Deep-Strip or ‘Trench-Fill’ Foundation
A deep-strip foundation, also sometimes referred to as a ‘trench-fill’ foundation, is particularly useful in sub-soils such as clay, which can move quite violently with changes in groundwater content. A foundation trench is excavated to the width and depth needed and then all of the trench is filled with concrete to a level that stops within two brick courses of finished ground level (see Fig 42). The depth (‘D’) is as for a traditional strip foundation, however, the width (‘W’) can be reduced to 450mm as there is no longer any requirement for working space within the trench itself.
Deep-strip foundations require more concrete but have certain advantages over traditional strip foundations. Excavation is faster as less material is dug out (although most of the spoil has to be disposed of elsewhere, since little back-filling is required). Also, time and materials and skilled labour are saved by not having to construct walling below ground level.
Fig. 42 Concrete deep-strip or ‘trench-fill’ foundation.