pile groups

[terzaghi83]

When looking at pile groups in clay:

assuming individual pile capacity: The friction term is : Qf= Perimeter*Length*alpha*cohesion or Qf= Perimeter*Length*adhesion

where adhesion=Ca=alpha*c

References I see show the following for group action:

Qf= Group perimeter*length*cohesion (No alpha term is used).

Is this correct? Das , fundamentals 3rd edition example 14,9 shows the above to be true.e.

However, in the Goswami PE Practice Exam Book, problem 338, they use the adhesion factor when looking at friction of the group.

Does anyone know which method is correct?

Thanks.

 

[YMZ PE]

The value of the adhesion factor depends on the cohesion of the clay and can be obtained from the Tomlinson (1979) charts for the alpha method. It sounds like the Das example assumed a factor of 1.0.

 

[terzaghi83]

Thanks for the reply.

In the equation listed for pile group friction in both V.N.S. Murthy and various Das texts, the alpha*Cu (or Ca) term is not present at all... not just in the example problems. Instead, it just uses the cohesion only for pile group friction.

The alpha term (or Ca) IS used for individual pile friction capacity equations in both V.N.S. Murth and Das, so I was under the impression that one should not use alpha at all when looking at group skin friction. Is this correct?