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Chapter 5 Soil and rock description and characteristics

Hot desert soils and rocks possess a number of distinct characteristics that relate to: 

Such characteristics have an important bearing on soil and rock engineering behaviour (see Chapter 8), and warrant specific consideration and careful characterization before, during and after construction.

Rock weathering, disintegration and soil formation under arid conditions tend to reflect the low amounts and rates of physical, chemical and biological processes typical of such settings (see Chapter 3). High annual temperatures and large diurnal temperature ranges, coupled with an annual net moisture deficiency, results in a hot desert landscape usually with little or no vegetation (Chapters 2 and 4). Soil formation is dominated by in situ physical breakdown of soil and rock, with the resultant soil typically being ‘thin’ and with a low organic content.

When moisture is available it can collapse or swell a soil, often involving salt or carbonate dissolution or crystallization. Soil transport and deposition is dominated by wind-blown (aeolian) processes and, thus, many hot desert soils are composed mainly of fines (sand or finer). Post-depositional modification of soils can involve the displacive introduction and cementation by minerals precipitated by water moving through the ground or through biogeochemical processes leading to the development of hard layers called ‘duricrusts’. While these processes are typically restricted to inland depressions and coastal flats in hot desert environments, there may be more widespread relict examples of their activity in wetter pluvial periods during the fluctuating climatic conditions of the Quaternary.

The aim of this chapter is to provide guidance on how to identify and describe the characteristics (i.e. characterize), and then classify, hot desert soils and rocks for engineering geology purposes.

Soil and rock characterization involves the field examination of in situ …