Currently, China has emerged as the leading country worldwide in terms of scale and complexity of tunnel and underground construction (Song et al., 2021; Xu et al., 2022). The 21st century has witnessed significant advancements in underground engineering in China (Ding and Xu, 2017; Liu et al., 2020; Huang et al., 2022). Over the past two decades, numerous tunnels have been successfully constructed (Wang et al., 2022a; Zhou et al., 2022; Wang et al., 2023a). The shield tunneling technology, known for its safety and efficiency, has been extensively utilized in the construction of urban subways and municipal tunnels (Langmaack and Lee, 2016; Xu et al., 2020; Jia and Gao, 2022). However, when a shield machine traverses clayey strata, the excavated clayey muck tends to adhere to the cutter head and cutters (Hollmann and Thewes, 2013; Wang et al., 2023b; Gao et al., 2023). The force exerted during shield pushing causes the attached muck to consolidate, resulting in the formation of a layer of hardened muck on the surface of cutter head (Thewes and Hollmann, 2016; Zeng et al., 2020; Li et al., 2021). As the hard muck rubs against the ground, heat is generated at the tunneling face. This heat accelerates the consolidation of muck adhering to the steel instruments. Consequently, the cutter becomes completely engulfed in consolidated muck, rendering it incapable of cutting and leading to a clogging in the shield machine. The shield clogging can have various adverse effects. In milder cases, it can lead to a loss of cutting efficiency of the cutting tools, significantly impacting the tunneling construction efficiency. In more severe instances, it can cause surface subsidence, posing risks to surface structures and personnel safety. For example, a large diameter shield tunneling machine was clogged when it was passing through a railway station in Shenzhen, China. The on-site construction personnel used frequent cabin cleaning to remove the soil adhered on the cutter head, resulting in a significant increase in ground settlement and settlement rate, posing a huge threat to the safety of surface railway station.

Based on current engineering experience, both slurry pressure balance (SPB) and Earth pressure balance (EPB) shields can face clogging issues in clayey strata (Chen et al., 2022; Li et al., 2022). To mitigate shield clogging during EPB tunneling, soil conditioning chemicals are frequently injected ahead of the cutter head (Martinelli et al., 2015; Zumsteg and Langmaack, 2017; Liu et al., 2018; Wang et al., 2021). However, the injection holes for soil conditioning chemicals in the cutter head are sparsely distributed, making it challenging to evenly distribute the injected chemicals across the entire cutter head. Consequently, certain areas may still experience shield clogging. During SPB shield tunneling, the central position of the cutter head is washed with slurry sprayed from the pipeline to prevent muck adhesion (Xiao et al., 2020). Nevertheless, due to limited flow rates, the cutter head may still encounter clogging issues in cases of high adhesion strata or poorly designed cutter heads (Yang et al., 2023).

Shield clogging is primarily caused by the strong adhesive strength between metal and clay, also known as the clay-metal interface strength (Sass and Burbaum, 2009; Wang et al., 2020; Liu et al., 2022). The clay-metal interface strength can be divided into the normal direction perpendicular to the contact and the tangential direction along the interface (Zumsteg et al., 2013; Burbaum and Sass, 2017; Li et al., 2023). The cutting tool used in shield tunneling tears through the stratum, with the muck predominantly flowing tangentially along the cutter head. When the tangential strength, referred to as the shear strength of the clay-metal interface, is too high, it causes the flow of muck to halt and results in shield clogging. Therefore, the shear strength of the clay-metal interface is the primary factor contributing to shield clogging (Zumsteg and Puzrin, 2012; Liu et al., 2018; Liu et al., 2019). Several researchers have conducted relevant studies on the shear strength of the clay-metal interface. Zumsteg and Puzrin (2012) developed a rotating shear device to evaluate the shear strength between metal and clay in the interface. They investigated the impacts of metal surface roughness, water content, and applied pressure on the soil specimen. The findings demonstrated that an increase in surface roughness or normal pressure applied to the soil specimen led to a steady increase in the shear strength of the clay-metal interface. However, the shear strength dramatically decreased as the water content rose. Basmenj et al. (2016) used a direct shear apparatus and inserted the bottom half of the shear box into a metal block to test the shear strength of the clay-metal interface. They analyzed the variation in shear strength for soil specimens with different clay contents using the consistency index. The results showed that when the consistency index was low, the shear strength of the clay-metal interface remained relatively constant, but it increased rapidly when the consistency index surpassed a certain threshold. Bircha et al. (2016) investigated the relationship between water content, compactness, and the fluctuation of shear strength between sand, silt, clay, and metal. The study demonstrated that the shear strength of the clay-metal interface initially increased and subsequently decreased as the water content increased. Furthermore, the shear strength of the clay-metal interface decreased with the level of sand and silt compactness. Wang et al. (2022b) investigated the effects of the consistency index, normal pressure, clay plasticity, contact angle, and metal surface roughness on the shear strength of the clay-metal interface. They found that normal pressure, consistency index, and hydrophilicity of the metal surface significantly influenced the shear strength. In shield tunneling, the normal pressure is related to the tunnel face support pressure. Lower face support pressure reduces the likelihood of shield clogging but may lead to tunnel face collapse (Cao et al., 2020; Wang et al., 2022c; Niu et al., 2022). Therefore, simply decreasing the normal pressure is not a viable solution to avoid shield clogging. The natural water content and Atterberg limits of the soil in the stratum, which are challenging to modify, determine the consistency index. By applying an anti-adhesion substance to cover the cutter head and cutter, it is possible to reduce the hydrophilicity of the metal surface and prevent shield clogging. Preventing the formation of shield clogging can significantly improve shield tunneling construction efficiency and eliminate construction risks in cohesive strata. Coating technology is widely employed in various industries. For instance, in the medical field, anti-adhesion substances are coated on the surface of medical devices to reduce bacterial adhesion (Yang et al., 2022). In the glass manufacturing industry, anti-adhesion coatings are used to enhance the surface roughness of products by applying them to the mold surface (Zhang et al., 2020). Similarly, coatings are utilized to prevent corrosion in offshore wind power plants (Eom et al., 2020). However, there has been limited recent research specifically focusing on the application of anti-adhesion coatings to address shield clogging in tunnel engineering.

