Group Efficiency for Pile Rows in Water Tank

What happened to all of the easy questions?


Are you worried about axial - or lateral - group efficiency?

I am worried about axial capacity. Further research indicates little to no change in axial capacity for piles in this configuration (a long, continuous row with 3d spacing). But I feel that I am at my limit of comfort with the load we are asking the piles to carry.

A working load of 120 Tons/pile has been assigned with a factor of safety of 2 (actual maximum load considering dead and seismic/wind loads is 118 Tons/pile). I am worried that the hard tills will fracture, causing reduction in load capacity to subsequent piles. I have indicated that the piles are to be restruck, and we will be using PDA during installation, which gives me some comfort. I have used the till before with good results, just never to 120 Tons on a 12x53. I'm a little concerned about the pile itself, too(40,000 ft-lb energy hammer).

Thanks for responding, I know it is not an easy one.

Okay, now I understand your concerns.

I'm not concerned about group effects; but if you are still worried, treat the bearing forces as a series of point loads and run some elastic settlement analyses.

How does the 120 ton design capacity break out - how much friction? From which layers (break out by layer)? How much bearing?

I don't have any experience with glacial till; but the crush pressure for silica sands is about 100 ksf. Check the contact pressure of your H-pile (I'm too lazy to look up the bearing area) and see how close you are to this "fine grain soil" limit. (It doesn't apply specifically to glacial till, but it is a good "rule of thumb".)

I suspect that a "punch-through" failure is unlikely.

Do not overdrive your piles! Trust the PDA (assuming that you have good equipment and an experienced operator.)

dch337:

The design has obviously been made based on structural capacity of pile. In regard to the capacity it depends on what allowable steel stes that was used some use 9000 psi others 12,000 and yet others 18,000. The end area of the steel section is used. You may wish to check the capacities for each of these stresses. In some cases it has been argued that 20,000 psi is acceptable as well. Many use 12,000 psi or 18,000 psi.

Regarding geotechnical capacity, again this is a numbers game for tills. You can have the design based on granular till which invokes friction values. If you look at the bearing capacity factors N sub q for a friction angle of 35 degrees can vary from 40 by Terzaghi and Peck to 200 by Meyerhoff. Berezantsev provides 60 which seems to be acceptable. Higher friction values give higher bearing capacity factors. Some would use 40 degrees which puts the Berezantsev value to about 200. The end area of the plugged pile is used and not the steel area.

However despite the above and possibility of lower geotechnical capacities than the allowable structural capacity, the pile in the soil described would not fail and the proposed loading, although a bit high, is reasonable.

Some more comfort may be had with a heavier section as the area increases and so will the capacity. However, experience in the area with similar structures is what may govern.

I have no problems in recommending 100 ton capacity for the piles for the conditions described. If you change the section weight then the cost goes up as well for material, I would think. You may want to do some simple number crunching to defend your position of comfort.

Failure of the till by punching is unlikely to occur.

The piles will be driven to the depth prescribed without much difficulty so I would not worry about problems unless large cobbles are encountered.

[Cheers]

Thank you for your comments.

Yes, the H-pile section will be 50Ksi steel, with a ~17500 psi working stress max, approximately 13Kpsi at the tip assuming only the cross section. However, and as you mentioned, VAD, I have assumed a box area at the end, thus providing one square foot of area.

I did use a phi of 40 to model the till, as modeling with cohesion was not as realistic. Furthermore, I assumed only the flange widths for skin friction friction, which produced an ultimate capacity of ~140kips.

I agree with wanting a larger section. But I feel better about my calcs now.

Thanks again for the responses.