* 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 andapplication.
Key construction issues for good workmanship (which also relate to preventing cracking/seepage) and costs for specific lining types are detailed below:
#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). <br>
#Floor to be made from concrete which needs to be laid with vibration in order to be sure they are leak-proof.
#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
