Samuel Washington McCallie.

A preliminary report on the roads and road-building materials of Georgia online

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face. The following is a short description of some of the most im-
portant rocks employed in road-construction: —

Trap Rock. — The term, "trap," is from the Swedish word
*'trappa," meaning a stair or step. It was first applied to igneous
rocks, weathering in the form of massive steps. The term, as now
commonly used, has a rather indefinite meaning. However, it is
frequently applied to any dark, massive, eruptive rock, whose
mineral constituents are not readily made out, without the use of
the microscope. This class of rocks has received various local
names, given from their natural or fancied resemblance to certain
minerals, or other object. A common name applied to them
throughout the Southern States is iron-stone or iron-rock. " Nig-
ger-head " rock is also frequently used. The last of these seems to

American Highways, by N. S. Shaler, p. 54.

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be somewhat descriptive, and evidently has reference to its pecu-
liar rounded mode of weathering, and its unusual hardness. All
the trap rocks are of igneous origin, and commonly occur in the
form of dikes, which were originally fissures in the earth's crust
extending to great depth, through which the trap rock welled up
in the form of melted lava. The great rapidity, with which these
rocks cooled, caused them to be very fine grained and compact.
Many of them have such exceedingly fine structure, that it is often
impossible, to identify the individual mineral constituents, without
the aid of the microscope. The most common varieties of trap
rock used in road-construction are diabase and diorite. These
rocks differ from each other, both in mineral composition and in
physical structure.

The diabases are made up of plagioclase and augite, and possess
a peculiar interlocked structure, which gives to these rocks their
remarkable toughness.' The feldspar occurs in the form of long,
narrow, lath-shaped crystals, surrounded and enclosed by broad,
angular plates of augite. All the diabases are of a dark color, and
are usually very homogeneous in texture. As a general rule, they
have a fine texture, and always break with great difficulty. The
fragments, whether broken by hand or by the crusher, are usually
irregular and angular, which permits them to become easily con-
solidated into a compact, hard mass, by the action of the roller.
The diabases, which are the most durable of all rocks for road-sur-
facing material, are pretty generally distributed throughout the
eastern part of the United States. The trap rock of the Palisades
on the Hudson river, and many of the intrusive rocks of the Con-
necticut valley. New Jersey, Maryland, Virginia, the Carolinas and
Georgia, are diabases. In Georgia, the diabases occur as dikes,
varying in thickness from an inch to one or more hundred feet.
They are readily distinguished from all other rocks by their mode
of occurrence, great toughness, high specific gravity and their
peculiar manner of weathering into rounded boulders.

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The diorites resemble the diabases very closely, in general ap-
pearance. They differ from them, however, in containing horn-
blende, in the place of augite ; also, in their granular structure.
The diorites, as a general thing, are not so tough as the diabases ;
and, as a consequence, they are not so suitable for road-surfacing.
Nevertheless, they are extensively used for this purpose, and are
said to possess excellent cementing and wearing qualities. This
class of rocks generally occurs with granites and gneisses, and
often has a laminated or schistose structure. The massive variety
is always preferred for road material ; as it forms a more complete
bond, and the fragments are not so easily crushed by action of the
wheel. A number of other varieties of dark-colored rock, used
for road material, are often called trap rock. The most common
of these are the gabbros, the hornblende-schists and the norites,
all of which make good road material, imless they are laminated
or are very coarse-grained.

Granite. — The granites, including the gneisses and syenites,
have a more or less extensive use in road-construction. They are,
however, inferior to trap rock, both in toughness and in binding
quality. Many of the true granites, especially those having a fine
grain and a homogeneous texture, make a fair road material. One
of the principal defects in nearly all granites, one that affects their
wearing quality when used as road-surfacing, is the unsound con-
dition of their feldspars, one of their essential minerals. By an
examination of a thin section of almost any granite under the
microscope, it will be seen, that many of the feldspar crystals are
more or less altered to kaolin, the most commonly occurring
member of the clay group of minerals. This altered condition of
one of the leading mineral constituents of a granite greatly reduces
its power to withstand abrasion ; and, as a result, it is readily
ground to powder by the action of the wheel. Mica, when it is
abundant in granite, is also an element of weakness ; as it is readily
affected by atmospheric agencies, thereby causing a disintegration
of the stone. It is a good rule, in constructing a broken-stone
road, never to use granite for surfacing material, unless it has a

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fine texture and the feldspar is in an unaltered condition. Coarse-
grained granite with the feldspar partially decomposed is but little
improvement on common coarse sand, in the construction of a
hardened way.

