Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels SHANG Kejian
Physical and Numerical Simulation of Geotechnical Engineering 2nd ISSUE, March 2011 Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels SHANG Kejian1, JIANG Zhaohua2, ZHOU Lin3 1. Wuhan Polytechnic University, Wuhan 430023, Hubei, China 2. College of Civil Engineering, Chongqing University, Chongqing 400030, China 3. China Communications the Second Highway Consultants, Wuhan 430056, China ABSTRACT：According to the practical engineering of rock deep excavation at depths about 30 m adjacent to the large span light railway tunnel, the finite difference numerical model is established for the foundation pit project. Reserved rock wall thickness and foundation pit supporting scheme by stages have been presented to protect the adjacent existing tunnel. Different supporting construction methods are simulated by the numerical method. The Fish program of point safety factor for tensile criterion of rock mass is compiled. The results indicate that foundation pit supporting scheme method by stages can effectively control the deformation of the adjacent existing tunnel. KEYWORDS: tunnel, foundation excavation, two stages supporting, point safety factor, numerical modelling 1 INTRODUCTION With the development and utilization of underground space, the number of deep excavation has become more and more. There are structures around the foundation before the excavation, and many of the deep pits are close to the light rail tunnel [1-2]. The excavation would break the original stress equilibrium, making the rock stress re-distribution and leading to internal forces and deformation. Light rail tunnel should meet the requirements of strength and deformation, especially deformation control requirements so as to keep safety of the Tunnel. Thus its impact on the adjacent tunnel should be considered during excavation process. A reasonable choice of foundation and excavation support structure should be selected. In recent years, many scholars at home and abroad has studied this issue, mainly in the following aspects. A method is presented for estimating the maximum bending moment for continuous or rigidly jointed pipelines affected by tunnel-induced ground movement[3-4]. The estimation can be made based on the knowledge of tunnel and pipeline geometries, the stiffness of soil and pipeline, and tunnel-induced ground deformation at the pipeline level. The method takes account of soil nonlinearity by an equivalent linear approach. The approach is conservative and promises that the bending moment is not underestimated. The validity of the method as an upper bound approximation is evaluated against centrifuge test results Combined with the practical engineering problem of tunnel excavation of Shenzhen metro by non-water lowering with adjacent pipeline, the construction scheme is illustrated; and evaluation standards of buried pipelines are given. Firstly, the effect of interval tunnel excavation on the buried pipelines is simulated by means of centrifugal model test. Secondly, the © St. PLUM-BLOSSOM PRESS PTY LTD 3-D FEM coupling analysis model of tunnel pipeline is established. Deformation and internal force of buried pipeline are analyzed specially by numerical simulation of tunnel excavation and also the safety of the pipeline is forecasted. The rationality and reliability are proved by comparing the results of centrifugal model test, numerical simulation and in-site measurement. Theoretical base and guidance are provided for real engineering and some meaningful results are achieved . Engineering activities are inevitable in the urban region adjacent to the current metro tunnel with the continuous city development and increased needs for land of building. But the deep excavation of the foundation pit adjacent to the tunnel will change the stress state of the neighboring soil. And the deformation of tunnel will be induced which cause impair to the daily operation and the safety of tunnel. According to the practice of deep excavation neighboring the tunnel in Shanghai，the application of plastic finite element method with Mohr-Coulomb model to simulate different construction stage of deep pit excavation is presented. The results show good agreement with the field test data which show that the finite element method can provide good simulation for this kind of engineering activity and provide convincing computing results for the design and on striation. Different construction method， furthermore，is simulated by the numerical method and the calculated results suggest that the rational excavation method will reduce the negative influence of the tunnel and ensure the success of the whole project . Therefore, this paper is on deep excavation of a rock near the tunnel. It’s different from other general excavation. The main features of excavation are depth; large span tunnels near the station, in addition to the excavation of rock blasting ensure the integrity of rock itself, minimizing damage or weakening the surrounding rock strength. For these reasons, this paper establishes a three-dimensional Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels DOI: 10. 