2017 International Conference on Transportation Infrastructure and Materials (ICTIM 2017) ISBN: 978-1-60595-442-4

Safety Control and Mechanical Response of Existing Tunnels Induced by Excavation of New Shallow Tunnel in

Jianchen Wang1,2,3, Yunliang Liu3, Dingli Zhang1, Sulei Zhang4, Nanxin Du3, Fanwei Meng4

1Key Laboratory for Urban Underground Engineering of Ministry of Education, Beijing Jiaotong University, Beijing 100044, *Corresponding Author, Email: [email protected] 2Beijing Urban Construction Group Co., Ltd., Beijing 100088; 3Beijing Urban Engineering Design and Research Institute Co., Ltd., Beijing 100037， China 4CCCC-SHEC Second Engineering Co., Ltd., Xi'an, 710119, China

ABSTRACT: In crossing engineering, new tunnels’ section and construction methods are numerous, geological conditions are complex, and existing tunnels’ structure forms are various. New tunnels, geological conditions, and existing tunnels have a complex composition relationship which is no corresponding research. 18 sets of measured data from 13 projects in Beijing were found to summarize the construction methods of the new tunnels, analyses the existing tunnel’s deformation law and study the existing tunnels’ mechanical response by the two stage method. Research shows: (1) the new tunnels have three types of municipal pipelines, subway stations and metro tunnel. The new tunnel construction method have guiding hole method, bench method, cavern-pile method and center drift method. The corresponding relationship between the construction method and excavation area is clear. (2) The probability distribution of the existing tunnels’ maximum settlement accorded with normal distribution. The mathematical expectation is 4.89, and the variance is 16.4. The existing tunnels’ average maximum settlements are 2.56mm, 3.82mm and 11.07mm, which caused by excavation of municipal Pipeline, metro tunnel and subway stations. The research results can provide reference for similar engineering of new subway tunnels excavated beneath adjacent existing underground structures in urban areas.

KEYWORDS: Crossing engineering; existing tunnel; mechanical response; safety control

INTRODUCTION

Existing underground structures gradually increased with rapid developments of underground engineering in Beijing, which making it’s inevitable for new tunnels to cross below the existing structures.[1] Undercrossing construction reduce the intensity of underground structures foundation that further change the settlement and internal force and eventually effect the normal application and structural safety. Thus, selecting suitable schemes of crossing construction to reduce the disturbance on existing underground structures has becoming an urgent issue in the construction of urban underground engineering. Numerous researches on deformation laws of underground structures and safety controlling technologies in crossing engineering have been carried out by domestic and foreign scholars. Ma[2] analyzed four examples of measured deformation of existing underground structures which was conformed to the normal distribution in undercrossing projects. Han[3] counted undercrossing projects of London (10 cases) and Beijing (2 cases), considering that Peck formula could analyze and predict the deformation of existing underground structures. Zhang, Yao et al.[4-6] proposed a grouting uplift method after investigating the deformation controlling technologies and measured displacements of crossing engineering in Station of . Simultaneously, they also collected ten cases of closely undercrossing projects in Beijing confirming the flexibility of existing underground structures in global deformation and rigidity in single segment between two deformation joints.[7] Chen et al.[8] investigated controlling technology and standard of deformation in closely undercrossing engineering based on the Subway Station of Line Airport above and beneath in Beijing. Zhang[9] analyzed deformation and stress of existing tunnels under the effect of adjacent construction in accordance with two-phase method. Among the current researches, safety controlling technologies and measured deformation analysis in single engineering were targeted lacking universality while there are so many types of crossing projects. Deformation of underground structures can be predicted by modified Peck formula which seems incapable of analyzing its mechanical characteristics. Both of deformation and stress characteristics can be solved by two-phase method and finite element analysis method but only for specific projects. With numerous kinds of new tunnel forms and construction methods as well as varieties of existing underground structures in Beijing area, systematic analysis of deformation controlling and mechanical response does not yet exist. Started from the Chongwenmen station of Beijing subway Line 5 beneath the interval of in 2005, a large number of crossing engineering have emerged in 10 years which providing possibility for systematically analyzing the safety controlling technologies and the mechanical response regularities of underground structures. This paper collects and fits 18 groups of deformation data in 13 projects of crossing engineering in Beijing area. The results can provide reference for future similar crossing engineering.

