Natural or artificial ground catchment and Lined sub-surface tanks

These are natural, artificial or modified catchments that have low to relatively high runoff coefficients. Water from these catchments is captured and stored in lined sub-surface reservoirs excavated below ground level. The reservoirs are known by different names (berkeds in Somaliland, taankas in India, hemispherical sub-surface tanks in Kenya – also included in this category are excavated water cellars such as the shuijiao in China) and have been lined with many different materials. These tanks normally have a larger depth to surface ratio compared to open ponds and their scale means a roof of some description is a possibility. When the lining is constructed well, there will be no leakage, and water will either evaporate or be abstracted. These tanks are often privately-owned by one or more families, but can be communal.

Suitable conditions

• Site in a place where run-off is seen to flow after rains.
• Artificial catchments are created where infiltration of runoff zone is high.
• Care should be taken when siting in clay areas, but the type of clay is more important.
• Do not site tanks near big trees whose roots might crack the walls.
• Do not site tanks where heavy vehicles will pass close to tank wall.
• Do not site sub-surface tanks in areas of high water tables to reduce risk of flotation.

Construction, operations and maintenance

The reason for constructing a sub-surface tank is to retain the water. Therefore one of the most important aspects is that seepage and cracks must be avoided. Therefore good quality construction work with adequate supervision is vital to create a sound structure – this is especially important in areas with swelling soils that can affect the integrity of the lining. While ownership and management of tanks is important, such privately-owned tanks have often failed due just to the technical construction component. Construction materials vary and include the natural soil formation itself, clay, stone masonry, bricks/cement, ferrocement, anthill/lime/cement and plastic/rubber lining. Material may affect cost but choice may also depend on what is available and the type of surrounding soil. To generally prevent cracking/seepage:

• Round tanks are inherently stronger than rectangular ones. Hemispherical and cylindrical designs are commonly used.
• Type of tank will vary depending on the swelling ability of the surrounding soil – a problem in clay areas, but type of clay is more important – montmorilonite, calcium-containing clays (in marls/gypsum sediments) and black cotton soils are all prone to swelling and can crack sub-surface tank walls that are not built robustly enough. Therefore it is important to construct the right type of tank for the area. When in doubt, avoid making sub-surface ferrocement or anthill/lime/cement tanks in unstable soil.
• Admixtures can be added to the concrete mix in order to reduce the amount of water needed. Research has shown that superplasticizers work best by reducing the amount of water that needs to be added when mixing concrete, which results in 35% less shrinkage. The resulting end material is stronger and can reduce the amount of micro cracks in mortar by half compared to normal mortar while resulting in 76% fewer leaks. In general, the amount of plasticizer to be added should not be greater than 2% of the dry material weight. A plasticizer that can be used that is possibly available is household washing up liquid. In hot climates though, more research is needed in the field application of plasticizers, since the reduction of water used (and increased strength of product) may not be that great due to more water needed to prevent drying out between mixing and

application.

Key construction issues for good workmanship (which also relate to preventing cracking/seepage) and costs for specific lining types are detailed below:

• Stone Masonry:

1. In Somaliland it cost between $39 - $43 per m3 of storage for a new berked and $8 per per m3 for a rehabilitated berked, excluding about 30-45% of local contribution (e.g. 493m3 new berked = $19,550; rehabilitated existing berked = 4,000 USD). In India, stone masonry sub-surface tanks cost $28 per m3 of storage (35m3 tank cost $990).
   
2. Floor to be made from concrete which needs to be laid with vibration in order to be sure they are leak-proof.
3. In clay areas, be sure to build the tank robustly enough to resist cracking. Sample dimensions & mixtures for walls and floor for stone masonry tanks in an area of swelling clay are: Walls: 0.4m wide, 2 blocks thick Floor leveling mixture: 0.05m thick, ratio 1:4:6 (not used in rehabilitated berkeds as level floor foundation already exists) Floor: 0.16m thick, unreinforced concrete, ratio 1:21⁄2:4