Different pile loads within a group 2

[TigerGuy]

We are working on a project with multiple pile/drilled shaft foundations in the design. We have 3 different piles, each with its own loading configuration, within a few feet of each other.

Does anybody have some guidance on how to analyze the "group" effect on these elements? One pile will need a diameter of 5 feet (150 cm) and is within 6 feet center-center with 2 others with diameters of 3 feet (90 cm). Each has a different load combination. Would it be best to design a pile cap connecting the three and making it all work together to handle the loads?

I haven't found this situation in any other threads.

Thanks

 

[SlideRuleEra]

Can you post a plan of the area showing all the nearby piling?

http://www.SlideRuleEra.net

 

[TigerGuy]

Attached is a rough diagram of the situation. Each element supports independent structural component, and therefore has different axial, lateral and bending load.

I'm just hoping to learn how to analyze these with the obvious interference between them.

 

[SlideRuleEra]

Three independent loads on each of three closely and irregularly spaced independent piles of different sizes... I don't know how to analyze interference for this case. Group action is normally calculated for multi-pile groups with a common pile cap. All piles are the same size, arranged in a rectangular pattern and all having the same spacing between piles.

I do know that even if these piles are point bearing, they are too closely spaced for good practice. If they are friction piling the situation is much worse, probably unacceptable.

Also know what I would do about this situation (based on the limited info given).

1) As you mentioned, use a common pile cap for the three unequal loads.

2) Pile pattern, piling diameter and number of piling will have to be determined so that the resultant of the three unequal loads are shared equally by all supporting piles.

3) All piling will be the same size.

4) The pile pattern will not necessarily be rectangular.

5) All piles will be uniformly and appropriately spaced (at least 2.5 diameters, preferably 3 diameters).

6) The pile pattern and therefore the pile cap will likely have a larger footprint than the triangular pattern in the sketch.

7) There will not be a single "answer", numerous combinations of pile pattern, size and spacing will work... some may be more suitable than others for various reasons.

http://www.SlideRuleEra.net

 

[GeoPaveTraffic]

I agree with SlideRuleEra, I would under no circumstances use three separate pile caps and different pile sizes. There is just no way to determine the pile capacities, settlements, load transfers, etc. that will occur. Much better to use a single pile cap that combines the various loads and then size the piles to take the resulting load.

Mike Lambert

 

[TigerGuy]

Thanks for the responses. I think we need to go back to our client and discuss options for the situation. We can analyze piles in a group, but this configuration has us scratching our heads.

 

[EireChch]

It is possible to construct it as youve shown i.e. different pile sizes in a combined pile cap, however it would need 3D FEM such as plaxis to determine settlements, and load in each pile.