Soldier piles may be driven or they may be installed in drilled holes. Drilled holes may be backfilled with concrete, slurry, sand, pea-gravel or similar material after the soldier pile has been installed in the hole. Some soldier pile drilled holes are backfilled with concrete to the depth of excavation and then the remainder of the hole is filled with slurry to ground level. Slurry is generally considered to be a sand-cement mix placed wet enough to fill all voids. Backfill materials other than concrete are used when it is desirable to extract the soldier piles. When materials other than concrete are used to backfill drilled soldier pile holes some vibratory methods and/or jetting procedures need to be used. The backfill should be as compact as the native soil into which the pile is set. The use of pea gravel backfill in lieu of concrete substantially decreases the passive resistance on all sides of the soldier pile. The pea gravel does not permit the soldier pile to act as a unit until sufficient soldier pile deflection compacts the pea gravel against the soil. Similar reasoning is true for compacted sand backfill. Soldier piles offer a number of advantages over conventional retaining wall and soil stabilization approaches, including: