Displacement Characteristics of an Urban Tunnel in Silty Soil by the Shallow Tunnelling Method

3e urban shallow tunnelling process in silty soil is easy to cause large displacement of surface and tunnel. Obviously, if the strata and the tunnel face are not treated by reasonable reinforcement method, instability and collapse phenomenon will be encountered during the tunnel excavation. 3ere are a series of studies on construction methods of shallow tunnels, but these methods have limitations in silty soil. In this study, a comprehensive construction plan of the urban shallow tunnel in silty soil was proposed and applied to a case study in Fuzhou, Fujian Province in South China. 3e in situ monitoring tests and numerical simulation were employed to address displacement characteristics of surface and tunnel. Results indicated that the urban shallow tunnelling process could achieve good effect by dewatering of silty soil, reinforcing surface by vertical jet grouting piles, and advanced small pipes and circumferential grouting in the tunnel face; surface settlement during dewatering process accounted for about 30% of total surface settlement in silty soil; the excavation of the top heading, the middle, and lower benches had great effect on displacement of surface and tunnel for three-bench seven-step excavation method in silty soil; surface settlement troughs in silty soil were deeper and wider; lock-feet bolts had good effect on restricting horizontal convergence; and ratio of total crown settlement and total horizontal convergence was in range of 1.43∼1.59 when b/h was 0.88 in silty soil. 3e construction plan proposed in this paper is helpful for further study of shallow tunnel tunnelling process in silty soil.


Introduction
In recent years, there are more and more urban shallow tunnels in silty soil. Silty soil has characteristics of high compression, high sensitivity, high creep, high water content, low strength, and so on [1,2]. Many problems need to be considered in construction process of the urban shallow tunnels in silty soil, for example, treatment of groundwater, selection of reinforcement schemes, and controlling of surface settlement [3]. Meanwhile, silty soil is widely distributed in mainland and coastal areas of the world, especially in China, such as Guangzhou, Nanjing, Shanghai, Hangzhou, and Fuzhou [4][5][6]. e support system of the urban shallow tunnels in silty soil in recent years in China is shown in Table 1. At present, strata reinforcement can also be carried out by cement mixing piles and jet grouting piles; most of advanced supports in the tunnel face are advanced large pipe-sheds, advanced small pipes, and the tunnel face grouting. To improve stability of silty soil shallow tunnels passing through, it is necessary to seek more effective methods to reduce underwater, strengthen stratum and the tunnel face.
Up to now, many studies about shallow tunnelling process have been conducted. Kirsch [7], Chen et al. [8], Oreste and Dias [9], Zhang et al. [10], Vu et al. [11], and Li et al. [12] adopted numerical simulation and theoretical calculation methods to analyze stability of the excavation face of shallow tunnels in soft soil and sand. Following Peck [13] and Schmidt [14], Wang et al. [15], Pinto and Whittle [16], Dindarloo and Siami-Irdemoosa [17], and Khademian et al. [18] modified Peck formula to calculate surface settlement troughs and predict the maximum surface settlement due to shallow tunnelling process in soft soil. Fang et al. [19], Yang and Huang [20], Ocak [21], and Bian et al. [22] used field tests and theoretical calculation to study large deformation and rock mass pressure in soft rock of shallow tunnels in soft soil. Additionally, Paternesi et al. [23] and Cao et al. [24] employed finite element analysis and field measurement to analyze surface settlement of shallow tunnelling method (STM) for subway station in soft soil. Fetzer et al. [25], Nazari et al. [26], and Bryk [27] carried out laboratory tests and microscopic photographs to analyze the granular characteristics and permeability of silty soil.
According to the previous literature, the explorations on shallow tunnels have concentrated on studying stability of the tunnel face and characteristics of surface settlement and rock mass pressure in soft soil and sand by shield method; there are several researches about STM of subway station in soft soil; there are a few researches about characteristics of silty soil [28]. Meanwhile, most of existing studies are constructed by shield method, and few studies have focused on displacement characteristics of shallow tunnelling process in silty soil by STM. However, STM is often used in the urban shallow tunnels in silty soil, and it is prone to surface cracking and even surface collapse [29], as shown in Figure 1. Hence, this paper retrospects the case study of STM in silty soil of the Hudonglu tunnel in Fuzhou, Fujian Province in South China, compares the in situ monitoring results and numerical results, analyzes displacement characteristics, and summarizes construction method and supporting measures for urban shallow tunnelling process in silty soil.

