Pretreatment for Tunnel Karst Cave during Excavation: A Case Study of Guangxi, China

Hunan Provincial Key Laboratory of Geotechnical Engineering for Stability Control and Health Monitoring, Hunan University of Science and Technology, Hunan, Xiangtan 411201, China School of Civil Engineering, Hunan University of Science and Technology, Hunan, Xiangtan 411201, China National Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science & Technology, Changsha 410114, Hunan, China


Introduction
Karst landscapes are formed as a result of the dissolution of soluble rock-particularly limestone. ey have a significant degree of unpredictability and heterogeneity, by which various hazards may be caused [1][2][3][4][5][6]. It is understood that karst caverns are mainly filled with groundwater and filler [7]. During tunnel construction, water inrush, mud inrush, and collapse may occur due to karst caverns [8][9][10][11][12][13]. Hence, it is important to prepare worksites with inhomogeneous karst cavern treatment prior to tunnel construction. By this, safety and mitigation of hazards will be guaranteed during construction. erefore, it is essential to study karst geology and take effective countermeasures to mitigate geohazards during deep excavations in karst regions with caverns [14][15][16].
Karst is widely distributed throughout southern China [14]. With the rapid development of infrastructure engineering in China-particularly in the tunnel and metro systems [17][18][19]-it is ineluctable that karst caverns will be encountered during construction in karst regions. Karst caverns can be discriminated into two types, according to the internal filler content (with or without water), or three types, according to the cavern position (at the tunnel vault, arch waist, or bottom). Besides these characteristics, high permeability, variable shapes, and collapse are features of tunnel caverns in Guangxi Province.
China's tunnel engineering is in a period of rapid development. Not only has tunnel construction technology seen many innovative developments, but also numerous construction project problems and natural obstacles have been effectively dealt with in karst areas. e impact of karst caverns on tunnels is immediate. For example, in the Yanglin Tunnel in Jiangsu Province, excessive water and mud inrush occurred during construction and caused large settlement [20]. More than 505 karst caverns were predicted in a case of karst regions in Guangdong Province. Finding a way to address these caverns became the most important consideration during construction. Today, the most widely used methods include the clearance-filling and beam-bridge methods, dynamic compaction, jet grouting, and sleevevalve-pipe grouting [3,4,21]. However, these methods have defects in construction and cannot be universally applied to tunnel caverns [14]. As such, specific treatment methods for specific karst caves are needed.
In this study, a karst tunnel was investigated and reinforced by grouting. It was known that the tunnel construction route encountered caverns and hazard mitigation needed to be performed quickly. e objectives of this paper are as follows: (i) to describe the general situation of the tunnels on the Hechi-Baise (Hebai) Expressway in Guangxi Province, as shown in Figure 1, (ii) to discuss potential geohazards and pretreatment techniques for karst caverns during tunnel construction in karst regions, and (iii) to introduce a karst cavern treatment technique for the specific position of the tunnel. is study provides feasible suggestions and concrete construction methods for similar projects and karst cave treatments in the future.

Construction Site Description
ere are 4 middle and 3 short tunnels on the Hebai Expressway that are run through karst regions of China's Guangxi Province. ere are small clear distance and separated left and right lanes of tunnels. Full-line tunnels use composite lining structures as flexible support systems-including reinforcing steel mesh, shotcrete, and I-shaped or grille steel frames-as initial supports, which allow full play to the bearing capacity of the surrounding rock. A secondary lining is made from molded concrete (or molded reinforced concrete), and a Chemical Bonding and Physical Crosslinking Synergism (CPS) reactive-bonded prepaved high polymer membrane is laid between the initial support and secondary lining as a water-proof layer to prevent the surrounding rock water from seeping into the tunnel. e tunnel area is located in a karst peak-cluster depression geomorphic region without large topographic fluctuations, surface water, or stable groundwater in the tunnel body or above. e proposed tunnel passes through a steep mountain, and its entrance and exit each are located near the foot of the slope on opposite sides of the mountain. e terrain is relatively low, but there is loose-slope cumulose soil. Open-pit excavation is adopted in the tunnel design. At the entrance to the cavern, pipe shed and small pipe pregrouting has been used to reinforce the fractured rock mass. e development of cracks in the rock surrounding the tunnel body and local corrosion phenomenon may cause water dripping or gushing during the tunnel excavation process. e physical parameters of the proposed tunnel are shown in Table 1.

