Research on the Design and Application of a Novel Curved-Mesh Circumferential Drainage Blind Pipe for Tunnels in Water-Rich Areas

Abstract

To address the issues of low permeability, clogging susceptibility, and insufficient circumferential bearing capacity of traditional drainage blind pipes behind tunnel linings in water-rich areas, this study proposes a novel curved-mesh circumferential drainage blind pipe specifically designed for such environments. First, through engineering surveys and comparative analysis, the limitations and application demands of conventional circumferential annular drainage blind pipes in highway tunnels were identified. Based on this, the key parameters of the new blind pipe—including material, wall thickness, and aperture size—were determined. Laboratory tests were then conducted to evaluate the performance of the newly developed pipe. Subsequently, the pipe was applied in a real-world tunnel project, where a construction process and an in-service blockage inspection method for circumferential drainage pipes were proposed. Field application results indicate that, compared to commonly used FH50 soft permeable pipes and F100 semi-split spring pipes, the novel curved-mesh drainage blind pipe exhibits superior circumferential load-bearing capacity, anti-clogging performance, and deformation resistance. The proposed structure provides a total permeable area exceeding 17,500 mm², three to four times larger than that of conventional drainage pipes, effectively meeting the drainage requirements behind tunnel linings in high-water-content zones. The use of four-way connectors enhanced integration with other drainage systems, and inspection of the internal conditions confirmed that the pipe remained free of clogging and deformation. Furthermore, the curved-mesh design offers better conformity with the primary support and demonstrates stronger adaptability to complex installation conditions.

1. Introduction

With the rapid development of modern transportation and urban underground spaces, the demand for tunnel construction under complex geological conditions is steadily increasing. However, groundwater infiltration poses a significant threat to the long-term safety and durability of tunnel lining structures. In many in-service mountain transportation tunnels, lining leakage frequently occurs during the service phase [1], becoming a critical factor that compromises structural integrity and traffic safety. Several studies [2,3,4] have shown that tunnel lining leakage is closely related to insufficient drainage performance or internal clogging within the tunnel drainage system. When the drainage system becomes blocked, groundwater flowing from rock mass fractures accumulates behind the tunnel lining, increasing the hydrostatic pressure borne by the lining. This phenomenon can cause groundwater to penetrate through weak points in the waterproofing layer, leading to lining cracks and even potential structural instability. Therefore, one of the core objectives of tunnel waterproofing and drainage design is to effectively control groundwater pressure exerted on the tunnel lining [5].

To date, scholars have conducted extensive research on the design and material selection of tunnel drainage systems. In terms of theoretical analysis, Zhu et al. [6] investigated the distribution pattern of external water pressure in shallow non-circular tunnels based on the concept of the equivalent permeability coefficient. Qi et al. [7] developed a simplified model for groundwater inflow in deep tunnels, considering the internal surface of the support structure bearing hydraulic head, and derived calculation formulas for inflow rate and water pressure on the support structure. Qing et al. [8] analyzed the mechanism of drainage systems and clarified the relationship between drainage performance and groundwater inflow. Liu et al. [9] established a drainage seepage model that incorporated drainage pipes, waterproof membranes, and geotextiles, and provided a theoretical guidance for estimating the permeability of primary supports.

In terms of engineering application, Li et al. [10] proposed an inspection method for highway tunnel drainage systems based on the visual technology and identified the clogging issues in some operational tunnel drainage systems. Zhou et al. [11] developed new inspection methods and technical procedures for tunnel drainage system in operational highway tunnels. Liu et al. [12] conducted a systematic investigation on the functionality of drainage systems and tunnel defects in a highway tunnel, and proposed effective countermeasures for dealing with the drainage system clogging issues. Jiang et al. [13] developed a serial evaluation model to assess the drainage capacity of tunnel drainage systems and analyzed its influence on the water pressure distribution on tunnel linings. Urbanowicz, K., et al. [14] reviewed analytical solutions for the accelerated flow of an incompressible Newtonian fluid in a pipeline. Karpenko, M., et al. [15] presented theoretical research on hydraulic processes in hydraulic drives of transport machinery, offering valuable references for the design of pipeline connections. Chen et al. [16] documented several cases of crystalline clogging in the drainage systems of railway subgrades and tunnels in France, identifying geological conditions as a key factor and proposing an assess index. Thomas [17] suggested that bicarbonate ions in groundwater reacted with free calcium ions produced during cement hydration in shotcrete. Zhang et al. [18], Shin et al. [19], and Khormali et al. [20] further demonstrated that chemical reactions between groundwater and shotcrete resulted in the formation and crystallization of precipitates, which could block the tunnel drainage pipes.

