Stone/Gravel Piles.

The stone, or gravel, pile is mainly used as a means of strengthening sub-strata by introducing a series of stone columns using vibration or jetting methods which compact the ground around the stone and replace the void created with a compacted stone column (see Fig. 9.33).

There are basically three main applications which require quite different design judgement and approaches to site testing, and these aspects are dealt with in detail later. In general terms, stone or gravel piles are used in areas of soft sub-strata, or fill, where sufficient upgrading of thebearing capacity or reduction in differential settlement can be achieved. In such situations the stone/gravel pile is  usually much cheaper and in some situations much more suitable than the concrete pile alternative. For example, the gravel pile has a particular advantage in mining areas where the use of concrete piles could result in the foundation picking up unacceptable ground strains and/or possibly the piles shearing off during subsidence due to the brittle form and limited capacity to resist horizontal ground strain. The gravel pile can be used incorporating a slipplane between the top of the pile and the underside of the foundation in a manner similar to that described for the slip-plane raft and Fig. 9.25.

Fig. 9.33 Stone/gravel piles (vibro).

Fig. 9.25 Slip plane raft.

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.