Site classification investigations for domestic structures are carried out under Australian Standard AS 2870-1996.
An investigation usually involves a visit to a site by a trained technician and the excavation of two test holes either with an auger or a backhoe. The test hole is logged with the various materials being visually classified.
Generally a single representative sample is taken from the most plastic material observed in the near surface layers which are likely to underlay the proposed foundation. The sample is taken back to the laboratory for testing.
The area to be occupied by the proposed structure/dwelling is usually tested with a penetrometer to a depth of 1500mm within the area to be occupied by the proposed building.
The penetrometer consists of a stainless steel rod which is calibrated and driven into the ground with a standard drop weight. The number of blows to drive the rod into the ground each set distance of 300 is recorded. The penetrometer test gives an indication of the in-situ density of the site materials.
The history of the site and the presence of existing uncontrolled fill material is relevant.
The presence of uncontrolled fill will generally lead to the classification of the site as a Problem Site (Class P).
An uncontrolled fill is one for which there are no test certificates available.
It is generally required that fill underlaying a structure be rolled and achieve a given density (Usually 95% SDD). It is required that the testing be carried out at a set frequency as the fill is placed. The number of tests carried out during the placement of fill is related to the volume of fill placed and the surface area of the fill.
Other aspects which can cause a site to be classified as a problem site are:
- the presence of rock over part of the site and possibility of differential settlement;
- the presence of loose or soft material and the possibility of excessive settlement;
- the possibility of foreign material or potential for slip or general instability.
It should be noted that the carrying out of slope stability analysis is generally very involved and outside the scope of the investigations carried out by some laborites
The time to carry out a site classification investigation is following site clearing or following the execution of excavation and filling of the site.
Where the filling of the site will not exceed a depth of 300mm then the investigation may be carried out before the placement of fill. Where a site is to be extensively cut and filled then a geotechnical specialist should be contacted before work commences so that a testing program can be planned.
The site is classified based upon the expected movement of the foundation.
Where the foundation materials contain clay this is related to the capacity of the clay to expand. The following classifications may be given:
- CLASS S - Slight
- CLASS M - Medium
- CLASS H - High
- CLASS E - Extreme
The site classification is presented in a standard format report which generally forms part of a building application.
The foundation classification report is provided to the building designer or engineer.
The report may also include other recommendations which are relevant to the building process.
Site Preparation Checklist
- Has the site been cleared. (Holes left by the removal of tree stumps have been filled with the fill being compacted with a vibrating roller and the fill placed in 200mm layers).
- If The
site is to be cut and filled :
*The topsoil has been removed.
*The area to be filled has been rolled
*The fill is to be placed in 200mm layers - The geotechnical specialist has been contacted to arrange testing during the filling operation.
- The builder is familiar with the required procedure for site filling and preparation. (Adoption of wrong practices may lead to a PROBLEM classification)
- Has a site classification investigation been arranged with geotechnical specialist.
