The Earth Pressure Balance Shield (EPB) is widely adopted in urban tunnel construction. Its core working principle is to take excavated muck inside the soil chamber as pressure medium. Thrust cylinders apply thrust force to build earth pressure so as to balance stratum pressure at the excavation face. The screw conveyor discharges muck to keep the excavation face stable.

The complete EPB shield integrates tunnelling & cutting, muck removal, segment support and attitude adjustment. Key hardware includes cutterhead, shield body, cutterhead drive unit, manlock, thrust cylinders, articulation seal assembly, segment erector, screw conveyor, central rotary joint, soil chamber, articulation cylinders, tail seal, segment handling device, belt conveyor and screw conveyor gate. Auxiliary systems such as soil‑conditioning unit, synchronous grouting, data acquisition and tunnelling guidance are also equipped. The back‑up system provides auxiliary support for the whole machine.

During shield operation, a front partition connects the soil chamber with the screw conveyor, and the soil chamber is filled with excavated muck. Thrust cylinders advance and pressurize muck in the soil chamber. The earth pressure generated by muck acts on the excavation face and stabilizes the stratum.

Ⅰ Cutterhead System

As the critical component for shield tunnelling, cutterhead design shall give top priority to geological conditions on‑site. Key parameters such as structural pattern, support mode, opening ratio and cutter layout shall be comprehensively evaluated to adapt to construction requirements of different strata.

Functionally, the cutterhead fulfils three major tasks: soil excavation, excavation‑face stabilization and muck stirring inside the soil chamber.

1.Cutterhead Structural Types

Two mainstream cutterhead configurations are panel‑type and spoke‑type, with obvious differences in applicable strata.

（1）Panel‑type Cutterhead: It features high structural rigidity and is suitable for rock strata and mixed ground. Its disadvantage is that excavated soil tends to adhere and clog when entering the soil chamber, and mud cake may form on the cutterhead panel.

（2）Spoke‑type Cutterhead: It consists of several spokes fitted with stirring blades at the rear. Muck flows smoothly with low clogging risk. It delivers better comprehensive performance than panel‑type cutterhead in homogeneous soft ground such as sand and clay.

💡 Engineering Selection Tip: When spoke‑type cutterhead is adopted in soft clay strata, soil conditioning shall still be matched to further reduce the risk of mud‑cake formation.

2. Cutterhead Drive Modes

Available drive solutions cover variable‑frequency drive, hydraulic drive and fixed‑speed motor drive. Selection is determined by equipment power and site working conditions.

3.Cutterhead Support Modes

Support structures are divided into central support, intermediate support and peripheral support for different shield diameters and geological conditions.

（1）Central Support: Mostly applied to small‑ and medium‑diameter shields. During cutting, sufficient flow space is available for soil inside the soil chamber with wide stirring coverage. Muck mixing performance is favourable, clay adhesion and clogging rarely occur, excavation‑face pressure remains stable, which is beneficial for ground‑settlement control. Nevertheless, limited internal space restricts its capacity to handle large boulders and cobbles.

💡 Engineering Selection Tip: If boulders exist in strata for shields with central support, pre‑treatment or crushing measures are recommended in advance.

（2）Intermediate Support: It achieves uniform structural stress and is mainly used for medium‑ and large‑diameter shields. If adopted for small‑diameter shields, mud‑cake accumulation at the cutterhead centre shall be strictly prevented. This support divides the soil chamber into inner and outer zones, among which the central zone accounts for a large proportion. During cutterhead rotation, soil in outer zones flows and stirs well, while muck in the central zone shows poor flowability. In high‑cohesive soil layers, continuous muck accumulation will form mud cake that completely loses flowability. Significant flowability difference exists between inner and outer zones, making it difficult to guarantee overall stirring effect.

💡 Engineering Selection Tip: Intermediate‑support structure should be avoided for small‑diameter shields passing through thick cohesive soil layers.

（3）Peripheral Support: Commonly used for small‑diameter shields with ample internal space and good performance in gravel‑bearing strata. Its shortcoming lies in easy soil adhesion on cutterhead outer edge. Anti‑adhesion technical measures must be prepared in advance for construction in cohesive soil strata.

Ⅱ Soil Conditioning System

Soil conditioning is a vital process for EPB shield construction. It reduces tool wear, stabilizes the excavation face, guarantees smooth muck discharge, and lowers torque and thrust required in tunnelling. Three common conditioning agents including foam, bentonite and polymer are applied for respective strata.

1.Foam Injection System

Foam is the most‑frequently‑used conditioning agent for EPB shields, applicable to sandy soil and clay strata. Being environmentally friendly, foam degrades within several days after mixing with soil with low residue, and original soil properties can gradually recover.

