1. Introduction
As an effective way to alleviate the pressure of urban traffic, urban underground rail transit has sprung up nationwide and even worldwide. The construction of rail transit is an important way for urban sustainable development. First of all, it can greatly reduce the urban land use area, thus alleviating the congestion of ground traffic. Secondly, the efficient operation of infrastructure provided by rail transit reflects the economic sustainability of the city. Finally, rail transit has relatively little damage to the environment in various transportation methods, which can save a lot of one-time resources. That reflects its environmental sustainability.
In the coastal soft soil areas, with the rapid development of urban rail transit, the surrounding deep and large foundation pit projects continue to emerge. At present, the foundation pit engineering in soft soil areas is developing in the direction of deep excavation. At the same time, with the construction of a large number of underground infrastructure such as urban subways and urban underground pipe corridors, more and more excavations are adjacent to existing subway tunnels. The influence of excavations on adjacent existing tunnels has become a prominent problem worthy of attention in urban underground engineering construction. At the same time, the protection of the existing tunnel to ensure its long-term sustainable operation is also an embodiment of sustainable development.
There is often a certain distance between the existing tunnel and the retaining structure. In engineering practice, partition piles (walls) are often set at this position to block the transmission of the influence of excavation deformation on the soil, so as to reduce the deformation of the existing tunnel [
1,
2,
3].
However, relevant research shows that the influence of the existence of the partition piles on the displacement field of the deep soil outside the excavation varies greatly with the depth [
1,
2]. Based on the field-measured data, Zheng et al. [
1] used the numerical analysis method to study the influence of partition piles on the soil displacement field in the active area outside the excavation. As shown in
Figure 1a, The numerical analysis results show that after the completion of the excavation, the soil in the red area will slide into the excavation significantly because of the excavation unloading effect.
Figure 1b shows that the partition piles significantly affect the soil displacement in the active area, and have the opposite effect in different areas. This showed that a further study of the deformation control effect of partition piles on deep tunnels is needed.
In recent years, many studies focused on the protective effect of partition piles have been conducted [
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16]. In recent years, numerical simulation is a very common, simple and effective research method in geotechnical engineering. Marta [
4] studied the influence of the deep excavation of an office building in Prague on underground tunnels. Two-dimensional finite element model simulated the whole process of tunnel construction, and calculated the deformation and stress changes caused by excavation. The comparison shows that the calculation results of the tunnel deformation by the numerical simulation are in good agreement with the actual measurement results. Based on a deep and large excavation protected by the partition piles, Zheng et al. [
6] have studied the protection mechanism of partition piles by the finite element method. The results showed that when the traction effect was large, the partition pile would increase the horizontal displacement of the soil and the tunnel within a certain depth.
The centrifuge model test is a real and reliable research method in geotechnical engineering, so it is necessary to use the centrifuge model test to study the protective effect of partition piles. Some scholars have carried out some tests [
17,
18,
19,
20]. Ng et al. [
17] designed and carried out two three-dimensional centrifuge tests in dry sand. It was found that the influence range of excavation on the vertical displacement of the tunnel was about 1.2 times the length of the excavation. Xu et al. [
18] carried out a series of centrifuge model tests. It was found that the influence range of the tunnel heave was about 2.5 times the width of the excavation from the excavation boundary. Chen et al. [
20] carried out the three-dimensional centrifuge model test and numerical analysis of the influence of excavation on the side tunnel in dry sand and analyzed the protective effect of partition wall. The test results show that the partition wall can reduce the surface settlement, the change of soil pressure outside the tunnel, the horizontal displacement of the retaining wall and the bending moment of the tunnel.
The above studies show that partition piles can play a positive role in tunnel deformation protection, but systematic research about the protection effect of partition piles can not be founded. The finite element method was used in most of the research, and the centrifuge model test was relatively limited. This is mainly because compared with the conventional model test, the actual operation of the centrifuge model test is more difficult. It is necessary to further carry out centrifuge model tests on the deformation of adjacent tunnels protected by isolated piles and optimize the test design from the technical aspect.
This paper mainly focused on the centrifuge model test design and optimization of the deformation of the adjacent existing tunnel caused by the excavation in terms of the partition pile under the dry sand. At the same time, the numerical simulation of the centrifuge model test was conducted. The numerical simulation results show that the actual tunnel displacement of the model is too small, and the measured value in the model test should reach the accuracy of 0.01 mm, which leads to the difficulty of actual measurement. Therefore, a series of optimization designs were carried out in the technical aspects, which made the experiments more real and effective and provided a reference for the subsequent centrifuge model tests.
3. Design of the Centrifuge Model Test
3.1. Layout of Test Materials and Components
In this test, Japanese Toyoura sand will be selected to prepare the foundation with the “artificial sand rain” method. The falling distance is 0.5 m and the dry density of the soil is about 1.50 g/cm3. Since the test is a two-dimensional excavation, the partition pile, tunnel and retaining wall in the model will be pre-embedded in the soil layer and arranged along the whole width of the model box in the process of preparing the foundation soil.
Magnesium-aluminum alloy will be selected to simulate the retaining wall, partition pile and tunnel. The thickness of the retaining wall and the tunnel lining is 500 mm and 200 mm. The test will use the partition wall to simulate the partition piles. The equivalent bending stiffness method is used to calculate the thickness of the partition wall as 300 mm. The model box profile and other dimensions are shown in
Figure 9. In addition, the similar relationship of the parameters in the centrifuge model test is shown in
Table 3. EVA sponge tape is pasted on both sides of the retaining wall, partition wall and tunnel, and Vaseline is smeared on the sponge to ensure that the edge of the structure can slide freely in the vertical direction without the leakage of sand.
3.2. Sensor Arrangement
This test mainly will focus on the tunnel displacement caused by the excavation under the protection of partition piles. The measurement targets mainly include vertical displacement of the tunnel, horizontal displacement of the tunnel, horizontal displacement of the retaining wall, tunnel internal force and settlement behind the retaining wall.
The digital image correlation (DIC) technology will be used to measure the horizontal displacement, vertical displacement of the tunnel and horizontal displacement of the retaining wall. The left and right arch waist, vault and arch bottom of the tunnel will be taken as the measuring points. The depths of the retaining wall are 0, 80, 160, 240 and 320 mm as the measuring point of the horizontal displacement. The internal force and strain of the tunnel will be measured by arranging a half-bridge strain gauge, which is used to measure the circumferential bending moment of the tunnel monitoring section. In the test, the laser displacement sensor will be used to measure the surface settlement. The white plastic sheet with grooves is fixed at the measured point on the surface to reflect the laser. As shown in
Figure 8, five monitoring points will be set up behind the retaining wall corresponding to the middle section of the longitudinal tunnel, which are, respectively, located at 0.1 H, 0.5 H, 1.0 H, 1.5 H and 2.0 H (H is the excavation depth).
3.3. Test Procedure
In this test, the excavation will be simulated by discharging heavy liquid. The heavy liquid uses a ZnCl2 solution with the same soil density. In each group of tests, the heavy liquid is guaranteed to be used repeatedly, which well reflects the concept of sustainable development of green environmental protection.
The steps are as follows: (1) the preparation of the soil foundation and the arrangement of components and the installation of sensors, (2) excavate the soil to the predetermined bottom and lay silica gel bags in the excavation and pour in the heavy liquid, (3) start the centrifuge and turn it to 50× g. After the data in each sensor is stable, discharge the heavy liquid (the discharge speed of the heavy liquid is equivalent to the excavation speed in the prototype). (4) After the excavation is completed and the data in each sensor is stable, gather the data and then stop.
4. Optimization of the Test Design
4.1. Selection of Test Materials
In the former research, the materials used in the model test were basically aluminum, steel, plastic and so on [
15,
16,
17,
18]. Considering the particularity of the centrifuge model test, the size and geometric characteristics of components should be chosen following a certain similarity ratio. When the elastic modulus of the model material is not consistent with that of the actual material, the size in the model was usually obtained by equivalent conversion. The converted size of the material is too small to make the model components many times. In this centrifuge model test, magnesium-aluminum alloy was used to make components innovatively, as shown in
Figure 10.
The experimental results showed that the alloy has good machinability and is not easy to rust. Through the static test, it was found that magnesium-aluminum alloy belongs to the metal material without an obvious yield platform. There was no obvious boundary between the linear elastic stage and the strengthening stage, as shown in
Figure 5. By selecting the straight line in the first half of the curve for data processing, the elastic modulus of the material was about E = 40 GPa. After processing, the elastic modulus could basically reach 30 GPa, which was close to that of the common reinforced concrete. The tensile strength was about 185 MPa and the elongation rate was about 3–10%. According to the result of the performance testing, the alloy had good physical and mechanical properties. Using magnesium-aluminum alloy as material to make structural members had the following two advantages: first, the elastic modulus was close to concrete, and the size of model members could be calculated directly according to the geometric similarity ratio; second, it could simulate the actual project more accurately and make the test more authentic.
4.2. Excavation Simulation
In this centrifuge model test, the method of discharging heavy liquid in the centrifuge process will be used to simulate excavation. This method has the advantages of simulating excavation under operating conditions and controllable excavation speed. Before the test, the sand has been prepared by artificial sand rain method, and the dry density of soil was about 1.50 g/cm3. The heavy liquid will adopt a ZnCl2 solution with the same density as the dry sand soil. The solution with the target density will be prepared by solid ZnCl2 according to the ratio. After calculation, the molar density needed to prepare the solution is 3.67 mol/L, and the test requires 2.56 L of solution in total.
The heavy liquid discharge system used in the test mainly includes a discharge rubber tube, heavy liquid collection box, solenoid valve, flow control valve, silicone bag and so on. The solenoid valve will be responsible for remotely controlling the opening and closing of heavy liquid discharge. The flow control valve will be used to control the discharge speed and to simulate the real excavation speed of the actual project. The silicone bag will be installed at the bottom of the excavation and fixed on the inner surface of the excavation. The bottom of the bag will be connected to the discharge rubber tube. The side of the model box will be perforated and the discharge tube will be led into the heavy liquid collection box. In order to prevent the discharge of liquid from being unsmooth in the process of centrifugation, the height of the heavy liquid collection box should be reduced as much as possible. Finally, the PP (Polypropylene- a plastic material resistant to chemical corrosion) box of 80 mm × 400 mm × 100 mm will be selected as the heavy liquid collection box, and the bottom of the box and the base of the centrifuge will be fixed by bolts. The specific installation location is shown in
Figure 11.
4.3. Optimization of Measurement
According to the preliminary numerical simulation, the horizontal and vertical displacements of the tunnel are roughly 3–7 mm. If converted to the model test, the tunnel displacement is roughly 0.06–0.14 mm, and the measurement accuracy needs to reach 0.01 mm. The general displacement measurement sensor can not achieve such high accuracy. In addition, the settlement of the pre-test also shows that the model displacement of soil and structure is very small.
The laser displacement sensor has high accuracy and is responsible for measuring the surface settlement behind the wall in this test. However, it is difficult to install in tunnel measurement. Therefore, in this test, digital image correlation (DIC) technology will be selected to measure tunnel displacement. It needs to mark points on the surface of the tunnel and the retaining wall in the glass side of the model box. When shooting, it is necessary to ensure that the camera is stable and perpendicular to the glass side of the model box to guarantee the accuracy of the later data. Finally, the image can be imported into the MATLAB program for analysis. The principle of DIC measurement technology is to import the images taken during the experiment into the MATLAB program. The pixel coordinates on the marked points in the initial image are identified by a special algorithm, and then the changed coordinates are automatically identified in other images. Thus the displacement value of the marked points in the experiment is obtained. In theory, the higher the resolution of the picture, the higher the displacement accuracy of the DIC calculation.
In order to meet the requirement of high accuracy, a high pixel camera was prepared to photograph the test process (
Figure 12). Considering the camera bracket and the size of the model box, this test chose a 48-megapixel Shan Gou A8 camera with a size of 26 mm × 40 mm × 60 mm.
In order to ensure the accuracy of the subsequent test data, the DIC measurement accuracy test was carried out. Firstly, fix the Vernier caliper on the desktop, mark its surface, and fix the camera perpendicular to the Vernier caliper. Secondly, the mobile phone is used for the remote control to shoot the Vernier caliper before and after displacement respectively. Finally, the two photos are imported into the MATLAB program for calculation, and the test photos are shown in
Figure 13.
The calculation result in the DIC program is in pixels. After calculation, the lateral displacement value at the mark point is 1.57 pixels. The actual lateral size of the photo is 273 mm, and the lateral pixel value is 3072. The final calculated displacement value is (273÷3072) × 1.57 = 0.1395 mm. Compared with the 0.14 mm displayed by the Vernier caliper, the error is 0.36%, which can meet the accuracy requirements of the test.