Covers the requirements for shop fabricated cylindrical, horizontal steel tanks, single and double skin for the underground storage of water polluting liquids (both flammable and non-flammable) within the following limits: - from 800 mm up to 3000 mm nominal diameter and, - up to a maximum overall length of 6 times the nominal diameter and, - for liquids with a maximum density of up to 1,9 kg/l and, - with an operating pressure (p[o]) of maximum 1,5 bar (abs.) and, - for double skin tanks with a vacuum leak detection system where the kinematic viscosity does not exceed 5 x 10[-3] m[2]/s.