(i) It must have sorne specificd safety against ultimate failure.
(ii) The settlements under working loads should not exceed the allowable limits for the super-structure.
The bearing capacity of the soil, used for the design of foundations (i.e. for determining the dimensions of the foundations) is determined on the basis of the above two criteria.
In general, the supporting power of a soil or rock is referred to as its bearing capacity. The term bearing capacity is defined after attaching certain qualifying prefixes, as defined below:
1. Gross pressure intensity (q). The gross pressure intensity q as the total pressure at the base of the footing due to the weight of the super-structure, self weíght of the footing and the weight of the earth fill, íf any.
2. Net Pressure intensity (qn). It is defined as the excess pressure, or the difference in intensities of the gross pressure after the construction of the structure and the original overburden pressure. Thus, lf D is the depth of the footing
where γ is the unit weight of soil aboge the level of footíng.
3. Ultimate bearing capacity (qf). The ultimate bearing capacny is defined as the minimum gross pressure intensity at the base of the foundation at which the soil fails in shear.
4. Net ultimate bearing capacity (qnf). It is the minimum net pressure intensity causing shear failure of the soil. The ultimate bearing capacity qf, and net ultimate bearing capacity (qnf) are evidently connected by the relation
5. Net safe bearing capacity (qns). The net safe bearing capacity is the net ultimate bearing capacity divided by a factor of safty F:
6. Safe bearing capacity (qs). The maximum pressure which the soil can carry safely without risk of shear failure is called the safe bearing capacity. It is equal to the net safe bearing capacity plus original overburden pressure
Sometirnes, the safe bearing capacuty is also referred to as the ultimte bearing capacity qf divided by a factor of safety F.
7. Allowable bearing pressure (qa). It is the net loading intensity at which neither the soil fails in shear nor there is excessive settlement detrimental to the structure in question. The allowable bearing pressure thus depends both on the sub-soil and the type of building concerned, and is generally less than, and never exceeds, the safe bearing capacity.
Methods ot Estimating Bearing capacity
The bearing capacity of soil can be determined by the following methods:
(a) Analytical methods involving the use of soil parameters
(b) Plate load test on the soil
(c) Penetration test
(d) Presumptive bearing capacity values from codes.