University of Virginia Library

LECTURE 11- SLOPES

Consider the elements which make up the cross-section of a roadway:
the cut-slope extends from the feather edge above the excavation
downward to the ditchline; the fill-slope extends from the
shoulder edge downward to the feather edge of the embankment; other
elements are ditches, shoulders, and pavement. Slopes take up most
of the road prism; they are the transitions from the ditches and
shoulders to the limits of grading. The appearance of the roadside
and the views to distant features beyond the roadway are largely
the result of the landscape architect's skills in drawing the grading
plan.

Construction manuals show standard sections with slopes of 1:1
or 2:1 as though the slopes are mathematic conveniences for estimating
earthwork. In my observations there are no standard slope ratios to
fit all soil conditions along the roadway. Look for the steepest
natural slope in the adjacent area; that slope ratio is stable for
similar soils in that segment of the roadway. If we use a slope
steeper than the angle of repose, we will be cleaning ditches from
now on!

Observe the cut-slopes in the Piedmont of Virginia; 2:1 slopes
are unstable because of the high clay content. Slopes of 3:1 or
flatter are more stable.

It is essential that we show on the drawings the material to be
encountered in the excavation; therefore, study the soil maps and the
boring charts before you begin a grading plan. And go to the field
to find indicators of changes in soils! Rock outcrops, rock strata
in streambeds, sediments deposited at grade-breaks won't show on
topo drawings.

Bidders will increase their bid prices for unclassified excavation
to play it safe. Estimators will allow for rock excavation
because it requires blasting. Therefore, your bid prices will be
more realistic if you show the slope ratio for given soil types
according to the conditions you expect to encounter. Field inspection
will show the stable slope of shale or decomposed granite
more accurately than the standard slope ratio shown in the construction
manual.

Howland's Theory:

I have a theory to share with you. It is that a slope resembling
a natural curve is more stable than an artificial curve. If we
determine the angle of repose for a particular soil, a Catenary
Curve taken from the midpoint of the slope downward through the
ditchline to the shoulder will be stable. If the same curve is
reversed and extended upward to the feather edge of the cut-slope,
the face of the slope will then be in equilibrium. That convex/
concave slope is what we see in the landscape. If we accept the
definition that the angle of repose is that gradient at which a
particular soil will stand when fully saturated, the slope will
then be in equilibrium. I intend to test the theory to prove its
logic and will tell you the results later.

Therefore, slopes are transitions in elevation. A drop of
rainwater falling at the crest of a convex slope will increase in
velocity as steepness increases. If the lower slope is made concave,
the acceleration will be reduced until the drop of rainwater arrives
at the ditchline.

The Seedbed:

Road slopes are a giant seedbed beyond the pavement on which
seeds will germinate under conditions of sunlight, moisture and
sufficient nutrients to support growth.

Moisture which reaches the root systems of plants is in part a
function of steepness. The steeper the slope and the smoother the
texture of the soil surface, the faster will be the rate of runoff.
Runoff will penetrate the soil surface over a longer time period
on flatter slopes because the rate of acceleration is lower. We
know that snowpack will be retained longer on north and east facing
slopes than on south and west facing slopes. Therefore, the former
will be wetter longer.

We know, too, that moisture is a factor in the success of
volunteer species which will occupy a slope; and that dry site
species will occupy the upper slopes and that wet site species will
occupy the lower slopes. In the Yellowstone we find that lodgepole
pine germinates quickly on slopes exposed to sunlight, and that fir

and spruce will germinate on duff covered, moist shaded slopes.
The succession of species following forest fires is the same.

CLIMATIC FACTORS:

Exposure to wind, sun and extremes of temperature will affect
available moisture for plant growth. Seed retention and ultimate
germination will depend on the roughness of the slope where slight
cracks and crevices provide harbors for the seed. Mulch will shelter
the seed from the sun's heat, and also will absorb the impact of
rain from dislodging the seed.

The practice of hydroseeding promotes the growth of turf on
slopes, especially in situations where equipment and planting
crews are impractical. Hydroseeders are capable of spreading mixtures
of seed, fertilizers, and mulch simultaneously. Be especially
cautious in the fall of the year...Seed the slopes (the seedbed)
before the ground freezes. Root growth will occur over the winter,
and the degree of erosion will be reduced.

Remember that the lighter colored mulches will reflect the sun's
heat while darker colored mulches will absorb heat. Deep mulches
(three to four inches) will insulate the soil against freezing and
thawing better than will thin mulches (one to two inches). Frost
heave results from alternate freezing and thawing. Voids created
by frost heave cause roots of plants to wither and die. The choice
of mulch materials is important, in that the process of decomposition
extracts nitrogen from the soil. Inorganic mulches (asphalt or
fiberglass) will not deplete soil nitrogen needed for plant growth.

Maintenance:

You will be asked for recommendations for the maintenance of
road slopes. Please remember that slopes steeper than 2:1 are
hazardous to equipment operators, and that wet ditches cause tractor
wheels to sink. Designate areas to be mowed to grass heights of
four inches to reduce browning-off during the summer droughts and
set aside areas as seedbeds for wildflowers...these to be mowed
after the seeds drop in late summer.

Aesthetics:

Vista clearing is a designed operation, not "eye-balling" as
some folks accuse us of practicing. First, decide on what features
you want to interpret along the roadway (the nature of the mature
forest, for instance) and then decide on those features which will
enhance the scenic quality beyond the roadway (the snowpeaks of a
mountain range, for example). Prepare a plan showing the locations
of these "openings" along the roadway. Flag those dominant trees
to be removed. Step back to the motorists' travel path to study
the trees selected. Consider the effects of the scene, the consequence
of removal on the remaining vegetation. Make the final
decision based on your best aesthetic judgment and your knowledge
of silviculture. Vista clearing is a combination of the two arts:
aesthetics and silviculture.

Running the job:

Working over a slope, especially a wet slope, is sometimes
hazardous and sometimes damaging to the slope itself. Removing
deadfalls and loose rock can leave scars, and a fall or a slip of
the foot can cause an injury. Use ropes and gloves when working
on steep slopes... and never work alone!

Be especially careful with handtools and instruct new workmen
in the use of saws, axes, and pulaskies. Gloves, goggles, chaps,
stout shoes, and hard hats are essential.

Controlling traffic along the roadway is necessary to reduce
accidents. Warning signs must be posted at least 500 feet in
advance of the work site so that motorists can slow down, and flagmen
must be stationed to control cars approaching the work site. Safety
for both work crews and motorists is part of the job!