EFFECTS OF LARGE-SCALE UNLOADING ON EXISTING SHIELD TUNNELS IN SANDY GRAVEL STRATA

The influence of large-scale unloading of soil on existing metro tunnels is a difficult problem in the operation of Metro in sandy gravel strata. In order to predict and control the tunnel deformation and ensure the safety and normal operation of the metro, three-dimensional numerical models are proposed in this study. These models based on the line 5 of Chengdu subway engineering and the adjacent unloading projects analyse the deformation and stress characteristics of existing metro tunnels under large-scale unloading of soil. In conclusion, after comprehensive reinforcement measures such as advanced tubal curtains and horizontal beams, anti-floating anchor cables, the elastic modulus of strata is improved to a certain extent, and the rebound of pit bottom soil is effectively reduced. After the excavation of the foundation pits are completed, the maximum longitudinal uplift of left and right tunnel is 8.33 mm and 9.56 mm under reinforced condition, which is less than the control standard value of 10 mm.


Introduction of Engineering
The total length of line 5 of Chengdu Metro is about 49 kilometres. The section works between Saiyuntai Station and Dafeng Station whose stake is from K0+760 to K0+800 underneath pass throat area of Baoji-Chendu railway, and the building map diagram is shown in Figure 2. The relative spatial position of the municipal tunnel and line 5 of Chengdu Metro is shown in Figure 3. The municipal tunnel will be constructed by cut-and-cover method and divided into two parts: the south side foundation pit and the north side foundation pit. The foundation pit on the south side has been excavated before the excavation on Line 5 of Metro,so the unloading of the foundation pit on the north side will affect the structure of the lower subway.  Figure 4. The plane dimension of the north side foundation pit is 35m×59m, including the west side relief road, the main foundation pit I, the main foundation pit II and the east side relief road. The foundation pit excavation unloading depth is about 12m, and the minimum distance from the bottom of the foundation pit to the vault of the shield tunnel below is only DOI 10.14311/CEJ.2020.04.0046 538 2m. The burial depth of shield tunnels is 10.8 to 17.8 m, in which inside diameter is 6 m and outside diameter of 6.4 m.

Fig.4 -Plane Position Diagram
In this project, the unloading of a large range of soil will affect the tunnel below [31]. Considering the lack of construction experience of short distance unloading above Metro in gravel strata, the safety control standard of Chengdu Metro Line 5 is formulated in combination with the Technical Specification for Monitoring Urban Rail Transit Engineering.

Advanced Tubal Curtains and Horizontal Beams
In order to reduce the influence of municipal tunnel construction unloading on the additional deformation and internal force of the existing shield tunnel structure, the soil above the arch of line 5 of Chengdu Metro shield tunnel is strengthened by using the joint support of advanced tubal curtains and horizontal beams. There are two layers of advanced tubal curtains, the lower side advanced tubal curtains whose longitudinal length is 51.5 m, diameter is 800 mm and DOI 10.14311/CEJ.2020.04.0046 539 circumferential distance is 850 mm are semi-circular along the tunnel vault, and the upper side advanced tubal curtains are horizontally placed under the bottom of the municipal tunnel, which layout diagram is shown in Figure 5. The advanced tubal curtains that use locks to ensure the stability of the interface are 67. After the construction of the advanced tubal curtains is completed, the micro-expansion concrete is filled to enhance the longitudinal bending stiffness of the advanced tubal curtains.

Anti-floating Anchor Cables
In order to further strengthen the safety of the subway operation, the prestressed anchor cables and the tunnel segments are connected to control the floating of the shield tunnel. The affected area under the foundation pit of the municipal tunnel and within 100m of the longitudinal tunnel segment, the prestressed anchor cables are arranged at the bottom and side of the tunnel segments and the layout of anchor cables of each ring tunnel segment is shown in Figure 6. Seven tension points are set for each segment, a total of 933 tension points are set in the range of 100 m. Each prestressed anchor cable is about 10m, the anchor end is about 8m, and the free end is 2m. Through mechanical analysis, the prestressed anchor cable that is flexible structure is composed of 3 steel strands whose diameter is 15.2mm and pre-stress force is 200 kN.

