Missing:
UNDERGROUND TANKS Training Notes by Prof. Bancy Mati 1.1 Ground Catchments and underground tanks 1.1.1 Ground Catchments Ground catchment is a broad term that describes all types of ground surfaces that serve as source of surface runoff used in water harvesting. These can range from open land surfaces whether paved or unpaved, including rocky areas, roads, home compounds, pasture lands, forests and other open areas. The sizes could vary from a few square metres for micro ground catchments to extensive land areas and whole watersheds used for large storages such as dams. Most ground surfaces are covered by soil, thus, water harvesting is hampered by losses due to infiltration, soil moisture storage, deep percolate losses, evaporation and transpiration by plants. Therefore, the runoff coefficient of ground catchments is low. Furthermore, ground catchments yield runoff that is laden with sediments, dirt, as well as other pollutants. But due to the vastness of available runoff, e.g. in watershed, they offer the best option for large scale water harvesting projects. Small/micro scale ground catchments are used to collect rainwater for domestic use at places where appropriate roofs are not available. Bigger catchments are used for collecting higher flows in huge storages, which can be used for large-scale water supplies and irrigation. The runoff coefficients of small catchments covered with soil can be improved as applicable by: Covering them with plastic or metal sheets, tiles or other non-toxic impervious material; and plastering them with cement. Such improvements are however relatively expensive and thus limited to situations where water is much needed. Treating/adding material to the soil surfaces to seal them and reduce infiltration. Such material includes cement, lime, clay soil, etc. The durability of such treatments is however limited due to erosion, weed growth and such other factors, thus need closer follow up and maintenance. When compared with roofs, ground catchments in general have the advantage that water is collected from larger area; this is particularly useful in regions where rainfall amounts are small. Ground catchments are particularly useful for harvesting water for agricultural uses, such as irrigation and livestock watering. They have however apparent limitations as follows. The water is easily contaminated. If the water is to be used for drinking purposes, it has to be boiled, chlorinated, or filtered say with slow sand filter. It may however be used for other domestic purposes without treatment. The water can only be stored below the ground surface such as ponds, dammed reservoirs, or sub-surface tanks where they normally need some kind of an abstraction device such as pumps; Ground catchments have low runoff coefficients; and The water is silt laden, and thus has poor physical qualities such as color, taste and turbidity. The excess sediment may need to be removed with silt traps. 1
Water collected from ground catchments can be stored in various types of structures such as dams, weirs, pans, ponds and underground tanks. This Chapter focuses on underground tanks. 1.1.2 What is an underground tank? An underground tank (or sub-surface tank), is a water storage structure constructed below the ground. The term also includes structures that are partially below ground. In most cases, underground tanks collect and store runoff from ground catchments such as open grasslands, hillsides, home compounds, roads, footpaths, paved and unpaved areas (Figure 1.1). However, in certain circumstances, roof catchments can also be channeled into underground tanks.
Figure 1.1 (a) Rainwater harvesting from (b) Road runoff harvesting into geohome compound with underground tank membrane lined tank (photo by Bancy Mati) Underground tanks are especially suited to homesteads having thatched roofs and other traditional structures or for areas where a roof catchment may not be feasible. However, it may be necessary to pump (lift) water, except where the ground gradient permits and where gravity outlets are constructed. Underground tanks can be designed as spherical or cylindrical and constructed using bricks. Since underground tanks get support from the surrounding ground, thus they can be built with less reinforcing material. Thus, underground tanks have lower construction costs and therefore, are more suited for storing agricultural water than surface tanks. 1.1.3 The case for underground tanks Compared with other ground storage structures, underground tanks are generally small, with volumes ranging 20 to 150 m3. They permit irrigation of small plots of land and livestock watering and are suited to smallholder irrigation as individual farm storages. A major limitation is the lack of adequate expertise at village level to design and construct underground tanks. Also, the design and construction faces more challenges of functionality, safety, water abstraction and possibility of failure as compared to surface tanks. However, underground tanks share much of the features of surface tanks regarding construction materials and methods. Water harvesting from traditional huts In the case of grass thatched or other traditional houses e.g. “manyatta”, where roof catchment with surface tank is not feasible, underground tanks can be used to supply domestic water as illustrated in figures 1.2(a) and (b). For grass-thatched houses, circular trenches can be dug and positioned to receive runoff directly below the roof eves and trench gutters dug around
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hemispherical traditional houses. The mitre drain from the trench acts as a gutter and should be positioned on the side or at the back of the house to be accessible. The mitre drains convey runoff into an underground tank.
