Bearing Capacity of Soils

The bearing capacity of soils is perhaps the most important of all the topics in soil engineering. Soils behave in a complex manner when loaded so, it is important to know the bearing capacity of soils. Soil when stressed due to loading, tend to deform. The resistance to deformation of the soil depends upon factors like water content, bulk density, angle of internal friction and the manner in which load is applied on the soil. The maximum load per unit area which the soil or rock can carry without yielding or displacement is termed as the bearing capacity of soils.

When excessive load is transmitted to the soil by structural foundation, the settlement of the foundation takes place which can endanger the stability of the structure. The settlement due to load is caused basically on account of two factors, namely,
(i) the soil below footing gets compressed by certain amount and
(ii) since the foundations cover only a limited area there is a possibility that the concentrated stresses developed are so high as to cause actual rupture (shear failure) and displacement of soil below.

The loading intensity which causes soil rupture and lateral displacement resulting in rapid sinking of the loaded area into the ground is termed as ultimate bearing capacity of soils. 

Past experience shows that very often a structure fails due to unequal settlement or differential settlement. This happens when a part of building is founded on compressible stratum and the remaining part rests on firm soil strata. Thus the part of the building on compressible soil settles at a rate well in excess of the part of building on firm soil leading to the differential settlement. Differential settlement can also occur when one part of the building is loaded much more than other or intensity of load is varying and is more than the bearing capacity of soils. In case, however, the settlement is uniform and small in magnitude it does not endanger the structure in any way.

Based on the survey of data concerning the damage due to differential settlement, it has been observed that no structural damage or damage to interior finish or partition walls etc. occurred in buildings investigated at angular distortion smaller than 1:300. For any given structure there is certain amount of settlement (either uniform or diffeiential or both) which can be tolerated without creating unsafe conditions. This is termed as permissible settlement. It is on this account that almost all design codes permit certain maximum value of permissible settlement in the design of structures. IS 1904-1978 gives details of maximum permissible settlement which can be allowed in the design of structures. This value varies from 50 mm to 100 mm for different type of structures, foundations (isolated or raft) in different types of soils.

The various terms which are used in connection with the bearing capacity of soils are summarised under,

1. Ultimate Bearing Capacity of Soils:

The intensity of loading, at the base of foundation, at which soil support fails in shear is called ultimate bearing capacity of soils.

2. Safe Bearing Capacity of Soils:

The maximum intensity of loading that the soil will safely carry without risk of shear failure is called safe bearing capacity of soil. This is obtained by dividing the ultimate bearing capacity by a certain factor of safety, and it is the value which is used in the design of foundation. The factor of safety normally varies from 2 to 3.

3. Net Pressure Intensity:

Let B = width of the footing,
W1 = weight of soil that was existing above the base of foundation prior to excavation.
W = total load on the base of the foundation {This being sum of (i) total dead load upto footing top + (ii) Self weight of footing + (iii) Weight of backfill soil located above the footing + (iv) Live load on the foundation due to floors above, and due to snow, wind, seismic forces etc. (wherever applicable)}
qn = net pressure intensity

Net Pressure intensity= (Net load on base of foundation/Area of footing)

Net load on base of foundation = (W— W1)

qn = (W-W1)/Bx1

Hence net pressure intensity refers to loading acting on the bottom of the foundation trench, in excess of the weight of soil removed from the trench. Net pressure intensity is normally considered in the design of foundation, thereby utilising the advantages of reduction of loading brought about on account of removal of soil from the foundation trench.

4. Allowable Bearing Pressure:

It is the maximum allowable net loading intensity which can be applied to the soil taking into account the ultimate bearing capacity, the amount and kind of settlement expected and the ability of the given structure to withstand the settlement. It is, therefore, dependent upon both the sub-soil and the type of building proposed to be erected thereon. The allowable bearing pressure adopted in the design of foundation is lesser of the following two values,
(a) The safe bearing capacity of soil, or
(b) The maximum allowable bearing pressure that the soil can take without exceeding the specified limits of permissible settlement.