Some characteristics of soft rocks

Chapters 3 and 4 cover fine (cohesive) and coarse (granular) soils used as engineering fill. This also includes excavated rock material used as granular fill, and highly to completely weathered rock utilised as cohesive and granular engineering fill, depending on the parent rock type.

A number of ‘soft’ rocks also occur in the UK and on continental Europe that have been used in earthworks in the past. The strata are generally of younger than Permian age.

The Mercia Mudstone is a series of red-brown mudstones with subordinate siltstones and sandstones of Triassic age. In the UK it crops out on both sides of the Pennines, in the Midlands and extends to the Bristol Channel. There are also outcrops in the Cheshire Plain and in the Mersey area. Only highly weathered Mercia Mudstone has soil-like consistency and engineering properties. Chandler (1969) produced a descriptive classification, grading the material into five zones, depending on the degree of weathering. This scheme is reproduced in Table 5.1, where it has been supplemented with index properties for various Mercia Mudstone zones published by Chandler and Davis (1973) and updated by Chandler and Forster (2001).