N Value Reduction in Large Gravels

[garrettk]

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.

 

[gandersen]

garrettk:

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

 

[stochas]

I too am unaware of any standard practice for evaluating SPT blow counts in gravel soils. Whenever I have encountered such soils, I look at all three six inch intervals of the SPT test. If the blow counts were 5-5-25, I would likely use an N-count of 10 rather than 30. Also, a comparison of blow counts between various borings in the same layer is useful in determining trends.

 

[VAD]

Hello garrettk:

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.

 

[BigH]

VAD has a good point that using the Becker Hammer is more appropriate (this was discussed in a previous thread or two - could search for Becker Hammer). I ran into a situation where we had "N" values of 8 to 10 for a gravel with SPT, but when I used pentest (51mm dia cone with 60deg apex angle driven by standard 65kg hammer dropping 750mm) - we got penetration blows of only 4 to 5.

 

[garrettk]

Thanks for the input, all. For the purpose of calculations, I'll likely reduce the N value somewhere between my 30% and 50% and go from there.

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.

 

[Focht3]

It sounds like the biggest risk is erosion of the gravel/cobbles. One solution could be to grout the gravel, then bear on/in the cemented zone. (This is impractical if the gravel is fairly thin or 'dirty'.) This may be a lot cheaper than drilling or driving through a fairly thick gravel zone. Anyway, it's something to consider.



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?"

 

[VAD]

Focht 3:

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

 

[cbosy]

Be careful when using elevated blow counts in gravely soils to estimate driven pile depths. I have seen piles go a lot deeper than they should have.

 

[Robert168]

If the soil is a gravelly sand, silt or clay, the gravels, when larger than the sampler, should be bypassed for SPT. I think the surrounding soil plays a dominant role in the properties of the soil as a whole. However, the "erroneous" counts can still be used to develope a feel of the firmness of the soil, especially for evaluating the easiness of pile driving.

If the soil is a sandy, silty or clayey gravel, then SPT is not applicable as made clear in many manuals and text books.

 

[DRC1]

This is a situation where you really need to stare at the boring logs to get a sense of what is happening. In the perfect world the geotechnical designer should haul himself out to the site and watch some (heaven forbid all) of the drilling. But that is another topic for another day. I agree that the best bet is to look at each 6" interval for what appears to be the unobstructed blow count. This where I often find the casing blows useful. Since they are driven continuously, you can see where the are driving through cobbles and where they are not. Then you can match up the blow count to these casing blows. The good thing about gravel is that no matter what it usually is pretty good bearing material, so it s not as critcal as say a soft clay.
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.