M. Sonou
FAO Regional Office for Africa, Accra, Ghana
Relative advantages and disadvantages of small and large diameter wells
Shallow tube well construction techniques
Shallow tube well programmes
Shallow tube well costs
Conclusions and recommendations
References
One of the main constraints to irrigation development in West Africa is the mobilization of water resources and its associated high costs. At times, these costs become prohibitive, especially when groundwater is tapped for irrigation. The deeper the well or borehole, the higher the capital, operation and maintenance costs of the scheme.
While West Africa has developed only 34% of its potential for irrigation, more than 95% of the developed area relies on surface water. In view of the high evaporation rate that characterizes the sub-region, surface water is not always available at the right moment and in adequate quantity for crop requirements. Supplementary irrigation may therefore benefit from groundwater resources where economically feasible. In high potential areas year round irrigation could rely on groundwater resources, provided that abstraction remains within sustainability limits.
Traditionally, farmers lift water from shallow dug-outs and dug-wells for individually-managed micro scale irrigation in the dry season. Discharge is low and can only allow small irrigated areas. However, in Nigeria, the introduction of low-cost shallow tube well technology combined with small engine-driven water pumps triggered off the development of fadama irrigation. The total number of shallow tube wells drilled by the Bank funded Agricultural Development Projects (ADPs) in Bauchi, Kano and Sokoto State between 1983 and 1990 was over 15 000. The cost of constructing shallow tube wells was reduced by about two-thirds, with a commensurate increased return on tube well investment. In 1992 the Bank prepared a new project which would construct about 50 000 shallow tube wells in Nigeria, would privatize drilling, simplify drilling technology for shallow tube wells, conduct aquifer studies and upgrade irrigation technologies.
In West Africa, large wells are traditionally of 80 cm to 100 cm diameter. Modern large wells frequently have 140 cm or 180 cm diameter.
Theoretically, the flow of water into a well is given by the following Dupuit formula:
TABLE 1 Relative advantages and disadvantages of small and large diameter wells
|
CRITERION |
SMALL |
LARGE |
|
Equipment Required: |
|
|
|
(1) For Construction |
(1) Specialized equipment such as augers and bailing buckets required. |
(1) Little specialized equipment is absolutely necessary. |
|
(2) For Raising Water |
(2) Specialized equipment such as pumps or small diameter well buckets are necessary. |
(2) Ropes and buckets are frequently used. |
|
Cost of Construction |
Lower, because relatively little material is required. |
Higher, because much more material is required. |
|
Sanitation |
Potentially good, especially when a hand pump is used. |
Poor, since top of well is open. Buckets and ropes which may be dirty are used in the well |
|
Safety |
Danger during construction and use negligible. |
Construction: Danger of cave-in may be eliminated by proper construction. Danger of something dropping on worker in well always possible. |
|
Use: Proper construction of top of well can minimize danger of people falling in. |
||
|
Maximum Number of People Able to use the well concurrently |
One |
Three or Four |
|
Rate of Discharge Possible |
Potentially better since well can be made almost any depth below static water level. Good possibility of putting perforated part of casing in material of high permeability. |
Depth to which well may be excavated below static water level is limited by equipment. Therefore, rate of discharge is limited. |
|
Skill Required: |
|
|
|
(1) Well Construction |
(1) Somewhat more, since tools are special and work cannot be seen |
(1) Somewhat less. |
|
(2) Water Raising Equipment |
(2) More, must be able to maintain and repair pump and/or small dia. Well buckets. |
Little required |
|
Reliability: |
|
|
|
(1) Well |
(1) Excellent |
(1) Good only if certain precautions are taken in constructing the bottom of the well. |
|
(2) Water Raising |
(2) Frequently a problem under village use (requires trained maintenance personnel). |
(2) Good |
|
Ability to Store Water For Hours of Peak Demand (of Possible Importance When the Permeability of the Aquifer is very low) |
|
May be increased by increasing the diameter and depth of well |
|
Limitations on when well may be constructed. |
None |
Should be done at the time of year when the water level is at its lowest. |
|
where |
Q = yield or rate of pumping (e.g. m3/sec.) |
|
|
K = permeability of the aquifer (m/sec.) |
|
|
H = Static height of water (m) |
|
|
h = height of water during pumping (m) |
|
|
R == radius of the "cone of depression" (m) |
|
r = radius of the well (m) |
Based on the expression given above; and assuming that R = 25 m, that depths of penetration are the same for two wells with respectively 15 cm and 150 cm diameters. The large diameter well will yield 1.8 times as much water as the small diameter well and requires 100 times as much excavated material. Increasing depths is frequently a more efficient way of increasing the yield of a well than is increasing the diameter. However, the deeper the well, the more difficult the working conditions during the construction phase, and the higher the costs. For large diameter wells, once the aquifer has been reached, further deepening is limited by equipment.
