becoming inevitable for new stations to cross below the existing tunnels. Thus how to control the effect of construction on existing tunnels has becoming the key technical problems in construction of urban rail transit engineering [1-3]. Datun Road tunnel which is a large-scale tunnel among existing urban tunnels was closely crossed in parallel by Olympic Park Station of Beijing subway line 15. There three characteristics in crossing engineering according to the statistics of 23 crossing projects in Beijing area [4]. Most of crossing projects are perpendicularly crossed. The spacing of new tunnels roof and existing structures floor are 0~10.025m among which 6 cases are zero-spacing, 3 cases are closely crossing(<5m ) and only one of them is m. Shallow tunneling method is mainly applied of which the excavation areas are 9m2~130.95m2. The length, width, height of the Olympic Park Station are 205.5m, 16.3m

2 and 23.1m respectively. It s the first time in China to closely cross large-scale tunnels in parallel with such large area and long distance (205.5m). At present, numerous researches on deformation laws of existing tunnels and safety controlling technologies in crossing engineering have been carried out. Ma [5] analyzed measured deformation of existing tunnels in 4 crossing projects which was conformed to the normal distribution and the maximum deformation generally generated near the center line of new tunnels. Han [6] counted undercrossing projects of London (10 cases) and Beijing (2 cases), considering that Peck formula could analyze and predict the deformation of existing tunnels and calculation method of formation loss rate and settling tank width coefficient was given. Zhang, Yao et al. [7-9] investigated the measured displacements of existing subway structures crossed by the Chongwenmen Station of Beijing subway line 5. They considered that the settlement was the main deformation while the torsional deformation and transverse settlement of tunnels were small. The existing underground structures represent the characteristics of rigidity body and the maximum settlement was in deformation joints. The station of Beijing subway line 2 crossed by Xuanwumen Station of line 4 was investigate by the author confirming the flexibility of existing underground structures in global deformation and rigidity in single segment between two deformation joints [10]. Chen et al. [11] investigated controlling technology and standard of deformation in closely undercrossing engineering based on the Dongzhimen Subway Station of Line Airport above and beneath Line 13 in Beijing with the application of numerical simulation. However, there are all perpendicular or large angle crossing in aforementioned studies and no mature experience of deformation laws and controlling technologies in parallel crossing the existing underground structures. 2 Project overview The new Olympic Park Station is located in the Olympic central area directly below the Datun Road tunnel and in east-west parallel with the tunnel. It s an island-style transfer station with subsurface excavation. The buried depth of roof is 11.4~13.9m and the main structure is flat-topped walls with two layers of three spans which is shown in the Fig of the floor plan. Figure 1. The engineering plan figure. The tunnel was closely crossed by the roof of tunnel station. The buried depth of Datun Road tunnel is 12m and is cast-in-place concrete closed frame structure with net width of m, net height of 7.4m and thickness of roof, bottom plate as well as side and middle wall are 1.0m, 1.0m, 0.9m and 0.8m respectively. The sectional view of tunnel standard section is illustrated in Fig 2

3 and the location relationship between new station and existing tunnel is illustrated in Fig 3. Figure 2. The cross-section profile of datum tunnel. Figure 3. The cross-section profile of new railway station. Crossing engineering geology consist of artificial accumulation layer, quaternary alluvial deposits and late quaternary alluvial deposits. There are mainly silt fill, miscellaneous fill, silt, silty clay and fine sand layer from top to bottom. 3 Construction method research of new stations 3.1 Method selection The original design of Olympic Park Station was PBA method with 8 guide holes that were four up and four down (Fig 4). PBA method [12] was developed based on the combination of shallow tunneling method and top-down method. In conventional PBA method, upper and lower small guide holes are first excavated and overall support system including sidewall support structure and arch support is formed through the space of guide holes. Then the tunnel could be excavated. This method can guarantee the enough strength and rigidity of support system when partially excavate the tunnel. Meanwhile, ground deformation can be greatly controlled by multi-guide-hole distributed construction. PBA which is good in deformation controlling of ground and existing structures has becoming popular among digging station in Beijing area [13 14]. However, in the Olympic Park Station of line 15, the settlement of Datun Road tunnel dramatically increased when the lower guide holes of new station were excavated. According to the measurement, only one guide-hole footage caused maximum accumulative settlement of 34mm which indicated the incapability of PBA in closely parallel crossing engineering. Crossing engineering are the most risked in the underground construction of which the essence is the safety problems generated by the disturbances on existing underground structures. The ground deformation induced by tunneling

