Excavation Support or Earth Retaining Structures: Essential Techniques and Applications

Excavation support systems, or earth-retaining structures, are critical in construction where deep excavation is necessary, and stable slopes can’t be maintained. These support systems prevent soil collapse, ensure worker safety, and enable accurate vertical cuts. Excavation supports come in various types, each suited for specific soil conditions and project needs, allowing efficient and safe construction of below-ground structures.

What Is Excavation Supports?

Excavation supports are temporary structures installed to prevent soil collapse when excavating deep below-ground levels. Whether for constructing basements, tunnels, or retaining walls, these supports keep the excavation site safe and stable. In deep cuts, soil from the sides tends to collapse due to low stability, necessitating earth-retaining structures that protect workers and infrastructure. These supports provide temporary stability until permanent structural elements can be built to retain soil and control ground movement.

Soil Stability and Safe Excavation Limits

Soil’s stability depends on its unconfined compressive strength and cohesion—two factors that determine how much weight soil can bear without reinforcement. As excavation depth increases, soil may become unstable beyond a certain limit, potentially causing failure. Here’s a table outlining safe height limits for different soil types based on unconfined compressive strength and cohesion:

This information is essential for selecting the right excavation support method, ensuring that soil’s natural stability is supplemented where needed.

Types of Excavation Supports

There are several types of excavation supports, each with unique characteristics tailored to different soil and project requirements. The most common methods include Soldier Pile and Lagging and Soil Nailing.

a. Soldier Pile and Lagging

The Soldier Pile and Lagging method is a frequently used support technique in construction. Soldier piles, which are essentially steel I-beams with a central web between two flanges, are drilled around the excavation perimeter. These piles are driven slightly deeper than the required excavation depth and then grouted with weak concrete for stability.

• Installation Process: The piles are spaced approximately 10 feet apart. After excavation, the soil between the piles is supported by wood lagging, which is typically composed of timber planks between 2 to 4 inches thick. This wood lagging fits behind the flanges, requiring some manual excavation to secure it in place. Welded studs or clips enhance the connection between the horizontal wood planks and the front flanges.
• Support Techniques for Different Excavation Widths:
  • Struts: In narrow cuts with greater depth, horizontal members called struts are used to connect the opposite sides of the excavation and enhance stability.
  • Rakers: For wider excavations, inclined struts, known as rakers, are deployed to distribute soil pressure effectively.
• Advantages and Limitations: Soldier piles and lagging are versatile and relatively easy to install, making them cost-effective. However, they may be limited in very deep or waterlogged soils.

b. Soil Nailing

Soil Nailing is a modern, economical excavation support technique that stabilizes in-situ soil. This method reinforces soil by installing a series of steel bars, or nails, which hold the soil mass together.

• Installation Process:
  • Shotcrete Application: Soil nailing begins with the application of shotcrete, a type of concrete sprayed onto the excavated surface.
  • Nail Installation: Once the shotcrete hardens, steel nails are drilled and grouted in a grid-like pattern. This configuration helps distribute load evenly, making the soil stable.
  • Layered Installation: For deep excavations, soil nailing is repeated in layers until the required depth is achieved.
• Applications: Soil nailing is often used in road and rail embankments, tunnels, and high-rise foundations. This technique is particularly suitable for seismic-prone areas, as it offers resistance to earth tremors.
• Benefits and Considerations: Soil nailing is rapid and efficient but requires skilled labor knowledgeable in geotechnical engineering. Since it’s most effective above the groundwater table, soil nailing may not be suitable for all sites.
• Tie-Back System for Improved Stability: For areas where standard soil nailing may disrupt operations, a tie-back system may be added. Here, anchor bars are installed behind the support structure, secured at a 10-20 degree angle based on soil characteristics. These tie-backs help transfer load to deeper soil or bedrock, providing added stability and preventing lateral movement.

Comparison of Soldier Pile and Lagging vs. Soil Nailing

Both methods have unique advantages suited to particular situations:

• Application Suitability: Soldier pile and lagging are effective for shorter-term, lower-budget projects or where less reinforcement is required. Soil nailing, however, is ideal for projects demanding higher reinforcement and stability, especially in seismic areas.
• Cost and Construction Time: Soldier pile and lagging are typically less expensive and quicker to set up, while soil nailing may incur higher costs due to skilled labor and specific equipment.
• Labor and Skill Requirements: Soil nailing requires specialized skills in geotechnology and craftsmanship, while soldier piling can be done with more general construction knowledge.

Conclusion

Excavation supports, including soldier pile and lagging and soil nailing, are indispensable in modern construction, providing stability, safety, and efficiency in deep excavation projects. Selecting the right support system depends on soil characteristics, project depth, and budget. By employing the correct method, construction teams can ensure safer work environments and prevent costly failures, helping to meet structural goals effectively.