Table 1 (Koegel, 1985), gives some relative advantages and disadvantages of small and large diameter wells.
The decision to have a large or small diameter well will depend on many factors including:
· the geology of the location;· materials available and their cost;
· skills available and their cost;
· the need to store water, particularly in the poorly permeable rocks where wells cannot obtain flows even in the best conditions; and
· end use of the well
In any case, two or more adequately spaced small wells will provide a cheaper and more reliable water source than a single large well.
There are a variety of techniques which are used to construct small diameter wells. Table 2 provides a summary of seven such techniques.
As mentioned earlier, the deeper a well, the higher its construction costs and the cost per unit volume of water abstracted, irrespective of the type of water lifting device used. For these reasons, in West Africa, the use of groundwater for irrigation, especially from deep aquifers has not been very successful. However, over the last 15 years, shallow aquifers have been making an increasing contribution to the expansion of small-scale irrigation, particularly in Nigeria. The presence of groundwater resources at shallow alluvial depths, less than 20 metres in most of the fadamas throughout the dry season plays a key role. These aquifers are recharged annually with the onset of the rain and the river flow since they are in hydraulic continuity with the river channel. More importantly, the introduction of low-cost techniques for the construction of shallow wells has been a major contributing factor.
Five main well construction techniques are currently used in West Africa. They can be grouped under two headings as follows:
|
· Drilling Techniques: |
Small Rotary rig method |
|
|
Percussion bailer (Cable-tool) method |
|
|
Vibro-bailer method |
|
· Jetting Techniques: |
Clear water washboring method |
|
|
Mud washboring method. |
Small rotary rigs
In Nigeria, most of the ADPs use small rotary rigs in drilling tube wells. This is the most versatile method since it is restricted only by hard rock formations. Drilling completion time for a 20 metre deep hole is one day (Kaduna State Agricultural Development Project). These rigs are designed to penetrate to a maximum depth of 60 metres. The capital cost of a rig and associated spares and accessories is high, over US$ 50 000 (Wardrop Engineering, 1992). Due to its highly mechanical nature, a rotary rig is more susceptible to breakdown than other methods. It is likely that a finished 20 m borehole drilled by these rigs will cost around 2 000 Naira if the machines continue to be well maintained and operated. This cost compares with a few hundred Naira for a simple washbore well and with roughly 15 000 Naira for the village boreholes being installed in Bauchi State by Mitsui" (Carter, 1984).
Tube wells are drilled (in fadama) where washboring is not feasible because of the depth of the aquifer or the resistance of the overlying materials. At this stage in the development of tube wells for small irrigation in Ghana, investment in small rotary drilling equipment may not appear warranted.
