Similar to Grout, Controlled Low Strength Material (CLSM) is sometimes referred to as flowable fill, and is a lower strength type of high slump concrete often used for non-structural applications such as filling old tunnels or unused sewer lines. Flowable Fill is a self-compacting low strength concrete with a flowable consistency, similar to a liquid, and is often times substituted for compact gravel. Flowable Fill should not be used in place of standard concretes, but is often used for retaining wall backfill, tunnels, sewers, and utility trenches.