Pile Capacity in Deep Excavation

[moe333]

Hi all,

I have a project where I will need to evaluate liquefaction potential and provide geotechnical pile capacity (driven or bored) at an elevation up to 40 feet below the elevation of the explorations (CPTU and Borings). The site will be excavated to this depth to construct 4 levels of underground parking. In addition, one portion of the building may not have underground parking. The soils are anticipated to be loose to medium dense silty to clayey sands, some relatively thin clay layers, with weak bedrock at a depth of about 80 feet.

I have not yet explored the site, but I am anticipating groundwater may be at about 30 feet so dewatering may be required. In addition, the site will likely be liquefiable.

I have the following questions:

I typically evaluate the liquefaction potential from the CPTU data using Robertson's method in accordance with the NCEER guidelines. I also evaluate it from the SPT data and fines content as a check. But I am at a loss as to how I would evaluate the liquefaction potential for site conditions that will be 30 to 40 feet below the exploration elevation? The CPT readings and SPT's are affected by the overburden, so once you remove the overburden, these values would change. However, near the edge of the excavation, the soil below the excavation bottom would probably feel some effect from the overburden

Similar for the geotechnical pile capacity, how would I evaluate the pile capacity for a condition where the site will be excavated 30 to 40 feet? I don't think I could simply subtract the capacity from the upper 30 to 40 feet since the soil capacity below this depth is partly due to the overburden above as well. An added complexity is that one portion of the building would not have a basement so the pile caps for this portion of the building would be 30 to 40 feet above the basement level.

I cannot find any references for this scenario for either liquefaction analysis or pile capcity design.

For a site where there is insignificant cut or fill below the exploration depth, I would typically evaluate the geotechnical pile capacity using the LCPC method.

Has anyone had experience with a similar scenario?

Thanks for any comments.

 

[BigH]

moe333: Just a question and comment. Are you going to recommend additional borings so that you have the information at the depth you are interested in? I would think that to try to design without borings of sufficient depth - relying only on the borings that are too short might not be such a good idea. And, I am sure that you have thought of this as well. Another question is as to why you think that you might have to use piles in the building part with the underground parking. If the design could accomodate, you might be able to design the underground areas as a compensated (bouyancy) raft foundation - whereby the 30'x120pcf+10'x60pcf = 4200psf might be enough to offset the weight of the structure. Just a thought (see Tomlinson's book on foundation design). As for the design of the part without basement - I can see this going on piles if needed - it will also prevent the building from loading your basement walls - but I would keep the non-basement section "separate" from the basement section. I would think that the unloading might not have "that much" effect on your SPT values. But, you could base your design on your best information and then after the excavation is completed, carry out some boring checks to see if there really was an effect - if so, either positive or negative, you could then adjust your foundation design - but make sure, as I am sure you would, have varoius scenarios already plotted out to minimize 'down time'. Just some initial thoughts. Can you give a link to the NCEER guidelines you have noted - I've not run into these before (but again, I am over a decade removed from North American practice).

 

[moe333]

BigH: Thanks for your comments. Some clarification on my post; The CPT's and borings that I would use would extend to about 80-100 feet or to a depth of at least 10-20 feet into good bearing material. I believe the piles may still be required for the basement structure due to the presence of liquefiable soil below the basement elevation. Typically, the maximum depth the design procedures were based on is 50 feet, but this is I have seen many sites where the depth can extend to 75 feet. But for my case, the basement may only extend to 30-40 feet which could potentially leave 20 or more feet of liquefiable material below the basement. If it were not for the liquefaction issue, I would go with a mat foundation.

But the liquefaction analysis is based on normalized overburden pressures among other factors, and I do believe the CPT and SPT values and will also vary significantly with overburden depth. The analysis procedures were designed for free field conditions; no cut or fill. So I was looking for a procedure that may have been developed to modify the free field condition to my excavation scenario, both for liquefaction and pile design which I believe there is a similar issue in terms of overburden effects. I know of a procedure to handle a fill condition for liquefaction analysis, but not for excavation.

It would be nice to perform additional CPT's/borings at the base of the excavation, but I doubt I would have that luxury; too many logistical/financial issues there.

The NCEER report is a semi-concensus document for liquefaction analysis from various experts who gathered in 1996, and was edited by Youd. Here is the link to the NCEER Report. It costs $35 through the site but someone may be able to help you out.

http://mceer.buffalo.edu/publications/workshop/97-0022/

 

[BigH]

moe333: Sorry I am not much help on 'procedures' - but I would say you have a pretty cr*ppy site if the liquefication potential extends to 40 to 60 ft! One other thing that you could do is to drive wood piles as compaction piles - to compact the soils enough to prevent liquefaction - or use vibrofloatation to densify. I'll give the real meat of your query some weekend thought - the Bicardi and diet coke might help!! Cheers.

 

[VAD]

moe333:

Just some partial info on your problem. For pile design in the basement section for the situation described it would be best to base design on effective stresses. This would take the excavation aspect into account. The non basement area I would do pile design based on effective stress approach as well. It is also worthwhile to do design based a total stress approach and invoke what would be the ultimate scenario re ground conditions when construction is completed.

 

[moe333]

VAD, I agree, for pile design I would use a design method that uses effective stresses such as the LCPC using the CPT data, or the Lambda method. One of the problems is the design strengths calculated obtained from the CPT or SPT correlations will be affected by the existing overburden pressure. The strengths I would obtain from the CPT or SPT if I were to perform them at the bottom of the excavation would be different, and probably much lower. The strengths closer to the edge of the excavation may be higher since the soil may "feel" the adjacent overburden at the edge.

 

[VAD]

Ok. Got your drift. I do not use the cone method. Anyhow I did a quick check in Foundation design and Construction by Tomlinson sixth Edition. He has a brief discussion on your case and refers to a paper by Brough. N.W.A "The effect of vertical unloading on cone resistance, a theoretical analysis and a practical confirmation'. Proc 1st International Geotechnical Seminar on Deep Foundations and bored Auger Piles, Ghent 1988. According to Tomlinson Brough has in his paper a method to predict the reduction in cone resistance due to reduction in overburden pressure.

Interesting stuff. I do not have the paper. If you get the paper it would be a useful addition to anyone's collection.

See if this solves some of your problems.

 

[KarlRoche17]

Hi all,

SL Kramer book will be a good source for evaluating liquefaction.

BY the way,
anybody knows how analyze the barrete/rectangular pile??? like capacity, lateral load capacity? do we need to analyze same as circular pile with equivalent diameter, is this suitable?

Thanks