Building on Fill – Screw Piles

I have previously posted about building on fill using Concrete Piers.

Well one alternative to constructing concrete piers is to use Screw Piles such as the one to the right.

These are made from a thick steel tube with:

  • A cutting edge on the bottom.
  • A  spiral plate close to the bottom.
  • A keyway at the top to lock into the drive tool.

The pile is screwed into the ground using a drive tool mounted on an excavator.

Screw piles will normally be more expensive than concrete piers when only a few shallow piles are needed.

They can be cost effective, and fast to install, when a larger number of piles are required or soil disposal is a major problems.

Other advantages are:

  • There is minimal spoil to be removed from site.
  • You don’t have to worry about rain as much during the installation.
  • They can be easily installed when the base of the pile is below the water table.
  • They can be installed in very restrictive locations.

Edit based on question I received

If you are worried about corrosion normal screw piles are fairly thick steel to stand the driving forces (individual thickness of material may vary)

Typically corrosion for plain steel piles in natural ground can be in the range 0.12 – 0.3 mm in 10 years, although galvanised steel will be less.

See Ground Conditions for more about your new house foundations.

Building On Fill – Concrete Piers

Its not unusual for the Site Investigation for your new house to reveal fill materials on top of the more stable clay or rock.

Even if there is no fill the builder may need to level a sloping site using ‘Cut and Fill’ methods, or the top layers of soil are generally poor.

To deal with the problem of building on poor ground concrete piers are commonly used.

These are basically a hole that is excavated through the fill and into the top of good ground. The hole is then backfilled with concrete to the level of the underside of the slab.

When they are finished you have a flat site with concrete pads (like the photo below), ready for the slab to be constructed

You don’t have to have Concrete piers over the whole site, just the parts where there will be fill between the bottom of the slab and the stronger soils.

The following diagram shows a typical slab on a cut and fill site with piers excavated through the fill to the good ground.

See the following link for an alternative to Concrete Piers: Screw Piles.

The Slab is likely to be either a Waffle Pod Raft or a Conventional Raft Slab

When the site investigation indicates fill the builder will often put in a Provisional Sum for so many metres of concrete piers.

At the time of construction they will drill down to  good ground in the fill areas and calculate the actual metres you will need to pay for.

If your whole house is on ‘Controlled Fill‘ You may not need to use concrete piers.

 

Lots more information in the anewhouse Guide to Buying a Block for only $4

See Ground Conditions for more about your new house foundations.