Numerical investigation of slurry shield cutterhead design for clogging mitigation using a coupled computational fluid dynamics–discrete element method approach

Abstract

Cutterhead clogging remains a significant challenge in slurry shield tunneling, particularly in clay-rich strata, leading to reduced efficiency and increased operational costs. This paper presents a comparative numerical study of three common slurry shield cutterhead designs, atmospheric soft-soil cutterhead, atmospheric mix-ground cutterhead, and conventional cutterhead, using a coupled computational fluid dynamics–discrete element method (CFD–DEM) approach. The integrated CFD–DEM model allows for a detailed examination of slurry flow dynamics and granular material behavior within the cutterhead chamber. The analysis focuses on muck discharge efficiency, quantifying the volume of excavated material effectively removed, as well as particle accumulation patterns within the slurry chamber, identifying areas prone to clogging. Furthermore, the study examines cutterhead torque and thrust requirements for each design, providing a comprehensive performance assessment. Results demonstrate variations in performance across the three types, with the conventional cutterhead exhibiting the highest muck discharge rate under the tested conditions, while atmospheric-pressure designs show a higher risk of clogging due to increased particle accumulation in specific zones. This work contributes to a more informed selection and design methodology for slurry shield cutterheads, enabling engineers to optimize designs for specific soil conditions and minimize the risk of clogging in challenging geological conditions.

Graphical abstract

Keywords

slurry shield / cutterhead design / cutterhead clogging / CFD–DEM modeling

1 Introduction

As an efficient ground excavation technique, slurry shield tunneling has been widely applied in tunnel engineering construction. Its characteristics of high efficiency, environmental friendliness, and safety have established it as a mainstream technology for urban tunnel projects. However, with the continuous expansion of project scales and increasing geological complexity, conventional shield technologies have demonstrated growing limitations in addressing muck retention issues under complex geological conditions. For instance, in heterogeneous strata such as gravel and cohesive soils, the timely discharge of muck has become a critical challenge constraining tunneling efficiency and safety [1], significantly impacting project timelines and economic viability. The mechanisms underlying cutterhead clogging involve multifaceted factors spanning geological conditions, construction control, and design considerations. Geologically, clay content, mineral composition (e.g., montmorillonite > 15%), and moisture content are primary contributors to cutterhead clogging [2,3]. From a construction perspective, thrust parameters, slurry material composition, annular flow velocity, and flushing measures critically influence clogging risks. Additionally, cutterhead structural design tailored for clay-rich formations plays a decisive role in clogging mitigation [4]. The clogging process entails complex interactions, including compression, shearing, adhesion, and friction, between the cutterhead and excavated clay [5,6]. Optimized cutterhead geometry has been proven to reduce clogging probability by 20%–35% [7]. Conventional engineering practices typically increase cutterhead opening ratios to mitigate clogging risks [8]. For example, in the Suez Canal Subaqueous Tunnel project, the opening ratio was elevated from 31% to 46% to address long-distance clay stratum penetration [9]. Nevertheless, a systematic methodology for cutterhead selection and structural design targeting clogging issues remains absent.

To overcome this technical bottleneck, the engineering community has explored advanced cutterhead configurations. Current mainstream designs include spoke-type, panel-type, and hybrid spoke-panel structures [10]. Spoke-type cutterheads feature high opening ratios with cutters mounted directly on spokes. Panel-type designs exhibit lower opening ratios but enhance face stability through integrated cutter panels. Hybrid configurations balance structural reinforcement and opening optimization. Earth pressure balance shields typically adopt these three types, while slurry shields predominantly utilize panel-type or hybrid designs to ensure excavation face stability.

Despite extensive engineering applications, systematic comparisons and quantitative analyses of muck discharge efficiency across cutterhead types in complex geology remain lacking. Current selection practices rely heavily on empirical judgments, increasing project risks. Recent studies have advanced understanding through experimental and computational approaches. Yang et al. [11] developed a laboratory apparatus demonstrating that higher cutterhead rotation speeds and lower advance rates reduce clay adhesion. Wan and Jin [12] identified opening ratio and maximum aperture size as critical parameters. However, physical testing faces limitations in cost and scale effects, hindering full characterization of cutterhead-muck-slurry multiphase interactions.

Beyond physical and numerical experiments, data-driven approaches such as intelligent algorithms and multi-objective decision models have also been explored for cutterhead optimization and performance prediction [13–19]. However, while these methods excel at identifying patterns from existing data, they often lack the physical interpretability to elucidate the underlying mechanisms of clogging, particularly the complex fluid-particle-structure interactions within the slurry chamber. This limitation underscores the necessity for a mechanistic modeling approach that can explicitly simulate the multiphase flow dynamics and granular behavior responsible for clogging. Discrete element method (DEM) effectively models particle-scale interactions, revealing that particle-to-screw pitch ratios critically govern discharge efficiency [20,21]. Computational fluid dynamics (CFD) captures slurry flow characteristics but struggles with discrete particle behavior [22]. To address this, coupled computational fluid dynamics–discrete element method (CFD–DEM) frameworks with dynamic meshing and bidirectional force transfer enable multiscale simulations of fluid-particle-cutterhead interactions [23,24]. In light of this benefit, the CFD–DEM coupling technique is being increasingly adopted for the design, optimization, and refinement of slurry shield machines [25].

In this study, a CFD–DEM coupled model was developed to investigate three typical cutterhead configurations. The framework integrates FLUENT for slurry flow simulation and EDEM for granular dynamics, connected via API interfaces for real-time data exchange. This approach enables accurate prediction of clay migration patterns under pressurized slurry environments while accounting for cutterhead geometry and kinematic parameters. By analyzing thrust responses and muck transport characteristics across designs, structural optimization strategies for clogging mitigation are proposed, advancing scientific cutterhead selection and design methodologies.

2 Geometric modeling of cutterhead

This chapter focuses on the geometric modeling of typical cutterhead structures used in slurry shield tunneling, aiming to investigate the performance differences among various cutterhead designs. We begin by summarizing the mainstream cutterhead structures, including spoke-type, panel-type, and hybrid configurations, analyzing their geometric features and applicable conditions. Subsequently, we select three representative cutterhead designs, a soft-soil cutterhead, a mixed-soil cutterhead, and a conventional cutterhead, and create simplified models for numerical simulations. These models not only provide a foundation for subsequent analyses but also offer theoretical insights into how parameters like opening ratio and structural thickness affect tunneling performance.