Presentation.
e Hudonglu tunnel locates in Fuzhou, the provincial capital of Fujian province in South China, as shown in Figure 2. It is the main part of Hudonglu engineering and starts from the Hudong road, ends to the two ring road; the total length of the line is about 1900 m, of which tunnel project is about 1650 m, the tunnel is mainly constructed by open excavation method and STM. e tunnel is arranged with two holes and double lanes, eastwest, width (b) of tunnel clearance is 9.2 m, and height of tunnel clearance is 6.75 m. e left tunnel enters shallow tunnel section at ZK0 + 965 and ends at ZK1 + 350; the right tunnel enters shallow tunnel section at YK1 + 060 and ends at YK1 + 340. e left tunnel ZK1 + 180∼ZK1 + 350 enters into silty soil, the right tunnel YK1 + 175∼YK1 + 340 enters into silty soil, and longitudinal section of the left tunnel is shown in Figure 3. According to the geological survey report of the earlier stage of Hudonglu tunnel, the geological condition and depth of the right tunnel are similar to that of the left tunnel. e tunnel depth (h) of ZK1 + 180∼ZK1 + 350 is 7.9 m∼11.2 m. e geological characteristics of strata where tunnel passing through are presented as follows: (1) Filling soil (Q4 ml ): the soil in this layer is dark gray with loose structure and contains a small amount of gravel, organic matter, and biological debris, and particle size is generally 2.00∼4.00 mm. is layer is widely distributed, mainly on surface, with a thickness of 0.60∼2.5 m. (2) Clay (Q4 al+pl ): the soil in this layer is light yellow, plasticity, slightly wet, and contains more clay and silty particles, loose to slightly dense, with a thickness of 0.7∼8.1 m. (3) Strong weathered granite (c 5 2(3)c ): the soil in this layer is brownish yellow, massive, and short-columnar and has been completely weathered into gravel clay with poor cohesion and disintegration, with a thickness of 8.2∼11.7 m. (4) Medium weathered granite (c 5 2(3)c ): the soil in this layer is generally lighter in color and often rendered with dark brown ferromanganese. Most minerals are severely weathered and metamorphosed. Some have become clay minerals. Original rock structure is clear, and weathering degree of rock mass is often uneven, with a thickness of 6.3∼15.8 m. (5) Silty soil (Q4 dl+el ): the soil in this layer is yellow, wet, and contains clay and organic matter, with average water content 56% and the void ratio 1.54, thixotropy

Construction
Scheme. STM is used in Hudonglu tunnel in silty soil. Special construction scheme is adopted for Hudonglu tunnel in silty soil. Well-point dewatering is carried out on surface. Vertical jet grouting piles are used to reinforce strata. Advanced small pipes grouting and circumferential grouting are applied to the tunnel face. reebench seven-step excavation method (TSEM) is used for tunnel excavation.

Well-Point
Dewatering. Well-point dewatering is a method of lowering groundwater level by setting dewatering wells first and then by pumping equipment, and it is suitable for dewatering in shallow and medium strata within the range of 6∼15 m [30,31]. Well-point dewatering on surface is arranged as in Figure 4, and target dewatering layer is clay and silty soil [32][33][34]. Dewatering wells enter weak weathered granite 2 m [35][36][37][38]. A total of 51 dewatering wells are set up, and the minimum distance between well and tunnel should be determined by actual construction. e minimum distance between two wells should not be less than 14 m; monitoring dewatering wells are set up with 12 ports; and tunnel direction is arranged with distance to 280 m. According to actual situation of in situ monitoring of water level, increase number of dewatering wells if necessary. Because permeability coefficient of clay and silty soil in the site is small, when the effect of dewatering is bad, negative pressure dewatering should be carried out. Construction of Advances in Civil Engineering negative pressure dewatering wells before tunnel excavation can be adjusted according to the site to avoid buildings on surface.

