ENGINEERING GEOLOGY/Problematic Rocks 543 Problematic Rocks F G Bell, British Geological Survey, Keyworth, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction As far as engineering behaviour is concerned, a distinction has to be made between rock as a material and the rock mass ‘Rock’ usually refers to the intact rock, which may usually be considered as a continuum, that is, as a polycrystalline solid consisting of an aggregate of minerals or grains with void or pore space The properties of intact rock are governed by the physical properties of the materials of which it is composed and by the manner in which they are bonded together The properties that influence the engineering behaviour of rock material therefore include its mineralogical composition, texture, fabric, minor lithological characteristics, degree of weathering or alteration, density, porosity, strength, hardness, intrinsic (or primary) permeability, seismic velocity, and modulus of elasticity Swelling and slaking are taken into account where appropriate, for example in argillaceous rocks On the other hand, a ‘rock mass’ includes the fissures and flaws as well as the rock material and may be regarded as a discontinuum of rock material transected by discontinuities A discontinuity is a plane of weakness within the rock mass, across which the rock material is structurally discontinuous Although discontinuities are not necessarily planes of separation, most of them are, and they possess little or no tensile strength Discontinuities vary in size from small fissures to huge faults The most common discontinuities in all rocks are joints and bedding planes Other important discontinuities are planes of cleavage and schistosity, which occur in some metamorphic rock masses, and lamination, in some sedimentary rock masses Obviously, discontinuities will have a significant influence on the engineering behaviour of rock in the ground Indeed, the behaviour of a rock mass is, to a large extent, determined by the type, spacing, orientation, and characteristics of the discontinuities present As a consequence, the parameters that should be used when describing a rock mass include the nature and geometry of the discontinuities, as well as overall strength, deformation modulus, secondary permeability, and seismic velocity of the rock mass The Influence of Weathering on Engineering Behaviour The process of weathering represents an adjustment of the constituent minerals of a rock to the conditions prevailing at the surface of the Earth (see Weathering) Importantly, in terms of engineering behaviour, weathering weakens the rock fabric and exaggerates any structural discontinuities, thereby further aiding the breakdown processes Rock may become more friable as a result of the development of fractures both between and within grains Also, some weathered material may be removed, leaving a porous framework of individual grains Weathering is controlled by the presence of discontinuities because these provide access for the agents of weathering Some of the earliest effects of weathering are seen along discontinuity surfaces Weathering then proceeds inwards, so that the rock mass may develop a marked heterogeneity, with cores of relatively unweathered material within a highly weathered matrix (Figure 1) Ultimately, the whole of the rock mass can be reduced to a residual soil Weathering generally leads to a decrease in density and strength and to an increase in the deformability of the rock mass An increase in the mass permeability frequently occurs during the initial stages of weathering owing to the development of fractures, but, if clay material is produced as minerals break down, the permeability may be reduced Widening of discontinuities and the development of karstic features in carbonate rock masses lead to a progressive increase Figure Weathered basalt showing corestones with spher oidal weathering; basalt plateau, northern Lesotho