(also known as Site Investigation Reports)
For residential building these reports are prepared in accordance with the current Australian Standard AS 2870 2011 "Residential Slabs and Footings - Construction".
For commercial and industrial sites the investigations and reports are undertaken in accordance with accepted geotechnical practice and include the information to enable a suitably qualified and experienced engineer to design an appropriate footing system.
WHAT IS A SOIL REPORT?
The most important component of any structure is the footing system. The footing system is the method by which the entire structure is supported. The footing system is commonly and incorrectly referred to as the "foundations". It should be noted that the foundation is the material (i.e. soil or rock) beneath the footing system which provides support for the footings. The footing system is a constructed component of the building and involves excavating trenches or holes into the ground and then installing reinforced concrete.
There are a number of different systems. The most common types include strip and stump footings; which consist of a perimeter strip of concrete supporting the outside walls in conjunction with internal stump or pad footings which support the internal floor. Another common type is the concrete slab on ground; which consists of a slab of concrete with thickening beams around the perimeter and internally in order to provide support and rigidity.
Residential buildings constructed in Australia must comply with Australian Standard AS2870 [Residential Slabs and Footings], which applies to the construction of footing systems. All soil reports should be undertaken in accordance with The Standard. The purpose of The Standard is to provide information that allows for the adoption of the appropriate footing system in the construction of residential buildings.
The purpose of a soil report is to provided site-specific geotechnical information that determines the most appropriate and cost effective footing system alternatives for the subject site, in accordance with the requirements of AS2870. It is essential that this information is based on a geotechnical site investigation, undertaken on the specific subject site.
There are a vast number of factors that can affect the performance of a footing system and these should form the basis of a comprehensive soil report. Important factors include:
the type of soil, in particular the clay type;
abnormal soil moisture conditions; often caused by the drying action of trees or wetting action of poor site drainage/leaks;
the presence of fill material such as back fill of an old dam or similar; and
the local climatic zone.
The adoption of the correct footing system for every individual project and site is critical. In addition to the vast savings in costs associated with installing the most effect footing system, the consequences of poor advice when adopting the footing system can be significant.
As a result, soil reports should only be carried out by qualified, registered and experienced personnel. Although not currently required under the Building Act, it is advisable to ensure that soil reports are undertaken by Registered Building Practitioners who carry current and appropriate professional indemnity insurance.
SLOPE STABILITY &
LANDSLIDE RISK ASSESSMENTS
Various local Councils require some sites within their municipalities to be assessment with respect to slope stability or landslide risk.
The Shire of Yarra Ranges in particular requires that sites that are within the Erosion Management Overlay (EMO) are assessed in terms of Slope Stability. Recent amendments require that these Geotechnical Assessment Reports are undertaken in accordance with The Australian Geomechanics Society Practice Note Guidelines for Landslide Risk Management 2007. The Council requires that these Geotechnical Assessment Reports are undertaken only by highly qualified and experienced Geologists or Geotechnical Engineers who are professionally recognized by their peers for their expertise in the field.
DMLawrance Soil Testing Pty Ltd undertakes Geotechnical Assessment Reports on slope stability in the Shire of Yarra Ranges. In recent times, our reports have been successfully used to obtain Planning Permits in the EMO affected areas of the Shire of Yarra Ranges.
DISTRESSED OR CRACKED BUILDINGS
Movement and distress in low rise residential and commercial buildings most commonly occurs as a result of interactions between the footing system and the ground. As a consequence such problems fall within the Geotechnical field of expertise rather than the structural engineering field.
The soil profile across almost all areas of metropolitan Melbourne and most of country Victoria contains reactive clays. Reactive clays are clays that change volume with variations in moisture level. When a reactive clay becomes wet it will swell and when it dries it will shrink.
Reactivity or the shrinking or swelling of foundation clays caused by moisture variations is the single most important factor that influences footing movement and therefore damage and distress to low rise residential and commercial buildings across Melbourne and country Victoria.
There are a vast number of factors that can influence soil moisture levels to the extent that is sufficient to cause movement and damage to buildings. These factors can be interrelated which further complicates the effects and increases the difficulty of determining the main causes of the movement and damage to buildings.
Unfortunately there is a lot of incorrect information with regard to the main causes of movement and damage to low rise residential or commercial building across Melbourne and Victoria. This information comes from a variety of usually non expert sources and circulates mainly through the media.
One of the more common misconceptions is that the natural clay soil beneath a footing will wet up to the extent that it will deform beneath the load from a low rise residential or commercial building. Even when a clay is close to its maximum natural moisture content it is still likely to provide adequate bearing capacity to support the normally expected loads from a low rise residential or commercial building. As a result of this, bearing failure which is the soil not being strong enough to support the footings of a low rise residential or commercial building almost never occurs.
Our Geotechnical Distress Investigation Reports identify the underlying cause or causes of the movement and distress that has occurred in the building. These reports then set out a method of action to fix the problem. The method of action to fix depends on the cause but usually rectification measures consist of a number of integrated steps. The first of these steps is obviously to identify and address the underlying cause. The second step is usually to monitor the movement and distress until it can be established that most significant movements have stabilised this normally takes approximately six to eighteen months. The final step is the actual repairs to the damage which can usually be completed reasonably economically, particularly compared to underpinning which often does not address the underlying cause of the movement.
After considering the above it is clear that Distressed or Damaged Building Investigations and Reports should only be undertaken by qualified and highly experienced professional practitioners who must have a detailed understanding and knowledge of all of the Geotechnical factors that are likely to influence the movement of a building. These Investigations and Reports should only be undertaken by Registered Building Practitioners who carry current and appropriate Professional Indemnity Insurance.
Common Signs of Movement and Damage:
Cracks in bricks, cracks in brickwork, cracks in plaster, cracks in walls, diagonal cracks in plaster, door frame distortion, window frame distortion, gaps in walls, gaps between window frames and brickwork, gaps between door frames and brickwork, gaps between window sills and brickwork.