What is a site classification?
A Site Classification is carried out in accordance with the Australian Standard AS2870: “Residential Slabs and Footings”. The available Classes include S (slightly reactive), M (moderately reactive), H1-H2 (highly reactive), E (extremely reactive), or P (problem site). Classes S, M, H1-H2, and E refer generally to sites in which clayey soils will form the founding strata.
The classification indicates how reactive the clay subsoil is to changes in moisture content. The reactivity (shrinking and swelling) of the clay can have a significant impact on the footings / slabs of a building slab which need to be designed to counteract the movements of the clay soils, otherwise damage (cracking) of the house may occur.
Sites classified as Class P may present difficulties for the proposed construction. The P classification more often than not suggests deep and/or uncontrolled fill, which cannot provide suitable bearing for the house. In these situations, the house is either founded on the stable materials beneath the fill (i.e. deep footings / piers), or the fill is removed and replaced with compacted, controlled fill.
CQ Soil Testing are able to provide site classifications for either large subdivisions or individual allotments. We have the facilities to carry out both the fieldwork and laboratory testing necessary for a site classification, whilst our association with professional engineers are able to interpret the results and provide the appropriate footing designs in a short period of time.
Some of the soil properties analysed when preparing a soil site investigation include:
- Texture Analysis
- Moisture Content
- Estimated Bearing Capacity
- Water Table Level
Other factors that need to be included in the classification of a site are external influences such as:
- Locating Fill Material
- Locating Soft Soil
- Other structures
|A||Sand and rock site with no ground movement from moisture changes.|
|S||Characteristic Surface movement 00 mm – 20 mm.Slightly reactive clay sites with only slight ground movement from moisture changes.|
|M||Characteristic Surface movement 21 mm -40 mm.
Moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes.
|H1||Characteristic Surface movement 41mm – 60mm.Highly reactive clay sites, which can experience high ground movement from moisture changes.|
|H2||Characteristic Surface movement 61 mm – 75 mm.Highly reactive clay sites, which can experience very high ground movement from moisture changes.|
|E||Characteristic Surface movement > 75mm.Extremely reactive sites, which can experience extreme ground movement from moisture changes.|
|P||Sites which include uncontrolled fill, soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.
For Class M, H and E this includes sites with existing or recently removed/altered dwellings which are likely to have created abnormal moisture conditions across the site.
For Classes M, H1-H2 and E further division based on the depth of the expected movement is required. For deep-seated movements, characteristic of dry climates and corresponding to a design depth of suction change, Hs, greater than 3m, the classification shall be M-D, H1-D, H2-D, or E-D as appropriate.
How is a site classification carried out?
A general inspection of the site is conducted to assess potential factors which may influence the classification of the soil. Boreholes (generally 2-3) are drilled to assess the soil properties, and gain a profile of the existing stratification. Most bores are drilled to 3m depth, with the capability of going to 10m. An undisturbed core up to 1.5 m can also be taken if required. Soils are analysed in our laboratory to determine the soil site classification.
SITE LOCATION AND IDENTIFICATION
As part of the site investigation and the design, the site classifier and/or the engineer must take into consideration all of the following information provided by the contractor as a requirement of the Queensland Building Services Board Policy:
- The property description and site address;
- Plan and/or survey;
- Contours of the site;
- The location of trees on the site and adjoining sites and any other impediments;
- Location and identification of existing overland flow;
- The footprint location of the proposed building and indication of platform levels;
- Location of proposed cut and fill
- Land searches (where available) to establish impediments to the proper performance or function of the footing or slab system having regard to the site’s location and condition. These searches may include flooding, underground infrastructure, easements, subdivisional fill and vegetation protection. The nature of the searches will always be dependent on the site’s location and condition.
GUIDELINE FOR THE SITE CLASSIFIER
The classifier is the person or organisation responsible for classifying the site. Classification of a site should be carried out by a Registered Professional Engineer (Queensland) (RPEQ) or licensed Soil Tester under the Queensland Building Services Authority Act 1991. As part of the site investigation the classifier must take into consideration the information provided by the contractor as detailed in Part 1. The site classifier shall visit the site and take soil samples for the purpose of laboratory testing where required. The site investigation and testing shall have regard to the following:
- Sufficient number of exploration positions or bore holes within or in close proximity to the proposed footprint of the building and below final platform level to ensure the testing is representative of the site.
- Unless otherwise prescribed under AS2870, undertake a minimum of two exploration positions or bore holes as part of the site investigation and, where necessary, undertake any other additional investigation to determine the characteristics of the site. Bore holes shall extend to a depth to refusal on rock or to the depth of zone of seasonal influence and below final platform level.
- For sites containing clay soils, the site classification report shall state the Iss (shrink swell index tests results(s) to AS 1218.104.22.168) and calculate the Ys for the site.
- The method of testing shall be “shrinkage index tests” (in accordance with AS2870 and AS1222.214.171.124, AS12126.96.36.199 and AS12188.8.131.52) carried out on undisturbed core samples for clay sites rather than plastic index and linear shrinkage tests, unless a reliable correlation exists with shrinkage index.
- Where abnormal site conditions exit on a site the classifier shall classify the site as Class “P” and also take into consideration the expected movement potential depending on the reactive soil characteristics i.e. A, S, M, H1, H2 & E classifications.
Please note: The above are some relevant exerts from the BSA’s Subsidence Policy and associated documents. The whole of the documents are available at Queensland Building Services Authority. Please do not hesitate to call us at CQ Soil Testing for our interpretation of these guidelines. The above exerts are provided for information purposes only. It’s the responsibility of the reader to ensure that the information is current and that the text is read in context.
CQ Soil Testing pride ourselves in supplying
independent soil testing services.
independent soil testing services.