Suitability of Silt Deposits of Irrigation Tanks in Angunakolapelessa, Sri Lanka for Engineering Applications

Dry zone of Sri Lanka is the main contributor to the rice supply in the island nation. Since this zone is prone to long droughts, a large network of agricultural tanks had been built over 1500 years ago to overcome the adverse impacts of the droughts experienced in the zone. Due to poor maintenance and negligence, there is siltation in some of these tanks and as a result of this the water storage of the tanks is not at the optimum level . This study was focused mainly on ten such tanks situated in the Angunakolapelessa Divisional Secretariat Division in the Hambantota District . These tanks require dredging to be at their optimum capacities. It has therefore become necessary to investigate the possible uses of dredged material , i.e., for erosion mitigation, since ad-hoc dumping of erodible dredged materials would in turn adversely affect the capacities of the other tanks located downstream of the cascade. Thirty soil samples were collected from a depth of 300 mm and laboratory tests were conducted on them to study their soil properties. Particle size distribution and Atterberg Test results show that the samples contain mostly clay-silt sediments and according to USCS and USDA classifications, most of them can be classified as silty loam. Standard Proctor Compaction Tests conducted on the samples indicate that the soils have a low maximum dry density and a high optimum moisture content which indicate that the soils tested cannot be recommended for use in embankment/road construction. However, these soils could be reused in erosion barriers/berms erected in agricultural fields. Proper management of irrigation tanks avoiding siltation caused by erosion is recommended to ensure food security and sustainable water management.