Finite Element Model and Selected Parameters
The excavation foundation pit of the municipal tunnel, which is 12m high, 59m long and 35m wide, is built above line 5 of Chengdu Metro. The numerical calculation model was established by using Midas NX finite element software. The length was 180m (X direction), the width was 130m (Y direction), and the calculated depth was 50m (Z direction), which can eliminate the influence of boundary conditions. The overall model is shown in Figure 7(a). The modified Mohr-Coulomb constitutive relation is adopted in the model. The model soils are simulated by solid elements, the tunnel segments adopt the shell elements, the foundation pit retaining structures are simplified to be equivalent to the 2D plate elements, the advanced tubal curtains and internal supporting structure of steel pipe are made of beam element, and the anti-floating anchor cables are simulated by embedded truss. The reinforcement structures are shown in Figure 7(b-d). The boundary conditions restrict the normal displacement, the bottom boundary restricts the vertical displacement, and the upper boundary is the free surface. The specific soil and structure parameters were determined by the "Special Exploration Report of sandy gravel strata of Shield Tunnel of Line 5 of Chengdu Metro " and the empirical values of mechanical calculation parameters of each rock layer in Chengdu area. Detailed formation mechanics calculation parameters and structural parameters are shown in Tables 2 and  3. DOI 10.14311/CEJ.2020.04.0046 541

Simulation of Construction Conditions
When the foundation pits are excavated, the foundation pit supporting structures are first applied, and then the soils are excavated. The excavation sequence is from the west side relief road of foundation pit to the east. The foundation pit is divided vertically into multi-layers. The depth of each excavation is 2 m. When the main foundation pit II and the east side relief road are excavated, the method of vertical stratification and horizontal segmentation is adopted to excavate the soil within 6 meters above the bottom of the pit. The test section of 6 m long foundation pit excavation above the tunnel is set up, as shown in Figure 8.

RESULT ANALYSIS
Deformation and Stress Figure 9 shows the changing rule of the deformation of the bottom of foundation pit under different excavation stages. After the excavation of the west side relief road is completed, the maximum uplift deformation at the bottom of the pit is 4.75 mm, while the soil above the left side tunnel has a certain degree of sinking, with a maximum of about 3.9 mm (Figure 9a). After the excavation of the main foundation pit I is completed, the bottom uplift of the west side relief road foundation pit continues to increase to 7.95 mm, and the maximum uplift at the bottom of the main foundation pit I is 5.17 mm. At this time, the settlement of the soil above and around the left-line tunnel gradually becomes smaller. (Figure 9b). After the excavation of the main foundation pit II, the maximum bulge at the bottom of the pit is mainly located at the bottom of the main pit I, which is about 9.69 mm (Figure 9c). After the excavation of the municipal foundation pit is completed, the whole bottom of the pit is bulged, and the maximum bulge is 11.1 mm (Figure 9d). From the bottom bulge value of the foundation pit shown in Figure 10, it can be seen that the maximum bulge of the foundation pit occurs at the centre of the main foundation pit I, about 11.1 mm, and the ridge change of the main foundation pit II is 8-10 mm. The main foundation pits have a relatively large degree of uplift, while the foundation pits of east and west side relief road are relatively small. This is mainly because the main foundation pit has a larger excavation size and wide unloading range than the auxiliary road foundation pit, so the bulge deformation of the bottom of the pit is increased to some extent. Figure 11 shows the vertical displacement of the left and right line tunnels under different working conditions. Since the west side relief road and the main foundation pit I are relatively far from the line 5 of subway, the vertical displacement of the tunnel is mainly settlement, and the maximum settlement is 3.8 mm. After the excavation of the foundation pit II above the right-hand subway tunnel is completed, the maximum uplift of the right-line tunnel is 6.37mm, while the deformation of the left-line tunnel is basically zero, and the tunnel undergoes upward uplift deformation after settlement. After the excavation of the foundation pits are completed, the maximum ridge of the right-line tunnel is 9.56mm, and the maximum ridge of the left line is 8.33mm, which occurs below the centre of the pit bottom and above the tunnel axis. It can be seen from the above analysis that the maximum settlement and uplift deformation of the tunnel in each working condition occur above the tunnel vault. In order to visually study the effect of soil unloading on the vertical deformation of the subway tunnel along the axis, a research point is selected every 2m along the longitudinal direction of the tunnel to extract the vertical deformation at the position of the tunnel vault, as shown in Figures 12. After the excavation of the west side relief road is completed, the left and right tunnels will undergo settlement deformation in the area below the foundation pit, and the main settlement is about 3-3.8 mm. As the foundation pit gradually excavated in the direction of the tunnel, the left and right tunnels began to bulge upward. When the excavation of the main foundation pit II is completed, the right-line tunnel in the area below the foundation pit exhibits a bulging deformation, and the maximum bulge amount is about 6 mm. However, the left-line tunnel deformation is almost zero, which means that the settlement and bulging of the tunnel cancel each other out. After the excavation of the foundation pit, the maximum ridge deformation of the right tunnel is about 9.56 mm, and the maximum ridge of the left line is about 8.33 mm, which is less than the proposed 10 mm deformation control standard. It can be seen from Figure 12 that the maximum deformation position of the left and right tunnels occurs at the middle of the tunnel (Y=55m), and also near the centre of the Y-direction of the municipal frame foundation pit. Therefore, the section is the most unfavourable section showing the safety of the tunnel structure when the soil is unloaded. The vertical displacement of the joint at the vault position of the section is extracted with the deformation value of the excavation step of the foundation pit as shown in Figure 13. It is noteworthy that when the construction of the main foundation pit II begins, the right tunnel begins to rise upward, the curvature of the tunnel uplift curve increases, and the deformation rate accelerates. At the same time, with the increase of the excavation depth of the foundation pit, the right tunnel has a larger increase rate and uplift value than the left tunnel, and the uplift deformation increases nonlinearly. When the east side relief road starts construction, the rate of change of the right-line tunnel uplift is relatively reduced, but the growth rate of the left-line tunnel is significantly increased. During the construction, the deformation of the subway tunnel should be monitored closely in real time, and effective and reasonable unloading methods should be adopted in time to ensure the safety and controllability of the subway structure.

