1. Introduction
Due to the influence and limitations of geographical location and surrounding environment, problems such as poor geological conditions, shallow burial depth, tight engineering land, complex surrounding environment, and strict control standards are common [
1,
2]. To overcome these challenges, double-arch tunnels are widely used in urban tunnels because of their advantages of convenient portal selection, smooth route, land conservation, beautiful section shape, and relatively small impact on the surrounding environment [
3,
4]. However, urban double-arch tunnels also have limitations, such as long construction period, high cost, various procedures, complex supporting structure, more intersection of excavation and supporting construction surfaces, and difficult quality control [
5,
6,
7]. Additionally, with the construction of special underground projects involving environmental protection and surface cultural relics, an increasing number of ultra-shallow tunnels have been constructed. Hence, during the excavation and support process of an urban ultra-shallow buried double-arch tunnel with complex geological conditions, the stability of tunnel surrounding rock and the deformation control of surrounding underground pipelines have become issues that need special attention [
8,
9]. According to modern tunnel theory, the surrounding rock is the main body to bear the unloading stress caused by excavation. In the process of tunnel construction, effective technical means are necessary to adopt to maximize the self-supporting capacity of the surrounding rock. Such means improve the stability of the surrounding rock and tunnel structure, which can not only improve the safety of a project during construction, but also improves the quality and service life of the tunnel structure during operation [
10,
11]. To strengthen the bearing capacity of the rock mass and avoid possible risks in construction, various methods and measures are often adopted in the project, such as advance support, small pipe reinforcement, and grouting reinforcement to reinforce the surrounding rock [
12,
13,
14,
15,
16,
17]. Compared with other treatment methods, grouting reinforcement technology has the advantages of high efficiency, convenience, low cost, wide treatment range, and good reinforcement effect, especially in rock and soil with high fragmentation degree [
18,
19,
20]. The principle of grouting reinforcement technology is to use cement slurry, water glass, cement mortar, chemical grout, and other materials to fill the voids of broken rock masses effectively to improve the compactness, integrity, tension, compression, shear, and creep of the rock mass, so as to maintain the stability of rock mass [
21]. The strength and stability of grouted surrounding rock formed after grouting reinforcement of broken rock mass is not only an important index to evaluate the effect of grouting reinforcement, but also a key parameter to determine the deformation and the self-supporting capacity of the surrounding rock.
Extensive research has been carried out on grouting and reinforcement of surrounding rocks and remarkable results have been achieved. Zong et al. [
22,
23] found that the stiffness, roughness, peak shear strength, residual strength, and shear strength parameters of structural surface after grouting reinforcement were significantly improved. Niu et al. [
14] studied the grouting diffusion law, splitting grouting reinforcement mechanisms and grouting reinforcement effects using 3D simulated grouting test system. The uniaxial compressive strength of filled soil after grouting increased by 186%, the permeability coefficient decreased by 47 times, and the cohesion and internal friction angle increased by 45.3% and 44.9%, respectively. Evdokimov et al. [
24] compared and analyzed the shear strength parameters of fractured rock before and after grouting consolidation, and considered that grouting significantly increased the shear strength of fractured rock. Moosavi et al. [
25] found that when the water–cement ratio was 0.4 and 0.5, respectively, the peak and residual shear strength of grouted surrounding rock increased with the increasing normal stress. Taking the grouting reinforcement project of a large section cavern surrounding rock as the background, Cheng et al. [
26] found a linear relationship between grouting volume and grouting pressure, hydraulic conductivity, and grouting time, and a square root relationship between slurry diffusion radius and grouting pressure, hydraulic conductivity, and grouting time through numerical simulations. On the basis of the tunnel crossing railway stations, Lu et al. [
27] used ANSYS software to simulate and analyze the grouting reinforcement effect of tunnel surrounding rock according to the equivalent continuous medium theory. The results show that grouting reinforcement has a good effect on reducing surface settlement and horizontal convergence. Through grouting experiments on fully weathered granite, Yang et al. [
28] studied the diffusion law of cement rule of different viscosity and the influence of grout viscosity on the reinforcement effect, and found that an increase of slurry viscosity strengthens compressive strength and shear strength. Liu et al. [
29] found that the tensile strength of rock cracks increases with increasing grouting viscosity, but the overall tensile strength was low. Ma et al. [
30] found that broken fine sandstone reinforced by compaction grouting slurry has obvious ductility, strong plasticity, and deformation resistance, and can remain stable in a large deformation range. Li et al. [
31] took the compressive strength, deformation modulus and hydraulic conductivity of grouting reinforced rock mass as the evaluation index of grouting reinforcement effect, tested the grouting plus solid effect under different water–cement ratio and curing time, and found that slurry water–cement ratio has a significant impact on grouting reinforcement effect. Wang et al. [
32] found that grouting reinforcement can effectively suppress the stress concentration at the crack tip, improve the integrity of the specimen, avoid the formation of stress redistribution, and enhance the integrity of the specimen.
