STRATA SUBSIDENCE CHARACTERISTICS OF SHIELD TUNNELING IN COASTAL SOFT SOIL AREA

In order to study the strata subsidence characteristics caused by large diameter shield tunneling in soft soil area, based on the project of North Oujiang shield tunnel, the displacement field, seepage field and stress field of surrounding rock soil considering fluid-solid coupling effect during shield tunneling were analyzed using finite difference method. The results show that when the shield passes through the monitoring section, the surrounding rock soil in a certain range above the tunnel will be uplifted. Shield tail grouting can effectively control the settlement of the strata, and the increasing range of the strata subsidence gradually decrease. With the advance of the shield the pore water pressure increases, and the pore water pressure in the soil will rise sharply due to the shield tail grouting. When the shield passes through the monitoring section of the tunnel, the strata stress above the tunnel increases due to uplift extrusion, and the strata stress below the tunnel decreases due to stress release. When the shield tail grouting is completed and the shield machine gradually moves away from the monitoring section, the stress release leads to the decrease of the soil stress around the tunnel, and the strata stress distribution is funnel-shaped.


INTRODUCTION
With the continuous development of shield technology, slurry balance shield technology is widely used in the construction of the submarine tunnel in soft clay strata. During the process of shield tunneling, the disturbance to the surrounding rock soil will inevitably occur. If the disturbance is too large, it will pose a threat to the safety of the surrounding buildings or adjacent structures, and it is not conducive to the stability of the tunnel structure in the later period. Therefore, it is of great engineering significance to study the displacement field, seepage field and stress field in the surrounding rock soil during the shield tunneling.
Researchers have studied the ground settlement during tunnel construction using different methods. Based on the measured data of the project, Peck [1] induced and analyzed the surface settlement during tunnel construction, and obtained that the shape of settlement trough in non-cohesive soil and cohesive soil strata presented error function or normal distribution curve shape. Due to its clear concept and simple calculation, this formula has been widely applied in practical engineering and has been continuously improved by some scholars [2][3][4][5][6][7][8]. For example, Zhu [9] considered the fill rate of grouting, shield eccentricity and support pressure ratio during shield tunneling, the modified Peck formula was established based on the modified gap parameters. Sagaseta [10] assumed that the soil was homogeneous, isotropic and linearly elastic incompressible material in a semi-infinite space. Combined with the Mindlin solution and the virtual mirroring technology, the analytical solution of formation displacement caused by formation loss was obtained. Based on Mindlin solution and Loganathan formula, Lu et al. [11] solved the additional load on the tunnel face, the friction force on the side wall and the loss of the soil at the tail of the shield by numerical integration, respectively, and obtained the elastic displacement field of the soil.Based on the Mindlin solution, Liang et al. [12] considered the influence of the support force on the shield tunnel face, the lateral friction between shield shell and soil, the shield tail grouting pressure and other factors on the ground settlement. The vertical and horizontal displacement prediction formulas of the ground during shield tunneling were obtained. Liu and Zhang [13] applied the stochastic medium theory to the prediction of ground subsidence caused by tunnel construction, and obtained two-dimensional and three-dimensional analytical formulations for settlement prediction. Later some researchers analyzed the ground settlement of shield tunnel under different soil layers and construction conditions, and further developed the theory [14][15][16]. Rowe and Kack [17] employed the finite element method to simulate the ground settlement during shield construction, and analyzed the influence of segment weight, soil parameters, formation loss, and shield tail grouting on the ground settlement. Jallow et al. [18] combined with the shield tunnel of Taipei MRT system, the influence of consolidation on long-term settlement was evaluated by using PLAXIS 3D software and small strain soft soil and hardened soil models. Taking the overlapping section of the left and right lines of the Ludao Lake-Liantang segment of Foshan Metro Line 2 as the main research object, Wu and Liu [19] effectively predict the ground settlement of the overlap section caused by construction using the numerical method and the field measured data, and the results showed that reasonable reinforcement measures could be taken to effectively control the ground settlement before the construction of the overlapping section. Jin et al. [20] analyzed the influence of new shield tunnel construction on the deformation characteristics of existing tunnel and ground through a large number of monitoring data of undercrossing construction examples in Shenzhen metro area. An empirical formula of the settlement of existing tunnels caused by new shield tunnel excavation was proposed. Combing with practical engineering, Chen et al. [21] used empirical formula (assuming that the settlement curve above the tunnel is a Gaussian distribution curve) and finite element analysis to conduct reverse analysis to the ground settlement of the tunnel. Based on the shield tunnel project of Guangzhou Metro Line 8, Lv et al. [22] investigated the settlement of shield construction in upper-soft and lower-hard strata by using the numerical simulation and the monitoring data of the project. They concluded that with the decrease of the height ratio of soft and hard rocks (soft rock height/hard rock height), the ground settlement decreased and the settlement trough became shallower. Groundwater seepage is also an important factor affecting ground settlement during shield construction. Zhang and Huang [23] summarized stratum response caused by excavation disturbance, synchronous grouting, seepage and creep behavior of soft soil during shield tunneling. Wei and Zhu [24] assumed that the groundwater inside and outside the tunnel was one-dimensional steady seepage, and the seepage drainage model was established. Combined with practical engineering, the influence of seepage on the effective stress of stratum around the tunnel was analyzed and the ground subsidence caused by seepage was calculated. For water-rich loess tunnel, Wei and Zhu [25] introduced the method of sealing groundwater seepage and controlling surrounding rock masses deformation by base grouting and curtain grouting technology. He and Wei [26] established a three-dimensional finite element model of reinforced soil during shield launching, and analyzed the influence of seepage on surface settlement during shield launching.
Scholars have made many rich achievements in the research on the strata subsidence characteristics during tunnel excavation. However, most studies do not consider the influence of the seepage effect of pore water on the strata subsidence, and there are few studies considering the fluid-solid coupling effect in surrounding rock soil on the strata settlement. In addition, for the crossing-river tunnels, it is necessary to systematically investigate the displacement field, seepage field and stress field in surrounding rock soil during shield tunneling. Therefore, based on the North Oujiang tunnel of S2 railway line in Wenzhou city, the displacement field, seepage field and stress field of surrounding rock soil considering the fluid-solid coupling effect during shield tunnel construction were analyzed by using finite difference method. This paper will provide theoretical guidance for the construction and design of similar underwater tunnel engineering.

