Introduction
EC Engineers was sourced by a national real estate agency to assist their client (a prospect purchaser) by completing a pre-purchase building inspection. Concerns were raised by the prospective purchaser based on the outcome of a building and pest report. Our services were engaged to conduct a site inspection and report on the cause of the cracking to the dwelling.
Methodology
Our Structural Engineer assessed the site itself and identified the allotment had been cut and filled to create a level building platforms. An inspection of the dwelling was undertaken to determine the extent of cracking and possible foundation movement that was identified in a building and pest report provided to the potential purchaser.
The inspection commenced in the grounds and included the external walls of the house and ancillary structures within the property. After a thorough inspection of the exterior of the property, the inspection continued internally.
A hydraulic floor level survey was undertaken to measure the extent of any possible floor level differentials. The measurements taken from the floor level survey are used to formulate a contour profile of the house, which is often used as an indicator for the cause of the foundation movement. The results of the floor level survey indicated floor level differentials from the rear to the front of the dwelling. The extent of the difference in floor levels was measured to be 75mm.
The building and site were assessed to determine the probable cause/s of foundation movement:
Construction
The front of the dwelling is constructed primarily on fill. The settlement and resultant slopes of the floors match the natural topography of the site, and it is likely that the front of the dwelling is where the junction of the cut ground and the fill material meet.
Foundations are to be installed to a depth such that the ground they reside on is firm enough to support the weight of the dwelling without gross subsidence. A site with reactive soils, or if the dwelling is constructed on fill will require a more substantial foundation design than that of a dwelling constructed on a flat and stable site. The evident subsidence that has occurred indicates the dwellings foundations are to an insufficient depth and have subsided under the weight of the dwelling.
Founding soils – Fill
Often fill soils are not adequately compacted, or foundation piers have not been installed through the fill. Once a structure is placed on uncompacted fill soil, the additional weight of the dwelling will result in consolidation or “squashing” of the soil. This results in foundation subsidence. Consolidation of fill soil is a major contributing factor in the localised subsidence of the dwelling.
Lack of protection of foundations from surface water
The two causes as described above have been aggravated by the weakening of the foundation soil by uncontrolled surface water.
Solution/Outcome
Inspection of the building confirmed the presence of cracking to the walls caused by differential foundation movement. According to the Australian Standard tolerances, the extent of foundation movement is significant and undesirable.
It has been recommended that an engineering solution such as underpinning is utilised to stabilise the dwelling and possibly, if practical, raise its floor levels back to within acceptable tolerances. Once stabilisation works are complete cosmetic crack repairs can be undertaken. The cost of underpins for this property is estimated to be a minimum of $20,000 – $30,000 plus GST, plus additional costs such as soil tests and an engineer’s design and certification.
Prior to acceptance of purchase, our report recommended the prospective purchaser obtain official quotations from builders and underpinning contractors. Our report included contact details of well-respected underpinning contractors to contact.
By engaging our services, we were able to identify serious structural issues with the property in question. These structural issues were not minor and repair works were estimated to cost tens of thousands of dollars. The small outlay of the engineering inspection fee has ensured the prospect purchaser is aware of the implications of these structural issues and is better placed to negotiate with these issues in mind.