Soil and site conditions are a key consideration when planning, building, and maintaining a new home. The soil conditions on your building site will dictate the type of foundation that may be used. Soil conditions may also have a significant impact on the cost to build your new home.

Why are soil conditions important?

Structural engineers need to know the type of soil on a site so they can design the foundations correctly.

Foundations transfer the weight of a home (and other forces acting on the structure) to the ground. Soil that is loose, of uneven quality, likely to move and shift, or is “reactive” (swells and contracts as moisture levels vary), may affect how the foundation supports your home and transfers these forces. An incorrectly designed foundation could lead to excessive movement and cracking, and in extreme cases failure of the structure itself.

The impact of reactive soils has been an issue during the drought. This ABC article provides a real world insight into the importance of well-designed foundations.

Soil Types

Australian Standard 2870 sets out the different soil classifications:

S – Slightly reactive clay sites, which may experience only slight ground movement from moisture changes

M – Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes

H1- Highly reactive clay sites, which may experience high ground movement from moisture changes

H2 – Highly reactive clay sites, which may experience very high ground movement from moisture changes

E – Extremely reactive sites, which may experience extreme ground movement from moisture changes

P – Problem soil – Sites with inadequate bearing strength or where ground movement may be significantly affected by factors other than reactive soil movements due to normal moisture conditions. P sites include soft or unstable foundations such as soft clay or silt or loose sands, landslip, mine subsidence, collapsing soils and soils subject to erosion, reactive sites subject to abnormal moisture conditions and sites that cannot be classified in accordance with the above classifications.

The most common type of site we encounter around the Coffs Harbour region is “M” class.

How are Soil Conditions Tested?

Engineers have a number of different methods to analyse the strength of soil on a site and the thickness and depth of subsurface soil layers. The most commonly used method on residential building sites in our area is Dynamic Cone Penetration testing. The DCP testing apparatus is inexpensive and easily transported, and can be operated by one person.

The test is performed by driving a metal cone into the ground by repeatedly striking it with a 9kg weight dropped from a consistent height. The testing engineer records how many blows of the hammer are required for the cone to penetrate a certain depth. If the cone penetrates the soil easily with few blows, that signals to the engineer that the soil is not particularly strong at that level.

The testing process is continued down to deeper levels. This allows the engineer to build up a picture of the subsurface soil layers. Once the engineer has a clear understanding of the soil layers on a site, he/she can design the footings and foundations to suit.

How are foundations designed for different soil types?

A structural engineer will use the soil classification to help determine the depth and type of foundation and the grade of steel reinforcing.

For most sites around the Coffs Coast, a waffle pod slab is an effective solution (we’ve a whole article about waffle pods if you want more details). Depending on the soil conditions, our engineer will specify the depth of the ribs and the size and placement of the reinforcing steel. For more difficult sites, the slab will be thicker, the ribs will be deeper, and there may be more steel used.

In some cases, concrete piers may be needed under the slab to provide additional support. A concrete pier is simply concrete poured into a specially bored holes at a specified location under the slab. The piers must be deep enough to reach solid natural ground, so any loose or inconsistent soil near the surface does not impact the strength or longevity of the foundation. Reinforcing may be required at some depths, and steel screw piers may be used in some circumstances.

Financial Impact of Soil Conditions

Poor soil conditions can add to the cost of a new home. At Balance Design and Construction, we include an M Class slab in our standard specification. When the soil type is more reactive than M Class, or piers are required, there is likely to be a cost impact. The extra cost of dealing with poor soil conditions varies based on the quality of the soil, the size of the new home, contours of the block, and other factors.

Because we are a local company with years of experience building throughout the Coffs Coast, we often know about difficult site conditions in advance and can help our clients understand the likely financial impact at an early stage.

We also offer our clients the option to obtain a soil test before finalising their quote, so the site conditions are a known quantity before signing a contract.

A Word of Warning

Soil Conditions are sometimes used by unscrupulous builders as an excuse to pile on extra unjustified costs. If your builder notifies you that the soil conditions on your block are poor, ask to see the engineering report and do not be afraid to ask detailed questions. If piers are required, ask why, what sort of piers will be used, and how deep they will be. If a different slab is required, ask how it will be different from the builder’s standard option. A good builder will take the time to transparently explain the site conditions and what extra steps are required to construct a solid, code-compliant house.

It’s also helpful to check a builder’s inclusions list to find out what sort of slab is included as standard. If the builder only includes an S Class slab as standard, expect to pay for an upgraded slab, because almost all sites around the Coffs Coast are a minimum of M Class.

Maintenance Considerations

Even with the highest quality foundation, poor maintenance could eventually lead to problems. Foundation maintenance will be the subject of a future blog post, but below are a few key tips to keep in mind:

  • Ensure the soil around your home drains well. If necessary, install a drain network connected to the stormwater system (your builder should install yard drainage, but homeowners should monitor the performance of this drainage over time).
  • Protect the building perimeter – pave or concrete around your home, or at the very least ensure the ground is sloped away from your home. Avoid gardens close to your walls, and avoid heavy watering of plants or turf next to your foundations.
  • In buildings with a subfloor, ensure good ventilation under the house.
  • Do not plant large trees with extensive root structures near your home. If such a tree is present, seek expert advice about managing the root structure or removing the tree.
  • If excavation around your home is required, seek expert advice from a structural engineer to ensure you do not undermine your foundations.

The CSIRO has a very helpful guide on maintaining your new home’s foundations. We provide a copy to each of our clients, but you can also download the guide direct from the CSIRO.

Are you planning a new home? Not sure about the soil conditions on your block? Give us a call!

Soil Conditions


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