The choice
of the appropriate type of foundation is governed by some important factors
such as
1. The nature
of the structure
2. The loads
exerted by the structure
3. The subsoil
characteristics
4. The allotted
cost of foundations
Therefore to
decide about the type of foundation, subsoil exploration must be carried out.
Then the soil characteristics within the affected zone below the building
should be carefully evaluated. The allowable bearing capacity of the affected
soil strata should then be estimated.
After this
study, one could then decide whether shallow foundations or deep foundations
should be used.
Shallow
foundations, such as footings and rafts, cost less and easier to execute. They
could be used if the following two conditions are fulfilled;
1. The
superimposed stress (Dp) caused by
the building lies within the allowable bearing capacity diagram of different
soil strata as shown in Fig.1.
This condition is fulfilled when , in Fig.1, is
smaller than and smaller than and smaller than
and smaller than and so on.
2. The building
could withstand the expected settlement estimated for that type of foundation
If one or
both of these two conditions cannot be fulfilled the use of deep foundations
should be considered.
Deep
foundations are used when top layers of the soil are soft and there exists a
good bearing stratum at a reasonable depth. Soil strata lying beneath the
bearing stratum should be of ample strength to resist the superimposed stresses
(Dp) due to the loads transmitted to the bearing
stratum, as shown in Fig.2.
Deep
foundations are usually piles or piers which transmits the load of the building
to the good bearing stratum. They usually cost more and require well trained
engineers to execute.
If the
explored soil layers are soft for considerable depth and no bearing stratum is
found at a reasonable depth, floating foundations could be used.
To build a
floating foundation, a mass of soil, approximately equal to the weight of the
proposed building, is to be removed and replaced by the building. In this case,
the bearing stress under the building will be equal to the weight of the
removed earth (γD) which is less than
(qa = γD +2C)
and Dp will be equal to zero. This means that the bearing
capacity under the building is less than ( qa ) and the expected settlement equals
theoretically to zero.
Finally, the
engineer should prepare an estimate of the cost of the most promising type of
foundation which represents the most acceptable compromise between performance
and cost.
