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* Consultant Engineering Geologist, 47 Crescent Road, Caterham, Surrey CR3 6LH;
Department of Applied Earth Science, Queen Mary College, London E1 4NS;
Sir William Halcrow & Partners, 25 Windsor Place, Cardiff CF1 3BZ (formerly Halcrow International Partnership, Dubai)
Many parts of the world's extensive hot drylands pose significant problems for the civil engineer because of chemically aggressive and changing ground conditions. The most important areas are generally the broad flat coastal strips of more recent sediment, for these commonly contain abundant sulphate as gypsum and anhydrite and have widespread surface or near-surface crusts of halite.
The coastal water and groundwater in these regions are usually sulphate and chloride rich, moving slowly through the sediments because of rapid evaporation of the groundwater via the capillary fringe into the drier atmosphere. This movement of groundwater accelerates solution and precipitation of the more soluble minerals, the greater the flow rate the more rapid the compositional change in the sediments—sufficiently rapidly to be of engineering significance. The engineering works themselves can effect significant changes in the ground chemistry and this is most often brought about by changing the position of the water table or the capillary fringe, by dewatering, or raising ground levels by depositing fill. In addition to the significant effect of salts on results of standard engineering tests, the other most likely problems created by the chemical changes are: settlement due to solution in flowing groundwater, heave due to crystallization from groundwater or by reaction between soils and groundwater, and the creation of a chemically aggressive environment in foundations.
The properties of these groundwater-soil systems are reviewed from the point of view of field observations and some large-scale experiments which give indications of the magnitudes and rates of the chemical processes. Two case histories are cited by way of illustration-Dubai Dry Dock and the Harbour Works at Mina Jebel Ali, Dubai. The Dry Dock represents an example of settlement effected by the solution of gypsum from lithified sediments through which seawater was passing, and the Harbour Works show how groundwater and fill chemistry have been changed in the few years required for the construction works.
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