Page 71 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
66 Geotechnical Engineering

Counting tree rings therefore can help date the first start of a landslide. Large
trees that move with a landslide usually die if their tap roots are broken.

Soil underneath a landslide is weakened by sliding so one scarp frequently leads to
another farther up on a hillside. Because the slip surface typically is concave, each
new sliding block tends to tilt back in the direction of the scarp and trap rainwater
in shallow ponds. Infiltrating water then further aggravates sliding. Pockets of
standing water therefore should be drained as a first step toward stabilizing
a landslide.

Soil in a shear zone is weakened because cohesive bonds are disrupted, and as soil
grains roll over one another they dilate and increase in volume, and suck in more
water. As this process continues, the basal soil in a landslide can turn into a
viscous mud that squeezes out in the toe area and can make access very difficult.
Test boring often requires the use of track-mounted drilling machines, and even
then the toe areas may be inaccessible.

Creeks or rivers running at the toe of a landslide often are pinched and may be
temporarily dammed by the moving mass of soil. Since the dam is loose, uncon-
solidated soil, as soon as it is overtopped by flowing water, it will be breached by
erosion, causing a flood downstream. Cutting a channel to prevent damming also
is risky because removal of restraint at the toe will allow more sliding.

4.3.6 Landslide Abuse and Retaliation

When a landslide occurs the reaction is automatic—all one has to do is hire
a bulldozer and put the soil back. This is about the worst thing that can happen
unless one makes a hobby of nurturing and aggravating landslides. If the soil was
not stable the first time it slid, it certainly will not be stable the next time around.
As previously mentioned, soil in the slip zone is remolded so cohesion is lost,
and it sucks in water to further reduce its strength. Pushing soil back up the
slope makes reactivation of the landslide a virtual certainty.

Another way to abuse a landslide is to remove soil from the toe, because it
is the weight and resistance from the toe area that stops sliding. This caper often is
accomplished when the landslide is old and covered with vegetation so that its
existence is not recognized or appreciated. Landslides also are triggered by natural
removal of soil from the toe by erosion, in particular by lateral cutting by a river.

4.3.7 Concealing a Landslide
A landslide decimates property values because stopping it and repairing
a structure often costs more than the structure is worth. Residential lots on land-
slides are virtually worthless, and laws require that such deficiencies be revealed to
a potential buyer. A few developers or real estate salespeople who are either

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