Model Test on the Influence of Surcharge, Unloading and Excavation of Soft Clay Soils on Shield Tunnels

Abstract

In view of the influence of symmetrical surcharge, unloading and excavation of soft clay soils on shield tunnels, the surface settlement, tunnel settlement, vertical additional earth pressure and tunnel additional confining pressure are measured by an indoor model test. The changes of confining pressure, tunnel settlement, surface settlement and vertical earth pressure caused by surcharge, unloading and excavation are studied and analyzed. The results show that, in soft clay, surcharge will cause tunnel settlement, and unloading and excavation will generally cause tunnel uplift. There is hysteresis in surface settlement and tunnel settlement above the shield tunnel caused by surcharge and unloading; the change of additional confining pressure of the shield tunnel caused by surcharge is mainly concentrated in the three directions of 3, 5 and 7 of the radar chart. These points belong to weak points. Unloading and excavation can effectively reduce the additional confining pressure of the tunnel in these three directions; excavation will cause the increase of vertical cumulative additional earth pressure and tunnel confining pressure in local directions. The influence range of surcharge in soft clay is wider than that in sand, but the depth is relatively shallow.

1. Introduction

Due to the shortage of urban land, there are usually many construction projects near existing shield tunnels, and surcharge, unloading and excavation often occur above existing shield tunnels [1,2,3]. Surcharge, unloading and excavation inevitably have adverse effects on the existing shield tunnel and surrounding soil, including tunnel segment cracking, joint opening, uneven settlement of soil, etc., thus affecting the service life of a subway. Therefore, it is of great significance to study the influence of surcharge, unloading and excavation on existing shield tunnels.

In view of this engineering problem, local and international scholars have done a lot of research. The research methods of the influence of surcharge, unloading and excavation on shield tunnel of adjacent subway are mainly divided into field measurement method [3,4,5,6], theoretical analysis method [7,8,9,10,11,12,13,14,15,16,17], numerical analysis method [18,19,20,21,22] and model test method [23,24,25,26,27,28,29], among which the cost of model test method is controllable and the obtained test results are relatively reliable, which is a better method to study the influence of surcharge, unloading and excavation on the shield tunnel of an adjacent subway. Xian Liu et al. [23] conducted a full-scale experimental study on the bearing capacity of the shield tunnel structure under overload conditions. Qing Wu et al. [24] studied the influence of buried depth and surcharge position on an existing shield tunnel structure through an indoor model test. Minggao Zhang et al. [25] carried out an indoor model test on the vertical earth pressure of a tunnel caused by surface overload and established a finite element model. Huangsong Pan et al. [26], in order to study the distribution and development law of pressure arch in unlined loess tunnel, carried out the model test of straight wall dome tunnel excavation under different loads, and the numerical simulation method was used for comparative analysis. Xiaojian Chen [27] used the model test to study the longitudinal differential settlement of the shield tunnel in soft and hard uneven stratum. Dawei Huang et al. [28] measured tunnel deformation, earth pressure and soil settlement under surface overload through the model test in order to clarify the impact of surface overload on existing shield tunnels in soft and hard strata. Fayun Liang et al. [29] studied the influence of local surcharge in soft and hard strata on the transverse deformation of tunnel by means of an indoor model test. In the research of the model test method, the existing tunnel model generally adopts a simple model, and there are few refined tunnel models assembled by bolts between segments; most of the soil is sand, and there is no research on pure clay. Therefore, it is necessary to use the indoor model test method for further research.

In this test, a large proportion of indoor model tests were carried out with soft clay, and the effects of surcharge, unloading and excavation on existing shield tunnels and surrounding soil were studied. The surface settlement, tunnel settlement, additional strain, vertical additional earth pressure and additional confining pressure of tunnels were monitored. Meanwhile, the comparative tests under surcharge conditions were carried out with sand, which has reference significance for engineering practice.

2. Indoor Model Test

2.1. Test Device

In this test, a total of 23 segments of a shield tunnel were selected as the research object. The outer diameter of the shield tunnel is 6.2 m, the ring width of the segment is 1.2 m, and the segment thickness is 0.348 m, with a total length of 27.6 m. The geometric similarity ratio of indoor model test is 1:15.5.