4 construction caused the disturbances. Selecting a suitable deformation controlling methods is necessary in this project. There are four kinds of controlling measures: 1 reduce the ground deformation, 2 cut off the propagation of ground deformation, 3compensate the deformation of ground and existing structures, 4 underpinning of the bearing capacity in the foundation, 5improve the resistance to deformation of structures. Control measures are counted in the following Table 1. ID Table 1. Control measures of crossing engineering in Beijing area. Control Measures Cases methods 1 [15,16] reduce the ground deformation cut off the 2 [9,10] propagation of ground deformation 3 [9,10] compensate the deformation of ground and existing structures transfer the 4 [3] bearing capacity of the foundation improve the resistance to 5 deformation of 1large area to small area 2increase the spacing of old and new tunnels 3optimize construction methods, reduce the formation loss rate Install isolated pile, large pipe curtain or large pipe shed to cut off the propagation 1grouting uplift the existing tunnels 2lift the existing tunnels by jacks apply underpinning method to provide the capacity of the foundation Reinforce the existing tunnels, Improve its rigidity. 1Wukesong station of line 1 crossed by South-to-North Water Diversion (doublehole) 2intervals of line 1 crossed by Military Museum Station of line 9 1intervals of line 1 crossed by Chongwenmen station of line 5 2station of line 2 crossed by Xuanwumen station of line 4 1intervals of line 1 crossed by Chongwenmen station of line 5 2station of line 2 crossed by chao-dong interval of Line 6 Dongzhimen Subway Station of Line Airport above and beneath Line 13 none structures Adaptive analysis of control measures in table 1: (1) The excavation area could not adjust due to the function and line limit. (2) There is no space for large pipe curtain and shed because of closely crossing. (3) The parallel crossing distance reaches 205.5m making economic pressure in grouting uplift and jack lifting. (4) Do not choose the method of reinforcing existing structures due to the limitation of building boundary. In this project, canceling the lower 4 guide holes and forming the pile foundation supporting system as soon as possible were proposed based on the idea of reducing construction disturbances and underpinning of the bearing capacity (pre-supporting strut method of single-layer guide). Numerical

5 Maximum settlement/mm simulation was applied in the analysis of deformation induced by the two methods. Results are shown in Fig 5 and Table 1. Figure 4. The cross-section profile of new railway station by PBA method. Figure 5. The numerical simulation model. construction stage pilot tunnel supporting arch lobby floor platform floor PBA method monolayer pilot-tunneling pre-support pillar method Figure 6. Different construction method of maximum settlement of existing tunnel in numerical method. Table 2. Maximum differential settlement of existing tunnel using different construction method in each stage. construction stage S 8i (mm) S 4i (mm) S 8i -S 4i (mm) pilot construction supporting arch lobby floor platform floor Note: S 8i is the maximum see element value of 8-holes PBA, S 4i is the maximum sedimentation value of each stage of single-layer guide-hole pre-supporting strut method.

