The construction of modern roads

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The construction of modern roads

Chugunova A.A.

Vladimirskiy State University named after the Stoletov brothers

Vladimir, Russia

Since the beginning of the 20th century, as the automobile and truck have offered ever higher levels of mobility, vehicle ownership per head of population has increased. Road needs have been strongly influenced by this popularity and also by the mass movement of people to cities and thence to suburban fringes -- a trend that has led to increasing travel needs and road congestion and to low-density cities, which are difficult to service by public transport. Often the building of new roads to alleviate such problems has encouraged further urban sprawl and yet more road travel. Long-term solutions require the provision of alternatives to car and truck transport, controls over land use, and the proper pricing of road travel. To this end, road managers must be concerned not merely with lines on maps but also with the number, type, speed, and loading of individual vehicles, the safety, comfort, and convenience of the traveling public, and the health and welfare of bystanders and adjoining property owners.

Ideally, the development of a major road system is an orderly, continuous process. The process follows several steps: assessing road needs and transport options; planning a system to meet those needs; designing an economically, socially, and environmentally acceptable set of roads; obtaining the required approval and financing; building, operating, and maintaining the system; and providing for future extensions and reconstruction.

Planning transport road construction

Road needs are closely associated with the relative location of centres of population, commerce, industry, and transportation. Traffic between two centres is approximately proportional to their populations and inversely proportional to the distance between them. Estimating traffic on a route thus requires a prediction of future population growth and economic activity, an estimation of their effects on land use and travel needs, and knowledge of any potential transport alternatives. The key variables defining road needs are the traffic volumes, tonnages, and speeds to be expected throughout the road's life.

Once the traffic demand has been estimated, it is necessary to predict the extent of the road works needed to handle that traffic. A starting point in these calculations is offered by surveys of the origins, destinations, and route choices of present traffic; computer models are then used to estimate future traffic volumes on each proposed route. Estimates of route choice are based on the understanding that most drivers select their estimate of the quickest, shortest, or cheapest route. Consideration in planning is also given to the effect of new traffic on existing streets, roads, and parking provisions.

Where feasible, the next step in planning a road system is to refine the selected route to a narrow corridor. The various alignment options are drawn, considering the local terrain and conditions. The economic, social, and environmental benefits and costs of these options are discussed with relevant official and community groups until an acceptable specific route is determined.

Road design: alignment and profile

After a route has been selected, a three-dimensional road alignment and its associated cross-sectional profiles are produced. In order to reduce the amount of earth to be moved, the alignment is adjusted where practical so that the earth to be excavated is in balance with the embankments to be built. Computers allow many options to be explored and realistic views of the future road to be examined.

In order to fully understand the design stage, a few standard terms must be defined. A traffic lane the longitudinal part of the carriageway width sufficient for the movement of vehicles in a row, the lane can be indicated by road markings. The shoulder is an element of the road adjacent to the carriageway on the same level with her, different type of coverage or selected using markup.

In order to maintain quality and uniformity, design standards are established for each functional road type. The number of traffic lanes is directly determined by the combination of traffic volume and speed, since practical limits on vehicle spacing means that there is a maximum number of vehicles per hour that pass through a traffic lane. The width of lanes and shoulders, which must strike a balance between construction cost and driver comfort, allows the carriageway width to be determined. Standards also specify roadside barriers or give the clear transverse distances needed on either side of the carriageway in order to provide safety in the event that vehicles accidentally leave the carriageway. Thus it is possible to define the total right-of-way width needed for the entire road, although intersections will add further special demands.

Design standards also help to determine the actual alignment of the road by specifying, for each design speed, the minimum radius of horizontal curves, the maximum vertical gradient, the clearance under bridges, and the distance a driver must be able to see the pavement ahead in order to stop or turn aside.

Drainage

Adequate drainage is the single most important element in pavement performance, and drainage systems can be extensive and expensive. Drainage involves handling existing watercourses, removing water from the pavement surface, and controlling underground water in the pavement structure. In designing the system, the engineer first selects the “design storm”--that is, the most severe flood that can be expected in a nominated period of time (as much as 100 years for a major road or as little as 5 years for a minor street carrying local traffic). The drainage system must be able to carry the storm water produced by this design storm without flooding the roadway or adjacent property.

