Minimum Pile Spacing and Group Effect

[humanengr]

Generally, Pile Spacing is limited to a minimum of 3 X Diameter.
Question: I have a case where the Pile Spacing is less than 3 X Dia. (approx. 2-1/2 X Dia.), and the Group capacity exceeds the sum of the individual pile capacities, therefore, the individual pile capacity is used as the limit for each pile load.
In such a situation, is a reduction of the individual pile capacity required due to the spacing?

 

[SlideRuleEra]

You did not mention which pile group efficiency formula you are using, there are several. All of them give approximate answers at best. A popular one is the Converse-Labarre formula, shown below. It takes into account the pile spacing with the term Theta which is the arc tangent (in degrees) of diameter / spacing. For example, for a spacing of 3 diameters, Theta is 18.4 degrees. For spacing of 2.5 diameters, Theta is 21.8 degrees.

You will probably note that even for large pile spacing, say 10 pile diameters, the formula still shows group efficiency lower than 100%... that's one indication of the limitations of these calculations. Use them with that in mind.

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[humanengr]

SlideRuleEra - thank you for the response.
I simply calculated the Pile Group Capacity based on friction along the perimeter of the pile group block.
It seems the Converse-Labarre equation is more conservative (33% less Group Efficiency).

However, what I was trying to determine, is this:
The Pile Group Capacity exceeds the sum of the individual pile capacities.
Therefore, Individual Capacity governs and the capacity of the group = Sum of Individual Piles.

In this case, is any reduction factor (even if Group Efficiency > 1.0) required simply
because of the pile spacing?
If so, that would imply there is an upper bound pile capacity somewhere between the individual capacity and the (group capacity divided by no. of piles) as a function of pile spacing.

 

[SlideRuleEra]

Ok, I now understand how you are making the calcs. Everything that I've seen about the pile group capacity block failure is that it applies for a pile spacing of 2 to 3 pile diameters. This is another method that is based on limited field data. IMHO, there is no way to fine tune the calculation to give more accurate results. You will have to use judgment and perhaps make individual pile load test(s). Despite advances in pile driving monitoring, it remains as much art as it is science.

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