A model box with the size of 1.8 m × 1.8 m × 1.5 m is used in the test, as shown in Figure 1. A model tunnel with a diameter of D = 0.4 m and a length of about 1.78 m is adopted. The model tunnel is made of plexiglass with E = 2.06 GPa. Each ring tunnel consists of five segments with 67.5° and one segment with 22.5°, the segment thickness is 22 mm. In order to truly simulate the connection between the actual tunnel segment and segment ring, the model tunnel segment and segment ring are connected by bolts, as shown in Figure 2. The similarity constants of the indoor model test are shown in

Table 1. Similarity constants of the indoor model test.

Physical Quantity	Similarity Relation	Similarity Constant	Physical Quantity	Similarity Relation	Similarity Constant
Geometric dimensions	E (Basic quantity)	15.5	Pressure	$C_q$	16.75
Elastic modulus	D (Basic quantity)	16.75	Axial force	$C_N=C_E\cdot C_L^2$	4024
Strain	$C_{\epsilon }$	1	Bending stiffness	$C_{EI}=C_L^4$	57,720
Stress	$C_{\sigma }$	16.75	Axial stiffness	$C_{EA}=C_L^3$	3724
Displacement	$C_{\delta }$	15.5	Shear stiffness	$C_{GA}=C_L^3$	3724

, the geometric parameters and material properties of tunnel model are shown in

Table 2. Geometric parameters and material characteristics of the tunnel model.

	Outer Diameter of Segment (m)	Segment Inner Diameter (m)	Segment Thickness (m)	Ring Width (m)	Elastic Modulus of Segment (MPa)	Segment Poisson’s Ratio (-)
Prototype	6.200	5.504	0.348	1.200	34,500	0.2
Model	0.400	0.356	0.022	0.077	2060	0.3

, and the geometric parameters and material properties of tunnel connecting bolts are shown in

Table 3. Geometric parameters and material properties of the tunnel connecting bolts.

	Bolt Length (m)	Diameter of Bolt (m)	Number of Bolts	Elastic Modulus of Bolt (MPa)	Poisson’s Ratio of Bolts (-)
Prototype	0.400	0.030	17	200,000	0.30
Model	0.027	0.002	6	33,800	0.32

[30].

2.2. Test Soil Sample

The soft clay used in the test is taken from a construction site in Hangzhou, and the soil sample is gray, which is a common soft clay in Hangzhou. The physical indexes of soft clay are shown in

Table 4. Physical indicators of soft clay.

Moisture Content	Unit Weight of Soil	Porosity Ratio	Specific Weight of Soil	Liquid Limit	Plastic Limit	Cohesive Forces	Angle of Internal Friction
ω (%)	γ (kN/m3)	e0 (%)	Gs	ωl (%)	ωp (%)	c (kPa)	φ (°)
45.0	17.2	1.258	2.73	43.2	23.6	13.7	9.4

.

2.3. Test Conditions

According to the influence of surcharge, unloading and excavation of soft clay soils on shield tunnels, three groups of tests were carried out. See

Table 5. Test conditions.

Test Number	Working Condition	Test Soil
1	Surcharge	Soft clay
2	Unloading	Soft clay
3	Excavation	Soft clay

for three groups of test conditions.

In case 1 and case 2, five measuring points (measuring point 1 to measuring point 5) are set up to measure the surface settlement, and five groups of dial indicators are placed on the soil surface at a horizontal distance of 0.3 m from the center of the model tunnel every 0.2 m longitudinally. In order to measure the vertical settlement of the tunnel, five measuring points (vertical settlement measuring points 1–5) are set up, and five displacement meters are placed in the middle of the tunnel from the middle to one side, with intervals of 0.154 m, 0.154 m, 0.231 m and 0.231 m, respectively. In order to measure the vertical additional earth pressure of tunnel top soil and tunnel bottom soil, four measuring points (measuring points 1–4) are set respectively, four earth pressure boxes are respectively placed in the top soil and bottom soil in the center of the tunnel, and the spacing between adjacent earth pressure boxes is 0.2 m (the earth pressure boxes in the top soil and bottom soil in the center are shared with the earth pressure boxes in confining pressure section 5). In order to measure the additional confining pressure of the tunnel, confining pressure section 5 is set in the center of the tunnel, confining pressure section 3 is set at 0.308 m away from confining pressure section 5, eight measuring points are set at confining pressure section 5 and 3, respectively, and eight earth pressure boxes are evenly arranged along the tunnel section. The layout plan and section of surcharge, unloading and excavation measuring points in soft clay are shown in Figure 3 and Figure 4.