2.1 Main cutterhead structural types

According to their main structural type, shield cutterheads can be classified into spoke type, panel type, and spoke-panel type. The spoke type cutterhead’s main structure consists of spokes, with pilot cutterheads, scraping cutterheads, and other soft soil cutting tools directly mounted on cylindrical or box-shaped spokes. It is characterized by a relatively large opening ratio, as shown in Fig. 1(a). The panel type cutterhead’s main structure is composed of panels of various shapes, equipped with rolling cutterheads or ripping cutterheads, while the panel edges are equipped with cutting and scraping tools. It generally has a smaller opening ratio, as shown in Fig. 1(b). The spoke-panel type cutterhead’s main structure incorporates both spokes and panels. However, the main difference from the panel type is that its panels are not equipped with main cutting tools such as ripping cutterheads or cutting tools; the panels only serve as sealing plates to strengthen the cutterhead structure and reduce the opening ratio, as shown in Fig. 1(c). From the cutterhead structure itself and the arrangement of cutting tools, the spoke-panel type cutterhead is a combination structure type between the spoke type and the panel type. Since earth pressure balance shield tunneling is applicable to a wide range of strata, often encountering multiple geological conditions within the same cross-section, all three structural types are used in cutterhead design. For slurry shield, which rely on slurry pressure to balance the earth pressure on the excavation face, the panel type or spoke-panel type is usually chosen for cutterhead design to ensure excavation face stability.

In highly permeable strata with poor stability, such as water-rich sandy layers and sandy cobble strata, and in underwater tunnels, slurry shield is typically employed. The cutterhead structure commonly adopts a panel type design with an opening ratio of 10% to 30%. Based on a survey of existing international slurry shield construction cases, the widely adopted slurry shield cutterhead structures can be broadly classified into three categories according to their opening shape.

Type A slurry shield cutterhead is a spoke-panel type structure, as shown in Fig. 2. The main features of this type of cutterhead are: its main structure consists of five or six square or circular spokes. To control the opening ratio, triangular panels are arranged between the spokes at the cutterhead periphery. Each opening is ‘Y’-shaped, and there are five or six ‘Y’-shaped openings on each cutterhead. This type of cutterhead generally features ripping cutterheads and cutting knives on the main spokes; rolling cutterheads are typically not equipped. The triangular panels generally do not have cutting tools. This type of cutterhead is an atmospheric soft-soil cutterhead, generally allowing for atmospheric replacement of ripping cutterheads, scraping knives, etc. In strata with high clay content, the area of the triangular panels is often reduced to increase the cutterhead opening ratio, while additional circumferential ribs are added to assist in excavation face stability, as shown in Fig. 2. This type of cutterhead is mainly suitable for soft soil strata characterized by silty soils, clays, and sandy layers.

Type B slurry shield cutterhead is also a panel-type structure, as shown in Fig. 3. The main characteristics of this type of cutterhead are: six main arms are distributed across the cutterhead panels. A ‘V’-shaped opening is located between adjacent arms. To prevent excessive opening size, radial and circumferential ribs are arranged in each opening to maintain excavation face stability. The type B cutterhead is also an atmospheric mix-ground cutterhead, but compared to the type A, the type B typically incorporates rolling cutterheads. This type of cutterhead is suitable for both soft soil layers such as weak silty clay, loose sandy soil, and gravelly soil, and composite strata including cobble layers and hard rock. Due to the need to replace rolling cutterheads in the central area, the type B cutterhead usually cannot have openings in the center, thus hindering central slag discharge. In contrast, the type A cutterhead, equipped only with scraping knives and ripping cutterheads (soft-soil cutting tools), can have openings in the central area, resulting in relatively better spoil mobility. Due to the space requirements for the cutterhead replacement chamber, the thickness of atmospheric cutterheads is greater than that of conventional cutterheads.

Type C slurry shield cutterhead is a panel-type structure, as shown in Fig. 4. Its main characteristics are: the main structure can be considered as a panel composed of eight main spokes and eight secondary spokes arranged in a crisscross pattern. Rectangular openings are distributed on both sides of the spokes. The entire cutterhead generally has 16 rectangular openings. The panels are equipped with ripping cutterheads, cutting knives, and other main cutting tools. Rolling cutterheads and other hard rock cutting tools can also be equipped as needed. Compared to the first two types of cutterheads, the type C has more spokes, and cutting tools can be configured on both the main and secondary spokes as needed, unlike the type A where the triangular panels generally do not have cutting tools. The type C cutterhead is a conventional cutterhead and is widely used in earth pressure balance shields and Slurry balancing shields. Because it can be equipped with rolling cutterheads and other hard rock cutting tools, this type of cutterhead is suitable for both soft soil layers such as weak silty clay, loose sandy soil, and gravelly soil, and composite strata including cobble layers and hard rock. Compared to the type A cutterhead, its application range is broader. Compared to atmospheric cutterheads, conventional cutterheads do not require space for cutterhead replacement, resulting in a larger number of arms, smaller individual arm areas, and therefore a larger number of openings, although the individual opening area is smaller.

2.2 Simplified geometric model of slurry shield cutterhead

Based on the summary of existing slurry shield cutterhead structural types in Subsection 2.1, existing slurry shield cutterheads can be roughly divided into three types: atmospheric soft-soil cutterheads, atmospheric mix-ground cutterheads, and conventional cutterheads. To compare the excavation response of the three types of cutterheads in clay strata, three typical representatives of the structural types, named Cutterheads A, B, and C, were selected for numerical modeling, as shown in Fig. 5. Cutterhead A represents an atmospheric soft-soil cutterhead, i.e., the cutterhead shown in Fig. 2. Cutterhead B represents an atmospheric mix-ground cutterhead, i.e., the cutterhead shown in Fig. 3, and Cutterhead C represents a conventional cutterhead, i.e., the cutterhead shown in Fig. 4. The opening ratios of the three types of cutterheads are 31%, 31%, and 33%, respectively. Since the opening ratios are similar, the influence of the opening ratio on the calculation results is eliminated.

