Rest Easy

Melbourne has many different soil types which affects the base for your extension – and dont forget the Angle of Repose

If you are considering doing an extension to your home, site conditions and constraints need to be taken into account. Supa Group’s Design Consultants and contracts administration staff are well qualified to do this examination.

Site constraints include any easements on the property. Easements usually contain government assets such as a Melbourne Water sewerage or stormwater pipes, or the easement may exist for access. They are typically wide enough to fit a small excavator or tractor.

Properties adjoining your own property also have to be protected and not undermined, so this is a further constraint.

How closely you build to these assets, and how deep you can excavate for your own foundations is determined by a concept called the Angle of Repose.

The term comes from the Italian verb Reposare “to rest” – when you rest you want to rest comfortably and securely. In other words your foundations must be built on solid ground. Furthermore works on your house extension cannot compromise adjoining properties or government assets.

When you are excavating near easements or neighbouring properties there are engineering considerations, including soil testing that are essential to ensure the neighbouring property or your own foundations are not compromised or damaged as a result of your works. As part of the soil test a footing probe needs to be performed. The depth of assets located within easements also need to be established prior to any excavation.

This is particularly the case where the trend is to incorporate basements or cellars into an extension or new home, as well as building closer to boundaries. It is also the case for multi-unit developments where a basement is excavated to allow for car parking spaces.

Soil type is a major factor in this determination. Geotechnical Engineers take samples from your property by drilling into the soil. They use this information to determine the soil type and this in turn will determine the engineering requirements for each site. The builder and the building surveyor have an obligation to ensure that the Angle of Repose is taken into consideration.

Melbourne soil types vary from suburb to suburb.

A classic example of this is the delineation caused by the Darebin Creek between Northcote, Fairfield and Thornbury on the west side and Ivanhoe and Heidelberg on the east. The Geological Survey of Victoria, Ringwood Sheet (1:63,360) shows this delineation. This is also bourne out by the type of native vegetation evident, as native plant communities differed by the type of soil encountered.

On the western side the soil is shallow silt with highly reactive silty clay. There can also be “floaters” under the soil, rocks that look like giant chook eggs not immediately obvious until excavations commence. Soil classification is typically H Class – highly reactive clay sites, which can experience high ground movement from moisture changes. This soil type continues through parts of Brunswick, Fitzroy, Collingwood and Richmond

Floaters excavated from a site in Alphington. Soil type was Quarternary “Newer Volcanics” of shallow silts underlined by highly reactive silty clays with basalt underneath. The slab had a Class H classification common throughout this part of Melbourne.

On the eastern side the soil is Silurian and could be “Dargile Formation” or “Anderson Creek Formation” with the more typical M Class soil classification – moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes. This soil type is continued on the other side of the Yarra River from Ivanhoe, in suburbs like Kew, North Balwyn, Bulleen and Lower Templestowe.

So let your building rest easy by ensuring that proper examination of your site has been carried out by qualified professionals. They will ensure the Angle of Repose and correct engineering is incorporated into your design for the protection of both you and your neighbours, and also to ensure your extension lasts for many years to come.