The drainage system in mountain tunnel linings typically serves as the primary barrier for groundwater discharge. This system comprises circumferential drainage blind pipes, longitudinal drainage blind pipes, and transverse diversion pipes. Among them, the circumferential drainage pipe plays a critical role in guiding seepage water and effectively reducing groundwater pressure behind the lining. In engineering practice, the spacing, diameter, and permeability of circumferential drainage blind pipes directly influence the efficiency of the tunnel lining drainage system. However, traditional circumferential drainage pipes are often prone to clogging, material aging, or construction deviations, all of which reduce drainage efficiency and lead to groundwater pressure buildup behind the lining [18,19,20]. Huang et al. [21] pointed out that quality control during tunnel drainage pipe construction lacks standardized procedures, and that pipe selection is largely based on experience. They emphasized that both drainage pipe materials and construction techniques need to be optimized. Tan et al. [22] reported quality issues in railway tunnel construction, where longitudinal drainage pipes suffered from severe deformation and damage. Although the existing studies [23] have acknowledged the problems of clogging—especially those related to crystallization and pipe diameter reduction in tunnel lining drainage systems—most research still focuses on performance degradation due to internal crystallization and its effects on the structural safety of tunnel linings. To enhance the drainage capacity of tunnel linings, Fu et al. [24] developed a refined calculation model for tunnel drainage systems considering the spacing parameters of circumferential blind pipes. In France, the ULTI-FLOW drainage system, composed of stacked corrugated polyethylene pipes wrapped with polypropylene geotextiles, has been applied [25]. Double-wall corrugated pipe [26], drainage mat and blind ditch materials [27] have also been developed and used in geotechnical engineering. Additionally, capillary permeable drainage belts have been introduced as a novel drainage material [28]. These products actively remove seepage water and exhibit superior drainage performance compared to traditional sand-gravel filter layers and geotextiles. Nevertheless, research and practical applications of new drainage blind pipe materials in tunnel engineering remain limited.

To address this research gap, this study identifies and analyzes the key limitations of existing circumferential drainage blind pipes used in highway tunnel engineering—namely, poor permeability, high susceptibility to clogging, and insufficient load-bearing capacity. In response, a novel corrugated mesh-type circumferential drainage blind pipe is proposed, featuring an enhanced structural configuration and optimized material composition. Laboratory tests are conducted to evaluate and compare the ring stiffness, drainage capacity, anti-clogging performance, and ultimate bearing capacity of the proposed and conventional pipes. The new pipe is further applied in a highway tunnel project to validate its practical feasibility, during which a standardized construction procedure and an in-service clogging inspection method are also developed. This work aims to provide a theoretical foundation and technical guidance for performance-oriented design, efficient installation, and long-term maintenance of tunnel drainage systems under complex hydrogeological conditions.

2. Limitations and Demands of Drainage Blind Pipe

2.1. Circumferential Drainage Blind Pipe

According to the China’s specifications [29,30] and several international standards [31,32,33], under conditions of high water pressure, a hydraulic pressure control system can be installed to limit discharge, ensuring that the local water pressure does not exceed the design value. In Chinese tunnel design standards, the drainage system for mountain tunnels follows the principle of “integration of prevention, blocking, interception, and drainage, with comprehensive management adapted to local conditions.”

As illustrated in Figure 1, the typical lining drainage system configuration for mountain highway tunnels is installed between the initial lining and the secondary lining. Groundwater accumulating behind the lining can be discharged directly through circumferential drainage blind pipes and the gap between the two linings. The water then flows down to the tunnel sidewalls, where it is collected by longitudinal drainage blind pipes. Finally, it is discharged through transverse diversion pipes into drainage ditches along the tunnel roadway. This drainage design effectively reduces the hydrostatic pressure exerted on the tunnel lining and helps mitigate structural deterioration caused by groundwater.

The tunnel lining drainage system, roadway drainage ditches, and road subgrade drainage together constitute the three-dimensional drainage system in highway tunnels. Among these components, the design of circumferential drainage blind pipes is the most complex. Groundwater can be effectively discharged in a timely manner through the circumferential blind pipes, thereby reducing seepage pressure to a relatively low level.

During the operation period, the drainage system of highway tunnels is prone to clogging, mainly due to the crystallization and hardening of calcium carbonate deposits, as well as the infiltration of particles such as silt and gravel carried by groundwater flow. Once the tunnel drainage system becomes blocked, groundwater cannot be discharged promptly, leading to increased water pressure behind the tunnel lining. In the areas between adjacent circumferential pipes, pore water accumulates, resulting in the highest seepage pressure [34]. As groundwater pressure continues to rise, it may cause leakage or even cracking of the lining structure, as illustrated in Figure 2.

2.2. Limitations of the Existing Circumferential Drainage Blind Pipe

In tunnel lining drainage systems, circumferential drainage blind pipes are typically installed in close contact with the inner surface of the initial support, with a spacing of 5 to 10 m. In water-rich surrounding rock sections, this spacing can be reduced to enhance groundwater drainage efficiency. These circumferential pipes are generally designed as permeable elements. Currently, the two most commonly used types of circumferential drainage blind pipes in Chinese transportation tunnels are the FH50 soft flexible permeable pipe and the F110 half-split semicircular spring pipe. The performance of these pipes must comply with the relevant Chinese specifications, namely the national standard Flexible Permeable Hose (JC 937—2004) [35].

