Pile group reduction factor

[EQguys]

We have serious debate going on in our office regarding the above mentioned topic. Let me explain the problem.

The Geotech firm has given us the Modulus of Horizontal subgrade reaction at various soil layers in terms of

(Kh*B)/(D*C) where
Kh = modulus in lbs/in^3
B = pile size
D = reduction factor for effect of pile group action
C = initial loading factor assumed as 1.0 (no debate here)

They have also given us the reduction factor D for various pile spacing in terms of pile size B. For example

Spacing of 3B, D=0.25
Spacing of 4B, D=0.40 and so on.

We have battered piles as foundation and the normal practice is to take a weighted average (over the different layers of soil) of the above KhB/DC values over a 10B depth and use it for pile force calculations. In doing so, since the piles batter (and hence pile spacing changes), we take the pile spacing at 5B depth (average depth) and multiply the final value of KhB/DC with the D value at 5B depth.

The debate is whether to take the D value at 5B depth OR do we have to take the D value at top/middle/bottom of each layer of soil since the KhB/DC for each layer is changing.

This debate is going to affect all our force computations and hence an educated opinion is very much required and appreciated.

"Does the man make the journey or does the journey make the man" - Mark Twain

 

[kslee1000]

Make a sample study use & compare results of both methods. Then decide by reasoning the outcomes.

 

[oldestguy]

Not to, throw cold water on the issue,but I may be too removed from reality in my over 50 years of experience. but I have never heard of a case of pile failure due to making a wrong decision on group action or efficiency.

Matter of fact, the great majority of pile cases I have been involved with have neglected the possible effect of group action efficiency reduction, for lack of a better word.

Those rare few jobs that have failed usually were due to some other effect, such as artesian water flow or a soft layer below the tips.

In summary, neglect the possible group efficiency and rely on a safety factor for the many unknowns not dealing with pile group action, such as hammer problems and the like.

 

[msquared48]

I can understand a reduction factor if the pile are spaced closer together than recommended, but not farther apart. Furthermore, with the pile farther apart at 4D than 3D, why a HIGHER reduction factor? It should be lower, if any. This makes no sense to me.

Furthermore, I can see a reduction with friction pile, but not end bearing.

Mike McCann
MMC Engineering