Precast concrete pile of size 50 cm × 50 cm is to be driven into clay...
Soil is clay ≡ point bearing
Qu = CubNcAb + aCu avAs
Qu = 500 × FOS = 500×3 = 1500
1500 = 110/2 × 9 × 0.52 + 0.6 × 110/2 × 4 × 0.5 × L
L = 20.8522 m.
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Precast concrete pile of size 50 cm × 50 cm is to be driven into clay...
Given data:
Size of precast concrete pile = 50 cm × 50 cm
Unconfined compressive strength of clay strata = 110 kN/m²
Safe working load = 500 kN
FOS = 3
α = 0.6
To find: Length required to carry the safe working load
Solution:
1. Calculate the ultimate bearing capacity of the clay strata:
qult = C × Nc + γDf × Nq + 0.5 × γB × B × Nγ
where,
C = Unconfined compressive strength of clay strata = 110 kN/m²
γ = Unit weight of soil = 18 kN/m³ (assumed)
Df = Depth factor = 1 (for depth less than 3 times the width of pile)
B = Width of pile = 0.5 m
Nc, Nq, Nγ = Bearing capacity factors (values can be obtained from standard tables)
For clay,
Nc = 5.7, Nq = 1, Nγ = 0
qult = 110 × 5.7 + 18 × 1 × 1 + 0.5 × 18 × 0.5 × 0
qult = 672.6 kN/m²
2. Calculate the safe load carrying capacity of the pile:
qs = qult / FOS
qs = 672.6 / 3
qs = 224.2 kN/m²
3. Calculate the area of the pile:
A = 0.5 × 0.5
A = 0.25 m²
4. Calculate the ultimate load carrying capacity of the pile:
Qult = qs × A
Qult = 224.2 × 0.25
Qult = 56.05 kN
5. Calculate the safe working load of the pile:
Qsafe = Qult / α
Qsafe = 56.05 / 0.6
Qsafe = 93.42 kN
6. Calculate the length of the pile required to carry the safe working load:
L = Qsafe / σ
where,
σ = Allowable stress in the pile = Qult / L
Assume a length for the pile and calculate σ
If σ > qs, increase the length and repeat the calculation
If σ < qs,="" decrease="" the="" length="" and="" repeat="" the="" />
Let's assume the length of the pile as 20 m
σ = Qult / L
σ = 56.05 / 20
σ = 2.8025 kN/m²
L = Qsafe / σ
L = 93.42 / 2.8025
L = 33.33 m
Assuming a pile length of 20 m gives a σ value less than qs, so we need to increase the length of the pile.
Let's assume the length of the pile as 25 m
σ = Qult / L
σ = 56.05 / 25
σ = 2.242 kN/m²
L = Qsafe / σ
L = 93.42 / 2.242
L = 41.65 m
Assuming a pile length of 25 m gives a σ value greater than qs, so we need to decrease the length of the