Contents preface (VII) introduction 1—37

4.9. MEHODS OF WELL CONSTRUCTION
The operations involved in well construction are drilling, installing the casing, placing a well screen and filter pack, and developing the well to ensure maximum sand-free water yield. Shallow wells, generally less than about 15 m deep, are constructed by digging, boring, driving or jetting. Deep wells are constructed using drilling methods. Wells used for irrigation purposes are generally deep.
4.9.1. Digging
Wells in shallow and unconsolidated glacial and alluvial aquifers can be dug by hand using a pick and shovel. Loose material is brought to the surface in a container by means of rope and pulleys. The depth of a dug well may vary from about 3 to 15 m depending upon the position of the water table. Dug wells usually have large diameter ranging from about 1 to 5 m. Dug wells must penetrate about 4 to 6 m below the water table. The yield of the dug wells is generally small and is of the order of about 500 litres per minute.
4.9.2. Boring

Hand-operated or power-driven earth augers are used for boring a well in shallow and unconsolidated aquifers. A simple auger has a cutting edge at the bottom of a cylindrical

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container (or bucket). The auger bores into the ground with rotary motion. When the container is full of excavated material, it is raised and emptied. Hand-bored wells can be up to about 20 cm in diameter and about 15 m deep. Power-driven augers can bore holes up to about 1 m in diameter and 30 m deep (4).

4.9.3. Driving
In this method, a series of connected lengths of pipe are driven by repeated impacts into the ground to below the water table. Water enters the well through a screened cylindrical section which is protected during driving by a steel cone at the bottom. Driven wells can be installed only in an unconsolidated formation relatively free of cobbles or boulders. The diameters of driven wells are in the range of about 3–10 cm. Such wells can be constructed up to about 10 m, if hand driven, and up to about 15 m when heavy hammers of about 300 kg are used. The maximum yield of driven wells is usually around 200 litres per minute. The main advantage of a driven well is that it can be constructed in a short time, at minimum cost, and by one man.
4.9.4. Jetting
The jetting (or jet drilling) method uses a chisel-shaped bit attached to the lower end of a pipe string. Holes on each side of the bit serve as nozzles. Water jets through these nozzles keep the bit clean and help loosen the material being drilled. The fluid circulation system is similar to that of a direct rotary drilling method. With water circulation maintained, the drill rods and the bit are lifted and dropped in manner similar to cable tool drilling but with shorter strokes. Jet drilling is limited to drilling of about 10 cm diameter wells to depths of about 60 m, although larger diameter wells have been drilled up to about 300 m by this method (6). Other drilling methods have replaced jet drilling for deep and larger diameter wells.