Joints have been surveyed in the London Clay at Wraysbury, south Buckinghamshire, and fissures have been studied at Wraysbury and at two sites near Edgware, Middlesex. In all cases the observations are limited to the top 40 ft of the clay. The joints at Wraysbury are predominantly normal to the horizontal bedding at this site and show a moderate degree of preferred orientation in the strike directions N 60°W and N 30°E. They range in height from a few inches to 8 ft and in length up to 18 ft. There is no reason to suppose that they differ essentially from joints in other sedimentary rocks. Fissures within the depth examined are rarely more than 6 in. in size. Their strike directions show little or no preferred orientation, but the dips tend to concentrate at inclinations to bedding of less than 20° and between 70° and 90°. The mean size of the fissures decreases and the number per unit volume correspondingly increases as the upper surface of the clay is approached, suggesting that stress release and weathering play an important role in fissure genesis. However, detailed observations are required at greater depths and in other geological situations before general conclusions can be drawn. Some fissures are slickensided and small relative movements have occurred along some of the joints. The great majority of fissures and joints, however, exhibit a matt surface texture without particle reorientation, indications of brittle fracture rather than shearing. A summary is given of laboratory tests to measure the shear strength along joints and fissures in the London Clay at Wraysbury. © 1969, Thomas Telford Ltd. All rights reserved.
