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