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Re: Geotechnical opinions

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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.

Am I off base?

Charles Laines, S.E.
Long Beach, California