Pile foundations

As time has moved on and technology has become more advanced, we have developed ideas to help us overcome foundation problems associated with poor ground conditions. If a site investigation has highlighted that the ground is unsuitable and unworkable to a depth of more than 2m, a pile foundation may be used. Piles first came into practice approximately 30 years ago but were not widely used for domestic construction until the mid 1980's. Traditionally they were used for large construction/civil engineering projects but have now been developed on a smaller scale for residential development.

A pile transmits the load deeper into the subsoil where denser material can be found to prevent collapse and uplift. They are thin columns of reinforced concrete, spaced out at regular intervals and at the intersections of walls, that go down deep into the ground where the bearing capacity is greater. The main reason for piles being used so often in modern construction is due to the pressure to re-develop Brownfield sites where strip foundations are not always appropriate.

Pile foundations are only generally used when:-

1. There is a high water table
2. Removal costs for the excavated material are expensive
3. Soils such as clay are likely to swell or shrink excessively due to water content
4. Trenches are not very stable and health and safety is an issue

Pile foundations undergo the same principles as any other foundation. Firstly a site strip is excavated, removing the first 150mm of soil and debris. This strip is then deepened to get a working level and to rid the ground of any contaminants. The ground is then marked out to show where the piles will go.

The piles are then placed in the ground in one of two ways as follows:-

1. Driven
2. Augured

Driving a pile into the ground uses a large machine that acts as a hammer, forcing the piles into the ground to their required depth. These can be either pre-cast reinforced concrete or steel.

Normally piles come in standard 3m lengths. If the required depth is larger than this, piles are fixed on top of each other on site and continue to be driven into the ground.

Piles can also be augured into the ground, this consists of a large machine drilling to the required depth. The drill itself is designed to remove the material as it pushes further into the earth. Once the hole is drilled concrete is then poured and the tube is manoeuvred out of the hole as it fills with concrete. A steel tubular framework is then placed into the concrete to act as the reinforcement.