Effect of increased overburden pressure on SPT blow counts

[rochplayer]

I am doing some preliminary liquefaction evaluation of a flat site in a high-seismicity area. The site soils are loose to medium dense, clean sands and gravels of glacial outwash and alluvial origin. I have good blow count, groundwater, ground acceleration, and earthquake magnitude information.

My question is that we are going to be removing the upper ~5-ft of soil that is silty sand and silts and also raising the grade ~5-ft above existing, so in effect we are placing ~10-ft of engineered fill. The minimum stress increase will be generally 5-ft x 135 pcf over the entire site. If a sample has a field N value of 10 now, what would it have after 5-ft of fill has been placed?

(Note: I am using a spreadsheet developed to calculate liquefaction factors of safety based on blow counts, the input values being field blow counts, unit weight, depth to layer bottom, soil classification, and % fines.)

My initial thought is that for a given layer, determine the blow count corrected to N1(60)without fines (prior to fill placement), then add the fill to the ground surface. For the same layer, modify (increase) the field blow counts so that the new "field" blow counts for the given layer are corrected to the same N1(60) without fines as previously determined. The result is that the soils show at least the "same" density as determined during the original field investigation, but not show a density increase due to the fill placement, the conservative case.

Is there an accepted way to quantify a change to SPT due to placing fill so that I can be consistent with standard practice?

Thanks, rochplayer

 

[BigH]

Yes, there are correction factors for overburden pressures -e.g., Cn=0.77xlog10(20/p'o) where p'o is the effective overburden pressure in t/sf or kg/cm2. You could determine the "N" value for the existing overburden pressure, then determine the "N" value for the new overburden pressure. It will probably drop - 5ft of added o/b is not likely to change the N value much in itself. The most likely assistance to increasing the N value and hence the chances for liquefaction is in the compaction of the base after removal of 5ft of soil. Assuming that the groundwater is low or can be lowered, you can pound the excavated base with a large vibratory roller - then place your 5 ft plus 5ft in layers and pound away. This will increase the relative density more than the change of 5ft of o/b pressure.
In the end, while you might be able to increase the upper zone of the liquefiable sand, there still may be zone near the bottom that cannot be increased and would still be a problem. Then, you would need to look at other methods of ground improvement - stone columns, dynamic compaction, vibroflotation, etc.

 

[ashjun]

Hi,
I have never come across any method to "evaluate on paper" the new (and probably improved) N value after re-compaction and possible placement of compacted fill.
The only way that I see is to take the SI team back to the site and perform SPT tests and compare with the previous results.

BigH has cited good point on the corrections that also need to be applied.

Regards

 

[rochplayer]

Thanks for the input. I did some more research and most of the guidelines in the literature is to determine the N1(60)-clean sands under the free-field condition and use the same N1(60)cs for the layer of concern in the post-fill condition.