How the two walls are built
A contiguous wall is a line of reinforced piles with a small gap between each. The gaps are often closed later with shotcrete or grout as the excavation goes down.
A secant wall is built in two passes. Primary piles, in weaker or unreinforced concrete, are installed first. Reinforced secondary piles are then drilled between them, cutting into the primary piles on both sides so that the wall is continuous.
Side-by-side comparison
| Factor | Contiguous wall | Secant wall |
|---|---|---|
| Gaps between piles | Yes | No: piles overlap |
| Water tightness | Not watertight | Largely watertight when built to tolerance |
| Soil that stands between piles | Needed, or gaps must be sealed | Not needed |
| Construction tolerance | Normal | Tight: verticality controls the overlap |
| Programme and cost | Faster and usually cheaper | Slower and usually more expensive |
Where groundwater sits
If the excavation stays above the water table and the soil can stand in the gaps for a short time, a contiguous wall is often enough.
If the excavation goes below the water table in sands or other permeable soils, water and soil can flow through the gaps. A secant wall, or a contiguous wall combined with dewatering or grouting, is then considered.
Other factors in the choice
- Excavation depth and the number of support levels (anchors or struts).
- How close neighbouring buildings, roads and services are, and how much movement they can tolerate.
- Whether the wall is temporary or becomes part of the permanent basement.
- Hard layers or obstructions that affect drilling and verticality.
- Whether groundwater can be lowered outside the wall without affecting neighbours.
What is checked during and after construction
Pile position and verticality are recorded for every pile, because in a secant wall they decide whether the overlap is achieved at depth. During excavation, wall movement, groundwater levels and neighbouring settlement are monitored against agreed limits.