Foundation depth
With increasing depth the bearing capacity increases not only because of the overburden pressure but also due to the failure pattern of deep foundations. The effect of overburden is the main consideration in deciding the depth of shallow foundation.
Inclination of the load
The bearing capacity decreases rapidly with larger inclination of the load from the vertical and this reduction is more pronounced for horizontal bases than for inclined bases so that with inclined loads there is some advantage in adopting inclined
bases.
IS CODE RECOMMENDATIONS
Both Hansen and Meyerhof have given their equations for the various effects given above. Based on these values, IS 6403-1981 has recommended the following
equations for the calculation of the ultimate bearing capacity of shallow footings.
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where,
B = width of foundation
c = undrained cohesion of soil
q0 = effective overburden pressure at foundation level = D γ0 = effective unit weight of soil above foundation level γ1 = effective unit weight of soil below foundation level
Nc, Nq and Nγ are the bearing capacity factors sc, sq and sγ are shape factors
dc, dq and dγ are depth factors
ic iq and iγ are inclination factors which depend on inclination α of load to the vertical W’ is a factor for effect of water table which is 1 if water table is at a
depth B below the foundation level and linearly varies to 0.5 if it is at the base of foundation.
According to IS 6403 the value of Nc, Nq and Nγ is to be taken as those derived by Vesic. They may be also calculated from the following equations (Vesic's equations).
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The values of these modifying factors as recommended in IS 6403-1981
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Effect of Footings on Sloping Ground
For a gradually sloping ground, the bearing capacity for a depth equal to that of the centre of gravity of the foundation can be used for design.
When the footing is very near to the edge of a steep slope, it is better to make a stability analysis of the slope to determine the safety conditions taking into account the variation of the water level also.
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Effect of Shape of Base of Foundation
In cohesionless soils, the bearing capacity is greater under concave foundation (looking from below) than under convex shapes. The increase can be as much as 20% depending on the relative density of the soil and curvature of the foundation.
General Equations for Bearing Capacity
Meyerhof's method is more popular in North America and Hansen's factors in European countries IS 6403 recommendation (Hansen's values) can be safely used for all practical designs.
The following procedures can be used for calculation of bearing capacity of footings:
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Uses of piles
To carry loads which are too heavy to be supported by a shallow foundation and are to be transferred to deeper, stronger and less compressible strata or over a larger depth of the foundation soil.
To carry part of the load to deeper soil for reducing the settlement as in piled raft foundations.
To carry horizontal loads as in bridge abutments or retaining walls and also to increase the stability of tall buildings.
To withstand uplift forces in foundations as in expansive soils and floating foundations.
To avoid loss of support by scour as in bridges.
To produce large differential settlement in situations where there are large variations of column loads.
To compact foundation material such as loose sands.
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Types of Piles
On the basis of their size (diameter)
Piles larger than 600 mm in diameter are called large diameter piles.
Sizes 300 to 600 mm are called normal or small diameter piles.
Piles of 150 to 250 mm in diameter are called mini piles while those below 150 mm diameter are classified as micro piles.
On the basis of the method of installation
driven cast in-situ
bored cast in-situ
precast driven
precast piles driven in pre-bored holes
On their action (that is, the purpose they are intended to serve)
Displacement piles (driven piles)
Non-displacement piles (bored piles)
Small displacement piles (driven steel H pile)
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Codes on Piles
The specification for the four types of commonly used concrete piles are covered by Indian Standard IS 2911 (Second revision) under the following heads:
Driven cast in-place (displacement) piles—Section 1 Bored cast in-situ (non-displacement) piles—Section 2 Pre-cast driven (displacement) piles—Section 3
Pre-cast piles driven in pre-bored (non-displacement) piles—Section 4.
In addition, the following Indian Standards also pertain to pile design and construction:
IS 2911 Part II—Timber piles
IS 2911 Part III—Under reamed piles IS 2911 Part IV—Load tests on piles.
Factors affecting Choice of Type of Pile
Disturbance of nearby old structures: Vibrations are caused during pile driving.
Length and size of pile: Precast R.C driven piles are small in size and are usually of length up to 16 m and size less than 550 mm. Bored piles can be taken very deep provided they are reasonably large. They can also be of large diameters.
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Time taken for piling: Driven precast and cast in-situ piles, can be more quickly executed than bored cast in-place piles. However if ground heave is expected, driven cast in-place piles will pose problems involving the integrity of the pile.
Loss of bearing at pile tip: In bored cast in-place piles, the success in washing the base of the pile depends on the availability of good equipment, workmen and
experienced contractors.
Surface water currents: In sandy areas near large water bodies subsurface flow
channels may exist. In such cases, the concrete in cast in-place piles can be washed out before it sets, thus causing local weakness.
Difficulty in pulling out casing: In pure sand deposits while using driven cast in-place piles it will be difficult to pull out the casing after concreting. Defects like necking occur in such cases.
Quality of concrete and its capacity to withstand deterioration: In bad environmental conditions with chlorides and sulphates precast driven piles are superior to cast in-place concrete which needs very good care and supervision in its in-placement. The dumping of concrete especially in driven cast in piles from large height and with the pile reinforcement in place, segregation of concrete cannot be avoided. It discourage the use of driven cast in-situ piles.
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Probability of negative skin friction: It is claimed that this can be reduced in precast piles by bituminous coatings. However because of larger disturbances produced while driving, driven piles produce more negative friction.
Possibility of pile damage during driving: If the driving is hard, precast driven piles tend to get damaged in the body due to driving stresses and at head due to
inadequacy of equipment or lack of strength at the top.
Possibility of socketing: For bearing piles in weathered rock, bored cast in-place piles are ideal and, if necessary, socketing of piles can also be carried out. This will increase pile capacity considerably in soft and weathered rock formations. When good rock is available at reasonable depths, bored piles taken to rock present the best solution.
Load Carrying Capacity
The static method based on soil properties for all types of piles.
The dynamic method using pile-driving formulae based on the resistance observed in the field in driving the piles for driven piles.
The wave equation method for driven piles. (Both the theoretical method and case method using field values are used.)
IS 2911 incorporates only the first two methods
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Effective Length Point Of Inflection
If the pile is projecting free above the ground level, the following criteria can be used to fund the point of inflection or contraflexure to find effective length [IS 2911 part I/sec 1 1999 clause 6.5.1]:
If the ground is firm, the depth of PI is taken as 1/10 the projecting pile
length or lm subject to a minimum of 3D (B.S. Code CP 2004 recommends it as 1.5 m).
If the top embedded stratum is soft below 0.1 kg/cm2 in undrained shear strength, the depth of point of inflection is to be taken as one-half the, depth of penetration but not more than 10D or 3 m, whichever is less.
If the stratum is liquid, mud is to be treated as water.
If the top end is fixed, both in position and in direction, the upper point of inflection may be taken as one-fourth the exposed length below the top of the pile.
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