In working condition 3, the tunnel settlement, the vertical additional earth pressure of the top soil of the tunnel and the vertical additional earth pressure of the bottom soil of the tunnel, and the additional confining pressure of the tunnel at the confining pressure section 5 are measured, and the arrangement positions of these measuring points are the same as those in working condition 1. A schematic diagram of the excavation is shown in Figure 5.

2.4. Test Steps

(1) Put soft clay soil samples in 0.1 m/layer in the model box. When the thickness of the soil samples reaches 0.2 m, put the model tunnels and earth pressure boxes with measuring parts. Continue to put the soil samples in 0.1 m/layer until the soil layer height reaches 1.2 m, then cover the soil surface with plastic wrap and let stand for 7 days. Place a dial indicator for measuring surface settlement on the surface; a bearing plate with a size of 0.4 m × 0.4 m is placed in the soil sample directly above the middle of the model tunnel, and 34.4 kg, 68.8 kg, 103.2 kg and 137.6 kg are loaded on the bearing plate with weights step by step, corresponding to the surcharge loads of 36 kPa, 72 kPa, 108 kPa and 144 kPa, respectively, and each loading lasts for 1 h.

(2) After all loading is completed, unload from 137.6 kg to 0 kg in turn, and each stage of unloading lasts for 1 h.

(3) After the surcharge and unloading tests are completed, cover the surface with plastic wrap and let stand for 5 days, then excavate the soil layer by layer along the longitudinal direction of the tunnel. Support it in the form of steel plate plus support while excavating, with 0.1 m as a layer and 5 layers excavated, and let stand for 1 h after each layer is excavated.

3. Analysis of Test Results

3.1. Analysis of Surface Settlement

Figure 6 and Figure 7 show, in the process of loading and unloading, the accumulated surface settlement measured by each measuring point at the horizontal distance of 0.3 m of the tunnel (Figure 6), and the accumulated surface settlement of each measuring point under different loads (Figure 7), where a negative value indicates surface settlement and a positive value indicates surface uplift.

As can be seen from Figure 6 and Figure 7:

(1)

The reason why the soil at measuring point 5 keeps settling all the time is that measuring point 5 is closest to the surcharge place, and settlement occurs at the surcharge place. The settlement of measuring point 5 increases gradually in the early stage, and reaches the maximum value when unloading to 72 kPa, with the maximum settlement of 0.405 mm.

(2)

The soil at measuring point 4 keeps rising all the time, which is related to the settlement at the piled load, which causes the soil to squeeze around. The maximum rise of measuring point 4 occurs when it is loaded to 36 kPa, and the rise amount is 0.360 mm, and then it gradually decreases, then slowly increases when it is unloaded to 72 kPa.

(3)

The soil at measuring points 1, 2 and 3 heaved to a certain extent at the beginning of loading, but maintained settlement during subsequent loading and unloading. The maximum settlement of measuring point 3 is 0.060 mm when unloading to 72 kPa. The maximum settlement of measuring point 2 is 0.135 mm when unloading to 0 kPa. The maximum settlement of measuring point 1 is 0.070 mm when unloading to 36 kPa.

(4)

According to the ground settlement at each measuring point, there is obvious lag in the ground settlement above the shield tunnel caused by surcharge load in soft clay geology. This is consistent with the result that Gao Jinglian [31] found that the strain development of soft clay under load lags behind the stress development by studying the hysteresis loop in the stress–strain relationship of soft clay.

