SAND-CLAY AND GRAVEL ROADS
In Chapter IV, mention was made of the variation in serviceability of
road surfaces composed of the natural soil existing on the
right-of-way of the road. It has been found that soils of a clayey
nature in which there is a considerable percentage of sand usually
afford a serviceable road surface for light or moderate traffic,
especially in areas where climatic conditions are favorable. A study
of these soils, together with the construction of experimental roads
of various mixtures of sand and clay, has led to a fairly
comprehensive understanding of the principles of construction and
range of capacity of this type of road surface, which is known as the
sand-clay road.
The sand-clay road surface consists of a natural or artificial mixture
of sand and clay, in which the amount of clay is somewhat greater than
sufficient to fill the voids in the dry sand. It may be assumed that
the sand contains 40 per cent of voids and that at least 45 per cent
of clay is required to fill the voids and bind the sand grains
together, because the clay spreads the sand grains apart during the
mixing, thus having the effect of increasing the voids. As a matter of
experiment, it is found to be impractical to secure by available
construction methods mixtures of sufficient uniformity to render it
necessary to exercise great exactness in proportioning the components,
but reasonable care in proportioning the materials is desirable.
Successful utilization of this type of surface requires considerable
study of available materials and investigations of their behavior when
combined. Extensive and exhaustive experiments have been conducted
with sand-clay mixtures in various places where they are widely used
for road surfaces and the following general principles have been
deduced.
=The Binder.=--In the sand-clay road, stability is obtained by
utilizing the bonding properties possessed to some degree by all
soils. Naturally this characteristic may be expected to vary widely
with the several types of soil. It is generally considered to be a
common property of clay, but the term clay is a general one that is
often applied to soils differing greatly in physical characteristics
and the term therefore loses its significance in this connection.
Those soils that are properly and technically called clay are
decidedly sticky when wet and are the best materials for sand-clay
construction. Of the clays, those that produce a tough sticky mud are
best. This can be tested by mixing a small quantity into a stiff mud
and molding it into a ball and immersing in water. If the ball retains
its shape for some little time, it is likely to prove a very
satisfactory binder, but, if it becomes plastic and loses its shape,
it will be an inferior binder, as a general rule. The ball clay, as
the former is called, may be of any color common to soils, not
necessarily yellow or reddish as is sometimes supposed. Likewise,
balls of mixtures containing varying percentages of sand and the
binder to be used may be made up and immersed in water. The mixture
that holds its shape longest is of course the best combination of the
materials and indicates the mixture to use in the construction.
An ideal, or even a fairly satisfactory soil for a binder may not
exist in the vicinity of a proposed improvement, and consequently an
inferior binder is frequently the only material available.
Sometimes deposits of clay or gravel contain a considerable percentage
of gypsum which serves as a binder and is particularly effective when
used in combination with clay and sand or gravel.
In many places a soil of the type used for adobe and called "caliche"
may be found and this is an excellent binder for sand or gravel.
=Top-Soil or Natural Mixtures.=--Deposits consisting of a natural
mixture of sand and clay in which the ingredients happen to exist in
about the correct relative proportions for sand-clay road surfaces are
found in many localities. These mixtures are commonly referred to as
top-soil. If the deposits are somewhat deficient either in sand or
clay, they can be utilized if the proper corrections in the
proportions are made during construction. Very satisfactory road
surfaces are sometimes constructed with mixtures that appear to be far
from ideal in composition, but experience and frequent trials are
needed to determine the best way in which to handle these mixtures.
=Sand-Clay Surfaces on Sandy Roads.=--Sand-clay surfaces may be
constructed on naturally sandy roads either by adding clay and mixing
it with the sand to secure the desired composition, or a layer of a
natural sand-clay mixture, caliche or sand-clay-gypsum may be placed
on top of the sand.
