In design of underwater crossings primary consideration peculiar to problem are counterbalancing of buoyancy, protection from external damage, and provision of strength to resist structural failure; type of river and its geological, hydrological, and historical characteristics must be taken into account in selection of site, design of crossing, and determination of best working season; design considerations and testing. (See also Oil and Gas J v 55 n 26, 28, 30 July 1 1957 p 107-10, July 15 p 96-7, 100, July 29 p 200, 202; Petroleum Engr v 29 n 7, 9 July 1957 p D29-36, Aug p D42-4)