Predicted settlement to be used in establishing foundation pad height should be specified.
Pads equipped with a ringwall are required for the following conditions:
- Tanks larger than 120 ft (36.5 m) in diameter
- Tanks higher than 48 ft (14.6 m) with diameters of 100 ft (30 m) or larger
- When a cohesionless soil will be used as general fill, the minimum size tank requiring a ringwall should be specified.
Acceptable materials for stone ringwall construction, below the top 4 in. (100 mm) zone of graded crushed stone or crushed gravel, include any crushed stone or crushed gravel size numbers 2, 24, 3, 357, 4 or 467 (ASTM D 448) that meets the requirements of ASTM D 693 for crushed aggregate. Proposals to use other gradations or materials should be approved prior to design. Crushed slag should not be used.
A reinforced concrete ringwall is acceptable, provided a tank shell settlement of 2 in. (50 mm) or less is expected. If site-specific experience has demonstrated satisfactory performance of concrete ringwalls for larger shell settlements, then the use of concrete in lieu of crushed stone can be considered in these cases as well.
The average elevation of finished pads, measured at the tank shell, should be within a tolerance of 1 in. (25 mm) of that shown on the construction drawings.
Tank pad elevation differences, measured at the shell, should not exceed any of the following:
- 1:720 in any distance of 30 ft (10 m) or more.
- 1 in. (25 mm) between the highest and lowest elevation points.
To achieve these tolerances, it may be helpful to either choke surface voids in a crushed stone ringwall with fine aggregate of gradation Size No. 10 (ASTM D 448), or excavate a portion of the crushed stone and place a layer of unreinforced or fiber-reinforced mortar having maximum dimensions of 1 ft (300 mm) wide by 2 in. (50 mm) thick.
Deviations from the specified slope of the pad within the tank shell should not exceed 1:720 in a horizontal distance of 30 ft (10 m) or more.