Numerical model and boundary conditions
The shield construction process at the typical section in the middle section of the river is simulated. The excavation diameter of shield machine is 14.9m, the outer diameter D of the tunnel is 14.5m, and the inner diameter d is 13.3m. The tunnel is buried 20m below the riverbed, and the water depth above the riverbed is 12m. A three-dimensional numerical model of tunnel construction is established by using the finite difference software FLAC3D. Due to the symmetry of the structure, half of the structure model is selected for calculation and analysis. In order to eliminate the influence of boundary effect, the distance between the left and right boundary, and the lower boundary of the model and the tunnel axis is 3~5 times tunnel diameter. Figure 1 shows the numerical model of shield construction and its dimensions.

Fig. 1 -Numerical model of shield construction
The elastic constitutive models are used for the lining, grouting layer and shield of the tunnel, and the Mohr-Coulomb model is used for the surrounding rock soil. The 8-node solid element is used for the lining, grouting layer and surrounding rock soil, and shell element is used to simulate shield shell of the shield machine. Physical parameters of surrounding rock soil are shown in Table  1. The material parameters and elements of the shield shell, shield tail grouting body and lining are shown in Table 2. Considering the influence of staggered assembled segments and bolt connection on the stiffness of lining structure, the reduction coefficient of bending stiffness is selected as 0.

