CHOICE OF THE TYPE OF FOUNDATION

Choice of the Type of Foundation

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

(q_a = γD +2C)

and Dp will be equal to zero. This means that the bearing capacity under the building is less than ( q_a ) 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.