An analysis of crack pattern in clay soil: its density and orientation

The structure of much clay soil is angular blocky, and the pore space consists of a three-dimensional network of narrow, approximately planar cracks. This paper presents a means of describing such a network in terms of its density and orientation. Undisturbed samples are impregnated with resin containing a fluorescent dye. Plane sections of known azimuth and elevation are cut through them and photographed under ultra-violet light. The photographs are projected onto a screen and the spacings between successive cracks along linear probes are measured in several directions. The sets of distances obtained are then analysed. The method was applied to subsoil of the Windsor series. The distribution of inter-crack distances appeared to be exponential but with a dearth of distances less than about 1 mm. The crack pattern was judged to be the outcome of a Poisson process, but one in which some of the very narrow wedges of soil had collapsed. The crack network was perceptibly anisotropic, and was modelled as two superimposed patterns, one isotropic and the other a set of approximately parallel cracks. The orientation of the parallel component was estimated from the sections. The cracks were found to be aligned parallel to the contour of the land and to dip into the hill slope at angles ranging from about 25° to 75° to the horizontal. The intensities in the soil of the two components were estimated as 0.18 mm^?1 for the isotropic component and 0.2 mm^?1 normal to the parallel component, equivalent to 0.1 mm^?1 for a random direction.