problem pilecap 6

I would be interested in opinions on a situation I have encountered as a checking engineer:

Rectangular pile cap, 4 large diameter piles, supporting R.C. walls.
60% of building loading walls built when engineer notes mistake that means piles will be overloaded in final condition.
Engineers proposal is that current load (D1) is now locked into the existing piles. 4 mini-piles are to be added to the pile cap in a roughly central position. The remaining load to be added (D2 + L1) would then be distributed to the original piles and the new mini-piles according to their stiffness. The load D1 would remain solely within the original piles.

I am not sure whether this last assumption is fully correct. Would the addition of mini-piles not affect stiffness and bending stresses of pile cap and therefore rredistribute load? Thoughts please.

 

[JAE]

Initially, the engineer is correct in that the D1 load is already "in" the original piling. With the addition of the new piling, the additional D2 + L would be distributed according to the relative stiffnesses of all the piling.

However, two things to consider, in my opinion, would be

1. The true stiffness of a pile is difficult to know with any degree of accuracy. Especially so with two different kinds of piling. So need to beware and be careful to add some conservatism to account for the unknown.

2. Over time, the piling will settle in at different rates due to different load/stiffness interaction. As the piling settles in, the original D1 load may be re-distributed out to the new piling.

 

[jeg]

Loopybear
Some thoughts to add to what JAE has provided:

Pile capacity usually requires a spacing of 2 to 3 diameters between piles. Will the mini piles infringe the spacing and thereby reduce the overall capacity?

If the mini piles are to be bored through the pile cap they will cut the cap rebar at the critical centre of span. The anchorage lengths of the rebar and the "beam" capacity should be checked.

 

JAE,

It is your point 2 that concerns me. 2 of the original piles are still overloaded in the final case with the argument that load can redistribute and the total capacity of piles > applied load?!??!

 

[redhead]

loopybear:
Can you extend the pile cap? This would allow you to place the added piles around the perimeter and not affect the structural integrity of the original cap.

 

[Ron]

In my opinion, both of JAE's points are right on target. Considering JEG's point of pile spacing is very important to the distribution effect.

Has anyone considered a full scale load test on the cap, with some additional instrumentation to check distribution and interaction? StatNamic (sp) or static load test could be done.

The magnitude of the mistake seems large. Has anyone else checked the Structural Engineer's work? Are you sure that he's right about the mistake? Is his design credible in the other areas of the building?

 

JAE,

Thank you for your excellent clear and concise reply which is usual for you.

 

[JAE]

I would second Ron's thought on load testing. At the very least, you would get a better handle on load/deflection (= stiffness) of the current piling. My second point was basically aimed at suggesting a higher level of conservatism in the additional piling added.

Ron's other point: What else is there in the design that may have been missed by the engineer? When we test composite studs, we usually check about 10% of them and if some are bad we extend the testing to more studs. Similarly, in this case, one mistake may be a warning sign that others are out there yet to be discovered.

 

[austim]

loopybear,

To help with your problem of estimating possible redistribution of load between 'old' and 'new' piles, we would need full details of everything below ground level.

Details of old and new piles - material, size, length, design assumptions (end bearing on impenetratable stratum, friction, combination ?)

Details of foundation materials over the full length of your piles.

Basis of your 'allowable' pile load. Is this the structural capacity of a single pile, the limiting load capacity of a single pile as controlled by the foundation materials, a limit imposed by considerations of permissible settlement, etc? Does it include any allowance for group effect?

Depending on your particular conditions, it is quite possible that the proposed additional piles will achieve nothing whatsoever.

 

[asdf]

I take it you have evaluated the feasibility of reducing the load on this section of wall, or considered utilizing an additional load bearing wall. I hope you have taken the time to send the engineer a letter letting him know he is responsible for the cost of whatever rework and modifications are necessary.

 

[PM]

Snipped from loopybear:"I am not sure whether. . . [the] assumption, [of load D1 remaining solely within the original piles] is fully correct. Would the addition of mini-piles not affect stiffness and bending stresses of pile cap and therefore redistribute load? Thoughts please.

The load interactions are anything but clear in your original post and they might be more complex than some assume. Without a sketch or free body diagram, its impossible to be sure of the orientation of the various pile sizes and types, as well as their proximity to the centre of load.

[ul][li]I think the previous comments, including those from Ron and particularly JAE are excellent, but they don't seem to address the specific question posed.[/li] [li]I think jeg is on to the central point which is the stiffness of the pilecap rather than the stiffness of the piles. Obviously both are relevant.[/li][li]Whatever method is being applied to the analysis of these piles / pilecaps, I suspect there may be an assumption that the pilecap is rigid. Doubtless, this may not be the case, particularly for a deep beam with mini or micro piles driven through it.[/li][li]Pile proximity to the 'elastic centre' of the foundation becomes increasingly relevant. I assume there is no significant eccenticity between the load centre and the elastic centre, but I wonder.[/li][li]Nobody mentioned if mini-pile pre-loading is / was considered either. This could reduce loads on original piles and balance the final laods nicely.[/li][/ul]Who ever suggested full scale load testing of the pilecap / pile reaction, has the definitive answer. Without these data, there are too many unknowns to be unequivocal and therefore significant conservatisim must be applied to any remedial work in order to consider the foundations safe.
Regards,

 

[LPPE]

How overloaded are the piles? Dont forget the factor of safety. If you have a 100 ton design capacity pile and your loading it with 102 tons actual, not a big deal.

 

[billyrayjimbob]

asdf,

Do you know for a fact that the engineer is responsible for the rework and modifications? Do you know whether it is loopbear's responsibility to write such a letter?
No, I didn't think so!

I have been following this thread from the start and it is a pity that loopybear has been censored. The fact is that there is lot of unecessary guff in this thread which dilutes some excellent responses, particularly PM and JAE.
Stay focussed guys.

 

[ayfarm]

I would re-investigate the pile capacities first, now that they have attained setup time, using modified PDA instruments and weap re-runs. If still found inadequate, depending upon soil conditions, I would "densify" the soil below by injecting cement/sand/gravel or concrete mix along pile to up the "resistances". Email me for details on a similar project I did.

 

[PFCIVIL]

It seems to me that a good geotechnical engineer may be of value here. That person would be familiar with the exact theoretical mechanisms of transfer of load from the piles and mini-piles to the surrounding subsurface. He (she) may be able to offer some assistance on expected differential rates of settlement and effect of pile capacity on close spacings.