16 A SUMMARY OFACCESSIBILITY STANDARDS FOR OUTDOOR DEVELOPEDAREAS accessibility standards for Federal outdoor developed areas CONSTRUCTION TIP—Stable materials Generally, the following materials provide firmer surfaces that are more stable than the alternative: ¡ ¡ Crushed rock (rather than uncrushed gravel) ¡ ¡ Rocks with broken faces (rather than rounded rocks) ¡ ¡ A rock mixture containing a full spectrum of sieve sizes, including fine material (rather than a single size) ¡ ¡ Hard rock (rather than soft rock that breaks down easily) ¡ ¡ Rock that passes through a ½-inch screen (rather than larger rocks) ¡ ¡ Rock material that is compacted in 3- to 4-inch layers (rather than thicker layers) ¡ ¡ Material that is moist (but not too wet) before it is compacted (rather than material that is compacted when it is dry) ¡ ¡ Material that is compacted with a vibrating plate compactor, roller, or by hand tamping (rather than material that is laid loose and compacted by use)
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