ENGINEERING GEOLOGY/Problematic Soils 561 other materials When a hardened crust is present near the surface, then the strength of laterite beneath decreases with increasing depth Red clays and latosols are residual ferruginous soils formed primarily by chemical weathering of the parent rock This results in the release of iron and aluminium sesquioxides, increasing loss of silica and increasing dominance of new clay minerals such as smectites, allophane, halloysite and, with increasing weathering, kaolinite The microstructure also is developed by chemical weathering processes and consists of an open-bonded fabric of silt and sand size peds, which are formed mainly of clay minerals and fine disseminated iron oxides Relatively weak bonds exist between the peds and are formed of iron oxides and/or amorphous aluminium silicate gels Such soils differ from laterite in that they behave as clay and not possess strong concretions They do, however, grade into laterite Allophane-rich soils or andosols are developed from basic volcanic ashes in high temperature-rainfall regions Allophane is an amorphous clay mineral These soils have very high moisture contents, usually in the range 60% to 80% but values of up to 250% have been recorded; and corresponding high plasticity The soils also are characterized by very low dry densities and high void ratios (sometimes as high as 6) Moisture content does affect the strength of andosols significantly as the degree of saturation can have an appreciable affect on cementation Soils containing halloysite, or its partially dehydrated form metahalloysite, have high moisture contents (30% to 65%) and can possess high plasticity Some of these soils are susceptible to collapse Soils of Hot Arid Regions Most soils in arid regions consist of the products of physical weathering of rock material This breakdown process gives rise to a variety of rock and mineral fragments that may be transported and deposited under the influence of gravity, wind, or water Many arid soils are of aeolian origin and sands frequently are uniformly sorted Uncemented silty soil may possess a metastable fabric and hence be potentially collapsible The precipitation of salts in the upper horizons of an arid soil, due to evaporation of moisture from the surface, commonly means that some amount of cementation has occurred, which generally has been concentrated in layers, and that the pore water is likely to be saline High rates of evaporation in hot arid areas may lead to ground heave due to the precipitation of minerals within the capillary fringe Where the watertable is at a shallow depth the soils may possess a salty crust and be chemically aggressive due to the precipitation of salts from saline groundwater Occasional wetting and subsequent evaporation frequently are responsible for a patchy development of weak, mainly carbonate, and occasionally gypsum cement, often with clay material deposited between and around the coarser particles These soils, therefore, may undergo collapse, especially where localized changes in the soil–water regime are brought about by construction activity Collapse is attributed to a loss of strength in the binding agent and the amount of collapse undergone depends upon the initial void ratio Loosely packed aeolian sandy soils, with a density of less than 1.6 Mg m 3, commonly exhibit a tendency to collapse Silts may have been affected by periodic desiccation and be interbedded with evaporite deposits The latter process leads to the development of a stiffened crust or, where this has occurred successively, to a series of hardened layers within the formation Loosely packed aeolian silty soils formed under arid conditions often undergo considerable volume reduction or collapse when wetted Such metastability arises from the loss in strength of interparticle bonds resulting from increases in water content Thus, infiltration of surface water, including that applied during irrigation, leakage from pipes, and rise of water table, may cause large settlements to occur Low-lying coastal zones and inland plains in arid regions with shallow water tables, are areas in which sabkha conditions commonly develop Sabkhas are extensive saline flats that are underlain by sand, silt, or clay and often are encrusted with salt Groundwater is saline, containing calcium, sodium, chloride, and sulphate ions Evaporative pumping, whereby brine moves upward from the water table under capillary action, appears to be the most effective mechanism for the concentration of salt in groundwater and the precipitation of minerals in sabkha Salts are precipitated at the ground surface when the capillary fringe extends from the water table to the surface One of the main problems with sabkha is the decrease in density and strength, and increased permeability that occur, particularly in the uppermost layers, after rainfall, flash floods, or marine inundation, due to the dissolution of soluble salts that act as cementing materials Changes in the hydration state of minerals, such as calcium sulphate, also cause significant volume change in soils There is a possibility of differential settlement occurring on loading due to the different compressibility characteristics resulting from differential cementation of sediments Excessive settlement also can occur, due to the removal of soluble salts by flowing groundwater This can cause severe disruption to structures within months or a few years Movement of groundwater also can lead to the dissolution of