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4.2 SUBSURFACE INTAKES 69
The performance of the well screen is enhanced by a gravel (filter) pack, which consists of clean, uniform, and well-rounded gravel and sand placed between the borehole wall and the well screen to prefilter the groundwater entering the well. Typically, the gravel pack depth extends at least 1 m (3 ft) above the well screen.
A well seal is installed above the filter pack to prevent soil and contaminants from entering the well-screen area. The well seal is a cylindrical layer of cement, bentonite, or clay placed in the annulus of the well between the well casting and the borehole. Typically, the well seal extends at least 0.6 m (2 ft) above the top of the gravel pack and usually through the elevation of the soil frost zone. The above ground portion of the well is finished with a concrete surface seal. The surface and well seals protect the well from surface runoff contamination and supports the casing.
Once constructed, the vertical well has to be monitored frequently to secure its long-term performance and identify early signs of potential malfunction and failure. The most common causes of well failure are borehole collapse, corrosion of the casing, improper or defective construction techniques, growth of organisms within the well borehole, and formation of mineral deposits or crusts in the open-hole or screened section of the well borehole.
The 80,200 m3/day (21 MGD) Sur SWRO plant in Oman is the largest plant with vertical intake wells in operation at present. The intake area consists of limestone formations, which have average transmissivity of 7000 m3/day m (David et al., 2009). The well field includes 33 (25 duty and 3 spare) beach wells capable of producing 70e100 Lps (1.6e2.3 MGD) each. Well depth is 80e100 m (260e330 ft) and the diameter of the wells is 14 in. Each well is equip- ped with 14-in. diameter PVC casting and screen with slot size of 3 mm. The wells are surrounded by gravel packs. Each well is equipped with a duplex stainless steel submersible pump and has an average drawdown on 12 m (39 ft).
4.2.2 Horizontal Ranney Wells
This type of wells consists of a concrete caisson that extends below the ground surface with water-well collector screens (laterals) projected out horizontally from inside the caisson into the surrounding aquifer (Fig. 4.2). Since the well screens in the collector wells are placed horizontally, higher rate of source water collection is possible than with most vertical wells. This allows the same intake water quantity to be collected with fewer wells. Individual Raney intake wells are typically designed to collect between 90 and 450 Lps (2 and 10 MGD) of source water.
The caisson of the horizontal collector well is constructed of reinforced concrete of 2.7e6.0 m (8.9e19.7 ft) inside diameter with a wall thickness from approximately 0.5e1.0 m (1.6e3.3 ft). The caisson depth varies according to site-specific geologic conditions, ranging from approximately 10 to over 45 m (33e148 ft).
The number, length, and location of the horizontal laterals are determined based on a detailed hydrogeological investigation. Typically, the diameter of the laterals ranges from 0.2 to 0.3 m (8e12 in.) and their length extends up to 60 m (197 ft). The size of the lateral screens is selected to accommodate the grain-size of the underground soil formation. If necessary, an artificial gravel-pack filter can be installed around the screens to suit finer-grained deposits. Usually, one well has 2e14 laterals oriented toward the source water body (ocean, river).