Keywords

1 Introduction

Nowadays, the scale of the city is increasing rapidly, and the development of underground space is also developing rapidly. with the improvement of the degree of development, the relationship between the projects of underground space is more complex, and the mutual influence can not be ignored. Shield construction as an important way of underground space development, it is very important to study the disturbance law of shield construction to the surrounding soil, so as to reduce stratum deformation and ensure the safety of the surrounding buildings and tunnels.

In the research on the disturbance of surrounding soil caused by tunnel shield construction, numerical simulation [1, 2] and theoretical analysis [3,4,5,6] are more common, in which the numerical simulation is generally based on the finite element calculation of continuum [7,8,9], while the theoretical analysis is basically aimed at saturated clay, and the undrained assumption [10, 11] is adopted at the same time, and there is little research on sand layer [12]. The research on the field measured data based on the actual project is less, and it is not systematic enough. At present, most of the existing studies on the displacement of the surrounding soil measured in the field only focus on a single deformation of the soil around the tunnel in the shield, such as surface settlement [13, 14], deep horizontal displacement [15, 16]. There is a lack of comprehensive research on the short-range and high-density soil displacement monitoring data for the same shield tunnel project.

Relying on the actual engineering, this paper carries out high-density monitoring of the surface settlement, layered settlement and the horizontal displacement of the deep soil around the shield tunnel, and studies the disturbance law of the surrounding soil caused by the shield construction in the upper soft and lower hard composite stratum. Combined with the synchronous grouting pressure, the fluctuation of the horizontal displacement of the deep soil when the tail of the shield machine leaves the monitoring section is discussed, and the disturbance law of the shield construction to the soil is studied.

2 Background

Shijing ~ Huanxi Power Tunnel (Xiwan Road ~ Shisha Road Section) is located in Liwan District and Baiyun District of Guangzhou City. The type of shield machine is mud-water balance shield, the length of shield machine is 9.62 m, the inner diameter of shield segment is 3.6 m, the thickness is 0.25 m, the outer diameter is 4.1 m, the diameter of cutter head is 4.35 m, and the buried depth is 415 m.

The monitoring section arranged in this project is located on the south side of Xiwan Road Crossing Zengpohe Highway Bridge in Guangzhou City. The strata of the tunnel are sand layer and silty clay layer. The buried depth of the tunnel axis is 14 m. The geological profile of monitoring section is shown in Fig. 1.

Fig. 1
A diagram includes the components of the geological profile, which are the Zengbu River, artificial fill, mucky soil, silty and coarse sand, silty clay, and medium-weathered limestone at the bottom.

Geological Profile

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The monitoring contents include surface subsidence, layered settlement and deep horizontal displacement. The distribution of each measuring point on the monitoring section is shown in Fig. 2. The surface subsidence measuring points are arranged perpendicular to the tunnel axis and are located on the same tunnel section. The layered settlement measuring points and deep horizontal displacement measuring points are basically symmetrically arranged along the tunnel axis. Due to the influence of the actual construction level and complex strata, the actual measuring points can not be strictly symmetrically arranged on both sides of the tunnel.

Fig. 2
A layout of monitoring points depicts the position of 6 surface subsidences, 13 layered settlements, and 2 inclinometers from surface subsidences C J 2 and C J 3.

Layout of Monitoring Points

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3 Study on Disturbed Deformation of Soil Mass

3.1 Surface Subsidence

The surface subsidence shows the surface subsidence of the monitoring section. There are six surface subsidence monitoring points in this project, numbered as CJ1, CJ2, CJ3, CJ4, CJ5 and CJ6,6. The measured surface subsidence is shown in Fig. 3. The Abscissa is the distance between the cutterhead and the monitoring section. The two red dotted lines in the Fig. divide the whole monitoring process into three stages. The dotted line on the left indicates that the cutterhead of the shield machine reaches the monitoring section. The dotted line on the right indicates that the tail of the shield machine is separated from the monitoring section.

Fig. 3
A line graph of the surface deformation versus the distance between the cutter head and the monitoring section plots 6 curves all in decreasing trend for C J 1 through C J 6.

Surface Subsidence

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As can be seen from Fig. 2. and Fig. 3, the settlement of the CJ4 measuring point with the smallest horizontal distance from the tunnel axis is the largest, and the settlement values of the other measuring points decrease with the increase of the horizontal distance between the location and the axis, which is consistent with the transverse surface subsidence curve proposed by Peck [17].