The syenites^ which differ from the granites, mainly in the ab-
sence of quartz, generally consist of orthoclase an<J hornblende.
These rocks are always granular like the granites ; but generally
they are fine-grained, more compact and better suited for road-
construction. The best varieties of syenite for road material are
those having a very dark color, which indicates the presence of a
large quantity of hornblende, a mineral constituent, to which the
rock in a great measure owes its toughness.

The gneisses^ which are more abundant than either of the above
named rocks, are also quite extensively used for road purposes.
The only difference between the gneisses and the granites is the
banded structure of the former, which gives to it a stratified ap-

Closely related to the gneisses, is another class of rocks, known
as schists. These rocks also have a limited use in road-building.
They are, however, a very inferior road-metal ; and they should
never be used, if any other material is at hand. The various
rocks, here included under the general head of granites, are of very
common occurrence in the eastern part of the United States.
They make up, in a great measure, the majority of the crystalline
rocks, of ancient though doubtful age, forming a narrow belt, ex-
tending from Vermont to Alabama. All the rocks, with few ex-
ceptions, belonging to the Crystalline Area of North Georgia, are
of this class.

Limestone. — The limestones, together with the dolomites, are
widely distributed, and are probably more extensively used for
macadamizing purposes than all other kinds of stones combined.
When compact and semi-crystalline, they make a fair road mate-
rial, the cementing property of which is of the highest quality.
The crystalline varieties called marble are not so suitable for road-
construction, as their granular structure causes them to be easily

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crushed by the wheel. Many of the limestones also contain a high
percentage of clay, and are quite heterogeneous in structure. Such
stones possess an excellent cementing property ; but they are
almost invariably defective, in weathering qualities, which renders
them undesirable for road-surfacing. The dolomites are generally
harder and more compact than the true limestones ; and, as a con-
sequence, they are preferable for road-construction. Prof. Shaler,
in speaking of the durability of limestones, says : — ' '' In practice,
limestone wears, under a given amount of traffic, at least twice as-
rapidly as the trappean rocks. In cases, particularly where the
slopes are steep, so that the dust readily washes away, or where
the road is exposed to a strong wind, the rate of wear is about
four times as rapid as it would be, if the road were covered with
the best quality of trap."

Sandstone. — Sandstones are indurated or hardened beds of
sand. The individual granules, of which they are composed, are
cemented into a compact mass by a ferruginous, calcareous, argil-
laceous or siliceous matrix. The sandstones usually possess little
or no binding property ; and, furthermore, they are generally soft
and easily crushed. These qualities render this class of stone
almost worthless for road-surfacing material. Quartzite, a meta-
morphic sandstone, is far more valuable as a road material. This
variety of sandstone is frequently quite tough, and makes a very
durable road material. It is essential, however, in most cases, ta
add some foreign material to the quartzite, in order to make the
broken fragments unite into a hardened surface.

Chert. — Chert is a name, applied to a cryptocrystalline va-
riety of quartz, closely akin to flint, and occurring as layers or
nodules in many of the older limestones.' When the limestones
are dissolved and carried off in solution by water, these flinty
layers and nodules remain unaltered, and often accumulate, in
thick deposits. When this material does not contain too much
clay, it makes an excellent road-covering, especially when the
traffic is light. The cementing quality of chert is probably sur-

Op. cit., p. 60.

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passed only by limestone and diabases ; but its wearing quality is
not so satisfactory, owing to its greater brittleness. This class of
road material is quite common throughout the Southern States,
wherever the Silurian limestones are present. It occurs in the
northwestern portion of Georgia in great abundance, where it has
been used more or less extensively for several years for surfacing

Shale and Slate. — Shales and slates, which are indurated
or hardened clays, are of very common occurrence; but, as a
general rule, the>' are brittle, and so easily aflEected by atmospheric
agencies, that many of them are but little better than common
clay for constructing hardened ways. When these rocks are of a
sandy nature and contain a considerable amount of lime, they may
be used for road-surfacing ; but, even then, it is advisable to mix
with them some other material, which will give to the hardened
way a more lasting surface.