Figure 3 illustrates the installation schematics of the FH50 soft flexible permeable pipe and the F100 half-split semicircular spring pipe. The FH50 pipe is a geosynthetic product composed of steel wire springs and woven polymer fiber mesh [36], with a permeability coefficient exceeding 0.1 cm/s. The permeability coefficient refers to the seepage velocity perpendicular to the pipe wall under a unit hydraulic gradient. However, the effective pore size of the FH50 pipe ranges from 0.06 mm to 0.25 mm, making it susceptible to clogging by fine particles in groundwater. Additionally, its 50 mm diameter limits its water conveyance capacity.

The F100 half-split semicircular spring pipe consists of thermally bonded semicircular steel springs wrapped in a geotechnical composite membrane. Although it conforms well to the initial support surface, this pipe lacks a standardized product specification. Moreover, the cross-section is not a true semicircle, with a vertical height of only about 3 cm, which significantly limits its drainage capacity. The geotechnical composite membrane, composed of fibers and plastics, also exhibits a relatively low permeability coefficient [37].

Engineering experience indicates that the selection and installation of circumferential drainage blind pipes in tunnels should meet the following requirements:

(1)

Circumferential drainage blind pipes must withstand the loads imposed during the secondary lining concreting process, including pressure generated by heat of cement hydration. They should not undergo significant deformation or cracking under localized concentrated loads [35].

(2)

These pipes must be perforated to allow effective water infiltration. Although no formal specification stipulated the minimum permeable area, the newly issued Technical Code for Drainage and Waterproofing of Highway Tunnels (T/CECS G:D72-01—2024) [38] recommended that the whole permeable area of tunnel drainage pipe should not be less than 40 cm²/m (4000 mm²/m). In sections with high groundwater inflow, the spacing of circumferential drainage pipes may be reduced.

(3)

The circumferential drainage blind pipes should conform closely to the surface of the initial support and must not interfere with the placement of the waterproof membrane between the initial support and the secondary lining.

3. Design of Novel Curved-Mesh Annular Drainage Blind Pipe

3.1. Structural Design

Rigid plastic permeable pipes adopt a mesh structure, with water-permeable holes evenly distributed across the tube surface. These holes are typically designed in diamond or circular patterns, which effectively leverage both the siphon principle and capillary action [39]. Consequently, the drainage rate shows minimal degradation over time, and engineering applications have demonstrated that this type of pipe exhibits strong resistance to clogging by sediment. The diameter of the permeable holes generally ranges from 1 mm to 5 mm, resulting in significantly greater permeability than that of the FH50 soft flexible permeable pipe and the F100 half-split semicircular spring pipe.

To overcome the limitations of existing circumferential drainage blind pipes in terms of permeability, deformation resistance, and anti-clogging performance, a new type of pipe was developed based on the mesh structure of rigid plastic permeable pipes (see Figure 4). This new design features a semicircular profile with a maximum thickness of 55 mm, aligning with the dimensional specifications of conventional drainage pipes. It incorporates a reticular (mesh-like) permeable structure on both the circumferential surface and bottom face. The circumferential and bottom components are integrally connected by a series of corrugated ribs. The bottom structure consists of two stacked layers of ribs, enabling a uniform distribution of water-permeable openings across the entire pipe surface. Accordingly, the pipe was designated as the Corrugated Reticular Circumferential Drainage Blind Pipe.

The new reticular permeable drainage pipe offers several notable advantages:

(1)

A large number of water-permeable holes are distributed across the entire pipe body, providing excellent drainage performance and enabling efficient discharge of groundwater accumulated behind the secondary lining.

(2)

During the installation phase, since the primary support is constructed with shotcrete and typically has an uneven surface, the circumferential blind pipe must be closely fitted to the irregular contours of the initial support.

(3)

During the concreting of the secondary lining, the pipe is subjected to non-uniform stress, which may lead to deformation. Therefore, the pipe should be composed of materials that combine both rigidity and flexibility, allowing it to conform to the surface of the primary support while withstanding the mechanical stress imposed during secondary lining construction.

3.2. Material Selection

At present, plastic pipes have been widely adopted across various industrial and engineering fields due to their lightweight, excellent durability, environmental friendliness, and long service life [13]. Commonly used materials in these applications include high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), acrylonitrile–butadiene–styrene copolymer (ABS), and polybutylene (PB) [40,41]. The advantages and disadvantages of these materials are summarized in

Table 1. Characteristics of different types of plastic materials.

