Pile Foundation Design for Difficult Ground

What This Requirement Covers

Piled foundations are used where the near-surface soil does not have adequate bearing capacity to support a building on conventional strip or raft foundations. Common situations include very soft clay, made ground, former landfill sites, and areas with a high water table.

Key Requirements

Types of Pile

1. Driven piles: Preformed piles (concrete, steel, or timber) driven into the ground by a piling hammer. Suitable for non-displacement piles in granular soils.
2. Bored piles (CFA or rotary): Drilled into the ground and filled with concrete and reinforcement. Suitable for most ground conditions and cause less vibration than driven piles.
3. Mini-piles (micro-piles): Small-diameter (150-300 mm) bored piles suitable for restricted access sites and domestic projects.

Design Requirements

1. Piled foundations must be designed to BS EN 1997 (Eurocode 7) and BS EN 1992 (for concrete piles) or BS EN 1993 (for steel piles)
2. The design must account for:

- Bearing capacity of the pile (shaft friction and end bearing) - Settlement under working loads - Negative skin friction (downdrag) where soft soils are consolidating - Lateral loads (wind, earth pressure) - Group effects where piles are closely spaced

1. Pile test loads (static or dynamic) may be required to verify the design

Pile Caps and Ground Beams

1. Pile caps (reinforced concrete caps) transfer loads from the building columns or walls to the piles
2. Ground beams span between pile caps to support walls
3. The pile cap and ground beam design must be coordinated with the superstructure design

Practical Compliance Tips

1. Commission a ground investigation (boreholes to the expected pile depth) before designing the piling scheme
2. Select the piling method based on ground conditions, site access, and proximity to neighbouring buildings (vibration sensitivity)
3. Mini-piles are often the most practical solution for domestic projects with restricted access
4. Ensure the piling contractor provides pile installation records (pile depth, set, concrete volume) for every pile
5. Building Control and the warranty provider will require pile test results and installation records
6. Coordinate the pile layout with the drainage, services, and any existing foundations
7. Budget for the cost of pile testing; it is a necessary quality assurance measure