ROAD CONSTRUCTION

Progress, Volume X, Issue 1, 1 September 1914, Page 27

ROAD CONSTRUCTION

By ROBT. WHITSON. Engineer

Why Not Test the Different Methods ?

The extraordinary amount of attention that is being devoted to the question of road construction all over the world at present, brings home to us very forcibly the enormous influence of the motor in the present day methods of transport.

For centuries past in all countries, the cry has been for better roads, but under the slower conditions of travel that existed in the past, the need was not a very insistent one, and it has needed the acceleration of pace brought about by the use of the motor vehicle, not only to lay bare the defects in existing roadways, but also to establish the fact that drastic changes Of method are necessary in the laying down of new roads if they are to stand up to modern requirements.

To the unthinking man it is probably a surprising fact that a rubber shod motor should prove such a destructive agent on an ordinary roadway, but without attempting to give the reasons for the fact, it may be well to point to several of the causes which transpire to bring about the results attained. In the first place the diameter of motor wheels is very much less than that of the old horse drawn vehicles, which means that a small irregularity in the road surface that would have been bridged by the larger diameter wheel, does not escape the motor wheel, with the direct consequence that further abrasion takes place. Again, owing to the increase of pace of the motor over ordinary traffic the shock or jar given to any irregularity in the roadway is enormously increased, and it has long been established that the percussive effort is the most destructive factor in road surface disintegration.

Lastly, the enormous increase of weight per wheel carried by the modern motor vehicle was never dreamed of or conceived to be possible when most of our existing roadways were laid down. It is nothing unusual to see a motor lorry with a three-ton chassis, and a five-ton load on our city streets nowadays, giving an effective weight of two tons per wheel, and when it is seen that each wheel when steel-tired has only perhaps twelve square inches of bearing on the road surface, or in other words, that the crushing weight on the road surface equals some 370 lbs. per square inch, it will be realized that our Dominion roads were not built to stand up to this class of work.

One hears constantly, glib remarks about the old Roman roads, and their marvellous wearing and lasting qualities. Viewed from the point of view of our modern constructive methods of roads, which are expected to give good service, it would be an astonishing thing if the Roman roads had not lasted. Here is a brief description of the construction of the famous Appian Way, and be it remembered that this roadway was laid about the year 312 B.C.

with a view to handling a traffic (as far as we know) which consisted mainly of horse and foot soldiers, and war chariots. After the route was laid out, drains were cut the full length of the roadway at each side, well below the level of the excavation in which the road was to be built. All loose soil was then removed until a good foundation was obtained, and this was left standing until the foundation was thoroughly drained. On this foundation, were laid two or three courses of flat stones, snugly fitted by hand but leaving ample room between for free drainage, next came a course of rubble masonry, or in some cases very coarse concrete. The third layer was a dressing of finer concrete on which were bedded hard stone blocks, handfitted with great nicety. There is some diversity of opinion as to whether the stone blocks were grouted in the joints, but the point is immaterial. The roadway was in many places three feet in thickness when laid on the ordinary soil, but varied in proportion to the hardness of the subsoil as it proceeded. The Eoman roadways and their method of construction have not been referred to casually, for they convey a message of enormous importance to the present day road engineer. This old roadway put down some 2226 years ago, was undoubtedly centuries ahead of its needs, but the very fact that some of the finest traffic avenues of the world for both horse and motor traffic, are to-day built on the foundations of the old Roman roads, both in France and England, goes to prove that when all is said and done, it is the foundation of the road that counts, when length of life and good service are demanded. Macadam maintained in the face of all opposition that if the ground was thoroughly drained, a thickness of 9 inches of broken metal, water bound, well rolled and top-dressed with metal screenings, would stand up to all the ordinary demands of traffic, And for the traffic of his day, it undoubtedly would, and did, but not even the wildest dreamer of Macadam's day would have predicted a wheel load of 370 lbs. per square inch. The "Main Arterial Roads" that are claiming so much attention at the hands of the daily papers, Motor Clubs, and particularly members of Parliament at present are, in this Dominion, for many miles at a stretch innocent of all road metal, and these portions do not of course come within the pale of criticism. But on those portions that are metalled, it would appear to be only common sense for those responsible for the work, to see that such work that is done, is done as well as possible. Macadam based the success of his method on good drainage, and yet in our local road construction, this is about the last portion of the work to receive any attention. In the country districts the authorities in charge

are of course faced with an alternative problem. Shall they merely top dress the roadways of the district, so making a yearly patchwork job of the traffic ways, or shall they each year undertake a section, and make and form it thoroughly? In the former case the available funds are usually more than absorbed in making the roads passable for winter traffic, and in the latter case, if a section is to be taken in hand and thoroughly formed, there will probably be no surplus of funds to maintain even a passable surface on the other portions of the district. With the paving or traffic surface of city streets this article is not greatly concerned, for from the diversity of opinions held by the leading engineers of the various countries who are concerned about the road traffic problem it is very evident that road engineers are themselves still searching for the ideal surface that will stand best for all classes of city traffic.

We have lately had before us in the daily papers, long reports of the impressions of various Dominion engineers who have lately tripped over the Continent and England, gathering data as they went, as to the most favoured class of road surfacing in the various large cities, and as might be expected, even among these authorities, there is a very great diversity of opinion as to the best method to adopt in our Dominion city streets.

Wood blocks, stone setts, asphalt blocks, and the numerous bitumen surfaces all seem to have their advocates, and we can only suppose and hope that the long suffering public who move on wheels, will eventually get a good road surface in the cities by the slow process of the elimination of the unfit.

All those engineers however who have been abroad to study the road problem are unanimous on the subject of the road foundation, and Mr. F. W. Furkert, Inspecting Engineer of the Public Works Department, who attended the International Congress of Road Engineers held in London last year, is reported to have said in the course of an interview with a reporter of the "Dominion" on his return to New Zealand:"The exceptionally fine roads of England and France are due not so much to any magic power contained in the up-to-dateness of their engineers, but to the foundations laid by hundreds, and in some cases even thousands of years of consolidation under traffic, with, in places feet of metal." In that statement we are given the prime factor of road construction, and as the trend of motor development is to still further increase the wheel load of vehicles, the question of road foundations is apt in the future to demand even more care than it is receiving in the present.

In the matter of pavements for city streets there are of course many things to be taken into consideration. There is first the question of the class of material that is available in the neighbourhood, the cost of laying, cost of upkeep, and the amount of skilled labour that each class of roadway requires in the laying. . ■ Taking into consideration the number of paving systems offering,. each doubtless with some points in its favour, and after reading the latest reports from our engineers, which all differ more or less, it would seem that the only way in which the vexed

question can be finally settled, will be by resorting to the old rule of thumb method of trying them out. Why has this method not been tried? Older countries than ours have freely admitted that without experiment they have been unable to arrive at definite conclusions, and our road engineers in order to keep abreast of the times must surely fall into line. The only satisfactory method of arriving at a definite conclusion, as to which system of road paving is best suited to our city streets is by selecting a thoroughfare carrying all classes of traffic, and laying sections of the various pavements along it. This will ensure that the same amount and class of traffic passes over all sections alike, and if first cost, maintenance, life, and suitability for tractive purposes are carefully noted, the question would in a comparatively short time be settled at first hand.