CASE STATISTICS OF CROSSING ENGINEERING IN BEIJING AREA

Case statistics of crossing engineering 18 groups of measured data in 13 cases of undercrossing projects were collected in this paper where Peck formula was applied to fit the measured deformation of existing underground structures. Results are shown in Table 1. The transverse stratum settlement of single-hole tunnel in fitting can be explained as:[7] 2 AVl x sx( ) exp(2 ) （1） i 2 2i As shown in the eq.(1), A is the excavation area, sx()is the settlement of x which means the horizontal distance from the tunnel centerline; i is the distance from the symmetry center of settlement curve to the inflexion point of the curve; Vl is the loss rate of the stratum which indicates the tunneling-induced cavern convergence rate of deformation that not considering the consolidation and it’s related with construction method.[3,7] The transverse stratum settlement of double-hole parallel tunnel is formulated as: DD ()()xx22 AVAV sx( )1ll 1 exp[ 22 ]  2 2 exp[  ] （2） 22ii22ii22 1212 In eq. (2): A1, A2 are the areas of the first and second tunnel; Vl1, Vl2 are the loss rate of the stratum respectively induced by the first and second tunnel construction; i1, i2 are the width of the settlement troughs induced by the first and second tunnel construction too; D is the distance between the two tunnels’ centerline.

Table 1. Crossing engineering statistics in Beijing. engin Existing geological Construction eerin New tunnel L H D Z underground h L ×H S V conditions 1 1 method o 2 2 max l g structure The south to Line [10] find sand、 18.3×7.8 0.28 1 North Water 4.2 5.1 8.2 6.22 bench method 1wukesong 4.83 4.83 5 4 Transfer Project pebble Metro station Silty clay、 pilot tunnel + 19.85×7. 0.14 2[2] 14.1 9.3 63.2 4.66 1gongzhufen 4.51 2.91 find sand、 jack 75 2 Metro station Metro station pebble

[11] junbo pebble、 16.0 Metro 10.4×5.8 0.02 3 9.6 10.5 14.3 6 pilot tunnel 5.3 2.35 Metro station conglomerate 5 tunnel 5 4 silt、Silty Line 2 Metro clay、find tunnel Chongwenmen Chongwenmen 0.29 4[2] sand、 3.6 2.5 - 1.25 bench method 3.87 9.9×5.9 1.74 thermal tunnel Metro 9 medium station~beijing coarse sand、 Metro station pebble silt、Silty Line 2 Metro clay、find tunnel Chongwenmen Chongwenmen 1.18 5[2] sand、 1.9 1.8 - 0.9 bench method 3.87 9.9×5.9 2.6 sewage tunnel Metro 3 medium station~beijing coarse sand、 Metro station pebble Line 1 Metro tunnel Zheng Chang Silty clay、 wukesong 12.05×7. 0.11 6[2] Zhuang thermal 5.8 3.2 - 1.6 bench method Metro 2 1.08 sandy clay、 52 6 tunnel station~wansho pebble ulu Metro station pile 0.72 silt、find 4.62 up .95 airport line foundatio 5 Lin 13 back- 12.05×4. 7[12] dongzhimen sand、 13.3 9.2 - n 8.8 down turning line 18 Metro station medium 4.62 underpinn 11.39 0.64 line coarse sand ing+jack Conside ring the 0.10 final 4.41 2 deforma N tion ort Elimina h te lin jacking e deforma 0.18 8.56 tion 7 before 4 pilot construc Line 2 [13] Silty clay、 tunnel tion 19.7×7.9 8 9.8 9 13.9 6.4 xuanwumen 4.5 +pipe Conside 5 Metro station pebble Metro station roof ring the 0.19 final 5.9 5 deforma So tion ut Elimina h te lin jacking e deforma 0.27 10.27 tion 3 before construc tion Line 5 center Excavation Line 2 Metro 9[6] find sand、 7.2 11.5 - 7.71 5.1 5.9×5.9 31.26 0.89 chongwenmen drift complete of tunnel