Surface Reinforcement.
Vertical jet grouting piles are selected to reinforce surface of Hudonglu tunnel in silty soil. rough reinforcement (Φ � 0.8 m, L � 3 m) of soil on the top of tunnel, crown of tunnel forms a closed shell structure, which can reduce the effect of excavation deformation around tunnel, as shown in Figure 5(a). Both sides of the tunnel use vertical jet grouting piles (Φ � 0.8 m, L � 8 m) as curtain for cutting off water, and plane layout of vertical jet grouting piles is arranged as the plum flowers [39][40][41], as shown in Figure 5(b). ere are two rows of piles of curtain for cutting off water on both sides of tunnel; there are 13 rows of piles on the top of the tunnel. Construction technical parameters of vertical jet grouting piles are as follows: they adopt 42.5 R composite Portland cement, water-cement ratio is 0.8∼1, air pressure is 0.7 MPa, slurry pressure is 25∼30 MPa, lifting speed is 10∼15 cm/ min, rotation speed is 10 r/min, slurry flow rate is 100 L/ min, length of jet lap is 10∼15 cm, cement dosage is initially 300∼400 kg/m, and cement proportion is controlled 25%∼ 30% [42][43][44].

Tunnel Support
Parameters. Support structure of Hudonglu tunnel in silty soil is shown in Figure 6. Firstly, a single-layer advanced small pipes are used in the tunnel face, diameter of steel pipe is 42 mm, extension length is 2 m, the lapping length of each cycle should be controlled within 1 m, and the scope is 150°of crown of tunnel; at the same time, circumferential grouting in the tunnel face is set with extension length of 4 m. Secondly, steel frame and double-deck steel meshes are constructed; lock-feet bolts (steel, Φ � 42 mm, length � 4.5 m, longitudinal spacing � 1 m) are installed at arch feet to guarantee the stability of the tunnel face, and then 30 cm fiber concrete is sprayed. e deformation allowance between primary support and secondary lining of the tunnel is 5 cm. Finally, 50 cm C35 waterproof concrete is set as secondary lining.

Construction Sequences of TSEM.
TSEM is one of the sequential excavation method (SEM), based on the bench cut method, and reserving core soil and staggering excavation from the left to right, which is conducive to stability of the tunnel face [45,46]. Longitudinal perspective of TSEM is shown in Figure 7 [47][48][49][50].
(1) Firstly, the top heading (Stage 1 in Figure 7) is excavated after construction of advanced support, core    soil is reserved, length of core soil is not less than 3 m and width is more than 1/2 width of tunnel excavation, and distance between the top of core soil and the top of the tunnel face is 1.8 m. Excavation distance of the upper bench is 4∼6 m, ratio of rise to span is 0.3, excavation step is 0.5∼0.6 m, 3∼5 cm concrete is sprayed immediately after excavation of the top heading, then steel frame and steel meshes are set up simultaneously. At the height of 30 cm above arch foot of steel frame, lock-feet bolts are set at the edge of 30°on the side of steel frame, and lockfeet bolts are welded firmly into steel frame, then concrete is sprayed to the design thickness (30 cm).
(2) Secondly, the left and right middle benches (Stages 2 and 3 in Figure 7) are, respectively, excavated: excavation step is 1.0∼1.2 m, excavation distance is 8∼10 m, and the left and right middle bench is stagger 2∼3 m. e next construction procedure is the same as the first stage. (3) irdly, the left and right lower benches (Stages 4 and 5 in Figure 7) are, respectively, excavated: excavation step is 1.0∼1.2 m, excavation distance is 5∼8 m, and the left and right lower bench is stagger   Advances in Civil Engineering 5 2∼3 m. e next construction procedure is the same as the first stage. (4) Fourthly, core soil (Stages 6-1, 6-2, and 6-3 in Figure 7) is, respectively, excavated. Excavation step is consistent with cyclic step of each bench. (5) Fifthly, tunnel bottom (Stage 7) is excavated and construction of inverted arch is carried out immediately, and inverted arch is used for each construction of 4∼6 m. (6) Lastly, secondary lining is constructed according to excavation distance.

Numerical Simulation
According to geological survey report of the Hudonglu tunnel, the average tunnel depth (h) between ZK1 + 180 and ZK1 + 350 is 10.5 m, and scope of impact radius of the tunnel excavation is considered.

