In the construction of subways and underground structures, diaphragm walls play a pivotal role in ensuring structural stability and preventing soil and water ingress. However, given the challenging environmental conditions these structures face, effective waterproofing of diaphragm walls is essential to guarantee their durability and functionality. In this blog, we will explore the importance of waterproofing for diaphragm walls and the solutions available to address this critical aspect of underground construction.

The Role of Diaphragm Walls in Underground Structures

Diaphragm walls are reinforced concrete walls constructed in-situ to serve as temporary or permanent retaining walls. They are commonly used in:

• Subway systems.
• Underground parking structures.
• Tunnels.
• Deep basements.

These walls not only provide structural support but also act as barriers against groundwater, making them indispensable in projects where water ingress could compromise safety and longevity.

Why Waterproofing Is Crucial for Diaphragm Walls

The underground environment poses unique challenges for diaphragm walls:

• Groundwater Pressure: High levels of groundwater exert pressure on the walls, increasing the risk of leakage.
• Hydrostatic Forces: Continuous exposure to water can weaken the structural integrity of concrete over time.
• Soil Contamination: Aggressive chemicals present in the soil and water can corrode reinforcement bars and degrade concrete.

Waterproofing solutions are therefore vital to protect diaphragm walls from these challenges, ensuring that they perform as intended for decades.

Key Waterproofing Techniques for Diaphragm Walls

1. Membrane Systems

One of the most common methods of waterproofing diaphragm walls involves the use of membranes:

• PVC and HDPE Membranes: Flexible sheets are applied to the surface of the wall to form a watertight barrier.
• Bituminous Membranes: Known for their durability and ease of application, these are often used in high-pressure environments.
• Composite Membranes: Combine multiple materials to enhance resistance to water and chemical attacks.

2. Integral Waterproofing Solutions

Integral waterproofing involves modifying the concrete mix itself:

• Crystalline Additives: These additives react with water to form insoluble crystals that block pores and micro-cracks in the concrete.
• Water-Repellent Admixtures: These reduce the absorption of water into the concrete, enhancing its impermeability.

3. Injection Grouting

Injection grouting is a technique used to seal leaks in diaphragm walls after construction:

• Epoxy Grouts: Ideal for structural cracks, they restore strength while sealing the wall.
• Polyurethane Grouts: Expand to fill voids, providing a flexible and watertight seal.

4. Surface Coatings

Surface coatings provide an additional layer of protection:

• Cementitious Coatings: Applied as a slurry, these adhere to the concrete surface and create a waterproof barrier.
• Polyurethane Coatings: Offer high elasticity and durability, making them suitable for areas with significant movement.

5. Geosynthetic Clay Liners (GCLs)

GCLs are prefabricated layers of bentonite clay sandwiched between geotextiles. They swell upon contact with water, creating a watertight barrier and are particularly effective in areas with high groundwater pressure.

Factors Influencing the Choice of Waterproofing Solution

The selection of a waterproofing technique depends on several factors:

• Environmental Conditions: The presence of aggressive chemicals or high groundwater levels can dictate the type of waterproofing required.
• Wall Design: The thickness and reinforcement details of the diaphragm walls influence the choice of solution.
• Cost Considerations: While some methods like crystalline additives are cost-effective, others such as composite membranes might be more expensive but offer higher durability.
• Ease of Application: Certain techniques, like integral waterproofing, are easier to implement during the construction phase.

Common Challenges in Waterproofing Diaphragm Walls

Despite the availability of advanced waterproofing solutions, several challenges persist:

• Site Constraints: Limited space in urban areas can complicate the installation of waterproofing systems.
• Complex Wall Geometries: Irregular shapes or joints in diaphragm walls can make uniform application difficult.
• Quality Control: Ensuring consistent application and adherence to standards is critical to achieving effective waterproofing.

Best Practices for Effective Waterproofing

To ensure the success of waterproofing in diaphragm walls, the following best practices should be adhered to:

1. Comprehensive Site Analysis: Understand soil conditions, groundwater levels, and chemical properties to choose the appropriate solution.
2. High-Quality Materials: Use certified and tested materials to ensure long-term performance.
3. Skilled Workmanship: Employ trained professionals for the application of waterproofing systems.
4. Regular Maintenance: Periodic inspections and repairs help address issues before they escalate.

Innovations in Diaphragm Wall Waterproofing

The construction industry is continuously evolving, bringing new technologies to enhance waterproofing:

• Smart Membranes: Equipped with sensors to monitor the integrity of the waterproofing layer in real-time.
• Self-Healing Concrete: Contains bacteria that produce limestone to fill cracks when exposed to water.
• Robotic Application Systems: Ensure precision and efficiency in the application of waterproofing materials.

Case Study: Successful Waterproofing of a Subway Diaphragm Wall

In a recent subway construction project, engineers faced high groundwater levels and contaminated soil. By employing a combination of PVC membranes and crystalline admixtures, they successfully:

• Prevented water ingress.
• Enhanced the durability of the diaphragm walls.
• Minimized long-term maintenance costs.

This project highlights the importance of integrating multiple waterproofing techniques to address complex site conditions.

Conclusion

Waterproofing diaphragm walls in subways and underground structures is a critical aspect of construction that cannot be overlooked. With the right combination of materials, techniques, and expertise, it is possible to protect these walls from water ingress and ensure their long-term performance. As technology continues to advance, the future of waterproofing promises even greater efficiency and effectiveness, making diaphragm walls a reliable solution for challenging underground environments.

For builders and engineers, investing in robust waterproofing systems for diaphragm walls is a decision that enhances safety, reduces costs, and ensures project success in the demanding world of underground construction.