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Geotechnical Considerations in Designing Diaphragm Walls for Soft Soils

December 10, 2024

In construction, diaphragm walls are indispensable when deep excavations are required in challenging ground conditions. These reinforced concrete walls, constructed in situ, serve as retaining walls, load-bearing elements, or groundwater barriers. Their role becomes even more critical when dealing with soft soils, which often pose significant geotechnical challenges.

This blog delves into the key geotechnical considerations for designing diaphragm walls in soft soils and highlights strategies to ensure structural stability and safety.

The Challenge of Soft Soils

Soft soils, such as clay, silt, and peat, are characterized by low shear strength, high compressibility, and poor load-bearing capacity. These properties make them susceptible to excessive deformation, settlement, and instability under excavation and construction loads. Designing diaphragm walls for soft soils requires a thorough understanding of soil behavior, coupled with advanced engineering techniques to address these challenges.

Key Geotechnical Considerations

1. Soil Investigation and Characterization

A comprehensive geotechnical investigation is the foundation of diaphragm wall design. Understanding the properties of soft soils is critical to predicting their behavior under excavation and loading conditions.

Parameters to Assess: Soil type, shear strength, compressibility, permeability, and groundwater levels.
Techniques Used: Borehole drilling, Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), and laboratory tests for soil samples.

Accurate soil characterization helps determine the required depth, thickness, and reinforcement of diaphragm walls.

2. Groundwater Control

Soft soils are often associated with high groundwater levels, which can lead to seepage, instability, and increased hydrostatic pressure on diaphragm walls.

Key Issues: Seepage can cause soil erosion and compromise the stability of the excavation site. High water pressure may require thicker walls and advanced waterproofing.
Solutions:
- Use of bentonite or polymer-based slurry during excavation to maintain stability.
- Installation of dewatering systems like well points or deep wells to lower the water table.
- Incorporating water-resistant concrete or waterproof membranes in the wall design.

3. Wall Stability and Lateral Earth Pressure

The design of diaphragm walls must account for lateral earth pressure exerted by soft soils. These pressures can vary significantly depending on soil type, moisture content, and excavation depth.

Calculation Methods:
- Rankine’s Theory for simplified conditions.
- Coulomb’s Theory for considering wall friction and soil cohesion.
- Advanced numerical models for complex soil behavior.
Reinforcement Design: Adequate reinforcement, often in the form of steel cages, is crucial to withstand lateral forces.

4. Settlement and Heave Control

Soft soils are prone to settlement under loading and heave during excavation, leading to deformation that can compromise the integrity of diaphragm walls.

Settlement: Caused by soil compression due to excavation or loading from nearby structures.
Heave: Occurs when the removal of overburden pressure during excavation causes the soil to expand upward.

Mitigation Techniques:

Pre-consolidation or soil improvement methods, such as preloading, vacuum consolidation, or chemical stabilization.
Designing diaphragm walls with sufficient stiffness to resist deformation.

5. Construction Techniques in Soft Soils

The method of construction plays a pivotal role in ensuring the success of diaphragm walls in soft soils. Traditional excavation techniques may lead to instability or excessive deformation.

Trench Stability: Soft soils require trench support during excavation to prevent collapse. Using bentonite or polymer slurry provides temporary stability until concrete is placed.
Sequential Excavation: Constructing diaphragm walls in segments minimizes the risk of soil collapse and ensures continuous support.
Tremie Method: This method involves placing concrete underwater through a tremie pipe, ensuring a continuous pour and avoiding contamination by soft soil or slurry.

6. Load Transfer and Anchoring

In soft soils, transferring loads from diaphragm walls to stable strata is critical to avoid excessive deformation or failure.

Base Anchoring: Diaphragm walls may require anchoring into deeper, more stable layers using end-bearing piles or rock sockets.
Tiebacks and Struts: For additional lateral support, tiebacks or struts are used to anchor the wall and distribute loads more effectively.

7. Durability and Long-Term Performance

Soft soils often contain organic matter, chemicals, or moisture that can affect the durability of diaphragm walls over time.

Chemical Attack: Soft soils may contain sulfates or other chemicals that can degrade concrete.
Corrosion: High moisture levels can accelerate the corrosion of steel reinforcements.

Solutions:

Using sulfate-resistant cement and additives to enhance concrete durability.
Coating steel reinforcements or using corrosion-resistant materials like epoxy-coated rebar.

Advanced Techniques for Soft Soil Conditions

Modern construction techniques and technologies have revolutionized the design and implementation of diaphragm walls in soft soils.

1. Soil Improvement

Improving the properties of soft soils before construction reduces risks and enhances stability. Methods include:

Jet Grouting: Injecting cementitious material into the soil to increase strength and reduce permeability.
Deep Soil Mixing: Mechanically mixing soil with binding agents to create stabilized columns.

2. Finite Element Modeling (FEM)

Advanced numerical simulations help engineers predict the behavior of diaphragm walls under complex loading and soil conditions. FEM allows for optimization of wall thickness, reinforcement, and construction sequence.

3. Monitoring Systems

Real-time monitoring of soil movement, wall deformation, and groundwater levels ensures immediate detection of potential issues. Instruments like inclinometers, piezometers, and strain gauges are commonly used.

Conclusion

Designing diaphragm walls for soft soils is a complex task that requires a deep understanding of geotechnical principles, soil behavior, and construction techniques. Challenges such as groundwater control, settlement, lateral earth pressure, and soil improvement must be addressed with precision and innovation.

By leveraging advanced technologies, thorough planning, and continuous monitoring, diaphragm walls can be successfully constructed in soft soils, ensuring structural stability and long-term performance. Whether for urban infrastructure, underground transportation, or deep basements, diaphragm walls remain a cornerstone of modern geotechnical engineering.