TABLE 2 Summary of methods for drilling small diameter wells (after Koegel)
|
Method |
How penetration is accomplished |
Minimum equipment required |
Removal of material from hole |
Advantages/disadvantages, limitations |
|
Augured or bored |
Cutting lips of a rotating auger shave or cut material loose from the bottom of the hole. |
Auger, detachable tubular extensions, and a handle for rotating. |
Auger must be removed from the hole whenever it is full of cuttings This necessitates uncoupling extensions. |
Equipment is simple and can usually be fabricated or adapted locally Cannot penetrate hard formations Uncoupling extensions slows work at greater depths Usually cannot be used below the water table. |
|
Driven |
A point on the lower end of a string of pipe allows the pipe to penetrate as it is driven on the upper end. |
Drive point which usually also includes a well screen above it, special drive pipe with couplings, drive cap, and driver. |
Material is not removed from the hole, but is forced through it. |
Fast and simple Special well points and heavy drive pipe may not be available locally Hard formations cannot be penetrated limited to small diameters, but multiple well pointsmay be connected to a common pump. |
|
Jetted |
A high velocity system of water coming out of the bottom of a vertical pipe washes away material ahead of it as it is lowered. |
Pipe equipped with jetting orifice(s) at lower end, couplings, suitable rump (hand or powered), flexible connection between pump and pipe, and supply of water. |
The water is used for drilling returns to the ground surface by way of the annular space around the jetting pipe carryings the material removed with it. |
fast cannot penetrate hard formations Difficulty in bringing large gravel or stone to the surface Drilling equipment can be fabricated locally, but a pump and a source of water are required. |
|
Hydraulic percussion |
The hole is kept full of water The alternate raising and dropping of a string of pipe equipped with a cutting bit at the bottom allows penetration by a combination of mechanical and hydraulic action. |
Hollow drill bit with water inlets and a check valve, string of pipe, devices to aid raising and dropping A hand over the top of the drill pipe may be substituted for the check valve. |
The raising and dropping action in conjunction with the check valve causes water to be pumped up in side of the drill pipe carrying the cuttings with it. |
Equipment can be fabricated locally or purchased Water is required Traditionally used in some areas, thus understood by local well drillers Hard formations cannot be penetrated Difficulty in bringing large gravel or stones to the surface. |
|
Cable |
A heavy cylindrical weight equipped with a cutting edge at the bot-torn with rope or cable attached to the upper end is alternately raised and dropped Impact pulverized material at the bottom of the hole. |
Heavy drill bit, rope or cable, devices to aid raising and dropping. |
The pulverized cuttings mixed into a slurry with water during drilling These are removed using a bailer. |
All formations can be penetrated at varying rates Some water required Commercially built rig expensive and requires consider able skill to operate, but a simple set of tools can be fabricated locally and adapted to man or motor power. |
|
Bail down |
A long, cylindrical bucket with a check valve at the bottom and a rope or cable attached to the top is alternately raised and dropped in a hole partially filled with water Penetration is accomplished by combination of hydraulic and mechanical action. |
Bailer, rope, devices to aid raising and dropping. |
Slurry of cuttings and water enter the bailer as it is repeatedly dropped These are prevented from leaving the bucket by the check valve The bucket is raised to the surface for emptying. |
Equipment can be fabricated locally Frequently used in conjunction with other methods, such as percussion Hard formations cannot be penetrated by the bailer alone. |
|
Hydraulic rotary |
A hollow drill bit with either a fixed cutting edge or toothed rollers is rotated at the bottom end of a string of pipe Material is scraped, abraded or chipped away by mechanical action. |
Drill bit drill pipe circulating pump, device for rotating drill pipe. |
Water or "mud is pumped down the hollow drill stem to lubricate the bit and to carry the cuttings up to the surface through the annular space around the drill pipe Circulation may also be in the reverse direction. |
Commercially built rig is expensive and requires considerable skill to operate However, small adaptations using either man power or small engines have been devised A water supply is necessary It is difficult to drill in loose formations. |
Percussion bailer (cable-tool) method
The equipment is made up of a heavy drill bit, a bailing bucket consisting of a tube with a check valve at the bottom and a bail for attaching a cable or rope to the top. The method consists of alternatively raising and dropping the chisel-edged bit to break loose and pulverize material from the bottom of the hole.
The pulverized material mixes with the small amount of water kept in the hole. The slurry thus formed is removed by lowering the bailer periodically in place of the percussion bit. Drilling and bailing are alternated until the desired depth is reached. In unstable geological formations, a casing is lowered and the driving of the casing is alternated with the other two processes.
