Instability in jointed and fissured rock

Abstract

The strength of essentially intact rock was discussed extensively in Chapter 4. However, most rock masses consist of aggregates of blocks of rock material separated by structural features such as bedding and cleavage planes and joints. These structural features, especially joints, have a critical influence on the behaviour of rock and the landforms developed on them. Joints are conspicuous rock weaknesses, but other, less noticeable structural weaknesses occur. Subjoints are minute fractures branching off main joints. Many are filled with secondary quartz, feldspar or biotite and have been described as joint fringe and feather fractures. They allow water to penetrate the rock, causing preferential weathering and considerably weakening the rock. Many igneous rocks, especially granite, possess numerous microcracks which may occur completely in one grain, or cross or follow grain boundaries. Bisdom (1967) makes the important distinction between structural microcracks, which are present in unweathered rocks and are straight with angular intersections, and weathering microcracks, which are sinuous and exhibit dendritic patterns. As weathering progresses, grain boundaries become stained and new microcracks are formed. These may be stained grain boundaries, open grain boundaries, stained microcracks in quartz and feldspars, infilled microcracks in quartz and feldspars, clean, transgranular microcracks, filled or partially infilled microcracks and pores in plagioclase feldspars (Man & Dearman 1978).