Set in Stone

So we know a soil test report is needed in order to get your building approval to start construction on your project, whether it's a new build or renovation where new footings &/or slab are required the soil test will verify that the design is correct for the conditions relating to your site. We asked our resident "soil guy", soil technician and Managing Director of Geo-Serve P/L Ben Mullins to answer a few of the more common questions relating to soil testing.

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images courtesy of Geo-Serve P/L


So at Cyber you are affectionately known as our “soil guy” but what is your technical title and what does that mean?

Well firstly, my name is Ben Mullins, Managing-Director of Geo-Serve P/L. Our company is primarily involved in soil testing and we are essentially what are known as soil technicians: professionals trained to assess soils for building and construction.  My personal background is as an Environmental Scientist with an Earth Science major, being mostly the study of soils and geology.  In terms of soil testing, I am a licensed soil tester under the Queensland Building and Construction Commission (QBCC).  This means I have met their requirements for licensing with regards to having the skills and experience to carry out soil classification investigations and subsequently write soil classification reports for housing construction.  I have approximately 15 years of experience working in the soils consulting and engineering industry.

That’s all very technical sounding Ben, you must use some interesting equipment then when completing a soil test, what are the main tools of your trade?

The main thing we need to do is dig a hole to observe, record and sample the soil on a site.  To do this, we use a 4WD Landcruiser mounted auger drilling rig.  If we are unable to access the site with a 4WD then we need to conduct the digging by hand, using a hand auger or similar type of tool.  It is obviously preferable to use the drilling rig as it can go deeper if required and through harder material and also requires a fair bit less elbow grease!  There are various other tools we use on site to sample and test on site soils properties including DCPs, pocket penetrometers and shear vanes.  We also have a soil laboratory in our office to test the soils sampled to assist in classifying the sites.

We know that we need a soil test in order to get a building approval from a certifier but what impact does the soil type really have on a build?

Soil conditions and types have a direct impact on the design of the slab and/or footings of a building.  Soil and site conditions can therefore have a major impact on build cost, which is the reason this is included as a condition of sale in contracts for most new lot sales.  Regardless of this, proper assessment of onsite soil conditions, through the soil investigation and report, is essential to assist in the suitable design of footings and slabs to ensure the structural integrity of a building.  This is the main purpose and goal of soil testing.

…but if my neighbour had a soil test done on his place, shouldn’t mine just be the same?

This will generally give a good indication of what is likely to be found on your site, though soil conditions can change dramatically over short distances for both geological and historical reasons (e.g. fill having been placed on the site).  As a reflection of this, assessment and testing within the proposed building footprint (or for practical reasons, as close as possible) and assessment of overall site conditions to Australian Standards in a soil classification report is required by Council and the QBCC for virtually any new residential build.

When you come on site to do the soil test how much of my yard are you digging up exactly?

Disturbance to the site is usually minimal.  The drilling rig uses a 100mm diameter auger to drill and sample the soil down to generally between 1.5-3m depth. The drill cuttings are put back down the hole after the soil has been logged.  The client will generally barely be able to tell we’ve been there.  In fact, this has happened on the odd occasion!

In terms of the soil lottery what type of soil are we hoping for and why?

Soil classifications primarily relate to the prediction of the amount of surface movement (in millimetres) that is expected via the shrinking and swelling of clay soils on the site due to seasonal moisture variation (the full range of moisture change over time) down the soil profile.  This is calculated by comparing the onsite soil profile and laboratory results for shrink/swell potential of reactive clay soils and also factoring in the effects of such things as fill and large or potentially large trees, which generally act to amplify the amount of surface movement.  Soil classifications range from Class A to Class E sites (from lowest to highest in terms of surface movement, and usually cost).  Class E sites are sites that experience extreme surface movement due to seasonal moisture variation (>75mm), these sites require heavy slabs and footings and are a common classification in the black cracking clays around Ipswich.  Typical sites have a Class M and H1 classification and are generally considered not a bad result, as slabs are lighter.  There is another classification which is outside the surface movement component, which are a “P” Class or problem sites.  These are sites which have either large or potentially large trees, underground services, fill, low bearing capacity (“soft”) soils, aggressive (saline/acidic) or erodible soils, sites subject to slope stability or mine subsidence issues or those which experience abnormal moisture conditions.  A “P Class” site is not necessarily bad, as for example the presence of a large or potentially large tree triggers this classification and are present on most sites.  The “P” classification is just a flag to the design engineer to consider something in particular in the design.  This is not necessarily something that will add cost to the build, though obviously it also can. This should be assessed on a case by case basis and discussed with your design engineer and/or builder if you are nervous.