Enhancing communities’ adaptive capacity to climate change in drought-prone hotspots of the Blue Nile Basin in Ethiopia
ILRI-UNEP-Wollo University pilot project
TRAINING MANUAL ON WATER RESOURCES DEVELOPMENT (HAND DUG WELLS, SPRING DEVELOPMENT AND DISCHARGE MEASUREMENT) FOR EXTENSION WORKERS AT KABE WATERSHED, WEREILU, ETHIOPIA
MEZGEBU MEWDED (MSc)
e-mail:[email protected]
1 APRIL 2012
Acknowledgements
The initiative which led to the development of this training manual grew out of discussions with UNEP, ILRI & Wollo University. Therefore, I gratefully acknowledged UNEP/ILRI for their financial assistance to undertake this work. Special and sincere gratitude goes also to Mr. Derbew Kefyalew the Project Coordinator of ILRI at Kabe watershed, and Teklemariam Bekele – research director of Wollo University who encouraged me during the compilation of this training material. I also wish to express my gratitude to Ms. Fentanesh Tsegaye who helped me in writing the script of the document.
2 Table of Contents
Acknowledgements…………………………………………………………………………………1 Introduction ………………………………………………………………………………………….4 General Objectives…………………………………………………………………...... 4 PART 1 -HAND DUG WELLS…………………………………………………………………….5 1.1 Introduction…………………………………………………………………...... 5 1.2 Types and Parts of Wells………………………………………………………………………5 13 Why Opt For Shallow Well Technology...... 6 1.4 Advantages and Disadvantages of Hand Dug Wells………………………………………6 1.5 Selecting a Well Site…………………………………………………………………………...7 1.6 Designing Hand Dug Wells……………………………………………………………………10 1.6.1 Size and Shape………………………………………………………………………...... 10 1.6.2 Lining the Shaft ……………………………………………………………………….....10 1.6.3 Intake……………………………………………………………………...... 11 1.6.4 Materials, Equipments and Tools……………………………………………………….11 1.7 Constructing Hand Dug Wells………………………………………………………………...13 1.7.1 Constructing Well Shaft ………………………………………………………………....13 1.7.2 Constructing Intake ………………………………………………………………………16 1.7.3 Finalizing the Well Shaft ………………………………………………...... 17 1.7.4 Developing & Deepening the Well……………………………………………………...18 1.8. Finishing Wells/Head Works………………………………………………………………….18 1.8.1 Description of Head Works ……………………………………………………………..18 1.8.2 Finishing a Hand Dug Well ……………………………………………………………...20 1.9 Improving Traditional Wells……………………………………………………………………21 1.10 Management, Operation and Maintenance………………………………………………...23
PART 2- SPRING DEVELOPMENT………………………………………………………………25 2.1 Introduction………………………………………………………………………………………25 2.2 Types of springs………………………………………………………………...... 26 2.3 Parameters to Be Considered For Spring Development ……………………………….....27 2.4 Design Features of Spring Development…………………………………………………….27 2.4.1 Spring Box Designs ……………………………………………………………………...28 2.4.2 General Construction Steps of Spring Box …………………………………………..29 2.4.3 Storage Reservoir ………………………………………………………………………..30 2.4.4 Storage Reservoir Design……………………………………………...... 31 2.4.5 Storage Reservoir Construction………………………………………………………...31 2.4.6 Tap stands ……………………………………………………………...... 32 2.4.7 Materials Lists……………………………………………………………………………..32 2.5 Post Construction Monitoring Spring ………………………………………………………..33
PART3- DISCHARGE MEASUREMENTS………………………………………………………34 3.1 Introduction…………………………………………………………………………...... 34 3.2 Ways to Measure the Flows of springs and Small Streams……………………………….34 3.3 Measuring Yield of a Well……………………………………………………...... 36
4. REFERENCES……………………………………………………………………………...... 38
5. GLOSSARY………………………………………………………………………………………39
3 List of tables
Table1.1: Estimated yields of aquifers……………………………………………………………9 Table1. 2: Water consumption and distance to water source………………………………….10 Table 1.3: Dimensions of shaft, lining and caisson rings……………………………………...11 Table 1.4 uses and mixes of concrete and mortar……………………………………………...12 Table 1.5 Densities of materials…………………………………………………………………..12 Table1.6. Sample Material List……………………………………………………………………13 Table 1.7Conditions when to use different lining methods ……………………………………14 Table 1.7 the main differences between a traditional and improved well…………………….22
Acronyms & Abbreviations
HDW: Hand Dug Well SP: Spring Development
4 Hand dug wells, Spring Development and Discharge measurement Manual
Introduction For small towns & rural communities in developing countries, piped water supplies with house- connections are often not economically feasible. In such instances, the realistic option is the protected well fitted with a hand pump, a spring tapping structure or perhaps a rainwater catchments and storage system. To carefully select a technology, which is simple, reliable, and adapted to the available technical and organizational skills, it is necessary to build the capacity of the engaged expert.
As a lecturer and researcher, I realize that Wollo University should deliver services to the community. One of the services is to train experts using an up to date training manual on water resources development and management. This realization gives me an incentive to prepare training manual on Hand dug wells, Spring Development and Flow Measurements through the support of UNEP & ILRI.
It could be known that the extension workers at the wereda/district level have difficulty to get references on the water resources development projects. As a result, a considerable effort has been made to compile this manual.
I hope this manual will help all water resources professionals in grasping and integrating better the skills and concepts on water development works. Others who are related professionals would also find it extremely useful, because this module is an important working tool for all actors in the water sector.
About the manual
The manual consists of three parts: Part I- Hand Dug Well (HDW) Part II-Spring Development (SD) Part III- Discharge Measurement
General Objectives of the Manual
After each manual training, the trainees will be able to: Know the site selection, designing, construction, operation and maintenance of hand dug wells Have better understanding and skill on spring development Know methods of measuring flows of streams and yields of springs & wells.
5 PART 1 HAND DUG WELLS
1.1 Introduction
Ground water is water that is stored underground in porous layers called aquifers. It is found in most parts of the world and can be a reliable source of drinking water. Sources of ground water are usually free from disease causing bacteria. There is usually less seasonal variation in groundwater quantities than in surface water. Wells are used to develop or extract ground water. A well is simply a hole that pierces an aquifer so that water may be pumped or lifted out. Wells can be classified according to their method of construction. Five types of wells are: hand-dug, driven, jetted, bored, and cable tool. This technical note describes only hand dug wells methods.
Objective After the training, the trainees will be able to know: Ways of ground water formation Factors in site selection for HDWs How to design HDWs Methods of construction of HDWs Techniques of digging HDWs How to operate and maintain HDWS
Methods of Delivery Lecture the essential points of the manual Open discussion on the issues raised by the trainees Experience sharing among trainees Field visits
1.2 Types and Parts of wells
Types of wells Definition: A well is a hole or shaft, usually vertical, excavated in the earth for bringing ground water to the surface.
Two major types of wells based on depth 1. Shallow wells 2. Deep wells
Shallow wells: Shallow wells may be large diameter hand dug wells (diameter 1-4m) and depth 20m. Or machine drilled wells of small diameter (diameter 8-60cm) and depth 60m.
Deep wells: Deep wells are most large, deep, high-capacity wells constructed by drilling rig. Construction can be accomplished by cable tool method or rotary method. Drilling rigs are capable of drilling wells 8 to 60cm in diameter and depth 600m.
Parts of Wells Wells have five basic parts as shown in Figure 1.1: shaft, casing, intake, wellhead, and water-lifting device.
6 The shaft is the hole that is sunk from the surface of the ground down into, and sometimes through, the aquifer. The purpose of the shaft is to provide access to ground water. The method of sinking the shaft differs with each type of well. The casing lines the sides of the shaft. The purpose of the casing is to prevent the shaft from collapsing, to provide an impermeable barrier to store ground water in, and to keep polluted surface water out. For dug wells, the casing supports the wellhead. Casings are usually made from concrete or metal. For dug wells, they may be brick or masonry. Casings are installed either after the shaft is sunk, or as it is sunk. The intake- this is the part of the well in contact with the aquifer. It is either the lower portion of the casing which is perforated or made from porous material or the open bottom end of the casing. In either case, the intake is within the aquifer, and its purpose is to allow ground water to flow into the casing. The wellhead is a structure built on and around the casing at ground level. It is usually made of concrete. The purpose of the wellhead is to provide a base for a water lifting device, to prevent contaminants from entering, to keep people and animals from falling in the well, and to drain away surface water. The wellhead should be built on an earthen mound 15-20cm above the original surface so water will drain away from the well. The water-lifting device can be a pump, windmill or other method of extraction. The purpose of the device is to get water out of the well.
Figure1.1 Parts of a well
1.3 Why Opt For Shallow Well Technology?
In most cases, communities will go for shallow wells because of the following reasons; They can acquire a well using their own resources (low cost and locally available technology compared to borehole or piped water technologies). Some Areas needing water supplies are not accessibility by big drilling machines. Some areas have naturally polluted deep water (eg too salty for human consumption), while shallow water is of better quality). When external support is relatively low, compared to the magnitude of need, some communities opt for digging many shallow wells for the same amount of money, which would have gone into much fewer boreholes.
Limitations of Shallow Wells Despite the above advantages of shallow wells, the following are some of their typical limitations; Shallow wells dry quickly in protracted dry season since the water table of the annually recharged perched water aquifer in the superficial deposits (eg the sand, clay formation etc) goes down fast. Easy water pollution due to poor disposal of human and industrial waste on the ground. The large diameter wells are hazardous to people when they collapse. If unprotected, animals can also cause pollution when they drink from it.
Generally, in many areas of the world, shallow wells have proven very effective due to their locally available technology and low cost. Where well constructed and fitted with hand-pumps, they have proven sustainable in supplying potable water. Therefore, with proper consideration for local hydrogeology (ground water quality, fluctuations levels and potential for pollution), the shallow wells will effectively provide potable water to many people.
Quantity of water available from shallow wells is limited as their source of supply is uppermost layer of earth only and sometimes may even dry up in summer. Hence they are not suitable for public water supply schemes. The quantity of water obtained from shallow wells is better than the river water but requires purification. The shallow wells should be constructed away from septic tanks, soak pits etc because of the contamination of effluent.
7 The shallow wells are used as the source of water supply for small villages, undeveloped municipal towns, isolated buildings etc because of limited supply and bad quality of water.
1.4 Advantages and Disadvantages of Hand Dug Wells:
Advantages of hand dug wells 1. The community of users can become involved from the very beginning of the process. This can lead from an information campaign to the submission of a request by the community, to the planning and construction steps, and to preparation for the operation and maintenance phase. The relatively slow pace of hand-dug well preparation and construction allows plenty of time for valuable contact with the community. 2. The technology provides a perfect opportunity for community participation in contributing unskilled labor to the preparation of the construction site and the excavation of the well. Depending on the arrangements in a particular case, villagers may also assist in the prefabrication of lining rings or bricks, or in the mixing of concrete for in-situ placement. 3. In most cases where hand-dug wells are an option, excavation is relatively easy and does not require sophisticated equipment. 4. Where construction is concerned, it is cheap in comparison with other technologies such as the mechanical construction of boreholes. 5. Construction and maintenance do not require very sophisticated equipment. Routine ground level maintenance such as the repair of cracks in the apron can be done by somebody within the community, thus eliminating the need for an extensive, centrally-controlled corrective maintenance network. 6. Construction teams require minimal technical and logistical support in comparison to other methods. However, this is not intended to diminish the importance of having competent field and supervisory staff. 7. Given the relative simplicity of the technology, the involvement of the private sector at local level is encouraged. 8. Apart from cement, the materials needed for construction are normally available locally. The provision of these materials from nearby sources is another opportunity to increase community involvement in the construction phase. 9. When construction takes account of local soil conditions and proper construction standards are applied, the well will rarely require any down the whole structural maintenance. 10. A number of options are available to increase the yield of the well if the need arises. 11. Depending on the water-lifting device installed, it can provide years of trouble-free water supply. 12. If a hand pump is installed, the quality of water supplied can be brought to a high level. 13. Where technical conditions permit, and in a situation where the construction of water supply infrastructure is demand-driven, it is a good midrange option between traditional sources and mechanized systems, to offer a community. A positive experience with a low-technology, easy to manage system will encourage the community to develop its water supply system as demand and economic capacity increase. 14. Wells can be excavated in harder soils where hand-drilling is difficult. 15. A large-diameter, hand-dug well exposes more of the aquifer, thereby allowing a greater volume of water to flow into the well, and creates a larger reservoir for water storage. 16. In the case of a closed well, if the hand pump experiences a serious breakdown, there is alternative emergency access to water via the inspection cover. 17. While the construction phase may be longer than with other technologies, the longer time will be useful in assuring the acceptance of the new system by the villagers since the community will have more time to witness the development and “arrival” of the new system. 18. Hand-dug wells are in many cases very similar in form to traditional water collection systems. As such, the technology can be readily accepted by the community, and provides an ideal basis for future development of the system (for example, from an open well to a covered well with a hand pump, and from there to a system with motorized pump).
Disadvantages 1. Community participation may be difficult due to safety considerations. Excavating a well is a hazardous undertaking, even in ideal conditions, and the work cannot be passed lightly to inexperienced workers. If, on the other hand, the work is given to a private contractor, he or she may not wish to be dependent on a
8 supply of voluntary labor over which there is very little control, given the many demands on the time of rural farming and fishing families. 2. Excavation can be dangerous for a number of reasons. At depths of over 2 meters, access to the well by those doing the excavation must be subject to strict safety controls. Also, in some areas, the process of excavation may release harmful gases. 3. During excavation, a method of keeping the well pumped dry after reaching the water table is required. This will normally involve a motorized pump, which in turn will require a power source. The capacity to dewater the well will limit the extent to which excavation can continue below the top of the water table. 4. The lowering of lining rings weighing up to 900kg can be a dangerous operation. 5. Supply is greatly influenced by water table fluctuations. 6. Since most hand-dug wells exploit shallow aquifers, water in the well may be susceptible to pollutants infiltrating from the surface. 7. In open hand-dug wells, the water can be contaminated by mud, vegetation, bird and animal droppings or even by rubbish thrown into the well. 8. Again in an open well situation, the use of multiple buckets and ropes can lead to contamination of the water. 9. An open and unprotected well can be dangerous for the users. Small children, especially, can fall in. 10. A programme of shallow well construction needs much more supervision capacity per person served, since it is normally necessary to make several visits to a construction site over a period of weeks during the construction phase.
1.5 Selecting a Well Site
Selecting a well site properly is important to ensure that the well will tap into a reliable source of good quality ground water, and to ensure that the water will not be contaminated in the future. Selecting a site involves considering existing wells, local geography, quality and quantity of ground water, possible sources of contamination, accessibility to users, and proposed methods of well construction.
Existing wells The primary objective when sinking a new well is to sink it where ground water is likely to be found. Existing wells are the best indication of the presence of ground water. Where possible, sink a new well near an old one--ground water will probably be reached at about the same depth. The history of the old well will provide information on seasonal changes in the water table, which may indicate that the new well should be deeper than the old one.
If the new well is to be used in addition to the old one, care must be taken not to sink it too close to the existing well. Otherwise, the yield of one or both wells may be adversely affected. This is due to the effect that a well has on the surrounding water table.
When water is pumped or lifted out of a well, the water level in the well falls below the original level, called the static level, until it stabilizes at a new level, called the pumped level. The distance between the static level and the pumped level is the drawdown. The water table surrounding a wells curves down to the pumped level, forming a cone of depression. See Figure 1.2. If the cone of depression of two wells over lap, the pumped level in one or both wells will be lowered and the yield will be decreased. Draw all existing wells on your map similar to Figure1. 3.
Figure1.2. (a) Cone of depression (b) well interference
Local Geography If no wells exist, the presence of ground water can be indicated by surface water, topography, and certain types of vegetation.
Surface water: A successful well can generally be sunk near a river because the river will replenish the ground water and reduce changes in the water table. Water taken from such a well is usually cleaner and
9 cooler than water taken from the river. If the well is deep, water may be avail able even when the river is temporarily dry.
Topography: Ground water gathers in low areas. Therefore, the lowest ground is generally the best place to sink a well. In hilly areas, valley bottoms are the best places for wells. An exception to this could be where there is a spring on the side of the hill. The spring may indicate lateral movement of ground water over a layer of impermeable soil. If so, a successful well could be sunk uphill from the spring. This may have the advantage of bringing the source of water closer to the community or dwelling. On your map, draw all rivers, springs, and topographical feature.
Vegetation: Certain types of vegetation can indicate that ground water lies near the surface. The most useful indicators of ground water are perennial plants (those present year-round), especially trees and shrubs. Annual plants, such as grasses, are not good indicators, because they come and go with the seasons. The dry season is probably the best time to survey vegetation for indications of ground water.
Quality of Ground Water: Once ground water is located, its quality must be tested before constructing permanent wells. The water must be clean, clear, and good tasting, and be free from disease causing organisms. If a water sample is analyzed and the ground water is contaminated, another source may have to be found.
Quantity of Ground Water: The quantity of a groundwater source is nearly as important as its quality. Unfortunately, the only way to test the yield of an aquifer is to dig a well and pump it. You can, however, make a rough estimate of the yield by identifying the sediment and rock which compose the aquifer. The two most significant elements of an aquifer are its porosity and permeability. Porosity governs the amount of water that an aquifer can contain. Permeability governs the amount of water that can be brought to the surface.
For example, some aquifers may contain large quantities of water, but their rate of yield is too slow to suit the needs of the user. Porosity and permeability depend on a number of factors including particle size, arrangement and distribution. Table 1.1 shows the estimated yields of aquifers composed of different types of sediment. The table should not be used for exact calculations but only for indications of yield.
Table1.1: Estimated yields of aquifers:
Sediment composing the aquifer Estimated yield ( liters/minute) Sand and gravel 11400; could be less based on pump and well design Sand , gravel and clay 1900-3800 Sand and clay 1900 Fractured sandstone 1900 Lime stone 38-190; more if near stream, or if there are under ground caverns Granite or hard rock 38 or less shale Less than 38 If the quantity of ground water is insufficient, another well site will have to be found. The new site may replace or supplement the old site.
Possible Sources of Contamination: A well should not be dug in areas where the ground water is likely to be contaminated. A well site should be uphill and at least: 50m from a seepage pit or cesspool; 30m from a subsurface absorption system; 30m from a pit privy; 30m from animal pens, barns, or silos; 15m from a septic tank;
10 7m from a drain ditch, or house foundation The well site should not be subject to flooding during the wet season or any other time. This will be of greatest concern where the well is in a low area or near a river that yearly overflows its banks. The site can be protected from flooding by building small dams or ditches to prevent flooding the well. If not, another site should be considered. Draw all possible sources of contamination on your map, as in fig.1.3
Accessibility to Users: The well site should be as close as possible to the village or dwelling. As the distance between the well and the user increases, the per capita water consumption decreases. This is shown in Table1. 2. The table should not be used for exact calculations but only for indications of consumption. Political considerations may influence accessibility. There may be pressure to put the well near the dwelling of the village chief or other influential member of the community. A compromise may be necessary.
Table1. 2: Water consumption and distance to water source Estimated consumption ( liters per Distance to source person per day) More than 1000m 7 500-1000m 12 Less than 250m 20-30 In the yard of the dwelling 40
Methods of Well Construction: The proposed method of well construction must be suitable to the soil conditions at the well site. For example hard rock, large boulders are unsuitable soils for hand dug method of construction.
1.6 Designing Hand Dug Wells
Proper design of hand dug wells is important to assure a year-round supply of water and to assure efficient use of personnel and materials.
Designing involves determining the size and shape of the well; the method of lining the shaft; the type of intake; and the necessary personnel, materials, equipment, and tools. The products of the design process are drawings of the shaft and lining and a detailed materials list. These, along with a location map similar to Figure1.3 should be given to the construction foreman before construction begins.
Figure1.3: Location map
1.6.1 Size and Shape When viewed from the top, wells can be any shape but most of them are round. This is because a round well produces the greatest amount of water for the least amount of excavation, and a round lining is stronger than any other shape. The size of the well refers to its depth and diameter. Although it is impossible to know the depth of a well before it is dug, an attempt should be made to estimate it. This will allow you to roughly calculate the quantities. Use information from test holes or existing wells in the area to estimate the depth of the water table.
For practical and economic reasons, well diameters are between l.0m and 1.5m. The smaller diameter results in a savings in materials costs, and it requires less soil to be excavated. The larger diameter means a higher materials cost but a more efficient work output, since two men rather than one can dig the shaft. A larger diameter provides a greater storage capacity and allows more water to enter the well. If pre-made forms or precast concrete rings are used, their size will determine the diameter of the well. When the depth and diameter of the well shaft have been determined, write the dimensions on a design drawing similar to the sample in figure1.4.
1.6.2 Lining the Shaft
11 Although various materials have been used to line well shafts, concrete is the best and most common lining. It is strong, long-lasting, and widely known. The two basic methods of lining well shafts are dig-and- line and sink lining or caissoning. In dig-and-line, a portion of the shaft is excavated, shutters are set in place in the shaft, and concrete is poured behind the shutters. When the concrete hardens, the shutters are removed and the next portion of the shaft is excavated.
In sink lining, concrete rings called caissons are cast and cured in special molds at the surface. The rings are stacked on top of each other and attached together with bolts. As soil is excavated from beneath the rings, they sink into the earth and line the shaft. Often, both methods are employed in a single well: dig- and-line is used until the water table is reached, then caissoning is used to sink the well into the aquifer. The lining is usually 75mm thick and the caisson rings are 125-150mm thick. The outside diameter of the rings is from 50-100mm less than the inside diameter of the lining to allow the rings to freely move downward. Table1.3 shows common dimensions of shaft, lining, and rings. Write the dimensions that you determine are best for your well on the design drawing similar to Figure1.4. For example when more people are dependent on a dug well or for irrigation purposes, 2-3m diameter may be provided
Figure1.4. Designing of well lining and caisson
1.6.3 Intake The caisson rings are sunk into the aquifer as far as possible; that is, until the water becomes too deep to continue the excavation.
Table 1.3: Dimensions of shaft, lining and caisson rings
Feature Dimension Shaft diameter 1.45m Lining , outside diameter 1.45m Lining , inside diameter 1.30m Lining , thickness 75mm Caisson, outside diameter 1.20m Caisson, outside diameter 0.90-0.95m Caisson, thickness 125-150mm Caisson, height 0.50m
Ground water may then enter the well either: (1) Through the opening under the lowest caisson ring, or (2) Through the rings themselves. In the first case, the rings are made of standard concrete which does not allow entry of water. In the second case, the rings are usually made of porous concrete (mix ratio of cement: aggregate = 1:4) which allows water to pass through voids in the concrete. Another way to allow water to enter through the caisson rings is to build the rings from standard concrete and perforate them with seepage holes. For all types of intakes, the bottom of the shaft should be covered with a porous base plug made from porous concrete or layers of sand and gravel. The plug prevents aquifer material from rising into the well. The type of caisson ring used depends on the nature of the aquifer.
Normally, rings are made of porous concrete. However, if the aquifer is composed of fine sand, which would clog the pores or flow through the seepage holes, the rings should be made of standard concrete without perforations.
It may not be possible to know which type of intake is needed until the aquifer is reached. But an attempt should be made to anticipate the necessary intake, based on test holes or other wells in the area. When the type of intake has been determined, indicate it on the design drawing similar to Figure1.4.
1.6.4 Materials, equipments and tools
12 Concrete is the mixture of cement, sand and aggregate, mixed with water. Mortar is the mixture of cement and sand only (with water).Depending on the use; these ingredients are mixed in different proportions to achieve a mixture with the desired properties. In general, the more cement used in the mixture, the stronger the resulting material. Concrete is used to make structural or mass objects, whereas mortar is used to smooth, plaster or seal a surface, or to glue bricks or stone together. Examples of uses and mixes of concrete and mortar are shown in the table below.
Table 1.4 uses and mixes of concrete and mortar
Material Use Example Mixture
Rough Plastering Outside of tanks 1:6 Mortar Masonry (bricks and stone) Masonry well shaft lining
Smooth plastering Well head external surface 1:4 Water tight sealing Inside of tanks
General Structural Concrete Lining; Concrete Rings; Apron; 1:2:4 Concrete drainage channel
Porous Concrete Porous Intake Rings 1:4
The materials needed to line a hand dug well are concrete mix and reinforcing steel. The common mix ratio of concrete is 1:2:3.The quantities of these materials needed can be roughly estimated using the same method used in spring development.
Table 1.5 Densities of materials
Material Density( Kg/m3) Material Density( Kg/m3) Cement 1400 Lime 1900 River sand 1840 Steel 7800 Stone/aggregate 2250 Cement mortar 2300 Reinforced concrete 2300 basalt 2700 Un reinforced concrete 2300
The main equipments needed are head frame, a heavy duty stretcher with a U belt in the center, steel shutters, two kibbles etc.The worker needs tools for measuring, plumbing, excavating, and trimming the shaft; mixing, pouring and finishing concrete; and positioning and securing re rods.
13 Table1.6. Sample Material List
Item Description Quantity Estimated Cost Personnel Foreman 1 -- Worker, skilled in sinking well 1 -- Workers, experienced with concrete 1 -- Workers, unskilled 2-4 -- Supplies Cement (Portland) -- kg -- Sand (clean; fine to 6mm) -- m3 -- Gravel (clean; 6-36mm) -- m3 -- Water (clean and clear) -- -- Re-rod for lining:8mm diameter -- m -- Re-rod for caissons: 15mm diameter -- m -- Materials for storage shed -- -- Equipment Head frame Rope for caissons; 100x12cm diameter, steel wire -- -- with fiber core, tensile strength 7kg/cm2 Rope for kibbles: 100x6mm diameter -- -- Rope for trimming rods: 100mx3mm diameter -- -- Steel shutters(1.3m diameter x0.5m high) with wedges -- -- and bolts Steel shutters (1.3m diameter x1.0m high) with -- -- wedges and bolts -- -- Steel molds for caisson rings (1.2m outside diameter, -- -- 0.95m inside diameter, 0.5 high) --- -- Template for molds Stretcher for caisson Total estimated cost= ______
1.7 Constructing Hand Dug Wells
Proper construction of a hand dug well is important to ensure a year round supply of water and to protect the water from contamination. Construction involves assembling all necessary personnel, materials, and tools; preparing the site, excavating the well shaft; and lining the shaft. There are several good methods to construct a hand dug well; if you are familiar with a specific method, use it. This technical note describes one method of construction, using locally available materials, that has been employed successfully in a number of countries. Read the entire technical note before beginning construction. After the project designer has given you location map, design drawing and material list documents, you begin assembling the necessary workers, supplies and tools.
1.7.1 Constructing Well Shaft
Hand Dug Wells can be: Lined, Unlined or
14 A combination In all wells, however, at least the top 3 meters should be lined to prevent (potentially dirty) surface water seeping in. There are several options available for lining a well shaft. These are: • Pre-cast concrete rings, cast on the surface and lowered into the well shaft • Concrete rings cast in-situ in the well shaft • Masonry lining using bricks or local stone
The choice of lining method will depend on the ground conditions, cost and the availability of local materials. A summary of when to use which method is shown in the following table: Table 1.7 Conditions when to use different lining methods
Lining Method Conditions when used Pre-cast concrete rings Collapsing ground (sandy, loose gravel etc) Unstable ground (wet, sand or silt layers) Once the aquifer has been reached. Can be used in all conditions. In-situ Cast Concrete Stable, solid ground Masonry Lining Stable, solid ground where there is an abundance of local stone Unlined Solid rock
A well shaft (or a section of a well shaft) can only be left unlined when the surrounding ground is solid rock (including pumice), although even in these circumstances it may be desirable to line the shaft anyway.
Pre-cast Concrete Rings: Pre-cast concrete rings are used for a number of purposes in Hand Dug Well construction: 1. Pre-cast concrete rings form the basis of the caisson excavation method 2. Porous or perforated pre-cast concrete rings are used to line the intake section (except when there is a very high inflow rate)
Figure1.5 precast concrete rings
Features of Pre-cast Concrete Rings Dimensions: Pre-cast concrete rings can be made in a range of sizes, and with several methods for joining them. It is recommended that if concrete rings are used to line the shaft above the water table a larger diameter ring is used than below the water table. The standard sizes for a concrete rings currently used in Ethiopia is: Large Diameter: • Outside diameter (OD) 1.5m (1500mm) • Inside diameter (ID) 1.3m (1300mm) • Wall Thickness (WT) 100mm • Height (Ht) 0.5m (500mm)
Small Diameter: • Outside diameter (OD) 1.2m (1200mm) • Inside diameter (ID) 1.0m (1000mm) • Wall Thickness (WT) 100mm • Height (Ht) 0.5m (500mm)
Figure1.6 Features of Pre-cast Concrete Rings
An alternative (lightweight) design for pre-cast concrete rings (used for Caissoning from the surface) is: • Outside diameter (OD) 1.1m (1100mm) • Inside diameter (ID) 0.95m (950mm) • Wall Thickness (WT) 75mm • Height (Ht) 0.6m (600mm)
15 This size of pre-cast concrete rings must be such that they can easily fit inside the shaft of a well with in- situ concrete lining (with an ID of 1.3m).
If the well has to be deepened and the existing intake lining cannot be lowered by the caisson method, then a smaller diameter concrete ring may need to be made. These small rings can then be caissoned into the aquifer within the larger concrete rings using a technique known as Telescoping. However, the digging space is extremely confined and so should only be used as a last resort.
Construction & Excavation Techniques The ground conditions in a well shaft will vary from site to site, and also within one shaft. You need to be familiar with a range of excavation techniques and technologies so that you can handle whatever circumstances you find in a safe and efficient manner. Ground may be collapsing (meaning as you dig the adjacent soil falls into the hole) or self supporting (meaning as you dig the adjacent soil stays where it is). Soils containing high levels of gravel or sand will tend to be collapsing, whereas soils with high clay content will tend to be self supporting. In some cases, ground that is self supporting when dry can become collapsing when it is wet (for example after rain).
There are two main excavation techniques that can be employed when constructing a lined well shaft. They are: 1. The Dig-Down-Build-Up Method 2. The Caisson Method
The Dig-Down-Build-Up Method can only be used in self-supporting soils. It involves digging a shaft down a certain way (usually no more than 5 meters; less when the ground is less stable) then installing the lining up to the surface. This is known as the first lift. Once the lining is complete, the shaft excavation continues below the first lift for a similar distance (3 -5 meters again) or until either solid rock or the aquifer is reached. At this point the well is lined back up to the bottom of the first lift, creating the second lift. This process continues until solid rock or the aquifer is reached. The Caisson Method can be used in either self supporting soils or collapsing soils. The process involves digging a shallow hole and placing a pre-cast concrete ring in the hole. The ground underneath the ring is excavated which allows the ring to settle further into the hole at which time a second concrete ring is placed on the first one. This process continues until the bottom of the well, or solid rock is reached. The first ring placed in the hole is the cutting ring. This is a specially designed concrete ring with a beveled edge and a wider outside diameter than the concrete lining rings. The purpose of the cutting ring is to make excavation easier, and to create a larger diameter excavation hole so the rings above have space to move down easily.
The Caisson Method is safer than the Dig-Down-Build-Up Method because the ground is never left unsupported.
Which technique should you use? Deciding which excavation method to use is something that comes with experience. The recommended process for excavating a well shaft is as follows: Begin excavating shaft. When you have excavated a meter or so, assess whether the ground is self supporting or collapsing.
If the Ground is Collapsing type, then, • Use the Caisson excavation method, using large diameter caisson rings (for example ID1.5m, OD 1.7m). • Continue excavating using this method until the ground water (aquifer) or solid rock is reached. • Below the ground water the caisson excavation method is used using smaller diameter caisson rings (for example ID1.0m, OD1.2m).
If the ground is self supporting, then, • Excavate carefully down approximately 5 meters. If the ground is stable, then continue excavating until the ground water (aquifer) or solid rock is reached.
16 • At this point line the well shaft using in-situ concrete lining, masonry lining or large diameter pre-cast concrete rings. • If the ground becomes unstable, or you are not sure whether the ground is stable, then STOP excavating, and line back to the surface using one of the techniques listed above. Continue excavating using either the Dig-Down-Build-Up excavation technique or the Caisson technique (using large diameter pre-cast concrete rings), depending on which lining method you choose.
Figure 1.7 caisson method
Note: Once the aquifer or water table has been reached, the Caisson Method should always be used. If you reach solid rock, you need to decide whether it is worth continuing with the excavation using rock breaking techniques, or whether you should abandon the site and start excavating in a new site.
Keeping the Shaft Vertical: It is extremely important to keep the well shaft vertical. The distance that the shaft can be out of vertical is 10mm for every meter of depth. You should check this regularly during construction as a well shaft that is not vertical is difficult to construct and difficult to use when it is finished. The recommended method for checking verticality during excavation is by using offset pins, a plumb rod and line and trimming rods. Alternatively, the plumb line and trimming rods can be suspended from a tripod.
Figure 1.8 Keeping the Shaft Vertical
Digging in Rock: Very often when excavating a well shaft, rocks and boulders and even a solid layer of rock will be encountered. Small rocks and boulders (up to about 30 – 40 kg) can generally be extracted and removed without the need for any special techniques. However if larger boulders or solid rock is encountered then it may be necessary to use techniques and/or special equipment to break the rock into manageable pieces.
Techniques for breaking rock include: • Using a crowbar, sledgehammer, rock chisel or similar. • Using a mechanical tool such as a jack hammer or hammer drill • Heating and rapidly cooling the rock to cause it to crack • Using explosives
Before raising boulders or large pieces of rock from the well shaft, all people should leave the well shaft. The technique employed will depend on the type and size of rock encountered, and the availability of equipment. Explosives are not recommended.
1.7.2 Constructing Intake The intake is the section of the well shaft that lies within the aquifer. The intake is designed to allow water to enter the well, but to exclude sand, silt and other particles. The aquifer will be either pervious (meaning it can hold and transmit water) ground comprised of soil, sand, clay or a combination, or rock with fissures (small cracks through which water flows).
Pervious ground is inherently unstable as the movement of water loosens particles and allows them to move over each other. This is particularly so in sand and gravel (which are generally excellent water bearing materials). It is therefore very important that the lining method be structurally sound and the excavation method safe. For this reason, it is recommended that the caisson method be used to excavate into the aquifer. In fissured rock, an intake lining may not be needed; however, unless it is absolutely sold it should be lined anyway. Only the Supervisor can make the decision not to line a well shaft.
There are three lining options for intakes: .Perforated Concrete Rings
17 . Porous Concrete Rings .Solid Concrete Rings (flow from underneath) Note: Perforated Rings should be used unless there is a clear reason to use one of the other options. Perforated Rings are normal Pre-cast Concrete Rings except that they have small holes cast into them through which water flows into the well. The holes are generally 10 – 20mm in diameter and so have the disadvantage that sand and small gravel can pass through them into the well. The advantage of perforated rings is that they are strong so there is no danger they will break during construction. Porous Rings are made from a concrete mix that contains cement and gravel but no sand. The resultant concrete has spaces between the gravel which allows water to flow through. The main advantage of the porous ring is that the voids are generally small enough to allow water to pass whilst filtering out most particles (sand, gravel etc). The main disadvantage of the porous rings is that they are weak (compared to normal concrete rings) and so can easily break when they are being handled during construction. Solid Concrete Rings are used when the aquifer has a lot of water and/or the ground in the aquifer contains fine sand or silt. Under these conditions water flows into the well from underneath or through the joints in the concrete rings. Solid Concrete Rings are identical to the Pre-cast Rings used in the well shaft.
Construction & Excavation Techniques One of the main differences between traditional hand dug wells and improved hand dug wells is that the later continue to supply water to the communities even in the driest seasons. This is achieved by ensuring that the well shaft penetrates the aquifer by at least 3 meters, that an adequate yield is achieved.
The excavation method used to dig 3 meters or more into the aquifer is the Caisson Method. The only difference being that when excavating below the water table, De-Watering is required. The only variation of this technique that is sometimes used to deepen wells if they dry up is ‘Telescoping’. In this technique, smaller diameter concrete caisson rings that fit within the rings used in the original construction are used to excavate further into the aquifer.
De-Watering De-watering is extremely important for allowing excavation to penetrate 3 meters or more into the aquifer. The lack of effective de-watering is the main reason why traditional hand dug wells have dried up in dry seasons.
De-watering can be done by bucket and rope or with a pump. In general the bucket and rope method should be used in the first stages of excavation into the aquifer, and only when the water flows into the well faster than it can be effectively bucketed out should pumping begin. In some cases where the flow into the well is slow or can be restricted (for example if perforated concrete rings are used in the intake section, the holes can be temporarily blocked with wooden plugs during digging to limit the flow of water into the hole) then bucketing may be adequate for the whole excavation. In most cases, however, pumping will be required for the deepest section of the well shaft.
