Excavations beneath the slab that are not to be filled with concrete (usually plumbing or other mechanical trenches) should be brought back to grade in compacted lifts. This means a 24" trench would be backfilled 6" at a time, each "lift" being mechanically vibrated so it is left compact. Learn more about subgrades and subbases for concrete slabs.

If these excavations are not compacted when backfilled, this loose soil will settle over time leaving the concrete over that area with no earth under it. This becomes a prime place for concrete to settle. Since the soil next to these excavations is native soil, the uncompacted trench can literally become a thoroughfare for water.

Most rental yards have compacting equipment and it is worth the investment to use it. Learn how to compact soil before pouring concrete.

Excavations from the house to the street for utilities should also be backfilled and compacted in the same manner so areas under the driveway concrete don't sink. Uncompacted areas under lawns can be identified by areas of sunken grass-so it is good idea to compact trenches even under areas not receiving concrete.