Key Engineering Materials ISSN: 1662-9795, Vol. 723, pp 782-788 doi:10.4028/www.scientific.net/KEM.723.782 © 2017 Trans Tech Publications, Switzerland Submitted: 2016-06-08 Revised: 2016-08-09 Accepted: 2016-08-30 Online: 2016-12-12 Uplift Model Analysis for Shallow Overlaying Slurry Shield Tunnel Based on Time-Varying Behavior of Cement Grouting Material Yu Haung1, a *, Lin Li2, b, Jing Ni1, c, Leibiao Chen1, d 1 Department of Environment and Architecture, University of Shanghai for Science and Technology, No. 516 Jungong Road, 200093 Shanghai, China 2 Shanghai Tunnel Engineering Co., Ltd, Shanghai 200082, China a * sunlitaurora@126.com, bl57488110@126.com, cwendy_1943@163.com, d 1126033976@qq.com Keywords: Slurry shield tunnel, Uplift, Shallow, Numerical analysis. Abstract. As a necessary and key procedure of shield technology, synchronous grouting affects ground layer deformation, and controls the tunnel uplift and ground subsidence, usually exerting negative influence on underground engineering construction and surrounding buildings. This paper establishes the 2D uplift model of the soil and lining by ABAQUS, based on a static buoyancy varying with solidification of grout and dynamic buoyancy produced by grouting pressure. The effect of buoyancy on lining is simulated in the lining and soil through a series of specific magnitude of interference fit. Effectiveness of the model is validated by comparison of data measurement of South Hongmei Rd. tunnel with predicted results for vertical displacement of ground surface. East line of South Hongmei Rd. tunnel with super large diameter is chosen for the simulation and central burial depths vary as a key parameter. The results show t4hat the differences between predictions and data measurement are limited, hence providing a basis of numerical analysis for the design and optimization of shallow slurry shield tunnel with super large diameter. Introduction Usually, shield tunnel lining is under buoyant stress composed of time dependent static buoyancy due to setting and hardening of grouts and dynamic buoyancy produced by grouting pressure [3]. When the overlaying soil pressure and weight of lining cannot resist the buoyant stress, partial or whole tunnel may float upward and cause soil failure and uplift of ground surface. Without proper measures, tunnel may suffer cracking and leakage problems, Meanwhile, surrounding buildings and underground pipelines may be under negative influence as well [4]. However, some part of tunnel can be under shallow overlaying soil during shield tunnel construction, such as the portal section of the tunnel. With the increase of tunnel diameter, buoyancy grows significantly. When the thickness of soil cover is less than 1~1.5 times of excavation diameter, further research on safety problems caused by uplift of tunnel must be carried on [1, 2]. A lot of research work has been done on buoyancy in the recent year, Ye [5] presented that buoyancy mainly consists of static buoyancy, such as underground water buoyancy, grout buoyancy, and dynamic buoyancy, as well as grouting pressure, hydraulic pressure, jacking force, and unloading rebound. Most of scholars believe that the value of static buoyancy is so small as to be ignored, which would cause underestimation of practical devastation and inappropriate analysis of the safety of shield tunneling. Case Study The South Hongmei Road Tunnel, a main project in this paper, which connects Fengxian and Minhang is located in south region of Shanghai Huangpu River. Project is carried out by super large diameter slurry balance shield at a diameter of 14.93 m. Tunnel diameter is 14.5m. The lining is 2m wide and 0.6m thick made by C60P12 precast concrete. Average Grouting pressure is about 5.5 Bar. