Compaction

Compaction squeezes air out of soil by mechanical effort: rolling, ramming or vibrating. It raises density and with it shear strength, while reducing settlement and permeability. Every embankment, road subgrade, backfill behind a wall and earth dam core depends on proper compaction.

The key laboratory finding, by Proctor, is that for a given effort there is an optimum moisture content (OMC) that gives the maximum dry density (MDD). Too dry, and grains resist rearrangement; too wet, and water fills space that soil could occupy. The compaction curve rises to a peak then falls, staying below the zero-air-voids line. The standard (light) test uses a 2.6 kg rammer; the modified (heavy) test uses a 4.9 kg rammer dropped further, giving higher MDD at lower OMC, closer to modern heavy rollers.

On site, specifications require a relative compaction, such as 95 or 97 percent of laboratory MDD, within a moisture range around OMC. Layer thickness, roller type and number of passes are set in field trials. Smooth wheel rollers suit granular bases, sheepsfoot and padfoot rollers suit clays, and vibratory rollers suit sands and gravels. Density is checked by the sand replacement or core cutter methods, or with nuclear gauges, and stiffness by plate load or light weight deflectometer. Clays compacted dry of optimum are stiffer but more permeable and prone to swelling; wet of optimum they are more flexible, which suits dam cores.