If the shaft is too deep for a single de-watering pump and a higher capacity pump is not available, it is possible to connect two pumps in series (one after the other). This is done by suspending one pump in a suitable bucket half way up the shaft. The pump at the bottom pumps into the bucket from where the suspended pump pushes the water to the surface. The flow rates of the two pumps must be balanced using gate valves. Sometimes the Dig-Down-Build-Up Method is used for excavating into the aquifer. This is NOT recommended as the risk of the walls collapsing on workers is very high.
1.7.3 Finalizing the Well Shaft Once you have established that the well shaft is deep enough and it has been fully lined, there are a number of things that need to be done to finalize the well shaft. These are: . Install the Base Plug . Gravel pack and seal the shaft .Develop the well
18 Base Plug: The base plug is used to stabilize the bottom of the well to prevent erosion and control the flow of water into the well from underneath during periods of heavy use to ensure that sand and silt are not carried into the well. It can be constructed of perforated concrete, or formed by placing graded layers of gravel at the bottom of the well.
Gravel Pack and Sealing: The void between the concrete rings (if used) and the surrounding ground should be packed with uniformly graded gravel. This helps to stabilize the surrounding ground and control the flow of sand, silt and water into the well.
If a combination of the Dig-Down-Build-Up and the Caisson excavation methods was used, then there will be a gap in the joint between the two. This should be sealed. The top 3 meters of the well shaft must also be sealed to prevent surface water seeping into the well.
Figure 1.9 Gravel packing
1.7.4 Developing & deepening the Well: Developing the well: Developing the well involves pumping or surging a bailer up and down in the water in the well. This forces water to move backwards and forwards through the small pores and voids in and around the intake. This effectively cleans them, removes loose particles and establishes the pathways for the water to enter the well.
Well Deepening: Water tables and aquifer levels can vary from the dry season to the wet season and from year to year. It is not uncommon for wells to dry up in a dry season, particularly in times of drought or exceptionally dry years. The aim of constructing improved Hand Dug Wells is to ensure that water is available all year round, even in times of drought, and so if a well dries up in the years subsequent to its construction it will be necessary to deepen it.
To deepen a well that has been completed, unless the intake is in solid rock, the Caisson Method is used. Depending on the well the process will involve excavating and extending the Caisson Lining that was originally placed in the well, or telescoping a smaller diameter Caisson Lining inside the original one.
Notes: 1. Whatever the construction and excavation method used, the top 3 meters of the hand dug well must be sealed to prevent surface water from entering the well. 2. If a combination of the Dig-Down-Build-Up method and the Caisson method has been used, then there will be a gap between the outside of the top pre-cast concrete caisson ring and the inside of the lining used above that. This gap will need to be sealed to prevent soil and other materials entering the well. 3. The well should go 3 meters into the aquifer and yield a minimum of 10 liters per minute. If higher yields are achieved before you have dug 3 meters into the aquifer, then you may be able to stop digging sooner, subject to the following guide. Acceptable aquifer penetration depths and yields: A. 2 meters and 20 liters per minute B. 2.5 meters and 15 liters per minute C. 3 meters and 10 liters per minute 4. It may be necessary to check the quality of the water once the aquifer has been reached.
1.8 Finishing Wells/Head Works Finishing a well is important to protect the water from contamination, to prevent people and animals from falling into a hand dug well, and to ensure that water can be drawn from the well maximum efficiency. How a well is finished depends on whether it is hand dug or drilled. The term “drilled” includes driven, bored, jetted, and cable tool wells. Finishing a hand dug well involves constructing a headwall, an apron, and perhaps a cover; and installing a water-lifting device other than a pump. Finishing a drilled well involves constructing an apron and developing the well. For drilled wells and for hand dug wells that will have a pump, the pump must be at the well site before the well can be finished.
19 1.8.1 Description of Head works The headwork consists of all of the above ground components of a hand dug well. This includes the well head, the cover slab, the apron and drainage, and, depending on the technology chosen, an optional windlass for use with a bucket and rope, or the hand pumps.
The Community will have chosen one of the following options for extracting water from their hand dug well. 1. A bucket and rope 2. A hand pump Many of the features of the design of the headwork is the same for both options, and even if the community chooses to install a hand pump, provision should be made to withdraw water using a bucket and rope when the hand pump is broken.
Well Head The well head is simply the extension of the well lining above the ground level. The well head should extend at least 300 – 400mm above the ground. This ensures that no surface water can enter the well. If Pre-cast concrete rings are used, then the well head is constructed by adding additional rings to the well lining to achieve the desired height above the ground. If In-situ concrete lining is used, then this can be extended above the ground by using an outer mould as well as the inner mould, and if the lining is masonry, then this can simply be extended above ground to the desired height.
Cover slab The cover slab is a cast concrete cover that sits on top of the well head, and effectively closes the well. It will have an access hole with a removable hatch that is used for entering the well for maintenance or deepening. If no hand pump is installed then this hatch will also be used for withdrawing water from the well using a bucket and rope. In this case the access hole should be raised by 500 – 600 mm above the cover slab level. The cover slab will also have a 150mm diameter hole and associated bolts for fitting a hand pump. The spacing and arrangement of the bolts will depend on the type of hand pump chosen. If no hand pump is installed, then this should be closed with a cover, however it should not be permanently closed in case the Community chooses to install a hand pump at a later date.
Apron & Drainage A concrete platform, known as the Apron, is constructed around the Well Head to create a clean, solid surface on which to collect water, and to ensure any spilled water (or other surface water) drains away from the well. The Apron should be founded on solid ground, and should be strong enough so that it does not crack or move over time. It should have a raised lip all around to prevent surface water from flowing onto it from adjacent ground, and it should incorporate a drainage channel leading off at least five meters so that spilled water can flow away. The drainage channel may feed a small vegetable garden or an animal water trough if so desired.
Figure1.10 Typical layout of apron & drainage
Hand pump Hand pump installation procedures will vary from pump type to pump type. If a hand pump is installed, however, then a concrete plinth should be constructed under the pump spout onto which water jars etc can be placed.
Windlass If water is to be drawn from the well with a bucket and rope, then a dedicated bucket must be used. This will minimize the chances of contamination entering the well from user’s own buckets. To facilitate water collection using the dedicated bucket; some sort of windlass should be constructed. This consists of a frame constructed over the well (this can be a frame built during well construction for removal of spoil), with a pulley attached. This allows the user to stand back from the well head when drawing up water.
20 Completion Once the construction of the hand dug well is complete, there are still a number of things which need to be done. These are as follows:
(i)Disinfection and Cleaning The process of digging and constructing a hand dug well will almost certainly contaminate the water in the well, and so it must be disinfected before it can be used. This is done with chlorine which is a strong chemical that kills all organisms in the water.
Chlorine has a distinctive smell. Heavily chlorinated water should not be consumed and so once the chlorination process is complete, the well should be left for 24 hours and then pumped out. A small amount of residual chlorine in the water will not harm users, however in general people do not like to drink water that smells of chlorine, and so the usual practice is to continue pumping the water out of the well until no chlorine smell can be detected.
The inside walls of the well should also be cleaned. Any residual oil used to lubricate formwork should be cleaned off and the walls scrubbed with a weak chlorine solution.
(ii)Fencing and Well Protection The usual practice Communities adopt for using their well is to keep it closed except at certain times when access is controlled by a guard. It is also extremely important that animals are kept away from the well so that they cannot cause contamination or damage. It is therefore important that the well and surrounds are fenced. If the fence used to protect the site during construction is adequate then no more work need be done, however if this was just temporary fencing then a new or improved fence will need to be constructed.
Extra Features: Very often the Community will want to add some facilities and features to their hand dug well to make it easier to use or more useful. These include a modified headwork design that makes it easier for a woman on her own to collect water without assistance; a clothes washing rack, an animal watering trough and a small vegetable garden.
Figure 1.11 Clothes Washing Rack Figure1.12 Animal Watering Trough
1.8.2 Steps in Finishing a Hand Dug Well Steps: 1. Break away the weak mortar layer around the top of the well, being careful not to knock debris into the well. Bend the re-rods protruding from the well lining into a vertical position. 2. Scrape smooth a circular area extending 2.0m out from the well. This will form the bottom of the apron. The area should be 25-50mm below the top of the well lining. It should be well tamped and slope slightly downward away from the well. 3. Bend 2.9m-long sections of re-rod into right angles and tie them with wire to the protruding re- rods, so that 0.9m is in the vertical position and 2.0m is in the horizontal position radiating out from the well. Place small wooden spacer blocks under the horizontal portion of the re-rods, as shown in figure 1.13. 4. Fashion four circles of re-rod. The diameter depends on the diameter of the circle formed by the vertical sections of re-rod. Fix the circle of re-rod in a horizontal position around the vertical re- rods, and space them about 250mm apart.
Figure1.13 Building the apron for a hand dug well.
21 5. Make eight circles of re-rod for the apron. Fashion the smallest circle so that there is about 200mm between it and the outside edge of the well lining. Fashion each consecutive circle so that its radius is about 250mm larger than the one before it. The largest circle should fit just inside the ends of the horizontal re-rods. Tie the circles to the horizontal re-rods. Build up a small mound of soil around the ends of the re-rods to contain the concrete when it is poured. 6. Fix a set of steel shutters (1.0m high) around the inside of the well lining. Use metal shims or other means to tightly wedge the shutters in place. They should rise to nearly their full height (1.0) above the top of the lining. 7. Mix concrete in the proportions of one part cement, two-and-a-half parts sand, five parts gravel, and enough water to make a workable mix. Pour concrete into the apron and make it 75-100m thick. Trowel smoothes the surface of the apron so that it slopes gently downwards away from the well. Form a curb of concrete around the outside edge of the apron; make it about 25-50mm high to contain spilled water. 8. Cover the concrete with straw or wet burlap and keep it moist for seven days. Before the concrete has fully set, pour water on the apron to determine the low point at the edge. Cut a notch out of the curb at this point to allow water to drain away, as shown in figure1.14. 9. Dig a shallow ditch from the notch in the curb to a soakaway pit, a small pit filled with rocks, or other drainage area a few meters or more away from the well. Line the ditch with mortar.
Figure1.14.Finishing headwall and soakaway
10. When the concrete apron has firmly set, remove the cover materials. Position forms for the outside of the headwall. The radius of the forms should be 150mm greater than the radius of the steel shutters already in place. This will make the headwall 150mm thick. Pour concrete into the headwall forms and trowel smooth the top. See figure 1.14. If a water-lifting device other than a pump is to be fixed to the headwall, set the base of the device, or the bolts to hold it, into the fresh concrete. Cover the concrete with straw or wet burlap and keep moist for seven days. 11. If the well is to have a pump, build a concrete cover for the well. This can be done while the headwall is curing. 11a. Build a circular form 100m high and with the same diameter as the outside of the headwall. Place the form on a flat, oiled sheet of tin as shown in figure 1.15.
Figure1.15 making the well covers
11b. Make a circle of tin about 600mm in diameter and 200mm high. Set it inside the form for the cover at least 200mm from the outside edge. This will form the access hole. Fashion another circle of tin to form a hole large enough for removal of the riser pipe and pumping unit. 11c. Fill the form about one-third full with concrete. Set re-rods in place as shown in figure 1.15. Fill the form with concrete and trowel the top smooth. Before the concrete has set up, form a lip around the access hole to prevent spilled water from entering. Set bolts in the fresh concrete for the pump and the access hatch. Cover the concrete with straw or wet burlap and keep moist for seven days. 12. When the concrete headwall has firmly set up, remove the outside form and the steel shutters. 13. If the well is to have a water lifting device, set the device in place and bolt it to the headwall. See figure 1.16a, 1.16b, 1.16c, and 1.16d.
Figure1.16. Examples of finished hand dug wells
14. If the well is to have a pump, remove the forms from the well cover after the concrete has set up and set the cover on the headwall, as shown in figure 1.16d. Seal around the edges with concrete mortar. Bolt the access hatch in place.
1.9 Improving Traditional Wells In some locations, rather than digging a new well, it may be better to improve a traditional well. Whether or not this is possible will depend on the conditions at the well, and how it was dug in the first place. The main differences between a traditional well and an improved well are shown in the table below. Not all traditional wells will have the features (or lack of features) listed in the table, so when you are
22 assessing a traditional well, you should make a note of what improved features it does have and which are missing.
Table 1.7 the main differences between a traditional and improved well
Features Traditional Well Improved Well • May not have been sited properly • At least 10m from any latrines or rubbish pits • Not located in cemeteries, swampy or flood prone areas Site Issues • Generally small diameter • Standard diameter • May not be vertical • Vertical to 1cm for every meter of depth Well Shaft Lining • Usually unlined or only lined at the top • Lined with concrete or masonry
• Only penetrates aquifer by one meter or • Penetrates aquifer by at least 3 meters (or 2 meters if so very high inflow) • Unlined • Lined with perforated or porous concrete rings (unless • No base plug very high inflow) • Concrete or graded gravel base plug Intake • Open • Closed • No concrete apron or drainage channel • Apron, drainage and soakage pit • May not have raised headwall • Raised headwall • Nothing to prevent surface water • Top 3 meters of shaft sealed to prevent surface water entering the well entering well • No protection against animals • Diversion ditch to stop surface water flowing near well Head works • Fenced • Users drop their own buckets into the • Closed with dedicated bucket, or hand pump fitted well • Community rules and regulations on use of well • Nothing to prevent children dropping Water items into well Extraction
Assessing Traditional Wells The first thing you need to do when faced with improving a traditional well is to make sure that it is possible to improve it. Some characteristics of wells are able to be changed and some are not. Using the table above, the most important features that cannot be changed are: • The Well Site • The verticality of the shaft
The features that can be changed include: • The shaft diameter • The well lining • The intake section (including lining, base plug, aquifer penetration and so on) • The Head works • The water extraction method
23 If the well has been badly sited in the first place and so is liable to become contaminated, then no amount of improvement will solve the problem. Similarly, if the well shaft is not vertical, then it will be very difficult to make it vertical. If either of these things is not suitable, you should not attempt to improve the well.
If the well is in a good site, and the shaft is vertical (to within 1cm for every meter of depth) then it is likely that you will be able to improve the well. You should make a list of the features that need improving so that you can plan your work and make sure that the tools, equipment, materials and labor that you need are available.
Steps for Improving Traditional Wells: The following is a rough guide to the steps required for improving traditional wells. Every site will be different and so you will need to think through these steps for your circumstances.You may not necessarily need to complete each of these steps. 1. Plan and set out the site, and make the necessary preparations for excavation. You need to do the same preparation for improving a traditional well as for constructing a new well. Make sure during the site preparation that the well shaft is protected against items falling in (you should review all safety procedures. You also need to erect a suitable head frame for removal of spoil and lowering tools, equipment and lining materials into the shaft. All necessary tools, equipment and materials should be brought to site. 2. Excavate and Line the Shaft: You need to decide on the lining and excavation methods. Which method you choose will depend on the site conditions. Most traditional wells are unlined, which means the ground is self supporting, however the process of enlarging the shaft may cause sections to become unstable, therefore you should take all the same precautions as with excavating a new well shaft.
If the diameter of the existing shaft is greater than the outside diameter of the concrete rings then you can simply lower a column of concrete rings into the shaft and continue excavating using the Caisson Method to deepen the well. You should check the diameter of the shaft all the way down before lowering a concrete ring as a jammed concrete ring half way down the shaft is very difficult to move. Make sure also that the first rings lowered are porous or perforated for the intake, and the cutting ring is installed at the bottom (if used).
If the existing shaft is lined, and the lining is not suitable (if it is crumbling masonry or concrete, or rusting metal, then it should be removed. You should remove it in short sections (not more than 5 meters, and possibly less if the ground is not stable) and line as you go with whatever method you have chosen. . 3. Construct the Intake Section: Almost certainly the intake section will need to be deepened to ensure a year round and drought proof water supply. The well shaft will, by now, be lined and completed to the surface, and so further excavation can proceed as with a newly constructed well. Once excavation is complete, finalize the well shaft. 4. Construct the Head works: All the above ground components of the well should now be completed as with a newly constructed Hand Dug Well. This includes the headwall, the cover slab, the apron and drainage and any extra features chosen by the Community. 5. Complete the Hand Dug Well: Completion of the well is the same as for a newly constructed well. You should ensure that the well is properly cleaned and disinfected, that the well is properly fenced and protected, and that adequate training in operation and maintenance is provided to the community. As with a newly constructed well, you will be required to return and deepen the well should it dry up within 12 months of completion. The well is now ready to be handed over to the Community.
1.10 Management, Operation and Maintenance One of the key aspects in the long-term sustainability of any water supply system is the full and enthusiastic involvement of the community in all phases (including operation, maintenance and management) of the water supply process. In institutional terms, this is only logical, since most water authorities or water supply organizations in the developing world have, at best, sufficient resources only for the planning and construction phases. But an increase in construction activities implies an increase in the need for management of completed systems and it is here that the community has the most vital role to play.
24 Pay attention for the following cases; i) Hygiene and health considerations The following points relate specifically to safe and hygienic activities at the water point. 1. Upon completion, the water in the well should be disinfected. This is normally done by the addition of chlorine in proportion to the volume of water in the well. At regular intervals, and particularly if the well has recharged after lying dry for some time, the water should be checked and disinfected if necessary. 2. After completion, no new activities take place in the area of the well which could lead to contamination. The existence of new contamination risks for the well can be detected by carrying out regular sanitary surveys. 3. Fencing should be put in place to keep animals away from the well. This should be regularly maintained. 4. The area around the well should be kept clean of dirt, debris and stagnant water. 5. In the case of an open well, the container and rope being lowered into the well (there should only be one) should be checked regularly for cleanliness. Nobody should be allowed to stand on the headwall. This can be managed by ensuring that the headwall is too narrow to allow it. ii) Structural maintenance The following points will need attention from time to time in the life of a well. 1. Cracks in the apron Even seemingly harmless surface cracks in the apron should be dealt with as quickly as possible; to lessen the dangers of allowing dirty water from the surface to infiltrate back into the well. In repairing a crack, it is not enough merely to fill in the gap in the surface. Any loose concrete must be chipped away, and the crack thoroughly cleaned before it is filled with concrete in a 1:3 (cement: sand) mix. Care must be taken in analyzing the source of a crack. If it is due to normal wear and tear on the apron, the type of repair outlined above will suffice. However, the appearance of a crack may also be due to differential settlement in the underlying soil (if, for example, the apron was constructed without allowing enough time for the soil disturbed by the construction process to settle once more) or to erosion and undercutting of the apron. In these cases, the problem is more serious, and a complete reconstruction of the apron may have to be considered. 2. Security of inspection cover Whatever the material of which it is made, the inspection cover must be kept in place at all times during the normal use of the well. This is important from the point of view of hygiene and also in situations where water vending is practiced. Metal inspection covers must be painted in lead-free paint and regularly inspected to ensure that they are not rusting and contributing to the contamination of the well. If the inspection cover is made of concrete, the mortar which keeps it in place must be checked regularly, and any cracks repaired as soon as possible. A covered well which loses its inspection cover contributes very little of use to the health profile of a community. 3. Improving the yield of a well This may involve deepening the well or extending it horizontally. 4. Infiltration of sand If the excavation of the well concluded without reaching an impermeable layer, there is the danger of the well filling up with sand from the bottom. This can be treated by placing the bottom slabs as already mentioned. 5. Collapse of the well One instance in which collapse of a well may occur is in an area where ground conditions are hostile to concrete (for example, in soils with a low pH or excessive carbon dioxide). If this is known at the construction stage, the lining rings may be made using sulphate-resisting cement, but this may not always be readily available. When a collapse of this nature occurs, it can be because the aggressive element in the soil has broken down the weakest part of the shaft, namely the filter rings. This can be avoided by casting in holes to an otherwise solid ring.
25 PART 2- SPRING DEVELOPMENT
2.1 Introduction For small towns rural communities in developing countries, piped water supplies with house-connections are often not economically feasible. In such instances, the realistic option is the protected well fitted with a hand pump, a spring tapping structure or perhaps a rainwater catchments and storage system. A small community water supply system need not be difficult to design and construct. The engineer should carefully select a technology, which is simple, reliable, and adapted to the available technical and organizational skills. This is not easy but these problems present a fascinating challenge and rewarding field of work.
Small community water supply systems have been built for a long time, and recently such schemes have been constructed in considerable numbers. Some were successful but the overall record does not appear good, sometimes small water supplies proved to be unsuited to the conditions under which they have to operate. Several schemes have been completely abandoned within a few years after their construction. Frequent breakdowns are by no means uncommon. It is necessary to learn from past mistakes and to recognize the causes of failure. From these, guidelines can be developed for the planning, construction, operation and maintenance of small water supply systems. One of the water supply means is spring. Spring is the natural outflow/discharge of groundwater and it has been used as a source of water supply in many parts of the world. If properly capped it is the safest and the cheapest water supply source. Springs are found mainly in mountainous or hilly terrain. A spring may be defined as a place where a natural outflow of groundwater occurs.
Spring water is usually fed from sand or gravel water bearing ground formation (aquifer), or a water flow through fissured rock. Where solid or clay layers block the underground flow of water, it is forced upward and can come to the surface. The water may emerge either in the open as a spring, or invisibly as an outflow into a river, stream, lake or the see (fig.1). Where the water emerges in the form of a spring, the water can easily be tapped. The oldest community water supplies were, in fact, often based on springs.
Figure 2.1 Occurrence of springs
The best places to look for springs are the slopes of hill-sides and river valleys. Green vegetation at a certain point in a dry area may also indicate a spring, or one may be found by following a stream up to its source. However, the local people are the best guides, as they usually know most springs in their area. Real spring water is pure and usually can be used without treatment. Provided the spring is properly protected with a construction (e.g. masonry, brick or concrete) that prevents contamination of the water is really fed from the groundwater and not a stream that has gone underground for a short distance.
The flow of water from a spring may be through openings of various shapes. There are several names: seepage or filtration springs where the water percolates from many small openings in porous ground; fracture springs where the water issues from joints or fractures in otherwise solid rock; and, tubular springs where the outflow opening is more or less round. However, to understand the possibilities of water tapping from springs, the distinction between gravity springs and artesian springs is mot important. A further sub-division can be made into depression springs and overflow springs.
Objective: After the training, the trainees will be able to know:
26 Ways of springs formation and their types Some preparations done prior to spring development (SD) Design features of SD Construction, Operation and maintenance of SD
Methods of Delivery: Lecture the essential points of the manual Open discussion on the issues raised by the trainees Experience sharing among trainees Field visits
2.2 Types of springs There are three main types of springs that occur in nature: S Artesian springs S Gravity springs S Seepage springs
S Artesian springs: They are confined by two layers of impervious material. The water from artesian springs is likely to have been sufficiently filtered naturally through the ground, and typically has little to no chance of being contaminated with surface water that may infiltrate into the spring. There are three types of artesian spring.
Artesian depression spring: Artesian groundwater is prevented from rising to its free water table level by the presence of an overlaying impervious layer. That is the reason why artesian groundwater is under pressure. Artesian springs are the sites where the groundwater comes to the surface. Artesian depression springs are similar in appearance to gravity depression springs. However, the water is forced out under pressure so that the discharge is higher and there is less fluctuation. A drop of the artesian water table during dry periods has little influence on the artesian groundwater flow (Fig. 2.2a).
Figure2.2 Types of artesian spring: (a) artesian depression spring
Artesian fissure spring Artesian fissure springs (Fig. 2b) form an important variant of this type of spring. Again the water emerges under pressure, this time through a fissure in the impervious overburden. Fissure springs exist in many countries and are widely used for community water supplies.
(b) Artesian fissure spring (c) Artesian overflows spring
Artesian overflow spring Artesian overflow springs often have a large recharge area, sometimes a great distance away (Fig. 2c). The water is forced out under pressure; the discharge is often considerable and shows little or no seasonal fluctuation. These springs are very well suited for community water supply purposes. Artesian springs have the advantage that the impervious cover protects the water in the aquifer against contamination. The water from these springs is usually bacteriologically safe.
S Gravity springs: They rest on a single impervious layer, and can be thought of as an underground river. The unconfined aquifer will add many “tributaries” or input from local water and rain that seeps into the ground. Any
27 contaminated water that flows into the ground will only have the short flow distance before reaching the spring, giving the input water much less time to be filtered naturally. There are two types of gravity spring.