Types	Characteristics
Acrylonitrile–butadiene–styrene copolymer (ABS)	Well impact resistance strength, creep resistance capacity and corrosion resistance capacity, poor water resistance and poor flexibility.
High-density Polyethylene (HDPE)	The price is cheap, and the creep resistance capacity is poor.
Polyvinyl Chloride (PVC)	Well flame retardancy, low price, low strength and toxic monomer PVC.
Polybutylene (PB)	Well chemical corrosion resistance capacity, high temperature resistance and easy cracking.

.

Given the installation requirements of circumferential drainage blind pipes behind tunnel linings, the selected material must meet three key performance criteria: deformation resistance (i.e., circumferential load-bearing capacity), water permeability, and conformity to the primary support surface. These requirements are detailed as follows:

(1)

The circumferential drainage blind pipe must have sufficient circumferential load-bearing capacity to withstand the construction loads imposed during the secondary lining process;

(2)

The circumferential drainage blind pipe must possess a certain degree of flexibility to conform to the tunnel’s circumferential curvature during installation;

(3)

The circumferential drainage blind pipe should also exhibit adequate heat resistance and impact resistance to endure the temperature fluctuations and dynamic loads generated during the secondary lining construction stage. Even if partial deformation occurs, the pipe should not fracture.

Based on these requirements and the comparative properties listed in Table 1, HDPE and PP were found to meet the necessary performance criteria. Other materials lacked sufficient flexibility. Considering additional factors such as cost and manufacturability, high-density polyethylene (HDPE) was ultimately chosen as the raw material for producing the new circumferential drainage blind pipe. To enhance the pipe’s long-term durability, additives such as antioxidants, anti-corrosion agents (for acid and alkali resistance), and toughening agents were incorporated [38]. The thermoplastic resin content in the HDPE used for the fabrication of the pipe exceeded 80%. Based on the design parameters such as wall thickness and pore size (as detailed in Section 4), the test samples were produced using high-temperature extrusion in a single-step forming process. The resulting new reticular permeable drainage pipe is shown in Figure 4. The pipe featured uniformly distributed water-permeable holes, and the thickness of the corrugated ribs was 3 mm, resulting in a total wall thickness of 6 mm.

3.3. Parameter Selection of the New Circumferential Blind Pipe

The engineering performance of the newly developed semicircular corrugated circumferential drainage blind pipe primarily depends on its permeability and load-bearing capacity. Therefore, the aperture size and wall thickness of the drainage pipe must be carefully optimized to ensure that the pipe delivers practical value in real-world tunnel applications.

3.3.1. Permeable Capability Design

The circumferential drainage blind pipes installed behind the secondary lining in mountain tunnels must be designed with adequate water permeability. In this design, it is necessary to consider not only the permeability of the pipe itself, but also the seepage characteristics of the surrounding rock mass. To reduce the hydrostatic pressure acting on the tunnel lining, a portion of the groundwater must be effectively discharged. Therefore, the circumferential drainage blind pipes must possess sufficient infiltration capacity. In general, the total drainage capacity behind the secondary lining in each section should exceed the seepage inflow from the surrounding rock mass. The drainage capacity is partly determined by the total area of the permeable holes on the circumferential drainage blind pipes, which can be calculated using Formula (1) [42,43]:

$$Q_s={\displaystyle \frac{S_0\sqrt{K}}{15\xi }}$$

(1)

In Formula (1), Q_s represents the total design permeability per linear meter of the new reticular permeable drainage pipe, with a unit of m³. The equivalent permeability coefficient of drainage blind pipe was calculated by $\sqrt{K}/15$ [11]. K is the permeability coefficient of the surrounding rock mass, with a unit of m/s. ξ dimensionless reserve coefficient for the new reticular permeable drainage pipe, which can be determined based on blockage conditions using endoscopic inspection devices, generally, the reserve coefficient ξ is greater than 5. S₀ denotes the total area of permeable holes in the drainage blind pipe, expressed in mm² or m².

According to the literature [42,43], the water seepage of rocks can be calculated by Formula (2):

$$Q_c={\displaystyle \frac{2\pi H_{0}K}{\ln (r_0/r_1)}}$$

(2)

In Formula (2), Q_c represents the total amount of groundwater seepage per linear meter of the lining, with a unit of m³; $r_1$ is the outer radius of the lining, in meters (m); $r_0$ is the radius of the seepage influence zone, in meters (m); $H_0$ is the height of the groundwater level above the tunnel centerline, also in meters (m).

The spacing of circumferential drainage blind pipes can be set at fixed intervals based on the allowable structural displacement and the maximum water pressure permitted by the lining. Considering that a certain amount of drainage occurs between the primary support and the secondary lining, the seepage from the surrounding rock cannot be fully discharged through the circumferential drainage blind pipes alone [32,33]. Therefore, the total seepage capacity of the circumferential drainage blind pipes should exceed 50% of the seepage from the surrounding rockmass. In the design of the new reticular permeable drainage pipe, medium-permeability surrounding rock conditions were considered, with a permeability coefficient of 10⁻⁴ m/s [44]. According to Formulas (1) and (2), the reserve coefficient ξ of the new reticular permeable drainage pipe could be 6.0, then the total area of permeable holes per linear meter was 17,361.1 mm², i.e., 0.01736 mm².

