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Settlement is the change in elevation of the surface of the ground caused by deflection of the soil below the slab under load.

Technical Information

Settlement is a very important consideration when designing ground-supported slabs. Uniform settlement across the area of the slab doesn’t usually cause any structural problems. However, differential settlement can cause a variety of problems:
  • Damage to the slab – Because the slab support is effectively lowered in one area, the operational loads on the slab will cause it to experience bending moments for which it has not been designed.
  • Loss of levelness – Even a relatively small loss of levelness can cause serious issues for very high racks and precise MHE.
  • Loss of flatness – Cracks caused by any damage to the slab can affect its flatness. This can be problematic for some MHE.

Clays and silts have a low permeability and if they are fully saturated prior to loading, full settlement won’t happen immediately and will be delayed until the water dissipates. This process is called consolidation and could take a number of years to complete in some soils. Slow consolidation can be very costly if differential settlement occurs unexpectedly a long time after the structure is finished. Permeable or unsaturated soils should settle relatively quickly and should, therefore, not have this problem.

The rate of consolidation can be increased by the use of:

  • Drainage systems – create a permeable pathway allowing quicker dissipation of water from the clays and silts. Can be constructed from sand, coarse aggregate or geotextiles.
  • Surcharge loading – the application of a load greater than the anticipated service load will force excess water to dissipate quicker. The surcharge is removed once desired settlement is reached.

The settlement behaviour of the soil underneath a slab can be predicted if a thorough soil investigation is carried out by a Geotechnical Engineer prior to construction commencing.

Differential settlement can arise due to:

  • Lateral variation in strata. (example: leaning tower of Pisa)
  • Variation in loading on foundation.
  • Differences in construction time between different parts of structure.
  • Flexible foundations.


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