Which of the following statement is correct regarding negative skin friction?

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

II. The magnitude of negative friction for granular soil is 'p.c.Lf' (where p = perimeter of the pile, c = cohesion of soil in zone of Lf, Lf = depth of fill or soil which is moving vertically) 

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HPCL Engineer Civil 23 Dec 2023 Official Paper
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  1. Only II
  2. Both I and II
  3. Only I
  4. Neither I nor II

Answer (Detailed Solution Below)

Option 3 : Only I
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Concept:

Negative skin friction is an important concept in geotechnical engineering, especially in the context of pile foundations. It occurs when the surrounding soil moves downward relative to the pile, creating additional load on the pile.

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

  • This statement is correct. Negative skin friction occurs when the surrounding soil settles more than the pile, exerting a downward drag force on the pile.

Statement II: The magnitude of negative friction for granular soil is 'p.c.Lf' (where p = perimeter of the pile, c = cohesion of soil in zone of Lf, Lf = depth of fill or soil which is moving vertically).

  • This statement is incorrect. For granular soils (cohesionless soils), the magnitude of negative skin friction is generally expressed in terms of the unit weight of the soil and the effective stress, not cohesion. The expression involving 'c' (cohesion) applies to cohesive soils (clayey soils), not granular soils.

 

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