In the field of geotechnical and foundation engineering, retaining wall systems are essential for supporting excavations, stabilizing soil, and ensuring the safety of construction projects. Two widely used techniques in urban construction and deep excavation works are the contiguous wall (also known as contig wall) and the bored pile wall. These systems provide effective earth retention, especially in areas where space constraints or challenging ground conditions demand reliable solutions.
A contiguous wall is a type of retaining wall formed by installing a series of closely spaced bored piles. The spacing between these piles is intentionally small, but not fully continuous, allowing for minimal gaps between them. These gaps are often filled with sprayed concrete or left open depending on the soil type and water table level. This method is especially popular in city centers where excavations are made close to existing structures. The contiguous wall serves to retain soil during deep excavations and provides lateral support, reducing the risk of ground movement or collapse.
Contiguous walls are generally used in dry ground conditions where water ingress is not a major concern. In wet or unstable soils, alternative solutions like secant pile walls may be more appropriate due to their water-tight characteristics. The primary advantage of a contiguous wall system is its speed and cost-effectiveness. It allows for quick installation with less material usage compared to diaphragm walls, while still providing sufficient support for medium to deep excavations.
Construction of a contiguous wall involves the drilling of individual piles at regular intervals along the planned wall alignment. Reinforcement cages are placed in the boreholes before concrete is poured, creating a sequence of piles that form the structural wall. Once the wall is completed, excavation can begin, often supported with temporary props or anchors as the depth increases. This method is particularly advantageous in projects involving basements, underground parking structures, or metro stations, where space is limited and precision is key.
A bored pile wall, while similar in construction technique, can vary in its configuration and application. This type of wall is created by drilling large-diameter piles into the ground and placing reinforced concrete within the bored holes. Bored pile walls may be constructed as either contiguous piles (with gaps), secant piles (interlocking), or even as soldier piles with infill panels. The versatility of the bored pile method makes it suitable for a wide range of soil conditions and structural demands.
Bored pile walls are known for their ability to handle heavy loads and significant lateral pressures, making them a preferred choice for deep excavations and high-rise building foundations. The depth and diameter of the piles can be adjusted to meet project-specific requirements, offering a customized approach to foundation design. Additionally, bored piles produce minimal vibration and noise, making them ideal for construction in sensitive urban environments.
In water-logged areas, bored pile walls may incorporate water-sealing techniques or be paired with dewatering systems to ensure stability during excavation. Engineers often use finite element modeling to predict soil-structure interaction and optimize the design, ensuring safety and cost-efficiency.
Both contiguous walls and bored pile walls play a crucial role in modern construction, each offering unique benefits depending on the site conditions and project requirements. Their flexibility, structural strength, and adaptability to complex environments make them indispensable tools in the development of urban infrastructure.
As cities grow and space becomes more limited, the need for advanced retaining wall systems like contiguous and bored pile walls continues to rise. Understanding their design, construction process, and appropriate applications allows engineers and developers to make informed decisions that prioritize safety, efficiency, and sustainability in every project.
