Geotechnical Engineering

DHARM

BEARING CAPACITY 601

Example 14.27: Two load tests were conducted at a site—one with a 0.5 m square test plate
and the other with a 1.0 m square test plate. For a settlement of 25 mm, the loads were found
to be 60 kN and 180 kN, respectively in the two tests. Determine the allowable bearing pres-
sure of the sand and the load which a square footing, 2 m × 2 m, can carry with the settlement
not exceeding 25 mm.

qult = mx + σ

where x = Perimeter-area ratio, P/A

First test Second test

x 1 =

P

A

1

1

405

05 05

= ×

×

= 8 m–1 x 2 =

P

A

2

2

41

11

= ×

×

= 4 m–1

q 1 =

60

05 05..×

= 240 kN/m^2 q 2 =

180

11 ×

= 180 kN/m^2

∴ 240 = 8 m + σ ....(1) 180 = 4 m + σ ...(2)

Solving Eqs. (1) and (2) simultaneously,

m = 15 (kN/m) and σ = 120 (kN/m^2 )

Prototype footing:

x = P/A =

42

22

×

× = 2 m

–1

∴ q = mx + σ
= 15 × 2 + 120 = 150 kN/m^2
This is the allowable bearing pressure for a settlement of 25 mm. Load which the footing
can carry,

Qs = qs × Area = 150 × 2 × 2 = 600 kN.

Summary of Main Points

1. The load-carrying capacity of a foundation to transmit loads from the structure to the foundation
   soil is termed its ‘bearing capacity’. The criteria for the determination of the bearing capacity are
   avoidance of the risk of shear failure of the soil and of detrimental settlements of the foundation.
   Safe bearing capacity is the ultimate value divided by a suitable factor of safety; the allow-
   able bearing pressure is the smaller safe capacity from the two criteria of shear failure and
   settlement.


2. The factors on which the bearing capacity depends are the size, shape and depth of the founda-
   tion and soil characteristics, including the location of the GWT relative to the foundation.


3. The methods of determination of bearing capacity are selection from building codes, analytical
   methods, plate load tests, penetration tests, model tests, and laboratory tests.


4. Of the analytical methods, Schleicher’s is based on the theory of elasticity, Rankine’s and Bell’s
   are based on Rankine’s classical theory of earth pressure, Fellenius’, Prandtl’s, Terzaghi’s,
   Meyerhof’s, Skempton’s and Brinch Hansen’s methods are based on the theory of plasticity.