Secant & Contiguous Pile Walls for deep excavations
Secant and contiguous pile walls are rows of cast-in-place piles that form an embedded retaining (shoring) wall for basements, shafts and deep excavations. Contiguous walls have small gaps between piles; secant walls interlock primary and secondary piles to form a continuous, largely watertight barrier.
Suitable ground conditions and applications
Can be considered where…
- Basements and deep excavations next to existing structures, roads and services.
- Secant walls where groundwater must be cut off.
- Contiguous walls where groundwater is low or controlled separately.
- Temporary or permanent retaining walls.
Limitations
- Contiguous walls are not watertight, so groundwater control may be needed.
- Secant walls need tight control of position and verticality to interlock.
- Walls may need props or anchors as the excavation deepens.
Suitability is confirmed from the site investigation and the performance requirement.
How it works
A reinforced concrete guide wall sets out the pile positions. For a secant wall, primary piles are installed first, in weaker or unreinforced concrete; reinforced secondary piles are then drilled, cutting into the primaries to interlock.
A capping beam ties the pile heads together. As excavation proceeds, props or anchors are installed where the design requires them.
Design inputs
- Site investigation and groundwater.
- Excavation depth, surcharges and adjacent structures.
- Movement limits.
- Wall design: embedment, pile diameter, spacing and reinforcement.
- Support system: props or anchors.
Trial and calibration
- Construction sequence and tolerances confirmed on the first panels.
Execution controls
- Guide wall setting out.
- Pile position and verticality.
- Concrete mixes and timing for primary and secondary piles.
- Reinforcement installation.
- Excavation sequence against the design.
Verification and acceptance
Design intent, production control and acceptance verification are kept separate, so it is always clear what was required, what was done and what was proven.
What is required
Wall movement limits and design criteria agreed with the consultant.
What was done
Pile records, verticality and concrete records.
What was proven
Integrity tests, inclinometer and wall movement monitoring, settlement monitoring of neighbouring structures.
Adjacent assets
Embedded walls protect neighbouring structures, but the excavation can still cause ground movement.
- Neighbouring buildings and services are surveyed before work starts.
- Wall and ground movements are monitored against trigger levels during excavation.
Deliverables
- Method statement and risk assessment
- Wall layout and sequence drawings
- Pile records
- Monitoring reports
- Test reports
- As-built records
Common questions
What is the difference between secant and contiguous pile walls?
Contiguous piles have small gaps between them. Secant piles overlap, forming a continuous wall that is largely watertight.
When is a secant wall needed?
Where groundwater must be cut off, or where soil could flow through the gaps of a contiguous wall.
Do pile walls need anchors or props?
It depends on the excavation depth, the soil and the movement limits. Shallow excavations may stand as a cantilever; deeper ones are propped or anchored.
Discuss whether Secant & Contiguous Piles fits your ground conditions.
Share the site location, the investigation report and the performance requirement.