It seemed evident from visual examination of the soil at the site and possible sources of fill that could be imported to the site that there was no material available that would be satisfactory for the construction of water-retaining ponds.
This conclusion was born out by the results of the two soil test pits described in the FAO feasibility mission report mentioned previously and later confirmed by the Public Works Material Testing Division borings and laboratory tests designed by the consultant (see Section 3 on Soils Engineering).
Various methods have been used on projects elsewhere to make pond soils impervious in order to make their use practical and economical for pond construction. Methods considered by the consultant, with consultation with the aquaculture biologist and the soils engineers, included chemical soil methods (bentonite), use of soil cement, importation of clay for dike cores or lining, concrete and gunite linings, spray-on industrial membranes, nonwoven geotextiles and various impermeable plastic linings.
For various reasons, most of the methods considered were discarded as being either inappropriate, not compatible with fish culture, not practical from a construction standpoint or too expensive, either in first cost or maintenance. Plastic liners seem to be the best.
Plastic liners are available in many grades and thicknesses, are fish-compatible, can be furnished prefabricated to size thus eliminating field joining, and, when properly installed and protected, will give long, maintenance free performance. The liner which seems to offer the best technical solution at a reasonable cost is polyvinyl chloride (PVC) unreinforced lining. Various manufacturers have grades of lining which they certify as “fish-grade” that will produce no toxics detrimental to marine fish culture. Although the normal liner materials are available in thicknesses from 10 to 30 mm or over, a 15 mm thickness has been specified for these ponds for its strength and resistance to tear and puncture during installation by unskilled labour and for its medium price range.
Because of the small size of the ponds, it is assumed that a factory sealed liner can be fabricated in one piece and delivered folded accordingly for ease in installation.