Drainage

Drainage is also an important consideration when planning simple roads. If sufficient thought is not given to providing adequate drainage, then major damage may result. Large quantities of surface water can particularly be a problem with inclined terrain, so that it will pay to consider various ways to ensure flawless drainage. In this context, the first consideration should be to determine in which hydrological catchment area the road is situated. This depends on the adjacent area (upstream) and in particular its dimensions, inclination, and the ground cover. In mountain regions, the amount of melt water (from melting snow) is also important, as are the location and capacity of surface waterways in flat regions. Depending on the road environment, the following systems can be applied:

• In forests, open ditches are easy to manage and maintain. If the gradient exceeds 3% to 4%, or the water input is relatively large, fortification of the base is advisable.

• In agricultural areas, so that there is the least interference. With cultivation, rubble drains with pipes (usually plastic) should be used.

When it comes to drainage of the road surfaces, protection of the road body and the substructure from both surface and seepage water should be the foremost concern. Erosion of the cover layer is to be avoided. Aquaplaning should not be a concern, considering the low speeds normally involved on such roads.

To promote drainage, several types of carriage crossfalls, shown in Fig. 3.13, are proposed:

• Downstream carriageway crossfall. For ecological reasons (to avoid interference with natural circulation), an effort must be made to avoid concentrating the water drainage in a way that allows increasing high-water peaks in the drainage ditch; additional measures (retention basins) also should be avoided. A downstream road gradient of 1% to 3% is a good solution. It is also advisable to avoid long distances between discharges on the road surface because of the danger of erosion of the verges and of the downstream slope. Regular partitions should be made to allow drainage of the carriageway water. The subgrade surface also has to be slanted to ensure even thickness in the road body. A downstream carriageway crossfall of 3%,

Downstream carriageway crossfall

Downstream carriageway crossfall

Upstream carriageway crossfall

Upstream carriageway crossfall

Camber, top profile

Camber, top profile

Horizontal carriageway

Figure 3.13. Types of carriageway crossfall.

however, can lead to a certain amount of anxiety for drivers. In the event of ice formation, there might be a danger for vehicles (the liability of the owners of the road will need to be clarified).

• Upstream carriageway crossfall. In addition to the variations already mentioned, upstream longitudinal drainage (weep drain) also must be provided, although it will increase the cost. In poor weather conditions (ice on the road, slushy snow), this will provide a certain additional safety, because vehicles tend to skid in the upstream direction.

• Camber, top profile. This method, common in earlier days, is still appropriate for dirt roads. The amount of water that falls on the road is channeled evenly to both sides of the road. Runoff grooves should be provided on the verge downstream, and longitudinal drainage (ditches) will be necessary upstream. The subgrade surface must be made horizontal, so that the greatest road body thickness occurs between lanes.

• Horizontal carriageway. This is a commonly employed method for simple covered roads. The water drainage occurs sporadically at the curves thanks to the camber of the road surface. This technique is not recommended for dirt roads.

The general rule to follow is that water must be evacuated from the road surface as quickly as possible. Cross drains should be provided every 20 m to 50 m, depending on the steepness of the road, the amount of precipitation, and the frequency of bad weather. These will affect the road comfort as well as increase the cost of road maintenance.

Longitudinal drainage ditches, with or without pipes, also are commonly used systems. Other possibilities, however, include asphalt reinforcement, an asphalted ditch, or prefabricated concrete forms. The latter are expensive and therefore not widely used for rural roads.

Intake constructions (shafts for road water) should be placed so as to ensure that the surface runoff water does, in fact, flow into them, and to minimize the danger of obstruction (leaves, twigs, branches, and road debris) as well as to make maintenance easy.

Particular attention also must be paid to stream crossings. If possible, bridges should be avoided as a solution because they represent a significant cost factor. Depending on the topography and other marginal considerations (the danger of flooding), there nevertheless may be a need for bridges, support viaducts, or other major engineering structures, which we do not discuss further at this point. Some possible simple solutions for stream crossings are as follows:

• Fords. This is an appropriate solution for ditch crossings involving large inflows of water on a periodic basis, avalanches, and landslips. A longitudinally cut pipe culvert, accessible from above so that blockages can be remedied simply and quickly, is recommended as a normal outlet.

• Plate outlets made of local concrete.

• Plate outlets made of prestressed and glued wooden slabs.

• Outlets of corrugated steel (corrugated pipe outlet).

It is important that both the in and out ends of the outlet be secured and that the road be protected (from erosion in flumes and on the road surface).

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