Are Bay Muds (OL/OH) Class F Soils by UBC?

[gibsong]

Would San Francisco baymuds (OL/OH by USCS) be considered Class S-F Soil Profile Type per the UBC due to their organic content? If so, how do you provide a "site-specific evaluation". I am looking at a site with a bay mud thicknesses of less than 50 feet which are soft, highly compressible, with dry densities in the range of 40 to 60 pcf, and moisture contents between 50 to 120 %.

 

[coneboy]

To Gibsong

The bay muds which you refer may end being classified as sens fine grained using the Fr - Qc soil behavior chart developed by Robertson & Campanella with assistance from UBC. If they have some strength (more tip resitiance) to them then they be interpreted as clay.

If the b.muds had only occasionaly fiberous material (v.thin layers) then you might get small spikes in the Rf. If the bay muds had a consistent layer of organics I would expect the soil behavior type (SBT) to be "Organic".

If you absolutley need to classify the soils to a certain govt/federal soil classification standard then get a grab sample.

A dissipation test will tell you if the soil is clayey or silty in nature based on the length of timne to reach a t50 value.

.........of course the important item to remember is not whether the soil is a clay, silty clay or clayey silt, but it's measured properties. If you have determined the exact soil type.......well thats great, but the Su, OCR, cv and permeability are the important items to geotechnically think about.

I hope this helps as I am not sure what your "Site Specific evalution" means.
Coneboy

 

[gibsong]

Coneboy;

The UBC has six soil profile types designated SA through SF which are used with the seismic zone factor, Z and distance to known seismic source to determine the UBC Seismic Coefficients Ca and Cv, and Near Source Factors Na and Nv. Seismic coefficients for soil profile types SA through SE can be determined from the UBC tables. However, SF soils which include liquefiable soils, highly sensitive clays, collapsible weakly cemented soils, peats and highly organic clays > 10 feet thick, very highly plasticity clays > 25 ft, and very thick soft/medium stiff clays require a "site specific evaluation" to determine the seismic coefficients.

My question is: are organic clays and silts (OL and OH by USCS) defined as SF soil profile type if their thickness exceeds 10 feet, and how does one perform a "site specific evaluation" to determine the seismic coefficients?

Thanks,
Gibsong

 

[CISGeotechnical]

Gibsong:

I think your concern is to perform a "site-specific geotechnical investigation and dynamic site response analysis" as quoted from the UBC 97, chapter 16.

My recommendation is that you find a geotechnical firm who has an in-house engineering geologist and experience in the San Francisco area. You may want to ask an engineer you trust to recommend such firm. Be sure to define to them the scope of work you want in order to have a geotech scope and report that can be usefull for your purpose. You may quote the UBC and ask them to provide you with seismic parameters needed for your design.

Cone Penetration Tests may be helpful in the subsoil exploration method to use since this is far more technically and in-situ-properties-measurement-oriented than the SPT is.

Hope this helps. CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[CISGeotechnical]

Another though...

I think California has many guidelines developed for geotechnical investigations and report contents. For example, County of Los Angeles, Department of Public Works and the City of Santa Monica. you may find those on the web. Check for your specific area if such exist.

CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[pwtem]

Hi. Just found this site/forum so this response is v. late, but might be useful to others.

Measure the shear wave velocities using a P- S-wave velocity borehole tool, like the OYO P-S Suspension Logger. Those muds can be REALLY SLOW, like less than 30m/s (Vs).

 

[Focht3]

Okay, I'll throw in my two cents' worth.

Gibsong: with UDWs of 40 to 60 pcf and moisture contents of 50 to 120, it is highly likely that you are dealing with very sensitive soils. If you have access to miniature vane equipment, find your samples with the highest moisture contents and Liquidity Indices. Perform miniature vane strength tests on the undisturbed samples, then remold them without changing their moisture contents - and retest the molded samples. If the ratio of undisturbed strength to remolded strength exceeds 10, you are dealing with highly sensitive clays and the UBC SF soil profile classification applies.

CPT, SPT, shear wave - these are all useful, but don't directly measure the parameters needed to evaluate the SF classification.

 

[gibsong]

Thanks for the tips! However, I am still unclear what is required by the UBC to perform a "site-specific evaluation" and "dynamic site response analysis" if S-F soils are determined to exist at the site.

 

[CISGeotechnical]

Gibson:

We have try to give you some help on this, but lets get to the point. What is the purpose of your question? Are you designing a structure for compliance with seismic codes or need any other help, like research, thesis, etc.?

This info can be valuable in getting straight to the point. CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[gibsong]

CISGeotechnical:

I am a project engineer with a Bay Area geotechnical consulting firm working on several projects located on the margin of San Francisco Bay that are underlain by younger Bay Muds or localized pockets of peat that would appear to qualify as S-F soils per the UBC criteria. I would like to hear from someone with experience performing the "site specific investigation and dynamic site response analysis" required by the UBC to determine the seismic response coefficients Ca and Cv (Table 16-Q and 16-R)what is required to perform the analysis. We need to evaluate if we want to acquire the software and training to perform the analysis in-house or if we need to sub-contract it out to a firm specialized in performing the analysis. I am interested in what type of software is available to perform the analysis, what soil parameters are needed, and how much the analysis typically costs.

gibsong