Classification and Identification of Shales

Shale has been regarded as a notoriously troublesome and generally undesirable foundation material. However, all shales are not problem shales. A classification distinguishing the problem shales from the nonproblem shales is proposed. From an engineering viewpoint, a classification based on physical properties such as compressive strength, peak and residual shear strength, activity ratio, potential swell, elastic modulus, and predominant clay minerals is more useful in predicting probable in-situ behavior of shale than a purely geological classification. Shales can be broadly classified into two groups as compaction or soil-like shales and cemented or rock-like shales. Generally the rock-like shales provide satisfactory foundations with a minimum of problems; whereas case histories are repleat with failures of structures and slopes founded on compaction shales—particularly the clay shales. The clay shales are the major problem shales, and in many cases their in-situ behavior is unpredictable and puzzling in spite of thorough field and laboratory investigations. More research in laboratory and field testing of shales is needed before a completely satisfactory engineering classification for shales can be developed.