requirements, additional field tests (e.g., borehole seepage and pumping tests) may be performed to evaluate the quantity of the water likely to be encountered during dewatering operations. In the analysis of any dewatering system, the source of seepage must be determined, and the boundaries and seepage flow charac- teristics of geologic and soil formations at and adjacent to the site must be generalized into a form that can be analyzed. For example, the source of seepage can be modeled as a line or a circle; the aquifer as a homogeneous, isotropic formation of uniform thickness; and the dewatering system as one or two parallel lines or circles of wells or well points. According to the recommendations in the dewatering practices guidelines, the scope of the geotechnical investigation and laboratory testing for a project should include an evaluation of potential zones of cavity collapse. The formation of cavities involves natural processes of erosion or gradual removal of slightly soluble bedrock (e.g., limestone, gypsum or rock salt) by the percolation of water, collapse of a cave roof or lowering of the groundwater table. The geotechnical investigation should identify the existence of the collapsible or soluble materials. If such materials exist, which is likely in Abu Dhabi, the dewatering system and excavation should be designed to prevent the collapse of potential cavities, which could be influenced by the dewatering operations. There is a potential risk that dewatering may result in settlement. Compressible soils and loose granular soils have the potential to compress and consolidate when the groundwater table is lowered and the effective stress is increased. In such cases, settlement analyses are necessary to predict negative effects on nearby structures and then to plan measures to mitigate such effects. The dewatering guidelines provide the recommended Hazard Zone C 3 1,2 B 3 1,2 A 3 Proximity of Structures 1,2 Excavation/ Dewatering Type i i, ii, iii i i, ii, iii i i, ii, iii i i, ii, iii i ii, iii i, ii, iii i ii, iii i ii, iii i ii, iii Geologic/Hydrogeologic Hazard Zone Proximity of Structures Excavation Depth Excavation/Dewatering Type Pumping Capacity Analysis Settlement Analysis Third-Party Review Key - dewatering design scope A B C 1 2 3 Shallow Excavation Depth Shallow Medium Deep Shallow Medium Deep Shallow Medium Deep Shallow Medium Deep Shallow Medium Deep Shallow Medium Deep Hazard zones and dewatering design types High Potential Medium Potential Low Potential Sensitive or Large Structures Nearby Structures Could be Impacted by Project No structures that could be impacted 0-3 m Medium 3 m-10 m Deep i >10 m ii iii a b c I II X Open Cut (Sumps and Open Pumping) Cutoff Structure Wells and Ejectors Analytical Solution Flow Net Numerical Analysis Hand Calculation Numerical Analysis Third-Party Review Required Third-Party Review Not Required Pumping Capacity Analysis a a c a a b a b c a a b c c c c a a b b c c Settlement Analysis I I I I I I I I II I I II I I II II I I I I II II Third Party Review X X X X X X X X X X X 102 • DEEP FOUNDATIONS • MAY/JUNE 2017
