Trench fill foundation
Trench fill foundations are a type of shallow foundation that avoids bricklaying below ground by instead almost completely filling the trench excavation with concrete. Typically, concrete is poured to within 150 mm of the surface ground level. This type of foundation minimises the excavation required, as bricklayers are not required to access the trench to lay bricks or blocks. It can also help to reduce the width of the foundation where below ground brickwork would require a wider footing.
Trench fill is often used when soil is loose or in areas with a high water table, although in loose ground it can be difficult to predict the quantity of concrete required, and the quantity can be quite high if trenches meet or cross at right angles.
By getting ‘out of the ground’ relatively quickly, trench fill foundations can save on labour, time and temporary works.
Trench fill foundations can be taken deeper in areas with heavy clay or in the presence of trees, to a level where the subsoil moisture content is unaffected. In these situations, mesh reinforcement is often required. In addition, one or both trench faces may need to be lined with a compressible batt. This can also mitigate against the tendency in some situations for the trench strips to pick up longitudinal or lateral ground strains which may occur in the strata immediately around the foundation.
Another issue to consider is that the height of the concrete can create access problems for drainage and other services, as well as issues with landscaping and planting.
A variation on the trench fill foundation is the Rubble trench foundation.
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