Shield construction load parameter and construction simulation
The calculation results show that the support pressure at the center of the excavation face is 545.456kPa, and the change gradient of the support pressure on the excavation face is 15.537kPa/m. The grouting pressure is taken 1.1 times the original lateral water and soil pressure, and the grouting pressure at the central axis of the tunnel is 532.434kPa, with a gradient of 16.998kPa/m. The slurry balance shield machine is selected, the total weight is 2126t. The weight of the whole shield machine is evenly distributed on the whole shield shell, and the uniform weight is 71.342kPa.
The 30 rings excavation process of shield tunnel is simulated, and the length of each ring is 2m. The coupling calculation between fluid and solid is adopted to carry out mechanical calculation and seepage calculation at the same time, and the actual shield construction process is simulated and analyzed. Firstly, the in-situ stress balance is carried out for the model without considering the seepage in the surrounding rock soil. After tunneling and excavating, seepage calculation and mechanical calculation are carried out simultaneously, and the fluid modulus is set as 2×10 6 kPa. The empty model is used to excavate each ring of the tunnel. The support pressure is applied on the tunnel face, and the shell element is used to generate shield shell on the excavation ring. The excavation is carried out step by step. When tunneling to the 6th ring, the shield shell element of the first ring is deleted to generate the lining and the grouting layer of the first ring. Grouting pressure is applied on the soil and lining of the first ring, respectively. In this way, the excavation is carried out step by step, the grouting pressure is set behind the ring for the fluid-soild coupling calculation until the excavation of the 30th ring is completed.

CALCULATION RESULTS AND ANALYSIS OF RESULTS
The construction process of underwater shield tunnel is numerically simulated. According to the five stages of shield tunnel construction, the displacement field, seepage field and stress field of the surrounding rock soil are analyzed. In order to eliminate the influence of the boundary on calculation results of the surrounding rock soil, the 15th ring (Y=30m plane) is taken as the monitoring section.

Displacement field
Taken the 15th ring (Y=30m plane) as the monitoring cross section, the stratum settlement of monitoring section are analyzed when the 14th ring (Shield is close to the monitoring section), the 16th ring (Shield passes through the monitoring section.), the 18th ring (Shield passes through the monitoring section.), the 21st ring (The stage of shield tail grouting.), the 23rd ring (Hardening stage 1.), the 30th ring (Hardening stage 2.) are excavated, respectively. The formation depths of Z=0m, 10m and 18m are selected to analyze the laws of strata subsidence during shield tunneling, and the calculation results are shown in Figure 3.  The14th  ring  The16th  ring  The18th  ring  The21st  ring  The23rd  ring  The30th ring The Figure 3(a) shows that with shield advancing step by step, the stratum settlement (z=0 m plane) is slowly growing, the settlement curve is basically the same in each shield stage, which all fit to Gaussian normal distribution of Peck curve, and the largest settlement occurs in the center of the tunnel axis. And after the completion of the 14th ring, 16th ring, 18th ring, 21st ring, 23rd ring and 30th ring shield tunneling, the maximum settlements on the plane z=0m are 2.6 mm, 3.8 mm, 5.8 mm, 9.9 mm, 11.0 mm and 12.3 mm, respectively, which indicates that the settlement at the riverbed surface mainly occurs between 14th ring and 21st ring (the period of shield machine crossing and shield tail leaving the monitoring section) during shield construction, and the settlement at the riverbed surface is small during grouting completion and slurry consolidation, which shows the grouting pressure is appropriate, and strata subsidence can be controlled. Combined with Figure 3 (b) and (c), it can be seen that after the completion of the 16th and 18th rings of shield tunneling, the settlement curve in the ground does not conform to the Gaussian normal distribution, and the curve shows the shape of gradually decreasing around and bulging in the middle, which is related to the upward floating of the machinery caused by the stress redistribution in the construction process of the shield tunneling, so it is necessary to carry out counterweight on the shield machine in the process of shield construction to anti-floating. After the completion of shield tunneling in the 21st, 23rd and 30th rings, the settlement curve at any depth of the stratum still conforms to the Gaussian normal distribution because the formation stress has been released due to the existence of the grouting layer. The existence of buffer layer reduces the pressure difference between the top and bottom of the tunnel, so that the floating effect will no longer occur. As can be seen from the figure, in the whole process of shield construction, with the shield gradually approaching the monitoring section, the ground will be disturbed and vertical settlement will occur. With the shield tunneling passing through the tunnel section, A certain range of the surrounding rock soil above the tunnel will be uplifted, but the settlement of the riverbed will continue to increase. With the shield tail grouting gradually away from the tunnel section, the shield tail grouting can effectively control the subsidence of the ground. The subsidence of the ground is still gradually increasing, but after the condensation and hardening of the grouting layer, the subsidence of the ground will gradually tend to be stable.

