1. Introduction
After years of transportation construction, China’s total highway mileage had reached 5,198,100 km by 2020, including 21,316 road tunnels with a linear length of 21.9993 million meters. There are 2249 more tunnels, with an increase of 3.0327 million linear meters. Among them, 1394 extra-long tunnels have a total length of 6,235,500 linear meters, and 5541 long tunnels have a total length of 9.6332 million linear meters [
1]. Tunnel construction is gradually developing in remote areas with complex topography and geology. During the construction, high-pressure and water-rich tunnels are the most common and prominent sites of geological disasters. Sometimes, they cause huge economic losses and even casualties [
2,
3]. Water control often follows the guidance of “water plugging coordinated with limited drainage” [
4]. For the safety of people’s lives and property, curtain grouting, radial advance grouting, and other grouting techniques are recommended as the common solutions to these challenges. These techniques have become the main construction measures to deal with the problem of high water pressure in tunnels based on the construction concept of NATM [
5,
6,
7,
8,
9,
10,
11,
12,
13]. The tunnel’s surrounding rocks, reinforced by advanced grouting, can bear part of the water pressure and block the passage of groundwater flowing into the tunnel. However, regarding both curtain grouting and radial grouting, the influence of grouting scope on the construction of water-rich tunnels has not been specifically discussed. Technicians often use experience and judgment in the design, and the determination of grouting parameters has become a major task. Due to the thick anti-water-pressure lining, the construction can be inconvenient and difficult, and the tunnel cannot bear water pressure in time. Based on the engineering practice of double-layer initial support to control the large deformation of the tunnel [
14,
15,
16,
17,
18,
19,
20], a double-layer initial support (two-layer shotcrete and steel arch combined structure) is proposed to bear the high-water pressure, reduce the thickness of the secondary lining, and enhance the waterproof effect. However, previous designs adopted a single measure of grouting, without considering the damage and stress of the initial support. The research shows that the rich water in the later tunnel still has a great impact on the initial support. Therefore, the scheme of double-layer initial support design plus curtain grouting is proposed to support the surrounding rock area with high water pressure.
In previous tunnel construction cases, curtain grouting was used for the construction of water-rich tunnels. Although curtain grouting initially alleviated the risk of water inrush and gushing of the tunnel, the large seepage water pressure in the construction acted on the initial support, so engineers often used strong initial support, and later also used a thick secondary lining structure to control deformation and bear water pressure, which increased the construction investment to a certain extent. Referring to the deformation control technology of double-layer initial support used in the construction of soft rock tunnels, the authors adopts a strategy to bear the external load of the surrounding rock step by step and make full use of the support system formed by the surrounding rock and lining. The first layer of initial support bears the main load, and the second layer of initial support carries out reinforcement and offers a safety guarantee, so as to reduce the secondary lining structure, effectively speed up the project’s progress, and reduce investments. Therefore, based on the previous experience of water-rich tunnel treatment, this paper creatively puts forward the structural type of “curtain grouting + double-layer initial support” to provide the stability of the anti-water-pressure lining and improve the efficiency of water-rich tunnel construction. To obtain the effect of tunnel radial grouting and curtain grouting based on double-layer initial support, the finite difference method is adopted in this paper to analyze the radial grouting and curtain grouting technologies. These technologies are used in the Yongfutun tunnel of the Guilin–Liuzhou expressway. This paper aims to obtain the stress variation law of the surrounding rock deformation, shotcrete, and secondary lining under the operating conditions of different grouting ranges. The conditions can be with or without curtain grouting when the anti-seepage grade is P8 (according to 10.2.3 of “Specifications for Design of Highway Tunnels, Section 1 Civil Engineering” [
21], the impermeability grade of concrete should not be less than P8; P8 is used for fortification in this study). The research results can provide a reference for the grouting and lining support design of tunnels with high water pressure.
In general, this article summarizes the main construction measures of water-rich tunnels used in the past, and, combined with the large deformation control technology of soft rock, the structural type of “curtain grouting + double-layer initial support” is proposed to provide the stability of the anti-water-pressure lining and improve the efficiency of water-rich tunnel construction.
1.1. Tunnel Overview
The left line and the right line of the Yongfutun tunnel are located in Siding Town, Rong’an County, Liuzhou City, Guangxi. Their lengths are 5640 m and 5647 m, respectively. Their longitudinal slopes are −2.37% one-way slopes. As a separated extra-long tunnel, the Yongfutun tunnel zone belongs to the geomorphic area of tectonic dissolution with a peak-cluster depression and valleys, with a maximum buried depth of approximately 301 m. According to the drilling and geological mapping results, at the tunnel site, with a relatively thin overburden, the lithology of the underlying bedrock is dolomitic limestone of upper Liujiang formation (D3) and middle Donggangling formation (D2D) of the Devonian system. The dominating tunnel surrounding rocks are of Grade III and Grade IV. The unfavorable geology in the tunnel area is mainly karst. Several sinkholes were found in the tunnel during the survey, while geophysical exploration showed that the tunnel may have karst fissures, karst caves, and underground rivers, etc.
The water inflow section is located at the tunnel exit. The rock mass joints and fissures around the tunnel site are developed. There are two groups of main joint fissures at the exit. The occurrence of L1 is 250° ∠ 86°, closed, with an extension length of 2–3 m and a density of 1–2 pieces/m; L2 occurrence is 320° ∠ 80°, closed, with an extension length of 3–4 m and a density of 2–3 strips/m. Dissolution depression L42 is found at 175 m to the right of K66+450 and dissolution depression L43 is found at 210 m to the left of ZK66+643.
The geostructure of the tunnel site is located in the first Indosinian substructure (Devonian system). There is no tertiary stratum in the area. According to the development of quaternary river terraces and karst caves, the Himalayan movement involves mainly upward movement, which is manifested in terraces and karst cave layers of different heights at all levels. The underground water in the tunnel site area is mainly supplied by atmospheric rainfall and discharged in vein and linear form to the low-lying depression along with the pores, bedrock fissures, and dissolution fissures. The amount of karst water is controlled by the degree of karst development and supply source, and the seasonality is obvious. In particular, the surface flow formed in the rainy season easily infiltrates rapidly along the dissolution fissure and water drop tunnel, and the groundwater flow increases sharply, which has an important impact on tunnel construction.
On 17 June 2019, a mud outburst and water inrush disaster occurred at the K66+490 palm surface on the right line of the tunnel. This disaster was caused by a hidden cavern triggered by blasting construction. As of 30 June 2019, the cumulative amount of water and mud inrush reached 10,000 cubic meters (see
Figure 1 and
Figure 2).
1.2. Treatment Scheme Design
To ensure the construction progress and safety, the standard section lining was adjusted to a water-pressure-resistant lining. To optimize the thickness of the lining, a double-layer initial support scheme was designed (see
Figure 3). Two rows of Φ42 mm grouting with a small conduit were used as advance support, being 4.5 m in length. The extrapolation angles of the first and the second rows were 10° and 20°, respectively; the systematic bolts adopted Φ42 grouting small conduits, being 6 m in length. The spacing of the bolts was 50 × 50 mm, and they were in a quincunx arrangement; the initial support was a combined structure of C25 shotcrete (28 cm) + reinforcement mesh (Φ8, 20 × 20 cm) + I-beam steel frame (i22b, 50 cm/bay); the secondary lining was C40 reinforced concrete.