Introduction to Drilled Wells

A. Overview of Small Diameter Wells

1. Basic Features

Small diameter wells usually have diameters of less than 50 cm and can be as small as 2.5 cm. For the purposes of this manual, all wells that are sunk by using tools from the ground surface, and therefore do not require that people go down and work in the well to sink them, are small diameter wells. They are also called "drilled" wells, "tube" wells, or "boreholes."

Small diameter wells normally require a pump to supply water on demand at the surface. These wells are usually small enough so that a bucket used to lift water from a large diameter well will not fit into the well. A special water lifting device must then be built and installed in the well to allow people to draw water.

There are many different kinds of water lifting devices that may be used, ranging from a modified bucket arrangement that will fit in the well to a motor-powered turbine pump capable of delivering thousands of liters per minute. This manual emphasizes pumps that will comfortably supply enough water to meet the minimal needs of the local population.

Every small diameter well is sunk by using the same arrangement of equipment. At the ground surface is a power unit which supplies the necessary motion to sink the hole.

At the bottom of the hole is a cutting tool (drill bit) which, when moved in a certain way, loosens whatever soil or rock is beneath it in the hole. (See Fig.10-1.)

Between the power unit and the bit is a connecting mechanism that transmits the motion of the power unit to the cutting tool.


FIG. 10-1. DRILL BITS

2. Drilling Motions

Because of the size and shape of small diameter wells and the equipment required to sink them, there are only two different kinds of drilling motion that are used (Fig. 10-2 ):

a. up and down - called percussion

b. around and around - called rotary


FIG. 10-2. DRILLING MOTIONS

All well sinking equipment is intended to use one of these two different motions. However, all such equipment is designed to use one or the motions predominantly and often the other motion in some secondary capacity. Thus, tools that use an up-anddown motion are referred to as "percussion" tools because they rely on their downward fall to stike and loosen ground materials. Tools that must be turned to gouge and scrape ground materials are referred to as "rotary" tools for obvious reasons.

Both of the different types of equipment can be adapted for use in most ground conditions. However, when using simple hand powered variations of either of these techniques there are certain limitations which will be noted later.

3. Removal of Drill Cuttings from the Hole

In order for the cutting tool or drill bit to effectively do its job of loosening the soil at the bottom of the hole, the soil and rock that it has already loosened or chipped away must be removed so as not to hinder the bit's continued operation. There are several different techniques and tools that can be used.

A fluid can be used to continually pick up drill cuttings and carry them to the surface. Fluid is normally pumped down through the drill rod and out of holes in the bit where it picks up drill cuttings and carries them back up through the hole.


FIG. 10-3. BAILER


FIG. 10-4. AUGER BITS.

4. Equipment and Materials

Here are the equipment and materials that are usually employed (see Fig. B for illustration of Materials):

5. Casing

A pipe is installed in a well to prevent the hole walls from caving in and provide a conduit through which water can be brought to the surface.

If a well is sunk through anything other than consolidated rock, it will need to be cased to assure a permanent hole.

The casing extends from just above the ground surface to the top of the well screen which is at the bottom of the well.

Commercially drilled wells are usually cased with steel pipe which is either welded together or coupled, using specially threaded couplings. Although plastics are being used increasingly because they will not rust or corrode, nevertheless they cannot be driven into the hole as required by some drilling techniques and ground conditions. Basically, any cylindrical product which can be installed in the hole to prevent its collapse will work. For small diameter shallow wells, galvanized iron pipe, clay tile, bamboo and even hollowed logs have been used, although the last two are, for obvious reasons, not recommended.

NOTE: Before using steel or even galvanized iron casing, test the water to obtain some kind of measure of its corrosive properties. In Liberia, wells with steel casing had life spans of as little as six months due to corrosion.

6. Well Screen

This is the water intake section at the bottom of the well. It is about the same diameter as the casing and is made as long as is necessary a) for the depth of aquifer and 2) to produce the amount of water needed.

The screen itself acts both as a filter and as a medium through which the soil particles immediately surrounding the screen can be rearranged to permit easier and better water flow into the well, a process known as well development. Wells are usually developed by rapidly forcing water in and out of the well screen. This removes the fine soil particles from right next to the screen leaving only larger soil particles with larger spaces between them,thus permitting more and easier water movement into the well. (Fig. 10-5)

The different kinds of screens are discussed in detail in Chapter 15.


FIG. 10-5. CONTINUOUS SLOT WELL SCREEN IN DEVELOPED GROUND FORMATION

B. Design of Drilled Wells

The basic objective of well design is to achieve the best combination of materials, techniques, and cost to produce a well which is useful to a local community. Against a background of scarce material and financial resources, the design of wells for small rural communities in the developing countries requires a flexible approach to individual water supply needs that involves compromises on the allocations of community resources.

1. Design Problems

Two major problems in all water improvement projects: a) system maintenance and b) water testing and treatment should be considered in the design and planning of the well.

Water is rarely tested to determine its chemical and bacteriological characteristics. Chemical testing is necessary to determine possible appropriate materials for intake and pumping equipment, as well as potability. Bacteriological testing is needed to detect possible disease-carrying organisms. Many simple, inexpensive, and relatively effective methods of treatment are available and could easily be used to help ensure water potability and to lower disease rates. (See Appendix VIII.)

2. Diameter

Where large supplies of water are needed, the normal procedure for determining the well diameter is to:

For small wells, the same general considerations apply, although based on the selection of the most appropriate equipment available. The casings in most of these wells will be between 5 cm and 10 cm in diameter.

3. Depth

4. Choice of Pump (Also see Pump Appendix.)

C. Planning

Overview

Here is an overview of planning which presents major choices and their determining factors. Use it as a checklist in your planning activities.

a. How you sink the hole depends on:

b. How you case the well (reinforce the hole walls) will depend on:

c. How the intake section is constructed will depend on:

d. How the top section is constructed will depend on: 1) the degree of sanitary protection that is felt necessary and 2) users' preferences for a particular method of water delivery.

e. The maintenance performed on the well depends on:

D. Work Outline

Here is an outline of the essential construction activities, in working order: