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Re: Landslide

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ErnieNSE wrote:
> 
> 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.
> 
> Are there any provisions in the code for laterally restraining the house to
> the bedrock  to keep it from sliding downhill when a landslide(topsoil sliding
> on a sloping bedrock surface) occurs?
> 
> I also remember a new  L.A. City Code requirement with heavy, continuous and
> deep footings at the portion of the house sitting on bedrock which at first I
> thought was too conservative. Would this new code requirement  that came about
> after the Northridge Earthquake help keep the house from sliding downhill?
> 
> Or is this a Geologist's problem? Is the horizontal force of the sliding
> topsoil too large that no practical structural restraint is capable of
> resisting it? Can we design a connection of the house to the bedrock at the
> high point and just let the soil slide from under the house with the caissons
> capable of resisting the vertical loads? All these assume that all earthquake
> and wind loads were already taken cared of.
> 
> I'm just thinking of what I will say to a future client who wants to build on
> a hill side when he asks the question "will my house slide downhill?"
> 

________________________________________________________________________

It always come back to the same question, should we design the
foundation to take sliding soil forces, then if we do beat up the
caissons, they will attract more forces or we can try to stop or reduce
the soil sliding in the first place.  I usually take the latter
approach.

About 10 years ago, I designed a custom home (10,000 sq.ft.) at a
hillside of Santa Monica Mountain.  The owner wanted a swimming
pool,tennis court and a big house, all resting on hillside.  So we
located the swimming pool on the side yard, tennis court in the backyard
(overlooking the hillside).  The whole idea is to let the retaining
walls and slab of the tennis court be some sorts of buttresses for the
house.  If soil sliding do occur, the tennis court will be the first to
go, hopefully not the house and it should provide adequate time for the
owner to take action.  Also, planting lots of trees in front of the
tennis court will help as well.  The owner got what he wanted and did
not have to pay extra for additonal tie-back or caissons or whatever.  I
think the house is still standing there, I hope.

Tom Chiu, SE