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Soil Tests – What it Means

Soil tests are used to help structural engineers get some information about the soil on your block.

Structural engineers need to know if there are any problems with the soil on your block so that they can design your house footings correctly for those site conditions.

The main soil problems are:
Soft soil
Loose soil
Overly wet soil
Clay soil (reactive clays)

Of these, the soil problem that has been getting the most bad press lately is reactive clay soils resulting in Slab Heave. Reactive clays change in volume substantially when they absorb and release moisture. The change in volume results in the ground surface moving up and down and this can cause damage to a house.

Reactive clays are classed as follows:
Class “M”: a moderately reactive clay. We see this site classification about 35% of the time. Ground movement isn’t ‘too bad’ and house slabs can easily be designed for this soil. Ground surface can move vertically between 20mm and 40mm between wet and dry conditions (seasons).

Class “H”: a highly reactive clay. We see this site class around 20% of the time. Ground movement is a bit more serious and care needs to be taken designing the footings and maintaining good drainage around the house. The footing and slab code starts introducing special requirements for drainage and for protecting pipes from the movement that is likely to occur.

Class “E”: an extremely reactive clay. We see this site classification about 2% of the time. Very special care needs to be taken with the footing design and extra precautions are needed by the builder and homeowner (for the life of the building) to protect the house from slab heave. This is a life-long soil condition and future homeowners need to be aware of the limits and disclaimers in the footing design.

Firm sandy sites are classified Class “S”. These are my favourite sites. Nice and easy to design, there is no reactive clay movement to worry about. We see Class “S” sites about 15% of the time.

Soft soils, loose soils,wet soils and other problems site are classed as:
Class “P”: a problem site. We see this site class about 28% of the time. Usually the soil test has further clues that describe the problem with the soil. You’ll need an engineer to design your footings to solve these problems. Some of the common “Problems” are:

  • Uncontrolled fill. Extra soil that has been placed on your block that either hasn’t been compacted properly or that doesn’t have the paperwork (compaction tests) to show that it has been placed and compacted properly.
  • Soft soil. Soil could be soft because it is loose or unusually moist. Soft soil may not be strong enough to support the weight of your new building without extra precautions being taken.
  • Abnormal moisture conditions. If the soil tester has identified the potential for abnormal soil moisture changes on your block, they’ll explain the reason in the report.

Soil tests for house sites are carried out in Australia to comply with Australian Standard AS2870. You don’t need to be familiar with this standard unless you are a builder, an engineer or a certifier. However the disclaimers and information that is on the soil test and on the engineer’s plans is often derived from the rules in AS2870. You need to read these rules, understand them, and pass the information on to the next home owner. The soil under your house is there for the life of your building.

How to arrange a soil test

This is what you should do when you need to arrange a soil test for a new house or a building extension:

  • Get a copy of the site plan showing any existing buildings and any new buildings.
  • Email the site plan to your soil testing company. Ask for a quotation for a site classification (or soil test to AS2870).
  • Approve the quotation.
  • Be available to give the soil tester access to your site. Be aware of any underground services so that the drill rig does not damage underground infrastructure.
  • You should receive the soil report as a written report in pdf format. Forward a copy of the soil report on to your designer and/or engineer.

Find a soil tester in Mackay, Queensland.

Find a soil tester in Brisbane, Queensland.

Or use one of our recommended soil testers.

By Matt Cornell
For Cornell Engineers – Structural Engineers
A Structural Engineering Blog

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