Question 15.1: Compute pile capacity of identical piles in the same soil A ...
Compute pile capacity of identical piles in the same soil
A = cross-sectional of 20-in. square pile = 400 sq. in. (2580 cm²)
L = length of the pile = 40 ft (12.2 m)
W = weight of ram = 3.75 tons (33.4 kN)
P = weight of pile = 8.25 tons (73.4 kN)
h = fall of ram = 4.0 ft (1.22 m)
H = 12 h, the fall of ramin. = 48 in. (121.92 cm)
S = penetration per blow = 0.15 in. (0.381 cm)
R = resistance of pile according to various formulas
E = modulus of elasticity of concrete (2,000,000 psi) 2 \times10^{6} psi (13,789 MPa)
e = coefficient of restitution
k = hammer blow efficiency from Equation
1 By the Engineering News formula (Eq. (15.8))
R=\frac{2WH}{S+C} (15.8)
R=\frac{2\times 3.75\times 4.0}{0.15+0.1} =120 tons \left(1067 kN\right)2 By the Modified Engineering News formula (Eq. (15.9))
R=\frac{2WH}{S+0.1\left(P/W\right) } (15.9)
R=\frac{2\times 3.75\times 4.0}{0.15+0.1 \left(16,500/7500\right) } =81 tons \left(720 kN\right)
3 By the Terzaghi formula (Eq. (15.10))
\frac{AE}{L} =400\times \frac{2,000,000}{40\times 12} = 1,660,000 lb/in.\left(2907 kN/cm\right)WH\left(\frac{W+Pe^{2} }{W+P} \right)=7500\times 48\left(\frac{7500+16,500\times 0.5^{2} }{7500+16,500} \right) =174,000
Then substituting in Equation (15.10):
R=\frac{AE}{L} \left[-0.15\pm \sqrt{0.15^{2}+\frac{2WH\left(W+Pe^{2} \right) /\left(W+P\right) }{AE/L} } \right] (15.10)
R=1660,000\left[-0.15\pm \sqrt{0.15^{2} }+\frac{174,000\times 2}{1660,000} \right]
=1660,000\left(-0.15\pm 0.48\right)
Since the resistance cannot be a negative quantity,
R = 0.33 × 1,660,000 = 550,000 lb = 275 tons (2446 kN)
Comparison of computed pile capacities:
1 Engineering News formula 120 tons (1067 kN)
2 Modified Engineering News formula 81 tons (720 kN)
3 Terzaghi formula 275 tons (2446 kN).
Based on aforementioned results, there are significant variations, therefore, it should noted that pile driving formulas are only a guide to the engineer in predicting safe pile bearing capacities and should be used with caution and judgment.