Soil Types

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Soil types found in the Darling Downs & neighbouring areas are quite variable comprising a mixture of rock, sandy clays, sand & expansive clays. The more expansive the soil is the more movement you can expect to experience.

Soil movement is measured in two ways, Shrink & Swell. Swell occurs when clay layers absorb moisture and expand. Shrink occurs when these clay types dry out and lose its moisture.

The measurable degree of such movement is determined by the types & proportions of clay minerals present.

Australians standards AS2870-1966 have determined a classification for various site classifications.

ClassExpected Surface MovementOverview
A0 mmIncludes many sand, gravel and rock sites with no clays. These sites have no expected movement and as a result zero moisture variation.
S0 – 20 mmSlightly reactive sites which exhibit only small movement with moisture variation.
M20 – 40mmModerately reactive sites exhibit moderate amounts of movement with moisture variation. These sites often include red/ brown silty soils, some sandy clays & loamy soils.
H140 – 60 mmHighly reactive sites exhibit high amounts of movement with moisture variation. These sites include some silty clays in the Toowoomba area and many of the sandy & basaltic clays in the Lockyer valley.
H260 – 75 mmHighly reactive sites exhibit high amounts of movement with moisture variation. These sites include some silty clays in the Toowoomba area and many of the sandy & basaltic clays in the Lockyer valley.
E> 75 mmExtremely reactive sites which exhibit greater than 75mm of surface movement. Typically, these sites include deep reactive clay, such as black & dark brown soils on the darling downs, but also found throughout the Lockyer Valley & parts of Toowoomba. These sites generally require quite expensive footing systems.
P Class P sites may contain some of the features listed above & are an indication the engineer pay peculiar attention to the features of the site. ‘P’ sites may not be bad sites, but due to uncompacted fill, soil disturbance, unusually high moisture content resulting from water flow, ponds etc, & removal of trees cannot be classified as normal sites. An incorrect soil classification can lead to building damage.

Developers/estate agents may include a soil investigation report with the sale of land, however it is essential a full site investigation be undertaken. Ask if a Site Report has been prepared.

Good sites may be more expensive to purchase with poor sites by comparison being cheaper. Design Master Homes advice is, weigh up the pro’s & con’s before you purchase. Design Master Homes will arrange for qualified personnel to do a full site investigation & provide a report to enable an engineer to design your slab and foundation to suit.