BROKEN STONE ROAD SURFACES
The broken stone road surface, or macadam road as it is usually
termed, consists of a layer of broken stone, bonded or cemented
together by means of stone dust and water. The surface may or may not
be coated with some bituminous material.
=Design.=--It has been an accepted assumption that the macadam road
surface is somewhat more stable than the gravel road surface of equal
thickness, and since this is probably the consensus of opinion of
engineers familiar with both types, it may be accepted until
experimental data are available on the subject.
The thickness of the layer of macadam required for a road will depend
upon the same factors that were considered in connection with the
thickness of the gravel surface, i.e., kind of stone used, character
of earth foundation and nature of the traffic.
The standard macadam surface where good earth foundation is to be had
and where the loads do not exceed about four tons has for years been
eight inches thick. For heavier loads or inferior foundation, a
somewhat greater thickness would be employed, but the best practice
would probably provide a foundation course of the Telford type for
doubtful foundation conditions, especially for the extremely uncertain
cases. For soils of very good supporting strength such as very sandy
loam or deep sand or for arid regions where stable foundation is
always assured the thickness of the macadam might be reduced to six
inches. It should be borne in mind that the broken stone road is not
adapted to the traffic carried by trunk line highways in populous
districts, but is rather a type permissible on secondary roads and
usually adequate for local roads. It should never be employed for
roads carrying any considerable volume of passenger automobile traffic
or motor truck traffic. If surfaced with a bituminous material it will
carry up to 1200 passenger automobiles per day, but not to exceed
fifty trucks.
=Properties of the Stone.=--The stone employed for the broken stone
road should possess the qualities of hardness and toughness and should
be capable of resisting abrasion sufficiently well to have reasonable
life under the traffic to which it is subjected. Since the traffic may
vary from very light on some roads to far beyond the limit of the
economical capacity of this type of pavement on others, it follows
that any particular deposit of stone might be durable enough for some
roads, while for others it might be entirely inadequate. As a general
rule it has been found that stone that wears away at a moderate rate
will, when used for water-bound macadam surface, result in a smoother
trackway than one that will wear very slowly. It is not therefore
altogether certain that the most durable stone to be had should be
selected for a particular road. This is especially true now that the
water-bound macadam surface has been largely superseded for trunk line
highways and other heavily traveled roads, and is employed in
locations where service conditions are not severe.
The stone employed for the water-bound macadam surface must possess
good cementing properties, because the surface depends for stability
primarily upon the bonding action of the dust from the broken stone.
This is in contrast to the gravel road, where little dependence is
placed upon the bonding effect of the rock dust. In preparing the
stone for macadam surfaces, the ledge rock is crushed and screened,
and in that way a supply of the finer particles, which are a part of
the output of the crusher, is obtained for use in bonding the
surface. This finely broken material, usually called screenings, is
essential to the construction of the water-bound type of surface.
Rocks vary considerably in the cementing properties of the dust, but
usually the rocks classed as "trap," such as andesite, gabbro and
rhyolite, and schist and basalt possess good cementing properties.
Limestones usually possess good cementing properties, but some of the
dolomitic limestones are of low cementing value. Quartz, sandstone and
the granites are of low cementing value.
=Kinds of Rocks Used for Macadam.=--Limestone and chert are the two
sedimentary rocks, employed most extensively for broken stone roads.
These rocks are found in widely distributed areas and vary in physical
characteristics from very soft material of no use to the road builder
to materials possessing considerable durability. It is desirable to
carefully test out the deposits of these materials before using to
ascertain the probable value of the rock, for the construction of the
road surface.
Of the igneous rocks, those classed as trap are best known to the road
builder and many of the deposits of trap rock afford an excellent
material for broken stone roads where the severest conditions of
traffic are encountered. The trap rocks are tough and durable and
generally possess excellent cementing properties.
Granite and sandstone are seldom used for water-bound macadam as they
possess poor cementing properties and a binder of some kind must be
added to cement the pieces together. For this purpose clay or the
screenings from some other variety of stone may be utilized.