3.2. Settlement Analysis of Tunnel Bottom

3.2.1. Settlement Analysis of Tunnel Bottom during Surcharge and Unloading

Shown in Figure 8 is, in soft clay, the accumulated settlement measured at each settlement measuring point at the bottom of the tunnel when the pile is unloaded. In the figure, a negative value indicates settlement of the tunnel bottom, while a positive value indicates uplift of the tunnel bottom.

As can be seen from Figure 8:

(1)

At the beginning of loading, the settlement measuring points at the bottom of the tunnel all have a slight uplift. With the increase of surcharge load, each measuring point starts to settle, and the settlement increases gradually. The maximum value occurs when unloading to 108 kPa or 72 kPa, and then the settlement of each measuring point at the bottom of the tunnel decreases. After unloading to 36 kPa, the settlement of each measuring point at the bottom of the tunnel starts to increase slowly again. It shows that in soft clay geology, the settlement of the shield tunnel caused by surcharge load has a certain lag and will be repeated.

(2)

Except for measuring point 2, the cumulative settlement and settlement trend of other measuring points are basically the same; because the water content of the soft clay used in the test is greater than the liquid limit, the soft clay is in a flowing state, and the overall stiffness of the model tunnel is relatively large, resulting in the uniform settlement of the tunnel under load.

(3)

During the period from loading to 108 kPa to unloading to 36 kPa, the tunnel settlement changes greatly, which may be due to the existence of a soft clay with relatively low moisture content and relatively high hardness near measuring point 2, which produces a certain stress concentration effect in the process of loading.

3.2.2. Analysis of Tunnel Bottom Settlement during Excavation

Figure 9 shows, during the excavation, the accumulated settlement of each measuring point at the bottom of the tunnel.

As can be seen from Figure 9:

(1)

The accumulated settlement of each measuring point at the bottom of the tunnel is positive, which indicates that the bottom of the tunnel continues to uplift during the whole excavation process, which is related to the extrusion uplift of the tunnel caused by the soil piles on both sides being higher than the middle during the excavation process.

(2)

During the excavation process, the uplift at the bottom of the tunnel at measuring point 3 always remains the largest. The reason for this phenomenon may be that measuring point 1 and measuring point 2 are closest to the original surcharge position, and the nearby soil is consolidated under the surcharge paction; measuring point 4 and measuring point 5 are close to the box, with boundary effect. On the whole, there is little difference in the amount of uplift generated by each measuring point during excavation, which is consistent with the measured results of Fan et al. [6].

(3)

When the excavation reaches 0.3 m, the uplift of each measuring point at the bottom of the tunnel reaches the maximum value, and then the uplift of each point decreases gradually, which indicates that the effect of supporting with steel plate and support in the early stage of excavation is not as good as that in the later stage.

3.3. Analysis of Vertical Accumulated Additional Earth Pressure of the Top Soil of Tunnel

3.3.1. Analysis of the Vertical Cumulative Additional Earth Pressure of the Top Soil of the Tunnel during Surcharge and Unloading

Figure 10 and Figure 11 show, in the process of surcharge and unloading, the vertical accumulated additional earth pressure of tunnel top soil under different loads (Figure 10), and the vertical accumulated additional earth pressure of each measuring point of tunnel top soil (Figure 11).

As can be seen from Figure 10 and Figure 11:

(1)

During each stage of loading, the maximum vertical additional earth pressure of the top soil of the tunnel occurs at 0.2 m away from the top of the tunnel. This result is inconsistent with the result of the maximum vertical cumulative additional earth pressure in the center of the tunnel in the model test of Zhang Minggao et al. [25], which may be related to Zhang Minggao et al. using rubber particles instead of soft clay in the model test without considering the fluidity of soft clay. During each stage of unloading, the minimum vertical earth pressure of the soil on the top layer of the tunnel occurs at 0.2 m from the top of the tunnel, except when it is unloaded to 108 kPa.

(2)

In the whole test process, the maximum vertical additional earth pressure of the top soil of the tunnel occurs when measuring point 2 is loaded to 144 kPa, and the maximum value is 0.576 kPa. The minimum value of the vertical additional earth pressure of the top soil of the tunnel is −0.849 kPa when measuring point 2 is unloaded to 0 kPa.

