Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re[2]: Geotechnical opinions

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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). 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.

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. 

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.

(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 

(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.

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