Slip-Plane Raft.

The slip-plane raft consists of a concrete raft constructed on a slip-plane layer, such as sand of known friction or shear resistance, which is located between the raft and the substrata. The slip-plane is constructed in sufficient thickness to ensure that a straight failure plane could occur under excessive longitudinal ground strain (see Fig. 9.25). The depth of penetration of the raft into the ground is kept to a minimum to avoid picking up loading from ground strains.

However, the depth below finished ground level must take account of potential frost heave.

Fig. 9.25 Slip plane raft.