The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground.
The bearing capacity of a pile depends primarily on 3 factors as given below,
- Type of soil through which pile is embedded
- Method of pile installation
- Pile dimension (cross section & length of pile)
While calculating pile load capacity for cast in situ concrete piles, using static analysis, we need to use soil shear strength parameter and dimension of pile.
LOAD CARRYING CAPACITY OF PILE USING STATIC ANALYSIS
The pile transfers the load into the soil in two ways. Firstly, through the tip-in compression, termed as “end-bearing” or “point-bearing”; secondly, by shear along the surface termed as “skin friction”.
LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESIVE SOIL
The ultimate load carrying capacity (Qu) of pile in cohesive soils is given by the formula given below, where the first term represents the end bearing resistance (Qb) and the second term gives the skin friction resistance (Qs).
Where,
Qu = Ultimate load capacity, kN
Ap = Cross-sectional area of pile tip, in m2
Nc = Bearing capacity factor, may be taken as 9
αi = Adhesion factor for the ith layer depending on the consistency of soil. It depends upon the undrained shear strength of soil and may be obtained from the figure given below.
ci = Average cohesion for the ith layer, in kN/m2
Asi = Surface area of pile shaft in the ith layer, in m2
A minimum factor of safety of 2.5 is used to arrive at the safe pile load capacity (Qsafe) from ultimate load capacity (Qu).
Qsafe = Qu/2.5
LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESION LESS SOIL
The ultimate load carrying capacity of pile, “Qu”, consists of two parts. One part is due to friction, called skin friction or shaft friction or side shear denoted as “Qs” and the other is due to end bearing at the base or tip of the pile toe, “Qb”.
The equation given below is used to calculate the ultimate load carrying capacity of pile.
Where,
Ap = cross-sectional area of pile base, m2
D = diameter of pile shaft, m
γ = effective unit weight of the soil at pile tip, kN/m3
Nγ= bearing capacity factor
Nq = bearing capacity factor
Φ = Angle of internal friction at pile tip
PD = Effective overburden pressure at pile tip, in kN/m2
Ki = Coefficient of earth pressure applicable for the ith layer
PDi = Effective overburden pressure for the ith layer, in kN/m2
δi = Angle of wall friction between pile and soil for the ith layer
Asi = Surface area of pile shaft in the ith layer, in m2
The first term is the expression for the end bearing capacity of pile (Qb) and the second term is the expression for the skin friction capacity of pile (Qs).
A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity (Qsafe) from ultimate load capacity (Qu).
Qsafe = Qu / 2.5
IMPORTANT NOTES TO REMEMBER
- The value of bearing capacity factor Nq is obtained from the figure given below.
- The value of bearing capacity factor Nγ is computed using the equation given below.
- For driven piles in loose to dense sand with φ varying between 300to 400 , ki values in the range of 1 to 1.5 may be used.
- δ, the angle of wall friction may be taken equal to the friction angle of the soil around the pile stem.
- The maximum effective overburden at the pile base should correspond to the critical depth, which may be taken as 15 times the diameter of the pile shaft for φ ≤ 300and increasing to 20 times for φ ≥ 400
- For piles passing through cohesive strata and terminating in a granular stratum, a penetration of at least twice the diameter of the pile shaft should be given into the granular stratum.
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HOW TO INCREASE DURABILITY OF CONCRETE PILES?
DURABILITY OF CONCRETE PILES
Properly mixed concrete compacted to a dense impermeable mass is one of the most permanent of all constructional material and give little cause of concern about its long-term durability in a non aggressive environment. However, concrete can be attacked by sulphate and sulfuric acid occurring naturally in soils, by corrosive chemicals which may be present in industrial waste in fill materials and by organic acids and carbon dioxide present in ground water as a result of decaying vegetable matters. Attack by sulphate is a disruptive process whereas the action of organic acids or dissolved carbon dioxide is one of leaching. Attack by sulphuric acid combines features of both processes. The severity of attack by soluble sulphates must be assessed by determining the soluble sulphate content and the proportions of the various cat ions present in an aqueous extract of the soil. These determinations must be made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%.
A dense, well compacted concrete provides the best protection against the attack by sulphate on concrete piles, pile cap and ground beams. The low permeability of dense concrete prevents or greatly restricts the entry of the sulphates in to the pore spaces of the concrete. For this reason high strength precast concrete piles are most favorable type to use. However they are not suitable for all the site conditions and bored cast in situ / driven cast in situ piles if adopted must be designed to achieve the required degree of impermeability and resistance to aggressive action. Neither high alumina cement nor super sulphated cement is favored for piling work. Instead, reliance is placed on the resistance of dense impermeable concrete made with a low water cement ratio. Coating of tar or bitumen on the surface, metal sheeting or glass fibre wrapping impregnated with bitumen may be adopted.
Pile caps and ground beams can be protected on the underside by a layer of heavy gauge polythene sheeting laid on a sand carpet or on blinding concrete. The vertical sides can be protected after removing the form work by applying hot bitumen spray coats, bituminous paint, trowelled on mastic asphalt or adhesive plastic sheeting.
Precautions against the aggressive action by sea water on concrete need only be considered in respect of precast concrete piles. Cast in situ concrete is used only as a heart to steel tubes or cylindrical precast concrete shell pills. For precast concrete piles for marine condition, a minimum ordinary portland cement content of 360 kg/m3 and a maximum water cement ratio of 0.45 by weight should be adopted.
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