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M98168GE • • to develop behind retaining walls. A sketch of the drain system is shown on Figure 7. Subdrains should consist of a three (3) or four (4) inch diameter perforated rigid pipe surrounded by a filter. The filter should consist of a filter fabric or a graded material such as washed concrete sand or pea gravel. If sand or gravel is chosen the pipe should be placed in the middle of about four (4) cubic feet of aggregate per linear foot of pipe. The drain system should be sloped to positive gravity outlets. If the drains are daylighted the drains should be provided with all weather outlets and the outlets should be maintained to prevent them from being plugged or frozen. We should be called to observe the soil exposed in the excavations and to verify the details of the drain system. A drain blanket may be constructed beneath the basement concrete slab-on-grade floor slab to intercept water that may tend to rise in the beneath the floor slab area. The drain blanket should be at least one foot thick and consist of a free draining sand or gravel material which is compacted as discussed under Compacted Structural Fill above, section 7.0. The subgrade below the drain blanket should be sloped to collection points prior to constructing the drain blanket. A perforated pipe should be installed at the collection points and graded to discharge similar to the foundation drain discussed above. The drain blanket concept is shown on Figure 7. The under slab drain blanket may be considered as part of. the. structural fill intended to support the floor slab as discussed under Interior Floor Slabs, section 6.0 above.- we are available to discuss this concept with you. 10.0 BACKFILL Backfill areas and utility trench backfill should be constructed such that the backfill will not settle after completion of con- struction, and that the backfill is relatively impervious for the upper few feet. The backfill material should be free of trash and other deleterious material. It should be moisture conditioned and compacted to at least ninety (90) percent relative compaction using a modified moisture content-dry density (Proctor) relationship test (ASTM D1557). Only enough water should be added to the backfill material to allow proper compaction. Do not pond, puddle, float or jet backfill soil materials. Improperly placed backfill material will allow water migration more easily than properly recompacted fill. Improperly compacted fill is likely to settle, creating a low surface area which further 18 ~Lamberr ana ~,o~ociateo COMMIlTIMO OFOT[CMMK~L [NOOlllq HMO Y~Tll11~1 TEFTIMO