Although the method is frequently associated with large, motorized, truck-mounted equipment, it can be scaled down and used with small engines, or manpower. In the latter case, the method is still labour intensive, the equipment heavy and operations cumbersome. In the Kaduna State, Nigeria, experience showed that on average about 1 to 4 days were needed to complete one tube well in sandy soils and 4 to 6 days in clayey and gravely strata. It is less successful in strata where clayey soils and gravel are interbedded, and in basement complex with hard strata. In 1991, the cost of procuring one bailer set only was about US$ 3 700 in Nigeria. Sokoto, Niger, Adamawa, Taraba and Benue ADPs introduced manual bailer for drilling tube wells. Still, the technology is not very popular in fadama.
In Ghana, the method although tested successfully, was found slow and labour intensive, and the equipment heavy to move from site to site. Other available techniques are simpler. However, the cable-tool method may be used in conjunction with other methods when conditions such as hard or loose materials are encountered which make it more suitable.
Vibro-bailer technique
This is a combined hand augering and bailing method of drilling in sandy river beds. It is limited to shallow depths up to 10 metres and holes of 75 mm diameter. It is not suitable where layers of sand and clay are interbedded except, with use of special augers. Three semiskilled technicians can easily be mobilized to install four tube wells per day at close locations depending on lithology.
Jetting method
The jetting technique uses a high velocity stream of water to bore the well. The stream is generated by either motor or hand powered pump. The excavated material is washed out. The erosive action of water is however ineffective in cases of hard materials. Semi-hard materials may be penetrated by a combination of hydraulic and percussion effects which are obtained by raising and dropping a chisel-edged jetting bit. Moving coarse materials such as gravel vertically out of the hole requires a greater water velocity than do finer materials. Water pressure of 3 kg/cm2 for sand and 7-11 kg/cm2 for clay or gravel have been recommended. Basically, two jetting techniques are used:
(1) Water is pumped down a jetting tube or pipe which is used inside a temporary or permanent casing. When the required depth is reached, the final casing with screen attached is lowered inside the temporary casing, which is then jacked out of the hole. Alternatively, in case of permanent casing during the jetting operation, the well screen is lowered inside the casing, which is then jacked up far enough to expose the well screen to the aquifer.(2) Jetting may be done by pumping the water down through the casing itself. The excavated material is then carried up through the annular space around the outside of the casing.
Different versions of jetting techniques were transferred to Nigeria in the early 1980s. Three of them are presented below.
Clear water washbore method
Technique
This jetting technique uses a plastic temporary casing to support the wall of the hole until the slotted PVC lining has been placed. The temporary casing is then removed.
Technically the method appears to be quite satisfactory, except (common with all jetting methods) when penetrating clays. One solution used is hand-augering through the clay layer using a small diameter percussion tool inside the drilling pipe (having disconnected the water supply) or inside the casing (necessitating temporary removal of the drilling pipe).
In cases' of jetting through fine sands, it is often worthwhile continuing to greater depth in the hope of encountering coarser material. The static water level will still rest within suction depth of surface but the yield of the hole will be significantly greater than if drilling stops in the fine material.
Materials and equipment (Kana State Agricultural and Rural Development Authority):
· Petrol Engine Pump, Fuel, Oil, Suction Hose, Delivery Hose with same specification as for suction, Strainer, Hose Couplings and Hose Clips.· Medium Duty 2" GI Pipe 6m, 3 m & 1.5 m lengths for jetting (Total 20 m), 3" GI Cutting Edge, 3" X 2" GI Reduces Sockets, 2" GI Sockets, Bends Nipples, 24" pipe wrenches - 2 Nos.