Generally, the research on grouting reinforcement of surrounding rock has mostly studied the strength comparison and deformation analysis before and after grouting. However, only few studies have been conducted on the strength of grouted surrounding rock and how to determine grouting parameters of rock mass in engineering. With the land part of Haicang undersea tunnel as a background, laboratory experiments, literature analysis, and numerical simulation were performed to study the method and rationality of determining the grouting parameters of the surrounding rock of the ultra-shallow buried double-arch tunnel. The results can be referred by similar projects.
2. Overview of the Engineering
The double-arch tunnel of Haicang tunnel is located in Huli District, Xiamen City, Fujian Province, China. The tunnel is near to the Shigushan interchange. The entrance of the tunnel is located at the #4 working shaft and the exit is located at the #5 working shaft. The starting and ending mileage is BK 17 + 805–BK 18 + 045, with a total length of 240 m. The double-arch tunnel passes through Xinghu Road, a two-way six-lane urban expressway (
Figure 1). Xinghu Road has dense traffic and heavy vehicles. Various municipal pipelines such as water supply, drainage, and power cables are densely covered under the road. They are all buried within 2.5 m below the ground and about 1.8–3.0 m above the top of the tunnel. Thus, determining the impact of tunnel excavation on nearby pipelines is necessary.
The minimum buried depth of the main tunnel is about 5 m and the maximum buried depth is about 15.45 m. The designed single tunnel has a headroom width of 14.60 m and a headroom height of 10.15 m. The full cross-sectional area of the tunnel is 119.27 m
2.
Figure 2 shows the section of the double-arch tunnel. The tunnel is excavated by using the three heading excavation method. The side heading tunnel has a cross-sectional area of 26.7 m
2, an excavation height of 6.44 m, and an excavation width of 4.48 m. The excavation section area of the middle heading tunnel is 49.85 m
2, the excavation height is 8.1 m, and the excavation width is 6.2 m. The sequence of the three heading excavation method of the double-arch tunnel is to excavate the middle heading tunnel first, then the left heading tunnel, and finally the right heading tunnel. The construction step between heading tunnels is 20 m. The step method is adopted for the heading excavation method and the upper step is excavated 3 m to 5 m ahead of the lower step.
5. Conclusions
To study the reasonable grouting thickness and water–cement ratio of the surrounding rock of the double arch tunnel in the Haicang Tunnel, the ground settlement, deformation of the vault, and adjacent pipeline under different grouting parameters were studied through laboratory tests, literature analysis, and numerical simulation. The following conclusions were obtained.
(1) The viscosity of cement slurry decreased with the increase of water–cement ratio. The initial setting time of cement slurry increased linearly with the increasing water–cement ratio. The slurry bleeding rate gradually increased with time and tended to be stable after reaching a certain value, and the greater the water–cement ratio, the greater the slurry bleeding rate. The strength of the sample decreased linearly with the increase of the water cement ratio.
(2) Frictional angle φ and cohesive force c can be determined by water–cement ratio of slurry and UCS of rock mass before grouting.
(3) With the increase of reinforcement layer thickness and the decrease of cement slurry water–cement ratio, the ground settlement, vault displacement, plastic zone area, and pipeline deformation continued to decrease, but the reduction range increased first and then decreased.
(4) When the grouting reinforcement layer thickness h = 1.5 m and the water–cement ratio η = 0.85, the tunnel grouting reinforcement effect was best and more economical.