Seismic Site Class - Average or Individual Results When Looking at Liquefaction?

[TDAA]

I have not been able to filter this out of other threads so forgive me if this duplicates:

I have a site where we have some saturated sand and silt (non-plastic) materials with very low blow counts found in two out of ten borings to depths of about 65 feet. These two borings are in the same area, and both show an average blow count of less than 15 for the upper 100 feet which results in a Site Class E. The borings do not meet the other criteria for Site Class E (moisture is high for more than 10 feet, but Atterberg limits are low). Other borings within the site have higher average blow count, qualifying as Site Class D, with claystone bedrock much closer to the surface. The two boring are generally within the others indicating there is a “pocket” of loose soils. They are not confined by bedrock, which I know can cause some issues. The site measures about 500 feet across and will have multiple, relatively small buildings.

While the IBC / ASCE7-10 discuss dividing the site into strata, I do not believe they discuss how lateral extents of various materials effect the site class. From a structure standpoint, I would normally default to the lower site class - without having shear wave velocity testing. Liquefaction is a potential concern at this site. Calculations indicate it is not susceptible if there is a Site Class D, but it is susceptible if there is a Site Class E. While I could be conservative and assume the Site Class E, I would like to better understand the decision as it relates to liquefaction, as this is not normally a concern around here:
When looking at the site (using blow counts), are others using an average of all borings, or basing the decisions on individual borings?

When looking at the site (using shear wave analysis), are others using an average of all lines, or basing the decisions on individual lines?

At what point does a “pocket” (area) of loose material become an issue for liquefaction? I believe the greater risk here is due to settlement, as the soils are well confined. (Yes, I would check all aspects…just my gut feel at this point.) I ask this from the point of seeing lower blow counts, but I think shear wave velocity testing across the area would likely show a better Site Class due to the changes in material around the loose soils.

Thanks for insight on those questions or anything else that may be getting overlooked.

 

[MTNClimber]

From what I understand, it's an average of the blow counts in each boring or an average of each individual shear wave line, not an average of the blow counts or shear wave velocities of the entire site.

A pocket of liquefiable material is of concern when it is under the structure and potentially when it's near the structure, depending on the magnitude of settlement from liquefaction.

If the average SPT blow count (your other post in another thread said you use California Samplers) of the individual hole puts you into a Class E and you run an analysis and find that the soil is susceptible to liquefaction then its considered a Class F where you have liquefiable soils. If the "problem spot" only affects one of your buildings then only that structure is at risk not the entire site.

 

[TDAA]

Well, Modified California sampler blow counts can be correlated to SPT, or perhaps we used a standard split spoon for this. That aspect is irrelevant to the question, although your thoroughness is appreciated.

Also, I agree with your first points, for the most part. And I certainly agree that a pocket of liquefiable soil is a concern. The question is determining whether it is susceptible to liquefaction.

From my understanding, to determine if the site has liquefaction potential, a site-modified spectral acceleration value is used in the calculations, which involves a site amplification factor. That factor is influenced by the Site Class assigned to the site. In this case, where the site class is “D”, there is no liquefaction; however, if it is “E”, there is liquefaction, per the calculations. So, correct determination of the site class is important.

So, at what point does a single boring (or two) become an issue when looking at the site? While one boring may show poor blow counts, it may not be indicative of the site as a whole. While for my current site, it likely makes sense to treat the one area with more caution, would others treat the situation the same way if the borings are all in closer proximity? What if you redrilled holes close to, and surrounding the original suspect boring and the results were better? Does your engineering judgement tell you there is not an issue, or because you looked at the one, 4-inch diameter boring, it now controls the site?

 

[geomane]

I think it’s a judgement call. I usually take an average of the borings across the site to determine site class, but in my area, liquefaction is really not an issue. I would perform additional investigation (shear wave velocity or additional borings) if you are 50/50 and if you can justify it.

 

[GeoEnvGuy]

Subsurface conditions vary across a site, the engineering judgement comes in to interpret between borings. This is largely due to how the deposit of liquefiable material was formed.

If you found a boring that shows a zone susceptible to liquefaction it stands to reason that there is a geological deposit of liquefiable materials in the area. You can look at delineation with additional borings to determine the extent of the deposit.

 

[MTNClimber]

I 100% agree with GeoEnvGuy.

TDAA - I'm interested in what you are using for a correlation for CAL blow counts to SPT blow counts. During my previous research, I wasn't able to find any convincing correlation that was proven to actually work. My notes show that the conversions for a 2.5" OD/ 2" ID Modified California sampler blow counts to SPT blow counts were:

Burmister: 0.77 x N(CAL) = N(SPT)
Lacroix & Horn: 0.64 x N(CAL) = N(SPT)

But I remember David Rogers ran a test using SPT and 3" OD/ CAL samplers side-by-side and came up with something between Burmister's and Lacroix & Horn's equations. California Geological Survey tried running a similar experiment and the data was too scattered be conclusive.

What's your take on it and is it your company's standard approach? I've always been interested in running a test to compare results myself but never had a chance.

 

[TDAA]

Lacroix & Horn is typically recommended when working close to geologic contacts with stiffer materials.

I use the Burmister equation for the sampler diameters (1948): (2²-1.375²)/(D_O²-D_i²) for the split spoon diameters over the modified California diameters. At 2.5 OD and 1.935 ID, that puts the correction at 0.84.

I would not say that this is widely used throughout our company, but I have also not seen other companies correcting the modified barrel blow counts for general use either. Perhaps they are for seismic considerations. With the materials we have here, I do not often see the need to perform a correction. This goes back to the commonly used methods (local) in the area for most correlations for most design aspects.

From the seismic side of things, it is usually not required either since average blow counts (even at 0.64 times) are not at levels where a Site Class E typically comes into play and Site Class C can be difficult to achieve. This leads to most sites falling into a Site Class D. Is see Site Class C out of many other companies, but I question those due to the borderline shear wave velocity (C to D) of our claystone and sandstone bedrock.

For this project, we do have SPT as well as modified California. We are also going to perform some CPT and shear wave (MASW) testing, along with additional drilling, to gain additional information about the conditions.

 

[MTNClimber]

Please keep us updated of the results of the CPT and MASW testing. I'm genuinely interested if you'll get a better site class in the end.

 

[GeoEnvGuy]

If you are going to be doing shear wave MASW you would want to do this first for identifying changes in stratigraphy. Following those findings you should target your CPT program and I would also consider the downhole shear wave testing in one meter or one rod intervals during the CPT.

 

[GeoEnvGuy]

You may also want to determine if the entire area is unsuitable to withstand the design earthquake.

Thinking of Indonesia

https://www.google.com/amp/s/time.com/5413507/liquefaction-indonesia-earthquake-damage/?amp=true