(3)

The vertical additional earth pressure of measuring points 1, 2 and 4 respectively increases with the increase of surcharge, and decreases with the decrease of unloading vertical additional earth pressure. During unloading, negative vertical additional earth pressure appears at measuring point 1 and measuring point 2, which is due to the rebound of soil mass with the decrease of surcharge. The earth pressure box at measuring point 3 is not damaged, and the reason why the vertical additional earth pressure is almost always negative during the test needs to be further studied.

3.3.2. Analysis of the Vertical Cumulative Additional Earth Pressure of the Top Soil of the Tunnel during Excavation

Figure 12 and Figure 13 show, in the process of excavation, the vertical accumulated additional earth pressure of tunnel top soil at different excavation depths (Figure 12), and the vertical accumulated additional earth pressure of each measuring point of tunnel top soil during excavation (Figure 13).

As can be seen from Figure 12 and Figure 13:

(1)

During excavation, the vertical accumulated additional earth pressure at measuring points 3 and 4 hardly changes, because measuring point 4 is outside the excavation area, while measuring point 3 is in the excavation area, but it is very close to the trenchless area, which has obvious boundary effect.

(2)

With the increase of excavation depth at measuring point 2, the earth pressure increases negatively. With the increase of excavation depth at measuring point 1, the earth pressure increases positively. The main reason is that, during excavation, the excavated soil on both sides squeezes towards the excavation area, resulting in negative earth pressure at measuring point 2 and positive earth pressure at measuring point 1, as shown in Figure 14.

3.4. Analysis of Vertical Cumulative Additional Earth Pressure of Tunnel Bottom Soil

3.4.1. Analysis of Vertical Cumulative Additional Earth Pressure of Tunnel Bottom Soil during Surcharge and Unloading

Figure 15 and Figure 16 show, in the process of surcharge and unloading, the vertical accumulated additional earth pressure of tunnel bottom soil (Figure 15), and the vertical accumulated additional earth pressure of each measuring point of tunnel bottom soil (Figure 16). The hole position of strain gauge connected to measuring point 2 of tunnel bottom soil is damaged, which leads to the invalidation of the data measured at measuring point 2.

As can be seen from Figure 15 and Figure 16:

(1)

With the increase of heap load, the accumulated additional vertical earth pressure at each measuring point of tunnel bottom soil gradually increases, and with the decrease of heap load, the accumulated additional vertical earth pressure at each point of tunnel bottom soil gradually decreases. Under the same heap load, the vertical accumulated additional earth pressure of tunnel bottom soil under unloading is obviously larger than that under loading, which indicates that unloading can reduce the vertical accumulated additional earth pressure, but it cannot be completely restored.

(2)

When the distance is close to the tunnel, the distance has a great influence on the vertical accumulated additional earth pressure of the bottom soil of the tunnel, and with the increase of the distance, the influence of the distance on the vertical accumulated additional earth pressure of the bottom soil of the tunnel obviously decreases.

(3)

Compared with the vertical accumulated additional earth pressure of the top soil of the tunnel during surcharge and unloading, the vertical accumulated additional earth pressure of the lower soil of the tunnel is larger and the change trend is more regular, which shows that the soil compression trend of the bottom soil of the tunnel is relatively simple.

3.4.2. Analysis of Vertical Cumulative Additional Earth Pressure of Tunnel Bottom Soil during Excavation

Figure 17 and Figure 18 show, in the process of excavation, the vertical accumulated additional earth pressure of tunnel bottom soil at different excavation depths (Figure 17), and the vertical accumulated additional earth pressure of each measuring point of tunnel bottom soil during excavation (Figure 18).

As can be seen from Figure 17 and Figure 18:

(1)

During excavation, the vertical accumulated additional earth pressure at measuring point 4 hardly changes, because measuring point 4 is outside the excavation area.

(2)

With the increase of excavation depth at measuring points 1, 2 and 3, the earth pressure increases negatively, mainly because, during excavation, the excavated soil on both sides squeezes towards the excavation area, and negative earth pressure is generated at measuring points 1, 2 and 3, as shown in Figure 14.

