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The maximum size for crushed rock used as backfill material in underground storage tank installations is 3/4 inch. This size is optimal because it allows for proper drainage and compaction while preventing large voids that could lead to instability around the tank. The smaller size also minimizes the potential for damage to the tank and piping systems, ensuring that the entire assembly is adequately supported and protected against shifting or settling.
Using larger sizes, such as 1 inch or 2 inches, can create larger air pockets and reduce the effectiveness of the backfill in distributing loads and providing stability. In contrast, crushed rock that is too fine, like 1/2 inch, may impede drainage and promote water retention, which can lead to other issues related to corrosion or saturation of the surrounding soil. Thus, 3/4 inch crushed rock strikes a balance between stability, drainage, and support effectiveness in UST installations.