· OPTIONAL: Water Tank, Hand Auger
Procedures
|
Step 1: |
Drill pilot hole with hand auger, removing the top clay and silt preferably close to water bearing sand. Measure the depth of water table to decide whether or not to proceed further. The depth of the water table can also be determined if a reconnaissance survey is carried out in the area to establish that groundwater is available within 6 m from the surface. |
|
Step 2: |
After having established the site, decide whether to reuse the jetting water or not. If the water is to be reused, dig a pit 1 m x 1 m x 0.7 m approximately some 3 to 4 m form the pilot hole. |
|
Step 3: |
Assemble the drilling pipe 3 to 4 m with GI bend and drilling bit. |
|
Step 4: |
Assemble the pump with suction hose strainer and delivery hose connected to GI bend. |
|
Step 5: |
Hold the GI pipe vertically with the help of a plumber and 3 labourers at the top of the test hole or about 0.5 m inside the hole. |
|
Step 6: |
Start the pump. The drilling operation is performed by jetting a stream of water under pressure and washing the cuttings. Ensure all cuttings are removed before lowering the pipe further. If penetration is difficult, apply a downward reciprocating thrust by lifting and dropping the pipe continuously. By this method, it could be possible to penetrate through the lenses of clays when encountered. When the three-metre penetration is achieved, remove the 3 m jetting pipe and change it to 6 m and continue drilling, then change to 9 m and continue drilling. The change should be achieved with minimal loss of time. When penetrating the aquifer, the pipe will have the tendency to descend on its own. Examine the cuttings closely for the best available formation of coarse sand and gravel. When full depth has been penetrated either by hitting the rock or impermeable clay layer slow down the pump. The drilling pipe is then moved up and down while the pumping is continued so as to bring out all the fine material to the surface leaving the coarse sand and gravel where the screen is subsequently to be located. The pump is further slowed down. |
|
Step 7: |
Assemble the PVC casing and screen to match the location of the aquifer. |
|
Step 8: |
The drilling pipe is withdrawn and the assembled tube well installed immediately into the hole. |
|
Step 9: |
If the hole has collapsed and the screen could not be lowered to the required level, remove the drilling bit and insert the drilling pipe close to the casing and screen and start the pump again. With the help of further jetting, lower the screen to required position. |
|
Step 10: |
Cut off any length which is about 30 cm above the surface from the casing pipe and connect the PVC nipple with 2" GI bend. |
|
Step 11: |
Development:- The washbore is developed by over pumping. After the development is completed, operate the pump for two hours and test the yield. |
|
Step 12: |
After the tube well is operated for a fortnight, provide a concrete slab 40 cm x 5 cm around the well for permanent protection. |
Advantages of Method:
1) All the materials are available locally (in Nigeria)2) Simple technology, very cost-effective
3) No drilling mud is required. For higher depths, further improvements will be required like using a raised platform (scaffold) and using pumps to prevent holes from collapsing.
4) Excellent portability.
Disadvantages of Method:
1) A water tank pickup is required if water is not available locally.2) If clay is encountered below a thick sand layer, the drilling could be very difficult and it may not be possible to penetrate a thick clay layer.
3) This method cannot be used to penetrate coarse water bearing sands because of lost circulation of water. As a result the tube wells cannot be completed at sufficient depth to take full advantage of the available water. For this reason, Wardrop Engineering Inc. did not recommend it for use in Ghana.
Mud washbore method (bentonite method, KNARDA):
The mud washbore method should be considered where full penetration of the aquifer is not possible with the clear water washbore method because of lost circulation in coarse sands and gravels. There are 12 basic steps involved in the procedure.
|
Step 1: |
Drill a 10 mm diameter pilot hole down to the water bearing sand using either a motor hand-powered auger. The auger hole should be drilled until caving sand prevents further advance. |
|
Step 2: |
Dig pits for drilling fluid and place a screen in the flow path to remove sand that accumulates in the drilling fluid during the drilling process. |
|
Step 3: |
Mix drilling mud. |
|
Step 4: |
Install 50 mm drill pipe in hole. |
|
Step 5: |
Circulate drilling fluid using the pump and fill the hole. Once the hole has been filled it is normally necessary to add more water and drilling mud. |
|
Step 6: |
Advance drill pipe to final depth by circulating drilling mud to wash the sand from below the bottom of the drill pipe. |
|
Step 7: |
Remove drill pipe and install permanent casing and screen assembly. |
|
Step 8: |
Backwash through the bottom of the screen with clear water to remove the drilling mud. |
|
Step 9: |
Plug the bottom of the screen. Step 10: Develop the tube well by jetting or surging. |
|
Step 11: |
Test and measure yield and water level. |
|
Step 12: |
Install a cap over the top of the tube well. |
The Bentonite method, although more complicated, allows one to drill deeper (12 m) than the clean water method (max. 8 m until now). The friction around the temporary casing/drilling pipe becomes very high at that depth, and with the present equipment (plastic pipes) and manpower it is not possible to drill deeper.