Metro station pebble、 metho center hole d + cobble、 pipe Displaceme 14.95 0.38 medium roof nt grouting coarse sand Line 10 Metro tunnel guomao [14] find sand、 Line 1 Metro 0.33 10 Metro 6.1 7.8 77 5 6 pilot tunnel 10.6 5.7×6.1 4.69 tunnel 3 station~shuangji cobble ng Metro station Line 6 Metro tunnel dongsi find sand、 Metro medium 4 pilot tunnel + Line 5 dongsi 12.6 0.20 11 6.8 7.8 17 3.9 19×9.9 6.9 station~chaoyan coarse sand、 jack Metro station 8 4 gmen Metro pebble station Metro tunnel Silty clay、 Line 10 guangqumenwai 12 find sand、 6.3 6.6 17 3.3 bench method shuangjing 13.8 20×9.6 1.2 0.44 Metro medium Metro station station~shuangji coarse sand ng Metro station Line 6 Metro tunnel Line 2 Silty clay、 4 pilot tunnel + 22.7×11. 0.15 13 Metro 6.8 7.8 16 3.9 chaoyangmen 5.3 2.5 jack 0 8 station~dongdaq pebble Metro station iao Metro station annotate： L1； width of new tunnel （m）, L2； width of existing tunnel (m),

H1：height of new tunnel （m）, H2：height of existing tunnel （m）; ho: buried depth of existing tunnel （m）; D： center distance of new tunnel (m); Z： Distance from new tunnel axis to existing floor of underground structure

(m); Smax: Maximum settlement of existing tunnel (mm). When the double tunnel is separated, the excavation of double holes and the formation conditions and construction methods are the same, so Vl1=Vl2 ，

K1=K2.

Characteristics analysis of new tunnels According to the statistical results, there are following characteristics of tunneling crossing projects in Beijing area: (1) Complex types of new tunnels including municipal pipeline, subway section and subway station. Single and twin tunnels account for the majority and all were orthogonally crossing except Dongzhimen Station of Line Airport which was skew of 62°. The area of new tunnels were about 3.4~235.7m2 among which the municipal pipelines were 3.4~36.4m2 while subway section and subway station were 47.8~106.1m2 and 70.0~235.7m2 respectively. New tunnels had big difference in sectional area but consistency in form that were flat-roof and upright-wall as well as horseshoe section in addition to the Military Museum Subway Station of Line 9 (straight wall in vault) and Chongwenmen Subway Station of Line 5 (subsurface excavation in vault of single-hole station). In these cases, the thickness span ratios (the height of cavern/the width of cavern) of new tunnels were approximately 0.55~1.60, whose average was 1.0 and the ratio between 0.9~1.3 accounted for 70% that is nine projects. (2) Construction methods of new tunnels comprise bench method, multiple-drift method, hole-pile underpinning method as well as center drift method. Peck formula was applied in the fitting of deformation of existing underground structures, the value of Vl in each method are shown in Table 2.

Table 2. Vl of each construction methods. method guiding hole bench method pile foundation center drift method underpinning method method Vl 0.02~0.27% 0.12~1.18% 0.64%~0.73% 0.89%

As shown in Table 2, the value of fitting was 0.02%~1.18%. The average of Vl in each method are represented in Fig 1 that clearly indicating the disturbances on stratum of each construction method in Beijing area. (3) Widely application of auxiliary measures. The safety problems generated by the disturbances on existing underground structures during the propagation process of ground deformation which induced by tunneling construction is the essence of undercrossing engineering. Thus, crossing projects are dominated in four aspects: ① reduce the ground deformation originally, ② cut off the propagation of ground deformation, ③ compensate the deformation of ground and existing structures, ④ underpinning of the bearing capacity in the foundation. Controlling measures are counted in the following Table 3.

