The initial phase of construction concentrated on preparation of the pond dikes in anticipation of the arrival of the plastic liners.
Prior to the consultant's arrival the pond site had been brought up to the proper elevation and embankments of sandy fill were placed and compacted in the approximate configuration of the ponds. Considerable work remained in the shaping and sloping of the ponds after the consultant arrived, which considerably delayed the placement of the liners and partly invalidated the original schedule. The dike fill contained a large percentage of stones which had to be removed from the ponds or covered with finely crushed material and compacted. This operation took ten days and was also hampered by the contractor's initial insufficient and inconsistent supply of labourers and machinery. In addition, high wind conditions slowed the placement of the fine underliner material. Some of this delay could have been avoided if the department responsible for overseeing the pond dike construction (Public Works Department) had been more diligent in their inspection of the work.
Due to the porous nature of the soils in Bahrain it is necessary to line ponds with a water impervious membrane. Selected as the least expensive and most suitable material was an unreinforced polyvinyl chloride (PVC) plastic. This liner material is produced in rolls that can be factory solvent welded and shipped as one piece to the site. For this project, two liners, 20 × 45 m each (620 kg each) and 20 mil (0.65mm) thickness, were ordered from Staff Industries Inc. (see Appendix 2).
Once on site the liners were easily installed. Installation occurred at night when it was cool and there was no wind. The liners were accordion-folded in the shipping crate so each could be first extended the length of the pond in a 1.5 m strip and then opened widthwise to cover the pond bottom and sides. The liner was stretched along the berm to remove any wrinkles. The liner was held in place by burying its edges in a trench (30 cm wide × 30 cm deep) previously dug along the perimeter of the pond. This operation took two hours per pond and required 15 men. The PVC drainpipe penetrated the liner through a single slit cut in the appropriate location. The liner was solvent welded to the pipe and then a collar of PVC material was welded around the pipe to prevent leakage.
Sand was laid and compacted on the plastic liners (30 cm thickness) to protect them from puncture and deterioration due to sunlight. In addition, the plans specified that the sand be covered by a plastic cover (skirt) or rock rip -rap so as to prevent erosion of sand from the pond emankments by wave action. The selection of the plastic cover was left to the discretion of the present consultant after arrival on site. The criteria for this selection consisted of the following factors :
A number of materials suitable for the plastic cover were found in Bahrain (see Appendix 2 for suppliers).
The least expensive material (BD 0.165/m3)1 was UV resistant polythylene supplied by a greenhouse manufacturer. This material usually comes in rolls of 6 × 50 m and has a life expectancy of 1–2 years in Bahrain.
Another material selected for comparison was vinyl backed with asbestos (Marleydek Tropical Roofing) that is commonly used as a roofing material. Because of its white colour and thickness (1.016 mm) it holds up well against the sun and is guaranteed on roofs in Bahrain for five years. Rolls are typically 1.05×20 m, although other widths and lengths can be obtained by solvent welding strips of vinyl together.
Another material considered but disregarded because of cost (BD 4.110/m3) was PVC plastic (Rhenofol CT) which also is used as a roofing material. It comes in rolls of 2.05×20m and is 1.0 mm thick. It is also UV resistant and can be solvent welded.
Other roofing materials considered were bitumen based products but these were eliminated due to the potential toxicity of the hydrocarbon content. One proposal in the drawings was that a portion of the Staff Industries PVC liner not be covered with sand, however this option was not pursued because this PVC plastic is easily damaged by sunlight and heat.
The plastic covers were prefabricated into rolls of 3 m width so that 2.7 m of plastic would cover the upper portion of the embankment slope and 0.3 m could be buried in the berm ditch. The bottom 30 cm of the edge of the plastic covers was held in position by sand (15 cm thickness).
Pond A was covered with the polythene except for a 10 m section which was lined with pavement stones (60×60cm and 25 cm thick). The stones were placed in staggered horizontal lines with no mortar or spaces between them. Pond B was covered with the asbestos backed vinyl.
Until the electric pumps and seawell can be installed, provisions were made for a temporary seawater system which would use a diesel driven pump. The diesel pump was installed 10 m closer to the water than the site of the intended pumphouse and utilizes the permanent 4 in PVC pipeline. At the end of the suction line is attached a 4 in reinforced nylon hose and a brass footvalve. The footvalve keeps the suction line full of water when the pump is not operating. A valve at the pump allows the suction line to be filled should the pump lose its prime. It is important that the suction line be gradually sloping to the water with no peaks and valleys otherwise air blockage will result during priming.
It is recommended that a vacuum pump system be considered for the permanent pump installation so as to avoid problems with filling the suction line and with malfunctioning footvalves. It is also recommended that the PVC pipeline be covered with sand to protect it from deterioration and deformation due to sunlight.
1 US$ 1.00 = BD 0.380
The monks and drainage canal were constructed according to specifications. The canal, where not covered by cement, was lined with a reinforced PVC plastic (Rhenofol CVT). This material is resistant to sunlight degradation and is used as a roofing material guaranteed for 10 years in Bahrain. This material has been used in Kuwait to line aquaculture raceways and has given good service (Kneale et al., 1982). Despite the high locally supplied price (BD 4.800/m3), it should be considered as a viable alternative to earthern covered non UV resistant liners.