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APPENDIX D Split Backfill Guidelines L GENERAL I. I. Use select rounded or crushed stones for primary backfill material as specified in the Installation Manual in effect at time of installation, and Appendix C, Primary Backfill Requirements. 2. SPLIT BACKFILL INSTALLATION 2.1. Use the primary backfill material vertically up to at least 75 percent of the tank diameter. See Figure D-1. 2.2. Follow the instructions in section 5 of the Installation Manual on the placement of this backfill material. 2.3. Install a layer of geotextile fabric over the entire surface of the primary backfill material. See section 1 for more information about geotextile filter fabric. 2.3.1. All joints in the geotextile fabric should be overlapped a minimum of 12 inches [30 cm]. 2.3.2. Geotextile fabric must overlap onto the tank and excavation surface a minimum of 12 inches [30 cm]. 2.3.3. Installations with unstable soil may require that the fabric line the entire excavation. See section 1 for specific information about using geotextile or fabric in unstable in-situ (native) soil conditions. 2.4. Clean native backfill may be used as secondary backfill material above the geotextile fabric to subgrade. 2.4.1. Secondary backfill material must be clean, free-flowing, and free of large rocks, roots, organic materials, debris, ice and snow. Backfill material shall not be frozen or contain lumps of frozen material at anytime during installation. 2.5. Secondary backfill must be compacted to achieve a minimum of 85 percent standard proctor density. 2.5.1. Do not use rammer-type compactors over the top of the tank. 2.5.2. Some tank owners may require sample testing and written reports to verify the compaction of the backfill. 2.6. Material must be installed in 12 -inch [30 -cm] to 24 -inch [60 -cm] lifts compatible with the compaction equipment used. 2.7. In some conditions, frost heave may be encountered when using secondary backfill. Therefore, consider any frost- related problems that may occur. 2.8. Specifications for secondary backfill material and compaction above the geotextile fabric layer may be determined by the requirements of the piping, surface slab or roadway. 2.9. Referto applicable codes orstandards for base course and sub -base course material and compaction requirements. 2.10. The following are examples of acceptable secondary backfill material: • clean native backfill • coarse sand or gravel 2.11.100%of all backfill material must pass through a 1 -inch sieve. 44