Table 3. Control measures of crossing engineering in Beijing. Control Case Measure Method Single Tunnel Change Double Reduced 1、2、3、8、10、11 Tunnel; Optimizing Construction Deformation 、12、13 Method; Reinforced Stratum Barrier 、 Large screen or large pipe shed deformation 8 9 Compensating Grouting lifting or Jack jacking to 、 、 、 deformation 2 9 11 13 existing tunnel Transfer Replacing bearing capacity of foundation 7 existing tunnel base by underpinning bearing method capacity

As shown in the above table, the four methods are comprehensively applied in single projects instead of independent application. Among them: 1) Advance deep hole grouting reinforcement is the most common auxiliary measure which is almost used in every project. Grouting pressure is generally 0.5~1Mpa in the grouting range of 1.5-3m away from the hole week. 2) Barrier deformation measures that steel pipe shed of Φ600mm and Φ300mm were installed before excavation in vault or both sides was used in the Chongwenmen Station of Line 5 and Xuanwumen Station of Line 4. Due to the disturbances induced by pipe shed construction on stratum and existing underground structures as well as expanding the working section, heaving sand might be encountered in sandy soil strata. As a result, this measure was less used in subsequent crossing engineering. 3) It was general to apply grouting uplift in Beijing area which can be divided into pre-lifting and lifting. Pre-lifting was all found in Xuanwumen Station of Line 4, interval of Chaoyannmen to Dongdaqiao of Line 6, interval of Guangqumenwai to Shuangjing of Line 7. The existing line was uplifted about 4.37mm in the Xuanwumen Station while it was 16mm in the Chongwenmen Station. 4) Jack jacking method was applied in Gongzhufen Station of Line 10, interval of Dongsi to Chaoyangmen and interval of Chaoyangmen to Dongdaqiao of

Line 6. So far jacks were placed on steel frame to carry out lifting in aforementioned projects. Owing to the small cross section of steel frame, this method has high requirements on bearing capability of the foundation under the new tunnels. 5) The hole-pile underpinning method was applied in the Dongzhimen Station of Airport Line. This method was generally used in new tunnels with large section that the conversion of bearing capability of the foundation was achieved through guiding holes construction, digging into a pile inside the tunnel, tightly fitting of pile foundation and jacks. (4) In the crossing projects, due to the existence of underground structures, new tunnels were in complex deep-buried strata where involving the common layers in Beijing area: silt, silty clay, fine sand, coarse sand, pebble and round gravel.

Characteristic analysis of existing underground structures In Beijing area, there are following characteristics of existing underground structures in crossing projects: (1) Variety in structural form, including open cut station, mined underground metro station, open cut interval and mining section of which the shallow depth was 2.0~13.8m. In the open cut construction of structures, deformation joints were set in the range of 18m~30m. Existing underground structures were considered as box beam structures because most of crossing projects in Beijing area were orthogonal and equivalent method was applied in the analysis of bending rigidity, which is shown in Table 4.

Table 4. Flexural rigidity statistics of existing underground structure in Beijing. Equivalent Engineering Existing tunnel ﹒ 2 EIz/N m E/GPa Line 1wukesong Metro 1 1.7×1013 21.2 station Line 1gongzhufen Metro 2 1.79×1013 21.1 station 3 Line 1 Metro tunnel 3.65×1012 21.1 4 Line 2 Metro tunnel 2.24×1012 22.2 5 Line 2 Metro tunnel 2.24×1012 22.2 6 Line 1 Metro tunnel 3.65×1012 21.1 7 Lin 13 back-turning line 9.67×1012 20.3 Line 2 Xuanwumen 8 1.62×1013 19.7 Metro station 9 Line 2 Metro tunnel 2.24×1012 22.2 10 Line 1 Metro tunnel 9.06×1011 17.5 Line 5 Dongsi Metro 11 station 4.04×1013 17.0 Line 10 Shuangjing 12 Metro station 2.9×1013 17.7 Line 2 Chaoyangmen 13 Metro station 4.65×1013 18.8