Based on the three cutterhead structures shown in Fig. 5, three simplified geometric models for slurry shield cutterhead were established, as shown in Fig. 6. All three models have a cutterhead diameter of 6 m. Cutterheads A and B are atmospheric cutterheads; therefore, they have a thicker design. The main arm thickness is 800 mm. The triangular panel thickness of Cutterhead A is 200 mm, and the rib thickness at the openings of Cutterhead B is also 200 mm. Cutterhead C is a conventional cutterhead with a thickness of 400 mm. The dimensions of the slurry chamber are the same in all models, with a thickness of 600 mm. The inflow pipe is located at the top of the slurry chamber with a diameter of 400 mm, and the outflow pipe is located at the bottom of the slurry chamber with a diameter of 600 mm.

3 Computational fluid dynamics–discrete element method coupled modeling of shield driving

In this chapter, we employ a CFD–DEM coupled approach to develop a numerical simulation model for cutterhead tunneling in complex geological conditions. This coupling technique allows for a comprehensive examination of the interactions between fluid and granular phases, accurately depicting the multiphase flow dynamics of slurry movement and particle behavior. We detail the construction process of the coupled model, which includes CFD modeling for the fluid phase, DEM modeling for the granular phase, and the integration of fluid-particle interactions. By utilizing Ansys Fluent and EDEM for simultaneous computations, we achieve simulations of three typical cutterhead designs within a slurry environment, enabling us to analyze key performance indicators such as flow field distribution, particle accumulation patterns, and slurry discharge efficiency.

3.1 Computational fluid dynamics–discrete element method coupled model

Figure 7 shows the full-scale DEM models of the three simulated shield excavations, including half of the stratum and internal details. The model height (H) is 7 m, length (L) is 4 m, and width (W) is 7 m. The bottom, front, and back surfaces of the particle box in the model are set as rigid walls, while periodic boundary conditions are set on the left and right sides. This ensures that escaping particles re-enter the particle box from the opposite side, minimizing the boundary effect with the fewest possible particles. Particles with a radius of 0.05 m are used in all models. Given that the present study is designed to compare cutterhead performance under strictly identical conditions, all simulations share the same boundary settings and domain dimensions; consequently, any boundary effects are systematically imposed on all three cases. While the absolute values of accumulated mass or flow velocity may be subject to domain-scale perturbations, the relative performance discrepancies among Cutterheads A, B, and C remain both comparable and diagnostically meaningful.

Figure 8 shows the CFD models of the three slurry chambers. Each model comprises a rotating domain and a stationary domain. The rotating domain represents the boundary that rotates with the cutterhead, while the stationary domain represents the fixed boundary. In Cutterheads A and B, the rotating domain consists of the openings between the main arms of the cutterhead. Furthermore, due to the large space occupied by the central cutterhead changing chamber, the meshes for Cutterheads A and B are hollow structures. Cutterhead C, corresponding to the conventional cutterhead, only has four main support arms and a central rotating shaft responsible for cutterhead rotation; therefore, the flow space in its slurry chamber is larger than that of the atmospheric cutterheads.

3.2 Computational fluid dynamics–discrete element method coupling scheme

CFD–DEM coupled simulations were conducted using two software packages: Ansys Fluent for slurry flow calculation and EDEM for soil particle motion calculation. Figure 9 shows a detailed flowchart of the CFD–DEM coupled model. Ansys Fluent and EDEM provide a computational interface, namely the Application programming interface (API), for transferring energy and momentum between the CFD and DEM solvers. At the beginning of the calculation, Fluent calls the EDEM API to read information about particle velocity and position. Based on the particle state at the current time step, the CFD mesh porosity and flow state are calculated. After iterative convergence, a drag model is used to convert the slurry flow state into fluid-particle interaction forces, which are then transferred to EDEM via the Ansys API. EDEM then begins its time-step calculation, computing and updating the velocity and position of the DEM particles. Finally, the particle information is sent back to Fluent, initiating a new cycle to complete the two-way coupled calculation.

3.3 Computational fluid dynamics modeling of the fluid phase

In this study, the slurry within the chamber is modeled as a single-phase, homogeneous fluid. Consequently, the flow behavior is governed by the fundamental principles of mass, momentum, and energy conservation. The governing equations, representing these principles, are the continuity equation, the Navier–Stokes equations, and the energy conservation equation, respectively.

(1)\frac{\mathrm{\partial }{\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}}{\mathrm{\partial }t}+\mathrm{\nabla }\cdot ({\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}{\mathit{U}}_{\mathrm{f}})=0,

(2)\frac{\mathrm{\partial }}{\mathrm{\partial }t}({\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}{\mathit{U}}_{\mathrm{f}})+\mathrm{\nabla }\cdot ({\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}{\mathit{U}}_{\mathrm{f}}{\mathit{U}}_{\mathrm{f}})=-\mathrm{\nabla }{p}_{\mathrm{f}}+\mathrm{\nabla }\cdot {\alpha }_{\mathrm{f}}\mathit{\tau }+{\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}\mathit{g}+{\mathit{F}}_{\mathrm{p}\mathrm{f}},

(3)\frac{\mathrm{\partial }}{\mathrm{\partial }t}({\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}T)+\mathrm{\nabla }\cdot ({\alpha }_{\mathrm{f}}{\rho }_{\mathrm{f}}{\mathit{U}}_{\mathrm{f}}T)=\mathrm{\nabla }\cdot \left(\frac{k_T}{c_{\mathrm{P}}}\mathrm{\nabla }T\right)+s_T,

where α_f, ρ_f, p_f, and U_f denote the volume fraction, density, pressure, and velocity vector of the slurry, respectively; T represents temperature; s_T signifies viscous dissipation; c_P and k_T are the specific heat and thermal conductivity of the slurry; F_pf represents the interphase force exerted on the slurry by the solid particles; and τ is the viscous stress tensor. Crucially, the slurry density ρ_f and the viscous stress tensor τ require closure relationships to be defined. Specifically, for non-Newtonian slurries, the relationship between the viscous stress tensor τ and the strain rate tensor D can be expressed as

(4)\mathit{\tau }=\mu \dot{\gamma }\mathit{D},

(5)\mathit{D}=\frac{1}{2}\left(\mathrm{\nabla }\mathit{u}+\mathrm{\nabla }{\mathit{u}}^{\mathrm{T}}\right),

where the viscosity μ is a function of the shear rate, and u is the velocity tensor.