5503/J. PNSGE. 2010. 02.016 elastic-plastic model, proposes support structure foundation program in phases, compares analysis of different alternatives and calculate the displacement and the point safety factor, in order to optimize the design and construction of similar projects to provide useful Reference. pile reverse for the whole building for the underground structure along , K pile section size is 2m × 3.5m and length is designed to 36.5m, with 3 rows of 11φ15.2 cable, cable tensile force is 820KN,it is shown in Figure 2. Excavation scheme 1: In order to protect the tunnel and facilitate the support piles, construction of retaining structures can reduce the effect of excavation blasting on rock. Supporting scheme in phases is put forward. Inverse excavation, construction of underground structure along for the last order with H piles, the upper reserved rock wall thickness will set to 8m. That is to say, the distance is edges foundation and the outer edge of the second tunnel lining. Retaining pile section size is 1m × 1.8m, pile length is 22m. The down stairs is the pile anchor, then the rock wall thickness set aside 11m, I-pile section is 1m × 1m, pile length is 17m, and 3 rows of the lower pile 11φ15.2 cable, cable tensile force is 820KN, shown in Figure 3. 2 PROJECT OVERVIEW Foundation is located in central area of southwest, the upper part is for the square and the lower part is for the underground structures, which the number of layers is for the 5-6F.The excavation of foundation pit will form about 30m rock slope. The slope A1-A2 part is adjacent to the tunnel, as shown in Figure 1, the width of the tunnels is 21.08m, and height is 20.2m. It is key issues for tunnel safety to build a reasonable reserve rock wall thickness and a reasonable support structure approach. Model calculation parameters are shown in Table 1.. excavation foundation pit light railway rock wall lig ht rai lwa y Fig.2 The first supporting scheme of excavation Fig.1 Relative position of tunnel and excavation excavation Table 1 Rock mass parameters for computation layer thick /m γ 3 /kN/m C E / MPa /° / MPa ν light railway excavation 1 23 24.62 2.09 39.2 2800 0.08 2 32 25.66 0.8 33.4 1500 0.34 Fig.3 The second supporting scheme of excavation 3 EXCAVATION SUPPORT SCHEME 4 ANALYSIS OF THE TUNNEL EXCAVATION IMPACT Excavation scheme 1: The pile anchor program, the rock wall thickness between the excavation outer edge and adjacent buildings lining is set to 8m, excavation anchor When there is the existing structure around foundation pit, the initial stress field is calculated, and then excavation 79 Physical and Numerical Simulation of Geotechnical Engineering 2nd ISSUE, March 2011 and support is simulated. To simplify the analysis, the A-A section of Figure 1, the thickness distance is between piles, the bottom is all constraints, both ends is the horizontal constraints, the computational domain is 70m × 55m × 4m, a total of 14475 nodes, 11148 elements are shown in Figure 4-5. The thickness of secondary Lining is 800mm with a C30 concrete, without considering the anchor .To monitor the displacement of the tunnel, respectively, arch foot wall, the bottom center of the observation points is monitored during the calculation process, in Figure 5. Supporting piles and anchor of the calculated parameters is shown in Table 2, Table 3. Table 2 prestressed anchor cable parameters E /Pa A 2 /( m ) Y gr_per gr_coh / Pa /m / N.m-1 2.1E 1.52E 1.86 11 -3 E9 0.53 2.5E4 Fig.5 Calculation monitoring point of tunnel liner 4.1 Supporting schemes displacement analysis gr_k Comparison of the two programs in the tunnel excavation monitoring displacement, the larger horizontal displacement is calculated in arch foot point 2, the midpoint of the wall 4, the displacement are 3.5mm and 2.8mm.The results can be seen that the horizontal displacement is the larger point of the bottom wall of the center point of arch foot 6 and 2, displacement, respectively 2.4mm, 2.0mm;crown point a displacement of 1.4mm; maximum additional displacement of the dome is point 1, the displacement is 0.3mm. Through the above analysis it is not difficult to see the program in stages 2, the displacement of supporting can control the tunnel well, and the maximum horizontal displacement and vertical displacement is smaller than scheme, the displacement gradient small. In figure 6-7, point 4 displacement gradient is larger than flat displacement scheme 2. In addition, pit blasting process, due to the program reserved for rock wall thickness of 2 large, can reduce the damage or weaken the surrounding rock. At process of excavation, the tunnel runs in good condition. 