AACS Proceedings of the 8th Annual Convention
There seems to be at present a great awakening allover the United States on the question of road construction and maintenance. With this awakening, there has been launched upon the reading public by newspapers and tracts almost as much ignorance or knowledge misapplied as real knowledge. This has led to the employment of men who have neither the technical skill nor the practical appreciation for the subject in hand. Now whether it is always best to economize on the first cost of all engineering, namely the salary of the engineer, is a question open to debate. Some practical men, no doubt, can dispense with a civil engineer. Yet to the economist as well as to the practical man, the civil engineer can be of a great help and service. Therefore my first note of warning is employ a civil engineer to do your engineering work on roads. It will pay at the finish.
In all arts as well as sciences there apparently seems a conflict between the theoretical and practical sides of all questions: On the question of roads the art has so well blended in the science that there is very little, practically speaking, difference between them. To thoroughly understand all sides of the question we must of necessity be acquainted with many things, essential and non-essential. We hope tonight to give, however, rather a running summary of what might be termed the best essentials of road making in their application to all roads. In order to make ourselves a trifle clear on that subject, we might say a word on the non-essential. Much stress has been laid on the requirement that the descending grade shall not exceed angle of repose of the wheel but had we only two iron rails with freight cars the question would be comparatively speaking, an easy one for solution. But look what we have on any street or road, variation in the width of every tire, different sizes of the wheels and axles, state of lubrication and amount of load, which, together with wet, dry or frosty condition of the surface of the road prevent even an average solution of the problem. Experiments on traction therefore for this reason must give way before practical and actual experience.
In our opening sentence, we discussed the first essential, namely, the employment of competent engineers. The other essentials naturally group themselves under three heads: 1st, Laying out of the Road; 2nd, its Construction; 3rd, its Maintenance.
Laying out of the Road
Most of laying out of the road properly belong to the field. Before starting out on a survey, it would be best to thoroughly understand just what amount of money is appropriated and the character of the road to be built. A stone covered road can stand a heavier grade than a dirt one. A paved gutter can carry off water quicker than a dirt one.
A few hints from railroads may not come here amiss. Branch Railroads carry greater grades with fewer facilities than the Main lines. Most of our highways were laid out along property lines and we cannot always regulate the traffic as on a railroad. Yet nevertheless for any series of road, they should be laid out in a system, and the proper grades should be fixed accordingly.
If directness is wanted, get a straight line. Often a curved road will answer the purpose of a straight one and make up for its increase in length by its appropriateness and the lessening of its grades. In the matter of curves, never go below a fifty-foot radius, equalize the radii of compound curves as much as possible, put a tangent between reverse curves; these are about the only things of practical importance with regard to line questions.
In all cases, fix a maximum grade. As we have just said, a scientific solution of this question is beyond us. Hence we must seek experience and observation. Men naturally walk slowly up hill and quickly down hill while horses do the reverse. Consequently it would seem that the grade that would admit of the highest speed down hill would be the proper one to select. A grade of 1 in 30 for mixed traffic is about as near a solution as can be reached to this problem. A grade of one in twenty may do but one in thirty is nearer the average.
Also it is not always the traffic that will determine your grades. Remember your drainage all depends on your grade. A grade of one in twenty is apt to cause considerable damage in a heavy rain storm.
Avoid level stretches as there should be sufficient fall to carry off all surface water quickly otherwise, soft muddy spots will abound. As we have just cited a maximum grade we likewise have a minimum one. For paved gutter, 0.2 percent per 100 and 1 in 120 for unpaved, are the grades, and these grades should be limits in such cases.
Compensate on all curves. These might all seem trifles at first yet they never cost any more when put in effect, and in the end, decrease the cost of maintenance.
Drainage can next claim our attention. Too much importance cannot be attached to this subject. The worst enemies to out door structures are frost and water. If we draw away the latter, we can render the former ineffective. With the absence of water, we have less mud and also less dust as the stones having no chance to loosen, are not apt to break up into dirt and fragments.
Drainage naturally falls under two heads, surface and sub-drainage. Sub-drainage as we shall see later on is not absolutely necessary except in very wet soils, for a stone covered road; but on clay and earth road sub-drainage is absolutely indispensable. The crown, although it maybe steep, yet cannot possibly get rid of all the water into the side ditches. The remainder must gradually evaporate under the action of sun and wind. Hence the liability of frost. The French or blind drains are the best for this purpose and should always be located below the frost line. Most of the authorities agree that 3" unglazed tiles with loose collars with a covering to sub-grade of gravel, small stones, road metal or broken bricks are the best means for such drains. Trees as you all know grow as much under ground as overhead with the difference that instead of seeking the light as the branches naturally do, the roots drift towards moisture. The trees must be gotten rid of as they will continually block the drains. Keep your tiles to a uniform grade with the proper outlets open and then leave the rest to Heaven for you can do no more.
