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# Group Action in Piles

The efficiency of pile group depends upon __________

a) Characteristic of pile and Spacing of pile

b) Bearing capacity of soil

c) All of the mentioned

d) None of the mentioned

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Answer: a

Explanation: The efficiency of pile group depends upon the following factors: Characteristics of pile (i.e. length, diameter, material etc.), spacing of pile, total number of piles in a row and number of rows etc.

Which of the following formulae can be used for determining the efficiency of pile group?

a) Dynamic formulae

b) Static formulae

c) Feld’s formulae

d) Hiley’s formulae

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Answer: c

Explanation: Converse Labarre formulae, Seiler-kenney formulae, Feld’s rule are some of the common formulae available for determining the efficiency of pile group.

In which of the following rule, the value of each pile is reduced by one-sixteenth?

a) Converse Labarre formulae

b) Feld’s formulae

c) Seiler-Keeney formulae

d) All of the mentioned

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Answer: b

Explanation: According to Feld’s rule, the value of each pile is reduced by one-sixteenth on account of the effect of the nearest pile in each diagonal or straight row of which the pile in question is a member.

The bearing capacity of a single pile in clay is mainly due to _________

a) Friction

b) Shear strength of soil

c) Allowable load

d) Ultimate load

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Answer: a

Explanation: The bearing capacity of single pile in clay is mainly due to friction, and the friction and the point bearing resistance may be negligible.

The downward drag acting on a pile due to the movement of the surrounding is called ________

a) Skin friction

b) Negative skin friction

c) Frictional force

d) None of the mentioned

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Answer: b

Explanation: Negative skin friction is downward drag acting on a pile due to the downward movement of the surrounding compressible soil relative to the pile.

The area of the pile group along the failure surface is equal to __________

a) Perimeter × Area of cross section

b) Breadth × Length

c) Perimeter × Length

d) Perimeter/area of cross section

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Answer: c

Explanation: The area of the pile group, along failure surface is approximately equal to the perimeter P of the pile group multiplied by the length L of the pile.

The pile spacing of each pile is taken as __________ diameter of the pile.

a) Four

b) Five

c) Three

d) Ten

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Answer: c

Explanation: A spacing of three times the diameter of piles is commonly selected as trial spacing between the piles and checked against the criterion that the resistance Qs obtained.

The settlement of a group of friction piles can be computed on the assumption that ________

a) Clay is incompressible

b) Pile below the lower level is ignored

c) Bearing resistance is zero

d) None of the mentioned.

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Answer: a

Explanation: As a rough approximation, the settlement of a group of friction piles can be computed on the assumptions that the clay contained between the top of piles is incompressible.

The equation used for determining the bearing capacity of a group of friction piles is __________

a) Qup = Rf + Rp

b) Qug = n Qup . ηg

c) Qug = n Qup / ηg

d) None of the mentioned

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Answer: b

Explanation: A method of estimating the bearing capacity of a group of friction piles is to multiply the quantity nQup by a reduction factor called the efficiency of pile group.

Qug = n Qup . ηg.