The use of Bentonite does have the required effect of keeping the hole open prior to placing the casing/screen. However, there are several important disadvantages associated with its use. Chief among these are costs, availability and the problem of sealing of weak aquifers so reducing yield of developed holes.
It has been suggested that commercially available biodegradable drilling muds could be used in place of Bentonite. While this would reduce the problems of clogging and development, it would still be expensive. As suggested by J.R. Temple-Hazell at the Second Fadama Seminar, Bauchi, 6-8 March 1984, it may be possible to manufacture a suitable mud locally from cowpea flour. In Ghana, bags of biodegradable mud was used per well. The cost was US$ 25 per well 1992.
Washbore with double jetting pipes (KNARDA)
A third jetting technique involves the use of dual jetting pipes (in ABS plastic) and two pumps; one of the jetting pipes has a short (0.5 or 1.0 m) stainless steel Johnson type wellpoint on its lower end and is ultimately left in place in the hole. The second pipe has an open end and is used during the jetting process only, then removed.
This method uses a pre-fabricated solvent-cemented ABS lining tube and so is less flexible in terms of depth and positioning of the intake. It is nevertheless fast and relatively simple. The method has been found suitable in river bed sands.
Washbore linings
Generally the slotted plastic borehole linings used by Bauchi State Agricultural Development Programme performs satisfactorily in shallow (less than 10 m) 4 inch holes. Slotted steel was rejected earlier on in the programme. The stainless steel Johnson type screens used by KNARDA is expensive.
Problems arise in fine grained aquifers when, in addition to obtaining low yields, there is the problem of sand pumping. Little can be done to increase yield under these circumstances, except drilling and screening a greater depth, or using large diameter structures. Alternative well-linings may be considered.
Apart from using correctly designed and installed gravel packs, the only effective way to exclude fine sand is by using fabric or fibre wrapped screens. These screens can be made from locally available materials. In India, bamboo and coir well-screens have been used successfully for some time. These well-screens are constructed by wrapping coir ropes tightly round a cylindrical framework of metal hoops and longitudinal bamboo strips; the method is described by R.G. Koegel (FAO Irrigation and Drainage Paper 30, 1985). A metal framework of a quarter-inch reinforcing rod longitudinal on metal hoops, together with hemp or manila rope, could be tried.
Past programmes
In Nigeria, from 1983 to 1990, more than 15 000 washbores and drilled tube wells were constructed in the three States of Bauchi, Kano and Sokoto to irrigate 16 200 hectares. However, considering the 11 northern and middle zone states, more than 18 000 hectares were being irrigated with washbores and drilled tube wells. In Niger, the Lower Tarka Valley project had constructed 700 of such boreholes by 1993. The technology has also reached Chad and the northern zone of the Republic of Benin.
New programmes
The tube well and washbore technology for small-scale irrigation has proven to be very successful in Nigeria. It is much cheaper than large scale irrigation. The individual farmer's ownership of the wells and the pumpets provides considerable independence in selecting crops, cropping patterns and times of planting. It also allows adjustment of irrigation schedules in accordance with observed crop needs rather than being restricted to the strict rotation of large-scale irrigation schemes.
The success recorded by the various ADPs in Nigeria led to the National Fadama Development Project in 1992. The four year project aimed at constructing 50 000 shallow tube wells in Bauchi, Jigawa, Kano, Kebbi, Sokoto and other eligible states. The maximum irrigation potential would be 100 000 hectares. Actual realization was expected to be around 50 000 hectares. The project would simplify drilling technology, privatize drilling operations, conduct aquifer studies, upgrade irrigation technologies and organize fadama farmers for irrigation management.
The development of private irrigation in other countries of West Africa is expected to benefit from the Nigerian experience. Programmes recently formulated included 300 shallow tube wells in Niger, 100 in Burkina Faso. In Ghana, the Agricultural Sector Investment Project is currently implementing a programme of more than 2 000 shallow wells and some 100 deep wells. The washbore technique-is being extensively used.