Calculation of Composite Elastic Modulus of Composite
Soil. Composite elastic modulus is used to replace the elastic modulus of soil strengthened by vertical jet grouting piles above 3 m of the tunnel. Composite compression modulus of general composite soil can be calculated according to formulas (1) and (2) [51][52][53][54]: where E sp is the composite compression modulus of general composite soil, m is the area replacement rate, in equilateral triangle piles, d s � 1.05 s, s is the pile spacing, E p is the compression modulus of vertical jet grouting piles, and E s is the compression modulus of soil between piles. Foundation soil generally produces elastic-plastic deformation when subjected to loads. Composite elastic modulus reflects modulus of elastic deformation and partial plastic deformation, which is consistent with deformation of foundation soil in general [55][56][57][58]. Composite elastic modulus of composite soil can be determined by the following formula : where E 0 is the composite elastic modulus of composite soil and μ is Poisson's ratio of composite soil (here μ is 0.3).

Conversion of Elastic Modulus of Primary Support.
According to principle of simplified compressive rigidity, elastic modulus of medium-sized steel frame with primary support is converted into elastic modulus of shotcrete to simplify the calculation [59][60][61]. Calculation method is shown in the following formula: where E is the elastic modulus of converted shotcrete, E 0 is the elastic modulus of shotcrete, S g is the sectional area of steel arch, E g is the elastic modulus of section steel, and S c is the cross-sectional area of shotcrete.
Composite elastic modulus of composite soil above 3 m of the tunnel can be obtained from formulas (1)-(3), the transformation elastic modulus of primary support can be obtained from formula (4), and the rest parameters are based on geological survey reports and some similar studies, as shown in Table 2 [44,46].

Establishment of Numerical Model. Numerical model is based on the MIDAS-GTS (Geotechnical and Tunnel
Analysis System) software which is generally used to conduct geotechnical analysis. e layout and sizes of vertical jet grouting piles are referred to Figure 5. e depth of rock mass, the sizes of tunnel, and tunnel support parameters (including advanced support, primary support, and secondary lining) are referred to Figure 6. e construction sequences of TSEM are referred to Figure 7. In addition, the rock mass, composite soil is simulated by solid elements; lock-feet bolts and advanced small pipes are simulated by truss elements; primary support and secondary lining are simulated by shell elements; and vertical jet grouting piles are simulated by beam elements.
In the width direction, calculation range is 120 m, and height of whole model is 47.5 m, and whole numerical model is regular rectangle of 120 m × 50 m × 47.5 m, as shown in Figure 8(a). e support structure of the tunnel is shown in Figure 8(b). e first four construction consequences of STM are shown in Figure 9. According to the actual working conditions, the left tunnel is excavated first, and the right tunnel is excavated later; meanwhile, installing of lock-feet bolts and construction of primary support (Converted shotcrete) are treated as independent construction consequences. Simultaneously, the present three-dimensional model was based on the following assumptions: (1) All materials in this model are homogeneous, continuous, and isotropous. (1) e maximum surface settlement is 31.2 mm. When the top heading is excavated, surface settlement change is 4.6 mm. When the middle benches are excavated, surface settlement change is 3.8 mm and 3.2 mm, respectively. When the lower benches are excavated, surface settlement change is 3.5 mm and 2.8 mm, respectively. When primary support is constructed, surface settlement rate decreases rapidly. After filling of inverted arch, surface settlement nearly remains unchanged. (2) Behind the tunnel face, the surface settlement remains almost unchanged; that is, tunnel excavation has little effect in this region. Within 2b in front of the tunnel face, tunnel excavation has great influence on surface settlement. Within 3b∼3.5b in front of the tunnel face, tunnel excavation causes a small amount of uplift on surface. (3) After middle benches excavation, the maximum surface settlement is 17.2 mm; after lower benches excavation, the maximum surface settlement is 26.8 mm; after filling of inverted arch, the maximum surface settlement is 30.9 mm. e maximum surface settlement occurs above tunnel central line. ere is great effect on surface settlement within 1.5b on both sides of tunnel central line, and there is slight uplift between 3b and 3.5b on both sides of tunnel central line. (4) When double-lane tunnel is excavated, surface settlement shows a double-peak curve, but the curve is not completely symmetrical. After the right tunnel excavation, surface settlement above the left tunnel is slightly affected, which is about 0.2∼0.3 mm. Finally, the maximum surface settlement above the left tunnel is 31.2 mm and that of the right tunnel is 29.8 mm.

