Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 641

Excavation to compensate for part or all the weight of the structure may permit the use
of raft. This alternative should be considered along with others.

15.8.4 Irregular Deposits

If the subsoil consists of lenticular or wedge-shaped masses, it is rarely possible to make an
accurate estimate of bearing capacity or settlement. In such cases, it is better to determine the
general character of the deposit by means of numerous subsurface soundings supplemented by
a few boring and soil tests. The purpose is to form an idea regarding the size and distribution
of the softer elements and to judge the most unfavourable combination of elements that can be
reasonably expected. The estimate of settlement should be based on the assumption that the
most unfavourable conditions may occur in the most highly stressed portion of the soil. (Peck
Hanson and Thornburn, 1974).

15 .9 Illustrative Examples

Example 15.1: A building is supported on nine columns as shown in Fig. 15.29. and column
loads are indicated. Determine the required areas of the column footings:

6m 6m

6m

6m

123

456

789

Fig. 15.29 Building founded on columns (Ex. 15.1)

Column No. 123456789

Dead Load (kN) 180 360 240 300 600 360 180 360 210

Max. Live Load (kN) 180 400 210 300 720 360 120 300 180

At the selected depth of 1.5 m the allowable bearing capacity is 270 kN/m^2. γ = 20 kN/m^3.
Dead load plus maximum live load, maximum live load to dead load ratio, reduced live
load and dead load plus reduced live load are all determined and tabulated for all the columns
(A reduction factor of 50% is used for LL).