Secant walls and contiguous bored pile walls are two of the most widely used retaining wall systems in deep excavation and foundation engineering. These techniques provide lateral support to soil during and after excavation, particularly in urban areas where space is limited and soil stability is critical. Both systems are formed by bored piles—cylindrical concrete shafts drilled into the ground—but their design, construction, and application vary significantly depending on project requirements, ground conditions, and site constraints.

A secant wall is constructed by interlocking reinforced and unreinforced concrete piles. The term "secant" refers to the way the piles intersect or cut into each other. This interlocking system forms a continuous wall that prevents groundwater ingress and provides a watertight barrier. Typically, the construction begins with the installation of primary (usually unreinforced) piles at a regular spacing. Once the primary piles are set and have gained sufficient strength, secondary piles—usually reinforced—are drilled between and overlapping the primary piles. This overlapping creates a rigid and continuous structure that is highly effective in controlling both soil movement and groundwater.

Secant pile walls are especially useful in conditions where groundwater control is essential. Their watertight nature makes them ideal for deep basements, underground stations, and other below-ground structures where dewatering would be either costly or impractical. Moreover, secant walls can be installed in a variety of soil types, including sandy or silty conditions where water seepage could destabilize excavation work. However, their construction demands high precision in drilling to ensure the correct overlap between adjacent piles. Misalignment can lead to gaps or weakened sections, undermining the integrity of the wall.

Contiguous bored pile walls, by contrast, consist of a series of piles installed side by side with small gaps—typically ranging from 50 mm to 150 mm—left between them. Unlike secant walls, the piles in a contiguous wall do not intersect. This system is primarily used for retaining dry or cohesive soils where groundwater is not a significant concern. The gaps between the piles are often acceptable in conditions where minor soil migration is not problematic, or they may be sealed using sprayed concrete or shotcrete if a higher level of containment is required.

Contiguous bored pile walls are faster and less costly to construct than secant walls, making them a preferred option for many infrastructure and commercial projects. They are also easier to design and install, requiring less overlap precision and usually simpler reinforcement arrangements. Despite these advantages, they are not suitable for areas with high water tables or in soils prone to erosion through the gaps between piles. For these scenarios, the watertight nature of secant walls becomes a more reliable solution.

Both secant and contiguous pile walls can serve as temporary or permanent retaining structures. In temporary cases, they support the excavation during construction until the final structure is in place. In permanent scenarios, the walls may be integrated into the final design, often forming the basement walls of buildings. Their dual functionality can be particularly beneficial in projects with tight timelines or limited site access.

Construction of both systems requires specialized drilling rigs and concrete placement techniques. Reinforcement cages must be carefully positioned, and alignment must be continuously monitored to maintain structural integrity. Additionally, site-specific factors such as noise, vibration, and proximity to existing structures must be considered during planning and execution.

In summary, secant walls and contiguous bored pile walls are critical components of modern geotechnical engineering, offering versatile solutions for earth retention in complex urban environments. The choice between the two depends on ground conditions, groundwater presence, structural requirements, and budget considerations. With proper design and execution, these systems provide safe, effective, and durable support for deep excavations and underground structures.