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By "ferrocement", I presume you mean a 0.4: 1: 2 :: water: cement: sand mortar, hand applied to an armature of reinforcing bars and several layers of mesh, to form a thin (20-40 mm thick) shell deriving its strength mainly from membrane action rather than bending?
The benefits of this form of construction are that the materials and workmanship skills should be readily available in Nepal and easily transportable to the site that may be difficult to reach.
The optimum shape for such a form of construction is a membrane shell, like a boat hull, arch or, for liquid containment, circular with an inverted dome base. If a rectangular shape is required or desired for any reason, the bending is best resisted by an external structure (eg: a reinforced concrete base with gravity or reinforced concrete walls), using the ferrocement lining as a waterproof skin, able to bridge over local structural weaknesses (in the external structure). The composite action between the two is enhanced by shaping all external edges to form a curved bevel and internal edges to form a fillet, each of radius about 150 mm.
The other important thing is the mortar mix: the water cement ratio should be as low as possible that makes the mix workable; if the "sand" is too sharp (as crushed rock would be), the cement content should be increased maintaining the water: cement ratio at 0.4.
If you would like construction details, let me have details of the site, size of the tank, input flows and demand.
Look forward to hearing from you...
Kind regards, Leigh
RedR KnowledgePoint Moderator