The Advantages And Disadvantages Of Pile Foundations

Foundations, shallow or deep, support the structure of a building. When deep foundations are needed, you need to choose the right type of pile. Here are the options.

What is a pile foundation?

A pile is a long column of strong material inserted into the ground to support the building. Piles let you build where the ground near the surface is not strong enough to carry the building without undue settlement.

The right type depends on the ground conditions, environmental factors such as noise and vibration, and the load capacity and settlement criteria.

When to use piles

Piles are the answer where there is a layer of weak soil near the surface that cannot support the building, so loads must be transferred to stronger soil or rock below.

A ground investigation should establish the strength of the ground to a sufficient depth for a geotechnical engineer to design the piles.

Types of pile

1. Displacement (driven) piles

A piling rig hammers a pre-formed pile into the ground, keeping it vertical, with a cap protecting the pile head. The soil is displaced and compacted until the design depth is reached. A variation is the driven cast-in-situ pile: a closed-ended tube is driven in, filled with reinforcement and concrete, then withdrawn.

Advantages

• Piles made off site can be economical, though lengths must be measured accurately
• Long lengths can be installed using mechanical joints
• Almost no spoil to remove from site
• Quick to install

Disadvantages

• Limited load capacity
• Piles can be damaged while being driven
• Noise and vibration can affect neighbouring properties

2. Replacement (bored cast-in-situ) piles

Soil is removed with a short or continuous auger to the required depth, reinforcement is lowered in and the hole is filled with concrete. The soil is replaced with stronger materials.

Advantages

• High-capacity piles can be formed in large diameters
• No jointing needed to reach the design depth
• Minimal noise and vibration
• Can work through obstructions in the ground

Disadvantages

• Materials and workmanship need careful quality checks on site
• The soil is not compacted, so shaft resistance can be lower than for driven piles
• Can take longer to form
• Large amounts of spoil to remove

End bearing versus friction piles

An end bearing pile rests on a lower layer of strong soil or rock, acting as a column that carries the load through the weak layer.

A friction pile transfers the load to the surrounding soil along the length of the pile.

Most piles combine both. A geotechnical engineer designs them so the soil is not overloaded.

Embedded retaining walls

Piles can form temporary or permanent retaining walls, used to:

• Stabilise an unstable slope
• Retain ground where levels change across a site
• Allow deep excavations
• Form basements
• Protect river banks from erosion

There are three main types:

Contiguous piled walls: lines of piles that almost touch. Used where noise or vibration matters and groundwater is not present above the excavation depth.

Secant piled walls: overlapping piles with no gaps, suitable where groundwater needs to be held back.

Sheet piled walls: interlocking steel sheets driven into the ground, with joints that can be sealed to stop water flowing through.

The choice depends on the performance required, the geology and site constraints.

Reducing disruption from piling

Noise and vibration are the biggest concerns for neighbours. Continuous Flight Auger (CFA) piling is one of the quietest methods, with very little vibration, so it suits sensitive sites.

A hollow-stemmed auger drills the shaft, its rotation keeping the bore stable. Concrete is pumped down the stem under pressure as the auger is withdrawn, then reinforcement is plunged into the wet concrete.

What next?

Take professional advice when choosing piled foundations: everything rests on it. On the schemes we build, our in-house piling team handles foundations, from new build houses to airspace developments.