Geological Hazards.
e tunnel area is located in a karst peak-cluster depression geomorphic region without large topographic fluctuations, surface water, or stable groundwater in the tunnel body or above. e proposed tunnel passes through a steep mountain, and its entrance and exit each are located near the foot of the slope on opposite sides of the mountain. e terrain is relatively low, but there is loose-slope cumulose soil. Open-pit excavation is adopted in the tunnel design. At the entrance to the cavern, pipe shed and small pipe pregrouting has been used to reinforce the fractured rock mass. e development of cracks in the rock surrounding the tunnel body and local corrosion phenomenon may cause water dripping or gushing during the tunnel excavation process. e physical parameters of the proposed tunnel are shown in Table 1.
e engineering geology of the Hebai Expressway tunnels is complicated, such that it is difficult to ascertain the development, basic form, scale, and distribution of karst and faults. erefore, many unforeseen geological abnormalities have appeared during construction. Since tunnel excavation began, many hazards have occurred, such as collapse, water and mud inrush, and surface subsidence. internal fillers-particularly water and slurry. Because of being disturbed by tunnel construction, water and slurry may flow into the tunnel through cracks between the tunnel wall and karst cavern. If an effective treatment is not applied to prevent water and slurry from flowing into the tunnel, these cracks may develop into a large hole in the cavern and lead to water and mud inrush. (3) Surface Subsidence. Surface subsidence is a geological hazard that occurs during tunnel excavation around karst caverns. e construction can cause large disturbances, which may cause the soil above to collapse into the karst caverns [22]. is collapse can also induce large settlement and thereby cause surface subsidence. During Hebai tunnel construction, surface subsidence was seen around the tunnel area. As shown in Figure 4, a large pit approximately 2 Advances in Civil Engineering 2.4 m long and 1.1 m wide was observed that could lead to serious casualties and even threaten the safety of surrounding buildings. erefore, it is necessary to take action to prevent surface subsidence.

Construction Emergency Plan.
ese hazards-collapse, mud and water inrush, and surface subsidence-often occur unexpectedly during tunnel construction and have a great impact on construction safety. e new Austrian tunneling method is applied to tunnel excavation. e new Austrian tunneling method must implement a policy of "safety first, prevention first" and formulate corresponding emergency plans.

Construction Scheme under Special Geological
Conditions. Concrete, masonry, or dry masonry is generally used as backfill when a tunnel passes through a waterless karst cavern with a small span and no development. It is unnecessary to set up special drainage measures when a tunnel passes through a karst cavern with low water content. According to the difference of intersection position between karst cavern and tunnel, concrete is used to fill the cavern and protect the steel arch centering. Various measures, such as sandblasting and grouting with reserved grouting holes, can also be used to fill the cavern. e scale of a karst caves is typically large, and there are a large number of unstable fillings. As such, the support of a large pipe shed or pile foundation towing beam is often used to span the cave. Because the proposed tunnel section is located in the karst development area, there is karst cavern growth in the tunnel.    In the construction process, Tunnel Seismic Prediction (TSP) [23][24][25] should be strengthened within 15-30 m in front of the tunnel face such as palm face advance drilling and geological radar. Monitoring and measurement procedures for surrounding rock must be strictly implemented, and the construction waste must be removed in time. Construction should be guided by the monitoring and measuring of the surrounding rock, and the design should be revised promptly when necessary.

Treatment of Small Karst
Caverns. e proposed treatment scheme primarily aims at karst caverns outside the tunnel excavation surface-above the arch, and under the foundation and pavement-at a depth of less than 2.0 m, and those developed at the tunnel sidewalls. In principle, the backfilling method is used to fill the karst caverns.
(1) Cavern Development above the Arch Waist. In this scenario, as shown in Figure 5, C15 concrete is pumped to backfill the karst cavern to avoid excessive local pressure on the composite lining caused by backfilling with concrete. According to the size of the cavern, the bolt spacing was approximately every 1.2 m around the cavern, and each bolt was at least 1.0 m deep into the surrounding rock. After backfilling, shotcrete and steel mesh initial supports are applied. (2) Cavern Developed at the Tunnel Sidewall. In this scenario, as shown in Figure 6, C15 concrete backfill is used at a thickness of at least 1.5 m. A Φ100 highdensity polyethylene (HDPE) double-wall perforated bellow is also set up to connect with the side ditch every 2.0 m. On this basis, the shotcrete and steel mesh initial support can be omitted. (3) Cavern Developed under the Foundation and Pavement. In this scenario, as shown in Figure 7, C15 concrete is used to backfill. If there is filling material, it must first be excavated. A double-wall perforated bellow of Φ100 HDPE is arranged every 2.0 m and interconnected.

