The Slab Design

Unfortunately, the ‘P’ class site classification means that the standard ‘M’ Class slab will not be sufficient to meet the foundation requirements of our house. So what are the issues that need to be addressed in the slab design ?. As noted in the Soil Test report the slab must account for a differential settlement of the fill. Uniform settlement, typically does not result in damage to the supported structure.  However it is a problem if a part of the foundation settles while another part does not, which is what is meant by the term, “differential settlement”.  A foundation that has settled unevenly across the area of soil which supports it, has settled differentially.  This can result in bending and deflection of the house and much unhappiness.

In our case, two slab design decisions have been taken to address the risk of differential settlement. Firstly the slab has been upgraded from an ‘M’ class to an ‘H1’ class. Additionally, the Porch and Alfresco have been incorporated into the slab. Another issue for the engineer designing the slab was the drainage easement located on the north boundary of our property. In our site plan, the outer wall of our garage is located on this boundary. To ensure there is no load applied to the assets within the easement the engineer has provided for three piers which will pass through the 2.2m of fill to the clay based soil beneath.