Pile Capacity in Deep Excavation
Pile Capacity in Deep Excavation
(OP)
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.
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.





RE: Pile Capacity in Deep Excavation
RE: Pile Capacity in Deep Excavation
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:
RE: Pile Capacity in Deep Excavation
RE: Pile Capacity in Deep Excavation
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.
RE: Pile Capacity in Deep Excavation
RE: Pile Capacity in Deep Excavation
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.
RE: Pile Capacity in Deep Excavation
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