Pore water pressure
The positions above the central axis of the tunnel, which are 2 m, 6 m, 10 m, 14 m, 16 m, 18 m and 20 m from the riverbed surface, are selected to study the change of pore water pressure in surrounding rock soil caused by the whole excavation process, as shown in Figure 4. As can be seen from Figure 4, during the shield tunneling, as the cutter head gradually approaches the tunnel excavation face of the monitoring section, the pore water pressure in the formation will gradually decrease due to shield disturbance. When the shield passes through the tunnel face, the excess pore water pressure will be formed in the soil due to the disturbance, and the pore water pressure will increase with the shield advancing. When the grouting at the shield tail is completed, the pore water pressure in the soil within 4m of the upper strata above the tunnel will rise sharply, and the maximum excess pore water pressure is 39.5kPa. As the shield tail gradually moves away, the pore pressure in the upper strata above the tunnel gradually decreases, then increases and finally tends to be stable. The final pore water pressure is less than the original hydrostatic pressure. By contrast, the pore water pressure at the top of the tunnel eventually decreases by 18.4kPa. It can be seen from the figure that the influence range of pore water pressure is roughly within the range of 10m above.

Stress field
Distribution of transverse stress field According to Figure 5 (a), when the shield approaches the tunnel face (the 14th ring), the vertical stress of the stratum decreases slightly compared with the initial stress, and the shield has begun to affect the soil stress in front of the tunnel face. As can be seen from Figure 5 (b), when the shield passes through the tunnel face, the vertical stress of the upper soil layer increases due to the extrusion of the floating effect and presents an upward protrusion, while the bottom decreases due to the release of the stress. It can be seen from Figure 5 (c),and Figure 5(d) that when the shield tail moves away, there is a stratum loss. The existence of the gap leads decreases of the stress due to the release of the strata stress around the tunnel, and the disturbance also gradually decreases from the tunnel position to both sides, and the surrounding vertical stress presents funnel shape. In addition, it is seen from Figure 5 (d) that after the grouting body solidifying and hardening, the vertical stress of the formation is basically the same and tends to be stable. Figure 6 shows the vertical stress on the longitudinal section where the tunnel axis is located during shield construction, and the vertical stress of the stratum in the shield construction process is analyzed. The Figure 6 (a) shows as the tunneling approaches the tunnel face, the degree of disturbance increases, and the stress release of the formation in front of the tunnel face increases, and the formation stress distribution presents a funnel shape. In Figure 6 (b), the stress in the range around 8m in front of the shield face decreases due to the stress release, and the undisturbed area is still in the initial stress state. The formation stress at the shield tail gradually tends to be stable with the gradual advancing of the shield. When the shield passes through the strata, the stress of the upper soil above the tunnel increases due to the floating effect, and the stress of the bottom soil under the tunnel decreases due to the stress release. The distribution of vertical stress in Figure 6 (d) is almost the same as that in Figure 6 (c), and the formation stress tends to be stable as the shield tail gradually moves away.

CONCLUSIONS
(1) During the shield tunneling, as the shield gradually approaching to the monitoring section, the strata will be disturbed and vertical subsidence will occur. When the shield tunneling passes through the monitoring section, the surrounding rock soil in a certain range above tunnel uplifts, but the settlement of the strata under the riverbed will continue to increase. Shield tail grouting can effectively control the subsidence of the strata, the increasing range of the strata subsidence gradually decreases, and the strata subsidence tends to a stable state with the consolidating and hardening of the grouting layer.
(2) As the shield approaching to the tunnel monitoring section, the pore water pressure of the formation decreases gradually due to the shield disturbance. When tunnel shield passes through the monitoring section, excess pore water pressure will be formed in the stratum. With the shield advancing, pore water pressure will increase. The pore water pressure of the soil will rise sharply when the shield tail grouting. With the shield moving away from the monitoring section, the pore pressure in the stratum above the tunnel gradually decreases, then increases and finally tends to be stable.
(3) With the shield advancing the monitoring section, the vertical stress of the stratum around the tunnel decreases due to disturbance. When the shield passes through the monitoring section, the stress in the strata above the tunnel increases due to uplift extrusion, and the soil stress under the tunnel arch bottom decreases due to stress releasing. When the shield is away from the monitoring section, the formation loss occurs around the tunnel, and the stress decreases in the surrounding stratum due to the stress release and shows a funnel-shaped distribution.