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.scientific.net. (#103422207, Technische Universitaet Muenchen, Muenchen-13/11/17,04:17:06) Key Engineering Materials Vol. 723 783 The construction of the shield tunnel for west line starts from Fengxian to Minghang. Then the construction for east line starts from Minghang to Fengxian, indicating the completion of the whole project. The shallow-buried section of the tunnel is located mainly at 14m~18m depth. Parameters of the soil concerned are presented in Table 1. Table 1. Parameters of soil. Layer ②1 ③ ④1 ⑤1-1 Soil name Brown yellow silty clay Grey muddy silty clay Grey muddy clay Grey clay Dark green~straw yellow silty clay Gray yellow~ gray silt Gray silt and silty clay interbeded soil ⑥ ⑦2 ⑧2 unit weight γ (kN/m3) Compression modulus E(MPa) Void ratio e Poission ratio μ Cohesion C(kPa) friction angle Φ( °) 19.2 17.1 17.0 17.6 5.83 2.90 2.51 3.40 0.782 1.275 1.303 1.118 0.30 0.38 0.39 0.32 26 11 11 15 18.5 17.0 13.0 16.5 19.6 7.80 0.692 0.29 45 16.5 18.9 13.04 0.770 0.32 4 35.0 19.0 9.13 0.772 0.28 3 34.0 Calculation of Buoyancy Static Buoyancy. A generalized static buoyancy curve is acquired from the experiment based on the buoyancy test of tunnel model which is proposed by Lin [6], Yang et al [7]. And generalized static buoyancy can be expressed in terms of burial depth, tunnel volume, and solidification time of slurry cement as Eq. 1: - () - (1) where, is generalized static buoyancy (kN), is solidification time of slurry cement(h) , is tunnel volume (m3), is central burial depth (m). Dynamic Buoyancy. The dynamic buoyancy is closely related to the movement of grouting slurry in soil. The magnitude and distribution of dynamic buoyancy vary according to different diffusion models. In this paper, the most disadvantaged compaction diffusion model of dynamic buoyancy is considered [5] and proposed the following equation: ∫- ( ) (2) where, is dynamic buoyancy (kN), is grouting pressure (kPa), is dynamic buoyancy effective width (m), is tunnel radius (m), is an angle between the boundary of the grouting slurry distribution area and the vertical line. Dynamic buoyancy effective width is taken as a homogeneous, uniformly thick and elastic equivalent circular zone. According to the engineering experience, Poisson ratio of equivalent circular zone is 0.2, and its width, , is given by [8]: (3) where, is a material coefficient, =0.7~0.9 for hard clay, =0.9~1.3 for dense sand, = 1.3~1.8 for loose sand, =1.6~2.0 for soft clay, is void at shield tail. Based on Hongmei Rd.’s case, , and . Resultant Force of Buoyancy As static buoyancy decreases over time and dynamic buoyancy only exists during synchronous grouting, the resultant buoyancy is obtained by combining the static buoyancy and dynamic buoyancy one hour after synchronous grouting starts. The static buoyancy of unit length of tunnel lining is calculated for different depths (14m, 16m and 18m), with magnitude: 784 Proceedings of International Conference on Material Science and Engineering 2016 {- () () () (4) when, , static buoyancy: kN, Then the resultant force of buoyancy can be obtained: For H=14m, For H=16m, For H=16m, kN, kN, ; ; . Numerical Simulation The model for analyzing the east line of South Hongmei Road Tunnel is established by finite element software ABAQUS, and the hypotheses are expressed as followed: (1) 2D model is established for simulation; (2) All soil layers are horizontal; (3) Effect of excavation of existing tunnel(west line) is not considered in this simulation; (4) Grout is considered as a solid-like material which is called equivalent circular zone in simulation; (5) Resultant force of buoyancy acts through the interference fit between the equivalent circular zone and the lining. Materials. Concrete segments ,soil and grout(equivalent circular zone) are simulated in 2D planar. Isotropic elastic material is used for lining and equivalent circular zone, while Drucker-Prager model with non-associated flow rule is adopted for soil [9]. The lining is 2m wide and 0.6m thick. In order to compare data measurement of ground surface displacement with predicted data, three different central burial depths--14m, 16m and 18m are considered in numerical simulation. The center of the east line is 25 meters away from the center of the existing west line. Material parameters used in simulation model are given in Tables 2 and 3. Table 2. Drucker-Prager model Parameters. Layer Soil name ③ ⑤1-1 Grey muddy silty clay Grey clay Dark green~straw yellow silty clay Gray yellow~ gray silt Gray silt and silty clay interbeded soil ⑥ ⑦2 ⑧2 Non-associated flow rule friction Cohesion angle C(kPa) Φ( °) 6.70 26.9 8.82 26.2 unit weight γ( / 3) Compression modulus E(MPa) Possion ratio μ Layer thickness (m) 17.1 17.6 2.90 3.40 0.38 0.32 19.6 7.80 0.29 26.48 26.2 15 18.9 13.04 0.32 5.94 44.8 15 19.0 9.13 0.28 4.29 44.1 30 10 20 Table 3. Material parameters. Material Grouting slurry (Equivalent circular zone) Segment Elastic modulus (MPa) Possion ratio μ unit weight γ (kN/m3) Thickness (m) 3 0.2 20 0.387 30600 0.25 25 0.6 Soil and lining model is establish by FEM software ABAQUS. In this model, central burial depth of tunnel H varies from 14m, 16m to 18m. East line of South Hongmei Road Tunnel with 14.5m diameter is selected for simulation. Fig. 1 indicates that the odel di en ion i The horizontal constraints are imposed on both sides of the model. The bottom is fixed and the top is unconstrained. Excavation area is inside of east tunnel lining. Element types of soil and 0.6m-thick lining are CPE4I, CPE4 respectively. Key Engineering Materials Vol. 723 785 H 260m East Tunnel 14.5m 14.5m Existing West Tunnel 90m 10.5m Fig. 1 Model dimension. Simulation Methods. Fig. 2 shows a procedure in detail for a numerical simulation [10]. Initial geostress calculation The modulus of excavation area soil attenuate 30% Activate segment Remove soil of excavation area Applied Interference Fit Fig. 2 Process of Simulate excavation. Interference fit is conducted to simulate the complicated interaction between soil and lining, in which outer edge of lining is set as master surface and inner edge of equivalent circle as slave surface. When over-closure is applied between the lining and equivalent circle, contact force can be calculated. Meantime, buoyancy is equal to the vertical component of contact force of semi-ring lining. Case 1: For H=14m, magnitude of over-closure is -0.0230, and the vertical component of the contact force of the unit length of lining is 5541.03kN; Case 2: For H=16m, magnitude of over-closure is -0.0187, and the vertical component of the contact force of the unit length of lining is 5511.86kN; Case 3: For H=18m,magnitude of over-closure is -0.0172, and the vertical component of the contact force of the unit length of lining is 5499.24kN. Figs. 3, 4 and 5 respectively represent the contour of horizontal and vertical displacement of tunnel lining in depths of 14m, 16m and 18m. (b) soil vertical displacement (a) soil horizontal displacement Fig. 3 Soil displacement of 14m-burial-depth tunnel(m). (a) soil horizontal displacement (b) soil vertical displacement Fig. 4 Soil displacement of 16m-burial-depth tunnel(m). 786 Proceedings of International Conference on Material Science and Engineering 2016 (a) soil horizontal displacement (b) soil vertical displacement Fig. 5 Soil displacement of 18m-burial-depth tunnel (m). To verify the model, simulation of vertical displacement of ground surface for Case 2 and Case 3 are conducted and compared with data measurement of monitoring sections E1663 and E1687. Monitoring section E1663 and E1687 are on the ground with burial depth of 18.3m and 15.8m respectively. However, there is no measured data of ground surface for 14m depth in tunnel construction. For this reason, Case 1 is not considered to compare. Fig. 6 represents the location of monitoring points. Monitoring section at E1678 tunnel maesured depth: 15.8m Monitoring section at E1663 tunnel measured depth: 18.3m E1678-1 E1678-2 E1678-3 E1678-4 E1678-5 E1678-6 E1663-6 E1663-5 E1663-4 E1663-3 E1663-2 E1663-1 East Tunnel Axis West Tunnel Axis Fig. 6 Monitoring point position. Vertical surface displacement (mm) Vertical surface diaplacement (mm) Comparison of predicted results and data measurement are represented in Fig. 7. The difference of vertical surface displacement between numerical simulation and data measurement is less than 3mm. A “groove” is observed both in simulation profile and measurement profile. The minimum value of the groove is at center of tunnel. It can be seen that the actual profiles of surface uplift are in a fair agreement with those of numerical simulation. Therefore, this numerical model has a good reliability to practical engineering project. 