Engineering Geology

Chapter 6
Alluvial cones are found along valleys located at the foot of mountains. They are poorly stratified
and contain rock debris with a predominantly angular shape and great variety in size.
Gravels and sands of marine origin are used increasingly as natural aggregates. The winnowing
action of the sea leads to marine deposits being relatively clean and uniformly sorted.
For the latter reason, these deposits may require some blending. The particles are generally
well-rounded, with roughened surfaces. Gravels and sands that occur on beaches normally
contain deleterious salts and therefore require vigorous washing. By contrast, much of the
salt may have been leached out of the deposits found on raised beaches.
Wind-blown sands are uniformly sorted. They are composed predominantly of well-rounded
quartz grains that have frosted surfaces.
Glacial deposits are poorly graded, commonly containing an admixture of boulders and rock
flour. What is more, glacial deposits generally contain a wide variety of rock types, and the
individual rock fragments are normally subangular. The selective action of physical and
chemical breakdown processes is retarded when material is entombed in ice and therefore
glacial deposits often contain rock material that is unsuitable for use as aggregate. As a
consequence, glacial deposits are usually of limited value as far as aggregate is concerned.
By contrast, fluvio-glacial deposits are frequently worked for aggregate. These deposits were
laid down by melt waters that issued from or were associated with bodies of ice. They take
the form of eskers, kames and outwash fans (Fig. 3.30). The influence of water on these
sediments means that they have undergone a varying degree of sorting. They may be
composed of gravels or, more frequently, of sands. The latter are well sorted and may be
sharp, thus forming ideal building material.
Lime, Cement and Plaster
Lime is made by heating limestone, including chalk, to a temperature between 1100∞C and
1200∞C in a current of air, at which point carbon dioxide is driven off to produce quicklime
(CaO). Approximately 56 kg of lime can be obtained from 100 kg of pure limestone. Slaking
and hydration of quicklime take place when water is added, giving calcium hydroxide.
Carbonate rocks vary from place to place both in chemical composition and physical properties
so that the lime produced in different districts varies somewhat in its behaviour.
Dolostones also produce lime; however, the resultant product slakes more slowly than does
that derived from limestones.
Portland cement is manufactured by burning pure limestone or chalk with suitable argillaceous
material (clay, mud or shale) in the proportion 3:1. The raw materials are crushed and
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