As shown in Figure 5, the equivalent diameters of the permeable holes distributed across the surface of the circumferential drainage pipe can be approximated as circular. Considering that the total required water-permeable area should not be less than 17,361.1 mm², the equivalent diameter of the permeable holes ranges from 1 mm to 5 mm. The relationship between the total permeable area and the equivalent diameter of the permeable holes is illustrated in Figure 6. According to the results in Figure 6, when the equivalent diameter of the permeable holes is 3 mm, the total permeable area of the circumferential drainage pipe is approximately 18,075.5 mm², which meets the calculated requirements based on Equations (1) and (2). In this configuration, the corrugated mesh ribs were designed with a thickness of 3 mm, resulting in a total pipe wall thickness of 6 mm due to the superposition of the inner and outer ribs. The equivalent permeable hole diameter distributed across the pipe surface is 3 mm. In practical engineering applications, when the surrounding rockmass exhibits high groundwater inflow or a high permeability coefficient, the spacing of the circumferential drainage pipes can be reduced to enhance drainage efficiency.

3.3.2. Thickness Design of the Tube

The ring bearing capacity of the new reticular circumferential drainage blind pipe was primarily determined by its wall thickness. This capacity was evaluated by applying a vertical load to the pipe sample. As shown in Figure 7, the bearing capacity was measured using a ring stiffness testing machine. The test sample had a length of 30 cm and a wall thickness of 4 mm, and loading was performed in accordance with the relevant testing standards [45]. According to the test results in Figure 7, when the vertical displacement of the sample was less than 37 mm, the relationship between vertical load and displacement was approximately linear. When the vertical displacement exceeded 37 mm, the semicircular shape of the pipe began to flatten, and the vertical load continued to increase with deformation until the pipe was fully compressed. A bilinear trend was observed between vertical load and displacement, indicating the presence of an inflection point in the bearing behavior. At this inflection point, the pipe structure was essentially flattened, and its internal permeable space had significantly decreased. The load value at this stage can be considered the ultimate ring bearing capacity of the new circumferential drainage pipe. Therefore, when the pipe wall thickness was 4 mm, the ultimate ring bearing capacity was measured to be 0.86 kN.

The relationship between the circumferential bearing capacity and the wall thickness of the new circumferential drainage pipe is shown in Figure 8. As observed from the results in Figure 8, when the wall thickness of the new pipe was 6 mm, 8 mm, and 10 mm, the corresponding circumferential bearing capacities were 1.87 kN, 2.05 kN, and 2.17 kN, respectively. Compared to the pipe with a wall thickness of 4 mm, the circumferential bearing capacity was significantly enhanced.

According to the requirements for circumferential blind pipes to withstand the construction loads from secondary lining concreting, the ultimate bearing capacity must exceed 1.5 kN. Therefore, the wall thickness of the new reticular circumferential drainage blind pipe should be greater than 6 mm. Considering that the pipe is composed of two layers of curved ribs, when each layer contains approximately 13 to 15 ribs, the total wall thickness can be ensured to exceed 6 mm. Once the circumferential bearing capacity exceeds 1.87 kN, the pipe can effectively bear the construction load induced by the concrete pouring of the secondary lining.

4. Performance of Novel Curved-Mesh Annular Drainage Blind Pipe

4.1. Bearing Capacity Analysis

As shown in Figure 9, the circumferential bearing capacity of the new blind pipe with a wall thickness of 6 mm was tested, along with the traditional types of circumferential blind pipes (i.e., FH50 soft flexible permeable pipe and F110 semicircular spring pipe). In the circumferential bearing capacity test illustrated in Figure 9, the height of the F110 semicircular spring pipe was 30 cm, the height of the FH50 soft flexible permeable pipe was 50 mm, and the height of the new reticular permeable drainage pipe was 57 mm. Considering that all three circumferential blind pipes had a circular shape, for ease of comparison, the vertical deformation rate of each pipe was defined as the ratio of the maximum vertical deformation to the specimen height. When the vertical deformation rate reached 30%, the corresponding vertical load was defined as the circumferential bearing value of the pipe. The vertical deformation rate was calculated using the following equation [44]:

$$\epsilon ={\displaystyle \frac{W_{\max }}{D_1}}$$

(3)

In Formula (3), $\epsilon$ is the vertical deformation rate of the circumferential blind pipe. $W_{\max }$ is the maximum vertical deformation value; the unit is cm or m. D₁ is the outer diameter or equivalent height of the test sample; the unit is cm or m.