In Nigeria, the potential for shallow tube well was estimated as 237 000 hectares for the three States of Bauchi, Kano and Sokoto. Nation-wide potential is more than 2 million hectares.
Table 3 compares some technical and economic characteristics of some well technologies. Table 4 gives the unit cost estimate of shallow tube wells. The main cost factors to be considered are:
· Capital and depreciation
· Operating and maintaining the equipment
· Performance i.e. number of wells constructed per year
· Success rate.
The last two factors depend on training and experience gained by the construction team.
In Nigeria, experience has shown that (i) a washbore could be constructed in less than half an hour under favourable conditions; (ii) three to five wells could be drilled per day using a rotary or percussion (bailer) rig; (iii) about 10 percent of the wells would be abortive.
In Ghana, the following assumptions were made:
· mud washbore method of construction
· average depth of tube well 10 metres
· 140 successful tube wells completed by each team per year; and
· overhead allowance as 50 percent of manpower costs.
TABLE 3 Technical and economic characteristics of some well technologies
|
|
OFEDES Well f 100-200 ccm |
Génie Rural Large diameter well 180 cm |
Génie Rural Small diameter well 140 cm |
LWR f140 cm3 borehole (augered) |
Washbore |
|
Cost per depth unit (metre) |
140 000 F |
50 000 F CFA |
35 000 F CFA |
5 000 F CFA |
5 000 F CFA |
|
Discharge (m3/h) |
5 - 20 |
5 - 20 |
2 - 10 |
10 - 20 |
10 - 20 |
|
Duration of construction |
0 - 30 j (10 m) |
15 - 20 j (10 m) |
12 - 15 j (10 m) |
1 day |
1 hour |
|
Maintenance |
Very easy |
Very easy |
easy |
Little maintenance |
Little maintenance |
In the West Africa sub-region, the following unit costs can be quoted:
TABLE 4 Unit estimate cost of shallow tube wells
|
Country |
Washbore $ |
Drilled tube well $ |
|
Nigeria (1992) |
79 |
474 |
|
Ghana (1992) |
313 |
|
|
Niger (1991) |
200 |
|
|
Burkina Faso (1995) |
200 |
|
* Details are provided in Appendix 1
There are a large variety of simple, low-cost well drilling techniques. These techniques tend to be low capital and labour intensive. While each of them has its advantages and disadvantages, good combination of two or more techniques has proven very effective in overcoming some of the limitations imposed by internal or external factors as may relate respectively to the technique itself or the lithology of the well sites.
In the West African sub-region, the jetting method has a good prospect for expansion in view of the existing potential for small-scale irrigation in fadama areas. Many low-cost modifications of the basic jetting or washbore technique can be effected. It is unlikely that one universally suitable method will be found. Emphasis should be laid on methods which use locally available materials. Much effort should be put into setting up local private manufacturing firms for the equipment and the development of private contracting firms to offer washbores and drilled tube wells on a commercial basis.
Koegel, R.G. 1985. Self-help wells, FAO Irrigation and Drainage Paper 30, Rome.
Ghana Irrigation Development Authority. 1992. Report on Results of Demonstration Tube well Project, Wardrop Engineering Inc. June.
FAO. 1991. Projet de Promotion de la Petite Irrigation Privée au Niger, Rapport 42/91 CP-NER.23. avril.
FAO. 1992. Projet de promotion de l'irrigation privée au Mali, Rapport 73/92 CP-MLI.35. juin.
FAO. 1995. Projet de développement de l'irrigation privée et Agro-processing, Rapport 54/95 CP-BKF.37. mai.
Report on the Second Fadama Seminar, 6 - 8 March 1984.
Siddiqi, M.R. 1991. Strategy for Fadama Irrigation Development in Kaduna State.
Sonou, M. 1994. An Overview of Low-lift Irrigation in West Africa: Trends and Prospects, 1994, RAPA Publication 1994/95.
World Bank. 1992. Staff Appraisal Report, National Fadama Development Project, Federal Republic of Nigeria. February.
