Development of Roading
At the present time many new routes are being constructed which will give access to large areas of land for closer settlement. The West Taupo access road will open up fertile farming areas at present being broken in by the Departments of Lands and Survey and Maori Affairs. The Waipapa road scheme, which will connect est Taupo, via the west bank of the Waikato River, with Te Awamutu, will become a new main route between Auckland and Wellington. In the State forest area between Taupo and Rotorua hundreds of miles of roads have been built to serve the logging trucks which feed the paper mills at Murupara and Whakatane. In the South Island a particularly difficult problem is the completion of the Haast Pass road which will link Westland with Otago and so provide New Zeland with a wonderful tourist attraction. Along the Haast route, as in so many other districts, the bridging of numerous rivers and streams is not the least of the problems facing road engineers.
Road-building Materials
The best use of road-building material in New Zealand depends to a great extet on an understanding of the country's geology and petrology. Certain areas are volcanic in origin and provide good roading materials such as basalt or, as in the case of Taranaki, andesite, a softer rock. The scoria of the Auckland area is an asset. In the centre of the North Island, around Tongariro National Park, the brown ash is very poor and fails completely when wet. The rhyolite ignimbrite and pumice of the Waikato Valley are familiar to most travellers, but these have limitations. In general, the North Island is deficient in good roading materials, and the plastic clays and volcanic ash in the Auckland Province make remedal measures most expensive. Some areas, such as Hawke's Bay, Canterbury, and Southland, are fortunate to have excellent greywacke gravels, the reslt of the denudation of the main ranges. Although these gravels are most useful for road bilding tey require special consolidation to stand up to the stresses of modern traffic. Frost and ice are special winter problems in some places. In varous parts of the country roads have had to be built on poor swamp foundations, as, for instance, near Bluff, to the north of Wellington near Paraparaumu, and in the Waikato and Bay of Plenty districts.
In both islands there are high mountain ranges which binder road communications. There are no low alpine passes suitable for road crossings on the Southern Alps between the Lewis Pass and the Haast Pass, a distance of 180 miles. In the North Island the only low crossings are the Manawatu Gorge and the Waioeka Gorge. Throughout the country ribs from these ranges or foothills set the highway engineer an exacting problem.
Design of Highways
The earliest tracks, apart from those used by the Maoris, were probably made through the bush to enable kauri spars to be hauled down to the beaches for trading and whaling ships. As settlement grew, tracks were developed further afield to provide a rudimentary form of communication by foot or horse. With the discovery of the goldfields and the Maori Wars of the sixties, roadmaking assumed a new importance. The fact that many of these routes are still in use is a tribute to the skill of the early surveyors and engineers who carried out their work under most difficult conditions. Today the problem of the surveyor is less hazardous, owing to the use of a new aid called photogrammetry. By this method plans can be developed from aerial photographs which reveal contours at close intervals, thus enabling road location to be undertaken with sufficient accuracy for contracts to be let.
Before 1930 road engineers had largely to rely for earthmoving on such primitive methods as the pick and shovel, wheelbarrow, and horse-drawn cart, though motor lorries were used to carry away a great deal of the spoil. The coming of the crawler tractor, which was about 100 horsepower, rapidly changed the technique of roadmaking. Larger cuts and fills became both possible and economical and unit costs remained almost stationary, A further advance came with the introduction of the modern tractor (over 400 horsepower), so powerful that hard rock can be ripped up for handling without the use of explosives. Large rubber-tyred scrapers have also been developed as complementary units for moving large quantities of soil and rock at high speeds.
When traffic was horse drawn, speeds were really not of great importance; thus the curvature of the early roads is generally quite unacceptable by modern standards. Improvement of alignment is a matter of cost and the tendency today is to group substandard curves and to change speed value in steps. For many years the National Roads Board has used portion of a spiral or lemniscale for this purpose. In order to improve the riding qualities of highway curves, transitions have been introduced which change the curvature gradually and introduce superelevation. But good road designing is not in itself the answer to all the traffic problems of today. The human factor must never be overlooked. Thus special traffic signs warn motorists of the correct speed for taking curves, and “no passing” lines along certain sections give warning that overtaking is dangerous.
Easy grades were of great importance to the horsedrawn vehicle but were not always possible of attainment. But grades are not nearly so important to the modern high-powered motor cars, though on very steep inclines some commercial vehicles may be forced to drop to a low speed, thereby delaying other traffic. To avoid such congestion and to carry such slow-moving traffic, road engineers in Australia and the United States have evolved the system of an extra up-hill lane. This is now being used with considerable success in New Zealand on the Bombay Hills north of Pokeno and on the new Taihape deviation.
Design of Pavement
In determining the priority for highway construction, the National Roads Board uses a sufficiency rating which takes into account the various road features, such as width, alignment, grade, pavement strength, surface condition, and shoulder widths. These are compared with desirable roads standards and then grouped to show the sufficiency of the section in relation to safety, structural adequacy, and traffic serviceability. It is said that “in Roman times each legion had an engineer and the roads were well laid in layers with ditches on either side for drainage”. The construction of a modern highway remains much the same; it is still compacted in layers and drained on either side. But the engineer today has a more scientific approach. It is his responsibility to test the soil profile of the various routes and to decide how much use can be made of local materials. In some areas sealing chips, for instance, have to be brought long distances; costs can therefore be reduced if local materials are available. Well-equipped laboratories are available throughout the country to carry out such sampling and testing. The engineer also uses such aids as the California bearing ratio, a device in which performance of the pavements has been related to physical soil constants, or the Benkelman beam, which measures the curvature of the deflection of the pavement. From these tests the engineer is able to calculate the various thicknesses of the selected material and treatment to carry the loading in all weathers. Some materials, like clay, are particularly sensitive to moisture, and their ability to carry loads is greatly reduced when they become wet.
