Engineering and Construction

After the basement walls have been  waterproofed or dampproofed, insulating boards or protection boards  have been applied, drainage features  have been installed, and internal constructions that support the basement walls, such as interior walls and floors,  have been completed, the area around a substructure is  backfilled to restore the level of the ground. (A substructure built tightly against sheeted walls of an excavation needs little or no backfilling.) The backfilling  operation involves placing soil back against the outside of the basement walls  and compacting it there in layers,  taking care not to damage drainage  or waterproofing components or to exert excessive soil pressure against  the walls. An open, fast-draining soil  such as gravel or sand is preferred for backfilling because it allows the perimeter drainage system around the  basement to do its work. Compaction  must be sufficient to minimize subsequent settling of the backfilled area.

In some situations, especially in backfilling utility trenches under road- ways and floor slabs, settling can be  virtually eliminated by backfilling with  controlled low-strength material (CLSM),  which is made from portland cement  and/or fly ash (a byproduct of coal- burning power plants), sand, and water.  CLSM, sometimes called “flowable fill,”  is brought in concrete mixer trucks  and poured into the excavation, where  it compacts and levels itself, then hardens into soil-like material. The strength  of CLSM is matched to the situation:

For a utility trench, CLSM is formulated so that it is weak enough to be  excavated easily by ordinary digging  equipment when the pipe needs ser-vicing, yet as strong as a good-quality compacted backfill. CLSM has many  other uses in and around foundations.

It is often used to pour mud slabs, which  are weak concrete slabs used to create  a level, dry base in an irregular, often  wet excavation. The mud slab serves  as a working surface for the reinforcing and pouring of a foundation mat  or basement floor slab and is often the  surface to which a waterproofing mem- brane is applied. CLSM is also used to  replace pockets of unstable soil that  may be encountered beneath a sub- structure or to create a stable volume  of backfill around a basement wall.