Types of Fill

You will often find a reference to ‘Fill’ in your Geotechnical Report.

Your site may require ‘Cut and Fill’ to level the site.

Here is a bit of information about the various types of fill:

Ordinary Fill

Ordinary fill is normally excavated material from the site or from a unspecified location. After placement the excavator  tracks across it several times (known as Track Rolling) and then levels the top surface.

This fill is cheap but cannot be relied on for house foundations. You will either need to excavate through the fill to the underlying material, or use Concrete Piers.

Controlled Fill

Controlled fill is a known (tested) material either from the site or a specially imported material.

The filled area is constructed as follows:

  • The material is placed in layers, typically 150mm.
  • The water content is optimised, usually by adding water.
  • Each layer is compacted with specialised compaction equipment such as a vibrating roller.
Photo Courtesy of Coates Hire
  • At least 3 satisfactory tests of the compacted density of the filled area are carried out.

Providing the whole slab foundation will be on the controlled fill  your foundations only need to go down the required depth into the controlled fill.

No matter how well the fill was controlled I would not want to build where part of the house was to be on fill, and part on original ground…… In that case I would still like Concrete Piers, or Screw Piles installed through the fill to the original ground.

 

See Soil Classification for more information

 

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.

 

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