The characterization of sabkhas in the eastern parts of Saudi Arabia and its implications for engineering
The sabkhas of the Eastern Province of Saudi Arabia, some coastal some well inland, exhibit variation in clastic fraction, evaporite type, and brine composition. The clastic fraction largely reflects the local host lithologies with carbonate tending to be more important inland than on the coast. While halite is ubiquitous, its abundance is highly varied and it is derived sometimes by evaporation of brine, sometimes deposited as wind blown material. Gypsum forms mainly in layers and again has varied origins. It is particularly abundant where it occurs in the host rocks of the sabkha. The brines are of varied density and composition with the aquifer brines being sometimes relatively fresh while the shallow groundwater is nearly saturated in NaC1. The molar ratio of C1 to Na exceeds unity and varies up to about 1.4. Magnesium is also locally very high- especially where sodium chloride has been extracted from the brine. There are systematic relationships between the ionic concentrations with a numerical relationship occurring between sulphate, bicarbonate, magnesium, and sodium. Brine composition also relates to location but this appears to relate to brine/mineral equilibria with water origin and blending playing a lesser role. The water table has risen into a layer of some 3 metres of fill placed on Sabkhat A1 Fasl while the level, has remained unchanged in residual windows in the sabkha. Experiments show both seasonal and diurnal cycles in water level and that the rise probably relates mainly to lateral tlow of the groundwater. The flow reflects evaporation in areas where the capillary fringe broaches the ground surface. Where infiltration occurs it does not link up with the capillary fringe but evaporates to produce an embryonic salt crust where the salt is added from wind blown dust. Test beds show that the capillary fringe rises into fill by nearly a metre in a little over a year. The presence of salts in the fill, reduces the level to which the capillary fringe reaches. The fill is clearly ineffective and observed damage to construction and cultivation may be mitigated by the use of a more varied thickness of fill combined with the excavation of windows or drains in the original sabkha surface. Experiments in the use of capillary breaks show that though they slow the rise of mositure and salts they are not particularly effective and consideration needs to be given to the design of such breaks to make them more effective. They are however likely to prevent infiltration.