Piling types, and when each one is the right answer

Piling types, and when each one is the right answer

Piles exist for one reason. The ground near the surface cannot carry the building, so the load is taken down to ground that can, or into the ground by friction along the way.

Which type suits a site is decided by the ground, the neighbours and the access, in roughly that order.

Bored piles

A hole is drilled, reinforcement is placed, and concrete is poured.

This is the workhorse for larger loads and for sites where vibration matters. Diameters can be large, capacity is high, and the spoil comes out so you can see exactly what ground the pile is founding on, which is a genuine advantage.

Where the ground will not stand open, the hole is supported with casing or with a drilling fluid, which adds cost and complexity.

The disadvantages are spoil handling, the need for reasonable access for a rig, and the time each pile takes.

Continuous flight auger piles

A hollow-stem auger is drilled to depth, then concrete is pumped through the stem as the auger is withdrawn, so the hole is never left open. Reinforcement is pushed in afterwards.

Fast, quiet and low vibration, which makes it well suited to urban sites with neighbours close by. It also works in ground that would collapse in an open bore.

The limits are depth, reinforcement length, and the fact that you cannot inspect the base directly. Quality depends heavily on the rig instrumentation and on the operator, so the monitoring records matter more than on a bored pile.

Driven piles

Precast concrete, steel or timber sections hammered into the ground.

Fast, high capacity, and the driving record itself is a test of each pile, since resistance to driving correlates with capacity. On a large open site with suitable ground it is often the most economical option.

The problem is noise and vibration. On a tight Sydney site with masonry neighbours, driven piling is frequently ruled out before anything else is considered, and where it is used, vibration monitoring is essential.

Ground displacement is the other issue. Driving a pile pushes soil aside, which can heave the ground and affect adjacent structures and previously installed piles.

Screw piles

A steel shaft with helical plates is rotated into the ground.

Very low vibration, small plant, minimal spoil, and they can be installed with limited access, which makes them the common answer for underpinning, for residential work on difficult sites, and for jobs where the rig has to get down a side passage.

Installation torque correlates with capacity, so each pile is verified as it goes in.

The limit is load. They suit light to moderate loads rather than tower foundations, and they need ground the helix can grip.

What actually decides it

The ground profile. How deep the competent material is, what sits above it, and whether there is rock. A shallow rock profile suits bored piles socketed into it. Deep soft material may suit friction piles.

The neighbours. Proximity to existing buildings, their condition, and whether the consent conditions limit vibration. This rules options in and out faster than anything else.

Access. Headroom under an existing structure, gate width, and what plant can physically reach the pile locations.

Groundwater. A high water table makes some methods difficult and others straightforward.

Spoil. On a contaminated site, disposal cost can be the deciding factor, which favours displacement methods that generate none.

The investigation comes first

None of the above can be decided without knowing the ground. A geotechnical investigation with boreholes to sufficient depth is the input, and choosing a piling method before that exists is guessing.

We specify and design piled foundations across Sydney through structural and geotechnical engineering together, because the pile is the point where those two disciplines meet.