Proper drainage is critical to the performance of any retaining wall system. Drainage aggregate is used behind all walls to keep water pressure from building up behind the wall face. In addition, for walls over 3 feet high, perforated drain pipe is used to collect and eliminate large amounts of water from behind the wall.
Lay the drain pipe on a 1- to 2-inch layer of drainage aggregate directly behind the above-grade course. Next, place more drainage aggregate-- typically 3/4-inch clear, free-draining, angular gravel--on top of the drain pipe and to a depth of 12 inches behind the wall.
Place soil backfill directly behind the drainage aggregate and then compact both. Proper placement and compaction of backfill is critical to the stability of a segmental retaining wall.
Make sure you compact no more than 6 inches of backfill and aggregate at a time when using a power compactor, and no more than 2 inches at a time when hand compacting.
Additional courses are placed, pinned and backfilled in the same fashion. As you go along, remember to keep checking your bond, alignment and level.
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