3.5. Analysis of Accumulated Additional Confining Pressure of Tunnel

3.5.1. Analysis of Accumulated Additional Confining Pressure of Tunnel during Surcharge and Unloading

Figure 19 and Figure 20 show, in the process of loading and unloading, the radar diagram of accumulated additional confining pressure changes at each measuring point of tunnel section 5 (Figure 19) and section 3 (Figure 20).

As can be seen from Figure 19 and Figure 20:

(1)

In soft clay geology, the maximum points of additional confining pressure of section 5 and section 3 are mainly concentrated in three directions of the radar chart: 3, 5 and 7; the minimum confining pressure points are mainly concentrated in directions 4 and 6 of the radar chart.

(2)

The confining pressure of section 5 and section 3 is basically left-right symmetrical, but not completely symmetrical, mainly because the weights placement is slightly eccentric during loading, which leads to slight eccentric load, and the soft clay is very sensitive.

(3)

The confining pressure in direction 1 of the radar chart is much smaller than the additional confining pressure in direction 5, mainly because the measuring point in direction 1 is close to the pile load, and the soft clay will obviously squeeze around during the loading process, resulting in relatively small additional confining pressure at the measuring point in direction 1.

(4)

Although unloading can effectively reduce the accumulated additional confining pressure at each measuring point, under the same surcharge load, the accumulated additional confining pressure at each measuring point during unloading is basically greater than that during loading, and unloading cannot restore the accumulated additional confining pressure at each measuring point to its initial value.

3.5.2. Analysis of Accumulated Additional Confining Pressure of Some Tunnels in Excavation Test

Figure 21 and Figure 22 are respectively the radar diagram of cumulative additional confining pressure change of tunnel section 5 and the cumulative additional confining pressure change diagram of each measuring point during excavation.

It can be seen from Figure 21 and Figure 22 that with the increase of excavation depth, the cumulative additional confining pressure changes most obviously in directions 3, 5 and 7 of the radar chart, and increases greatly towards a negative value. The cumulative additional confining pressure in directions 1, 2, 8 as well as 4 and 6 has little change, in which directions 1, 2 and 8 slowly increase towards a positive value, and directions 4 and 6 slowly increase towards a negative value. Therefore, in soft clay, excavation can effectively reduce the additional confining pressure in directions 3, 5 and 7 on the radar map.

3.6. Analysis of the Influence of Soft Clay and Sand Surcharge on the Shield Tunnel

In order to better study the difference between soft clay and sand when they are piled up on the shield tunnel, the results are compared with the previous sand pile-up test results [32]. The sand used in the test is sea sand after being dried in the sun and screened out impurities. The physical indexes of sand are shown in

Table 6. Physical indexes of sand.

Density	Moisture Content	Angle of Internal Friction	Cohesive Forces	Modulus of Compressibility
ρ (g/cm3)	ω (%)	φ (°)	c (kPa)	ES (MPa)
1.495	0.23	29	0	2.89

.

When the sand is stacked, the tunnel settlement and confining pressure are measured, and the additional confining pressure of the tunnel at section 5 is measured. The layout position of these measuring points is the same as that of condition 1. The specific test steps are as follows: Put sandy soil samples in 0.1 m/layer and layer them in the model box. When the thickness of the soil sample reaches 0.2 m, put them into the model tunnel equipped with measuring parts, and then add the sand of the remaining height evenly two times to make the soil layer reach 1.2 m and let stand for 1 day. Place 0.4 m in the soil sample directly above the middle of the model tunnel. For the 0.4 m pressure bearing plate, 34.4 kg, 68.8 kg, 103.2 kg and 137.6 kg shall be loaded on the pressure bearing plate step by step with weights, corresponding to the surcharge loads of 36 kPa, 72 kPa, 108 kPa and 144 kPa, respectively, and each stage of loading shall last for 1 h.

3.6.1. Comparative Analysis of Tunnel Bottom Settlement

Figure 23 shows, under the same conditions, when the surcharge load reaches 144 kPa, the cumulative settlement of each measuring point at the bottom of the tunnel in sand and soft clay.