Gravity depression springs Gravity springs occur in unconfined aquifers. Where the ground surface dips below the water table, any such depression will be filled with water (Fig. 2.3a).Gravity depression springs usually have a small yield and a further reduction occurs when dry season conditions or nearby groundwater withdrawals result in the lowering of the groundwater table.
Figure.2.3a Gravity depression spring (2.3b) Gravity over flow spring
Gravity overflow springs A larger and less variable yield from gravity springs is obtained where an outcrop of impervious soil, such as a solid or clay fault zone, prevents the downward flow of the groundwater and forces it up to the surface (Fig. 3b). At such an overflow spring, all the water from the recharge area is discharged. The flow will be much more regular than the recharge by rainfall. Even so, an appreciable fluctuation of the discharge may occur and in periods of drought some springs may cease to flow completely.
S Seepage springs They occur where water simply seeps out of sand, gravel, and other porous material. Opposed to artesian and gravity springs where flow is directed to one point, seepage springs result from a somewhat unconfined aquifer, where an underground reservoir simply leaches out in different places. This gives seepage springs the highest susceptibility to contamination. Therefore seepage springs need periodic disinfection.
2.3 Parameters to be considered for spring development While conducting the feasibility study of spring development as a source of water supply, the following parameters should be considered: Discharge of the spring: the discharge of the spring can be measured using different locally available materials (container or vessel of known volume and stop watch or wrist watch). The discharge should be measured during the driest month of the year in the area. Spring type: Use one of the above mentioned parameters to classify the spring because spring development depends on its type. Possible aquifer: of the spring and the possible impermeable layer if any. In addition to these, the availability of construction materials, existence of potential contaminants in the area, location of the spring including altitude and distance from the beneficiary has to be mentioned. Further more, based on the water demand of the community and discharge of the spring, whether the night storage is needed or not has to be decided during the feasibility study. Night storage is required when the discharge of the spring is not high enough to suffice the water demand of the community with continuous flow.
Springs can be developed either for on spot water supply (with out distribution system) or with distribution. Spring development is site specific, i.e. it varies depending on geology, topography, hydrogeology, etc of the area. In the case of inadequate resources such as cement, steel bar, pipe etc for spring construction, communities can improve water tapping method as well as sanitation of the spring using locally available materials such as by fencing, cleaning and using bamboo as a pipe.
Figure 2.4 location map
2. 4 Design Features of Spring Development
Protective structures for springs and seeps assure a clean water supply for different purposes. The protective structure increases the volume of water, which can be diverted from the spring, and protects
28 the site from contamination by runoff and other foreign bodies including animals. Developing a spring or seep requires some understanding of ground water flow and preparation of a thorough construction plan.
The construction plan should include the following: (a) Map of the area, identifying the location of the spring, the locations of water use, and distances from source to use outlet points, and surveyed changes in elevations: (b) A complete list of all labor, materials, and tools needed; and (c) A spring box design with diagrams of the top, side, and front views, and the dimensions of a cover. Spring box structures can be costly in terms of the amount of time and finances invested, so careful planning is essential.
Figure 2.5 Spring Box Design
2.4.1 Spring Box Designs There are several possible designs for spring’s boxes, but, in general, their basic features are similar. Two basic design choices are a box with one pervious side for collection of water from a hillside and a box with a previous bottom for collection of spring water flowing from a single opening on level ground. To determine which design to use, dig out around the area until an impervious layer is reached locate the source of the spring flow, and design to fit the situation.
Spring box with open side A spring box with a pervious side is needed to protect spring flowing from hill sides. The area around the spring must be dug out so that all available flow is captured and channeled in to the spring box. After this has been done, a collection box can be built around the spring outlet as shown in figure 3. The dugout area should be lined with gravel. The gravel placed against the spring opening serves as a foundation for the box and prevents the spring water from washing soil away from the area. The gravel pack also filters suspended solids. The gravel-filled area should be between 0.5-1m wide depending on the size of the spring collection area.
Figure 2.6: Spring box with single pervious side for hillside collection
If the spring occurs at the base of a slope or hillside, the flow is likely to be gravity driven. Unlike an artesian spring, a gravity spring will most likely have just one impermeable layer (on the bottom).In this case, much less pressure will exist in the system. Due to the nature of the horizontal flow, and low water pressure, a gravity spring in a hillside will require a spring box with a side entrance for the water.
To ensure that no contamination of water, the gravel packs should be at least 1m below the ground surface. This is done either by locating the spring catchment in the hill side or by raising the ground level with backfill. Caution must be taken not to disturb ground formation when digging out around the spring. With out care, the flow of the spring may be deflected in another direction or in to another fissure. The area must however, be dug out enough so that the spring box fits in to impermeable material. In cases where the box does not reach impermeable material, puddle clay should be used to seal the area around the side of the spring box
Spring box with open bottom
If a spring flows through a fissure and emerges at one point on level ground, a spring box with an open bottom can be developed as shown in figure 4. The area around the spring is dug out until an impermeable layer is reached. The area around the spring is then leveled and lined with gravel. The spring box is placed over the spring and gravel to collect the flow, and clay or concrete is packed around the box to prevent seepage between the ground and the box. Some times a small sump can be built at the bottom so that sediment settles in one place.
29 Figure 2.7: Spring box with permeable bottom for collecting spring water flowing from an opening on level ground If the spring is naturally occurring on relatively flat ground, it is likely to be an artesian spring. Water flows vertically out of the ground due to the pressure that is accumulated within a confined aquifer. For this type of spring, a spring box with an open bottom is used, as illustrated below.
The design of both types of spring boxes is basically the same and includes the following features: (a) Water tight collection box constructed of concrete, brick, clay or other material. (b) A heavy removable cover that prevents contamination and provides access for cleaning. (c) An over flow pipe, and (d) A connection to a storage tank or directly to a distribution system, The spring box with an open bottom is simpler and cheaper to construct. Generally, on level ground, flow from only one source must be captured and collection of all available flow is much easier. Costs are lower because less digging and fewer materials are required.
Overflow pipe: The spring box should have an over flow pipe. The pipe is placed a little below the maximum water level and at least 0.15 m above the floor of the tank. If the pipe is above the maximum water level, water will not flow out and pressure is created in the tank. The pressure could cause a backup and a diversion of the spring. The overflow pipe should be covered with a screen fine enough to keep out small animals. The size of the pipe depends on the flow of the spring – A rock drain or concrete slab should be placed out side the tank below the overflow pipe to prevent erosion near the base and to carry the water away from the spring. A pipe which extends 3-5 m from the tank is desirable in order to keep the site free from still water.
Outlet pipe: An outlet pipe for connection to a distribution system should be located at least 0.1m above the bottom of the spring box to prevent a blockage due to sediment buildup. The pipe size depends on the grade to the storage tank and the spring flow. A general rule to follow is that at a 1% grade, a 30mm pipe should be used. A grade between 0.5 and 1% requires a 40mm pipe, while a 50mm pipe should be used for grades of less than 0.5%. In some cases the same pipe will be both outlet and overflow. The outlet pipe should slope downward for best flow.
After the spring box is installed, the space behind it must be filled with soil and gravel. The gravel is the bottom layer. On top of it, a water tight layer should be formed to prevent the entrance of surface water. This can be done with concrete or puddle clay.Puddled clay is a mixture of clay and water formed in to a layer 150mm thick. The layer is placed on the ground and worked in by trampling on it. Several layers of puddle clay should be placed behind the box.
After sealing the area, the box can either be completely covered with soil or stand above the ground surface. The box should be at least 0.3m above ground level so that runoff does not enter it. For further sanitary protection, a ditch should be dug at least 8m above the spring box to take surface water away from the area. The soil from the ditch should be piled on the downhill side to make a ridge and help keep surface water away. A fence around the area will keep animals from getting near the spring box and help prevent contamination and destruction of the area. The fence should have a radius of between 7-8m.
2.4.2 General Construction Steps of Spring Box The following steps are appropriate for either design choice: 1. Locate the spring site and mark out the area with measuring tape, cord, and wooden stakes or pointed sticks. 2. Clean out the area around the spring to ensure a good flow. If the spring flows from a hillside, dig into the hill far enough to determine the origin of the flow. Where water is flowing from more than one opening, dig back far enough to ensure that all the water flows into the collecting area. If the flow can not be
30 channeled to the collection area because openings are too diffuse, drains will have to be installed. Flow from several sources may be diverted to one opening by digging farther back into the hill. Always try to dig down deep enough to reach an impervious layer. An impervious layer makes a good foundation for the spring box, and provides a better surface for a seal against underflow. 3. Pile loose stones and gravel against the spring before putting in the spring box. The stones serve as a foundation for the spring box and help support the ground near the spring opening to prevent dirt from washing in. 4. Approximately 8 meters above the spring site dig a trench for diverting surface runoff. Use large stones, if available, to line the diversion trench and prevent erosion. 5. Mark off an area about 9 meters by 9 meters for a fence. Place the fence posts 2 meters apart and string the fence. Figure 2.8: Preparation of spring box site
2.4.3 Storage Reservoir Since a spring box is intended only to collect water and to protect a spring from contamination, it is usually also necessary to construct a storage reservoir also called collection chamber/night storage. It is good practice to construct a water collection point away from the spring box where community members can come and collect water. This has several advantages: 1. If the spring is in a gully or other location that is difficult to access, the water collection point can be located somewhere much easier for people to get to. This makes water collection easier and safer for women and children who normally do this activity. 2. It protects the catchment area and the area around the spring against contamination. 3. If necessary, it allows the construction of an overnight storage reservoir. This will make water collection much quicker than if collecting directly from the spring box. A storage tank can also be used to remove sedimentation from the water and to break pressure in a piped system. When designing a storage reservoir (collection chamber), one must specify: Placement of reservoir Basic construction materials of reservoir Dimensions of reservoir Type of cover for reservoir Type and placement of inlet, delivery, overflow, and scour pipes
Recommended specifications are described in steps 1-5 below, but they may need to be adjusted to suit your particular situation. 1. First you will need to decide on the exact placement of the reservoir. The reservoir should be located downhill from the spring box; it may be at any distance from the spring box, provided it is at least slightly downhill. It should also be located in an accessible place to allow for ease of construction and collection of water. The reservoir can be situated either at ground level or completely or partly buried. In any case, it should be situated in or on stable soil or rock to prevent shifting or sliding. If geologic conditions permit, it is usually cheapest and easiest to bury the reservoir, so that during construction, the sides of the hole can be used as a form for the concrete. 2. The next step in designing a storage reservoir is to choose the basic construction material: concrete blocks, Ferro cement, or cast-in place concrete. If the storage reservoir is to be buried, cast-in place reinforced concrete is preferable. If not, construction using concrete blocks, or Ferro cement will be cheaper and easier, since extensive forming would be required to pour a concrete reservoir above ground. When concrete blocks are used, they should be coated with an inch of mortar on the inside. 3. Next specify the dimensions of the storage reservoir. As a guideline the storage capacity should equal ½ of the average daily use of the system. More exact calculations for estimating desired storage capacity would be needed if the system involves an extensive piped delivery system. 4. The cover for a storage reservoir should be made of pre-cast, reinforced concrete. It should have a raised ‘’curb’’ over which a removable manhole cover fits tightly. The manhole cover should be placed near the inlet pipe to allow for inspection. 5. Suggested specifications for inlet, delivery, overflow and drainage pipes for storage reservoirs are as follows:
31 An inlet pipe should be located near the top of the reservoir. It may be fitted with a float valve to regulate flow into the tank, and a gate valve placed outside of the tank to shut down the flow if necessary. A delivery pipe should be located about three to five inches above the floor of the reservoir. It may be connected to more pipes leading to distribution points at different locations, or it may be a very short pipe, from which people can obtain water right at the reservoir: One reservoir may include both types of delivery pipes. The outlet end of a delivery pipe should point downward, and a self-enclosing valve should be provided to prevent water waste. An overflow pipe, larger than the delivery pipe, should be located very near the top of the reservoir, on level with the inlet pipe. Both the inside and outside ends of the pipe should be screened with copper, brass or other corrosion-resistant screen. The outlet should discharge downward. A drainage pipe should be placed on the bottom of the reservoir so that sludge can be drained out if necessary. It is helpful to leave a sloped indentation for the drainage pipe in the floor of the reservoir, and to slope the floor towards the pipe. The pipe should be closed with a tight cap on the outside end. Rock or concrete should be placed at all points where erosion is possible from overflow or delivery pipes. The tank should have good drainage on all sides and be fenced for protection.