For clay strata with high fine‑particle content, injected foam improves muck flowability, water retention and water tightness, optimizes conveyability of conditioned muck, reduces cutterhead torque and stratum water loss, and mitigates mud‑cake risks.

2.Bentonite Injection System

Bentonite injection is generally adopted for sand‑cobble strata low in fine particles. It supplements fine‑particle fractions to improve muck flowability and impermeability for smooth muck discharge, lower cutterhead torque and less stratum water seepage. Qualified bentonite shall be adopted and swelling time shall be strictly controlled on‑site to ensure conditioning performance.

💡 On‑site Operation Tip: Insufficient bentonite swelling will directly impair soil‑conditioning effect, so swelling duration shall be strictly controlled on site.

3.Polymer Injection System

Polymers are mainly used for non‑cohesive soil, especially water‑rich sand‑cobble strata. It binds internal soil moisture, mitigates water‑soil separation and increases soil cohesiveness, so as to optimize muck discharge, reduce cutterhead torque and cut down stratum water loss.

Ⅲ Muck‑Removal Conveying Equipment

1.Screw Conveyor

The screw conveyor is the core muck‑discharge device of the shield, composed of telescopic barrel, spoil‑discharge barrel, hydraulic motor, screw shaft and discharge gate. It transports muck out from the soil chamber.

2. Belt Conveyor

The belt conveyor receives muck discharged from the screw conveyor and transfers it to track‑mounted muck cars to complete intermediate transportation for outward muck removal.

Ⅳ Synchronous Grouting System

Synchronous grouting at shield tail is an indispensable procedure for tunnel formation. It serves three major purposes:

1. Timely fill the annular gap between shield tail and segments to support surrounding rock mass outside segments and effectively control ground settlement;

2. Hardened grout forms the primary waterproof barrier of the tunnel, blocking groundwater and fissure water from seeping into the tunnel and improving overall tunnel waterproof performance;

3. Provide early constraint and stability for assembled segments, integrate segments with surrounding rock mass, restrain serpentine deviation of tunnel structure, facilitate shield attitude control and guarantee long‑term tunnel stability.

💡 On‑site Operation Tip: Grouting pressure and grout volume of synchronous grouting shall be dynamically adjusted combined with on‑site monitoring data instead of directly adopting theoretical parameters.

Ⅴ Shield Sealing System

Sealing system determines tunnelling safety. Many safety hazards during shield construction occur at the shield tail. The sealing system for articulated shields consists of articulation seal and tail seal.

1. Articulation Seal

Three common structural schemes for articulation seal: single or multiple rubber lip‑type seals; graphite‑asbestos / rubber packing plus airbag seals; double‑row airbag seals. Structures shall be selected according to equipment design requirements.

2.Tail Seal

Water‑stop at shield tail relies on wire‑brush seal assembly integrating spring steel, wire brushes and stainless‑steel mesh. Shield‑tail grease pump continuously injects sealing grease into gaps between wire brushes to enhance water‑stop performance.

💡 On‑site Operation Tip: Insufficient shield‑tail grease injection easily causes water leakage at shield tail, which is a key inspection point on shield construction sites.

Ⅵ Segment Erector

Protected by the shield shell, the segment erector grabs precast segments transported from the tunnel rear and assembles them into complete rings in sequence to form tunnel lining. Segment erectors fall into two categories: mechanical gripper type and vacuum suction cup type.

Ⅶ Data Acquisition System

The data‑acquisition unit collects, processes, stores, displays and evaluates various shield operating parameters. It can generate shift reports, daily reports and weekly reports for construction. Functions include parameter setting, measurement, real‑time tunnelling monitoring and alarm triggering, as well as viewing of historical and dynamic curves. All operating data are archived for later condition review and problem analysis.

Ⅷ Tunnelling Guidance System

The guidance system collects and analyses real‑time attitude parameters of shield tunnelling, which is essential to keep the shield advancing along designed tunnel alignment. Its hardware generally includes theodolite, ELS target, back‑sight prism and computer. It continuously outputs real‑time shield attitude. Through deviation‑rectification operation, shield trajectory is kept within allowable tolerance of designed tunnel line. The primary reference datum of the guidance system is laser beam emitted by laser theodolite installed on segments behind the shield.

References:

DU Yanliang. Training Course for Shield Machine Practitioners[M]. Fuzhou: Fujian Science and Technology Press, 2022.

Note: This article is only for industry technical popular science. Part of technical principles refer to the above‑mentioned publicly‑available professional book. All viewpoints herein are for industry exchange only.