Figure 10 presents the test results of the circumferential bearing capacity for different types of pipes. The x-axis represents the cumulative vertical deformation rate, and the y-axis represents the vertical load. As shown in the results of Figure 10, when the vertical deformation rate of the specimens reached 30%, the corresponding vertical load values for the FH50 soft flexible permeable pipe, F110 semicircular spring pipe, and the new circumferential blind pipe were 500 N, 590 N, and 530 N, respectively. The test results indicate that the bearing capacity of all three types of circumferential drainage blind pipes was approximately the same when the deformation rate was less than 30%. However, when the deformation rate exceeded 40%, the bearing capacity of the new circumferential blind pipe significantly improved, and no cracking or delamination was observed on the pipe body during or after the test. In contrast, the FH50 soft flexible permeable pipe and the F110 semicircular spring pipe exhibited steel wire distortion and separation of the permeable layer during the loading process. Therefore, the new circumferential blind pipe demonstrated superior bearing capacity under external loads.

4.2. Permeable Capability Analysis

In tunnel engineering, circumferential drainage blind pipes are required to possess good water permeability. However, the water permeability index of such pipes is not clearly defined in many current engineering design standards. The F110 semicircular spring pipe is wrapped with a geocomposite membrane, which exhibits low permeability. The FH50 soft flexible permeable pipe is installed tightly against the surface of the primary support, forming a semicircular internal channel for drainage. The FH50 pipe is composed of a polymer fiber woven mesh wrapped around a steel wire spring, and its water permeability primarily depends on the properties of the polymer fiber mesh. The equivalent pore size of the fiber mesh ranges from 0.06 mm to 0.25 mm [38], and these small pores are easily blocked by fine particles carried by groundwater inflow.

Table 2. Permeability index for three types of circumferential blind pipe.

Type	F110 Semicircular Spring Tube	FH50 Soft Flexible Permeable Pipe	The New Reticular Circumferential Drainage Blind Pipe
section surface area (mm2)	1500~1750	1962.5	4750
water permeability area (mm2)	<5000	<5000	>17,500

summarizes the calculated water permeability indices of three types of circumferential drainage blind pipes. According to the table, the new circumferential drainage blind pipe features approximately 2500 permeable holes per meter, with aperture sizes ranging from 2 mm to 4 mm. Based on an average aperture of 3 mm, the total permeable area per meter is estimated to be 17,662.5 mm². Compared with traditional drainage blind pipes, the new design exhibits superior water permeability, with a total permeable area exceeding 17,500 mm², which is three to four times greater than that of conventional circular drainage pipes. Therefore, as a new type of tunnel circumferential drainage blind pipe, this reticular design significantly enhances water permeability behind the tunnel lining and improves the overall drainage performance of the tunnel lining system.

4.3. Impact Resistance Capacity and Ring Flexibility

The new reticular circumferential drainage blind pipe was fabricated using high-density polyethylene (HDPE), which provides excellent resistance to impact loads. According to the Chinese national standard GB/T 19472 [46], the impact resistance of circumferential drainage blind pipes can be assessed using the drop hammer impact test and the ring flexibility test. The standard sample length is typically 30 cm. After conducting the drop hammer impact test, the vertical deformation rate of the test sample must be less than 10%, and no cracks or delamination should appear on the pipe surface. During the ring flexibility test, when the vertical deformation rate reaches 50%, the sample must not exhibit reverse bending, fracturing, or delamination.

As described in Section 4.3, although the pipe has a circular cross-section, the new reticular circumferential drainage blind pipe was still subjected to both drop hammer impact testing and ring flexibility testing (see Figure 11a). In the impact test, the pipe exhibited a vertical deformation rate of approximately 7%, and no cracks or delamination were observed after loading. In the ring flexibility test (see Figure 11b,c), the specimen returned to its original shape after the load was removed. Even when the vertical deformation exceeded 50%, no reverse bending, cracks, or delamination were detected on the pipe surface. These results demonstrate that the new drainage blind pipe possesses excellent ring flexibility and strong resistance to impact loads.

4.4. Drainage Rate Analysis

To evaluate the anti-silting performance of the new reticular circumferential drainage blind pipe, small-scale model tests were conducted to compare the drainage rates of two types of drainage pipes. The test setup is illustrated in Figure 12. The model box had dimensions of 2.0 m in width, 1.0 m in height, and 1.0 m in depth. Both the new reticular circumferential drainage blind pipe and the FH50 soft flexible permeable pipe were buried in a sand layer at a depth of 0.6 m.

Due to the material characteristics of the F110 pipe—specifically, the geotechnical composite membrane composed of fibers and plastics—it has a relatively low permeability coefficient. Therefore, groundwater cannot fully pass through the membrane, resulting in an extremely low drainage rate for the F110 pipe.

For this test, each pipe specimen had a length of 1.0 m, and a 2% unidirectional slope was applied along the vertical thickness direction. A semi-open PVC drainage pipe was installed beneath both the new reticular circumferential drainage blind pipe and the FH50 pipe to facilitate water discharge. A sprinkler system installed above the sand layer simulated groundwater flow, delivering water at a constant rate of 20 L/day.

