Buying a Block: Can You Build On Fill?

by Greg on September 30, 2013

Buying a Block

It’s not unusual for a Site Investigation of a house block to reveal fill materials on top of the original soils.

Even if there is no fill the builder may need to level a sloping site using ‘Cut and Fill’ methods.

There are two types of Fill: Ordinary Fill and Controlled 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. Your builder will either need to excavate through the fill to the underlying material, or use concrete piers which go through the fill to more suitable material.

fill compaction
Controlled Fill

Controlled fill is a known (and 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 this vibrating roller.

At least 3 satisfactory tests of the compacted density of the fill are required. If someone says your site has controlled fill make sure you get a copy of the test certificate.

Precautions

Providing the whole foundation will be on the controlled fill your foundations can sit directly on 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 the 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.

avatar

Brian Ashworth

Brian is a Civil Engineer with over 40 years experience in the construction industry. He, and his wife, have had three new houses built. The first in England and the other two in Australia. One of the Australian houses was designed by Brian as a Passive Solar House and the other, a project home, orientated and extensively modified to make the best use of the sun. He has been running the Anewhouse blog for two and a half years.

This post is based on Brian’s latest book ‘Guide to Buying a Block’. The guide available as an E-book and can be purchased on his website.

{ 1 comment… read it below or add one }

Jenny February 2, 2016 at 2:15 pm

Hi Brian, hope you can help. We are knocking down and rebuilding a house in Sydney, and the project builder has asked for a “venom certificate”, which is because part of the slab is going over an in-ground pool which will be excavated and filled. Is this a VENM that EPA has on their website?Who does the certification, and it seems the demolisher is not doing the fill, so how do we go about it? The project builder has not got back to me recommending anyone. Will this mean extra piering over the excavated and filled pool?

Reply

Leave a Comment

Previous post:

Next post: