In modern construction, especially in urban and densely populated areas, the need for safe, effective, and space-efficient excavation support systems is more critical than ever. When building underground structures such as basements, tunnels, or foundations near existing buildings or infrastructure, engineers rely on robust retaining systems to prevent soil collapse and manage groundwater. Two widely used methods in this context are secant wall construction and contiguous pile shoring. Each technique offers unique benefits and is chosen based on site conditions, soil type, and project requirements.
Secant wall construction involves creating a continuous, interlocking wall formed by overlapping bored concrete piles. The term “secant” comes from the geometry of the wall, where one pile cuts into (or intersects with) the adjacent pile, forming a strong, nearly impermeable barrier. This method is particularly effective in challenging ground conditions, such as water-bearing soils or areas with high lateral pressure, where a watertight and structurally sound wall is required.
Secant walls are constructed using two types of piles: primary (often unreinforced) and secondary (reinforced). The primary piles are installed first, with a specific spacing, and allowed to cure. The secondary piles are then bored in between the primary ones, cutting into them to create the interlocking effect. Reinforcement is added to the secondary piles to provide additional strength, especially if the wall will bear significant loads or act as a permanent structure.
The key advantages of secant wall construction include its ability to control groundwater, provide high lateral support, and adapt to various geometries and soil conditions. Because the piles overlap, the wall has minimal gaps, making it nearly water-resistant. This is especially beneficial for deep excavations where groundwater control is a major concern. Additionally, secant walls can be installed in tight urban spaces with minimal vibration, reducing the risk of disturbing nearby structures.
On the other hand, contiguous pile shoring is a simpler and more economical method used primarily for temporary excavation support in dry or cohesive soil conditions. In this method, a series of vertical bored piles are installed in close proximity, with small gaps left between them—typically ranging from 50 to 150 mm. Unlike secant walls, the piles in contiguous shoring do not interlock, and the system is not watertight.
Contiguous pile shoring is ideal for excavations where groundwater is not a major concern and where speed and cost-efficiency are priorities. The gaps between piles allow for quicker installation and reduced material usage. If necessary, shotcrete or grouting can be used to seal the gaps during excavation, especially if minor water seepage occurs or additional strength is needed.
The construction process for contiguous pile walls involves drilling holes to the required depth along the planned wall alignment, placing reinforcement cages, and pouring concrete to form the piles. Excavation proceeds in stages, and as soil is removed, temporary supports such as struts or ground anchors may be added to ensure stability and prevent deflection.
While contiguous pile shoring is typically used as a temporary solution during construction, secant walls can be designed as either temporary or permanent systems. In many infrastructure projects, secant walls serve as both the excavation support and the permanent basement or foundation wall, reducing the need for additional structural components.
Choosing between secant wall construction and contiguous pile shoring depends on several factors, including soil and groundwater conditions, excavation depth, load requirements, and project budget. Secant walls offer superior performance in complex environments but come at a higher cost and longer installation time. Contiguous pile shoring provides a more cost-effective and faster alternative for less demanding conditions.
Both systems are integral to modern geotechnical engineering and play a vital role in ensuring the safety and success of underground construction. By understanding their capabilities and limitations, engineers and project managers can make informed decisions that align with technical needs and project goals.