The drainage rate at each pipe’s outlet was monitored to evaluate the degree of clogging in the pipe and its permeable openings. A notable decrease in the discharge rate was considered an indicator of internal clogging. The system was configured to automatically record the total outflow from both outlets at 1 h intervals, and the hourly discharge rates were then calculated to generate drainage performance curves.

Figure 13 presents the variation curves of drainage rate over time for the two types of circumferential drainage blind pipes. As shown by the experimental results, the drainage rate of the FH50 soft flexible permeable pipe remained relatively stable during the initial 48 h of testing. However, a significant decline was observed thereafter, with the rate falling below 0.01 m³/min, indicating partial clogging of the permeable holes.

By contrast, the new reticular circumferential drainage blind pipe exhibited only a slight reduction in drainage rate toward the end of the experiment (around the 90th h), with the drainage rate dropping to approximately 0.01 m³/min at that point, suggesting minor clogging. However, its drainage rate remained higher than that of the FH50 soft flexible permeable pipe throughout the testing period.

These results demonstrate that the permeability of both types of drainage blind pipes decreased over time. Throughout the entire test, the drainage rate of the new reticular circumferential drainage blind pipe consistently exceeded that of the FH50 pipe. In practical engineering applications, if the groundwater infiltrating behind the tunnel lining contains a high concentration of sand or silt, long-term inflow may lead to partial clogging of circumferential drainage blind pipes. The application of the new corrugated reticular circumferential drainage blind pipe can effectively delay clogging onset. The new pipe exhibits superior anti-clogging performance when used in groundwater environments with high sand or silt content.

5. Engineering Application of New Circumferential Drainage Blind Pipe

5.1. Project Overview

The G4 Expressway is a major transportation corridor connecting Beijing, Hong Kong, and Macau, spanning the entire length of China from north to south. In Guangdong Province, the G4 Expressway has been undergoing reconstruction and expansion due to its heavy traffic volume. The reconstruction and expansion project for the Guangzhou–Qingyuan section of the Beijing–Hong Kong–Macau Expressway (G4) commenced in 2025. Within this section, the existing Dan-Jia-Shao Tunnel was a twin-bore highway tunnel, each bore accommodating six lanes, with a total tunnel length of 807 m. Located in Qingyuan City, the Dan-Jia-Shao Tunnel serves as a key passage to Guangzhou. During the expansion, the route was upgraded to a twin-bore twelve-lane highway tunnel. As shown in Figure 14, the expansion plan involved the construction of two additional single-bore tunnels. One new tunnel (Left line) was built between the two existing bores, while the other was constructed on the west side. The distance between the newly constructed left-line tunnel and the existing left/right tunnels was approximately 25 m.

The newly constructed left and right bores of the Dan-Jia-Shao Tunnel are located in a denudation hill terrain, with a relative elevation difference of approximately 150 m. The slopes at the tunnel entrances and exits range from 25° to 35°, while the slope above the tunnel alignment varies between 30° and 50°. According to geological survey data, the groundwater in the tunnel area is primarily recharged by rainfall. Due to the high precipitation levels during the rainy season in Guangdong Province, the surrounding rockmass in this region contains abundant groundwater. During the operational phase of the tunnel, a substantial volume of groundwater must be discharged through the tunnel drainage system to reduce the hydrostatic pressure exerted on the lining, in order to ensure the structural safety of the tunnel.

5.2. Construction Technology of the New Blind Drain Pipe

Based on the structural characteristics of the new type of reticular circumferential drainage blind pipe, as can be seen in Figure 15, a finalized installation procedure was developed as follows:

(1)

Prior to installation, the drainage pipes should be pre-cut according to the tunnel’s circumferential length. The recommended length for each section of the reticular circumferential drainage blind pipe is 4 m.

(2)

During installation, the pipes are laid sequentially along the tunnel circumference and secured to the inner surface of the primary support using geotextile fabric and a powder-actuated nail gun. Two types of U-shaped clamps with different widths (2 cm and 10 cm) are used for fixation. The 2 cm-wide clamp is applied at the center of each pipe, while the 10 cm-wide clamp is used at the joints between two pipe sections to enhance the bearing stiffness at the connections.

(3)

A specially designed four-way connector is used to join the new reticular circumferential drainage blind pipes with other drainage components. The gaps at the joints between pipe sections are sealed with geotextile fabric to ensure system continuity and prevent leakage.

To enhance the drainage performance and construction quality of tunnel lining drainage systems, the new reticular circumferential drainage blind pipe developed in this study was implemented in the newly constructed Dan-Jia-Shao Tunnel. The engineering performance of this system is shown in Figure 16a. As illustrated in Figure 16b, compared with traditional types of circumferential drainage pipes, the new pipe features a transverse bottom structure that improves conformity to the surface of the primary support. This design adapts well to the uneven surface of shotcrete and prevents local deformation or damage during the secondary lining concreting process.