Reinforcement Advanced Tubal Curtains and Horizontal Beams
In order to further analyse the effect and degree of the active reinforcement measures of advanced tubal curtains and horizontal beams, the vertical displacement of left and right tunnel under three conditions is analysed by comparing the unreinforcement conditions and the reinforcement conditions of comprehensive measures. Figure 14 shows the variation of vertical displacement of left and right tunnel under different reinforcement conditions. The figure shows that the final vertical displacement of the left and right tunnels exceeds the stipulated 10 mm control standard under the unreinforced condition, which shows that the conventional unloading control measures cannot meet the control requirements of Metro deformation. After adopting advanced tubal curtains and horizontal beams reinforcement measures, the vertical uplift displacement of the tunnel decreases significantly, and the left tunnel uplift displacement decreases to about 10 mm, but the right tunnel uplift displacement still exceeds the limit, so the deformation of Metro Line 5 still does not meet the requirements.  Table 4, it can be seen that the maximum longitudinal uplift of left and right tunnel is 11.85 mm and 14.32 mm under unreinforced condition after all excavation of foundation pit has been completed. The vertical displacement of the left and right tunnel is reduced to 9.28 mm and 11.27 mm, which reduces respectively by 17.1% and 21.3% after adopting advanced tubal curtains and horizontal beams reinforcement measures. The vertical displacement of the left and right tunnel is reduced to 10.29 mm and 11.68 mm, which reduces respectively by 13.1% and 18.4% after adopting anti-floating anchor cables reinforcement measures. After adopting comprehensive reinforcement measures, the vertical displacement decreases by 29.7% and 33.2% respectively, and the reinforcement effect is more obvious. Comprehensive reinforcement measures can meet the control requirements of Metro deformation, but the two measures cannot meet the control requirements of Metro deformation when used singly.  (1) The comprehensive reinforcement measures such as advanced tubal curtains and horizontal beams, anti-floating anchor cables have obvious reinforcement effect. To a certain extent, the elastic modulus of the strata is improved, the rebound of the soil at the bottom of the pit is effectively reduced, and the excessive uplift of the tunnel structure is avoided. (2) After the excavation of the foundation pits are completed, the maximum uplifts of the left and right tunnels are 9.56 mm and 8.33 mm respectively, which are less than the control standard requirement of 10 mm, and have a certain safety reserve.

Tab.4 -Maximum vertical displacement of tunnels under three reinforcement measures
(3) When the construction of the main foundation pit II begins, the right tunnel begins to uplift upwards, the curvature of the uplift curve increases, and the deformation rate accelerates. At the same time, with the increase of excavation depth of foundation pit, the increase rate and uplift value of the right tunnel are larger than that of the left tunnel, and the uplift deformations increase nonlinearly. When the construction of the east side relief road begins, the change rate of the right tunnel uplift decreases relatively, but the growth rate of the left tunnel increases significantly, and the horizontal displacements increase nonlinearly. Therefore, it is necessary to monitor the deformation of metro tunnel in real time and adopt effective and reasonable unloading methods in time, which can ensure the safety of metro structure. (4) Compared with the unreinforced condition, the vertical displacements of the left and right tunnels are reduced respectively by 13.1% and 18.4% after the anti-floating anchor cables reinforcement measures are adopted. The vertical displacements of the left and right tunnel are reduced respectively by 17.1% and 21.3% by adopting the advanced tubal curtains and horizontal beams reinforcement measures. But, the two measures cannot meet the control requirements of metro deformation when used alone.