Consistent computational parameters were maintained across all three models to mitigate the influence of extraneous variables on the results. Table 1 details these parameters. The physical properties of the slurry within the CFD model were defined according to the data published by Yang et al. [26]. Field slurry shields typically use a suspension with 5%–10% bentonite to stabilize the face and transport debris, but current CFD–DEM studies simplify this due to computational costs. This study employs a Herschel–Bulkley rheological model to capture bentonite slurry’s yield stress and shear-thinning behavior, effectively reflecting the dominant influence of fine particles on flow characteristics. Owing to the enormous computational cost, this simplification is standard in current CFD–DEM studies [27]. While limitations exist, such as neglecting inter-phase interactions and spatial variations in viscosity, the comparative conclusions about cutterhead performance remain valid. The rotating domain was assigned a rotational speed of 0.5 r/min, and a time step of 0.01 s was selected for the transient simulations.

3.4 Discrete element method modeling of the particle phase

Within the slurry chamber, soil particles undergo a complex interplay of collisions and compressive forces, stemming from interactions with both the pipe walls and other particles. To model this particulate behavior, we employ the DEM, as originally formulated by Cundall and Strack [28]. The translational and rotational motion of individual soil particles is governed by Newton’s second law of motion, expressed as

(6)m_i\frac{\mathrm{d}{\mathit{U}}_{\mathrm{p},i}}{\mathrm{d}t}=m_i\mathit{g}+\sum _{j=1}^n({\mathit{F}}_{\mathrm{d},ij}+{\mathit{F}}_{\mathrm{c},ij})+({\mathit{F}}_{\mathrm{d},i\mathrm{w}}+{\mathit{F}}_{\mathrm{c},i\mathrm{w}})+{\mathit{F}}_{\mathrm{f},i},

(7){\mathit{I}}_i\frac{\mathrm{d}{\omega }_i}{\mathrm{d}t}=\sum _{j=1}^n{\mathit{T}}_{ij}+{\mathit{T}}_{i\mathrm{w}},

where m_i and U_p,i denote the mass and velocity of particle i, ω_i and I_i are the rotational velocity and moment of inertia of particle i, F_d,ij, F_c,ij, and T_i are the viscous damping force, contact force, and torque between particles i and j, respectively, F_d,iw, F_c,iw, and T_iw are the viscous damping force, contact force, and torque exerted by the pipe wall on particle i, respectively, and F_f,i is the force exerted on particle i by the fluid.

To accurately represent the cohesive interactions between clay particles, the Hertz–Mindlin with JKR contact model was chosen. This model incorporates the Johnson–Kendall–Roberts (JKR) theory for calculating the normal elastic contact force [29]. The Hertz–Mindlin with JKR model, often termed the JKR cohesive model, is well-suited for simulating adhesion and aggregation phenomena arising from electrostatic forces and moisture content, as demonstrated by Zhou et al. [30]. Within the JKR cohesive model, the tangential contact force, normal damping force, and tangential damping force are implemented analogously to the Hertz–Mindlin (no slip) contact model described by Mindlin [31]. The expression for the normal contact force is given by

(8)F_n^{\mathrm{J}\mathrm{K}\mathrm{R}}=\frac{4{\alpha }^3{E}^{\ast }}{3R^{\ast }}-\sqrt{8\pi {\alpha }^3\gamma E^{\ast }},

where γ denotes the surface energy, α is the contact radius between two particles, E* is the effective elastic modulus, and R* is the effective contact radius. E* and R* can be calculated using the following formulas, respectively.

(9)1/E^{\ast }=\left(1-{\nu }_i^2\right)/E_i+\left(1-{\nu }_j^2\right)/E_j,

(10)1/R^{\ast }=1/R_i+1/R_j,

where E_i and E_j represent the elastic moduli of particles i and j, while v_i and v_j denote their respective Poisson’s ratios. Additionally, R_i and R_j indicate the radii of these particles. This model goes further by including a cohesive force, which simulates attraction even when the particles are not touching. The maximum cohesive force sustainable before separation F_c and the corresponding critical separation distance δ_c under tensile loading are given by

(11)F_{\mathrm{c}}=2\pi \gamma R^{\ast },

(12){\delta }_{\mathrm{c}}=\frac{{\alpha }_{\mathrm{c}}^2}{R^{\ast }}-\sqrt{4\pi \gamma {\alpha }_{\mathrm{c}}/E^{\ast }},

where α_c can be expressed as

(13){\alpha }_{\mathrm{c}}={\left[\frac{9\pi \gamma R^{\ast 2}}{2E^{\ast }}\left(\frac{3}{4}-\frac{1}{\sqrt{2}}\right)\right]}^{\frac{1}{3}}.

Table 2 summarizes the computational parameters used in the DEM model. The particle and contact parameters were calibrated using direct shear tests, adhesive tests, and compression tests conducted on real soil samples from an engineering case study, as detailed in Ref. [24]. These experimentally derived parameters ensure the model’s accuracy in capturing adhesion and clogging behavior. The cutterhead rotational speed ω was set to 0.5 r/min, with an advance rate v of 30 mm/min. To ensure computational stability and efficiency, the DEM time step was set at 0.0002 s, resulting in a CFD time step 50 times larger. The CFD–DEM coupling in this study links the CFD time step t_c and DEM time step t_d via a sub-stepping scheme, where multiple DEM steps occur within a single CFD step (t_c = N × t_d). The DEM time step t_d is chosen based on stability criteria for particle contact interactions, ensuring that the Rayleigh time step constraint is satisfied. The CFD time step t_c is selected based on the Courant–Friedrichs–Lewy (CFL) condition to maintain fluid simulation stability. For this study, baseline time steps were set to t_d = 0.0002 s, t_c = 0.01 s, and N = 50.

