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Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)

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Bearing capacity of piles

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 also depends on the methods of installation

  1. Analytical Method
    (i) Qup = Qeb + Qsf
    (ii) Qup = qbAb + qsAs
    where,
    Qup = Ultimate load on pile
    Qeb = End bearing capacity
    Qsf = Skin friction
    q= End bearing resistance of unit area.
    q= Skin friction resistance of unit area.
    Ab = Braking area
    As = Surface areaDeep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)(iii) qb ∼ 9C
    where, C = Unit Cohesion at base of pile for clays
    (iv) Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, α = Adhesion factor
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) Unit adhesion between pile and soil.
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) Average Cohesion over depth of pile.
    (v) Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, Fs = Factor of safety.
    (vi)
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    F1 = 3 and F2 = 2
    ≈ F= F2 = 2.5
    (vii) For Pure Clays Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
  2. Dynamic Approach
    Dynamic methods are suitable for dense cohesionless soil only.
    (i) Engineering News Records Formula
    (a) Qup = WH/S + C
    (b) Qap = Qup/6 = WH/(S + C)
    where,
    Qup = Ultimate load on pile
    Qap = Allowable load on pile
    W = Weight of hammer in kg.
    H = Height of fall of hammer in cm.
    S = Final set (Average penetration of pile per blow of hammer for last five blows in cm)
    C = Constant
    = 2.5 cm → for drop hammer
    = 0.25 cm → for steam hammer (single acting or double acting)
    (c) for drop hammer
     Qap = WH/6(S + 2.5)
    (d) For single Acting Stream Hammer
    Qap = WH/6(S + 02.5)
    (e) For Double Acting Stream Hammer
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where P = Stream pressure
    and a = Area of hammer on which pressure acts.
    (ii) Hiley Formula (I.S. Formula)
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) 
    where, Fs = Factor of safety = 3
    ηh = Efficiency of hammer
    ηb = Efficiency of blow.
    ηh = 0.75 to 0.85 for single acting steam hammer
    ηh = 0.75 to 0.80 for double acting steam hammer
    ηh = 1 for drop hammer.
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, w = Weight of hammer in kg.
    p = Weight of pile + pile cap
    e = Coefficient of restitutions
    = 0.25 for wooden pile and cast iron hammer
    = 0.4 for concrete pile and cast iron hammer
    = 0.55 for steel piles and cast iron hammer
    S = Final set or penetrations per blow
    C = Total elastic compression of pile, pile cap and soil
    H = Height of fall of hammer.
  3. Field Method
    (i) Use of Standard Penetrations Data
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) 
    where, N = Corrected S.P.T Number
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) Average corrected S.P.T number for entire pile length
     Qap = Qup/Fs
    F= Factor of safety
    = 4 → For driven pile
    = 2.5 → for bored pile.
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    (ii) Cone penetration test
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, qc = static cone resistance of the base of pile in kg/cm2
    qc = average cone resistance over depth of pile in kg/cm2
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) Area of bulb (m)2

Under-Reamed Pile
An 'under-reamed' pile is one with an enlarged base or a bulb; the bulb is called 'under-ream'.
Under-reamed piles are cast-in-situ piles, which may be installed both in sandy and in clayey soils. The ratio of bulb size to the pile shaft size may be 2 to 3; usually a value of 2.5 is used.
Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
As1 = πbL1qs1 = αC α < 1.
As2 = πbuL2qs2 = αC α < 1.
where, bu = dia of bulb, Spacing = 1.5 bu.
Qup = qbAb + qs1As1 + Qs2As2

Negative Skin Friction
Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)

  1. For Cohesive sol
    Qnf = Perimeter. Lα C for Cohesive soil.
    where, Qnf = Total negative skin frictions
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, Fs = Factor of safety.
  2. For cohesionless soils
    Qnf = P x force per unit surface length of pile
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    (friction force = μH)
    Where γ = unit weight of soil.
    K = Earth pressure coefficient (Ka < K < Kp)
    δ = Angle of wall friction. (φ/2 < δ < φ)

Group Action of Pile

The ultimate load carrying capacity of the pile group is finally chosen as the smaller of the

  1. Ultimate load carrying capacity of n pile (n Qup)
    and
  2. Ultimate load carrying capacity of the single large equivalent (block) pile (Qug).

To determine design load or allowable load, apply a suitable factor of safety.
Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)

  1. Group Efficiency (ηg)
    ng = Qug/n.Qup
    Qug = Ultimate load capacity of pile group
    Qup = Ultimate load on single pile
    For sandy soil → ηg > 1
    For clay soil → ηg < 1 and ηg > 1
    Minimum number of pile for group = 3.
    Qug = qbAb + qsAs
    where qb = 9C for clays
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    (i) For Square Group
    Size of group, B = (n – 1) S + D
    where, η = Total number of pile if size of group is x.x
    They η = x2
    (ii) Qug = η.Qup
    (iii) Qug = Qug/FOS where, Qug = Allowable load on pile group.
    (iv) Sr = Sg/Si
    where, Sr = Group settlement ratio
    Sg = Settlement of pile group
    Si = Settlement of individual pile.Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
  2. When Piles are Embended on a Uniform Clay
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
  3. In case of Sand
    Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE)
    where, B = Size of pile group in meter.  
The document Deep Foundations - Notes | Study Foundation Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Foundation Engineering.
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