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Samuel Washington McCallie.

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

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the remaining two inches are to be put
on. The whole of this stone is to be
broken into pieces as nearly cubical as
possible, so that the largest piece, in its
longest dimensions, may pass through a
ring of two inches and a half inside di- Cross-section of a Drain.

ameter.

" The paved spaces on each side of the eighteen middle feet, are
to be coated with broken stones, or well cleansed strong gravel,
up to the foot-path or other boundary of the road, so as to make
the whole convexity of the road six inches from the center to the



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KOAD CONSTRUCTION



45



sides of it. The whole of the materials are to be covered with a
binding of an inch and a half in depth of good gravel, free from
clay or earth."

The modem method of constructing Telford roads differs slightly
from the above specifications. It is now customary for engineers
in constructing roads of this class, to crown the surface of the
roadbed, before the stone pavement is put into place ; and also to
use sand as binding material for the upper layers of broken stone.

The merits and demerits of the Telford system of road-building
is discussed at considerable length, in nearly all works on high-
way construction. Some of the prin-
cipal objections urged against the sys- Fig. i8
tem are as follows : —

1 The pavement foundation forms
a permeable layer allowing the water
from above to enter and saturate the
soil below, so that the rocks subside
and the soil rises ; and thus the con-
tinuity of the surface is finally de-
stroyed.

2 The foundation of larger stones

makes a hard, unyielding surface like Cross-section of a Tile Drain.

an anvil, upon which the smaller

stones above are easily crushed by the impact of the horses' feet.

3 The pavement foundation is expensive, on account of the
large amount of stones used in its structure, and the cost of putting
them in position.

The advocates of the Telford road meet these objections with
many plausible arguments, and, at the same time, bring forward
proof in support of this system over all others. Summing up the
various arguments for and against the Telford system of construct-
ing roads, the most plausible conclusion seems to be as follows : —

Telford roads should be used, where the roadbed is unstable and
diflBcult to drain ; also, when suitable material is not at hand, with
which to construct the entire hardened surface. In the latter



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46 J^OAD CONSTRUCTION

case, inferior stone, such as sandstone etc., can be used in the
foundation. Under such conditions, Telford roads can often be
most economically constructed.

Macadam Roads. — The Macadam method of constructing
hardened road surfaces, which differs from the Telford method
mainly in its foundation, is now most generally adopted by high-
way engineers. This method seems to be well-suited for country
roads, from a standpoint both of economy and durability. The
specifications, as originally laid down by Macadam, in construct-
ing the Bristol and Bath roads, England, may be summarized as
follows : The roadbed, after being properly graded and crowned,
was covered, to the depth of lo or 12 inches, with broken stones,
none of which exceeded two inches in their greatest diameters.
No binding material whatever was used. Splints, thin slices and
dust of all kinds were carefully separated from the stone, before it
was put in place on the prepared roadbed. Traffic alone, in
course of time, caused this layer of angular stones to form a com-
pact, hard surface, quite free from both mud and dust, and well
suited for travel of all kinds.

Practical road-builders of the present day, in constructing
macadamized roads, as in the case of the Telford road, depart
somewhat from the original plans adopted by the inventor. It is
now no longer considered essential, in making roads of this char-
acter, to free the broken stones from dust and thin chips ; but, on
the contrary, these materials are regarded as necessary, to fill the
interstices, and to bind the broken stones together into a hard-
ened surface. That this modern plan of constructing macadam-
ized roads is based upon a sound theory, is shown by a practical
experiment carried on some years ago in New York City by W. H.
Grant, Superintending Engineer of Central Park. Mr. Grant, in
discussing the system of road-building carried on in the park,
says : " At the commencement of the Macadam roads, the ex-
periment was tried of rolling and compacting the stone by a
strict adherence to Macadam's theory — that of carefully exclud-
ing all dirt and foreign material from the stones, and trusting to



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47



the action of the roller and the travel of teams to accomplish the
work of consolidation. The bottom layer of stone was suffi-
ciently compacted in this way to form and retain, under the
action of the rollers, an even and regular surface, but the top
layer — with the use of the heavy roller loaded to its greatest
capacity — it was found impracticable to solidify, and reduce to such
a surface as would prevent the stones from loosening and being dis-
placed by the action of the wagon wheels and horses' feet. No
amount of rolling was sufficient to produce a thorough binding
effect upon the stones, or to cause such a mechanical union and
-adjustment of their sides and angles together as to enable them
mutually to assist each other in resisting displacement The
Tolling was persisted in, with the roller adjusted to diflFerent
weights, up to the maximum load, 12 tons, until it was apparent,
that the opposite effect from that intended was being produced.
The stones became rounded by the excessive attrition, they were
subjected to, their more angular parts wearing away, and the
weaker and smaller ones being crushed. The experiment was not
I)ushed beyond this point. It was conclusively shown, that
broken stones of the ordinary size, and of the very best quality for
wear and durability, with the greatest care and attention to all
the necessary conditions of rolling and compression, would not
consolidate in the effectual manner required for the surface of a
road, while entirely isolated from, and independent of, other sub-
stance. The utmost efforts to compress and solidify them, while
in this condition, after a certain limit had been reached, were un-
availing."

Foreign materials, in many cases, are not only necessary for
binding the broken stones together into a hardened mass; but
Ihey are also essential, to make the road-covering. impervious to
the water, which falls on the surface. Broken stones alone are
the most permeable of all road materials ; and, in place of allow-
ing the water to be speedily carried to the side ditches, they retain
it ; and this thereby readily softens the foundation, and causes
the surface of the roadway to become disintegrated, in a short
Ttime. It frequently happens, that the unscreened stone, as it



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ROAD CONSTRUCTION



comes from the crusher, contains sufficient fine chips and dust to-
bind the angular stones together into a compact, hardened mass^
When such is not the case, sand, or some other foreign material,,
should be added. Some road-builders place the binding material
on the surface alone ; while others mix it with the entire mass of
broken stones forming the roadbed. Each of these methods has
its advocates; but a combination of the two, in most cases,,
would probably be more serviceable ; as it would insure a com-
plete bond throughout the entire road-covering.

In constructing a modem macadamized road, it is customary to-
prepare the roadbed by first giving it the proper grade and crown,
after which the roller, or traffic, is passed over it, until the surface
is thoroughly consolidated and hardened. The object of the last
named precaution is to prevent the lower layer of stones from being
pressed into the soil, and thus becoming useless in supporting the
layers of stone above. If the soil, on which the broken stones are
placed, is of a sandy nature, it is often found almost impossible to
secure, even by continuous rolling, or by traffic, a compact, hard-
ened road surface, on account of the intermingling of the sand and
broken stones. In such cases, a thin layer of straw, or some other
material, should be placed over the surface, before the stone is put
in position. The Chief Engineer of the Massachusetts Highway
Commission overcame this difficulty, by covering the surface of
the sandy roadbed with cheese-cloth. It is said, that the stones,
when placed on this cloth, behaved, under the roller, just as if it
were on a hard clay foundation, and were easily consolidated into an
even compact^ surface. The cost of this material (about $700 per
mile) will probably, in most cases, prohibit its use. Nevertheless,
it well illustrates the fact, that this difficulty may be easily over-
come, by covering the sandy surface with a thin layer of some com-
paratively fragile material. Another object in view in having a
compact surface, on which to place the broken stones, is to pre-
vent the water, that may percolate through the road-covering, from
entering the foundation clay, and rendering it unstable. The sur-
face of a sub-grade, when properly compacted, becomes, in a great
measure, impervious, and speedily conducts the water, which may



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49.



collect from above, to the side ditches, before it has time to enter
and soften the soil.

The surface of the sub-grade, having been prepared as above de-
scribed, is then ready to receive its covering of broken stones.
This material may be prepared, either by hand or by the crusher.
Some engineers prefer the former method ; while others prefer the
latter. It is claimed by the advocates of the hand-broken stones,
that they contain fewer incipient cracks and are less liable to go
to pieces under traffic. It is questionable, whether or not this
statement can be verified by actual tests ; and, if it can be, it is
probably not of sufficient importance, from an economic stand-
point, to make it of any pecuniary interest to the practical road-
builder. When the amount of stone to be broken is limited and
labor is cheap, it is economy in the majority of cases to have the
work done by hand ; but, on the other hand, when the quantity of
stone to be used is large and the cost of labor is high, it would
probably be economy to use the crushed stone.

In constructing first-class macadamized roads, it is always best
to place the broken stone on the prepared sub-grade surface, in two-
or more layers, in order that it may be more completely consoli-
dated by traffic or by the use of the roller. The total thickness of
these layers should depend both on the character of the traffic, for
which the road is constructed, and, to some extent also, upon the
nature of the road foundation. If the traffic is heavy and the in-
dividual loads are large, the thickness of the road-covering should
be greater than when the opposite conditions exist. Furthermore,
even when the traffic is light, the road-covering should be in-
creased in thickness, wherever the foundation is unstable or insuf-
ficiently drained.

The French engineers, on their most important highways, make
the macadam ten inches thick ; but, on the less important roads,
six, seven or eight inches is considered sufficient. In this country ^
the road-covering varies from 4 to 12 inches, an average being
about seven inches. There are said to be excellent macadamized
roads in the vicinity of Bridgeport, Connecticut, having a road -



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ROAD CONSTRUCTION



surface of only four inches in thickness. Such thin road-cover-
ings are not, however, advisable, unless the material is exception-
ally good and the roadbed thoroughly drained.

The lower layer of stones should be spread as evenly as possible
over the prepared roadbed, to the depth of three or four inches ;
and it should be thoroughly compacted by rolling or by traffic.
When the latter method is used in compacting the stone into a
solid bed, great care must be taken, to see, that the ruts are filled
by broken stones as soon as they are formed. It is always desir-
able to have the stones as nearly cubical as possible ; and, in no
<»se, should they exceed two and a half inches in their greatest
-diameter. The more cubical the stones, other things being equal,
the easier they are to become consolidated into a compact mass.
Some road-engineers screen the stone, taking only the larger size
for the foundation ; while others apply the stone just as it comes
from the crusher, with the larger stones, small fragments and dust
all combined. The last named method is usually adopted, when a
very compact and well-bonded foundation is desired ; but, gener-
ally, the stone is screened and the small fragments and dust are
used on the surface as a binder.

The lower layer of stone having been compressed from about
six inches to four inches, it is ready for the reception of the second
layer. This layer of stone, in first-class roads, is usually, when
consolidated, about three inches in thickness, and consists of frag-
ments one and a half inches, and less, in diameter. As the layer
-constitutes the actual wearing surface of the road, it is essential,
that the stone used should be as hard and tough as possible. The
.same care should be taken with this layer as with the first, in see-
ing that it is well compacted and hardened by means either of traf-
fic or the roller.

The road-covering is finally completed, by placing on the sur-
face a layer of binding material, a half inch or more in thickness,
which is sprinkled and continuously rolled, until it becomes thor-
oughly consolidated. One of the best and most satisfactory ma-
terials to use as a binder is the chips and dust obtained by screening
-the broken stones. When such material is not at hand, small



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51



gravel, sand or loam will answer the purpose ; but, in no instance,
should clay be used for top-dressing. The object of the thin
superficial layer is to form an impervious covering for the roadbed,
and, at the same time, to unite the fragments of stone into a
perfect bond. Prof. Shaler, in speaking of this superficial cover-
ing, says : — ^

" A coating of * fines ' or fragments from the crusher up to half
an inch in diameter, is to be spread to the depth of about half an
inch. The roller is then to be passed over this last layer, with the
result that the bits will be ground to powder. At this stage the
road is to be sprinkled with a watering-cart, but one with fine
apertures in the pipes, the work being done in several passages.
The roller is then again to traverse the way until in its move-
ment the water is forced upward or pushes before the drums of
the machine."