For completeness, a rigorous engineering validation of the CFD–DEM approach was conducted in our prior work [24]. This validation was performed on Beijing’s South-to-North Water Transfer project, using data from conventional slurry shield machines, including cutterhead torque and thrust. A direct comparison between field measurements and model predictions confirmed the reliability of the CFD–DEM methodology applied in this study.

The DEM model in this study uses spherical particles with a radius of 0.05 m to maintain computational efficiency, a common simplification in large-scale geotechnical simulations. While spherical particles deviate from the irregular morphology of real clay grains and underestimate inter-particle locking and cohesion, the JKR cohesion model was recalibrated to match clay-steel adhesion data from laboratory tests, ensuring the macroscopic adhesive behavior aligns with empirical observations. All cutterhead models adopt the same scaled particle size, ensuring valid comparisons of clogging potential, discharge efficiency, and flow patterns. The chosen particle size effectively reproduces clogging mechanisms governed by the particle-to-opening size ratio, and the slurry flow field captures overall fluid dynamics and pressure distribution at a larger scale. Thus, despite limitations in absolute values, the calibrated model reliably supports qualitative comparisons and mechanistic insights into cutterhead designs.

3.5 Particle-fluid interaction

The fluid phase exerts a force, denoted as F_f,i, on each particle. This force is a composite of three distinct interaction forces: buoyancy F_b, pressure gradient force F_p, and drag F_d. The expressions for the buoyancy force F_b and pressure gradient force F_p are given by

(14){\mathit{F}}_{\mathrm{b}}=-\frac{{\rho }_{\mathrm{f}}}{{\rho }_{\mathrm{p}}}\mathit{g},

(15){\mathit{F}}_{\mathrm{p}}=-\frac{1}{{\rho }_{\mathrm{p}}}\mathrm{\nabla }{p}_{\mathrm{f}},

where ρ_p denotes the particle density. The drag F_d can be calculated using the following equation

(16){\mathit{F}}_{\mathrm{d}}=\frac{18{\mu }_{\mathrm{e}}}{{\rho }_{\mathrm{p}}{d}_{\mathrm{p}}^2}\frac{C_{\mathrm{D}}R{e}_{\mathrm{p}}}{24}\left({\mathit{U}}_{\mathrm{f}}-{\mathit{U}}_{\mathrm{p}}\right),

where U_p is the particle velocity, μ_e is the effective fluid viscosity, d_p is the particle diameter, and the particle Reynolds number Re_p and drag coefficient C_D are calculated as follows

(17)R{e}_{\mathrm{p}}=\frac{\rho d_{\mathrm{p}}\left|{\mathit{U}}_{\mathrm{p}}-{\mathit{U}}_{\mathrm{f}}\right|}{\mu },

(18)C_{\mathrm{D}}=a_1+\frac{a_2}{Re}+\frac{a_3}{Re},

where Re is the fluid Reynolds number, a₁, a₂, and a₃ are constants with reference values as follows

(19)

4 Model calculation results

4.1 Particle phase results

Figures 10–12 show the velocity distributions within the excavation chamber at four different time points for Cutterheads A, B, and C, respectively. Observing the slurry chamber from inside the tunnel, the cutterhead rotates clockwise in the figures. As time progresses, an increasing accumulation of particles is observed in the slurry chamber in all three models, particularly in the lower left corner near the slurry chamber wall. This is because the clockwise rotation of the cutterhead causes particles on the right side to move toward the bottom of the slurry chamber under the cutterhead’s action and are discharged through the slurry discharge pipe; while particles on the left side are less likely to fall due to the cutterhead rotation and adhere to the slurry chamber wall. Over time, the area of adhesion gradually increases, leading to an increased risk of clogging. Therefore, when excavating in cohesive soil using a slurry shield, it is recommended to switch the cutterhead rotation direction after a certain excavation distance to prevent prolonged accumulation of excavated soil on one side of the slurry chamber. Furthermore, since Cutterheads A and B employed an atmospheric cutterhead with thicker main arms, particle accumulation was also observed between adjacent main arms. This indicates that the thicker main arm design in atmospheric cutterheads provides convenient conditions for the accumulation of excavated soil in the slurry chamber, potentially increasing the risk of cutterhead clogging.

The shield advance rate and cutterhead opening ratio were the same in all three models; therefore, the excavated soil volume per unit time was similar. The models used the same slurry flow rate, so the excavated soil discharge rate can reflect the smoothness of excavated soil discharge and indirectly reflect the risk of cutterhead clogging. To monitor the excavated soil discharge efficiency of the slurry discharge pipe, a mass flow sensor was placed at the slurry discharge pipe location in the DEM model. The monitoring results for the three models are shown in Fig. 13.

Figure 13 shows that in the first 50 s of excavation, the excavated soil is just entering the slurry chamber, and the discharge rate is generally low. After 50 s, the discharge rate gradually increases and stabilizes. Among the three models, Cutterhead C has the highest discharge rate, approximately 100 kg/s; Cutterhead A is next, at approximately 60 kg/s; and Cutterhead B is the lowest, at approximately 50 kg/s. The discharge rate exhibits a fluctuating cyclical pattern over time, particularly evident in Cutterhead B. This is due to changes in the fluid domain within the slurry chamber caused by cutterhead rotation. When a cutterhead opening faces the discharge port, the discharge rate is higher; when a main arm faces the discharge port, the discharge rate is lower. Cutterhead B has six main arms and six openings. With a cutterhead rotation speed of 0.5 r/min, the cycle for a single arm and opening to pass the discharge port is 20 s, consistent with the fluctuation period of the discharge rate in Fig. 13. Fluctuating cyclical patterns in discharge rate are also observed in Cutterheads A and C, but their fluctuation periods are shorter due to the larger number of openings. In comparison, the atmospheric cutterhead exhibits a lower discharge rate than the conventional cutterhead. Therefore, for slurry shields equipped with atmospheric cutterheads, measures such as increasing the slurry circulation rate or reducing the excavation speed should be adopted to reduce the risk of slurry chamber blockage.