As shown from the above table, 1) Larger cross section means larger rigidity. The rigidity of metro stations is much larger than intervals’ which is approximately 4~51 times. 2) C40 concrete was generally used in interval structures of metro in Beijing, whose elastic modulus is 32.5GPa. The elastic modulus of physical material is 17.0~22.2GPa after executing entity equivalent according to section size, which is decreased by 31.7%~47.7%; 3) The ratio of the equivalent elastic modulus of physical material to the deformation modulus of soil (take heavy clay as an example, 100MPa) is 170~220 indicating that the capability of resistance to deformation in existing stations or intervals are much stronger than natural formation’s in Beijing. (2) The statistics revealed that the maximum settlement of existing underground structures induced by under crossing construction was 1.08~31.26mm. Histogram and normal distribution curve are shown in Fig 2 in order to analyze the maximum settlement of existing underground structures (P.S. center drift method was applied in Chongwenmen Station underneath the interval of Line 2 in 2005, which caused large settlement and no longer be used later. Thus this method was ignored in this section; only final deformation of the Xuanwumen Station was investigated). In specific projects, the relationship of settlement and the types of new tunnel is shown in Table 5.

Table 5. Relationship between the maximum settlement of existing underground structure and the type of new tunnel. municipal type engineerin metro section subway station g job numb 1, 4, 5, 6 10、11、12、13 2、3、7、8、9 er A/m2 3.42~36.41 47.82~106.08 70.03~235.70 S / max 1.08~4.83 1.2~6.9 2.35~31.26 mm S均 2.56 3.82 11.07 /mm guiding hole meth bench guiding hole method, pile foundation method, bench od method underpinning, center drift method method

As shown in Tables 2 and 5: 1) There exists clear correspondence between the excavation area and construction methods. Bench method can be applied when excavation area is less than 40m2, both of multiple-drift method and bench method can be used when excavation area is about 40m2~100m2 while double-hole separated multi- pilot hole method, pile-supporting method for single as well as single-hole center drift method are applied when the area is larger than 100m2. The larger the excavation area, the higher formation loss rate in corresponding applied construction method. 2) At present the controlling standard of settlement in existing structures and rails is 3mm considering the normal operation of subway when new tunnels under cross existing rail lines in Beijing area. The excavation area is small in crossing engineering of municipal pipeline of which construction methods are unified. The maximum average settlement in existing underground structures was 2.56mm meaning it can basically satisfy with requirement of deformation controlling in crossing-rail projects. 3) The excavation area was relatively larger in new metro intervals. The flat-

topped straight wall section was applied in closely crossing while horseshoe cross-section was applied when isolated soil existed. The multiple-drift method as well as bench method were the main construction methods. The maximum average settlement of existing underground structures was 3.82mm which was slightly larger than the standard. These cases of which the value was less than 3mm accounted for 50% that was 2 cases. 4) The excavation area is relatively the largest in new metro stations. The construction methods have gradually transformed into double-hole multi guiding holes method from single-hole method for large scale and hole-pile underpinning method since the construction of the Chongwenmen Station in 2005. The maximum average settlement of existing underground structures was 11.07mm which was far bigger than the standard of deformation controlling in crossing-rail projects. Among them, cases of which the value was less than 3mm accounted for 40% that is 2 cases. So far the under crossing projects of new metro station are the most difficult and risked in crossing engineering.