6 As shown in Fig 5 and Table 1, pre-supporting strut method of single-layer guide has following characteristics compared with PBA. (1) Eliminate the group holes effect induced by guide holes construction and transfer the upper load to the pile foundation supporting system as soon as possible which has great effect on controlling the deformation of tunnels. The maximum settlement was 68mm after the construction of PBA with 8 guide holes while its 43.4mm in pre-supporting strut method of single-layer guide that decreased 36.5%. (2) During the construction of guide holes, settlement induced by the two methods is obvious. The maximum settlement was 23.1mm in 4 guide holes and pile foundation construction while it was 40.5mm in 8 guide holes. Hereby, the maximum settlement decreased 17.4mm after eliminating the group holes effect induced by guide holes construction which decreased 25.6% compared with PBA method. (3) During the construction of buckle arch, station hall and platform level, the settlement induced by pre-supporting strut method of single-layer guide was 20.1mm while PBA was 27.5mm. Hereby, the settlement decreased 7.4mm after transferring strips into pile foundation in PBA and forming the 4 pile foundations supporting system as soon as possible. The maximum settlement decreased 10.9% in the 4 pile foundations supporting system compared with PBA method. (4) Simultaneously, the settlement of existing tunnels induced by precipitation construction could be reduced in pre-supporting strut method of single-layer guide which can protect the groundwater resource. In this project, the groundwater level is located at the bottom of the existing tunnel which means precipitation construction is necessary in new station. The groundwater level only down to 1m below the invert arch of the upper guide hole in a long period of time after eliminating the lower guide holes construction in presupporting strut method of single-layer guide compared with PBA. The precipitation construction of station hall and platform level are carried out after the accomplishment of guide holes, pile foundation and buckle arch. 1) From the mechanical point of view, precipitation construction will cause soil compression deformation which represents stratum subsidence macroscopically. The settlement of the formation will result in the deformation of the existing tunnel due to the decrease of the bearing capacity of the basement beneath the existing tunnel floor. Subsequent precipitation construction in pre-supporting strut method of single-layer guide are carried out after the accomplishment of supporting system in new tunnels. Meanwhile, the bearing capacity of existing tunnels, which is reduced by precipitation construction, can be borne by the 4-pile support system in order to reduce the deformation caused by the construction of the existing tunnel deformation. 2) In terms of the precipitation cycle, it can save 50,000 tons of groundwater / day because of reducing the precipitation time in pre-supporting strut method of single-layer guide compared with PBA, which is benefit for groundwater resource. 3.2 Construction sequence of pre-supporting strut method of single-layer guide The main construction steps are shown in the figure. Step 1: excavate four guide holes in which the pile foundation, steel casing and steel column positioner were installed. Step 2: install the steel column and the roof stringer in middle pile to form

7 4-pile support system in bottom plate of Datun Road tunnel. Step 3: Excavate the upper structure of the new station and buckle arch. Step 4: Excavate the lower structure of the station to complete the main structure. (1) (2) (3) (4) Figure 7. The construction processes of method of monolayer pilottunneling pre-support pillar. 5 Conclusion The Olympic Park Station of Beijing subway line 15 is the first time in China to closely cross large-scale tunnels in parallel with such long distance. Deformation control methods and laws of existing tunnels are analyzed in this paper. Researches have shown that: (1) The pre-supporting strut method of single-layer guide has advantages in controlling the deformation of existing tunnels caused by the construction of the station, reducing the formation caused by precipitation and the settlement of existing tunnels as well as the protection of groundwater resources compared with conventional PBA methods. The numerical calculation shows that the maximum subsidence ratio of the existing tunnels is 36.5% less than that of the PBA method, after adopting the two measures of eliminating the lower guide hole and forming the underpinning support system. (2) The maximum measured settlement of the pre-supporting strut method of single-layer guide construction is 42.2mm. In the construction process, the largest increment is the construction stage of the four guide tunnels, followed by the deduction arch stage, while the construction stage of the station hall layer and the platform layer are the smallest. The optimization of excavation area as well as step and reducing the disturbance on ground are the key to control the deformation of existing tunnels based on strict injection, short footage, the concept of fast closure. (3) During the construction process, the deformation pattern of the existing tunnel changes gradually. The biggest differential settlement increment is the deduction arch stage, followed by the construction of four guide-way stage, and the station floor and the station floor construction minimum. The whole deformation of the existing tunnels is mainly caused by the construction of the four guide tunnels, and the overall settlement and deflection of the

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