Before the cutterhead of the shield machine reaches the monitoring section, the surface settlement and settlement rate are very small, so it can be seen that the shield tunneling has little influence on the front surface subsidence; during the period when the shield machine passes through the monitoring section, the surface subsidence increases and the growth rate is larger, at this time, the surface is disturbed by construction; within 4 m from the monitoring section, the surface settlement value is still increasing rapidly, and then the growth rate slows down until it tends to be stable. The settlement rate of the tail of the shield machine is larger before 4 m away from the monitoring section, and then becomes smaller and more stable, the reason is mainly affected by tail grouting before 4 m, during this stage, a ring of segments forgot to grouting (see Fig. 8.), resulting in increased settlement; after grouting, the deformation is mainly consolidation settlement, so the performance is relatively stable. The overall performance of the soil disturbance is that the settlement is basically stable after the tail of the shield machine is separated from the monitoring section about 2 times the length of the shield. And Lei Huayang [18], Bian [19] and Jiang [20]have come to the same conclusion when the diameter of the cutter head is close to the buried depth of the shield machine and the soil conditions are quite different. It can be seen that this conclusion can be used as a reference for the research of relevant shield engineering and the construction of projects with similar buried depth and cutter head diameter.

As shown in Fig. 4, the settlement of each measuring point varies with the distance between the cutter head and the monitoring section. The settlement law of the stratified settlement measuring point on both sides of the tunnel axis is the same as the surface settlement law mentioned above, but the three layered settlement measuring points directly above the axis have undergone a large displacement, in which the FC3-3 measuring point, which is only 2.6 m away from the top of the tunnel, sinks 0.6 m when the cutter head reaches the monitoring section, and then the measuring point can not continue to work. The FC3-2 measuring point also has a large settlement when the cutter head reaches the monitoring section, and the settlement is 20.24 mm, and then the settlement develops slowly when the shield machine passes through the monitoring cross-section. when the tail of the shield machine leaves the monitoring section, FC3-2 changes greatly again, develops to 98.94 mm, and then continues to increase rapidly to about 140 mm before it is basically stable. The variation trend of FC3-1, which is located in the lower buried depth and farther from the tunnel, is close to that of FC3-2. Because of the distance, the settlement is relatively small, and the maximum settlement is 67.34 mm.

Fig. 4
A line graph of layered settlements versus the distance between the cutter head and the monitoring section plots 13 curves that are linear toward the end. The shield machine passes lie between 0 and 10 meters.

Layered Settlement Curve on Both Sides of Tunnel Axis

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3.2 Layered Settlement

The three measuring points directly above the axis of the tunnel have a large displacement, because the upper part of the tunnel and FC3-2 and FC3-3 are located in the sand layer, the bonding between the soil particles in the sand layer is small, and the sand layer is easy to collapse in a small range in the process of tunnel excavation, so the settlement caused by construction is small and large, so the settlement increases sharply when the cutter head arrives and the tail of the shield machine detaches. This phenomenon should be paid attention to in shield construction in similar strata, especially when there are important buildings above the tunnel axis in the sand layer to avoid excessive settlement.

As shown in Fig. 5, the layered settlement monitoring point FC1 series and the surface settlement monitoring point CJ2 are on the same side of the tunnel and are almost the same horizontal distance from the tunnel axis. Similarly, the FC2 series and FC3 series correspond to the surface settlement monitoring points CJ3 and CJ4 respectively. The settlement difference △S is obtained by subtracting the measured values of the upper and lower adjacent points in each settlement measuring point, as follows:

$$ \Delta {\text{S = S}}_{{\text{D}}} {\text{ - S}}_{{\text{S}}} $$
Fig. 5
A line graph of the settlement difference of the measuring point versus the distance between the cutter head and the monitoring section plots 12 curves in fluctuating trends. The shield machine passes between 0 and 10 meters.

Settlement Difference Between Monitoring Points

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SD is the settlement of the deeper measuring points in the two adjacent measuring points, and SS is the settlement of the shallower measuring points in the adjacent two measuring points. A positive value of △S indicates that the soil between the two measuring points is compressed, while a negative value indicates that the distance between the two measuring points increases. Figure 5. shows the variation curve of the settlement difference between the upper and lower adjacent measuring points with the shield machine driving. Above the tunnel axis, the settlement difference of the CJ4 and FC3 series measuring points is relatively large. Therefore, the Y axis of the settlement difference curve of this series is the right axis. The Y axis of other curves is the left axis.

