554 ENGINEERING GEOLOGY/Problematic Soils gram of coal) What is more, coal may be oxidized in the presence of air at temperatures below its ignition point If heat is lost to the atmosphere, then the ignition temperature for coal is between 420 C and 480 C However, where the heat of reaction is retained, the ignition point falls appreciably to between 35 C and 140 C If the heat generated cannot be dissipated, the temperature rises, which increases the rate of oxidation so that the reaction becomes self-sustaining if there is a continuous supply of oxygen Some coals ignite more easily than others; for example, high-rank coals are less prone to spontaneous combustion than are low-rank coals It is not just coal seams that may spontaneously combust when exposed at or below the surface: colliery spoil heaps are also subject to spontaneous combustion Obviously, the spontaneous combustion of coal, especially when it is being worked by opencast mining, and of spoil heaps when being reclaimed present appreciable problems See Also Lava Pyroclastics Rock Mechanics Sedimentary Processes: Karst and Palaeokarst Sedimentary Rocks: Chalk; Evaporites; Limestones; Sandstones, Diagenesis and Porosity Evolution Tectonics: Fractures (Including Joints) Weathering Further Reading Bell FG (ed.) (1992) Engineering in Rock Masses Oxford: Butterworth Heinemann Bell FG (2000) Engineering Properties of Soils and Rocks, 4th edn Oxford: Blackwell Scientific Publications Bell FG (2004) Engineering Geology and Construction London: E & FN Spon Bell FG, Waltham AC, and Culshaw MG (2004) Sink holes and Subsidence Chichester/New York: Praxis/ Springer Goodman RE (1993) Engineering Geology: Rock in Engin eering Construction New York: Wiley Problematic Soils F G Bell, British Geological Survey, Keyworth, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Most civil engineering operations are founded in the uppermost layers of the ground and are therefore generally carried out in soil Poor soil conditions in terms of engineering increase the cost of construction by necessitating special foundation structures and/or mean that some type of engineering soil treatment is required Consequently, it important to understand the nature of the soil that is being dealt with However, all soil types at times can be problematic, depending upon the conditions existing at a particular engineering site For instance, if saturated gravel is to be excavated into, then it will have to be dewatered prior to the commencement of the operation in order to avoid flooding the site Furthermore, soils can be contaminated and thereby present special engineering problems if the area where they occur is to be developed Be that as it may, only the more troublesome soil types are dealt with here In the engineering sense, soil consists of an unconsolidated assemblage of particles between which are voids, which may contain air or water or both As such, soil consists of three phases, that is, solids, water, and air Under certain circumstances, soil can contain other gases such as methane or other liquids such as nonaqueous phase liquids The interrelationships of the weights and volumes of the three phases are important since they help define the character of a soil The solid phase of soil is derived from the breakdown of rock material by weathering and/or erosion, and it may have suffered a varying amount of transportation prior to deposition It also may contain organic matter, the total organic content of soils varying from less than 1% in the case of some immature or desert soils to over 90% in the case of peats The type of breakdown process(es) and the amount of transport undergone by sediments influence the nature of the macro- and microstructure of the soil that, in turn, influence its behaviour (Table 1) Furthermore, the same type of rock can give rise to different types of soils, depending on the climatic regime and the vegetative cover under which it develops Indeed, the character of a soil frequently is influenced to a significant extent by the climatic regime in which it is formed and exists This is especially the case with some soils formed in more extreme climates such as arid and semi-arid zones, notable examples being sabkha soils and dispersive soils, respectively, or quick clays in cold climates Such soils possess their own peculiar characteristics that provide problems for the engineer Time also is an important factor in the development of a mature soil