Types of Collection Points: There are two types of collection points 1 • Storage Reservoirs with taps 2 • Tapstands To decide whether a storage reservoir is needed, the flow rate from the spring must be measured. Note that the flow rate should be measured at the driest time of the year to determine the minimum flow rate from the spring. If the flow rate from the spring is less than 9 liters per minute then a storage reservoir will be needed. However, if the number of households using the spring is less than about 20, then a storage reservoir may not be required, even if the flow rate is less than 9 liters per minute.
2.4.4 Storage Reservoir Design The idea of the overnight storage reservoir is that it collects and stores water from the spring during the night when no one is collecting water so that in the morning when people come to collect water, there is plenty available that can be collected quickly.
Storage Reservoir Size: To determine how big the storage reservoir should be, calculate how much water will flow into the reservoir during the night (from when the last person collects water in the evening until the first person collects water in the morning). Volume (liters) = flow rate (liters per minute) x number of minutes Volume (cubic meters) = Volume (liters) ÷ 1000 It is generally a good idea to have some extra capacity in the tank, so multiply the volume calculated by 1.5: Storage Reservoir volume = Volume (cubic meters) x 1.5 Example: Flow rate = 3 liters per minute Spring closed at 7pm Spring opened at 6am Number of hours closed overnight = 11 hours Number of minutes closed = 11 x 60 = 660 minutes Volume (liters) = 660 x 3 = 1980 liters Volume (cubic meters) = 1980 ÷ 1000 = 1.98 cubic meters Storage Reservoir Volume = 1.98 x 1.5 = 3 cubic meters Once you know the volume, you can calculate the dimensions using the formula: Volume =Width x breadth x height
32 2.4.5 Storage Reservoir Construction: The storage reservoir can be made from concrete, masonry or both. It should have the following features: • It should be founded on solid ground • It should have a concrete base, reinforced with rebar, and sloped towards the scouring pipe. • It should be closed, with a removable hatch for cleaning and maintenance • The pipe from the spring box should enter near the top of the reservoir • There should be an overflow pipe no higher than 10cm from the top • The outlet pipe(s) should be at least 5cm above the bottom • There should be a scouring pipe at the lowest point of the base, fitted with a gate valve or removable plug. • It can have taps fitted directly into the wall of the reservoir, or it can be connected to a tapstands by an outlet pipe. • Outlet and inlet pipes should have gate valves fitted. If the reservoir has taps fitted directly into the walls, then there should be an appropriate concrete apron and drainage constructed.
2.4.6 Tap stands If there is no requirement for an overnight storage reservoir, or if the reservoir is not fitted directly with taps, then one or more tapstands will need to be constructed. Tapstands should have the following features: • They should be made of concrete, and founded on a solid base. • The riser pipe should be cast into a concrete plinth around 1 meter high. • The pipe from the spring box or storage reservoir should be buried until it reaches the concrete plinth. • A suitable concrete apron and drainage should be incorporated into the design. • The tapstands should be fenced to keep away animals and children, and to prevent unauthorized access.
2.4.7 Materials List Concrete is the major material used in the construction of spring boxes and cutoff walls. Concrete is the mixture of Portland cement, clean sand, and gravel in a fixed proportion. The proportion generally used here is one part cement, two parts sand and three parts gravel (1:2:3). Water is used to mix the concrete. 28 liters of water should be used for each bag of cement.
Example: Calculating quantities needed for concrete (Calculation for a box 1m*1m*1m with open bottom) Assume thickness of the wall = 0.1m, 1. Volume of top = 1m*1.2m*0.1m = 0.144m3 2. Volume of bottom = 0m*0m*0m = 0m3 3. Volume of two sides = 1m*1m*0.1m *2 = 0.2m3 4. Volume of two ends = 1m*1m*0.1m *2 = 0.2m3 5. Total volume = 0.54m3 6. Unmixed volume of materials = total volume *1.5 = 0.81m3 7. Volume of each material (cement,sand,clay,1:2:3) Cement: 0.167 *0.81 = 0.14m3; Sand: 0.33 * 0.81 = 0.27m3; Gravel = 0.5*0.81 = 0.4m3 8. Number of 50kg bags of cement = volume of cement / volume per bag Consider – l kg of cement = 0.00066m3 cement and 1bag of cement = 50kg of cement 0.14m3 *1.0kg Cement 212.12kg 212kg (kg) 0.00066m3 212kg *1bag Cement 4bags (bag) 50kg
9. Volume of water Consider - 28 liters of water for 1bag (50kg) of cement
33 4bags * 28liters 3 Volumeof Water 112liters 0.112m 1bag Note: 1) Do not determine volume for an open side or bottom 2) The top slab has a 0.1m overhang on each side 3) The same calculations will be used to determine the quantity of materials for construction of a seepage wall 4) To save cement a 1:2:4 mixtures can be used 5) An extra quantity of cement should be figured in to the total for use in grouting and sealing areas around the outlet pipes. 6) Extra gravel will be needed for backfill of areas behind springs. Graded gravel is preferable, but local materials can be used if necessary. Calculate the volume of the area to be backfilled by taking length* width*height of area.
Reinforced concrete: Concrete can be reinforced to give it extra strength. This is best done with wire mesh or specially made steel rods. Reinforced concrete sections must be at least 0.10cm thick. Reinforced concrete should be used for all spring box covers and for the walls of seep structures. If wire mesh is used, the quantity needed will be approximately equal to the area of the slab being constructed. If steel bars (re-rod) are used, they should be placed in the wooden form before the concrete is poured.10mm diameter rods should be used. The reinforcing rod should be located as follows: . So that the rods are at least 25cm (0.25m) from the form in all places; . So that rebar rests in the lower part of the cover; two-thirds the distance from the top or 70mm from the top of a 100mm slab; . So that a 150mm (0.15m) space lie between a parallel rods in a grid pattern as shown in fig. 2.9. Where the reinforcing rods cross, they should be tied together with wire at the point of intersection. To determine the number of reinforcing bars, divide the total length or width of the spring box cover by 0.15m (distance between bars). For example, 1.2m/0.15m = 8 bars.
Figure2.9 Placement of rebar in concrete slabs
2.5 Post Construction Monitoring Spring Close monitoring and follow up should be provided to spring of interest during utilization (Operation of the spring). In order to sustain the spring’s water quality and to maintain its good status during the operation of the spring, the following measures have to be taken: Provision of fence to avoid entrance of children and animals: Cleaning the surrounding of the spring and avoiding any storage of water in the surrounding; draining all the wasted water properly during taking water Check for leaks in pipes and the spring box. Check the overflow pipe to make sure it is not clogged Springs are often contaminated with bacteria during construction or maintenance. All new and repaired water systems should be disinfected using shock chlorination. If bacterial contamination occurs on a regular basis because of surface sources above the spring, continuous chlorination may be necessary (although this is not recommended). As a general rule, spring collection boxes and reservoirs should be cleaned and disinfected: 1 • Prior to use 2 • Anytime the spring or storage box has been emptied or opened 3 • When coli form test results are unsatisfactory 4 • Annually. Avoid plantation of trees with high evapotranspiration particularly eucalyptus trees at the surrounding of the spring.
34 PART3- DISCHARGE MEASUREMENTS
3.1 Introduction It is important to check that the discharge of the source is enough, throughout the year, to meet the community’s water requirements. This requires measuring or estimating the dry season flow, when least water will be available. If a small reservoir is to be built, more detailed flow data are needed: üYear round flows, to size the storage üFlood flows, to design the spillway
3.2 Ways to Measure the Flows of springs and Small Streams There are a number of methods to measure discharge, but I selected the common and easiest ways of flow measuring methods - volumetric (bucket and stop watch) and float methods
Volumetric method A very easy method to estimate discharge is to simply measure the time it takes to fill a container of a known volume. This method only works for systems with fairly low flow volume. Its main limitation is that the discharge must fall from a pipe or ditch in such a way that the bucket can be placed underneath it to capture all the discharge. Any size bucket can be used as long as it does not fill up too fast to get an accurate measurement.
In this method, all the flow from a spring or small stream is collected in a container whose volume is known (e.g. a bucket, jerrycan, 200-litre drum) and the time to fill the container is measured. The filling time should be more than five seconds, to give reasonable accuracy. Discharge or yield (I/s) = volume (liters)/time (seconds)
Figure3.1 Volumetric method
Equipments: Container to fill of known volume (a clean 5-gallon bucket works well) Timer (stopwatch) Paper and pencil for record keeping
Taking the Measurement: 1. Locate the site’s discharge pipe. If discharge occurs via a channel, then a temporary dam may need to be placed across the channel with the discharge directed through a single outlet pipe. 2. Place the container of a known volume (e.g., a 1 or 5 gallon bucket) directly under pipe. All of the discharge should flow into the container. Note: The 5-gallon line on the bucket may need to be measured and marked ahead of time. 3. Using a stopwatch, time how long it takes to fill the container. 4. Repeat this process three times to obtain an average.
Calculating the Discharge - Example Calculation
35 A 5 gallon clean paint bucket was placed under the spout of a discharge pipe. The bucket filled up in 15 seconds, 18 seconds and 14 seconds.
Calculate average time: Add the three recorded times together and divide by three to obtain the average fill time. Average time = (15 + 18 + 14)/3 = 15.7 seconds
Convert average time in seconds to minutes: Divide average time by 60 seconds per minute to obtain minutes. Average time = 15.7 sec/60 = 0.26 minutes
Calculate the site discharge: Divide the volume of the container (gallons) by the average time needed to fill the container (minutes). Discharge = 5 gal/0.26min = 19.2 gallons per minute (gpm)
The Float Method The float method is an adequate means of estimating flow especially in circumstances where a flow meter is not available or when the water in the stream is not wade able. This method is simple and inexpensive to perform. The concept is to time how long it takes for a buoyant object to travel a specific distance. Using the time, along with the estimated width and depth of the stream segment, stream flow can be calculated.
Figure3.2: float method
Selecting a Site The ideal site is where you can easily and safely access the stream. The stream section should be straight for at least3m, should be at least 6 inches deep and should represent the general flow conditions. In addition, the section should be relatively consistent in width and depth and should not contain any obstructions that may deter the float.
We need: Measuring tape Timer (stopwatch) Float (an orange or a plastic bottle filled with water – basically a buoyant object that is heavy enough to sit about an inch below the water line.) Paper and pencil for record keeping Three people (two will work); one at the top of your reach, one at the bottom, and someone to record data Taking the Measurement 1. If the stream is not wade able, estimate the width and depth at the end of the stream segment and record these measurements. 2. If the stream is wade able, measure the total width of the stream. Also, determine the average depth. To do this, record the depth at 1 or 2 feet increments across the stream. Add all these depth measurements together and divide by the total number of measurements taken. This will be the average depth. 3. Estimate the cross-sectional area of the stream by multiplying the total width by the average depth. 4. Measure off or mark a minimum of 50 feet along the stream bank. It may be useful to install a permanent measuring tape or apply marks to the bank so fl oat measurements can be taken at the same location. If the float moves too fast to get an accurate measurement, measure off a longer stretch (such as 75 or 100feets) 5. Gently release the fl oat slightly before the upstream end of the measured segment. This is done so the float will be moving at the speed of the stream when timing begins. Also, try to release the float towards the portion of the stream, which has the most representative flow.
36 6. Make sure the fl oat flows freely, without catching on rocks or branches. If the fl oat catches on something, you will need to repeat the process. 7. Begin timing when the fl oat crosses the upstream end of the measured segment and stop when it crosses the downstream end (using a stopwatch or digital watch). Record the time. 8. Retrieve the float and repeat the process at least two more times (and for a total of two years).