The change in particle accumulation in the slurry chamber over time for the three models are shown in Fig. 14. It shows that in the first 100 s, particles accumulate rapidly in the slurry chamber. After 100 s, the particle accumulation rate slows down, and the accumulated particle mass increases slowly. Among the three cutterhead designs, Cutterheads A and B show significantly higher particle accumulation than Model C. Because Cutterheads A and B use atmospheric cutterheads, their slurry chamber has a smaller fluid domain volume, yet experiences greater particle accumulation, thus increasing the risk of slurry chamber clogging. Therefore, for slurry shields equipped with atmospheric cutterheads, it is recommended to install high-pressure water jet nozzles in the slurry chamber to prevent blockage due to excavated soil accumulation.

4.2 Fluid phase results

Figure 15 shows the front pressure in the slurry chamber at 200 s for the three models. The cutterhead rotates counterclockwise in the figure. In all three models, the pressure on the left side of the chamber is significantly higher than that at the same height on the right side, a phenomenon caused by cutterhead rotation. Slurry enters the slurry chamber from the inflow pipe at the top and flows downward, ultimately exiting through the outflow pipe at the bottom. During this process, the counterclockwise rotation of the cutterhead affects the slurry flow. On the one hand, in the left half of the slurry chamber in Fig. 15, the cutterhead rotation direction is consistent with the slurry flow direction, resulting in lower pressure loss due to the cutterhead wall. On the other hand, in the right half of the slurry chamber in Fig. 15, the cutterhead rotation direction is opposite to the slurry flow direction, resulting in greater pressure loss due to the cutterhead wall. Therefore, the pressure on the left side of the chamber is significantly higher than that at the same height on the right side, and this phenomenon is more pronounced closer to the outer edge of the slurry chamber. Cutterheads A and B, i.e., atmospheric cutterheads, have thicker cutterheads. During rotation, the main arms of these cutterheads disturb the slurry more intensely than those of conventional cutterheads. Therefore, the pressure difference between the left and right sides of the slurry chamber in Figs. 15(a) and 15(b) is more significant than that in Fig. 15(c). The cutterhead rotation ensures sufficient pressure difference for excavated soil in the left region of Fig. 15 to be discharged from the slurry chamber, but it negatively affects the flow of excavated soil in the right region. This effect is more pronounced in slurry shields equipped with atmospheric cutterheads.

Figures 16 and 17 illustrate the velocity distributions on the front and back of the slurry chamber, respectively. As shown, the flow velocity on the positive Y-axis side is significantly higher than that on the negative Y-axis side. This disparity accelerates the accumulation of soil cuttings on the negative Y-axis side of the slurry chamber. Furthermore, a distinct low-velocity zone is observed beneath the baffle plate on the negative Y-axis side of the slurry chamber. Evidently, the accumulation of particles further reduces the slurry velocity in this region, creating a detrimental feedback loop. Therefore, periodically reversing the cutterhead rotation direction during tunneling operations is crucial to prevent prolonged accumulation of soil cuttings on one side of the slurry chamber, thereby effectively minimizing the probability of soil consolidation and cake formation.

As illustrated in Figs. 16 and 17, the velocity field upstream and downstream of Cutterhead C exhibits approximate symmetry about the Z-axis. This symmetry is a direct consequence of the cutterhead’s structural configuration. Unlike atmospheric-type Cutterheads A and B, which house a large central chamber for tool replacement, Cutterhead C is composed exclusively of slender, evenly spaced arms (Fig. 5(c)), creating a more open and uniform plenum. The symmetric obstruction pattern produced by these arms, together with the higher number of smaller apertures, prevents the formation of large-scale asymmetric vortices. Consequently, the resistance distribution from inlet to outlet is more balanced, yielding a symmetric velocity profile. Such hydrodynamic uniformity promotes homogeneous slurry transport throughout the chamber and mitigates local particle deposition and stagnant zones—issues that are pronounced in the asymmetric flow fields of Cutterheads A and B. This structural advantage underpins the superior muck-removal performance of Cutterhead C reported in Subsection 4.1.

4.3 Cutterhead torque and thrust

The torque and thrust of the cutterhead can serve as important criteria for mud cake formation. Higher torque often correlates with increased cutterhead resistance, particularly in cohesive soils, which may lead to more frequent cake formation on the cutterhead surface. Similarly, excessive thrust can contribute to clogging by compacting soil particles against the cutterhead openings, reducing the efficiency of muck discharge. For example, in some cases, an increment of 2–6 times in torque was observed when clogging events occurred [32], which identified excessive torque and torque as a key criterion in exacerbating clogging risks.

Figures 18 and 19 show a comparison of cutterhead torque and thrust for three models. As shown in the figures, under the same excavation conditions, the torque values of Cutterheads A and C are similar, approximately 120 kN·m, while Cutterhead B has the highest torque value, approximately 150 kN·m. The elevated torque of Cutterhead B stems from its atmospheric-composite configuration. The absence of a central aperture prevents immediate evacuation of spoil from the core region, prolonging particle–cutter interactions and further amplifying torque. With the same cutterhead opening area, Cutterhead B needs to overcome greater frontal frictional resistance, thus leading to more frictional heat generation and increasing the risk of excavated soil cake formation.

Regarding thrust, Cutterhead A exhibits the lowest thrust, approximately 750 kN, followed by Cutterhead B at approximately 850 kN, with Cutterhead C showing the highest thrust at approximately 900 kN. Cutterhead thrust comprises two components: the normal force on the cutterhead face and the tangential frictional resistance on the cutterhead opening walls. The three cutterheads have the same frontal surface area; therefore, under identical excavation parameters, the frontal normal forces are essentially similar. Since Cutterhead C has the most openings and thus the largest surface area of the opening walls, it experiences the greatest tangential frictional resistance, resulting in the highest thrust. Conversely, Cutterhead A, with its disc-cutterhead structure and thin triangular panels, exhibits the least frictional resistance on the panel side walls, resulting in the lowest thrust. Considering both torque and thrust performance, Cutterhead A demonstrates the lowest energy consumption and the lowest risk of clogging.

