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Re: Geotechnical opinions

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My comments are sprinkled througthout:

Benson, Tom wrote:

> Need some geotechnical input.
> My Van Nuys client has a soils report that includes the analysis of 4 test
> holes. The analysis found that 12-inches of fill and upper 2-feet of native
> soil that support the raised floor foundation are unconsolidated (less than
> 90% compaction).

What is your consultant comparing the "compaction" requirement to, Standard Proctor,
Modified Proctor, Relative Density?

> The fill and native soils are silty sand, sandy silt, and
> silty clays. The report concludes that this "somewhat loose and porous"
> material resulted from the 1994 Northridge quake, and this loose soil is the
> cause of the settlement of this 1950?s house. The site is basically flat.

What are the dry densities? Are they reasonable?  On a flat site how did these soils
spread?  Where was groundwater ?

> I thought liquifacation was the only mechanism that could cause foundation
> settlement in an earthquake, assuming a flat site and no slope failures. A
> basically granular soil would become more compact in a quake, not looser.

Could your Geotech consultant have meant that these soils were susceptible to
dynamic densification during the Northridge event and that this is likely a
contributing factor to the settlement, as described by Mr. Benson and simply didn't
communciate the point well?

> My opinion is that the soils have always been loose, and combined with poor
> site drainage, nearby trees, and age, the house has settled as the
> unconsolidated material has collapsed.
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> Here are some free comments from a California "entitled" Geotechnical Engineer
> (GE) with both Southern and Northern California experience.  With all due
> respect to the current "Geotechnical Engineer of Record", I must note that I do
> not know exactly where this site is, nor have I reviewed their boring logs and
> laboratory test results.  However:
> (1) In the 1950s it was unlikely that "engineered" or "documented" fill was
> compacted to 90% of the ASTM D 1557-91 laboratory maximum density.  Relative
> compaction in native/undisturbed soils is meaningless.

Not to mention that if more than 15% is passing the #200 sieve D1557 wouldn't be the
prefered method for meauring density..

> (2) Although soils can loose density due to lateral spreading and slope
> sloughing, confined soils on level ground are not likely to become less dense.
> (For example, tap the side of a coffee can and see if the coffee grounds
> consolidate or fluff up.)  Dynamic compaction resulted in significant fill (and
> sometimes loose alluvial and colluvial soil) settlement throughout the San
> Fernando Valley following The Northridge Earthquake.  Settlement and
> differential settlement following Northridge occurred as a function of fill
> thickness and variations (relative thickness and density) across building
> footprints.

> (3) Dynamic compaction is not liquefaction, but can result in differential

> settlement in sands, similar to liquefaction.  Dynamic compaction differs from

> liquefaction in that it can occur above groundwater, does not necessarily result
> in loss of shear strength, and will not have sand boils.
> (4)  Water leaks or other moisture changes (El Ni~no) can result in "collapse"
> (a geotechnical term) of "porous" silts and sands, and expansion of clays.
> In short, knowing only what you have told us about the site, it appears that you
> have some valid questions for your Geotechnical Engineer of Record.  Although
> the insurance companies may want to know what did happen, it may be more
> important for the homeowner to know what will happen, and what must be done to
> fix the problem.  Is there really only a few feet of loose soil?  If so,  it is
> unlikely that a significant settlement concern remains.  I hope you have enough
> borings to an adequate depth, with enough consolidation/expansion lab. data.

Well said Tim.

> Hope this was helpful, but it sounds like you have some work ahead to come to a
> consensus with your Geotechnical Engineer of Record.
> Good luck,
> Tom Benson (in Azusa Monday, Mountain View the remainder of the month)
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> Am I off base? Not likely.
> Charles Laines, S.E.
> Long Beach, California

Arvel L. Williams, P.E.