N Value Reduction in Large Gravels
N Value Reduction in Large Gravels
(OP)
Is there a standard, or common practice, for reducing N values when completing and investigation that goes through gravels that are a larger diameter than the split-spoon sampler?
I've encountered some larger gravels with a mixed silt and sand matrix within a perched watertable. The N value ranges from 21 to 37, but I feel that these values are inflated due to the gravel size. I'm already reducing my bearing capacity by half to account for the water.
Are there specific methods for determining this reduction, or is it completely a judgement factor (or should I not reduce them at all)?. Right now I'm considering a 30% reduction for the N-value.
I've encountered some larger gravels with a mixed silt and sand matrix within a perched watertable. The N value ranges from 21 to 37, but I feel that these values are inflated due to the gravel size. I'm already reducing my bearing capacity by half to account for the water.
Are there specific methods for determining this reduction, or is it completely a judgement factor (or should I not reduce them at all)?. Right now I'm considering a 30% reduction for the N-value.





RE: N Value Reduction in Large Gravels
I am not aware of specific reduction equations for large gravel or cobbles or boulders affecting the SPT values. We just finished a job in PA, USA where there were multiple cobbles in the granular deposit. I personally looked at all of the boring logs and at the driving records. When the logger indicated an obstruction during driving and the blow count was larger than the surrounding soil, I felt justified in questioning the high blow counts.
On another site we had a similar issue in Northeast Ohio. I again looked at the boring logs and driving records and where the logger noted a cobble or boulder and there was a blow count higher than the surrounding soil, I questioned the validity of the result.
In both of these cases, I didn't reduce the blow count but rather I did not use it in my analysis.
I'll be interested to see what others have to say.
Glen
RE: N Value Reduction in Large Gravels
RE: N Value Reduction in Large Gravels
When you speak of reducing the bearing capacity I presume that you are speaking of a shallow foundation unit to be proposed for the site. In such a case I would venture to do a tespit or two at the site to be able to examine the nature of the gravel sizes. Of course you may get sloughing quickly as a result of the high water table. The thickness of the deposit as well as a feel for its composition can aid in the overall judgement.
Often the Becker Hammer rig is used to explore gravel strata when it is known that these will be encountered and one would like to explore the stratigraphy with depth. There are correlations between Becker Hammer blow counts with SPT blow counts. Also the use of this equipment allows the opportunity to obtain an idea of the size of the deposit. However, some degrdation of sizes is inevitable as well.
This is one where the the type of foundation to be used, thickness of strata, loads etc and engineering judgement come together.
I think from your description reducing the bearing capacity by half unless the reduced value significantly influences the size of the foundation pad would be in order.
Play with the numbers and make a decision. This is part of the geowizardry, geomagic, or as a friend puts it mumbo jumbo of the geotec world.
RE: N Value Reduction in Large Gravels
RE: N Value Reduction in Large Gravels
This is for a small Con-Span style bridge crossing a small creek. I think we'll likely end up recommending bearing on some deeper soils that are not gravelly which will eliminate the guess work.
RE: N Value Reduction in Large Gravels
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: N Value Reduction in Large Gravels
Cute. I was discussing a similar problem where the geotech engineer was worried about scour or movement and suggested grouting and using footings at the pier location since piles would not be practical.
Regards
RE: N Value Reduction in Large Gravels
RE: N Value Reduction in Large Gravels
If the soil is a sandy, silty or clayey gravel, then SPT is not applicable as made clear in many manuals and text books.
RE: N Value Reduction in Large Gravels
One final note, have you considered drilling in some drains to remove the pearched water table? This may be a more effective answer than increasing the footing.