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Foundation Problems & Solutions

Slabs-On-Ground - Structural Deterioration And Damage

Problem: Cracking of slab - 1.

Cause: Differential or uneven settlement.

Differential or uneven settlement is caused by variations in the sub-grade. When loaded, the slab will tend to bridge over soft spots , such as underground utilities where soil hasn't been adequately compacted and rest on hard spots such as rocks or denser soil. This differential settlement often results in cracking as shown in Figure 36.

uneven settlement of a concrete slab
Figure 36 - Uneven settlement of a concrete slab

Solutions: Ensure that the sub-grade is completely uniform in firmness, grade level and dampness.

  • Remove all topsoil and organic materials to expose the natural ground. The strength of the soil is a function of the degree of its compaction and its moisture content.
  • The boundaries of the compacted area should be extended 5 to 10 feet (1.5 to 3.0 m) beyond the perimeter of the slab as shown in Figure 37. Compact the area around the slab-on-ground and slope it to promote drainage away from the slab.
sub-grade compaction
Figure 37 - Sub-grade compaction for a concrete slab
  • Backfill soil used around service trenches should be similar to the soils surrounding the utility trench. Compact it in layers to duplicate the compaction and moisture conditions of the adjacent soil.
  • After compacting the sub-grade, install granular fill and compact it across the entire area. A minimum of 4 inches (100 mm) of course, clean, granular fill is required beneath the slab as shown in Figure 38.
slab-on-ground sub-grade
Figure 38 - Slab-On-Ground Sub-Grade