The special joint design for the new drainage pipes does not require any dedicated customization, and the cost of the four-way connector is essentially the same as that of traditional straight-through joints. This indicates that the new connector is practical and cost-effective for engineering applications. Moreover, the four-way connector has minimal impact on overall project costs. On the other hand, the installation efficiency of the new reticular circumferential drainage blind pipe is comparable to that of traditional pipe types, such as the FH50 soft flexible permeable pipe and the F110 semicircular spring tube.

In addition to its enhanced drainage and load-bearing performance, the new corrugated mesh circumferential drainage blind pipe also demonstrates significant advantages in terms of ease of construction. Unlike traditional blind pipes such as the F100 semi-slit spring pipe and the FH50 soft permeable pipe, which often require precise alignment and additional reinforcement measures, the new pipe features a modular, standardized design that facilitates efficient transportation and on-site installation. Its flexible mesh structure allows it to better conform to tunnel curves and uneven installation surfaces, thereby reducing the need for manual adjustments. Furthermore, the streamlined connection method and reduced reliance on specialized installation equipment significantly reduce construction time and labor costs. These advantages make the new drainage pipe particularly suitable for rapid tunnel construction in challenging, water-rich geological conditions.

5.3. Maintenance Method for the New Blind Drain Pipe in Operation Period

When FH50 soft flexible permeable pipes and F110 semicircular spring tubes are used as circumferential drainage blind pipes behind the tunnel’s secondary lining, the issue of overlapping with longitudinal drainage pipes has long been overlooked—typically, joints were stacked with gravel only, as shown in Figure 17 and Figure 18. These pipes were commonly connected either by loosely placing crushed stones at the interface or by directly inserting the circumferential pipe into the longitudinal drainage pipe. Such connection methods hindered inspection and maintenance during tunnel operation.

To address the issue of inaccessible circumferential drainage pipes during the operational phase, a specialized four-way connector was introduced. This connector enabled the integration of the new reticular circumferential drainage blind pipe with other components of the tunnel lining drainage system, as illustrated in Figure 19a. As shown in Figure 19b, the improved connection method for the corrugated reticular pipe allows clogging conditions to be inspected during tunnel operation. By inserting a visual inspection device through the transverse diversion pipe, the internal condition of the circumferential drainage pipe can be effectively monitored

This connection scheme was successfully applied in the newly constructed Dan-Jia-Shao Tunnel. After the completion of the secondary lining concreting, a pipeline video detector was used to inspect the internal condition of the corrugated reticular circumferential drainage blind pipe through the exposed opening of the transverse diversion pipe, as illustrated in Figure 20. The inspection results shown in Figure 20 confirmed that the optimized connection design enabled efficient internal inspection of the circumferential drainage pipe, greatly improving maintenance accessibility.

In the field test, each section of the reticular circumferential drainage blind pipe could be fully inspected by the pipeline video detector within approximately twenty minutes. Furthermore, the results validated that the new pipe did not exhibit any significant deformation after secondary lining concreting.

In addition, the inspection also demonstrated that the new reticular circumferential drainage blind pipe exhibited stable structural performance and reliable long-term operation. Its load-bearing performance was well suited to the tunnel construction environment.

6. Conclusions

This study evaluated the advantages and limitations of existing circumferential drainage blind pipes used in mountain tunnels in China. To address the identified deficiencies, a novel corrugated reticular circumferential drainage blind pipe was developed. Its structural design and parameter selection method were proposed, and its performance was compared with that of conventional drainage pipes. Finally, the new pipe was implemented in a real-world tunnel project. The main conclusions are as follows:

(1)

To overcome the limited permeability and insufficient circumferential load-bearing capacity of traditional circumferential drainage blind pipes, high-density polyethylene (HDPE) was selected as the base material for the new pipe. A semicircular corrugated mesh structure was designed, and key parameters such as wall thickness and pore size were determined according to the functional requirements of tunnel drainage systems. As a result, a new type of circumferential drainage blind pipe was successfully developed for tunnel engineering applications.

(2)

Compared with traditional drainage blind pipes, such as the F100 semi-split spring pipe and the FH50 soft flexible permeable pipe, the new corrugated reticular pipe demonstrated significantly enhanced load-bearing capacity, permeability, and anti-silting performance. The total permeable area of the new pipe exceeded 17,500 mm²/m, which was 3–4 times larger than that of conventional pipes. These features make it particularly suitable for drainage systems in tunnels subjected to high groundwater inflow.

(3)

The newly developed corrugated reticular circumferential drainage blind pipe was successfully applied in the Dan-Jia-Shao Tunnel. A dedicated installation method was proposed, and a four-way connector was developed to improve the connection between the circumferential and longitudinal drainage systems. This optimized connection allowed for inspection of clogging conditions during the tunnel’s operational phase. Post-concreting inspections using visual detection equipment confirmed that the pipe exhibited no deformation or blockage. Moreover, the pipe demonstrated excellent adaptability to irregular primary support surfaces and maintained close conformity throughout installation.