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There is an old rule that the area of a follower should be one-fifth of precast concrete pile. Why?

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The rules of wave mechanics suggested that to avoid reflection of stress wave caused by different impedance values, acoustic impedance should be the same for the follower and precast concrete piles. As such, it enhances smoothest driving and prevent follower from bouncing on the head of piles which is undesirable as it may damage the piles and lowers the efficiency of driving.

Acoustic impedance = Elastic Modulus (E) x Area of Pile / wave velocity
Wave velocity (c) = Elastic Modulus / Density of Pile

For normal steel, E=205GPa, c=5,100m/s
For normal concrete, E=30GPa, c=3,800m/s

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For same acoustic impedance,
Area of concrete/Area of steel = (205/5,100)/(30/3,800) = 5

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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