I Working Principle of Slurry Pressure Balance Shield

Slurry Pressure Balance Shield (abbreviated as SPB shield) is a pressure‑balanced tunnel boring machine that employs slurry as the pressure‑transfer medium and transports excavated muck through hydraulic pipeline conveyance. Its applicable stratum scope ranges from soft sandy soils to thick sandy‑gravel and cobble strata.

The integrated slurry circulation system accomplishes excavation‑face support, rock‑soil cutting and muck removal simultaneously. The workflow is described as follows:

1.Pre‑conditioned slurry is delivered to the slurry chamber via slurry‑inlet pipelines, forming a low‑permeability filter cake on the surface of ground soil at the excavation face. Pressurized slurry inside the chamber transfers supporting pressure via the filter cake to counteract the combined earth‑water loads and maintain the stability of the excavation face.

2.Rock‑soil cut by the cutterhead is fully mixed with slurry within the slurry chamber to generate solid‑liquid two‑phase slurry mixture. Driven by slurry discharge pumps, the mixture is conveyed through discharge pipelines to the surface slurry‑treatment plant for solid‑liquid separation.

3.Separated slurry is re‑conditioned in terms of density, viscosity and other performance parameters, then pumped back to the slurry chamber for cyclic reuse. Separated muck is transported off‑site for disposal.

Ⅱ Classification of Slurry Pressure Balance Shields

Based on the pressure‑balance control mechanism for excavation faces, slurry shields are categorized into the direct‑control type (Japanese design) and the indirect‑control type (German design, D‑mode and composite air‑cushion mode).

1.Direct‑control Type (Japanese Design)

A pressure‑bearing bulkhead is installed inside the slurry chamber. The bulkhead and excavation face constitute a single‑compartment slurry chamber fully filled with slurry, with slurry inlet and outlet ports arranged on the bulkhead.

Pressure‑regulation principle: The tunneling rate and slurry‑inflow rate are kept basically stable. Chamber pressure is adjusted by modifying slurry‑discharge flow rate. Increased discharge flow reduces supporting pressure, whereas decreased discharge flow raises supporting pressure.

This machine configuration has no independent air‑cushion pressure‑regulating chamber. It features compact main‑machine layout and short shield body, delivering favorable performance for small‑radius curve tunneling and attitude correction. Its drawbacks lie in large chamber‑pressure fluctuation ranging from ±(0.05‑0.1) MPa. The discharge port is positioned close to the excavation face, granting good passing capacity for large‑size boulders; nevertheless, the rock crusher can only be mounted at the rear end. High content of large boulders may trigger pipeline blockage, and machine halts for troubleshooting interrupt tunneling operations. This type is more suitable for homogeneous strata composed of sand and clay.

2.Indirect‑control Type (German Design, Air‑cushion Type)

A partial bulkhead is fitted inside the slurry chamber, dividing the compartment into a front pressurized‑slurry zone and an upper compressed‑air air‑cushion buffer chamber to form a dual‑compartment air‑cushion structure. The front section is filled with pressurized slurry while the upper section is filled with compressed air, achieving continuous pressure at the gas‑liquid interface.

The system indirectly controls slurry supporting pressure at the excavation face by adjusting air pressure inside the air‑cushion chamber. Liquid‑level sensors monitor chamber liquid level in real‑time and interlock with feeding‑pump rotational speed to keep liquid level within the preset range.

Compressed air features rapid response and supports remote automatic pressure regulation. Pressure fluctuation inside the excavation chamber is limited to ±(0.02‑0.05) MPa, enabling higher precision of ground‑surface settlement control. As a disadvantage, the discharge port is located at the bottom of pressure‑regulating chamber far away from the excavation face. Constrained by bulkhead and crushing equipment, its passing capacity for boulders is limited. On the other hand, rock crushers can be installed at the front to achieve continuous rock breaking and reduce shutdown frequency. This machine type exhibits wide stratum adaptability, covering clay, sand, sandy‑gravel, mixed ground and even hard‑rock strata.

3.Core Differences between Two Types of Slurry Shields

（1）Decoupling performance of pressure control: For Japanese‑design shields, supporting pressure is strongly coupled with slurry inflow‑outflow parameters. Adjustment of supporting pressure inevitably alters inflow‑outflow working conditions. German‑design shields realize decoupling between supporting pressure and slurry circulation system. Excavation‑face pressure can be adjusted independently without changing tunneling and slurry‑circulation parameters, resulting in less stratum disturbance and superior excavation‑face stability.

（2）Pressure stability: Benefiting from the elastic buffering effect of air cushion, German‑design shields suppress pressure fluctuation induced by liquid‑level disturbance and deliver better supporting‑pressure stability compared with Japanese‑design counterparts.

（3）Muck conveyance characteristics: Japanese‑design shields adopt front‑positioned discharge ports with stronger passing capacity for large boulders; German‑design shields facilitate front‑mounted crushing units and are suitable for long‑distance rock‑breaking tunneling.

（4）Main‑machine configuration: Japanese‑design shields dispense with air‑cushion pressure‑regulating chambers and possess shorter shield bodies, showing outstanding capability for small‑radius curve tunneling and attitude correction.

Ⅲ Stabilization Mechanism of Excavation Face for Slurry Shields

Excavation‑face stability of slurry shields is governed by the coupling of filter‑cake formation effect and slurry supporting pressure, which constitutes its core mechanical mechanism.

1.Driven by differential pressure across the excavation face, conditioned slurry with preset pressure and viscosity permeates into surface soil at the working face. Fine slurry particles deposit within soil pores and form dense low‑permeability filter cake, preventing massive slurry leakage into surrounding ground.
2.Slurry pressure acts on filter‑cake surface and transmits supporting stress to ground soil through the filter cake, balancing in‑situ earth pressure and hydrostatic groundwater pressure so as to avert water inflow and collapse failure of excavation face.
3. Control of pressure fluctuation: Japanese‑design shields adjust chamber pressure via slurry‑flow regulation, hence pressure is susceptible to inflow‑outflow disturbance with large fluctuation amplitude. The air‑cushion chamber of German‑design shields takes advantage of gas compressibility to provide elastic buffering, counteract pressure variation caused by liquid‑level fluctuation, stabilize supporting pressure and mitigate stratum disturbance.

Key conclusion: Filter cake acts as the critical interface for pressure transfer. Without continuous and intact filter cake, slurry pressure will rapidly dissipate through ground pores and effective support for excavation face cannot be achieved.

Ⅳ Technical Differences between Slurry‑balance and Earth‑pressure‑balance Shields

Both slurry pressure balance shield and earth pressure balance shield belong to closed‑face pressure‑balanced shield machines. Nevertheless, essential distinctions exist in supporting medium, muck‑removal system and stratum adaptability, as summarized in the table below.

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.