Gravel. — Gravel is composed of rounded pebbles, varying
from the size of a pea to that of an ^%%, When they are of a
larger size, they are often called shingle. Gravel may consist of
almost any variety of rock ; but quartz is the most common, be-
cause of its hardness and great power of resisting abrasion.
These water-worn pebbles are frequently found forming extensive
deposits along sea-beaches, both ancient and modern, and they are
also quite abundant in the beds of streams, especially if the streams
are rapid and take their rise in the Crystalline area. Deposits of
water-worn gravel constitute one of the chief characteristics of
the LaFayette formation, which consists of a belt, many miles
wide, extending through Maryland, Virginia, the Carolinas and
the Gulf States. Gravel, obtained from this source, has quite an
extensive use for road purposes along the western margin of the
Coastal Plain. The great hardness of gravel, together with its
unusual toughness, especially fits it for surfacing material. How-
ever, it is at the same time, on account of its rounded form, very
defective in binding quality. To overcome this defect, it is often
necessary to mix with the gravel some foreign material as a binder,

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to secure a compact hardened surface. There is also another va-
riety of gravel, having an angular shape, often called pit-gravel,
which is more or less extensively used in road-surfacing. This
class of gravel originates as a detritus from the uneven weather-
ing of limestones, sandstones, shales and other rocks, and is
usually found in considerable beds at the base of hills or ridges,
where it occurs in the form of talus. To this class of gravel,
the chert spoken of above properly belongs.

Shells. — Shells are frequently used for surfacing roads, es-
pecially in the vicinity of the sea-shore, where they often occur
in beds several feet in thickness. This material makes an ex-
cellent hardened way for light traflfic. It binds well, wears
evenly, and is comparatively free from dust. Oyster shells, which
often accumulate in immense heaps about packing houses, are the
best shells for road-surfacing. They are not so fragile, as the
beach-washed shells ; and, as a consequence, they are more dura-
ble and freer from dust. Prof. Shaler places shells as equal in
value to ordinary limestone for road-construction, when the cost
of preparation is taken into consideration.

The following table, taken from the annual report of the Mas-,
sachusetts Highway Commission for 1896, gives the specific den-
sities, coefficients of abrasion, cementing value, and re-cementing
value of stone, as obtained from a series of laboratory tests : —

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*' In this table, the stones are arranged in the order of their
power of resisting abrasion. Column i contains the specific den-
sity of the stone ; column 2, the coeflficiency of abrasion ; the next
column gives the number of blows required to stress the 2.5-c.m,
briquets to their elastic limits ; column 4 gives the same data for
the first testing of 30gram briquets prepared for the re-cementation
test ; and the next column gives the number of blows, that the re-
cemented briquets will stand, before reaching their elastic limit."
The table, in a general way, shows the relative value of the stones
tested for road materials ; but Mr. Page says, that the data here
presented are too limited to be discussed in a judicious manner. ^

Thei«e tests have been greatly extended, since the above was written. See Report of Masa.
^Highway Commission for 1899.

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Many tools and machines have been devised, from time to time,
for the special purpose of constructing and maintaining highways.
The most useful of these implements are the plows, scrapers, road-
graders, rock-crushers and rollers. Of late years, these several
devices have played a very important part in the economy of road-
building. They not only greatly reduce the cost of constructing
roads ; but they also lessen the expense of maintenance in like
degree. It has been claimed, that the use of these implements
diminishes by one half the cost of building and maintaining
first-class roadways. That this statement is not overdrawn,
seems evident, if we take into consideration the fact, that much
of the work of grading etc., which was formerly done by manual
labor, can now be executed by horse or by steam-power, through
the invention of the above named implements. Improved and
modern road-making machinery- is as essential and important in
the construction and maintenance of highways as is the self-
binder in the harvest-field, or the mower and rake in the meadow*

Plows. — Plows constructed for road purposes should possess
unusual strength. The beam, when made of wood, should be
of the best hickory or white-oak, and protected by iron straps
on its under and upper sides extending from the clevis to the
standard. The handles should also be of like material, similarly
protected, where they are likely to be injured by contact with
the hard ground; and they should be securely bound to the
beam by means of iron rods. Some manufacturers construct
road-plows with steel beams and handles. Such plows are most


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economical, where they are constantly exposed to the weather,
as they cannot rot. The share, mould-board and coulter should
all be made of the best of plow-steel, and so patterned as to best
execute the special work, for which they are designed; There
are several varieties of plows manufactured for road purposes, each
of which is claimed by its agent to possess peculiar merits. The
plow most useful in common road-construction is the grading-plow,
which weighs from loo to 150 pounds, and is intended for four
horses. Lighter plows are frequently used ; but they are generally
unsatisfactory in hard ground.

Fig. 19 .

A Road-Plow.

Road Scrapers. — There are two different kinds of road-
scrapers in common use, namely, the drag scraper and the wheeled
scraper. The former is a steel or iron scoop, with a capacity of
from three to seven cubic feet, and weighing from 90 to 100
poimds. To the sides of the scoop near its anterior end is fastened
an iron bail, to which the horse is attached ; while to the rear, on
either side, are the handles, used in manipulating the scoop during
the process of loading and unloading. These scoops are simple
structures, easily repaired, and well suited for moving earth, when
the length of haul is limited to a few rods. Drag-scrapers are
made of three sizes, the smallest, for one, and the largest, for two

The wheeled scraper differs from the drag-scraper in having the
scoop mounted on wheels. This greatly reduces the tractive force

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necessary- to convey the load to the dump, and enables the team to
do much more work in a given time. The loading and unloading
process in the improved wheeled scraper is executed by means of
levers, while the team is in motion. Several sizes of wheeled
scrapers are made by different firms. They vary in capacity from
nine to sixteen cubic feet, and weigh from 300 to 700 pounds.
Road-scrapers are almost indispensable, where a large quantity- of
earth is to be moved. No recent invention has done more toward
cheapening the cost of road-grading than the scraper. It should
always be at hand, on all highways, whenever any earth is to be

Road-Machines. — Road-machines are recent inventions, de-
vised for the purpose of ex-
^'g- 20 cavating earth, transporting

it, and dumping it, where
needed. Several varieties of
these machines have been
placed upon the market in
the last few years. They
are all made upon the same
A Drag Scraper. general plan, differing only

in detail of construction.
They consist of a large iron or steel scraper, mounted on
wheels, so attached to a suspended frame as to be easily ad-
justed at any desired angle, by the movement of certain levers.
A road-machine, to give satisfaction for general use, should be
of light draft, well constructed of good material, easily and
safely operated, and readily adjusted for all kinds of road-work.
Mr. Isaac Potter, in speaking of the merits of road-machines, says : ^
"A good road-machine will do the work of twenty laborers ; and
when put to its best will do the work of forty. ***** To the
fair-minded, intelligent road oflScial of the present day, there is no
question as to the many advantages of road machinery over any
other method. of repairing dirt roads. Besides doing the work at

Country Roads, pp. 30-34.

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less than half the cost, vastly better results are obtained. The
difference between the result obtained by the use of a road-machine
and the old-fashioned plow and shovel method is clearly defined.
A road-machine of recognized merit will plow an even ditch of
uniform depth, and will plow, where, an ordinary plow will not
enter the soil. The scraping blade of the machine moves the
earth from the ditch to the middle of the road, distributing it
evenly and leaving the roadbed smooth, without any of those little

Fig. 21

A Wheeled Scraper.

lumps and holes which characterize a road repaired in the old-
fashioned manner.

''A good reversible road-machine with two or three teams and
two men will repair from three-quarters of a mile to a mile of road
a day, depending of course on the width of road and the amount of
earth to be moved. A conservative estimate shows, that to do
the same work in the old-fashioned way will require a plow, two
teams, and at least foity men. ******

'* Anyone, who has compared the road built by the old-fashioned
method with its rough, uneven surface, with the clean ditches
and smooth and even surface ofa road built by the machine method
by a good operator, will not hesitate long in passing judgment on
the two methods. A road-machine can also be made very useful

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in the work of under drainage, where side drainage is considered

In addition to the above described machine, there is another,
called the "New Era " grader, which is constructed on a some-
what different plan. This machine not only excavates the earth ;
but it also automatically carries the earth some distance, on an end-
Fig. 22

Champion Road-Machine.

less belt, and dumps it into the center of the road or loads it on
wagons. The earth is excavated by means of a large plow. This
class of machine is rather complicated ; and it will probably never
come into general use on ordinary country-roads.

Road-Rollers. — There are two types of road-rollers in common
use, one known as the horse-roller, and the other, as the steam-
roller. The horse-roller, which weighs from two and a half to
eight tons, is usually constructed in two sections, each revolving
on an independent axis. These sections, in standard rollers, are

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about five feet in diameter and twenty-six inches in length, giving
a total rolling width of fifty-two inches. The best rollers of this
class are reversible, so that the tongue may be changed from one
side to the other. They also have ballast-boxes, located on the
frame for the reception of pig iron or other heavy material for in-
creasing their weight. Some of these rollers, as the ** Addyston
Reversible " horse-roller, have solid heads ; and their weight is-
increased by filling the roller-drum with water.

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Online LibrarySamuel Washington McCallieA preliminary report on the roads and road-building materials of Georgia → online text (page 6 of 22)