This document specifies the product characteristics and test/assessment methods for workshop fabricated cylindrical, horizontal steel tanks, single (type S) and double skin (type D) intended to be used for the underground storage of water polluting liquids (both flammable and non-flammable), specifically used for storage and/or supply of fuel for building heating/cooling systems, and of hot or cold water not intended for human consumption at normal ambient temperature conditions (−20 °C to +50 °C) 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;-for liquids with a maximum density of up to 1,1 kg/l and;-with an operating pressure (Po) of maximum 50 kPa (0,5 bar(g)) and minimum – 5 kPa (–50 mbar(g)) and;-for double skin tanks with a vacuum leak detection system where the kinematic viscosity does not exceed 5 × 10−3 m2/s.Two tank types are distinguished:-Type S: Single skin;-Type D: Double skin.Tanks designed to this document allow for an earth cover of up to 1,5 m. If there are imposed traffic loads or a greater earth cover, calculation is required.This document is not applicable to tanks installed in industrial processes or in petrol stations, nor to loads and special measures necessary in areas subject to risk of earthquakes and/or to flooding.