Time-series analysis reveals the limitations of the initial energy consumption advantage of atmospheric cutterheads and clarifies the inherent contradiction with clogging risks. Taking the Type A atmospheric cutterhead (atmospheric soft-soil cutterhead) as an example, its energy consumption dynamics are remarkable: as shown in Fig. 18(a), this cutterhead exhibits the lowest torque during the initial excavation phase (< 50 s), confirming its initial low energy consumption advantage. However, as tunneling progresses, the torque shows a continuous upward trend. Correlation analysis between the torque time-history curve and particle accumulation data (Fig. 14) reveals that the rising trend in torque is highly synchronized with the rapid growth phase of particle accumulation (50–150 s), indicating that the sharp increase in particle mass within the chamber directly leads to a steady climb in torque. This demonstrates that while the structure of atmospheric cutterheads provides initial low torque, their propensity for clogging causes fluid resistance and mechanical friction resistance to increase significantly over time, ultimately requiring more energy to maintain rotation. Thus, the ‘initial low energy consumption’ and ‘long-term high energy consumption risk’ are actually two manifestations of the structural characteristics of atmospheric cutterheads at different time dimensions, resolving the apparent contradiction. It is worth noting that the Type B cutterhead does not exhibit the same trend due to structural differences (such as being equipped with disc cutters and having no central opening), indicating performance differentiation among atmospheric cutterheads. Nevertheless, the time-history data of the Type A cutterhead sufficiently confirms the core mechanism whereby clogging leads to dynamically increasing energy consumption.

5 Discussion

The results of this study provide a comprehensive understanding of the excavation performance and clogging risks associated with different slurry shield cutterhead designs. By employing a CFD–DEM coupled approach, the interactions between the fluid and particle phases were analyzed in detail, revealing key insights into the excavation dynamics and behavior of slurry shields under varying cutterhead configurations. This discussion synthesizes the primary findings, highlights the implications for cutterhead design and operation, and offers recommendations for improving tunneling performance in cohesive soil strata.

5.1 Particle accumulation and clogging risks

One of the significant findings of this study is the pronounced particle accumulation in the slurry chamber, particularly for atmospheric cutterheads (Cutterheads A and B). The thick main arms of these cutterheads create conditions conducive to particle accumulation, as the reduced flow domain and increased obstruction lead to the formation of low-velocity zones. In contrast, the conventional cutterhead (Cutterhead C) exhibits comparatively lower particle accumulation due to its thinner structure and higher number of openings, which facilitate better slurry flow and particle discharge. However, Cutterhead C also faces challenges, as its numerous openings generate higher frictional resistance along the sidewalls, increasing the thrust required during excavation. These results suggest a trade-off between minimizing clogging risks and maintaining efficient thrust performance, which must be carefully balanced during cutterhead design.

In slurry shield tunneling, clogging arises from two linked mechanisms: bulk particle accumulation and adhesive cake formation. Our CFD–DEM model, despite simplifying adhesion physics, still captures the dominant flow and accumulation dynamics driven by geometry. It uses a Hertz–Mindlin (no-slip) contact to simulate muck buildup in low-flow zones due to gravity and entrapment, representing the primary clogging pathway. While Fang et al. [2] note a second mechanism, physical-chemical adhesion of fine clay to surfaces forming a dense cake, our model does not explicitly account for mineralogical cohesion or pore-water effects, potentially underestimating long-term cake risk. Nevertheless, the model reliably differentiates cutterhead designs in terms of particle mobility and accumulation: configurations with less accumulation tend to reduce adhesive clogging risk, and the short screw conveyor effectively suppresses initial buildup, limiting cake formation potential. Thus, the framework offers robust design guidance to mitigate early accumulation and subsequent clogging.

5.2 Pressure and velocity distribution

The study also highlights the impact of cutterhead rotation on the pressure and velocity distribution within the slurry chamber. Cutterhead rotation causes significant pressure differences between the left and right sides of the chamber. On the side where the cutterhead rotation aligns with the slurry flow direction, the pressure loss is reduced, and the slurry velocity is higher, ensuring smoother particle discharge. Conversely, on the side where the cutterhead rotation opposes the slurry flow, the pressure loss is greater, creating low-velocity zones that promote soil accumulation and increase clogging risks. This asymmetry in pressure and velocity distribution is more pronounced in atmospheric cutterheads due to their thicker structural design, which intensifies slurry flow disturbances. These findings underscore the importance of optimizing the cutterhead geometry to reduce flow resistance and ensure uniform slurry distribution, particularly for atmospheric cutterheads used in cohesive soils.

The quantitative comparison in Table 3 reveals notable differences in the performance of the three cutterhead types. The traditional panel-type cutterhead exhibits the lowest inlet-outlet pressure difference (46.3 kPa) and slurry pipe velocity (1.65 m/s), indicating strong muck transport capacity. However, this comes at the cost of higher torque and increased susceptibility to blockages due to its smaller rectangular openings. The atmospheric composite cutterhead achieves a moderate pressure difference (52.7 kPa) and slurry velocity (1.45 m/s), balancing effective muck discharge with improved resistance to clogging. Its larger ‘V’-shaped openings reduce the risk of blockages, making it suitable for mixed ground conditions with both hard and soft layers. In contrast, the atmospheric soft soil cutterhead has the highest-pressure difference (58.5 kPa) and slurry velocity (1.20 m/s), reflecting its limited capacity in gravel-rich formations. The reliance on rippers and scrapers, combined with its large ‘Y’-shaped openings, reduces efficiency in handling hard or abrasive soils. This cutterhead is best suited for soft, homogeneous soils where clogging risks are minimal.

5.3 Cutterhead torque and thrust

The torque and thrust performance of the three cutterhead designs reveal critical differences in energy consumption and operational efficiency. Cutterhead A exhibits the lowest torque and thrust, indicating lower excavation energy consumption and reduced risk of excessive frictional heat generation. In contrast, Cutterhead B requires the highest torque, due to its thicker structure and inability to discharge particles from the center, which increases frictional resistance and heat generation. Cutterhead C demonstrates the highest thrust, attributed to its greater number of openings and the resulting increase in sidewall friction. While the high thrust of Cutterhead C may enhance its applicability to a broader range of geological conditions, it also poses challenges for cohesive soils, where excessive thrust can lead to soil compaction and consolidation. These results highlight the need for a balanced cutterhead design that minimizes torque and thrust while maintaining efficient particle discharge.

