Question

Q a) What are the factors to be considered, while making choice between coffer dam and caisson?

    b) Describe pile driving by drop hammer method . State and explain dynamic formula for determining load carrying capacity of piles.

Answer 1

Answer (a)

Factors to be considered while making choice between coffer dam and caisson are as follows:

• Based on economy:
  • In case of small foundation plan area with respect to water depth
    • Cofferdam is uneconomical
    • Caisson is more suitable
  • Heavy foundation works ( 12 - 15 m deep under the standing water surface level)
    • Cofferdam is uneconomical
    • Caisson is more suitable
• Caisson is preferred in situation where unsuccessful dewatering of cofferdam. These situations occur due to many factors which are listed below:
  • Water depth
  • Soil nature
  • Soil permeability
• Cofferdams are preferred where it is easy to drive sheet piles into soil
• Caisson are preferred where it is difficult to drive sheet piles into soil (boulders)

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Answer (b)

Pile driving by drop hammer method

• Pile driving is the operation of driving pile into ground
• Drop hammer is a pile hammer for driving the pile
• Drop hammer is a heavy metal weight
• ·Working :
  • Hoist line lift the drop hammer , release it making it fall on pile top
• Pile cap position :- between the hammer and pile head (large dynamic force)
• Standard drop hammer weight is as follows :- 500-3000 lb
• Applications :
  • Remote projects (no strict time completion)
• Speed :- 4-8 blows per minute

Dynamic formula for determining load carrying capacity of piles

The two important Dynamic formulas are as follows:

a. Engineering news formula

b. Hiley’s formula

Engineering news formula

Where,

W = weight of the hammer

H = height of the fall

F = factor of safety (usually taken = 6)

S = final set or penetration

C = empirical constant

(C = 2.5 for drop hammer and C = 0.25 for single acting and double acting hammers)

Note: In case of double acting steam hammers product of effective area of piston and mean effective steam pressure should be added to the weight of the hammer.

Hiley’s formula

C = total elastic compression (sum of temporary elastic compression of dolly and packing and pile and soil)

is calculated as follows:

  in case W is greater than ‘eP’

  in case W is less than ‘eP’

Where,

P = weight of pile and helmet and follower

e = coefficient of restitution which ranges from 0 to 0.5

Note: factor of safety of 2 or 2.5 applied to obtain allowable load