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# Re: Landslide

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Landslide
• From: "Martin Johnson" <mwj(--nospam--at)EQE.COM>
• Date: Tue, 10 Mar 1998 12:18:48 PST
• Priority: normal

```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

```