Requesting an independent soil test

An Independent Soil Test

A house’s movement and damage is tightly linked with the subsoil conditions under the house. Without an understanding of the soil conditions under the slab diagnosis and rectification methods are severely limited.

An independent sole test is most useful if you have an original saltiest comparative. However ordering an independent soil test is almost always one of the first things you should do when assessing and diagnosing Slab heave.

Do Not Rely on The Original Site Classification

Do not be tempted to rely on the original salt test when accessing diagnosing slab heave. The original Saltis may be used as a guide and may also be and will also be used to assess the accuracy of the engineering design for the footings however independent assessment of the soul conditions and soil moisture levels is critical in understanding cracks and damage observed in the house


I have heard owners and design engineers state that soil conditions do not change over time and therefore reinvestigating soil conditions is a waste of time and money. I’ve vehemently disagree. Too many factors are considered when classifying a site to ignore the possibility that a mistake or a lapse of judgement could have been made in the original soil test. That is not to say that and independent sole test cannot be flawed. However being able to compare two sets of measurements, borehole profiles, soil moisture contents, and soil classifications greatly increases your knowledge of the subject site.

Furthermore, greater weight can be given to the sigh classification that most matches the level of movement and damage Observed in the house.

Also shrinks well indices used to calculate a Ys value would be given preference over Atterburg calculations.

Then and Now

The original soil test should have been a standard site classification as required by AS2870. The independent soil test is your opportunity to gather information about the soil conditions at the time of the investigation to help you make a correlation between changes and damage in the house.

The original soil test should have been 2 holes within, or at least close, to the footprint of the building. The independent soil test will only be able to record conditions adjacenbt to the building.

The original soil test is a record of the site before construction commenced. The independent soil test is a record of the conditions now.

Ordering an Independent Soil Test

Your request for an independent soil test should specify the following aspects:

  • The number and position of the test holes. Two holes minimum and normally on opposite corners of the building. If you have completed your slab levels and there is a high point and low point on the slab, I normally choose to have the holes adjacent to these locations.
  • The depth of the investigation. test to at least Hs or to50% greater than the deepest footings on paper. The soil tester should be able to tell you Hs for the area, but don’t ask question you don’t already know the answer to.  See the section on Checking Hs.


Order an Independent Soil Test

The independent soil test needs to stand up to expert analysis. Only ever use a soil testing company that you trust. Preference should be given to a soil testing company that uses a geotechnical engineer if one is available.

The soil testing company should, as a rule, be able to prove they know the following information:

  1. The Hs for your area.
  2. That they use shrink swell index as a matter of course for testing clay soils.
  3. The depth of testing they recommend for the test area.
  4. Whether the drill rig is ute mounted, truck mounted or a hand auger. (Choose a company that uses a ute mounted auger for the best access).

Do not disclose the results of the original soil test to the independent soil testing company. Do not disclose the locations of the original soil test holes. Do not disclose the site classification, material logged or the nature of the problems in the house.

Ensure you request a site classification to AS2870-2011 and soil moisture contents in both (all) excavations at 500mm centres. Request Atterburg results also if these were provided in the original site classification.




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How to Identify and Fix Slab Heave