Keywords

1 Introduction

With the continuous development of China’s transportation industry, the living population of many large cities is increasing, and the urban area is expanding. In order to facilitate citizens’ travel, reduce citizens’ travel commuting time, and alleviate urban traffic pressure, subway tunnels may be in the process of construction or normal service. For some reasons (such as adjacent construction, vehicle load, etc.) have a certain uneven settlement. Many cities in China (such as Beijing, Shanghai, Guangzhou, Hangzhou, etc.) are increasing subway mileage.

The shield tunnel can well control the construction disturbance and has strong adaptability to different strata. However, in the process of shield tunnel construction, it will inevitably cause surface subsidence, which will have a serious impact on nearby buildings and structures [1]. Controlling the ground settlement caused by shield tunnel excavation is the key to the safe construction of the tunnel, especially the below-passed tunnel project. The grouting strength and grouting pressure of the shield tunnel gap have a significant impact on the ground settlement. Kasper [2] analysed the influence of parameters such as grouting pressure behind the wall and balance pressure on the surface settlement during shield tunnel construction, and concluded that the surface settlement decreased with the increase of grouting pressure and balance pressure on the surface. In the past, the research on ground surface settlement caused by shield tunnelling was usually carried out from two aspects: horizontal ground surface deformation caused by shield tunnelling and ground surface deformation along the direction of shield tunnelling. In recent years, researches on surface settlement caused by shield construction mainly include field measurement analysis [3, 4], theoretical analysis [5], numerical simulation [6] and machine learning [7]. The use of suitable grouting materials and grouting parameters will improve the effect of grouting and better control the settlement of soil above the tunnel. Among the existing grouting materials, cement-water glass slurry has good injectability and high strength after solidification. However, it has poor water resistance and is not suitable for high water-bearing strata. In this paper, the optimal ratio of secondary grouting materials is obtained through laboratory tests. The influence of secondary grouting on the soil above the tunnel is studied by monitoring the settlement of the soil above the tunnel before and after secondary grouting.

2 Engineering Background

The U-shaped groove between the railway station and the test section has a length of 3126.220 m on the left line and 3126.555 m on the right line. The left line of the section corresponds to ring numbers 1890 to 1945, while the right line corresponds to ring numbers 1892 to 1951. The test section (as shown in Fig. 1) is located before the subway line and passes through a silty sand layer with a full-section designation of 5–4 (as shown in Fig. 2). The depth of burial for the test section is 25.36 m. The horizontal section of the design axis is a straight-line segment, and it has a vertical slope of 15‰.

Fig. 1.
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Position diagram of test section

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Fig. 2.
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Stratum distribution graph

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The test section of the below-passed subway line is required to be located in an area with the same geological stratum and buried depth as the crossing section. Specifically, the left line test section is designated within the 1890 ring to 1949 ring. Both the test section and the crossing section are characterized by a full-Sect. 5–4 silty sand layer.

3 Indoor Test

The indoor test of secondary grouting was carried out for the silty sand stratum in the test section. According to the indoor test results, the relevant parameters and ratios of synchronous grouting in the test section were determined.

The test section is located in the silty sand layer. The silty sand layer is quite different from the clay layer in front of the excavation. The permeability coefficient of the silty sand layer is relatively large, while the hard slurry has the characteristics of good filling performance and good impermeability. Compared with the inert slurry, it is more suitable for the silty sand layer. However, the hard slurry is more prone to plugging in the actual grouting process, resulting in shutdown and delays in the construction period. Considering the good filling performance of the inert slurry and the difficulty of plugging, the project intends to add a small amount of cement to the inert slurry. In the case of ensuring fluidity, the formation of slurry strength is accelerated as much as possible, so that the slurry has early strength faster.

In order to improve the inert slurry obtained by optimization, different amounts of cement were added on the basis of the original site ratio to shorten the initial setting time and improve the compressive strength. Combined with the dosage of cement and water reducing agent of common hard slurry, the dosage of cement is 10kg/m3, and the dosage of 20kg/m3,30kg/m3,40kg/m3 is set respectively. The engineering performance of cement inert slurry with different cement content is studied. The research focuses on the fluidity, consistency, initial setting time and compressive strength of the slurry. The specific ratio is shown in Table 1.

Table 1. Proportioning Table.
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By studying the effects of different cement content and superplasticizer addition on the fluidity, consistency, initial setting time and compressive strength of cement inert slurry, the cement inert slurry with 20kg/m3 cement content was finally adopted. The fluidity is 28.3cm, the consistency of the slurry is 10.6cm, the initial setting time is 26 h, the compressive strength is 30–60 kPa, and the bleeding rate is reduced by 3%.

4 Monitoring System

For the test section of subway line, six layered settlement points of soil are buried in the test section, and the buried depth is 2 m above the tunnel vault to the ground. A monitoring section (ZD319-ZD329) is set up every 10 m within the test section, and an encryption axis point is added every five meters. The monitoring frequency is increased from the original 2 times/day to 4 times/day, and the monitoring points are arranged as shown in Fig. 3:

Fig. 3.
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Monitoring points graph

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5 Data Analysis

5.1 Without Grouting

The 1920 ring was not grouted, and after the completion of the crossing, the single settlement value fluctuated significantly (as shown in Fig. 4). The maximum single settlement was −1.29 mm, and the maximum single uplift was 1.58 mm. The stable period of the ZD323 ground axis reached 6 days (as shown in Fig. 5).

Fig. 4.
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Single settlement change at ZD323

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Fig. 5.
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Accumulated settlement change at ZD323

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5.2 After Grouting

The 1900 ring grouting volume is 0.8 m3, the grouting pressure is 0.9 Mpa. The monitoring data is shown in the Fig. 6 and Fig. 7. After grouting, the data basically stabilized, and after 36 h, the monitoring data remained stable.

Fig. 6.
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Single settlement change at ZD319

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Fig. 7.
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Accumulated settlement change at ZD319

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1915 ring grouting volume is 1 m3, the grouting pressure is 0.9 Mpa. After grouting is completed, the single variable is basically stable within 1 mm (as shown in Fig. 8), and the monitoring data can stabilize within 4 days after grouting is completed (as shown in Fig. 9).

Fig. 8.
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Single settlement change at ZD322

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Fig. 9.
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Accumulated settlement change at ZD322

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The grouting volume for the 1930 ring is 1.5 m3, the grouting pressure is 0.9 Mpa. After grouting, the settlement value was small. The settlement can be stable within 2 mm (as shown in Fig. 10), and there was no obvious uplift. However, later monitoring data showed that there was still some settlement (as shown in Fig. 11).

Fig. 10.
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Single settlement change at ZD325

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Fig. 11.
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Accumulated settlement change at ZD325

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6 Conclusion

Based on the background of Metro Line, the optimum ratio of secondary grouting material is determined by experiment, and the influence of secondary grouting on the settlement of soil above the tunnel is studied by monitoring the settlement value of soil above the tunnel during secondary grouting. The following conclusions are obtained:

  1. 1.

    The secondary grouting makes the ground settlement significantly reduced after the crossing, and the ground settlement can be stabilized within four days.

  2. 2.

    After the secondary grouting is completed, there is no large variable in the monitoring data above the soil. The grouting volume is 0.8–1.5 m3, and the grouting pressure is 0.9 Mpa and the ground stability period is different. When the grouting amount is 1 m3 and the grouting pressure is 0.9 Mpa, the effect of controlling the late settlement of the surface is the most obvious, and the stability period is the shortest.