To investigate the impact of anti-adhesion coating on preventing shield clogging, this study conducted a comprehensive series of direct shear tests to measure the shear strength of the clay-metal interface under varying water contents. The analysis focused on examining the influence of the consistency index on the shear strength of the clay-metal interface and comparing the shear strength with and without the application of an anti-adhesion coating. The results confirmed the effectiveness of the anti-adhesion coating and provided further insights into its underlying mechanism.

A water film theory for soil adhesion was proposed by Akiyama and Yokoi. (1972). In this theory, the interaction between clay and the metal block was modeled as a plate-water-plate interface, considering the lamellar structure of clay particles (Ren et al., 1990). This conceptualization is illustrated in Figure 6. The normal force acting on the clay-metal interface is calculated using the Laplace formula (2), which can be expressed as follows (Huang et al., 2013): W = π R 2 γ L cos ⁡ θ 1 + cos ⁡ θ 2 h (2) where h and R stand for the thickness and radius of the water film, respectively, θ₁ and θ₂ for the metal block and clay’s respective contact angles, and γ_L for the water’s surface tension. Thus, the following is the tangential force on the clay-metal interface: P = μ W = μ π R 2 γ L cos ⁡ θ 1 + cos ⁡ θ 2 h (3) where the coefficient of friction between the clay and the metal is denoted by μ. According to Eq. 3, the shear strength of the clay-metal interface is inversely proportional to the thickness of the water film (h) and directly proportional to the area of the water film (πR²). Zhang et al. (2004) reported that as the water content of clay increases from zero to the liquid limit, the water film at the clay-metal interface undergoes three stages: slow increase, rapid growth, and saturation. With an increase in water content or a decrease in consistency, the water film between the clay and metal interface gradually enlarges, leading to an increase in the water film area. Consequently, the shear strength of the clay-metal interface gradually increases during the first stage. When the water content reaches a certain level, the clay-metal interface becomes completely covered by water, and the water film area typically saturates during the second stage. The water film thickness rapidly increases, causing a rapid decrease in the shear strength of the clay-metal interface. In the third stage, the water film thickness at the clay-metal interface approaches saturation, resulting in a gradual decrease in the shear strength as the water content increases. Therefore, as the water content increases or the consistency index decreases, the shear strength of the clay-metal interface initially increases gradually, then decreases rapidly, and finally decreases slowly (Figure 4). The consistency index and normal pressure are factors that influence the area and thickness of the water film. Under the same normal pressure and consistency index, it can be assumed that the area and thickness of the water film between the clay and metal blocks are similar. In this case, the shear strength of the clay-metal interface is determined by the contact angle (θ₁) of the metal block. The uncoated and coated metal block surfaces have contact angles of 62.04° and 85.48°, respectively. The coated metal block surface has a higher contact angle than the uncoated metal block. However, as the contact angle θ₁ increases, the value of cosθ₁ decreases. Consequently, the adhesion between the coated metal block and clay has a lower strength compared to the uncoated metal block (Figure 5).

The shear strength of the clay-metal interface was investigated in relation to the consistency index using the large-scale direct shear test. A comparison was made between the shear strength of the clay-metal interface with coated and uncoated metal blocks. Furthermore, the mechanism of the anti-adhesion coating was further discussed. The main conclusions of the study are as follows:

(1) With an increase in shear displacement, the shear stress between the clay and metal blocks increased rapidly. However, it tended to stabilize after reaching a certain shear displacement magnitude.

This study introduces a novel approach to prevent shield clogging, which involves applying anti-sticking coatings on the surface of the cutter and cutting tools. However, the current research is limited to laboratory experiments, and the specific effectiveness needs further validation through field testing. Other properties of the coatings, such as wear resistance and high-temperature durability, also require further investigation.