Crown Settlement.
Crown settlement characteristics are shown in Figure 11. According to the numerical simulation, crown settlement characteristics are presented as follows:

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(1) e maximum crown settlement of tunnel is 74.8 mm.
When the top heading is excavated, the crown settlement change is 14.2 mm. Crown settlement change is 1.4 mm after installing of lock-feet bolts. When the middle benches are excavated, crown settlement change is 11.3 mm and 10.2 mm, respectively. Crown settlement change is 1.2 mm and 0.9 mm after installing of lock-feet bolts. When the lower benches are excavated, crown settlement change is 9.8 mm and 7.6 mm, respectively. Crown settlement change is 0.7 mm and 0.4 mm after installing of lock-feet bolts. (2) Tunnel excavation has a significant effect on crown settlement within 2b in front of the tunnel face. Tunnel excavation has slight effect on crown settlement beyond 3b from the tunnel face. Crown settlement behind the tunnel face is still effected by excavation of the tunnel face, and the effect is about 2∼3 m. e farther away depth from crown of the tunnel, the smaller crown settlement of tunnel and approximate to the linear reduction. (3) It can be seen that after reinforcement of vertical jet grouting piles on the surface, advanced small pipes and circumferential grouting in the tunnel face, the crown settlement and surface settlement converges gradually after the tunnel excavation, and surface and tunnel section reaches stable state. Construction scheme in this paper makes urban shallow tunnelling process in silty soil achieve good effect.

Monitoring Plan.
Monitoring points between ZK1 + 180 and ZK1 + 350 of Hudonglu tunnel in silty soil were shown in Figure 12, and longitudinal distance between two adjacent measuring sections is 5 m. To investigate the behavior of surface settlement, nine measuring points were laid on surface above typical cross sections of tunnel. One measuring point at crown and three measuring lines (arch shoulder, arch waist, and arch foot) at typical cross sections of tunnel were employed to address tunnel displacement [47]. According to the actual situation, there are 30 monitoring sections on surface and 32 monitoring sections of the tunnel. Table 3 shows statistics of total surface settlement of 30 sections; it could be seen that there were three sections which reached the maximum total surface settlement and that was in range of 7.0∼7.5 cm, and there were most sections in which total surface settlement was in range of 6.0∼6.5 cm. is phenomenon showed that surface settlement of tunnel construction in silty soil was relatively large and could not be neglected [47,49], and typical monitoring section was ZK1 + 220 and ZK1 + 337, among which ZK1 + 220 had the maximum total surface settlement, and ZK1 + 337 had the most representative time curves of surface settlement and surface settlement troughs. Measured time curves of surface settlement of typical section (ZK1 + 220 and ZK1 + 337) are shown in Figure 13. According to the monitoring data, surface settlement characteristics were presented as follows:

Surface Settlement.
(1) Time curves of surface settlement was divided into five periods: period 1, before dewatering, surface settlement changed slowly, about 2.5∼3.6 mm; period 2, in dewatering process, surface settlement changed quickly, about 20.9∼22.8 mm, the ratio was   Tables 4 and 5 show statistics of total tunnel displacement of 32 sections, it could be seen that there were two sections which reached the biggest total crown settlement and that was in range of 9.0∼10.0 cm, and there were most sections in which total crown settlement was in the range of 7.0∼8.0 cm. Meanwhile, most sections in which total horizontal convergence (arch shoulder) was in the range of 5.0∼6.0 cm, and the maximum total horizontal convergence was in the range of 6.0∼7.0 cm. Typical monitoring section was ZK1 + 260 and ZK1 + 310, among which ZK1 + 310 had the maximum total crown settlement, and ZK1 + 260 had the most representative time curves of crown settlement and horizontal convergence. Time curves of tunnel displacement of typical cross sections (ZK1 + 260 and ZK1 + 310) for different construction stages are shown in Figures 14 and 15. According to the monitoring data, tunnel displacement characteristics were presented as follows: (1) After excavation of the top heading, the crown settlement rate of two sections reached the maximum, about 8.9 mm/d and 9.4 mm/d. After excavation of the lower benches, the crown settlement (2) e total horizontal convergence of arch shoulder is bigger than arch waist and arch feet. e maximum total crown settlement reached 82.8 mm∼99.8 mm.  Figure 12: Layout of measuring points [47]. e maximum total horizontal convergence reached 57.9 mm∼63.3 mm. is showed tunnel excavation had greater effect on crown settlement than horizontal convergence in silty soil section.
(3) Due to the lower strength and higher compression of silty soil, the maximum horizontal convergence rate of arch foot was relatively large. After installing lockfeet bolts, horizontal convergence decreased rapidly, which showed that lock-feet bolts had good effect on reducing horizontal convergence for shallow tunnel in silty soil. (4) Crown settlement rate and horizontal convergence rate of tunnel decreased with monitoring time. e ratio of the maximum total crown settlement and total horizontal convergence was in the range 1.43∼1.59 in silty soil section. ere was still some tunnel displacement after excavation of inverted arch, about 1.1∼3.5 mm.

Comparison of Numerical Simulation and In Situ Monitoring Results.
Comparison of numerical simulation and in situ monitoring results are shown in Figure 16, and the measured surface settlement is induced by tunnel excavation. Monitoring results of surface settlement and tunnel displacement are slightly larger than those of numerical results, but the laws they reveal are similar. Surface settlement concentrates within 1.5b of the tunnel central line, and the measured maximum surface settlement is in the range of 3.0∼4.0 cm. Crown settlement is mainly caused by benches excavation of tunnel; the measured maximum crown settlement is in the range of 8∼11 cm. Both displacement of surface and tunnel are at a controllable and secure level.

Discussions
According to a large number of monitoring data of surface settlement during metro construction, Peck [13] used the "Gauss distribution curve" to fit it and proposed the Peck formula, as shown in formulas (5) and (6). Later, a large number of studies have revised the Peck formula, and it is found that the Peck formula is widely used in predicting surface settlement during metro and shallow tunnel construction [14][15][16][17][18]. In this paper, we make statistics on construction methods and support methods of urban shallow tunnels in silty soil in recent years in China, as shown in Table 1; meanwhile, we calculate the Peck formula parameters of surface settlement troughs of these tunnels, as shown in Table 6. After reasonable strata dewatering, reinforcement of surface, and the tunnel face, the construction of urban shallow tunnel in silty soil by STM can achieve the similar ground loss and small surface settlement as that by shield method, and good construction results have been achieved: where S max is the maximum surface settlement, V 1 is the ground loss, i is the trough width parameter, h is the tunnel depth, r is the radius of shield or equivalent radius of tunnel, and φ is the internal friction angle of soil. Generally, the urban shallow tunnel construction in silty soil is an urgent problem to be solved in tunnel construction in coastal areas. Many studies on tunnel construction in silty soil have proved the difficulty and complexity of the problem [23,24]. However, there are few studies on urban shallow tunnel construction by STM. In this study, we put forward a comprehensive construction plan for urban shallow tunnel in silty soil by STM and investigate the effect by numerical simulation and in situ monitoring. We found that change regulation of displacement of surface and tunnel, and both of them met requirements of safety and environment during tunnel construction. Our work extended construction method of urban shallow tunnel in silty soil, and it can provide reference for similar projects in the future.

Conclusions
is article combines an urban shallow tunnel in silty soil by STM and compares the in situ monitoring results and the FEM results. It can provide new insights into urban shallow tunnel construction in silty soil. e main conclusions are as follows: (1) For urban shallow tunnel passing through silty soil, a comprehensive construction plan of the well-point dewatering, surface reinforcement with vertical jet grouting piles, advance small pipes and circumferential grouting in the tunnel face, and TSEM can achieve better results, and displacement of surface and tunnel are both controllable.
(2) Numerical results show that surface settlement finally forms settlement troughs; tunnel excavation has obvious influence on surface settlement within 2b in front of the tunnel face; tunnel excavation has  show that silty soil has good elastic characteristics. Surface settlement caused by dewatering process in silty soil accounts for about 30% of total surface settlement in silty soil. e urban shallow tunnel in silty soil by STM could achieve similar construction effect with shield tunnel, including ground loss and surface settlement.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.