The Case of Incorrect Foundation Soil Classification and Footing Specification

As discussed in the previous blog post, footing is one of the most important parts of a structure, which transfers loads of a structure to the underlying soil. Therefore, one of the most critical factors in selecting the correct footing type and depth is the correct classification of the foundation soil that will support it. This classification is done by a geotechnical technician/engineer at the start of the project. Design Engineers rely heavily on the outcome of this geotechnical investigation and soil classification to design a suitable footing for the site that can withstand the stresses caused by the foundation soil movement. Therefore, incorrect foundation soil classification can be detrimental to the performance of the footing. 

In this blog, we’ll look at a real case where incorrect soil classification of the site has created a significant hassle for its homeowner and turned her dream home into her nightmare.  

The site in question is located in a newly developed western suburbs of Victoria. Its foundation soil is highly reactive, consisting mostly of Clay with ‘Ys’ value that ranges between 40mm to 75mm. ‘ Ys’ is vertical surface movement which is defined by AS2870 – 2011 as “Movement of the surface of a reactive site caused by moisture changes from characteristic dry to characteristic wet condition in the absence of a building and without consideration of load effects.” 

The owner decided to build a single-storey family home on this site and engaged a well-known Builder from Melbourne in 2009. As part of the building process, an engineering firm was hired to conduct a geotechnical investigation of the site and design the house. The investigation included drilling two boreholes, and the site was classified as class H (Highly Reactive) based on tactile assessment. No laboratory testing was carried out. Based on the outcome of the geotechnical investigation, the Engineer designed a 385mm thick waffle slab as per AS2870-1996 (the relevant Australian code at the time of construction).  

The homeowner started noticing cracks soon after the property was handed over. The cracks were mainly in the central region of the house. The ceiling and cornices started to show cracks, and the cornices were dislodged in several places. There were also gaps between walls and flooring. 

After several cosmetic repairs by the Builder, complaints by the owner about damage around her house continued. Not only did these damages reappear shortly after being repaired, but they also got worse over time. As a result, the Builder engaged the original design Engineer to assess the issue. The Engineer carried out a floor level survey throughout the house which identified slab ‘edge heave’ and a total variance in levels across the entire floor of 60mm. However, the design Engineer was unable to determine what was causing this damage.   

After years of unexplained movement in their floors, the homeowner reached out to MFS Engineering in hopes of finding answers and a resolution. Our survey showed that the pattern of slab movement had changed significantly and no longer demonstrates a pattern of edge heave. The slab has significantly deformed.   

The proximate cause of damage to this house became clear after reviewing all available documents, including the original geotechnical investigation report. The house was built at the end of a prolonged drought in Victoria, which left the ground unusually dry (drought period in Victoria from 1997 to 2010). Unfortunately, the Geotechnical Engineer did not consider this severe drought as an abnormal condition when classifying the site as ‘H’ (highly reactive) instead of ‘P’ (Problematic soil). Consequently, the design Engineer designed a footing that was not stiff enough to accommodate for the differential movements (i.e. ‘Ys’) caused by seasonal variations on a Class ‘P’ site. Our experience shows this is a common issue for waffle slabs built during drought or even very hot summers.  

To sum up, if foundation soil is not classified correctly, it can lead to serious performance issues and even structural failure down the line. With the current demand for housing leading to rapid development, it is important that builders and engineers do not neglect proper due diligence to ensure that the structures they design will meet expectations. Let us not sacrifice quality for speed – our homes and buildings should be built to last. 

Read more about forensic structural engineering here.