Because the settlement difference of CJ4 and FC3 series is larger than that of other measuring points above the tunnel axis, the Y axis of the settlement difference curve of this series is the right axis. The Y axis of other curves is the left axis. It can be seen from the diagram that most of the settlement differences of all measuring points occur in the interval between the arrival of the cutter head and the 3 m away from the monitoring section of the tail of the shield machine. The biggest difference in the Fig. is about-90 mm, which occurs between FC3-2 and FC3-1 above the axis of the tunnel. Because FC3-3 collapses when the cutter head arrives, resulting in about 600 mm settlement, it is inferred that the settlement difference between FC3-3 and FC3-2 is as high as-450 mm. The settlement difference of each measuring point above the axis of the tunnel is negative, indicating that the distance between the measuring points increases, forming a tensile loosening zone.

The final settlement difference distribution in the monitoring section is shown in Fig. 6. In the Fig., the red indicates that the vertical distance between the two connected measuring points increases, showing a stretched state, while the blue indicates that the vertical distance between the two connected measuring points decreases, showing a compressed state. The number at the segment represents the vertical displacement at the measuring point at both ends of the segment, “ + ” indicates compression, and “-” indicates tension. From this picture, it can be seen that the deformation of the upper soil caused by the tunnel shows obvious characteristics of “collapse arch”: the settlement and settlement difference of each measuring point (including CJ4 and FC3 series measuring points) on the tunnel axis (vault) is the biggest, and the settlement of the measuring point closer to the top of the tunnel is larger, and the whole is in a state of tension. It is noted that the settlement difference between FC3-3 and FC3-2 is about 0.45 m, and it can be inferred that there is a collapse crack between the two measuring points. At the measuring points on both sides of the tunnel, with the increase of the distance from the tunnel, the difference between settlement and settlement becomes smaller, so the arch effect is formed. Moreover, it can be seen that the range of the arch increases with the increase of the distance from the tunnel, but the smaller the settlement difference between the adjacent measuring points is, it tends to decrease gradually. It can also be seen from Fig. 6. that the settlement and differential settlement of the measuring points below the tunnel axis are very small.

Fig. 6
A graph of z versus x plots tension and compression curves for artificial fill, mucky soil, silty sand, coarse sand, and silty clay from top to bottom.

Settlement Difference Between Adjacent Monitoring Points

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3.3 Deep Horizontal Displacement

The deep horizontal displacement of this project is the radial horizontal displacement relative to the tunnel. The deformation curve of each inclinometer tube in the process of shield machine tunneling is shown in Fig. 7. The swing amplitude of the curve is: CX2 > CX1. It can be seen from Fig. 7. that the horizontal displacement mainly moves towards the tunnel, and changes significantly when the distance between the cutter head and the monitoring section is (-0.8 m ~ 0.4 m) and (6.4 m ~ 7.6 m). Among them, -0.8 m and 0.4 m correspond to the arrival and passage of the cutter head, and 6.4 m and 7.6 m correspond to the ring at the end of the shield that is about to arrive and is not grouted.

Fig. 7
A set of 2 change diagrams depicts waveform horizontal displacement curves for 23 values. The amplitude of deep horizontal displacement is greater in the second graph.

Horizontal Displacement (CX1, CX2) Change Diagram of Deep Soil

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Fig. 8
A line graph of the horizontal displacement over grout amount over pressure versus the distance between the cutter head and monitoring section plots curves for 8 values.

Comparison of Horizontal Displacement of Characteristic Points and Changes of Shield Construction Parameters

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The distance relationship between the two inclinometer tubes and the tunnel is: CX1 > CX2. The largest horizontal displacement occurs near -9.0 m, rather than 45-φ/2 angles upward from the center of the tunnel. The maximum horizontal displacement of CX2 is about 18 mm and the maximum horizontal displacement of CX1 is about 6 mm. There is a positive correlation between the distance between CX1 and CX2 from the axis of subway tunnel and the degree of disturbed deformation, that is, compared with CX2, the deformation degree of inclination curve of CX1 which is farther from the tunnel is smaller, which is consistent with the law of actual shield construction.

