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Re: Landslide

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>I read in The Register(Orange County's local newspaper) about the house in a
>hillside in Laguna Beach which went down on a landslide. A drawing shows a
>cross-section of the house in caissons extending into the bedrock to resist
>vertical loads.But it does not show anything to resist lateral load that why I
>think the house went downhill with the landslide when the topsoil above the
>sloping bedrock slipped.
>
>Ernie,

I had the occasion a few years ago to inspect a bridge over the North Umpqua
River that had one abutment under the influence of a landslide. Geologists
had installed an "inclinometric device" to measure the movement. My historic
research indicated that the initiator of the slide was excavation of
thousands of yards of the clay toe material of a mountain in order to build
a resort which was used by Zane Grey and others starting in the 1920's. 

Start with the assumption that somewhere below your structure the soils
coefficient of fri ction gets reduced to zero. Assume 120 pcf for the
material above that level ..... If your cliff or hill is say 40 feet high a
slip surface at 45 deg would give you an average of 20' depth of material x
120 pcf x 40' = 96000 lbs per running foot along the toe....that will yield
about 5760 kips of potential slide force!!!! .... Then recommend a
geotechnical report if they are truly concerned. My conclusion on the bridge
was that any money short of that expended on nuclear energy type structures
would not be enough to stop the abutments movement!!
Norb Volny, PE