Causes and Treatment Methods of Water-Stop Curtain Failure in Deep Excavation Projects

Introduction

The safety and stability of water-stop curtains have significant impacts on surrounding environments. During actual construction, failures may occur due to various factors such as design flaws, inadequate geological surveys, or poor quality control, leading to project delays or safety incidents. Therefore, studying the causes of water-stop curtain failure and effective treatment methods is crucial.

Requirements for Deep Excavation and Water-Stop Curtains

1. Deep Excavation

A deep excavation refers to digging projects with a depth of 5 meters or more, or those involving three or more basement levels. In cases where geological conditions or surrounding environments are complex, even shallower excavations may be classified as deep.

According to regulations in China:

• Excavations exceeding 5 meters (including 5 meters) in depth require specialized plans for earthwork, support, and dewatering.

• Excavations under 5 meters but with complex geological conditions, nearby structures, or underground pipelines also need strict safety assessments.

Due to high-risk factors, deep excavation projects require safety evaluations involving experts, project managers, supervisors, safety officers, and technical personnel from construction, survey, and design teams.

2. Water-Stop Curtain

A water-stop curtain is a critical component in excavation projects, designed to prevent or reduce groundwater seepage, ensuring construction safety and preventing post-construction settlement.

It consists of three main parts:

1. Retaining Piles – Similar to retaining walls, typically made of reinforced concrete piles with gaps between them.

2. Water-Sealing Layer – Reinforces soil and blocks water flow between inside and outside the excavation, often using cement mixing piles or compaction grouting.

3. Support Structure – Provides additional reinforcement, especially in large or ultra-deep excavations where diaphragm walls may be used.

Causes of Water-Stop Curtain Failure

Water-stop curtain failure can result from multiple factors, which must be analyzed based on excavation conditions and pile quality. Leakage types can be categorized as:

• By Depth: Above or below the excavation level.

• By Material: Reinforced concrete cracks or cement-soil gaps.

Common Causes of Failure:

1. Improper Cold Joint Treatment – Poor sealing at construction joints leads to localized leaks.

2. Low-Quality Mixing Piles – Rapid dewatering or excessive water extraction dilutes the unset cement slurry, weakening pile integrity.

3. Premature Excavation – Immediate large-scale digging after curtain installation causes structural displacement and damage.

Specific Failure Scenarios:

1. High Water Pressure in the Confined Aquifer Layer

If the water-stop curtain does not fully seal the aquifer, excavation reduces soil pressure, leading to piping or sand boiling near weak points (e.g., wells, tool piles, or retaining piles). This may cause base heave, endangering nearby structures and pipelines.

2. Non-Groundwater Sources Damaging the Curtain

• Leaking underground pipes

• Proximity to rivers

• Excessive rainfall saturates the oil
  These increase active earth pressure, causing deformation and cracks in the curtain, potentially leading to major accidents.

3. External Excavation Seepage

Despite initial surveys indicating no high-pressure aquifer, leaks may occur due to:

• Pipe leakage

• High soil moisture

• Poor-quality cement mixing piles
  Initial leaks may carry sediment, while later-stage leaks appear clear, indicating minor underground voids.

4. Other Causes

• Poor-quality deep mixing walls (insufficient overlap, weak joints, or uneven mixing)

• Soil reactivity affecting cement solidification
  These lead to minor but sediment-rich leaks, potentially forming sinkholes if untreated.

Leakage Treatment Strategies

1. External Curtain Grouting

• Install grouting holes in two rows (0.5m apart, staggered pattern).

• First-row holes are grouted vertically (6–8m depth).

• Use a 1:1 cement-sodium silicate mix at 1 MPa pressure.

2. External Dewatering Wells

• Install relief wells (10–12m deep, 20m spacing) to reduce the water head difference.

• Use intermittent pumping to minimize seepage.

3. Internal Shotcrete Sealing

• Install steel mesh (8mm bars, 15cm spacing) on pile gaps.

• Apply 6cm of C20 shotcrete in two layers (3cm each).

4. Internal Concrete Sealing

• Pour dense concrete at the base (6.8–7m depth) to block seepage.

5. Building Settlement Control

• Inject grout (6m depth, 0.5 MPa pressure) near affected buildings.

• Monitor ground movement; stop if uplift occurs.

6. Construction Process Summary

1. External grouting (first row, then second row).

2. Internal shotcrete application.

3. Leak sealing with cement bags if detected.

4. Concrete barrier at pile-base junctions.

7. Post-Treatment Evaluation

• Water Level Monitoring: Observe external water levels via boreholes. Stable readings indicate successful treatment.

• Settlement Monitoring: Install sensors (4 per building). Acceptable settlement: <2 cm.

Key Construction Management Points

1. Strict Quality Control – Regular inspections and timely repairs ensure curtain effectiveness.

2. Controlled Dewatering – Begin only after the curtain gains sufficient strength.

3. Site-Specific Solutions – Analyze causes before applying treatments.

Conclusion

Proper design, construction, and maintenance of water-stop curtains are essential for deep excavation safety. By understanding failure causes and implementing effective treatments, risks can be minimized, ensuring project success and structural integrity.