In conventional design, the allowable bearing capacity should be taken as the smaller of the following two values.The safe bearing capacity based on ultimate capacity

The allowable bearing pressure on tolerable settlement

The safe bearing capacity qs may be computed from Terzaghisanalysis: For strip footing: qs = (1/F) [c.NC + .D.(Nq-1).RW1 + 0.5 .B.N.RW2] + .D

For square footing: qs = (1/F) [1.3c.NC + .D.(Nq-1).RW1 + 0.4 .B.N.RW2] + .D

For circular footing: qs = (1/F) [1.3c.NC + .D.(Nq-1).RW1 + 0.3 .B.N.RW2] + .D

For rectangular footing: qs = (1/F) [c.NC (1+0.3(B/L))+ .D.(Nq-1).RW1+ 0.5 .B.N. (1-0.2(B/L))RW2] + .D

The net allowable bearing pressure q based on limiting the maximum settlement of individual footing to 25mm,

q = 34.3 (N-3) [(B+0.3)/2B]2 RW2. Rd

Rd, Depth factor = [1+ (0.2D/B)] 1.20

If permissible settlement is in place of (=25mm), the corresponding net allowable bearing pressure

q' = q x (/25)

Note: q or q' is the permissible net increase in the bearing pressure.

The permissible gross increase in the bearing pressure

qg = q + .D

The allowable bearing pressure (qa) will be smaller of two

Empirical relation

ITEMS FOR INSPECTION IN THE CONSTRUCTION OF FOOTINGS IN SOILS Minimum depth to base of footing: a

minimum depth prevents potential problems related to the weakening of the support for the footing that could result from any type of excavation, erosion, or ground loss in the immediate neighborhood of the footing. It is usually sufficient to require 0.75m as a minimum. An exception occurs near the property line, where it is prudent to adopt a minimum depth of 1.50m.

Certain minimum distances with respect to buried utilities, cavities, or other foundation elements

Sequence of construction of individual footings.

Construction inspection

Base elevation

Nature and type of soil at the base of the excavation

Base cleaning before concrete placement

Dimensions and cross sections of the footings

Concrete placement

Time for concrete placement

Footing integrity after concrete placement

CHOICE OF FOUNDATION TYPE AND PRELIMINARY SELECTION Information regarding the nature of the superstructure and the probable

loading is required, at least in a general way.

The approximate subsurface conditions or soil profile is to be ascertained.

Each of the customary types of foundation is considered briefly to judge whether it is suitable under the existing conditions from the point of view of the criteria for stabilitybearing capacity and settlement. The obviously unsuitable types may be eliminated, thus narrowing down the choice.

More detailed studies, including tentative designs, of the more promising types are made in the next phase.

Final selection of the type of foundation is made based on the costthe most acceptable compromise between cost and performance.

Appropriate foundation types for certain soil conditions

Causes of settlement

Settlement of footings

The total settlement of a footing in clay may be considered to consist of three components (Skempton and Bjerrum, 1957)

S = Sc + Si + Sswhere, S = total settlement

Si = immediate elastic settlement

Sc = consolidation settlement

Ss = settlement due to secondary consolidation of clay

The immediate settlement is the elastic settlement and can be computed from the following expression based on the theory of elasticity:

= 1 2

Where, q = uniform pressure on the foundation

B = least lateral dimension of footing

= modulus of elasticity of soil beneath the foundation

= Poissons ration of the soil

= influence factor = 0.88 for rigid circular footing= 0.82 for rigid square footing

= 1.06 for rigid rectangular footing with L/B = 1.5

= 1.70 for rigid rectangular footing with L/B = 5

Values of Influence Factor (IS:8009 PART I-1976)

Shape of loaded area Flexible footing Rigid footing Col. (5) = 0.8 x Col. (2)

Centre Corner Average

(1) (2) (3) (4) (5)

1. Circular 1.00 0.64 0.85 0.80

2. Rectangular (L/B)

1.0 1.12 0.56 0.95 0.90

1.5 1.36 0.68 1.20 1.09

2.0 1.52 0.77 1.31 1.22

5.0 2.10 1.05 1.83 1.68

10.0 2.52 1.26 2.25 2.02

100.0 3.38 1.69 2.96 2.70

Jambu, Bjerrum and Kjaernsli (1966) have proposed the following equation for computing the immediate settlement

= 01 1 2

Where, = 13

/

=1 2

Test 1

Test 2Test 3

Consolidation Settlement,

=

1 + 0. log10

0 +

0Where, 0 = effective initial overburden pressure due to soil overburden, measured at the centre of the layer

= vertical stress increment due to footing load, at the centreof layerCc = compression index = 0.009(wL 10)e0 = initial voids ratio

H = thickness of compressible layer

C = a coefficient or correction factor depending upon the geometry of the footing and the history of loading on the clay (i.e. on the pore pressure coefficient A)

In the absence of any other data C may be taken as unity for the clay

Permissible Settlements

Terzaghi and Peck (1948) specify a permissible differential settlement of 20 mm between adjacent columns and recommend that foundations on sand be designed for a total settlement of 25 mm.

Skempton and MacDonald (1956) specify that the angular rotation or distortion between adjacent columns in clay should not exceed 1/300, although the total settlement may go up to 100 mm.

Bozozuk (1962) summarised his investigations in Ottawa as follows:

Remedial Measures Against Harmful Settlements

Removal of soft soil strata, consistent with economy.

The use of properly designed and constructed pile foundations

Provision for lateral restraint against lateral expulsion of soil mass from underneath the footing of a foundation.

Building slowly on cohesive soils to avoid lateral expansion of a soil mass and to give time for the pore water to be expelled by the surcharge load.

Reduction of contact pressure on the soil; more appropriately, proper adjustment between pressure, shape and size of the foundation in order to attain uniform settlements underneath the structure.

Preconsolidation of a building site long enough for the expected load, depending upon the tolerable settlements; alternatively, any other method of soil stabilization