What is a Landslide?
Although rainstorms are extremely welcome to California, those same
storms also can bring a taste of what that welcomed water can do in hillside
areas. This article will deal with how geologists and engineers define and
recognize landslides and where a slide might be expected to occur.
A landslide is a downward movement of rocks and soil. However, it might
also include such things as vegetation, structures, or parts of roadways.
Sometimes the soil can liquify and the slide then becomes a mudflow which
acts much in the same way as a stream of water. Landslides may move as slowly
as two inches a year, such as the Penitencia Creek Landslide in San Jose,
however, speeds in excess of 20 miles as hour have occurred.
Landslides occur in all sizes and shapes. They can be backyard size, or
they can be a couple of miles long. Landslides can be shallow or deep, and
they can move a few inches or several miles. There are also numerous types
of landslides. They range from rock falls (such as those in Yosemite Valley)
to landslide-mudflows that can occur on very gentle slopes as low as 15
degrees. Homesites in hillside areas may experience landslide problems if
they were not properly designed or located relative to existing slope or
other conditions on the property.
Many landslides are man-made, caused by cutting roadways and building pads,
or placing improperly engineered fill on steep slopes. Earthquakes can trigger
landslides, but the most common triggering mechanism is the combination
of heavy rainfall, steep slopes, and loose or soft soil. Even slopes that
appear stable when dry can experience a landslide if the soil becomes saturated
by an intense storm or series of storms. Other sources of excess water such
as broken pipes, intensive landscape watering, or misdirected run-off can
also trigger a landslide. Hillside areas burned off by forest fires are
also particularly susceptible to landsliding because of the absence of vegetation
to bind the soil.
Can landslides be predicted?
Essentially all hillsides have the potential for a landslide; therefore,
they can only be predicted in the most general way. For instance, where
a landslide has occurred in the past, there is a higher probability that
it will reactivate sometime in the future. Where natural slopes have been
disturbed by cutting away at the bottom of the slope, there is a higher
chance of sliding in that area relative to the undisturbed hillsides. Hillsides
subjected to intensive watering or private septic systems may also be more
susceptible.
The type of soil a hillside is composed of can also affect its potential
for landsliding. Hillsides composed of poorly indurated or highly fractured
rock are more susceptible to landsliding.
We can also generally say that there will probably be more landslides in
a wet year than in a dry year, but an absolute prediction of where a new
landslide will occur would constitute an educated guess at best.
LANDSLIDES AND REAL ESTATE
The above discussion included how geologists and engineers recognize
and define landslides and mudflows. The following covers how real estate
brokers and agents might be affected by landslides.
Geologic maps for the San Francisco Bay Area show a large number of
landslides that have apparently not moved in hundreds or thousands of years.
Many of these landslides were considered to be stable enough for development.
During the "wet" winters we experienced back in 1981-1982 and
1982-1983, the rainfall was approximately "twice" that of normal.
This increase and the corresponding rise in groundwater levels were sufficient
to reactivate many old or ancient landslides. Even during drought conditions,
some of these ancient landslides were reactivated in the 1989 Loma Prieta
Earthquake.
Landslides are very complex. Homes built on top of or in the path of a landslide
can be damaged or destroyed. The owners of these homes can suffer financial
loss and, in some instances, personal injury or even loss of life.
Selling or buying property that has been affected directly or indirectly
by landsliding can be fraught with liability problems. Questions such as
the following must be answered:
- How severely is the property impacted by the landsliding?
- Will the landslide problem be fixed or will it continue to get worse?
- Do we have all the available information?
- Have serious problems been obscured by repair work?
- Has complete disclosure been made? Some attorneys advise that the client
must be made to comprehend the disclosure information, not just be presented
with it.
- Will the lender treat the property differently because of the landsliding?
- Will the City or County permit remodeling or an addition? If so, will
they require extensive geologic or geotechnical studies?
It is quite common for property affected by landsliding to be marketed after
the landslide problem has been mitigated.
There are two general approaches to landslide mitigation. The first and
most desirable is to eliminate the landslide, that is, remove all the sliding
soil and replace it with properly engineered fill. The second approach is
to contain the landslide by use of retaining walls or other structures.
Installing subdrains will aid in preventing one of the main triggers of
landslides-excess water. In the former case the landslide no longer exists
and the probability of future sliding is very low. In the second case, the
landslide is still present, has some remaining potential for movement, and
the mitigating measures may be visually obtrusive or affect the property
in other ways.
Marketing real property after landslide repair places an additional burden
of disclosure on the Broker and Agent. Many potential and recognized landslide
areas have been mapped and delineated on various county and city maps as
part of their and Seismic Safety Element plans. These should definitely
be included, if applicable, in the geologic disclosure of the property.
Discuss with the current owner if any landslide problems have ever occurred
on the property and include any of these problems in your geologic disclosure.
Be alert to any symptoms of distress resulting from a sliding problem during
the visual inspection and include these in the disclosure.
Prevention
Many landslides can be prevented if standard geologic and soil engineering
recommendations are followed. They can significantly decrease the possibility
of landsliding.
- Do not place loose soils on steep slopes.
- Do not channel or concentrate surface drainage onto slopes.
- Do not cut the toe or bottom of a slope without replacing it with a
retaining wall.
- Do not strip the vegetation from a slope. Deeply rooted plants that
do not require extensive watering make the best type of landscaping for
hillsides.
The analysis and repair of landslides is a tricky and specialized business.
Consult a soils engineer before attempting any landslide repair program
or altering the condition of an existing slope. The repair of an existing
problem may cost the Seller some money, but it will probably make selling
the house at the asking price that much easier.
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