Having constructed the roadbed as above described, and given
to its surface a sufficient crown to conduct the water quickly into
the gutters or side ditches, we have an excellent country- road, and
•one, if kept in proper repair, that will last for many years, even
under heavy traffic. Such roads are but little affected by the
seasons ; and they are as serviceable for traffic in winter as in
summer.

The following table shows the cubic yards of broken stone,
required per mile of road : — '



Depth of stone in
Inches



4.

6.

8.
10 .
12 .
14
j6.



Width op Macadam Roadway in Feet



8



645
968
1,290
1,613
1,935
2.258
2,580



16



1,290

1,935
2,580
3,225
3,870

4.515
5,160



24* 30



1,935
2,903
3.870
4.838
5,805
6,773
7,740



2,421
3.632
4,842
6,053
7,263
8,474
9.684



32



2,580
3,872
5,160
6,452
7,740
9.032
10,320



40



3.225
4.840
6,450
8,065
9.675
11,290
13,900



3,870

5,808

7.740

9,678

11,610

13,548

15,480



60



4,842
7,264
9,684
12,106
14,526
16.948
19.368



American Highways, p. 157.

Macadamised Roads, by Isaac B. Potter, p. 57.



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53



Angular, or what is usually called pit gravel, binds readily into
a compaet mass; and, on this account, it is commonly preferred to
water- worn gravel. This kind of gravel always contains a consid-
^erable amount of clay, which should be separated from it by
screening, before it is put on the road. It is, furthermore, desira-
ble, in order to obtain a uniform and smooth surface, to remove all
the stones having a greater: diameter than 2 % inches. When it is
not convenient to screen the gravel, the larger stones may be
readily removed with a rake, as they are distributed over the sur-
face of the road. Before the gravel is placed in position, the road-
bed should be properly prepared by giving it the necessary crown
and by compacting its surface, either by the roller or by traffic.
The thickness of the gravel forming the road-covering should be
greater than' for broken stones. Gen. Gillmore says, they should
have a total thickness of 10 or 12 inches, and should be put on
the prepared road-surface in two or more layers, each layer being
thoroughly rolled before the succeeding layer is placed in position.
Spalding, in his text-book on Roads and Pavements, in speaking
of similar roads, says,^ that with proper drainage three or four
inches of gravel, or even less, will frequently give very satisfac-
tory results. Such roads, however, would not withstand heavy
traffic and would be suitable only for pleasure drives or light
hauling.

One of the strongest arguments in favor of gravel roads is the
cheapness of construction. If the gravel has to be transported
only a short distance, this kind of road-covering is unquestionably
the most economical of all materials, and is especially well suited
for the construction of common country-roads, where the amount
of traffic is not unusually heavy. Even under the last named
condition, if the gravel covering is ten or twelve inches thick and
well bonded by sandy clay and oxide of iron, it will be equal in
wearing qualities to the macadam road, unless the material of the
latter is trap or some other very tough and durable stone.



A Text-Book on Roads and Pavements, p. 76.



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CordMrcn J^.nnc: — Cctrfiircy nnds are ziadc of small logs or
poles, plaisec a5c ty si5c across the roadway. They are usually con-
structed in STrarzpr fisricts. vbcie drainage is difficult and timber
abundant These roads are nscd to a consideiaKe extent in South
Georgia by lumbenncn. They form an excellent substitute for
the more costly roadvays. One of the main objections to be
urged against these roads is the nnevenness of the surface. This-
difficult}', however, can be overcome, in a great measure, by a
careful selection of the poles, and by seeir^, that they are so ad-
justed to each other as to leave the least possible ^lace between
them. When the poles used are several inches in diameter, it is-
often advisable to fill the ^aoe between them with poles of smaller
size split in quarters. H. upon a roadbed thus prepared, a thin
layer of brush be placed, or, what is better still, a layer of crushed
sugar-cane stalks, and the whole* cox^ered to the depth of two or
three inches with earth, the surface will become comparatively
smooth, and will form a pleasant roadway. It is always well, be-
fore putting the logs in positicm, to elex-ate the roadbed a few
inches above the maTJm-am water-level erf the swamp. This pre-
caution is usuaLy seoessary, in order to g^ve the drainage re-
quired for a dry surface : and also, to pre\-ent the logs from be-
coming loosened b>' the passage of hea\*\' loads. The earth taken
from the »de ditches is, in most cases, sufficient to raise the road-
bed to the desired height. The corduroy road, though usually
short-lived, frequently lasts for several years, when well con-
structed and made of good material, such as oak. A road of this
character between Yorktown and Williamsburg, Va., constructed
by Gen. McClellan, during the late war between the States, still
remained some ten years ago, when the writer \-isited that sec-
tion ; and it was then in fair state of preserx-ation.