CONCLUSION

(1) Most of crossing engineering in Beijing area is orthogonally under crossing. Single and twin tunnels account for the majority and types of new tunnels are complex including municipal pipeline, subway section and subway station. Construction methods comprise bench method, multiple-drift method, hole-pile underpinning method as well as center drift method. The average formation loss rate of each method is fitted by Peck formula: 0.165%、0.464%、0.683% and 0.890%. There is an exact correspondence relationship between excavation area of new tunnels and construction methods. (2) The maximum settlement of existing underground structures are 1.08~31.26mm and confirmed to correspond with normal distribution of which the μ is 4.98 and σ2 is 16.4 through probability distribution analysis. Among them, the maximum average settlement of existing underground structures is 2.56mm while new metro interval is 3.82mm. The metro station reaches 11.07mm which is the most difficult and risked engineering.

REFERENCES

Wang Zhansheng, Zhang Dingli. Key techniques on shallow embedded tunnel constructed beneath existing subway tunnel[J]. Chinese Journal of Rock Mechanics and Engineering，2007，26(Supp. 2)：4208-4214.(in Chinese) Ma Zhenchao. Research on the characteristics and the influence of the construction cross beneath the existing line in Beijing [M. S. Thesis][D]. Beijing： Beijing Jiaotong University，2012.(in Chinese) Han Xuan，Liu Chengwei，Standing J R. Structural settlement of existing tunnel caused by new tunnel excavated underneath[J]. China Civil Engineering Journal，2012，45(1)：134–141.(in Chinese) Zhang Chengping，Zhang Dingli，Wang Mengshu. Structural deformation of overlying subway tunnels induced by tunneling[J]. Chinese Journal of Geotechnical Engineering，2009，31(5)：805–810.(in Chinese) Zhang Chengping, Zhang Dingli, Wu Jiepu, et al.Construction control of a newly-built subway station under-crossing the existing subway tunnel[J]. Science,2009,35(5):69-73.(in Chinese) Yao Haibo. Research on construction technology of large section tunnel crossing under the existing subway tunnel by means of shallow tunneling method[D]. Beijing: Beijing Jiaotong University, 2005.(in Chinese) WANG Jianchen ， ZHANG Dingli, Zhang Chengping, et al. Deformation characteristics of existing tunnels induced by excavation of new shallow tunnel in beijing [J]. Chinese Journal of Rock Mechanics and Engineering， 2014，33(5)：947-956.(in Chinese) Chen Mengqiao，Yang Guangwu. Deformation control of a newly-built subway station crossing the adjacent existing subway tunnel[J]. China Railway Science，2011，32(4)：53–59.(in Chinese) Zhang Zhiguo, Huang Maosong, Wang Weidong. Responses of existing tunnels induced by adjacent excavation in soft soils[J]. rock and soil mechanics, 2009，30（5）1373.(in Chinese) Li Peng. Statistical analysis and prediction of the deformation of Wukesong subway station due to being crossed[M. S. Thesis][D]. Beijing： Beijing Jiaotong University，2010.(in Chinese) Xu Jimin. Study of influence for Beijing Junbo subway station pass through the existing line[M. S. Thesis][D]. Beijing ： North China University of Technology，2012.(in Chinese) Shen Liangshuai ， He Shaohui. Subsidence controlling and construction scheme optimization of cutting across and underpassing an existing metro tunnel under complex environmental conditions[J]. Chinese Journal of Rock Mechanics and Engineering，2008，27(Supp.1)：2893-2900.(in Chinese) Wang Jianchen，Zhang Dingli，Zhang Chengping，et al. Deformation of overlying subway structure caused by nearly shallow buried tunnel construction[J]. Chinese Journal of Rock Mechanics and Engineering ， 2014，33(1)：90–98(in Chinese) Bai Haiwei. Study on longitudinal deformation of existing metro tunnel caused by undercrossing construction and engineering measures[M.S. Thesis][D]. Beijing：Beijing Jiaotong University，2008.(in Chinese) Huang Yi，He Fangshe. Beam，plate and shell on elastic foundation[M]. Beijing：Science Press，2005：1-181.(in Chinese) Xu Ling．Research of the longitudinal settlement of soft soil shield tunnel[D]. shanghai: College of Civil Engineering of Tongji University, 2005.