Some other materials are occasionally employed for the construction of
macadam surfaces. Of these, oyster or marine shells, burnt shale, and
slag are most common.
Shells and slag are of rather low durability but possess good
cementing properties. Shale is a makeshift suitable only for very
light traffic roads.
=Sizes of Stone.=--The stone for the wearing course of a macadam road
should be as large as practicable, because the larger the pieces the
more durable the surface. If the individual stones are too large it is
difficult to secure a smooth surface, and large stones will be readily
loosened by tipping as the wheels roll over them. These considerations
limit the size to a maximum of that which will pass a 2-1/2-inch
screen. Stone of excellent wearing qualities may be somewhat smaller,
but never less than that which will just pass a 1-1/2-inch screen.
For the lower course, the size is not particularly important except
where the earth foundation is such as to require special construction.
It is not uncommon to use the same size of stone for both upper and
lower course and yet in many instances stone up to that which will
just pass a 3-1/2-inch screen is used for the lower course. Stone much
smaller in size may also be used successfully, but if the stone is
broken to a smaller size than is required, unnecessary expense is
incurred.
The bonding material is the finer portion of the product of the
crusher, which is called screenings. This material may be so finely
crushed as to pass a one-fourth inch screen, or may be so coarse as to
just pass a one-half inch screen, but in any case must contain all of
the dust and fine material produced by the crusher.
Where the soil and drainage conditions demand an especially stable
foundation course, the Telford type is used. The Telford foundation
consists of a layer of stones of various dimensions that can be laid
so as to give a thickness of 8 inches. These large stones are placed
by hand and therefore the size requirements are not rigid. Stones
having one dimension about 8 inches and the others not over 10 or 12
inches are satisfactory.
=Earth Work.=--A thoroughly drained and stable earth foundation is
essential to success with the macadam type of surface. Before placing
the stone, the road must be shaped to the proper cross section and
all grade reduction work completed. Preferably heavy fills should have
a year to settle before the macadam surface is placed. Side ditches,
necessary culverts and tile drains should be constructed as required
for drainage. The earth work is often carried out in connection with
the construction of the macadam surface, being completed just ahead of
the surfacing. In that case, the fills must be carefully rolled as
they are placed. The road bed may be shaped in connection with the
other earthwork. If the road has been brought to a satisfactory grade
some time prior to placing the macadam, the road bed for the broken
stone will be prepared as needed for placing the stone.
=Foundation for the Macadam.=--Macadam surfaces are quite generally
placed in a trench as described in the trench method for placing
gravel. It is an almost universal practice to compact the layer of
stone by rolling with an 8- or 10-ton power roller, and if the stone
is not held between substantial earth berms or shoulders, the rolling
merely serves to spread the stone out over the road bed instead of
compacting it. If an attempt is made to roll broken stone which has
been placed on a yielding foundation, no benefit results, but on the
contrary the stone is likely to be forced down into the soil. To
insure that the layer of broken stone can be compacted by rolling, it
is first necessary to roll the earth foundation until it becomes hard
and unyielding. If soft or yielding places appear during the rolling
these should be corrected by tile drains or by removing the earth from
the spongy place and back-filling with material that will compact when
rolled.
It is not always easy to determine why these soft places exist in what
appears to be a well drained roadway, especially since they are as
likely to be found on fills as anywhere else. Apparently they are due
to local pockets of porous soil held by denser soil so that the water
does not readily drain away. It is usually true that such places are
observed during the season of frequent precipitation more often than
during other seasons of the year.
In dry climates, the difficulties of securing suitable foundations for
the broken stone road are largely eliminated, but it may be observed
that this type of surface is not suitable for such climates unless
some sort of bituminous binder is employed to hold the stones in
place. The cementing power of the stone dust is inadequate when the
surface is continually dry.
[Illustration: Fig. 17.--Cross Section for Macadam]
=Telford Foundation.=--When the Telford type of foundation is
employed, the earth subgrade is prepared and then the Telford stone
placed carefully by hand. The spaces between the large stones are
filled with the spalls broken from the larger stones in fitting them
in place. When completed the base is rolled with a heavy roller to
secure a firm unyielding layer. The thickness is generally about eight
inches. Any fairly sound stone may be used for the Telford base.