36 32 28 -15 Simulation value-Case 2 Masure value-Section E1678 -10 -5 0 5 10 Distance from the center of the East Tunnel (m) 15 35 30 25 Simulation value-Case 3 Masured value-Section E1663 20 -15 -10 -5 0 5 10 15 Distance from the center of the East Tunnel (m) (b) Comparison of predicted and measured (a) Comparison of predicted and measured data at 18m-buried depth data at 16m-buried depth Fig. 7 Comparison of predicted and measured data. Key Engineering Materials Vol. 723 787 Case1-burial depth 14m Case2-burial depth 16m Case3-burial depth 18m 40 Horizontal surface displacement (mm) Vertical surface displacement (mm) Fig. 8 shows the curve of vertical displacement of ground surface under different central burial depths in the direction perpendicular to shield axis. The maximum upheaval value is on the left side of axis of east tunnel when grouting finished. In Case 1, the maximum upheaval on the surface appears at -6.53m, with a peak value of 38.34mm. In Case 2, the maximum upheaval on the surface appears at -6.22m, with a peak value of 35.80mm. In Case 3, the maximum upheaval on the surface appears -6.01m, with a peak value of 31.20mm. The reduce of the uplifting value starts at the left side of east tunnel and then increases when approaching to the existing tunnel, Therefore, there is the "groove" within the scope of 6 meters. The minimum value of groove appears on the center of the tunnel. When the buried depth is 14m, 16m, and 18m respectively, the minimum value for groove respectively is 35.18mm, 32.54mm, and 26.83 mm. The magnitude of the peak near existing tunnel is less than the maximum value. Therefore, a “di proportionate hump” is presented in Fig. 8. The maximum vertical displacement of surface decreases when overlaying soil thickness increases. For burial depth changing from 18m to 16m, a more reduced vertical displacement of ground surface at axis is observed when compared with that for depth changing from 16m to 14m. This is largely due to the fact that with the overlaying soil thickening, weight of overlaying soil increases, while resistance of buoyancy also increases, Furthermore, the surface uplifting value decreases. In Fig. 9, three simulated horizontal displacements of ground surface are indicated. The horizontal displacement of center of east tunnel is zero in three cases. When central burial depth is 14m, 16m and 18m, the maximum horizontal displacements appear at 14.54m, 18.60m, and 20.21m away from the center respectively, and corresponding values of maximum are 29.55mm, 25.76mm, and 22.20mm. 30 20 -100 -50 40 30 20 10 0 -10 10 -20 -100 -50 0 Case1-burial depth 14m Case2-burial depth 16m Case3-burial depth 18m 0 50 100 150 Distance from the center of the East Tunnel (m) -30 0 50 100 Distance from the center of the East Tunnel (m) Fig. 8 Stimulation data of vertical surface displacement of different central burial depth. -40 Fig. 9 Stimulation data of horizontal surface displacement of different central burial depth. Summary Numerical analyses have been performed on a shield tunnel with super large diameter. The effect of buoyancy which is taken as sum of static buoyancy based on one-hour consolidation and dynamic buoyancy due to grouting pressure on the surface uplift response has been evaluated. The main conclusions of this paper are summarized as follows: (1) Before the hardening of grout, lining is subjected to static buoyancy caused by grout buoyancy which is about 80% of dynamic buoyancy generated by grouting pressure and 44% of the resultant. Therefore, the static buoyancy can not be ignored when buoyancy is considered in shield tunnel floating and ground uplift problem. (2) Tunnel burial depth has a significant influence on surface deformation. The maximum vertical and horizontal displacement of surface decrease when overlaying soil thickness increases. As 788 Proceedings of International Conference on Material Science and Engineering 2016 Fig. 8 and Fig. 9 show, the maximum vertical displacement of surface varies from 38.34mm to 35.80mm and to 31.20mm when depth changes from 14m to16m, and to 18m. (3) For burial depth changing from 18m to 16m, a more reduced vertical displacement of ground surface at axis is observed when compared with that for depth changing from 16m to 14m. Thus, the overlaying thickness should be tightly controlled in shield tunnel construction. (4) In the numerical simulation, uplift of ground surface occurs due to the buoyancy. The uplift peaks are located in both sides of center of east line, and a “groove” is observed whose minimum is at center of tunnel. The difference of vertical surface displacement between numerical simulation and data measurement is less than 3mm.In this case, the prediction shows a good agreement with the actual profiles of surface uplift. (5) The maximum vertical displacement of ground surface appears away from existing tunnel (west line), while the other peak near the existing tunnel (west line) is smaller when compared with the maximum. 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