The maximum horizontal displacement of the two measuring points occurs near-9.0 m, and the soil layer in this depth is a silt layer with poor stability, and up to the middle line of the tunnel is a sand layer, which shows that the horizontal deformation of this depth is mainly the horizontal loss of the sand layer to the tunnel direction. The maximum horizontal displacement does not occur in the range of tunnel depth, but in the sand layer above the top of the tunnel, and the maximum horizontal increment caused by secondary under pressure of cutter head and shield tail grouting, and the maximum horizontal displacement in the monitoring process all occur at the same depth. This shows that the horizontal deformation of the soil around the tunnel in the water-rich sand layer may be caused by the flow of groundwater to the shield caused by the under pressure of the cutter head mud water bunker and shield tail grouting, resulting in the horizontal loss of the sand layer with poor self-stability. Therefore, when the top of the shield is dug in the sand layer, it may not only produce a large vertical displacement in the sand layer, but also lead to abnormal horizontal displacement. On the one hand, this abnormal horizontal displacement may have an adverse impact on the facilities adjacent to the shield.

Figure 8. shows the relationship between the measured values of deep horizontal displacement at three elevations in CX1 and CX2 and shield construction parameters. The three elevations are the tunnel center elevation -14.0 m, the maximum displacement elevation -9.0 m and the silt soft soil center -5.0 m. The pressure and amount of shield tail grouting and the pressure of the slurry warehouse before the arrival of the cutter head are also given in the picture. It can be seen from the plane layout of the measuring points in Fig. 2. that the position of the inclinometer tube is 1.2 m in front of the monitoring section (the forward direction of the tunnel). Therefore, the cutter head arrives at the position of the measuring tube 1.2 m later than it arrives at the monitoring section.

In the process of shield machine tunneling, the deep horizontal displacement is roughly as follows: when the cutter head reaches 2 m in front of the inclinometer section, the displacement is basically zero, and then it deforms rapidly to the inside of the tunnel, and the displacement above the tunnel increases gradually when the shield machine passes through. The deformation of the tail of the shield machine gradually slows down and tends to be stable after it is separated from about 4 m. On the other hand, near the central side wall of the tunnel (|CX2|-14.0 m|), due to being squeezed by the cutter head, the deformation occurs to the outside of the tunnel, and the displacement increment is 4 mm. From the point of view of the displacement direction, whether it is horizontal displacement (Fig. 7.) or vertical displacement (Fig. 3 and Fig. 4), except for a small amount of outward extrusion deformation near the tunnel axis during the passage of the cutter head, the soil above the tunnel is displaced to the tunnel direction. This shows that the soil deformation in sandy stratum is mainly affected by slurry warehouse pressure or grouting pressure, and a large amount of grouting does not necessarily lead to soil uplift or compression. This may be related to the easy escape of grouting in sandy water-rich strata.

4 Conclusions

Based on the actual project, combined with the shield construction parameters, the surface settlement, layered settlement and deep horizontal displacement of the soil around the shield tunnel are analyzed and studied, and the following conclusions are drawn:

  1. (1)

    Whether vertical displacement or horizontal displacement, the influence area of shield construction is mainly concentrated in the soil above the top of the tunnel, while the soil deformation in the range of shield is small. The overall performance of the soil disturbance is that the settlement of the tail of the shield machine is basically stable after the tail of the shield machine is separated from the monitoring section about twice the length of the shield machine.

  2. (2)

    In the shield construction of sandy stratum, the soil deformation is mainly affected by slurry warehouse pressure or grouting pressure. Although the amount of grouting is large, it does not necessarily lead to soil uplift or compression. And the sand in the upper part of the tunnel is easy to collapse in a small area. Due to the small diameter and large buried depth of the tunnel in this project, the formation of the collapsed arch does not have a great impact on the tunnel construction. Considering the large diameter tunnel engineering, this feature will be one of the causes of shield construction accidents.

  3. (3)

    A remarkable feature of soil disturbance around the tunnel in the water-rich sand layer is that the maximum horizontal displacement of the inclinometer tube with different distance from the tunnel occurs at the same depth, not from the upward 45-φ/2 diffusion angle of the tunnel center. It may be that the insufficient pressure of slurry warehouse or the insufficient pressure of shield tail grouting cause the flow of groundwater to the shield, which leads to the horizontal loss of the sand layer with poor self-stability.

  4. (4)

    When the pressure of slurry warehouse or shield tail grouting is insufficient in tunnel construction in water-rich sand layer, it will affect not only the vertical displacement of soil, but also the horizontal displacement of the surrounding soil. Compared with the settlement where the vertical displacement is greater than the order of 60 cm, the maximum horizontal displacement near the tunnel is only 5 ~ 18 mm, which is much smaller than the vertical displacement. Therefore, the actual influence distance of shield disturbance can be better analyzed by using horizontal displacement.