Safety requires that water be rapidly removed from the pavement surface. In urban areas, the water runs into shallow gutters and thence into the inlets of underground drains. In rural areas, surface water flows beyond the shoulders to longitudinal drainage ditches, which have flat side slopes to enable vehicles leaving the pavement to recover without serious incident. Cut-off surface drains are used to prevent water from flowing without restriction down the slopes of cuttings and embankments.

Vertical drainage layers, formed from single-sized aggregate or special sheets called geofabrics and geomembranes, are used to prevent groundwater from seeping laterally into the pavement structure. In addition, a horizontal drainage layer is often inserted between base course and natural ground in order to remove water from the pavement structure and stop upward capillary movement of any natural groundwater. Underground drains can also be used to lower the groundwater level by both preventing water entry and removing water that does enter the pavement structure.

Financing

The full design of a proposed road is analyzed with respect to its costs and its economic, social, and environmental effects. It may also be subjected to public review. This step can be lengthy, as new roads are usually popular with the traveling public but sometimes cause distress in the communities through which they pass.

Local streets and collector roads are usually administered by local governments and financed by local taxes. Arterial roads and highways, however, need a wider administrative and financial input in order to guarantee route continuity and uniformity. Since the 1920s the financing of roads has been largely transferred to the road user. A variety of taxes is employed: on fuel and oil, on road usage, on vehicle purchase and ownership, on driver licensing, on truck mass and mass times distance traveled, on tire and accessory purchases, and on the economic benefits provided by roads (e.g., higher property values or increased productivity). Fuel taxes usually provide the simplest source of revenue, but they are not necessarily intended solely for expenditure on roads. Many local roads are funded by property taxes.

Construction

After the road has been approved and financing found, surveyors define its three-dimensional location on the ground. Forming of the in-situ material to its required shape and installation of the underground drainage system can then begin. Imported pavement material is placed on the natural formation and may have water added; rollers are then used to compact the material to the required density. If possible, some traffic is permitted to operate over the completed earthwork in order to detect weak spots.

In countries where labour is inexpensive and less skilled, traditional manual methods of road construction are still commonplace. However, the developed world relies heavily on purpose-built construction plant. This can be divided into equipment for six major construction purposes: clearing, earthmoving, shaping, and compacting the natural formation; installing underground drainage; producing and handling the road-making aggregate; manufacturing asphalt and concrete; placing and compacting the pavement layers; and constructing bridges and culverts.

For clearing vegetation and undesirable materials from the roadway, the bulldozer is often employed. The construction of rock cuts is commonly done with shovels, draglines, and mobile drills. Shaping the formation and moving earth from cuttings to embankments is accomplished with bulldozers, graders, hauling scrapers, elevating graders, loaders, and large dump trucks. The material is placed in layers, brought to the proper moisture content, and compacted to the required density. Compaction is accomplished with tamping, seeps-foot, grid, steel-wheeled, vibrating, and pneumatic-tired rollers. Backhoes, back actors, and trenchers are used for drainage work.

In order to avoid high haulage costs, the materials used for base course construction are preferably located near the construction site; it is economically impossible to use expensive materials for long lengths of road construction. The excavation process is the same as for rock cuts, although rippers may be used for obtaining lower-grade material. Crushers, screens, and washers produce stone of the right size, shape, and cleanliness.

The placement of paving material increasingly involves a paving machine for distributing the aggregate, asphalt, or concrete uniformly and to the required thickness, shape, and width (typically, one or two traffic lanes). The paving machine can slip form the edges of the course, thus avoiding the need for fixed side-forms. As it progresses down the road, it applies some preliminary compaction and also screeds and finishes the pavement surface. In modern machines, level control is by laser sighting.

In producing a spray-and-chip seal surface (or a bituminous surface treatment), a porous existing surface is covered with a film of hot, fluid bitumen that is sprayed in sufficient quantity to fill voids, cracks, and crevices without leaving excess bitumen on the surface. The surface is then sprayed with more viscous hot bitumen, which is immediately covered with a layer of uniform-size stone chips spread from a dump truck. The roadway is then rolled to seat the stone in the sticky bitumen, and excess stone is later cleared by a rotary broom.

Bibliography

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