3.5E9 Table 3 retaining pile parameters pile E ν A Iy 2 Iz /m / m4 0.2 0.15 0.486 1 0.2 0.0833 0.0833 7 0.2 2.333 7.146 /Pa /( m ) H 3.0E10 1.8 I 3.0E10 K 3.0E10 4 4 3.5 点1 点5 点4 点6 点3 点2 displacement/mm 3 2.5 2 1.5 1 0.5 0 4 Fig.4 Flac3D model 7 12 16 19 Excavation depth/m 24 30 Fig.6 X-displacement of the first method monitoring point 80 Analysis of Influence of Deep Rock Excavation on Adjacent Tunnels DOI: 10. 5503/J. PNSGE. 2010. 02.016 2.5 点1 点5 点4 点6 点3 点2 displacement/mm 2 1.5 1 0.5 0 4 7 12 16 19 Excavation depth/m 24 30 Fig.7 X-displacement of the second method monitoring point Fig.9 Contours of z-displacement of the model In Figure 8-9, the crown point appear a larger horizontal displacement gradient, that is to say, the depth of the tunnel excavation are equal, when the excavation of the 12m or so. Open pit depth of the tunnel is at the very arch, arch horizontal displacement of point 2 feet is larger gradient, when the Ministry of excavation in the end wall of the central arch 2 and 4 points higher displacement gradient, vertical displacement is relatively stable of the excavation in the end. The crown displacement is large and more sensitive by the excavation. Gradient conditions in the location of the displacement are large and easy to strengthen the tunnel displacement monitoring. In figure 8-9, it can be seen that rock moved out after excavation, the horizontal and vertical displacement appear. Horizontal displacement is larger than vertical displacement. The tunnel deformation appears mainly in the central vertical wall lining, the horizontal displacement of tunnel is greater than the additional vertical displacement, and the deformation don’t exceed the required rail tunnel control displacement. Foundation itself, the next steps of the maximum horizontal displacement of additional rock is at the middle, because of the vertical displacement of unloading rebound. Fig.10 Point safety factor 4.2 Point safety factor calculation Hooke took the point safety concept safety in the slope for the analysis of stability. Point safety factor can be described stability of each unit, can quantitatively assess the degree of cell close to the plastic yield, the slope factor of safety usually only reflect the overall stability of the slope[7-8]. However, the current forms generally consider shear stress failure. Tensile failures often happen in lower tensile strength of rock mass characteristics. Based on the above view about the definition of safety factor, point safety factor in Mohr-coulomb yield criterion can be defined combined with Flac3D. Fs c cos 1 3 2 sin 1 3 2 (1) Where c, , 1 , 3 , is the cohesion, internal friction angle, maximum and minimum principal stress for the rock. The point safety factor of tensile failures Fig.8 Contours of x-displacement of the model 81 Physical and Numerical Simulation of Geotechnical Engineering 2nd ISSUE, March 2011 Fs t 3 REFERENCES (2) . Where, t is rock mass tensile strength. By comparing the point safety factor of shear stress and principal tensile stress, the smaller values is safety factor as the point. The point safety factor is compiled by Flac3D Fish. It is shown that a program of two phases Tunnel excavation in figure 10, the point safety factor is greater than 1. The calculation results of plastic zone does not appear, indicating that the two calculation methods are consistent and rock slope is stable. . . . . 5 CONCLUSION . Analysis of influence of deep rock excavation on adjacent tunnels and comparison of different support schemes, the conclusions is as following: (1) the horizontal displacement of the tunnel deformation is relatively large. Phased Support scheme can effectively control the tunnel displacement. Reserved rock wall by stages have been presented to protect the tunnel. (2) According to the definition of the point safety factor, tensile strength of rock mass calculation Fish program is compiled based on the modified Mohr-coulomb model. . . 82 CHENG Bin, LIU Guo-bin. The effection to the adjacent building and tunnel due to excavation[J]. Engineering Mechanics,2000(A03):486–491 LIU Guo-bin, HUANG Yuan-xiong, HOU Xue-yuan. The prediction and control of rebound deformation of the existed tunnels right under excavation[J]. 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