Figure 1.2: Illustration of RWH system for (b) Illustration of RWH system for a grass-thatched house (Source SWALIM, 2007) hemi-spherical house (manyatta) 1.1.4 Advantages and disadvantages of underground tanks Advantages of sub-surface tanks: Underground tanks offer a cheaper to install due to its lower cost of reinforcements needed during construction, as compared to surface tanks. This is due because ground support provides the strength needed to hold the water They are appropriate in places where space above ground is limited; and they can be made larger than surface tanks The water is sometimes cooler Larger volumes of water can be stored. Disadvantages: Pump or some kind abstraction device (such as rope and bucket) is required to lift the water. Except where the ground gradient permits and where gravity outlets are constructed. Higher possibility of contamination and sedimentation sediment inflow. They cannot be easily drained for cleaning. If not well managed e.g. properly covered, they pose danger to children and small animals. Leaks or failures are more difficult to detect Tree roots can damage the structure from beneath Flotation of the tank may occur if groundwater level is high, and Heavy vehicles driving over a tank or other weight can damage the structure. 1.2 Types of underground tanks There are many types of underground tank, categorized according to shape, size, capacity, lining material, construction and utilization. The most common types of tanks include the following: 3
1.2.1 Cisterns A cistern is a small underground reservoir of about 10 to 500 m3 capacity. The term is sometimes synonymous with underground tank. Cisterns are indigenous water harvesting systems commonly found in the Middle East and other dry areas. They are normally used for human and livestock water consumption, and are mostly located at or near homesteads. In many areas, they are dug into the rock, or they could be constructed as underground tanks lined with concrete. In this system, runoff water is collected from catchments such as roofs, home compounds, rocky surfaces, roads or open areas. Stilling basins are sometimes needed to reduce sediment entry. Since the water is stored below ground. A lifting device, e.g. pump, bucket and rope is used to bring water to the surface for use. Other than domestic use, cisterns are also used for irrigation of small gardens. Cisterns require little or no space above ground, and thus are unobtrusive, which is a safety feature. The main problems associated with this system include the cost of construction, the cistern’s limited capacity, and inflows of sediments and pollutants from the catchment.
Figure 1.3 (a) Square cistern with cover (photo by Bancy Mati)
(b) Cylindrical cistern for surface RWH (adapted from Alemu Seifu, 2011)
1.2.2 Rectangular lined tanks One of the easier ways of constructing underground tanks takes the shape of a rectangle or square. However, most rectangular tanks have a trapezoidal profile in volume. This shape accords good storage with easy design and construction features, as builders use straight lines. The tank can be lined with geo-membrane plastics, concrete, bricks, and other water resistant material. Lined underground tanks have the advantage of applicability on almost any soil type (Figure 1.4). The design also makes it easier to roof or cover the tank with galvanized iron sheets, grass, polythene, wood or other material. The tank is particularly popular for runoff harvesting for agricultural purposes, especially supplemental irrigation of small plots.
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Figure 1.4 (a) Open underground tank lined (b) Roofed rectangular underground tank with geo-membrane (photo by Jean Muhinda) (photo by Bancy Mati) 1.2.3 Housed excavated tanks In areas prone to high evaporation losses, housed excavated tanks (figure 1.5) and used for rainwater harvesting from the same roof or compound. Such water tanks can hold enough water to serve as the main source of water supply to a household or irrigation of small gardens. Excavated earthen water tanks can be lined to reduce seepage. The lining materials include clay, ferrocement, or plastic sheeting. Sandy soils/earth can also be excavated and lined with 15 cm concrete and serve as water tight tanks. The method is good for hot areas, and provides security since the house can be locked.
Figure 1.5 Housed excavated tanks within mud-walled rooms for RWH with polythene outlet (photos courtesy of MoANRM, 2010) 1.2.4 Concrete (reinforced) underground tank Underground tanks lined with concrete are used for storage of water for domestic use and agriculture including irrigation (Figure 1.6). They can store water from harvested rain or from river diversions and other sources. The tanks are usually rectangular shaped and can vary in size from a few cubic metres to about 5,000 m3. The larger tanks are built with reinforced concrete. Some of these tanks are usually covered with a concrete slab which can also serve as a catchment area for rainwater harvesting. Sometimes the catchment area, ranging about 750 to 1.000 m2 is paved with concrete to induce more runoff. In the absence of concrete, the catchment area can be graded and compacted to enhance runoff. These systems are used in areas with as low as 100 mm of annual rainfall.
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Figure 1.6 (a) Cylindrical reinforced (b) A rectangular reinforced concrete semiconcrete tank (photos by Bancy Mati) underground tank for communal use
1.2.5 Hemispherical tanks Hemispherical tanks can be built almost anywhere since they are usually lined with of ferrocement, clay or plastic sheeting. The shape is hydraulically efficient, but the construction can be complicated and requires expertise to achieve the correct curvature. Hemispherical tanks may be roofed and covered for safety and to reduce evaporation losses. Like with other ground catchment systems, sedimentation traps are necessary and some system of water abstraction. Hemispherical ground tanks can also be built of burnt bricks that are reinforced with chicken mesh nailed onto the interior of the tank (figure 1.7).
Figure 1.7 Hemi-spherical tank under construction (Source, MoANRM, 2009)
(b) Roofed hemispherical underground tank with silt trap (photo by B. Mati)
1.2.6 Spherical underground tank Spherical tanks take the shape of a ball and are considered the most hydraulically efficient shape for tank design. They are commonly used as underground tanks but good workmanship is required. Spherical underground tanks can be fully buried or partially underground (Figure 1.8). They are usually made of concrete, burnt bricks, masonry or other strong material. The main limitation with
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spherical underground tanks is the high labour demands and expertise to excavate the site and build the structure.
Figure 1.8 (a) Spherical underground tank (b) Spherical underground tank-nearing under construction (photos by R. K. Cherogony) completion 1.2.7 Cylindrical tank A cylindrical underground tank is made much the same way as in surface tanks. However, it requires a lot of digging to achieve appreciable storage volume (Figure 4.9). Generally, cylindrical tanks are made of concrete or bricks and have good hydraulic properties. They are easy to construct using local labour and the construction material can be minimized as compared to surface tanks.
Fig. 1.9(a) Brick lined, open cylindrical tank (b) Cylindrical tank partially buried (the (photos by Bancy Mati) visible pipe is for overflow) 1.2.8 Birkas A “birka” is a small underground tank, about 100 to 200 m2, usually used for water harvesting and storage (figure 1.10). It is an indigenous water harvesting technology in Ethiopia, and is usually family-owned and are thus located in home compounds. Birkas are used for animal watering and irrigation of small vegetable gardens. They are dug in a convenient depression and have walls lined with concrete, stones and mortar or impermeable clay tile. Birkas have different shapes such as circular, square or oblong. Collected runoff flows into a birka from the roof and surrounding areas. The main problem is the heavy labor demanded for excavating the 7
foundations. Also, there is the need to pump (lift) water except where the ground gradient permits gravity outlets. There is also the higher possibility of contamination and sedimentation, although the latter can be reduced by providing adequate siltation basins.
Figure 1.10 A birka for RWH with cover (photos by Bancy Mati)
(b) Inlet into birka through a buried pipe
1.2.9 Sausage tank A sausage tank is another name for a cylindrical tank which is constructed lying horizontal in the ground (Figure 1.11) and thus it is “sausage-shaped”. This arrangement saves labour since the depth of excavation is greatly reduced. Construction is also made easier. The length of the tank can be made longer than would be possible in a birka, whose cylindrical tank is vertical. Also, since the depth of the tank is shallower, it is possible to design a sausage tank with gravity outflow where land slope permits. The design is particularly sited for agricultural water since on sloping land, it can accord gravity fed irrigation.
Figure 1.11 (a)Sausage tank under (b) The completed sausage tank covered with construction (photos by Bancy Mati) soil with just the opening showing 1.2.10 Berkads A berkad is a cuboid-shaped ground tank, usually lined with masonry and/or concrete, which collects surface run-off during intense rainfall episodes. They also receive runoff water from upstream ground catchments via mitre drains or stone bund diversion and silt traps. Berkads are common in parts of the Horn of Africa and are used mainly for livestock and domestic use. The average size of berkads is larger than birkas, with volumes ranging 300 to 750 m3.
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A berkad consists of an inlet canal, masonry lining and a live fencing (figure 1.12). At the entrance of the berkad, stairs are constructed to facilitate access inside the tank. Fencing is done as a security measure for water conservancy or against dangers to livestock and human beings who may fall and get injured, sometimes fatally. Since berkads collect surface run-off, the risks of contamination is high. However, in some cases, silt traps are provided to check siltation. Also, many berkads are exposed and not covered by a roof, thus they have high evaporation losses.
Figure 1.12(a) Illustration of a berkad with conveyance(b) A typical berkad with fence system (Source: SWALIM, 2007) (Source, De Hass, 2010)
1.2.11 Partially below ground tank The partially below ground tank incorporates the merits of both above and below ground tanks in one simple, low-cost design. It is built as a hemi-spherical underground tank of ferrocement extending above ground level with a cylindrical wall made of blocks or stone masonry (Figure 1.13). It is suited for runoff harvesting from roof catchments which are too short to permit constructions of a surface tank. A partially below ground tank also takes advantage of the support given by the soil, to do away with the need for a structural component below ground level. At the same time protection is given against contamination by surface runoff and damage by vehicles is an added advantage. Water is drawn from the tank by either a hand pump or buckets. If the water is lifted out of the tank, some wastage may occur, but the tank can also have a tap outlet by excavating the water collection area.
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Figure 1.13 (a)Partially below ground tank (b) The completed partially below ground under construction (photos by Bancy Mati) tank (spherical) with roof catchment
1.3 Design of underground tanks 1.3.1 Major factors considered Planning is the first step and helps identify sites for suitability of water-impounding structure and other basic characteristics. Given water requirements, catchment/improved catchment runoff/yield, evaporation and leakage over a design period, calculate structure capacity, shape and dimensions. The layout of the water storage structure, catchment, inlet, outlet and safe overflow disposal are as shown in figure 1.14. The major factors considered in the design of underground tanks include:
Figure-1.14 Sketch of an underground tank arrangement (source: Gould and Peterson,1999)
Site characteristics The first step is to find a suitable site for the underground tank, pan or pond. The site of an underground tank should be appropriately located in terms of adequate catchment size, some gradient and positioning of the tank itself. The site should be on soils such as clay that retain water. Avoid sandy soils unless the pan will be lined to control seepage. The reservoir site should be a natural depression or small valley so as not to dig too deep to achieve required volume. A good source of runoff should be identified such as home compound, hillside, road or dry watercourse as close to the pan/pond as possible. The area from which water flows into the water pan should be have natural vegetation or grass, to minimize erosion and sediment deposits in the pond.
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A good site has the following characteristics: Close enough to the dwelling to avoid long lengths of guttering and downpipe (some suggest siting the tank mid way along the length of a building to reduce gutter size– this is fine if water from one side of the building only will be fed into the tank) Reasonably flat where possible – otherwise the ground will have to be levelled before marking out Away from areas where surface water will gather (i.e. depressions) Away from trees – the roots of trees can be problematic Away from areas where animals will wander – or else the tank should be fenced off Not so close to the dwelling that the foundations of the dwelling are undermined Somewhere convenient for extracting water e.g. close to the kitchen area The ground should be suitable for digging and for siting such a tank. There should be no large stones, bed rock or sheet rock close to the surface, and one should be sure that the groundwater table in the area is several meters below the bottom of the tank. This information can often be gleaned from locals who may have tried digging wells, sinking boreholes or digging garbage pits. Shape of the tank: Decision whether it will be circular, rectangular or square with consideration to having the embankment/walls constructed continuously around structure to reduce evaporation, control inflow and exclude paddock debris generated by severe storms. Consider the use of circular structures, as this shape has the smallest surface area for evaporation to affect. Depth of tank: should be adequate for provision of sufficient water supplies allowing for evaporation loss. Depth should be greater than annual evaporation, or greater than total evaporation for chosen design period if structure is for drought-proofing. Side slopes (batter ratio) of the retaining walls - usually, this should be 3:1 Freeboard: should be a minimum of 1 m above maximum water level. Overflow arrangements: The crest to be set at the maximum water level for the impounding structure. Where a silt pit is installed, overflow can be set out from the silt pit. Overflow to discharge clear of walls of structure. Where there is a risk of crest erosion, materials other than earth can be used — flumes and chutes being potential applications. Silt traps and inlets: These are very important as they affect the water quality and storage capacity. Silt traps are necessary in nearly all types of ground catchment structures including ponds and pans. They are particularly crucial where catchment to trap debris or other eroded materials. Many designs of silt traps exist (figure 1.15) either made in the form of drop structures, gravel pits, grassed or soak pits or by constructing a meandering entry channel into the tank.
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Figure 1.15 (a) Drop inlet structure for silt (b) Silt trap made of concrete sections control (photo by Jean Muhinda) (photo by Bancy Mati) 1.3.2 Determining harvestable water The assessment of harvestable water follows the same procedures as described in Chapter 2. However, the design capacity or volume of an underground tank can be either supply or demand based. In a supply-based approach, the equations that predict the discharge from the catchment are used. A demand-based approach, utilizes the expected water consumption and is matched to the dry periods for which this water shall be needed as well as the daily water requirements for domestic use, animal watering and irrigation. Three methods are commonly used as described below. Water balance method This is a month-by-month accounting method whereby inputs of rainfall are compared with consumption (and overflow if it occurs). It is assumed that the balance remains in the tank. Any size of tank can be considered using this approach. If a tank is too small, some rainfall will be lost as overflow and the demand may not be met. If it is too big, it may never fill completely. The choice of appropriate size in a given situation is not easy and the water balance method is not really as simple as suggested because the probability of getting a certain amount of rain needs to be considered along with many other factors. Computer modelling can help in decision-making. Cumulative supply or demand method This method involves calculating the cumulative supply and cumulative demand on a month-bymonth basis and showing them as lines on a graph. The maximum difference, during a 12-month period, between the cumulative supply line and the cumulative demand line indicates the optimum size of the tank.
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Dry season storage method This is the simplest and most preferred method for determining tank size for a RWH catchment. In this method it is necessary to estimate the longest period during the year without rain and estimate the daily consumption rate. The size of the tank that will be required is the number of dry days multiplied by the daily water use. However, if the volume of run-off estimated from the roof during the rainy season is less than the proposed tank size the latter is reduced accordingly. 1.3.3 Determination of storage capacity required Water Demand Water demand is the volume of water requested by users to satisfy their needs. A simplistic interpretation considers that water demand equals water consumption. However, conceptually, the two terms cannot be equated because, in some cases, especially in rural parts of Africa, the theoretical water demand considerably exceeds actual consumptive water use. Calculating Demand Calculating the water needs of the user is relatively easy and involves a simple formula which includes the average daily consumption of water from the tank per person (or livestock), the number of days in the dry season, and the numbers of people using the tank. Studies have shown that rural people with tanks next to their houses often use about 20 to 40 litres of water per person per day. This is high compared to people who must walk long distances for water who may use less than 10 litres per family per day. As an average, assume that each person will take 20 litres per day if it is a household tank, and 5 litres per day if it is a school or health centre tank. The formula is; Demand (litres) = Total dry days x water required per person x total number of consumers It is good to note that if the consumption is higher than estimated, the tank will run dry before the next rainy season. Volume of storage structure This is calculated differently depending on the shape of the tank. The general formula adopted for all shapes is the prismoidal formula expressed as follows: V = (A +4M + B)d/6 where: V = volume A = top area of excavation (area of water surface when full) B = bottom area of excavation (area of floor) M = area at ½ depth d = depth For convenient calculating, the following derivations of the prismoidal formula can be used for each particular excavation shape:
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Circular: V = π[R2 + (R x r) + r2]d/3 Rectangular: V = [(L x W)+(lf x wf)+[(L x lf)+(W x wf)]]d/6 Square shaped tank: The design volume is calculated with the following equation: V = [L2 + (L x lf) + lf2] d/3 where in all formulae: V = volume (m3) R = radius of water surface (m) r = radius of floor (m) d = depth (m) π = Pi or 22÷7 or 3.14159 L = length of water surface (m) W = width of water surface (m) lf = length of floor (m) wf = width of floor (m) d = depth of water from surface to floor (m). Treatment of Catchment area The catchment area can be made to yield more runoff by paving it with various materials. Concrete, plastic sheeting, butyl rubber and metal foil can also be used to cover the soil for rainwater harvesting. Gravel may protect the underlying membrane against radiation and wind damage. Also, wax, latexes, asphalt, bitumen, fiberglass and silicones can be used as sealants on soils which do not swell with moisture. Plots treated with sun-melted granulated paraffin-wax yielded about 90 percent of the rainfall as runoff, compared to 30 percent from untreated plots. In some cases, even when the soil contains a significant amount of clay and fine materials, excessive water losses may still occur due to well developed soil structure or arrangement of the clay particles. Applying small amounts of certain chemicals to the porous aggregates may result in rearrangement of the clay particles. The process is called de-flocculation. This dispersed or dissociated structure reduces soil permeability. The chemicals used are called dispersing agents. 1.4 Construction of underground tank Several of the techniques used for building surface tanks can also be used for underground tanks. For these the tanks are constructed in excavations with the soil being back filled around the outside of the tank on completion. Where impervious soils exist, such as clay or loess, it is often possible to construct unlined sub-surface reservoirs. Invariably these suffer from problems of seepage evaporation and poor water quality.
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1.4.1 Procedure Excavation of underground tanks is carried out in two ways i.e. “hollowing out method” and “open excavation method”. Hollow-out way of excavation is adopted only in areas where the sub-soil is firm enough and involves digging by starting at the top of the ground and hollow out/making wider the body of the cellar when gets down . Whereas the open excavation method is adopted in areas where the sub-soil is relatively weak and more loose, to avoid pit collapse when digging in. Casting (construction) of the top roof structure in the case of open excavation method will be undertaken after the construction of the tank is completed. The tank, constructed in areas with firm sub-soil can be plastered by using clay mud or mud mortar or cement mortar for seepage control. But if the tank is constructed relatively in area with weak and looser soil lining by using clay mud or cement mortar (thin wall) is not effective. In places where the soils are clayey, and impervious, it is possible to build unlined sub-surface tanks, but they suffer from seepage, evaporation and poor water quality. The following steps can be followed steps (i) Prepare site by pegging and referencing corners (square and rectangular shapes) or structure centre (circular shape). Measure fall across site for calculation of any storage volume above excavation. Install a temporary bench mark in a protected location. (ii) Remove topsoil and stockpile clear of embankment location. (iii) Excavate core trench under embankments (walls) if pervious materials are present under topsoil. Core trench must extend 1 m into impervious material. (iv) Build the embankments (walls) by excavating in ‘floors’ and pushing material to correct location (figure 1.16). Compact embankment with bulldozer weight in 50 to 75 mm layers or compact 150 mm layers with a sheep’s foot roller. Embankment side slope ratios can be confirmed by using an electronic builder’s slope finder or battometer. Install inlet and outlet pipes early in construction of embankments.
Figure 1.16 (a) Stone-pitched trapezoidal (b) Plastic-line tank with trapezoidal underground tank (photos by Bancy Mati) profile
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(v) Construct the roof of the tank (figure 1.17) (vi) Construct the overflow and ‘final trim’ structure. (vii) Topsoil outside batters and embankment top using stockpiled topsoil. Topsoiling encourages vegetation and helps retain embankment moisture and resist cracking.
Figure 1.17 (a) Roofing an underground tank (b) A roofed rectangular underground tank (source: Danida, 2007) (photo courtesy of Tewodros Teshome)
1.4.2 Choice of construction materials A good control over the quality of construction materials is a first important step in the construction of successful water tanks. Cost and availability are as well important initial considerations. Construction materials constitute a considerable proportion of the costs involved. It would thus be sensible to make use of available local construction material, such as bricks (figure 1.18) to help in cost saving; these include various types of soils, gravel/pebble, rubble stones and boulders/rocks.
Figure 1.18(a) Cross-sectional of a hemi-spherical (b) Hemispherical underground tank tank (adapted from Nissen-Peterssen and Lee, 1990) built with bricks Source: Danida, 2007)
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Cement Cement is packed in paper bags of about 50 kg and have a volume of 37 liters per bag. Cement contains principally lime (CaO) and silica (SiO2); it becomes plastic shortly after contact with water. After a couple of hours, a chemical reaction stiffens or sets cement to stone hard material, which bonds well with sand, aggregate and iron/steel, provided it is well cured for a minimum of three weeks., It is necessary to maintain the right proportion while mixing cement with other construction material. Water proof cement Water proof cement helps in sealing tanks; but it dries too quickly in hot and dry climates making fine cracks in the sealing coat. An alternative to water proof cement in such climates is a material called Nil. Nil is made by mixing cement with water to form a thin paste (cement slurry). It is applied to the final layer of plaster with a square steel trowel on the same day the plaster is applied. Nil is a cheaper option as well. Sand Sand is an important ingredient used for making concrete, mortar, blocks, etc. The main requirement on sand is that it should be free from organic or chemical impurities that would weaken the mortar/concrete. Sand should always be sieved before mixing, to remove organic materials which rot in tank walls and other parts of the structure. There should be a reasonable proportion of all grain sizes, without an excess of both fine or course sand particles. course sand with particle sizes of 1-4mm is the most suitable for concreting foundations and flat roof slabs; finer sand is useful for mixing mortar for plaster. Most clean sands are suitable for use in RWH structures. Aggregates Another name for aggregate is crushed stone; it is used for making concrete. The size aggregate should be 8-32 mm; it should be very hard with rough surface for a good bonding with other material. Porous, soft or easily weathered stone should not be used for aggregate; and as with sand, aggregate should be free from soil and organic matter. Water Although water may not have to be necessarily very clean for mixing with and curing cement, saline water should never be used for construction. Note as well that considerable amount of water is needed for construction and curing, and mostly it needs to be transported by women from far places. When construction is completed, some water should be filled in the tank to help in the curing process. More will be said in 6.2.2 regarding the ratio of water that should be used while mixing it with other construction material and for making ferrocement. Reinforcement There are different types of steel reinforcement mesh, which consist of thin wires either woven or welded in to a mesh. The main requirement on meshes is that they are handled easily, and that they are flexible enough to be bent around sharp corners. The wires should be tied and held firmly in place while the mortar is being applied/trowelled. Generally, various types of reinforcement 17
wire can be used such as weld mesh. BRC (No. 65), galvanized wire 3mm, barbed wire (guage 12.5), twisted 12 mm iron bar, and chicken mesh (25 mm). Cement Based Mixtures The preparation of a high-quality mixture of mortar/concrete from cement, sand aggregate/crushed stone and water is one of the most important stages in building water tanks. The following the very basic and crucial rules for mixing and applying mortar and concrete. If neglected, the strength and water proof properties of tanks are greatly reduced, leading to cracks and leaks. Cement, sand, aggregate and water should be mixed thoroughly well, without adding too much water; Mortar/concrete should be applied while fresh, within half an hour after mixing; and The cement work should be cured properly by keeping it moist and under shed for at least three weeks. Mortar and concrete should be mixed with the right proportions, which varies depending upon the tank component. It is necessary to mix cement and sand alone first (for mortar and aggregate as well for concrete), before adding water. Water should be added to the dry mix when everything else is ready for mortar/concrete application. This helps in making use of the cement mixture while it is fresh, and also avoids the extra heavy work of unnecessarily mixing water right at the start with the other ingredients. Water should be kept to a minimum, just enough to make the mortar/concrete workable. The lower the water content the higher the strength of the mortar/concrete. If too dry and stiff however, it will be difficult to work on to the formwork to achieve full compaction, and is likely to contain air voids and be imperfectly bonded to the reinforcement. A good mortar should be moist and never wet, that it spreads out like porridge, but have the consistency of mashed potato, water should never be visible and should not look shiny in the mixture. Ideally this should be for a 1:3 cement: Most sand mix ratio of mortar, a water to cement weight (not volume) ratio of 0.5:1 will be satisfactory. Under most conditions, and owing to varying reasons however, workability needs to be controlled by eye during mixing. Pumps Pumps are usually needed to lift water from underground tanks. They can be small motorized or manual pumps operated by hand or treadle. Pumps however need better management for operation and maintenance/repair which is generally not easily available, especially in rural areas. Communally owned tanks are more prone and complicated in this regard, than those individually owned. The advantage of pumps is however that when they fail/break water is not drained, and the tank emptied as is generally the case with gravity fed taps. A, rope and bucket can be used to abstract the water which requires to be cleaned out.
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Fencing Tanks should be fenced with suitable material, e.g. barbed wire or life-fence. A lockable door should be provided to exceptionally avoid the reach of domestic and wild animals and small children. Holes underneath made by burrowing animals, e.g. moles, should also be checked. Water take off from underground tanks should use low cost water lifting devices e.g. rope and washer pump or other manual and small motorized pumps. 1.5 Seepage control methods in underground tanks Lining with cement mortar, mud mortar or clay mud is applied when the sub-soil is firm and dense enough to keep the tank stable. It is usually thin lining. It can be kept stable in an excavated vertical section with a height up to 10 m without any support. The underground tank can work with thin wall. The stability of the structure mainly depends on the soil and role of the thin lining is to prevent from seepage. Depending on the firmness of the sub soil, the side slope is utilized varying from zero slope to 1:10 (horizontal to vertical) to stabilize the wall of the tank To make the seepage control activity more effective leave earth of the last 3-4 cm at the outer diameter of the tank unexcavated and to be compacted by wooden hammer to increase the density of the soil by which the cement mortar will be pasted. This is good for improving anti-seepage effect and strengthens the combination of soil/cement mortar, mud mortar and clay mud.
Lining with clay mud When cement is not affordable, clay may still be an option, for seepage control. Application of clay for lining involves (i) Dry up the clay; (ii) Crush the clay into fine particles by screening; (iii) Add water in proper quantity and mix hard; and (iv) Press the clay mud by hands or feet and squeeze it into dough-like mix. Cement mortar is common material for lining underground tank. It is more expensive than the clay mud, however it is less permeable. Clay grouting Clay grouting involves applying a clay blanket to cover the entire surface of the tank or pond over which water is to be impounded. The material for grouting should contain at least 20% clay particles by weight. The clay material should be at optimum moisture content and spread uniformly in layers of 15 cm to 20 cm thick, with each layer being thoroughly compacted before the next layer is added. There should be suitable clay borrow site that is close enough to get clay soil at reasonable cost. The minimum thickness of the clay blanket should be 30 cm for all depths of water up to 3 m. If the water depth is greater than 3 m, the thickness of the blanket should also be increased proportionally. Generally it is recommended to increase this thickness by 5 cm for every 30 cm of water exceeding 3 m depth. Clay blankets may require protection from cracking that may results from drying and from rupture caused by freezing and thawing. After completion of the clay-sealing, it is recommended to spread a cover of gravel 30 cm to 45 cm thick over the blanket. The method is suitable for underground tanks and ponds which have high percentage of coarsegrained soils but lacking sufficient clay to prevent excessive seepage.