Large Karst Cavern above Arch Waist.
is treatment scheme is mainly aimed at karst caverns located above the arch waist or outside the tunnel excavation surface that have a development depth of greater than 2.0 m, and width less than that of the excavation face of the tunnel. 1.2 m. e embedded depth of each bolt should be at least 1.5 m. Subsequently, the initial support-such as concrete and steel mesh-will be settled onto the original structure. Expansion of this tunnel section has karst cavern, and Φ800 steel pipe is preburied at the bottom of the tunnel to connect the original karst cavern waterway. (2) Empty Karst Cavern above Arch Waist. In this scenario, as shown in Figure 9, C25 concrete is pumped to a minimum thickness of 80 cm. e concrete on both sides of the cavern must be embedded at least 50 cm into the rock at a bolt spacing of approximately 1.2 m. e depth of each bolt should be at least 1.5 m. Subsequently, the initial support-such as concrete and steel mesh-will be settled onto the original structure. A double-wall perforated bellow of Φ100 HDPE is embedded in advance.

Large Karst Caverns above Arch Waist and below Base.
is treatment scheme is primarily aimed at karst caverns located above the arch waist and below the base of the tunnel.
e former have features such as a development depth of more than 2.0 m and width less than that of the tunnel excavation surface, while the latter have a development depth of more than 5.0 m and width less than that of the tunnel excavation surface.
(1) Karst Cavern under the Tunnel Base. In this scenario, as shown in Figure 10, such a cavern can be passed over by setting up a reinforced concrete slab bridge.
To ensure stable contact between the bridge slab ends and foundation, the bearing capacity of the base must exceed 1200 kPa.

Optimized Treatment for Karst Caverns in Tunnel
After an on-site investigation and discussion on the location and development of karst caverns, the following opinions have been proposed for the optimized treatment of karst caverns.

General Principle.
e excavation method must be changed to a "three-bench tunneling and reserved core soil method." Such method is necessary to strengthen the supports and form a closed ring as quickly as possible. e length of the upper, middle, and lower steps should be controlled within 3 m, and secondary linings should be installed simultaneously.

Optimized Measure
(1) Change the ickness of the Initial Support and Secondary Lining and Increase the Reserved Deformation. Because the karst cavern starts at the left sidewall, considering the safety of the construction process and minimization of the disturbance to the initial support and surrounding rock, the supporting methods are as follows.
e left side adopts a Φ108 large pipe shed lock foot and grouting plug. e length of the pipe shed is determined according to the actual situation on site. However, it is necessary to ensure that the depth of embedded bedrock is at least 2 m. e right arch foot is sealed with a Φ42 grouting catheter lock foot, and the nozzle is sealed after grouting. By this, the reserved deformation is increased to 20 cm, and the thickness of the second lining is increased to 60 cm (see Figure 12).
(2) Vault Grouting. ree layers of Φ42 advanced small pipe are used to consolidate the top and front surface soil of the solution cavity at the left side (the arc length of the solution cavity section is approximately 7 m). e lengths and elevation angles of the three small catheter layers are 4 m at 0°, 5 m at 15°, and 6 m at 45°( see Figure 13). Any construction technology-such as steel reinforcement mesh, steel arch support, shotcrete, and reserve pump piping-is applied in advance until the initial concrete strength meets the requirements, followed by sand blowing backfill.    Advances in Civil Engineering (5) Surface Collapse Treatment. After the karst section is passed smoothly, and the strength of the second lining reaches 100%, the surface subsidence pit is backfilled with clay to the original ground elevation, and a permanent water interception ditch is set around the original collapsed pit to prevent surface water from infiltrating the tunnel through the pit fissures. After backfilling, vegetation restoration on the surface of the original collapse pit can be performed. For this, Wei et al. [26] suggest that spray-on grass planting be used, and surface subsidence pit is backfilled with clay as shown in Figure 14.

Bolt Calculation of Treating Karst Cave.
For the bolt mentioned in this paper, the spacing and embedment depth need to be calculated. e empirical formula used in this project is as follows: (1) Calculation of Bolt Spacing where N is the spacing of each bolt; F is the safety factor of the project, which is determined according to the engineering specifications; P is the thrust (kN) bearing filler; P u is the ultimate tension load of the bolt, which is determined by experiment. (2) Calculation of Bolt Length. Calculating anchorage length L 1 of anchor cable according to bond strength between anchor rod and cement mortar where L 1 is the anchorage length (m), F 1 is the safety factor of ultimate resistance capacity, and d S is the external diameter of bolt; τ u is the bond strength between bolt and cement mortar where L is the length (m) of the bolt and L 2 is the length (m) of the exposed length of the anchor cable.

Optimized Treatment for Karst Caverns in Tunnel
e tunnel is built in the karst area and will encounter disaster accident of mud bursting and water bursting in front of the tunnel face ( Figure 15). In this section, when there is a pressure karst cavern in front of the tunnel face, based on the stress characteristics of the square rock wall in front of the tunnel face, the calculation formula of the safe thickness of the square rock wall in front of the tunnel face is proposed.

Computational Model.
When the square rock wall in front of the tunnel face is damaged by mud inrush and water inrush, the square rock mass in front of the tunnel face will be damaged by punching, and the failure structure is in the shape of cone platform, as shown in Figure 16. In this section, based on the principle of limit equilibrium, according to the failure shape of rock wall under the condition of water inrush and mud inrush, the calculation formula of safety thickness of square rock wall in front of tunnel face is derived [27,28].
As shown in Figure 16, h is the thickness of the square rock wall in front of the tunnel face, D is the diameter of the tunnel face, q is the cavity pressure, and θ is the pressure diffusion angle of the punching body, where θ � 45°− φ/2.

Entirety Punching Tensile Failure Model Calculation.
When the rock wall on the tunnel face reaches the limit equilibrium, only the tensile action on the failure surface of the rock wall cutting body is considered. By establishing the limit equilibrium equation on the failure surface (without considering the gravity action of the rock wall), the safe thickness of the rock wall can be obtained. e side area of the rock wall cutting body is where d is the compression diameter. According to geometric conditions,

Advances in Civil Engineering
According to the principle of limit equilibrium, there are where σ t is the tensile strength, MPa, and P t is the support pressure of the face, under the principle of NATM (0 kN). By substituting formulas (4) and (5) into (6), the minimum outburst prevention thickness of rock wall can be obtained.

Entirety Punching Shear Failure Model Calculation.
When the rock wall on the tunnel face reaches the limit equilibrium, only the shear resistance on the failure surface of the rock wall cutting body is considered. By establishing the limit equilibrium equation on the failure surface (without considering the gravity effect of the rock wall), the safe thickness of the rock wall can be obtained.
According to the principle of limit equilibrium, there are According to formulas (1)∼(2), the outburst prevention thickness of the square rock wall in front of the tunnel can be obtained under the shear failure mode.

Entirety Punching Shear Failure Model and Tensile Failure
Model Calculation. When the rock wall on the tunnel face reaches the limit equilibrium, considering the joint action of tension and shear on the failure surface of the rock wall cutting body, the safe thickness of the rock wall can be obtained by establishing the limit equilibrium equation on the failure surface (without considering the gravity effect of the rock wall).
According to the principle of limit equilibrium, there are According to formulas (4)∼(5), the outburst prevention thickness of the square rock wall in front of the tunnel can be obtained under the shear failure mode. e safety thickness of rock wall is affected by the diameter of tunnel, pressure of solution cavity, tensile strength, internal friction angle, and shear strength.
e safety thickness of rock wall is linearly related to the tunnel diameter; the safety thickness of rock wall decreases with the increase of tensile strength, shear strength, and internal friction angle; the safety thickness of rock wall increases with the increase of cavity pressure and tunnel diameter, and the safety thickness of rock wall is relatively affected by shear strength. After calculating the safe thickness of the tunnel face, the support parameters for the surrounding rock of the tunnel face can be carried out.

Effect of Optimized Treatment Applied for Karst Caverns
ere was a high-liquid-limit clay-filling karst cavern at the tunnel sidewall at sections K103 + 133 ∼ K103 + 137, and its direction was from left to right. Figure 17 shows the vault settlement at section K103 + 130, which was set up on November 15, 2018. e settlement progressed rapidly over 10 days and exceeded the tolerance value that was set by the local government. e settlement progressed rapidly with a daily variation of less than 5 cm and exceeded the 80 cm tolerance value set by the local government. e proposed optimized measure was applied for this karst cavern. After being treated, the daily variation value normalized, indicating that the settlement variation and risk of excavation had decreased.
Monitoring and measuring continued until the second lining was done. After 11 days, the vault settlement variation results indicated that the settlement had stabilized, and the optimized measure had produced a favorable result for the karst cavern at sections K103 + 133 ∼ K103 + 137.

Conclusions
(1) Tunnel construction in karst regions may result in serious geohazards. Tunnel construction in cavernous karst covered by sandy soil has induced such geohazards in Guangxi Province, including tunnel  collapse, water or mud inrush, ground collapse, and long-term instability. (2) In the present study, karst caverns in the Hebai tunnels were succinctly analyzed and investigated on the basis of size, shape, lithology, and internal filler. (3) According to the location, size, and internal filling of a karst cavern, different pretreatment methods were set for different situations. In the construction process, the cavern can be prepared in advance so as not to delay the project. (4) e calculation model of karst cave in front of tunnel face is established, and it is divided into three cases to deduce, and the safe rock thickness of actual tunnel face is calculated. According to the safe thickness of the tunnel face, some support is carried out for the surrounding rock of the tunnel face. (5) rough on-site inspection and characterization of karst caverns, specific and appropriate schemes were formulated to fill and reinforce the caverns in a timesensitive manner. e optimized treatment applied on the Hebai tunnel produced a favorable result.
Following treatment, the site should be monitored to determine whether the surrounding rock is stable. However, the treatments may be not suitable for karst caves filled with mud or soils.

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.