Stone Trench Fill.

Stone trench fill consists of stone deposited into the open trench excavation and compacted in layers. It is particularly useful in areas where poor quality sands, sandy silts, etc., exist. The material immediately below the topsoil is often suitable for the general floor slab loading but not for the more heavily loaded external and internal strip loadings.

Suitable sub-strata for the strip loads often exists at a  shallow depth and stone trench fill can be used down to these levels (see Fig. 9.7).

Fig. 9.7 Stone trench fill.