In-depth Analysis & Full-set Prevention Solutions for Water and Slurry Leakage during Shield Tunneling

Water and slurry leakage is a frequent hazard during shield tunneling for subway and tunnel projects. It mainly occurs at shield-tail seal, articulation seal, screw conveyor, segment joints, grouting system and soil chamber. Triggered by combined factors including equipment seal wear, improper construction operation, inferior material performance and complex hydro-geological conditions, severe leakage may lead to ground collapse, equipment damage and personal safety accidents. Treatment shall follow the principle of prevention-first with integrated prevention and mitigation. It is essential to maintain equipment sealing, standardize site construction, control raw-material quality and predict geological risks. Adopting supporting chemical products such as anti-erosion shield-tail grease, muck-conditioning foam agent and high-performance grouting material can effectively cut leakage risks and guarantee safe shield tunneling.

1. Introduction: Hazards of Water-slurry Leakage in Shield Tunneling

Shield tunneling is the mainstream construction method for urban subways, river-crossing tunnels and utility tunnels. However, water and slurry leakage is a frequent and tough problem in shield excavation under complicated underground conditions.

According to statistics, more than 60% of quality defects of domestic shield tunnels are related to water-slurry leakage.

● Minor leakage worsens tunnel working environment and increases construction delays.

● Severe accidents may lead to ground subsidence, surface collapse, damage to electrical & hydraulic equipment.

● Long-term seepage causes steel corrosion and concrete deterioration, reducing tunnel service life, and even brings fatal threats to on-site workers.

Leakage failures concentrate on several typical positions, with different hazard levels and root causes.

2. Main Leakage Positions & Root Causes

2.1 Shield Tail Seal System (Highest-risk Position)

The shield tail seal consists of multi-strand steel wire brushes and dedicated sealing grease, to block backfill slurry and underground water. Major causes for leakage:

● Improper shield-tail grease: insufficient volume, low injection pressure or poor-quality grease washed away by groundwater.

● Excessive simultaneous grouting pressure exceeding the limit of 0.5 MPa, which breaks through tail-brush sealing.

● Poor segment assembly: large segment offset, damaged segment corners, deformed or fallen waterproof gaskets.

● Unreasonable shield posture: frequent over-correction and snake-like driving causing uneven tail clearance.

● Wear, tear or deformation of shield tail brushes.

2.2 Articulation Seal System

The articulation joint enables shield machine to drive along curved alignments. In water-rich sandy ground, slight leakage may quickly evolve into water-sand gushing. Main triggers: worn sealing gaskets, insufficient articulation grease, excessive articulation angle during curve tunneling, loose connecting bolts and assembly defects.

2.3 Screw Conveyor Gushing

Gushing often occurs in high-permeability water-bearing strata. Core reasons: inadequate muck conditioning, empty chamber operation, too high screw rotating speed, low earth-chamber pressure and damaged shaft-end seals.

2.4 Segment Joint Seepage

It is a common long-term tunnel quality defect, covering segment manufacturing, on-site assembly and grouting:

● Defects of precast segments and unqualified water-stop gaskets.

● Dirty joint surfaces, segment offset and insufficient bolt pre-tightening force during assembly.

● Insufficient or poorly proportioned simultaneous grouting, segment floating after leaving shield tail shell.

2.5 Simultaneous Grouting System

Leakage comes from worn pipelines, failed flange seals, damaged mixer shaft seals and poorly-matched grout mix ratio.

2.6 Soil Chamber & Excavation Face

Threats mainly come from high-pressure confined water, unforeseen karst caves / fault fracture zones, insufficient support pressure and failure of main bearing seals.

3. Influence of Geological & Environmental Conditions

Geology is the fundamental external factor for leakage risk.

● Extremely high-risk geology: water-rich sand layer, sand-gravel stratum, confined water aquifer, fault fracture zone and karst formations, featured by high water pressure and large permeability coefficient.

● Environmental factors: seasonal fluctuation of groundwater level, nearby foundation pit dewatering and surface surcharge, which may reopen completed tunnel joints and trigger secondary seepage.

4. Comprehensive Prevention & Control Countermeasures

Following the principle “Prevention First, Combine Prevention with Treatment”.

4.1 Equipment Seal Management

● Maintain sufficient and stable injection of shield-tail grease and articulation grease, check injection status for every ring.

● Regularly inspect the wear condition of tail brushes, articulation gaskets and screw-conveyor shaft seals; replace damaged components timely.

● Complete roundness inspection and sealing test after shield reassembly.

4.2 On-site Construction Control

● Strictly control simultaneous grouting pressure below 0.5 MPa, ensure adequate grouting volume.

● Stabilize shield driving posture, avoid frequent large-range correction.

● Improve segment assembly quality, control segment offset, clean joint surfaces and guarantee bolt pre-tightening torque.

● Optimize muck conditioning formula; prohibit empty-chamber excavation in water-rich ground.

4.3 Material Quality Control

Carry out strict incoming inspection for key materials: anti-scour shield-tail grease, water-stop gaskets for segments, foam conditioning agent and grouting materials. Unqualified products shall be rejected.

4.4 Geological Risk Pre-warning & Emergency Preparation

● Perform advance geological prediction before tunneling; compile special construction schemes for high-risk water-rich sections.

● Store emergency plugging materials, organize regular emergency drills for water-slurry leakage accidents.

5. Conclusion

Water and slurry leakage during shield tunneling is a multi-factor coupling problem caused by equipment status, construction operation, material performance and geological conditions. No single measure can completely eliminate risks.

Selecting high-performance supporting chemical materials (anti-erosion shield-tail grease, muck-conditioning foam agent, high-strength grouting compound) plays a vital role in enhancing sealing reliability, improving muck property and guaranteeing backfill effect. Reasonable material matching together with standardized site management can effectively reduce leakage hazards and ensure safe and efficient shield tunneling.