5.4 Recommendations for cutterhead design and operation

Based on the findings of this study, several recommendations can be made to improve cutterhead performance and reduce clogging risks in slurry shield tunneling. First, for atmospheric cutterheads, increasing the opening ratio while maintaining structural stability can help reduce particle accumulation and improve slurry flow. The openings should be strategically positioned and sized to balance discharge efficiency and structural integrity. Secondly, during excavation, periodically reversing the cutterhead rotation direction can prevent prolonged particle accumulation on one side of the slurry chamber, mitigating the risk of soil consolidation. Periodically reversing the cutterhead rotation prevents particle accumulation in the slurry chamber. A combination of preventive periodic operations and warning-triggered interventions is recommended. Preventive operations, under tunneling parameters (advance rate: 30 mm/min, rotation speed: 0.5 r/min), should occur every 25–50 mm of advancement, as particle accumulation stabilizes after 50–100 s (Fig. 14). This disrupts incipient buildup and maintains muck removal without overloading the drive system. Warning-triggered reversals should activate if cutterhead torque exceeds historical averages by 15%–20%, shield thrust by 10%–15%, or the muck removal rate drops below 85%, with abnormal slurry density. In such cases, reversal (3–5 rotations) and slurry adjustments restore muck transport. This study focuses on a common combination of rotational speed (0.5 r/min) and propulsion speed (30 mm/min) to evaluate the performance of cutterhead designs, as the influence of other parameter combinations on clogging has already been discussed in detail in Ref. [11]. Nevertheless, the performance of the three cutterhead types may vary under different operating parameter combinations, which warrants further investigation in future studies.

Additionally, increasing the slurry circulation rate and reducing the advance rate can further minimize clogging risks. Finally, atmospheric cutterheads can benefit from the inclusion of high-pressure water jet nozzles within the slurry chamber. To address particle accumulation on atmospheric cutterheads, a high-pressure water jet cleaning system is recommended for the slurry chamber. Nozzles should target low-pressure, low-velocity zones (Figs. 15–17) with jet pressure controlled at 20–35 MPa to dislodge clay agglomerations without damaging the cutterhead or tools. Intermittent pulsed operation is advised, linked to the shield machine’s torque or thrust monitoring system. If cutterhead torque or thrust exceeds 15%–20% of normal under constant advancement speed, the jet system should activate automatically or manually to quickly address potential blockages. These jets can dislodge accumulated particles and prevent the formation of mud cakes, ensuring smoother excavation operations.

5.5 Implications for engineering practice

The findings of this study contribute to the development of a more systematic methodology for selecting and designing slurry shield cutterheads. By quantitatively comparing the performance of different cutterhead configurations, this work provides engineers with a deeper understanding of the factors influencing excavation efficiency and clogging risks. The insights gained from this study can guide the design of cutterheads that are better suited to specific geological conditions, reducing project risks and improving the overall efficiency of slurry shield tunneling operations. In conclusion, while each cutterhead design exhibits unique strengths and weaknesses, the integration of advanced numerical modeling techniques and thoughtful design optimization can significantly enhance their performance. By addressing the challenges identified in this study, engineers can achieve safer, more efficient tunneling operations in complex and variable geological environments. Future work could integrate the present cutterhead clogging model with dynamic models of other key subsystems, such as the air cushion chamber for holistic pressure control simulation [33,34], and consider the long-term performance of tunnel structures, including the time-varying behavior of sealing gasket materials under complex loads [35].

5.6 Extended analysis of cutterhead performance applicability under different geological conditions

The CFD–DEM model developed in this study primarily targets homogeneous clay formations, making its results most applicable to clay-rich strata. However, slurry shield tunneling often encounters heterogeneous mixed ground. The ‘atmospheric soft soil cutterhead’ is less suitable for gravel-rich formations due to its reliance on rippers and scrapers, which are ineffective at breaking hard gravel and cobbles. This leads to reduced efficiency and abnormal tool wear. Additionally, its spoke structure and large ‘Y’-shaped openings face strength and wear resistance challenges in abrasive conditions, limiting its use in hard rock formations. The ‘atmospheric composite cutterhead’ balances hard rock handling with clogging resistance, utilizing roller discs to fracture hard rock and ‘V’-shaped openings to reduce blockages from large gravel. However, its lack of central openings creates a bottleneck in muck discharge, making it a compromise in mixed formations with soft and hard interbedded layers. The ‘traditional panel-type cutterhead’ struggles in gravel- and cobble-rich formations with high hardness variability. Despite being equipped with roller discs and rippers for hard rock, its small rectangular openings are prone to blockage by large gravel, significantly reducing muck discharge efficiency. The high torque required to break coarse particles accelerates tool wear and increases cutterhead torque, further impacting efficiency and tunneling economy compared to clay-rich conditions.

6 Conclusions

This paper establishes three typical slurry shield cutterhead excavation models based on the CFD–DEM coupling method. By comparing the slurry chamber’s excavated soil transport characteristics and cutterhead excavation parameter responses under different cutterhead structural types, the following specific conclusions are drawn.

1) The relatively thick main arm structure of the atmospheric pressure cutterhead provides favorable conditions for the accumulation of excavated soil in the slurry chamber. In comparison, the discharge rate of the atmospheric pressure cutterhead is lower than that of the conventional cutterhead. Considering the amount of particle accumulation in the slurry chamber, the atmospheric pressure cutterhead exhibits a higher risk of clogging.

2) Significant pressure differences exist between the left and right sides of the slurry chamber. On the side where the cutterhead rotation direction is consistent with the slurry flow direction, the slurry velocity is faster, and the pressure loss caused by the cutterhead wall on the slurry is smaller. On the side where the cutterhead rotation direction is opposite to the slurry flow direction, the slurry velocity is slower, the pressure loss caused by the cutterhead wall on the slurry is greater, and soil accumulation and a higher risk of clogging are more likely to occur in this area of the slurry chamber.

3) In terms of cutterhead torque and thrust performance, the atmospheric pressure soft soil cutterhead exhibits the lowest excavation energy consumption, the atmospheric pressure composite cutterhead exhibits the highest excavation torque, and the conventional cutterhead exhibits the highest excavation thrust. The absence of openings at the center of the cutterhead is unfavorable for torque control, while a large number of small openings is unfavorable for thrust control.

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