Belled Pier Capacities

[FootNMouth]

Received a geotechnical report for a project site that has expansive clay soils. In the report is states that the belled pier capacity is to be reduced when the spacing of the bells is less than 3 bell diameters (clear space) based on the larger pier. The reductions are 40% for 2-3 diameters and 60% for 1-2 diameters.

Given that some of our columns are spaced rather closely together in certain areas we can't get many of the piers to converge on a solution. Does anyone have any literature on reduced capacity of belled piers based on proximity to other piers. I need some background knowledge so that I can convince the geotech to ease up on these reductions.

 

[BigH]

In the general, there is a reduced permitted loading because of zone of influence overlaps. It is not just for belled caissons. Check out Tomlinson or other foundation texts that discuss the topic.

 

[JAE]

Perhaps spread out the piers and use a pier cap between them to support the structure offset from the piers?

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[SlideRuleEra]

TXEng88 - Their may be an answer for you. You mention that the geotech's report states:

"...that the belled pier capacity is to be reduced when the spacing of the bells is less than 3 bell diameters (clear space)."

Spacing of 3 diameters is the commonly accepted spacing to prevent group action. This applies to both driven piling & drilled piers, whether they are point bearing or friction.

The loop hole you can present is that all credible references that I'm aware of measure the 3 diameters from center to center, not clear space - that is a big difference. Check out references such as "Principles of Foundation Engineering", by Braja Das. Also, make sure that you are reading the report correctly.

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[oldestguy]

I'd get back to the geotech and ask if a lowered elevation of support might be considered to use at the currently recommended bearing pressure. I suspect the support capabilities there would be higher, meaning overlap concerns would be reduced.

 

[Brad805]

Usually the limit we see is a spacing of 0.5 the bell diameter between bells. We deal with this a lot when we get into bin structures. These guidelines are usually quite arbitrary and if one is willing to pay the geotech to investigate a specific case it can save quite a bit of headache.

The pile diameter rule usually applies to skin friction piles, so it might be worth a call. The bell interaction should have more to do with the bulb around the bearing region. We put a lot of piles in CH or CM clay.