The most widely used method is to mix clay or other binder with the
sand. Since there is no need to provide for ditches to carry storm
water on a deep sand soil, the sand is graded off nearly flat across
the road and no ditches are provided. The clay is dumped on the road
in a layer about 8 inches thick and is then mixed into the sand. It is
desired to mix enough sand with the clay to produce a mixture composed
of approximately 1/3 clay and 2/3 sand. The mixing is accomplished in
various ways, the most common being to use a heavy plow at first and
to follow this with a heavy disc harrow. The mixing is a tedious and
disagreeable process, but its thorough accomplishment is
indispensable. The mixing is most readily done when the materials are
saturated with water and in practice it is customary to depend upon
rain for the water, although in the final stages water may be hauled
and sprinkled on the road to facilitate final completion of the
mixing. After the mixing has been completed, the surface is smoothed
with the blade grader and is kept smooth until it dries out. Repeated
dragging will be required, during the first year especially, and to
some extent each year in order to keep the surface smooth, but the
dragging can be successfully accomplished only when the road is wet.
[Illustration: Fig. 14.--Cross Sections for Sand-Clay Roads]
In regions where several months of continued hot, dry weather is to be
expected each year, the sand-clay mixture is likely to break through
unless it is of considerable thickness and generally the surface layer
is made much thicker than for regions where the annual rainfall is
fairly well distributed. This is especially necessary when the binder
is of inferior quality. It is not uncommon in such cases to make the
sand-clay surface as much as two feet thick.
As the mixing progresses it may appear that patches here and there are
deficient in either clay or sand and the mixture in these places is
corrected by the addition of a little sand or clay as may be
required.
If the top-soil is used it is deposited on the sand in the required
quantity and is remixed in place to insure uniformity. If either sand
or clay is needed to give a satisfactory mixture, the proper material
is added and mixed in as the work progresses. The surface is finally
smoothed by means of the grader and drag.
=Sand-Clay on Clay or Loam.=--If the existing road is of clay or loam,
ample drainage will be required as discussed in Chapter IV. The
surface may be constructed of a natural sand-clay mixture or of a sand
mixed with the natural soil. If the former, the surface of the
existing road is prepared by grading so as to insure good drainage and
the natural mixture is then deposited and the surface completed as
described in the preceding section.
If the surface is formed by mixing sand with the existing soil, the
sands may be deposited in a layer about six inches thick which will
gradually mix with the soil as the road is used. A second application
of sand may follow in a year or two if it is needed. Such a road
surface will lack uniformity of composition and it seems preferable to
mix the sand with the soil by plowing and discing as previously
described.
=Characteristics.=--Sand-clay road surfaces do not have sufficient
durability for heavily traveled highways, but will be satisfactory for
a moderate amount of traffic. These surfaces have maximum
serviceability when moist, not wet, and consequently are not as
durable in dry climates as in humid areas. They are likely to become
sticky and unstable in continued wet weather and to become friable and
wear into chuck holes in long continued dry weather. At their best,
they are dustless, somewhat resilient and of low tractive resistance.
GRAVEL ROAD SURFACES
[Illustration: Fig. 15.--Cross Sections for Gravel Highways]
=Natural Gravel.=--Gravel is the name given to a material consisting
of a mixture of more or less rounded stones, sand and earthy material,
which is found in natural deposits. These deposits exist in almost
every part of North America, being especially numerous in the
glaciated areas, but by no means confined to them. Gravel deposits
consist of pieces of rock varying in size from those of a cubic yard
or more in volume to the finest stone dust, but with pieces ranging in
size from that which will pass a 3-inch ring down to fine sand
predominating. The larger pieces are usually more or less rounded and
the finer particles may be rounded or may be angular. Many varieties
of rocks are to be found among the gravel pebbles, but the rocks of
igneous origin and possessing a considerable degree of hardness
generally predominate. Intermixed with the pieces of rock there is
likely to be clay or other soil, the quantity varying greatly in
different deposits and even in various places in the same deposits.
Often there are found deposits of material which are by the layman
termed gravel, which are really clayey sand or sand containing a few
pebbles, but which are of value to the road builder for the sand clay
type of surfacing. The term gravel is exceedingly general and unless
specifically defined, gives little indication of the exact nature of
to which it is applied.
TABLE 7
SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF
GRAVEL
-----------------+----------------------------
| Cementing Value
Per Cent Clay by +---------------+------------
Weight | As Received | Washed
-----------------+---------------+------------
4.4 | 276 | 43
6.4 | 105 | 285
5.1 | 241 | 70
14.5 | 500 | 279
8.5 | 500 | 112
10.1 | 300 | 267
14.8 | 500 | 107
7.5 | 184 | 198
16.5 | 500 | 428
2.0 | 185 | 239
1.5 | 500 | 500
4.5 | 212 | 204
2.5 | 116 | 363
-----------------+---------------+------------
The value of any gravel for road surfacing depends upon the degree to
which it possesses the properties of an ideal gravel for road
surfacing. Ideal gravel is seldom encountered, but a consideration of
its characteristics serves to establish a measure by which to estimate
the probable value of any deposit.
