A Residential Soil Classification Report, typically referred to as a soil test report or geotech report, is a report prepared by a qualified geotechnical engineer on the soil conditions of your site, assessed in accordance with Australian Standard (AS) 2870 ‘Residential Slabs and Footings’.
What does a Geotechnical Engineer do as part of a Residential Soil Classification Report?
As part of preparing the report, the geotechnical consultant will conduct an onsite investigation which includes the drilling of a borehole to take samples of the soil profile, usually down to 3 metres below the surface, or less if the auger is refused (ie. hits bedrock at a lesser depth). Testing of the samples by a NATA-accredited laboratory is then undertaken. Finally, the geotechnical engineer analyses the results in accordance with AS 2870 to determine a soil classification.
What is contained in a Residential Soil Classification Report?
A report prepared by a reputable consultant will contain a brief description of the site and surrounds and a specific description of the visual qualities of the soil samples taken, in terms of colour, moisture, plasticity and cohesion. The report will include a bearing depth and allowable bearing pressure, for the structural engineer to take into account when designing the footing system. As mentioned above, the report will also contain the site classification as determined by the geotechnical engineer in accordance with AS 2870. A summary of the possible soil classifications and their meaning is outlined below.
Classification Summary From AS 2870
A – refers to sandy or rocky sites with little or no ground movement
S – refers to slightly reactive clay sites with slight ground movement as a result of moisture changes in the soil profile
M – refers to moderately reactive clay sites with moderate ground movement as a result of moisture changes in the soil profile
H – refers to highly reactive clay sites with high levels of ground movement as a result of moisture changes in the soil profile
E – refers to extremely reactive clay sites with extreme levels of ground movement as a result of moisture changes in the soil profile
P – refers to sites with uncontrolled fill, loose sand, silt or abnormal moisture conditions.
In some cases, the letter ‘D’ will be add to the site classification and this refers to ‘deep-drying’ conditions, see examples below.
M-D – refers to moderately reactive clay sites with deep moisture changes
H-D – refers to highly reactive clay sites with deep moisture changes
E-D – refers to extremely reactive clay sites with deep moisture changes
The classifications of M and M-D are the most frequently encountered in the Wagga Wagga area. M classifications are typical because of the naturally occurring soil and bedrock profiles that are dominant throughout the area (moderately reactive red clay soils). Whilst an M-D classification is a combination of this dominant clay profile and the local climate (high evaporation rates which contribute to moisture changes in the soil and therefore the ‘deep-drying conditions’ that the ‘D‘ refers to).
Classifications such as A will typically be found on the very upper slopes of a residential area. Classifications, such as H, H-D and E are somewhat uncommon in the Wagga Wagga area, but not unique. These type of sites require additional concrete and reinforcement as part of their footing system to provide extra stability to the proposed building, given the more reactive or problematic soil, or foundation, that the building will be established upon.
What do these classifications mean for my property?
These classifications are important as they define the expected surface movement characteristics of the soil and therefore how the foundation of your home will act or perform under changing conditions. The following list gives you an idea of the surface changes that may be encountered for different soil classifications;
S – ground movement of 0 to 20mm
M – ground movement of 20 to 40mm
H1 – ground movement of 40 to 60mm
H2 – ground movement of 60 to 75mm
E – ground movement greater than 75mm
If you can picture the soil around your home heaving and moving up to 40mm for a typical M classification or even greater than 75mm for E classifications, you can probably understand why a soil test report is such a key piece of information!
Why do I need a Residential Soil Classification Report?
The report is used by a structural engineer to accurately determine the most appropriate footing and slab design for your new home, in order to withstand the expected ground movement on a particular site. As mentioned above, some of the poorer classifications (H, H-D and E) necessitate more detailed structural design for the footing system.
Let’s imagine the damage to your home if the footing system had been incorrectly designed and was experiencing ground movement of 40mm, 60mm or even greater. You might see significant cracking at cornices and other joints in the plasterwork, cracking in floors and tiles as well as movement in door frames so that doors will not open and shut properly. All of the lovely finishes and touches in your new home may be compromised if the basics aren’t right!
A copy of your report is also typically required to be submitted to council as part of the approvals process.
Where do I get a Residential Soil Classification Report from?
At MJM Consulting Engineers, we offer to organise a report from a local geotechnical engineering company on your behalf, as part of the service we provide to our clients. Alternatively, you may wish to do this directly yourself. We have worked with each of the companies listed below, which can be contacted for a quote to provide your soil test report.
These companies are based in Wagga Wagga, however if you are located in another part of southern NSW, we can recommend another company that may be able to satisfy your requirements, at a lower travel cost.
Aitken Rowe Testing Laboratories
Phone: (02) 6939 5555
McMahon Earth Science
Phone: (02) 6931 0510
Where can I found out further information?
If you have any queries in relation to a particular project, or the need for a soil test report, then feel free to contact MJM Consulting Engineers.
Wagga Wagga office: 6921 8333
Griffith office: 6962 9922
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