=Placing the Broken Stone.=--It has been found impracticable properly
to roll a greater thickness than about 5 or 6 inches of loose stone,
therefore, the stone for the macadam surface is usually placed in two
layers, the first or lower layer being rolled before the next layer is
placed. The stone is hauled in dump wagons, trucks or dump cars,
dumped on the road bed and spread by hand rakes or by means of a blade
grader and is then rolled. To insure the proper thickness the loads
are accurately spaced to spread to the proper thickness.
=Rolling.=--A three-wheeled or "macadam" type of roller, of the
self-propelled type, is best for compacting the broken stone road. The
weight varies from eight to fifteen tons, but for most conditions the
ten or twelve ton size seems to be preferable. On Telford base
construction, a heavier machine is desirable and for very hard stone
it may be successfully employed.
The first trip with the roller is made along the edge of the stone and
each successive trip is made a little nearer the middle until finally
one half of the strip of stone has been rolled. The roller is then
taken to the opposite side of the roadway and the operation repeated
on the other half. The rolling is continued until the stone is
thoroughly compacted, which is evidenced by the fact that the roller
makes but a slight track in the surface.
The second layer of stone is then placed and rolled in the same manner
as the first.
=Spreading Screenings.=--After the upper course has been rolled, the
screenings are spread on it from piles alongside the road, enough
being used to fill the voids in the layer of stone and furnish a
slight excess. As the screenings are spread they are rolled to work
them into the voids. When these are filled, the surface is sprinkled
thoroughly by means of an ordinary street sprinkling cart and again
rolled. In this way the dust and water are mixed into a mortar which
fills the crevices between the stones. This mortar hardens in a few
days, giving a bond that is weak, but sufficient for the purpose if
the traffic is not too heavy. A broken stone road finished in this way
is called a water-bound macadam, and is ready for traffic in three or
four days after completion.
=Bituminous Surfaces.=--On account of the inadequacy of the
water-bound macadam when subjected to motor traffic and to obviate the
tendency of broken stone surfaces to loosen in dry weather, there has
been developed a method of covering the surface with a bituminous
material such as tar or asphalt. This will be described in detail in a
later chapter.
=Maintenance.=--Even under favorable conditions as regards kind and
amount of traffic the macadam road requires constant maintenance. The
first effect of traffic will be to brush away the fine materials used
for bonding the surface, thus exposing the larger stones in such a way
that they are rather easily loosened and removed from the surface by
wheels and the hoofs of animals. This finer material must be replaced
as fast as it is removed so as to protect the surface. Either stone
dust or clayey sand may be used, but clay if used alone is likely to
be sticky when wet and prove to be worse than the condition it was
expected to correct. In time, ruts and depressions will appear, either
as the gradual effect of wear, which will inevitably effect some
portions of the surface more than others, or on account of subsidence
of the foundation. Uneven places are repaired by first loosening the
stone, then restoring the cross section by adding new material and
tamping or rolling it in place.
If a bituminous coating has been applied, it will eventually peel off
in places and these places must be recoated as soon as practicable.
Eventually the surface will be worn to such an extent that an entirely
new wearing surface must be added. This is done by loosening the
entire surface to a depth of 3 or 4 inches and then adding a new layer
of broken stone. The loosening is sometimes accomplished by means of
heavy spikes inserted in the roller wheels, and at others by means of
a special tool known as a scarifier.
The new surface is placed and rolled in precisely the same manner as
the wearing surface of the original construction, but the layer may
not be as thick as the original wearing course. A new course will not
bond to the old surface unless the old macadam has been thoroughly
broken up first.
=Characteristics.=--The water-bound macadam is a dusty, somewhat rough
surface of low durability for rubber tired vehicles. It has long been
the standard rural highway for steel tired vehicles, but cannot carry
any considerable amount of motor traffic. It is easily repaired. When
finished with a bituminous surface its durability is greatly increased
and the dust is eliminated. It does not seem to be sufficiently rigid
for truck traffic, unless placed on exceptionally good foundation.