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The backfill material surrounding a tank is a critical part of a fiberglass tank installation. <br />This brochure gives guidelines for choosing the best material to use when installing fiber- <br />glass tanks. <br />Xerxes specifies that gravel or crushed stone be used as a backfill material. <br />1. PEA GRAVEL (mix of rounded particles) must have a minimum diameter of and <br />a maximum diameter of 3/4'. <br />2. STONE CRUSHINGS should be washed and free flowing. The mix of angular <br />particle -size should be between /s' and 1h" and must meet ASTM C-33 paragraph <br />9.1 requirements. <br />3. No more than 5% of the backfill shall pass through a No. 8 sieve. <br />COARSE AGGREGATE is a more technical term for the material that meets Xerxes' <br />backfill size requirements. The American Society for Testing and Materials (ASTM) and <br />The American Association of State Highway Transportation Officials (AASHTO) have <br />specifications for standard sizes of coarse aggregate. The standard sizes of coarse aggregate <br />that meet Xerxes' pea gravel specifications are given in Table I. The standard sizes of coarse <br />aggregate that meet Xerxes' crushed stone specifications are given in Table II. <br />Some quarries may produce materials that meet Xerxes' requirements but are not iden- <br />tified by a standard coarse aggregate size number. The quarry should be able to provide <br />a specification that identifies the size or gradation of the material. If a specification for the <br />material is not available, an independent testing laboratory can provide a sieve analysis per- <br />formed per ASTNLC-136 on a sample of the backfill material. The sieve analysis or material <br />specification can then be compared against Xerxes' size requirements for the pea gravel or <br />crushed stone. <br />Another important characteristic of good backfill material is hardness or stability <br />when exposed to water or loads. Most materials have no problem meeting the hardness <br />requirement. Materials like soft limestone, sandstone or shale should not be used as backfill <br />because they break down over time. <br />1-4 R.1 <br />