=The Ideal Road Gravel.=--The ideal road gravel is a mixture of
pebbles, sand and earthy material, the pieces varying from coarse to
fine in such a manner that when the gravel is compacted into a road
surface the spaces between the larger pebbles are filled with the
finer material. The pebbles are of a variety of rock that is highly
resistant to wear so that the road surface made from the gravel will
have the quality of durability. The gravel possesses good cementing
properties, insuring that the pieces will hold together in the road
surface. The cementing property may be due to the rock powder in the
deposit or to earthy material mixed with the rock particles, or to
both. Table 7 shows the results of a number of tests made upon gravels
and indicates that the cementing property of the gravel does not
always depend upon the clay content.
=Permissible Size of Pebbles.=--The larger pebbles in the gravel are
less likely to crush under loads than smaller pebbles of the same sort
of rock, but if the rock is of some of the tougher varieties such as
trap, there is very little likelihood of even the smaller pebbles
crushing. If the pebbles are of rock of medium toughness, the smaller
pebbles might be crushed under the heavier loads. It is the usual
practice to permit gravel to be used for the foundation course in
which the pebbles are as large as will pass a 3-1/2-inch circular
screen opening, and for the wearing course, as large as will pass a
2-1/2-inch circular screen opening. If larger pebbles are allowed in
the wearing course, the surface is certain to become rough after a
time. If the gravel is to be placed in a single course as is a very
common practice, then the maximum size should not exceed that which
will pass a 2-1/2-inch circular screen opening.
The Wisconsin Highway Commission has constructed a very large mileage
of excellent gravel roads and the sizes specified for their roads are
as follows:
"_Bottom Course Gravel_.--Bottom course shall consist of a
mixture of gravel, sand and clay with the proportions and various
sizes as follows:
"All to pass a two-inch screen and to have at least sixty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate to be retained on a one-inch
screen; at least sixty-five and not more than eighty-five per
cent of the total fine aggregate to be retained on a two
hundred-mesh sieve."
"_Top Course Gravel_.--Top course shall consist of a mixture of
gravel, sand and clay with the proportions of the various sizes
as follows:
"All to pass a one-inch screen and to have at least fifty and not
more than seventy-five per cent retained on a quarter-inch
screen; at least twenty-five and not more than seventy-five per
cent of the total coarse aggregate (material over one-fourth inch
in size) to be retained on a one-half-inch screen; at least
sixty-five and not more than eighty-five per cent of the total
fine aggregate (material under one-fourth inch in size) to be
retained on a two hundred-mesh sieve."
"_Screened Gravel and Sand Mixtures_.--Where it is impossible to
obtain run of bank gravel containing the necessary binder in its
natural state, screened gravel shall be used and the necessary
sand and clay binder added as directed by the engineer. Gravel
and sand shall be delivered on the work separately. Clay binder
shall be obtained from approved pits and added as directed by the
engineer."
"_Run of Bank Gravel_.--When run of bank gravel is permitted
either for one course or two course work, the size shall not
exceed that specified for bottom or top course. If necessary, the
contractor shall pass all the material through a two-inch screen
for the bottom course, and through a one-inch screen for the top
course. When the work consists of only one course, the material
shall be of the sizes as specified for the top course. The
necessary binder shall be contained in the material in its
natural state, excepting that a small percentage of clay binder
may be added as directed by the Engineer."
=Wearing Properties.=--A certain amount of grinding action takes place
on the road surface under the direct action of wheels, especially
those with steel tires. Where rubber tired traffic predominates, this
action is much less severe than where steel tired vehicles
predominate, but the tendency exists on all roads. In addition, there
is distortion of the layer of gravel under heavy loads which causes
the pieces of stone in the surface to rub against each other and to
wear away slowly.
The gravel road in the very best condition is slightly uneven but
there is comparatively little jar imparted to vehicles, and,
consequently, little impact on the surface. When somewhat worn, the
impact becomes a factor of some importance and the pounding of
vehicles has a very destructive action on the surface. Soft pebbles
will be reduced to dust in a comparatively short time.
The degree to which any gravel resists the destructive action of
traffic depends upon the varieties of rock represented by the pebbles
in the gravel. If the pebbles are mostly from rocks of good wearing
properties, that quality will be imparted to the road surface. If
mostly from rocks of little durability, the same characteristic will
be imparted to the road surface. A very good general notion of the
probable durability of gravel can therefore be obtained by a careful
visual examination of the material and classification of the rock
varieties represented by the pebbles.
=Utilizing Natural Gravels.=--Gravel road construction is advantageous
only when it can be accomplished at low first cost. This usually
presupposes a local supply of gravel that can be utilized, or at any
rate a supply that need not be shipped a long distance. In the nature
of things, such deposits are likely to be deficient in some of the
desirable characteristics, and may be deficient in most of them. By
various means, the defects in the materials can be partially corrected
while constructing the road.
If the gravel deposit consists of layers of varying composition as
regards size and clay content, the material may be loosened from the
exposed face and allowed to fall to the bottom of the pit thereby
becoming mixed to a sufficient extent to produce a reasonably uniform
product. If deficient in clay, it often proves feasible to add a small
part of the clay over-burden, thereby insuring enough binder.
Sometimes adjoining deposits will consist one of relatively fine
material, the other of relatively coarse. These may be mixed on the
work by first placing the coarse material in a layer about 5 inches
thick and adding the finer material in a similar layer. The two will
mix very rapidly during the operations of spreading and shaping.
When deposits contain pebbles larger than will pass a 3-1/2-inch ring,
these larger stones will prove to be undesirable if placed on the
road, as they are almost sure to work to the surface of the gravel
layer and become a source of annoyance to the users of the road.
Oversize stone can be removed while loading the gravel or while
spreading it, if care is exercised and not too large a proportion is
oversize. It is preferable however to remove the oversize by means of
screens at the pit. Usually on large jobs the oversize is crushed and
mixed with the supply so as to utilize what is really the best part of
the material.
Gravels deficient in bonding material are often encountered in
deposits where there is insufficient overburden to give enough
additional binder or where the overburden is of a material unsuitable
for binder. Such materials may be utilized by adding binder in the
form of clay after the gravel has been placed on the road.
Almost any gravel deposit can be utilized in some way if the material
is of a durable nature, regardless of other characteristics. The
serviceability of a gravel road will depend largely on how nearly the
gravel approaches the ideal, but variations in the manipulations will
do much to overcome deficiencies in materials.
=Thickness of Layer.=--The thickness of the layer of gravel required
depends both upon the type of soil upon which it is placed and the
nature of the traffic to which the road will be subjected. Gravel
surfaces should not ordinarily be constructed on highways carrying
heavy truck traffic, but if gross loads of three or four tons are the
heaviest anticipated, the gravel will be reasonably stable. On such
roads, a layer of well compacted gravel ten inches thick will support
the loads if a well drained earth foundation is provided. If but
little truck traffic is anticipated and loads up to three tons on
steel tires are the average, a layer 8 inches thick will be
sufficient. In dry climates, a layer six inches thick will be
adequate if it can be kept from raveling.
On secondary roads, carrying principally farm-to-market traffic, and
not a great volume of that, the above thicknesses may be reduced about
one-fourth.
The exact thickness needed for any particular road is a matter for
special study on account of the variations in the gravels and in the
supporting power of the soil upon which they are placed.
PLACING GRAVEL
=Preparation of the Road.=--The roadway that is to be surfaced with
gravel is first brought to the desired grade and cross section. It
would be advantageous if this could be done a year before the gravel
is placed so that no settlement of the earth foundation would occur
after the gravel surface is completed. But if that is impractical, the
grading may be done just prior to placing the gravel, providing
appropriate methods are adopted for securing compacted fills.
=Trench Method.=--Two distinct methods of placing the gravel are in
general use, known as the trench method and the surface or feather
edge method respectively. The method to adopt for any particular road
will depend largely on certain conditions that will be explained
later.
In the trench method, a trench of the proper width and depth for
receiving the gravel is excavated in the earth road surface and the
gravel is placed therein.
