STEP Geological Conditions
Geological Conditions
Conditions in the Abu Dhabi area essentially consist of a linear coastline dissected by ancient channels/creeks. Superficial deposits consist of marine sands and silts in coastal zones. In addition, wind erosion, capillary action and evaporation has led to extensive sabkha deposits in certain areas, notably around the creeks forming hypersaline deposits. These superficial deposits overlay alternating beds of calcarenite, carbonate sandstone, gypsum, sands and cemented sands.
The preliminary results from initial site investigation work completed to date indicate that the tunnel alignment is likely to be in very weak to weak sedimentary rocks consisting of interbeded layers of Siltstone, Claystone and Gypsum. Ground water has typically been encountered close to existing ground surface. The unconfined compressive strength of the rock formation encountered to date generally varies between less than 1 MPa to about 15 MPa. The soils and ground water may be corrosive to concrete. The design of underground concrete structures such as tunnel lining shall take due consideration for corrosion protection of concrete on the extrados (the face of the structure in contact with the ground).
Due to the potential for the development of solution cavities in the rock formations, and/or zones where the rock matrix has deteriorated, advance probing ahead of face of TBM will be specified to investigate any potentially disturbed zones or solution cavities, which may or may not be encountered during geotechnical investigations.
The deep tunnel sewer will be constructed through a generally competent tunnelling material. The tunnel will, however, be deep below the groundwater table, requiring constant control of the tunnel face to prevent face instability. The use of pressurised face TBMs, either an earth-pressure balance (EPB) shield or a slurry face (SF) shield, will be required. The initial ground support will consist of pre-cast concrete segments, sealed and bolted together in rings. The annular void left behind outside the shield as it advances will be filled with grout under pressure to prevent the void from collapsing. After completion of excavation and primary (structural) lining, a corrosion protection lining will be installed.