It can be seen from Figure 23 that the settlement of the tunnel bottom 0 m and 0.154 m away from the tunnel center under soft clay condition is significantly less than that under sandy soil condition; the settlement of the tunnel bottom 0.308 m, 0.539 m and 0.770 m away from the tunnel center under the soft clay condition, which is significantly greater than that under the sandy soil condition. Because the water content of the soft clay used in the test is greater than the liquid limit, the soft clay is in a flowing state and has high cohesion, while the sandy soil has no cohesion and the soil particles are hard, which is conducive to the longitudinal transmission of force, resulting in the pressure diffusion angle θ of soft clay being obviously larger than that of sandy soil. The stress σ isolines of soft clay and sand are obviously different. The stress σ isolines of soft clay are flat, while the stress σ isolines of sand are slender, which makes the settlement change of each measuring point in soft clay (except abnormal measuring point 2) not particularly obvious, but remarkable. The schematic diagram of pressure diffusion angle θ of sand and soft clay is shown in Figure 24, and that of stress σ isoline of sand and soft clay is shown in Figure 25.

3.6.2. Comparative Analysis of Accumulated Additional Confining Pressure of Tunnel

Figure 26 shows, under the same conditions, when the surcharge load reaches 144 kPa, the change of accumulated additional confining pressure at each measuring point of sand and soft clay tunnels.

As can be seen from Figure 26:

(1)

The accumulated additional confining pressure in each direction on the radar chart under sandy soil condition has a small difference, among which the accumulated additional confining pressure in radar charts 1 and 5 is relatively the largest, which is consistent with the results measured by Liang Fayun et al. [29]. Under the condition of soft clay, the cumulative additional confining pressure of measuring points in each direction varies greatly. Among them, the cumulative additional confining pressure in direction 1 of the radar is significantly less than that in directions 3 and 7 of the radar, which is consistent with the cumulative additional confining pressure measured by Liang Fayun et al. [29] using sawdust soil instead of soft clay to simulate the tunnel passing through soft clay layer. This shows that it is feasible to use sawdust soil instead of soft clay to study the additional confining pressure of tunnel.

(2)

The accumulated additional confining pressure in directions 3, 5 and 7 of the radar chart in soft clay is obviously larger than that in sand. The accumulated additional confining pressure in directions 1, 4 and 6 on the radar chart is obviously less than that at corresponding points in sandy soil, which is probably due to the flowing state of soft clay used in the test. Under the action of surcharge load, soft clay is more easily squeezed, and soft clay in directions 1, 4 and 6 on the radar chart is squeezed to directions 3, 5 and 7, which leads to a big difference between the accumulated additional confining pressure at each measuring point in soft clay and in that of sand.

4. Conclusions

(1)

Under soft clay geological conditions, there is a certain lag between the settlement of the ground above the shield tunnel and the settlement of the shield tunnel caused by surcharge and unloading, and the extreme value occurs in the unloading stage.

(2)

When loading, the pressure diffusion angle θ of soft clay is obviously larger than that of sand. The stress σ isoline of soft clay is flat, while the stress σ isoline of sand is slender. The influence range of loading in soft clay is wider than that of sand, but the depth is relatively shallow.

(3)

In the whole process of surcharge and excavation, the vertical accumulated additional earth pressure in the top soil of the tunnel with a horizontal distance of 0.2 m from the top of tunnel changes the most, the vertical accumulated additional earth pressure in the bottom soil of the tunnel with a horizontal distance of 0 m from the bottom of tunnel changes the most, and the soil squeezing trend of the bottom soil of tunnel is simpler than that of the bottom soil of tunnel.

(4)

In the case of soft clay soil, the change of additional confining pressure of the shield tunnel caused by surcharge is mainly concentrated in the three directions of 3, 5 and 7 of the radar chart. These points belong to weak points, which need to be paid special attention in practical engineering. Unloading and excavation can effectively reduce the additional confining pressure of the tunnel in these three directions. However, excavation will cause an increase of vertical cumulative additional earth pressure and tunnel confining pressure at local locations, which should be paid attention to during construction.

(5)

When carrying out model tests under various working conditions under soft clay geological conditions, not only factors such as gravity and deformation modulus, but also the influence of fluidity of soft clay should be considered.