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Re: Landslide

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Responding to:
> From:          ErnieNSE 
> 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.
> 
> 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?
> 
IMHP, the caissons sheared because of the lateral force induced by 
the moving mass of earth.  Back when I was in night school (centuries 
ago) we worked a sample problem of determining shear forces and 
moments in a caisson subjected to a combination of superstructure 
and lateral earth movement.  The shear forces can become enormous, 
and when you try to increase the caisson size to resist it, the 
forces only get bigger. 

The problem here is one which seems to straddle the training of both 
geotechnical engineers and civil/structural engineers, and I think 
that current code provisions as a result are deficeint because of 
mis-communication between disciplines.

As a structural engineer, my view of the options to prevent this type 
of failure are as follows:

Option 1. Provide enough shear reinforcement in the 
caisson to develop a flexural hinge instead of a shear failure.  The 
caisson if free-head, will probably hinge just below the elevation of 
sliding ground, and start leaning over.  If fixed head (by a rigid 
beam at the top) a piar of hinges will form and the lateral 
displacement will be less.  I'm not sure if this is repairable but it 
wwill be safer.

Option 2. Tie the tops of the caissons back to a point where you 
presume is beyond the area of moving ground (ah, but where is that), 
with a big anchor at the end, plus add lots of shear reinforcement in 
the caissons. 

Option 3.  Add a permanent earth-retaining tie-back system of deep 
tension anchors to keep the ground from moving in the first place. 
-----------
Martin Johnson
EQE International, Inc.
mwj(--nospam--at)eqe.com