Dewatering the Desert
Despite being in a climactic environment where summer temperatures can reach in excess of 120 degrees, groundwater is generally very close to the surface in many developed cities throughout the Middle East (Dubai, Abu Dhabi, Bahrain, Doha, and Jeddah, in particular). The nature of the relatively high permeability, near surface geology in these areas means that up to five kilometers from the coast, there is a often direct hydraulic connection between groundwater and the adjacent sea. Typically in these areas, groundwater is within two meters of the surface. This means that almost every basement excavation and a large percentage of excavations for utility lines require some form of temporary dewatering to allow for construction in a dry environment. For shallow excavations in sand (up to around four meters below the water table), we typically utilize drilled or jetted wellpoint systems. For deeper excavations, a combination of staged wellpoint systems, or deep wells, are utilized, to draw the groundwater down anywhere from five to 25 meters.
Specific issues arise when we attempt to dewater areas where solution features exist, which is particularly problematic in certain parts of Abu Dhabi where layers of karstic gypsum exist. These solution features are irregularly distributed and flow rates can be extremely high, requiring large scale pumping to allow sufficient groundwater drawdown. In many cases, long term pumping can actually exacerbate the groundwater flows. Increased flows gradually increase groundwater velocities which causes further erosion of the gypsum karst, until the flow volumes effectively become unmanageable without resorting to ground treatment (i.e. grouting) to reduce flows to manageable levels. Cut-off walls are often ineffective in reducing flow volumes as flow is effectively vertical (rather than horizontal) through the base of the excavation.
The most notorious area in the Middle East with respect to groundwater issues is Jeddah, particularly along the ‘Corniche’ north of the city. A combination of proximity to the Red Sea and the presence of highly permeable porous coralline limestone at relatively shallow depth causes major issues when trying to control groundwater flows into a typical basement excavation. For example, it is common in Jeddah for a 50 x 50 meter footprint site with three basement levels to have groundwater flows as high as 5,000m3 per hour (1.1 million gallons per hour) without some form of grout-based ground treatment (again cut-off walls are largely ineffective as most flow is vertical rather than lateral). Even if pumping capacity is available, there are major issues associated with both water treatment and discharge of such large volumes of water. These issues among many others, certainly make life as a geotechnical engineer interesting in the Middle East.
About Paul Gildea
Paul is a geotechnical specialist with 27 years of experience as both a specialist contractor and consultant in the UK, Ireland, Hong Kong, China, Australia and throughout the Middle East. He currently manages international projects for Langan, primarily in the Middle East, Asia, and Africa with specific expertise in the design and construction aspects of foundations for high-rise structures and deep basements.