• Settlement • Collapse of subsurface cavities or voids • Transport of groundwater containing contaminants • Dissolution of soluble materials, such as rock salt or gypsum These aforementioned effects can signifi- cantly affect the area being dewatered and the structures located within the zone of dewatering influence. Several cases of ground failure and extensive settlements in Abu Dhabi have been attributed to nearby uncontrolled dewatering activities or poor construction practices. Ground failures and associated settlements that exceed about 5 ft (1.5 m) have occurred, especially in some residential sections of Abu Dhabi. To minimize potential subsidence/collapse problems related to dewatering, construc- tion companies and dewatering contractors need to be informed about the risks associated with dewatering and possible mitigation measures to reduce risks. Unde r the owne r ship o f the Municipality of Abu Dhabi City (ADM), a set of guidelines was developed that standardize dewatering practices to suppo r t the dewa t e r ing pe rmi t applications. These dewatering guidelines provide various dewatering project examples (suggested complete approach) to support dewatering applications in the city of Abu Dhabi. Special emphasis is given to control and to eliminate potential subsidence/collapse problems related to dewatering projects in the ADM area. The guidelines are designed to support designers, contractors, consultants and third-party reviewers regarding the technical aspects of the dewatering project according to three main phases in a dewatering project: (1) field investigation, (2) dewatering design and (3) monitoring program. The requirements addressed in these guidelines are scalable to the magnitude of the dewatering project. Subsurface Conditions Since much of the coastal strip has been reclaimed or developed, most surfaces in the ADM are covered with a variable amount of made ground. In the coastal strip, the made ground is often composed of carbonate sand dredged f rom neighboring lagoons. Typically, Quaternary sediments, which are deposited in the coastal areas, underlie the made ground. Landward of the coastal deposits is mostly aeolian sand, and fluvial sand and gravel deposits. In much of the ADM area, there has been extensive carbonate-evaporitic sabkha development, which consists of loose, silty, fine carbonate sands, and where cementation increases with depth. Sabkha is characteristically found in low-lying areas, which are prone to flooding during high spring tides. Stratigraphically, below the Quaternary sediments are rock layers, which are composed of sandstones in the southern areas; dolomitic conglomerates, sandstones and siltstones in the northern areas; and dolomites and limestones along with evaporitic mudstone and siltstone in the eastern portions of Abu Dhabi. The top of bedrock elevations within the ADM area typically range from about El. -66 ft to El. -328 ft (El. -20 m to El. -100 m) mean sea level (MSL). During various geotechnical investi- gations in Abu Dhabi, voids have been identified by tool drops in fractured calcareous mudstones and siltstones with gypsum inclusions, calcarenite and sands, between or above massive gypsum layers, and, in what is referred to as, the weathered/fractured top of rock. Intensive dewatering has also been interpreted as having the potential for either increasing the size of pre-existing voids in the subsurface or creating voids through the removal of the fine particles commonly found in the voids. Irrigation of gardens and inland farmland exacerbates the problem by increas ing the local groundwater head. This situation, in conjunction with construction-related dewatering within the urban area, is changing the hydraulic gradient, thereby creating one of the key triggers for sinkhole development. Sabhka distribution in Abu Dhabi 100 • DEEP FOUNDATIONS • MAY/JUNE 2017
