Tool drops and loss of water circulation have also been described in the calcarenite layers. Water loss is also commonly associated with highly-permeable soils (e.g., silty sand with gravel/shells, and gravels). Settlement has been observed in certain areas after the water level drops due to the intense dewatering occurring during construction activities, especially in those areas in which more permeable soils (e.g., gravel, sandy gravel and sand bars) are present. Dissolution of salt crystals in the fill material also contributes to settlement in some areas of the ADM. Analyzing and interpreting potenti- ometric surfaces indicated that ground- water elevations in the western and northcentral regions of Abu Dhabi are relatively flat, with elevations ranging from approximately El. -49 ft to El. -33 ft MSL (El. -15 m to El. -10 m MSL). The shallow groundwater system is recharged primarily from the ground surface via precipitation, irrigation and stormwater runoff detention ponds. A portion of the groundwater flow is discharged horizontally into the Arabian Gulf. However, a large portion of ground- water discharge can be attributed to evaporation to the atmosphere, especially in sabkhas and low elevation areas, where the surface of the groundwater table is very close to the ground surface. According to available literature, a water budget was estimated for the UAE coastal sabkhas based on data collected at two locations along the UAE coast. The calculations revealed that the average annual precipita- tion for Abu Dhabi is approximately 3.5 in (90 mm), and the average annual recharge to the sabkha groundwater is approximately 1.75 in (45 mm). The unconfined aquifer ranges about from 0 ft to 79 ft (0 m to 24 m) in thickness. In general, greater saturated thicknesses occur in the southcentral region of Abu Dhabi, within the aforementioned core geotechnical hazard area. Approximately 1,000 different hy- draulic conductivity field tests performed in the Abu Dhabi area were collected from various engineering reports. This data revealed that sands and gravels in the overburden and sandstones, calcarenites, and conglomerates in the bedrock forma- tions generally have the highest hydraulic conductivity values. Lower values of hydraulic conductivity were generally found in the finer grained sediments, gypsum, claystone, and mudstones. However, there is considerable variation and overlap within and between lithologic types. Natural hydraulic gradients within the surficial aquifer are generally low. However, when large-scale construction dewatering programs are implemented, water levels can be drawn down considerably and, thus, can create relatively steep hydraulic gradients, increase groundwater seepage velocities, and increase hydraulic uplift pressures. As a part of the preconstruction planning for a dewatering project, potential permeable zones should be identified based on borehole logging, monitoring of piezometers and wells, and geophysical measurements. Depending on the project Thickness of overburden (unconsolidated sediments and fill) DEEP FOUNDATIONS • MAY/JUNE 2017 • 101
