T. I. Gamon writes: The ‘completely weathered granite’ studied by Howat (1985) represents a transitional stage in the weathering sequence between fresh granite and residual soil. It has been recommended that the term ‘completely’ be superseded by the term ‘extremely’ (Dearman 1984), as the weathering processs at this stage is far from complete. Continued weathering results in a collapse of the soil skeleton with a loss of microfabric and continued discolouration of the material to a characteristic dark reddish brown. It is likely that the finer textured material and the discoloration observed in the mass structure is the residual soil picking out relict discontinuities.
Whilst the depth of weathering can extend to 100 m, the depth of the interface between the rock and soil zones, the weathering front (Mabbutt 1961), is generally at a depth of 30 to 40 m in the Hong Kong Granite. Below the weathering front the visible signs of weathering are generally concentrated parallel to existing discontinuities. A typical weathering profile for the Hong Kong Granite is illustrated in (Fig. 1) (Gamon & Finn 1984).
To compare the data obtained from several different boreholes, in which the weathering front has been encountered at different depths, it is more useful to plot the data against depth ratio rather than depth. The depth ratio is defined as the ratio of depth of the SPT test or dry density determination to the depth of the weathering front.
The relationship between SPT ‘N value and depth ratio (Fig. 2) is