Surface drainage must be provided in every case. This is partly affected by crowning the road, so as to throw the water into the side ditches or drains. Beyond merely mentioning this, so as to bring it under its proper head we will leave this for a time only to refer to later on. Bridges and culverts which naturally fall under this heading would demand too much space and time for our short paper.
The road next claims our attention. Contractors don't like to remove old stumps for nothing, and sometimes, if things can be covered up, so much the better. This is the reason of convenience and not sense as it eventually appears at the end. The base of all masonry rests below the frost line. In road construction this cannot always be done. The metal portion, if possible, should never rest on the ordinary surface as, the top soil is generally too porous and never compacted. Hence excavate about one foot below the natural surface and remove all stumps, brush, vegetable matter and rocks. Fill the resulting holes with suitable material, tamp and bring the surface to a proper grade by rolling. This surface should exactly correspond to the section of the metal portion. If on excavating treacherous or unsuitable material is found, it should be removed immediately and proper means substituted.
All authorities seem to agree on two points; the convexity of the road and the width of the metal portion. This convexity should always be greater or equal in grade to the longitudinal grade of the road, so as to avoid longitudinal flow of water. This cannot always be, for the greater the convexity, the more the tendency will be to drive in the centre which will soon wear the surface into ruts. Hence we must compromise. One in thirty for a metal road and one in twenty for dirt, for owing to its porosity, will fulfill most requirements.
On the width the multiple of 8 is best as this width gives ample play of all wagons and carriages.
What we have given before applies to all roads. Under the subject of road structure we will only take up, in a rather hasty sketch, what is really the best to satisfy all traffic. The Macadam Telford Road seems to satisfy these conditions best.
The Macadam Telford road is made up of two distinct portions, a uniform foundation of large stones which likewise act as a blind drain, and a superstructure of smaller stones acting as a durable water tight roof. Now, this distinction ought to be clearly made and thoroughly understood. A foundation and a superstructure, the former is never expected to wear out which the latter is expected to eventually.
The foundation, acting also as blind drain, should be porous but not pervious to water. It should never exceed 8" in depth for beyond this it is not possible to get a good bond. The foundation stones should be any tough durable stone about 8" in depth and 6 to 8" in width. They should be closely laid by hand on their broadest edge lengthwise across the road. All projecting points should be broken off, and the interstices filled with stone chips wedged by a hammer until the foundation presents an even surface of 8" in depth.
Upon this surface, the superstructure rests. The lower portion of this wearing coat should be of hard durable trap rock fragments whose longest dimensions should be able to, pass a 2½" ring. Machine broken stone is preferable as the edges of the fragments are apt to be sharper, and the screenings are the best for the top dressings. No gravel or round fragments should be tolerated, for even if it is possible to secure a bond with such, there is always the probability that the stones will eventually roll and work loose. The first layer should be 2½" thick and thoroughly rolled if possible. Another layer 1½" in depth of fragments 1" in diameter rests upon the first layer. After, being well watered a cover of fine trap rock screenings is added and rolled into the road as a binder until the surface becomes a hard water proof covering. It is absolutely necessary to keep the road well sprinkled under the action of the roller. The steam roller per inch run approaches the nearest, to the heaviest load per inch run of the vehicles and is preferred for this reason above horse roller. It should weigh not less than 5 tons. With regard to the rolling of binder the proper time to stop is when the fragments are apt to be crushed under the weight without being pressed into the road surface while for the other courses, the stones should not creep before the roller.
Having now got possession of your road give it a name, put up a few signboards so as to tell the people where it is, and on this board, likewise place a few numbers signifying the miles from some starting point. This is a great convenience and help. The maintenance of the roads seems to be a stumbling block to most systems. To some, the word maintenance, means in most cases, to clean out the side ditches, throw the refuse in the middle of the road and trust the rest to heaven and the driver. It matters not whether much or little is spent for road repairs, unless the repairing is done under a system, the road and your expense account is going to suffer. The system best adapted is called the continual repair system. Men do not allow a rent to remain in a coat for any length of time especially if the coat is wanted for service. They mend immediately, according to the nursery rhyme "a stitch in time saves nine." So with roads, fill up the ruts, remove the stones and sweep off the dust continually as fast as they are made and your road will always appear well and in good condition. Large amount of repairing should be done in the spring and autumn when there is plenty of rain. Also at that time there should be a good overhauling of the whole drainage system. Inlets and gutters cleaned out, weeds raked out and everything put in shape for the winter snows and rains. Briefly in conclusion or let me summarize a few facts that seem to be of value that we have tried to go over this evening:
1st, Employ an engineer to layout; 2nd, Never exceed a grade of 1 in 30 or go below 1 in 120; 3rd, Keep all drainage below frost line; 4th, Convexity grade of 1 in 20 to 1 in 30 and for the width of road metal, a multiple of 8; 5th, Stone fragments from 2½" to 1"; 6th, Continual Repairs.
From the publication:
AACS - Proceedings of the 8th Annual Convention
September 11, 12 and 13, 1894