The Hazards of Building on a Slope

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The view might be magnificent, but make sure the land is not on the move before building on a hillside.
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Understand the hidden dangers under the ground.

Click on the Image Gallery for referenced figures.

Before
you run off, buy a “bargain” piece of hillside, and start
constructing the home of your dreams on it, then, you
should know how to determine whether or not that slanted
acreage is suitable for the use to which you
intend to put it.

All Slopes Move, at Least a Little

Any hillside suffers a form of erosion known as mass
wasting
. This movement can take either the
form of a violent landslide (which, obviously, can be
catastrophic) or a slow creep (which, over a longer period
of time and not so obviously, can be just as
catastrophic). You should learn to recognize the evidence
left behind by both kinds of land movement and to predict
the probability of such movements in the future.

Landslides

Landslides that have already happened come in two
varieties: old and fresh. And you should run–not
walk–from any real estate agent who shows you either
type. Anyone who builds on a known landslide (old or new)
is asking for trouble. The added weight of a house and the
addition of water (on the lawn, on a garden, in a sewage
disposal system, etc.) can reactivate even a long-dormant
landslide with disastrous results. And there you’ll go to
the bottom of the hill, with your new home in splinters
around you!

Fig.1 shows a typical landslide and its most recognizable
scars. Whether fresh or hundreds of years old, any slide
you inspect should exhibit some or all of these
characteristics:

[A] SCARP. This is a cut in a hillside where a mass of
earth once pulled out of the slope. On a fresh slide, the
scarp will have the appearance of a recent excavation. On
an older one, the cut may appear to be nothing but an
innocent, shallow, rounded depression on the side of the
hill. Beware. Such a depression is sometimes not as
innocent as it first looks.

[B] SAG PONDS. Water tends to accumulate in the depressions
formed on the surface of a landslide. This is most evident
where the slide meets the base of the scarp.

[C] FRACTURES. As a mass of earth slides down a slope, its
surface is often ruptured into numerous cracks and
fissures.

[D] HILLY SURFACE. When a landslide comes to rest, it
slumps and–to varying degrees–is compacted.
This creates a hilly area that is often in marked contrast
to the surrounding, undisturbed portions of the slope.

[E] VEGETATION. The surface of any landslide–new or
old–usually differs a great deal (especially around
the bottom of the shifted mass of earth) from the topsoil
of adjacent, undisturbed portions of the hillside. And this
“new” surface (formerly subsoil) frequently supports a
community of plants that is noticeably different from the
vegetation growing on the surrounding slope. In short, a
lobe-shaped area of low shrubs, brush, and smaller trees
amongst a hillside of older and taller trees might very
well be all the “sign” you need to tell you that a
landslide has taken place sometime in the past. Be alert
for such clues.

Slow Creep

Unlike landslides, which let you know in a most dramatic
way when they move, the downslope creep of the earth and
rocks on a hillside is just what the name implies . . .
slow, deceptive, and insidious. Once started, it’s also
inexorable . . . and anyone who tries to pit the foundation
of a house against the gradual slide of thousands of tons
of soil is very foolish indeed.

It’s much more difficult to detect a creeping slope than it
is to spot an old or a new landslide . . . but it can be
done. Look for these signs:

[A] TREES WITH BENT TRUNKS. As soil inches down a hillside,
the top layers of the dirt will generally move faster than
lower layers of the shifting mass of earth. This can cause
trees growing in the creep to be deformed as shown in Fig.
2.

[B] TILTED POLES. The same action that bends tree trunks on
a creeping slope can also force telephone and other utility
poles to increasingly lean downhill (see Fig. 2).

[C] DISPLACED FENCE LINES. Fig. 3 shows what happens when a
fence is run across a creep zone. The section of posts and
wire within the moving mass of earth will–over a
period of time–“walk” right down the hill with the
sliding soil. It’s quite easy to see the resulting “sag” in
the fenceline.

Look for Signs of Danger Before Building on a Slope

Although the whole planet is plastic and its mass is
constantly changing, most of these changes take place so
slowly that–for all practical purposes–we can
ignore them. A great many hillsides and mountain slopes,
for example (although constantly and forever pulled
downward by gravity), erode at such an imperceptible pace
that their gradual wearing away need not concern us. We can
lay foundations on their slopes and construct buildings on
their faces with little more thought and skill than if we
were erecting the same structures on level land.

If, for some reason, you do want to put up a building or
establish a homestead on a mountainside or steep hill,
however, you’re wise to positively acquaint yourself with
the signs of troublesome mass wasting first
before you make even the down payment on your ‘dream” patch
of slanted soil. The shifting of millions of tons of
earth-in the form of either a sudden landslide or a slow
creep–is more than a match for anything you’re liable
to build plus any effort that you, your family, your dog,
and your mule–all together–are likely to
muster. Ignore the early warnings of mass wasting and you
endanger yourself, your whole homestead, and even some of
your downslope neighbors.