Calculations: Surface Velocity = Distance / Time Average Surface Velocity = Sum Surface Velocities / Number of Trials Average flow velocity = (0.85 to 0.95) * average surface velocity Stream flow = Area x Velocity x Correction factor Calculating Stream Flow - Example Calculation A river is approximately 30 m wide and approximately 4 m deep (too deep to wade). A float released upstream traveled 100 m in 28 seconds, 24 seconds and 30 seconds. Stream flow = Area x Velocity x Correction factor
Calculate area: Multiply the width of the stream by the depth. Area = 30 m x 4 m = 120 m2 Calculate average fl oat time: Add up all the individual times and divide by the number of times the fl oat was released (in seconds). Average fl oat time = (28 + 24 + 30)/3 = 27 sec Calculate average velocity: Divide the distance the item floated (i.e. the length of the segment measured in feet) by the average float time. Velocity = 100 m/27sec = 3.7 m/s Calculate stream flow: Multiply the average velocity by the area and by a correction factor (of 0.85). The correction factor takes into account the effects of friction from the stream bed, i.e. surface velocity is always grater than average flow velocity. Stream flow = 120 m2 x 3.7 m/s x 0.85 = 377 m3
3.3 Measuring Yield of a Well The yield of a well is the amount of water that can be removed from the well (by pumping or bucket) without the well running dry. You must know how to measure and calculate the yield of a well because this is how you know that you have dug the well shaft deep enough. The Project Supervisor will also measure the yield to confirm that you can stop excavating and start finalizing the well and constructing the head works. The Static Depth of Water is the depth after the well has been left overnight with no pumping or bucketing. This is also referred to as overnight storage. To determine whether the well is deep enough, you need to check both the yield, and the static depth of water. The following table indicates the yield that you need for different static depths of water.
Static Depth of Water (meters) Yield (in liters per minute) 2.0 20 2.5 15 3.0 10
Example: When you have excavated enough so that the static depth of water is 2.0 meters, check the yield. If it is 20 liters per minute or more, you can stop excavating. If it is less than 20 liters per minute, you must dig deeper.
37 When to measure yield: You should measure the yield when the static depth of water is approximately 2.0 meters. If you need to excavate deeper, check the yield for every half meter more you dig.
How to Measure Yield: Steps: 1. Check the static depth of water with a tape measure first thing in the morning before any work has begun. 2. Empty all the water out of the well by pumping (if you have a de-watering pump) or by bailing with a bucket. 3. Allow water to enter the well to a depth of 0.5 meters (check with a tape measure) 4. Wait for 30 minutes 5. Measure the depth of water in the well with a tape measure 6. Calculate the yield of the well as follows: A. First calculate the total quantity of water that has entered the well by multiplying the flat surface area of the well by the depth of water:
Area = Diameter x Diameter x 3.14 4 Depth = Final measured depth – Initial depth (0.5m) Quantity of water = Area x Depth x 1000 (to give liters)
Figure 3.3checking static water depths
Example1: Static depth of water = 2.0m Well diameter = 0.95m Final measured depth = 1.02m ∴Area = 0.95x0.95x3.14 = 0.71 square meters 4 Depth = 1.02 - 0.5 = 0.52 meters Quantity = 0.71x0.52x1000 = 369 liters
B. Next calculate the yield: Yield = Quantity / Time (30 minutes) Yield = 369 / 30 = 12.3 liters per minute C. Check the yield compared to the static depth to see whether it is acceptable. Static depth = 2.0m Yield = 12.3 liters/minute ∴ Not acceptable. You must keep digging deeper.
It is also possible to measure yield with a pump. The process is as follows: 1. Install a de-watering or similar pump in the well. Make sure the outlet hose is long enough to discharge at least 5 meters from the well shaft. Make sure there is a flow restricting valve on the outlet hose. 2. Measure the depth of the water in the well with a tape measure. 3. Turn the pump on full; making sure the flow restricting valve is fully open. 4. Keep measuring the depth of the water in the well every 10 minutes as the level drops. 5. When the water level stops dropping and remains the same for 30 minutes, measure the flow rate out of the hose as described below. If the level keeps dropping to 0.5 meters, reduce the flow rate by closing the flow restricting valve until the level remains steady at 0.5 meters. Once it has become steady, measure the flow rate out of the hose as follows:
A. Get a bucket of known volume and a stop watch or clock that measures seconds.
38 B. Place the end of the hose in the bucket and at the same time start the stop watch or read the seconds on the clock. C. When the bucket is full, stop the stop watch or read the seconds on the clock. D. Calculate the number of seconds to fill the bucket. E. Flow rate (liters per second) = Volume of bucket (liters) / number of seconds F. Yield (liters per minute) = Flow rate x 60
Figure 3.4 measuring yield of the well
4. REFERENCES
1. Ministry of Water Resources of Ethiopia (2007). Hand Dug Well and Spring Development Construction Manual: Technical manual No 3.
2. Seamus Collins (2000) Hand-dug shallow wells, SKAT Swiss Centre for Development Cooperation in Technology and Management
3. Water for the World: Methods of Developing Sources of Surface Water. Technical Note No. RWS. 1. M
4. Water for the World: Designing Structures for springs. Technical Note No.RWS. 1. D.1
5. Water for the World: Constructing Structures for springs. Technical Note No. RWS. 1. C.1
6. Water for the World: Designing hand dug wells. Technical Note No. RWS. 2. D.1
7. Water for the World: Constructing hand dug wells. Technical Note No. RWS 2.C.1
8. Water for the World: Finishing wells. Technical Note No. RWS. 2. C.8
9. Joy P. Michaud and Marlies Wierenga (2005). Estimating discharge and stream flows: Publication Number 05-10-070
39 5. Glossary
World/Terms Definition A concrete floor outside the head wall of a well. The apron provides a relatively clean environment around the well, and controls drainage of spilled water away Apron from the well. Water bearing channel or cavity in the soil or rock, aquifers may be relatively open Aquifer cavities, or consist of porous materials with water moving in the interstices. Bottom portion of a caisson ring mould, used to support the side form maintain Base Plate circular shape and spacing, and to form the lower rebate edge. Horizontal layer of consolidated rock formation extending much beyond the limits Bedrock of the well. A device for lifting water with a bucket without the removal and reinsertion of the Bucket pump bucket. Caisson Binding Rods Rods used to attach the bottom 3-6 caissons together during construction. A device for lifting caisson rings. The bar is inserted into two holes in the inner surface of the caisson ring. A training of steel rod allows lifting with minimal stress Caisson Lifting Bar on the bar. Also Caisson or ring. A cylindrical liner, usually pre-cast concrete or steel , which Caisson Ring may be placed in an existing hole, or sunk in place undercutting or other methods. A method of well digging consisting of undercutting pre-cast liners (caissons) to Caisson Sinking lower them in place and concurrently deepen the hole The upper portion of a caisson ring mould, used to maintain circular shape as well Capping Ring as form the upper rebate edge. Concrete A mixture of Portland cement, sand, and gravel. Additives are sometimes used. Contamination Introduction of pathogenic organisms or toxic chemicals into the water. A ring placed below the caissons to facilitate undercutting and sinking. Usually of concrete, it should have a concave bottom surface, and a slightly greater outer diameter than the caissons. It is usually not necessary if the caissons are suitably Cutting Ring designed and used. Soil which has been disturbed, and is therefore at risk of collapsing or washing Destabilized Soil into the well. The amount the water level is reduced below the static water level, at a given Drawdown pumping rate. Wells constructed with a variety of rotary and percussive mechanisms usually power driven. Drilled wells may have diameters as little as 5 cm. or as large as Drilled Wells 90cm, but are more commonly between 10 and 30 cm. Water which has been stored and filtered in the soil and rock below the surface. Such storage and filtration usually a results in purification from biological contaminants being carried to a well in groundwater contaminated by the mixing Ground Water of surface water or other contaminants at some distance away.
40 Wells excavated and lined by human labor, generally be entering the well with a variety of hand tools. They may be as small as 80 cm diameter, and in some Hand Dug wells traditional cultures, as large as 15 meters A device for lifting water from the well without the use of buckets and ropes, and Hand Pump powered by human labor. A structure placed over the well to prevent loose articles or soil being knocked in, to support a working platform, and usually to support a lifting apparatus such as Head Frame pulley or windlass. The portion of the well liner which extends above the ground level or above the Head Wall surface of the apron. In-Situ Lining Casting a lining in place between the soil and an inner mould. Lifting Calipers Devices used to grip the top edge of a caisson ring for lifting purposes. Light Lifting Holes left part way through the caisson wall to attach lifting bar to binding rods. Light Lifting Head A combination of a protective head frame with a windlass for lifting and lowering Frame materials and sometimes workers. The process of recording the soil and rock conditions, aquifers encountered, and other relevant data on the well construction which may be of relevance in contract Logging administration, maintenance, or in construction of other wells. A well improved depth, yield, lining materials, surface drainage, and/or other Modern Hand Dug Well enhancements. Moulds Forms used for casting shapes such as caisson rings from concrete. perforations See Weep Holes Porous Concrete mixture. It is unnecessary and dangerous in the ca se of caisson Permeable Mixture lining. Overlapping joint edge of a caisson ring or culvert tile, used to assure better alignment of one segment with the next. Does not provide a water tight unless Rebate Edge cement is used to join the segments. Inner moulds used in in-suit casting of lining, or a temporary liner to support soil Shuttering during excavation. Soil overburden The layers of various soils encountered above bedrock. The tendency for surface layers of concrete to chip off break away from the main body of concrete. One common cause for this is the corrosion and consequent Sprawl expansion of reinforcing steel. The level of the water in a well that has not been recently pumped. It is normal for Static Water Level the SWL to change seasonally. Water consisting of surface used for lifting heavy loads such as the weight of the caisson ring. The tripod is usually combined with lifting device such as winches or Surface Water block and tackles. A three-legged structure used for lifting heavy loads such as the weight of the caisson ring. The tripped is usually combined with lifting devices such as winches Tripod or block and deepened. Of caisson rings, the process of deepening the well by sinking the caisson as the Undercut hole is deepened. Sands. Silts and other soil mixture which may flow or collapse when one side is Unstable Soil unsupported. Holes formed all the way through concrete liners to permit the free flow of water. Weep Holes In the case of pre-cast or caisson linings these are usually not necessary. Usually a round concrete slab used to cover the well. The cap must be suitably reinforced. Pump mounting bolts and access hatches are cast into the cap as Well Cap appropriate. The structural components of the well above grade level. ie Apron, Head Wall, Well Head Cap, and drainage. A lifting device intended for greater lifting capacity combined with slower motion. May consist of a drum to wind cable, linked to a crank through a gear mechanism Winch to achieve greater mechanical advantage.
41 A lifting device consisting of a cylinder to wind rope on, and a crank mechanism. Windlass Usually ha a light to moderate lifting capacity combined with relatively fast motion. The quantity of water that can be drawn continuously from a well. The yield measured in liters per minute, or gallons per hour, must normally be specified at Yield some acceptable drawdown. Aquifer A water-saturated geologic zone that will yield water to springs and wells. Ground water Water stored below the ground's surface A large bucket for lifting materials when sinking a shaft; also called a hoppit or Kibble sinking bucket. Porous Having tiny pores or spaces which can store water or allow water to pass through. Water table The top or upper limit, of an aquifer Grout A fluid mixture of cement and sand The water which consists of undesirable substances which make it unfit for Polluted water drinking and domestic use. The water containing pathogenic organisms. Contaminated water can be polluted Contaminated water water but may not be the vice versa Discharge: Another term for stream flow, or the volume of water moving past a designated point over a set period of time. Flow Regime: The pattern of stream flow over time, including increases with storm water runoff inputs and decreases to a base-flow level during dry periods.
42 Lists of Figures
Figure1.1 Parts of a well
43 Figure1.2. (a) Cone of depression (b) well interference
Figure1.3: Location map
44 Figure1.4. Designing of well lining and caisson
Figure1.5 precast concrete rings
45 Figure1.6 Features of Pre-cast Concrete Rings
Figure 1.7 caisson method
Figure 1.8 Keeping the Shaft Vertical
46 Figure 19 Gravel packing
Figure1.10 Typical layout of apron & drainage
Figure 1.11 Clothes Washing Rack Figure1.12 Animal Watering Trough
47
Figure1.13. Building the apron for a hand dug well.
Figure1.14.Finishing headwall and soakaway
48 Figure1.15. Making the well cover
49 Figure1.16. Examples of finished hand dug wells
Figure 2.1 Occurrence of springs
50 Figure2.2 Types of artesian spring: (a) artesian depression spring
(b) Artesian fissure spring (c) Artesian overflows spring
Fig.2.3a Gravity depression spring (2.3b) Gravity over flow spring
51 Figure 2.4 location map
Figure 2.5 Spring Box Design
52 Figure 2.6: Spring box with single pervious side for hillside collection
Figure 2.7: Spring box with permeable bottom for collecting spring water flowing from an opening on level ground
53 Figure 2.8: Preparation of spring box site
Figure2.9 Placement of rebar in concrete slabs
Figure3.1 Volumetric method
54 Figure3.2: float method
Figure 3.3 checking static water depths
55 Figure 3.4 measuring yield using volumetric method
56