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.

National Roads Board

Since its inception the National Roads Board has endeavoured to develop a national system of rural highways to connect the county roads with those in urban areas. This policy has now reached the stage where the main difficulties lie within the cities' boundaries. Until recently, the Board had undertaken little or no work within the cities. When, however, the constitution of the National Roads' Board was amended to include municipal representatives, the Board assumed the responsibility for providing better routes through the municipalities. These roads have always had to accommodate the heaviest vehicles, because many cities are also seaports. Nevertheless, as public transportation now requires large buses with heavy axle loads, many routes have had to be upgraded. Meanwhile, the internal roading system in New Zealand has been improved to such an extent that hindrances to the free movement of traffic are becoming more pronounced only as such highways reach the city boundaries. The solution of the problems, particularly at intersections, is both complex and costly. Some of the work is being done by the cities and some by the National Roads Board, which acknowledges that “through” routes are part of the system. The Auckland Metropolitan Planning Authority has investigated the problem of traffic flow within its limits; in other cities the problem has been so complex and time consuming that overseas consultants have been engaged. Wellington city, for instance, has recently received a master plan for future development from such a firm.

Traffic Flow

In order that planning may be integrated more closely with transport needs the National Roads Board conducts traffic counts on all main routes at regular intervals, when the number of cars, as well as the number and weights of heavy vehicles, is recorded. The bulk of the traffic is local but on some of the major or arterial routes, through traffic is increasing to such an extent that by-pass roads have become desirable. Traffic counts taken on city streets usually show greater density than do those on nearby rural highways. Normally the pattern shows a marked increase as the road enters an urban area and a corresponding decrease as it leaves.

Recent traffic counts taken on the Auckland-Wellington main route give the following figures: at the south end of the Auckland Motorway density reaches 17,000 vehicles per day, falling to 2,800 south of Mercer and increasing again to 6,000 between Ngaruawahia and Hamilton. Between Hamilton and Cambridge the traffic averages 4,500 vehicles per day; on the Desert Road, 500; at Himatangi, 2,800; and north of Paekakariki, 6,100 vehicles per day. The Hutt Road, which connects the Hutt Valley with Wellington city, carries up to 30,000 vehicles per day. This is the densest traffic, outside city limits, on any road in New Zealand.

The traffic density in the South Island is generally much lighter than in the North. On the main route from Picton to Bluff the average varies considerably. Between Picton and Blenheim it is 1,400 vehicles per day; at Kaikoura, 400 vehicles per day; at Waimakariri Bridge, north of Christchurch, it is 6,500; at the Rakaia Bridge, 2,200; and north of Oamaru it drops to 1,000 vehicles per day At Milton, south of Dunedin, the average rises to 1,800 vehicles per day, while between Invercargill and Bluff it drops again to 1,400.

The situation in the larger cities is as follows. In Auckland, on the Great North Road just north of Point Chevalier Road, and also on Broadway in Newmarket, traffic density varies from 25,000 to 30,000 vehicles per day. In Wellington the traffic density south of Ngauranga is about 28,000 vehicles per day, increasing to 33,000 vehicles south of Ngaio Gorge. Waterloo Quay, north of Bunny Street, carries about 25,000 vehicles per day. In Christchurch the traffic in Blenheim Road is about 10,000 vehicles per day and the lower end of Papanui Road about 15,000 vehicles per day. In Dunedin trafic is about 20,000–25,000 vehicels per day along the Anderson's Bay Road, south of Cumberland Street, while the Main South Road in Caversham carries 10,000–12,000 vehicles per day.

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.