Plank Roads. — Plank roads, like corduroy roads, are gen-
erally constructed in swampy or bogg>' districts, where lumber
can be had at small cost. This class of roads was quite common
throughout the piney woods region of the South, during the days
of the stage coach ; but thej- are now rarely met with, except as



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ROAD CONSTRUCTION rr^

short cause-ways traversing marshy ground. Plank roads, when
first constructed, have all the essential qualities of a first-class
roadway. The surface is smooth, slightly elastic, and free from
dust or mud. These favorable conditions, however, are only of
short duration, unless the road is well constructed and is kept in
good repair. The effect of the sun upon the planks causes them
to warp, and the result is a very uneven surface, which in turn
induces rapid wear of the planks ; and the final consequence is a
very undesirable roadway. The method commonly adopted in
constructing plank roads is to lay down two parallel lines of sleep-^
ers, lengthwise with the road, on which the planks are to be
placed. The sleepers should be about five feet apart, and made of
durable wood. They should be of suflBicient size to form a stable
foundation for the road-covering above. Upon this foundation, a
floor is laid, of planks from three to four inches in thickness, and
about eight feet long. It is always best to have some of the
planks a few inches longer than others, so as to enable the wheel
to easily regain the plank-way, after being forced from it by the
passage of other vehicles. Yellow pine is probably the best ma-
terial for the construction of plank roads. It is very durable, and
at the same time, not so liable, as many other timbers, to be
warped by the sun.

Earth Roads, — Earth roads are the most unstable and short-
lived of all roadways. Nevertheless, on account of their cheap-
ness, they must of necessity constitute the great percentage of our
highways in the South, for many years to come. On this account,
the construction and maintenance of such roads should receive
more attention than is usually given to the subject in works on
highway-construction.

The common earth road, as long as it remains dry, smooth and
free from dust, is an ideal country-road. Its advantages are well
illustrated by the choice of the teamster, who invariably selects it
during the dry season in preference to the macadamized road,,
when the two lie side by side, and each is kept in good condition.
The earth road, when at its best, is elastic and easy upon the



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horses' feet, and is almost entirely free from the surface irregnlari-
ties, which are common to both broken-stone and gravel road&

One of the greatest difficulties to be overcome in the construe*
tion of earth roads is the question of drainage. This should re-
•ceive special attention, if the best results are to be obtained from
roads of this kind« Every observant person has noticed, that cer-
tain portions of earth roads are nearly always in good ccmdition,
regardless of the weather ; while other portions of the same road
are invariably in bad condition. This variation in the state of the
road-surface, at different points, is due, in the majority of cases^
almost entirely to drainage. An earth road is rarely ever in as bad
condition on a gentle slope, as it is on level ground. In oth^r
words, the quagmires and mud-holes are usually found in hollows
or on level ground, and not on the hills, where the drainage is
more perfect. This class of roads, in many places, especially
where the soil is somewhat sandy, frequently needs no further
drainage than that accomplished by shallow side-ditches, and by a


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Online LibrarySamuel Washington McCallieA preliminary report on the roads and road-building materials of Georgia → online text (page 4 of 22)