The trench is formed by plowing a few furrows and scraping out the
loosened earth with a blade grader. The loose material is generally
moved out laterally to build up earth berms or "shoulders" alongside
the gravel. Into this trench the gravel is dumped in the proper
quantity to give the required thickness after being compacted.
The greatest care must be exercised in spreading the gravel to
eliminate unevenness where the loads were deposited. An ordinary blade
grader is one of the best and most economical implements to use for
spreading the gravel. When the gravel has been deposited in the trench
for a distance of a thousand feet or more, the spreading is
accomplished by dragging the surface repeatedly with the blade grader,
the work being continued until all waviness disappears. The gravel is
then thoroughly and repeatedly harrowed with a heavy stiff tooth
harrow to mix thoroughly the fine and coarse gravel so as to produce
as nearly a uniform mixture as may be. The gravel is then finally
smoothed with the blade grader.
The gravel may be compacted by rolling or may be allowed to pack from
the action of traffic. The former is greatly to be preferred where
practicable. The rolling is performed with a three-wheeled
self-propelled roller weighing about 8 tons and must be done while the
gravel is wet. Generally a sprinkling wagon is used to wet down the
gravel, but advantage is always taken of rains to facilitate the work.
The gravel must be spread in layers not over 5 or 6 inches thick to
get the desired results, which means that for an ordinary gravel road
about 10 inches thick, the gravel will be placed in two layers of
about equal thickness, each of which will be rolled.
The gravel will compact slowly even if it is not rolled, but generally
does not become stable until the material is thoroughly soaked by
rains. Then it will begin to pack, but will become badly rutted and
uneven during the process. During this period the surface must be kept
smooth by means of the blade grader. The drag does not suffice for
this purpose, tending to accentuate the unevenness rather than to
correct it.
If gravel is placed in a trench in dense soil and rainy weather
ensues, sufficient water will be held in the trench to cause
unevenness from foundation settlement and the gravel will become mixed
with the soil to some extent and be thereby wasted. Trenches cut from
the road bed upon which the gravel is placed, to the side ditches,
will relieve this condition by affording an outlet for the surplus
water. Nevertheless some difficulty may be expected if the trench
method is used and wet weather prevails. If it is possible to close
the road against traffic until the road is dry the method is
applicable. Moreover, in long-continued dry weather, the dispersion
and loss of considerable gravel from the action of automobile traffic
is avoided because the gravel is held between substantial earth berms
and the gravel will pack better and hold its shape longer when
constructed by the trench method than otherwise.
=Surface Method.=--The surface method is one in which the gravel is
placed on the graded earth road surface without earth shoulders to
hold the gravel in place. It is also sometimes called the feather-edge
method. Except for the manner of placing as just mentioned, the
several operations are conducted in the same general manner as for the
trench method. The gravel does not compact as quickly as in the trench
method and a considerable loss of material is likely to result from
the effect of automobile traffic while the gravel is loose. But it has
the advantage of being free from difficulties in wet weather and in
some locations is therefore preferable to the trench method. It is
particularly applicable to those projects on which the placing of
gravel continues throughout the winter, the gravel being dumped and
spread, to be finally smoothed and finished in the early summer.
=Bonding.=--Where gravels deficient in binder are utilized, clay for
binder is sometimes added as the gravel is placed on the road. This
may be done by spreading the clay on top of the lower course of
gravel, placing the upper layer and sprinkling and rolling until the
clay squeezes up through the surface layer. It may also be
accomplished by spreading dry clay on the upper course before it is
harrowed and then harrowing to mix it with the gravel. Both methods
are practiced, but the former is believed to be preferable. A third
method is to separate the sand and pebbles and to mix the clay binder
with the sand and then spread the sand on top of the pebbles and mix
by harrowing.
=Maintenance.=--Gravel surfaces require careful maintenance,
especially during the first season the road is used. The gravel will
compact slowly and during the process will be rutted and otherwise
disturbed by traffic. It is important during this period to restore
the shape once a week or at least twice a month. The light blade
grader is usually employed for the purpose so long as the gravel is
somewhat loose. Later a drag of the type known as the planer will
prove to be the most effective. Figure 16 shows a type of drag that is
very satisfactory for use on gravel roads.
[Illustration: Fig. 16.--Road Planer]