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Sealing with Bentonite Bentonite is clay with a high shrink swell ratio. It is fine-textured colloidal clay, when wet; absorbs water several times greater than its own weight and, at complete saturation, swells to as much as 8 to 15 times its original volume. This action tends to seal soil that lack clay-size particles. Therefore, adding bentonite is another method of reducing seepage in soils containing high percentage of coarse-grained particles and not enough clay. Bentonite, like clay blankets, must also be protected against drying. Since it returns to its original volume when dry, bentonite is not recommended for sealing ponds with wide fluctuations in water levels. If considerable time elapses between applying the bentonite and filling the pond, protecting the treated area against drying and cracking may be necessary. A mulch of straw or hay pinned to the surface gives this protection. Lining with stone masonry/bricks Tanks are usually lined with stone masonry, bricks, or concrete. This involves using a concrete foundation and then building the concrete wall within the excavation. To ensure a water tight structure, it is very important to lay the masonry in a proper way. A layer of stone/bricks is laid and mortar is poured on top of the first layer of stones. The mortar should be pressed into all the voids between the stones to ensure a dense masonry. Then the next layer of stones is laid as before. It is important to make sure that the joints between stones should be arranged in an alternate manner and ‘straight’ joints in both vertical and horizontal directions should be strictly avoided. Once completed, soil is squeezed firmly in the space between in the outer space created during construction to ensure good fit with the ground. The tank can be roofed or covered with grass or canvas (figure 1.18).
Figure 1.19 (a) Underground tank lined(b) Underground tank lined with stone masonry and with concrete (photo by Bancy Mati) covered with canvas (Source; MoANRM, 2009)
Plastic geo-membrane linings Another option is to use plastic geo-membranes (figure 1.20). These are specially made plastic linings used in dam construction. Normally, a geo-membrane is made to measure in a factory and the tank is constructed to fit its dimensions. Its cost varies, being cost effective in some countries. 20
The advantage of plastic geo-membranes is that they can be installed by the user easily. Also, if well protected, they control seepage quite effectively, and can be used on almost any soil. A major limitation is the shorter lifespan, which rangers 5-10 years. They can also be easily damaged by agricultural equipment.
Figure 1.20(a) Plastic geo-membrane lining (b) Geo-membrane lining for RWH ready for sale (photos by Bancy Mati) tank Care is needed while installing geo-membrane linings to ensure that there is proper handling and storage i.e. transporting of the sheet in rolling pattern than sharp folding. Also, the surface of the pond should be smoothened to remove any piercing materials. Proper anchoring of the geomembrane in the trench at the top edges. Fencing of the pond in order to protect it from animals and children. Advantages of flexible geomembrane Compared to other sealing methods flexible geomembranes have the following advantages. They can be applied to various soil types (i.e. fine, medium and coarse course textured soils). The cost is getting lower as their popularity grows with more manufacturers and suppliers. Geo-membrane covers can be transported easily to place of use. If punctured, the membrane can be easily repaired by farmers themselves or local practitioners (by using used plastic products, heat, gluing using adhesives of bicycle inner tube maintenance techniques). Also, seepage losses can be completely reduced. 1.6 Water quality Although ground catchment systems are sometimes used to collect rainwater for drinking purposes, it is strongly recommended, where possible, that this water should be treated by e.g. boiling, chlorinated or passed through a slow sand filter before being consumed. Natural treated soil or compacted surfaces may form suitable catchment surfaces, although excess sediment may need to be removed from the harvested water. Due to the low runoff coefficients of many natural soil surfaces, especially where the slopes are small, various techniques have been developed to increase the amount of rainwater runoff. These basically involve three approaches – covering treating or compacting of the surface.
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The treatment of catchment surfaces should be done so as to reduce infiltration and hence increase the runoff from natural surfaces. Among the materials added to soil surfaces to try to seal and reduce infiltration are cement, butyl rubber, lime, paraffin wax oil, bitumen and asphalt. Sodium salts may also be used to encourage crusting in soils containing clay is of another approached that can be used. The compaction and shaping of natural soil surfaces using machinery to form catchments made of a series of cambered roadways known as roaded catchments. These catchments feed parallel drains, normally leading into a single surface reservoir. Although not generally used to provide community supplies, due to the poor quality of the water for domestic purposes. The potential for developing roaded catchments exists wherever road construction and earth-moving machinery are available. 1.7 Operation and maintenance As with other water harvesting systems, underground tanks require proper operation and regular maintenance. There should be regular checks on the tank for seepage, cracking and piping and movement cracks within embankment. Also, the side slopes, inlets and outlets should be inspected any damages. If present and attended to early, most of these problems can be treated. Regular cleaning of inlets, silt pits and the tank itself of any of debris and eroded soil material should be done. Vermin can burrow into inlets, outlets, embankments causing damage. Burrows should be dug out and repacked with clay. Vermin around the structure should be eradicated. The whole area around the tank should be fenced to improve safety.
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