Pavement Surface
Rigid pavements of cement concrete or bituminous concrete have been generally so expensive that these materials have not been used outside cities; rather than put a strong surface pavement on a relatively poor subgrade, New Zealand engineers have tended to build a very strong subgrade and cover the surface with a light treatment just sufficient to meet the needs of traffic. They have developed the bituminous seal with the large chip. This method produces tyre noise, but it does give at a low cost a strong nonskid membrane protecting the subgrade.
Where the traffic is heavy, or in districts where the supply of chips is not readily available, other types of pavement have been developed. For instance, on the Desert Road, which runs north of Waiouru across the North Island Central Plateau, local material was used to make a bituminous mix which has given very satisfactory smooth riding for many years. On the motorways around Auckland, where the traffic is extremely heavy and tyre pressures are high, an even stronger bituminous pavement has been laid down. On the more congested urban streets and highways the tendency is to put down the high-type concrete or bituminous surface. In cities the most permanent surface used is a bituminous mix on a concrete base. With modern equipment available it is possible to construct either type to a high degree of accuracy and with smooth-riding and long-lasting surfaces. To cope with the weight and thrust of heavy transport buses it has been necessary to provide concrete loading areas at bus stops.
Maintenance
Of equal importance is road maintenance. Even in the worst of weather, maintenance gangs keep open the traffic routes of the country. Modern highways are designed to stand up to the stresses of heavy traffic, but all roads require regular attention. The magnitude of this work may be appreciated by the fact that maintenance cost in recent years is approximately one-third of the total highway expenditure.
Bridging
The story of road engineering in a country such as New Zealand would be incomplete without reference to the bridge engineers. Recent advances in this field have been quite as outstanding as in those of roadway design. Like many of the early roads, some of the first bridges built in the colony are still open to traffic or have just been replaced, as, for example, the Kawarau Bridge (1962) on the Cromwell-Queenstown route. The old bridge (1880) was a suspension one with a 300-ft span. The Wanganui Bridge, which originally had lift spans, was erected in 1869 and still serves the city. Bridges of standardised design in steel and pre-stressed concrete speed up construction by pre-fabrication methods, and decrease the work to be done on the site. Again, new concepts and construction techniques, such as the use of steel piling in various forms, prestressed concrete piles, or bored piles, have enabled structures to be built where timber or ordinary concrete piles would have been inadequate. Construction by cantilevering out from piers has been recently used on a number of major bridges in New Zealand. This is a procedure which can be used on difficult sites.
As in road engineering, bridge design must be related to traffic requirements. In the early days the approaches to a bridge were of minor importance and every effort was made to secure the shortest and cheapest site. Moreover, bridges were designed for smaller loads than those required by modern traffic. Today a bridge is considered part of a highway and must be constructed in conformity with the curves and alignment of the road. But bridge construction is also designed for other purposes, equally important. The extension of overhead motorways into the heart of Wellington and of Auckland will require many major structures over land and rail.
The Future
Highway engineering is one of those branches of engineering which has been associated step by step with the progress of New Zealand. Today much has been achieved, but there remains just as much to be done. The engineer will be required to provide surfaces to carry heavier loads on more and more routes; at the same time manufacturers will be producing more and faster vehicles, which will require more road space.
by Harry Lancelot Hume, B.E., B.SC.(N.Z.), M.E.(YALE), M.I.C.E. Chief Civil Engineer, Ministry of Works, Wellington.
ROAD ENGINEERING
It is often said that engineering in some form or other has been associated with every step of progress which has been made in New Zealand. This statement is certainly true of the present, and one of the branches which is playing an increasingly important role in national development is highway engineering. Usually we think of highway engineering as one of the branches of civil engineering, but in some projects all branches of engineering are involved – civil, mechanical, electrical, and even electronic. For the purpose of this article, highway engineering is assumed to cover work associated with all public roads.
Road or highway engineering has two important aspects. First, it deals with people's reactions and, secondly, it deals with engineering problems. Closely involved in these two aspects are five basic transportation factors: safety, comfort, time, convenience, and economy.
Traffic Factors
In New Zealand the principal authorities responsible for roading works are the Government, the National Roads Board, and municipal and county councils. In the early years of the colony's development the county authorities had to provide some form of communication between farm and market; thus the original county roads were designed primarily for moving stock on the hoof and for light vehicles. Today, however, the farmer no longer moves his stock on the road; he must therefore be able to get a normal truck to his farm so that he can deliver his stock to the freezing works at the main centres. This has brought about a change in the character of country roads, with the result that bridges have had to be strengthened or rebuilt to carry heavier loads and the roads themselves made capable of carrying heavy vehicles, even though the frequency of traffic may be quite low.