THE (SOIL) TEST RESULTS ARE IN!


We walk over it, plant in it and build on it. It is the very earth beneath our feet but it can’t be relied upon to always support us. Made by the weathering of rock over millions of years, Soil is a thin layer on the earths crust which is in a state of constant movement, generally very slow but sometimes quickly and with great force.  In fact around 25 million years ago, the land in the area around the Glass House Mountains north of Brisbane was estimated to have a height 300mts above current levels. Weathering over millions of years has reduced the height so much that the intrusive plugs formed by volcanic activity were exposed to form the mountains we see today.

Soil is a vital aggregate of organic and mineral particles mixed with air, water and an astonishing array of living organisms. The importance of good soil to the survival of all land and sea animals can’t be understated, but it is also the reason why the most fertile areas are generally cleared for farming. My 55 acres of paradise near Kilcoy Queensland is classified as Class A farming land due to much of the block being on a rich flood plain fed by a natural spring creek. However, most of the valley and the areas around are only being used for cattle pasture perhaps due to the large amounts of water needed for crops.

The Queensland Department of Agriculture and Fisheries lists the agricultural land classes as follows –

CLASS A – Crop land – suitable for current and potential crops with limitations to production which range from none to moderate levels.
CLASS B – Limited crop land – marginal for current and potential crops due to severe limitations; and suitable for pastures.
CLASS C – Pasture land – suitable only for improved or native pastures due to limitations which preclude continuous cultivation for crop production.
CLASS D – Non-agricultural land – not suitable for uses due to extreme limitations. This may be undisturbed land with significant habitat, conservation and/or catchment values or land that may be unsuitable because of very steep slopes, shallow soils, rock outcrops or poor drainage.

Based on the above classes my land is currently being used as a class C but in my opinion, if it were not cleared by early pioneers, it would now be classed as class D significant habitat.

The first step to building on any land is a soil test. Not only is it a requirement for building approval but it is there to protect you and your building for decades to come. The soil tests are generally valid for 10 years but check with your local council. I have started the process of designing a tiny house and garage to live in prior to the main house being built so soil tests were done in both the tiny house/garage and main house locations.

Perhaps I am a little unusual but I found it very interesting to be onsite for the tests and would highly recommend it! As you will see by the first image below the initial drill of about 600mm was found to have a brown silty clay and then below that to the test depth of 2mts they found weathered rock (basically looked like sand – see the second image). In some of the tests sites, they also encountered rock which I dare say is the basalt of which there are scatters outcrops of near the creek.

Soil testing

The soil rating system is as follows –

Class A – is mainly sand and rock. Limited movement or changes in moisture.
Class S – is a firm sandy or slightly reactive clay. Little to no movement or changes in moisture.
Class M – a moderately reactive clay. Limited potential movement so house slab design is not complex.
Class H1 & H2 – a highly reactive clay. More movement potential so more care is needed in designing the house slab and other features such as drainage.
Class E – an extremely reactive clay. Serious movement and moisture change potential so some serious thought is needed when designing slabs, footings and drainage for these sites.
Class P – a problem site. Soft clay, silt or sand which is susceptible to subsidence and erosion. These are the sites which need a lot of thought from your slab designer (= higher costs!).

The below drill test results along with the laboratory swell (4.8%) and shrink (7.8%) tests resulted in a rating of Class P for my land. This, in turn, had a very real impact on the cost of the 6mt x 18mt slab – it went up in cost by $9000!

Soil test results

Interesting to note that most builders I spoke to in Brisbane all quoted based on a Class M or better however from my research most land in Brisbane is actually worse than Class M. This is often due to new land release sites being filled with mixed soils and rubble to make them flat or lift them above the floodplains which developers so dearly love.

Something to keep in mind if you are purchasing new land or budgeting based on builders quotes!

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