However, with modern alternatives to traditional underpinning methods emerging, subsidence no longer needs to be a costly and invasive issue. Even though modern solutions are helping to streamline the issue of subsidence, it is still important for those undertaking a renovation to understand what is happening in the ground to help identify the cause of any structural issues. This is because different soil types give rise to unique problems and require alternative solutions.
The soil types commonly encountered in ground remediation work include clay, sand and silt, fill and organic soils. Each have distinctive characteristics and require different solutions to address building subsidence issues.
Structural problems often result from varying moisture levels in reactive clay soil. It swells when wet and shrinks when dry causing subsidence in drier conditions, or heave in wetter areas. The soil can become oversaturated due to broken sewers, stormwater pipes or burst mains. Over-saturated clay also loses its bearing capacity causing building footings to settle in the weakened soil, which can sink even further as it dries out.
Sand and silt
Structures on sand and silt soils often experience settlement caused by ground water, a natural underground watercourse or a leaking sewer or stormwater pipe. The finer grains are washed away leaving larger grains to settle, and the building’s footings and foundations become unsupported, compromising the structure above.
Fill is earthen material placed and compacted in a hole or depression. It can consist of soil, and can also include aggregate, rock or crushed construction waste. Structures built on fill are susceptible to the same issues as those on sand and silt, especially when water flows wash-away finer material. Settlement is worse if the fill is poorly compacted.
Organic soils, especially peat, are weak and highly compressible making them prone to settlement, as the weight of any structure causes compaction. Primary settlement decreases over time, but can total hundreds of millimetres, resulting in significant structural issues.
It is vital to understand the type of soil a home is built on in order to correctly determine which solution is more suitable if subsidence does occur.