The drainage system employed in a road construction will depend on factors such as:
• The importance of the road;
• The amount of traffic;
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Fig. 13.2 A horizontal alignment designed to lead water away from sensitive areas |
• The zone (rural or populated);
• The sensitivity of the groundwater; and
• The sensitivity of streams, rivers and lakes.
Drainage systems can be classified as follows:
• Surface systems — these involve ditches and open channels in the surface of the ground;
• Subsurface systems — these are not directly accessible from the surface. Water is collected from water in the ground’s pore space and conveyed in trenches and pipes. Subsurface systems can be divided into shallow (interceptor) drains that collect percolating water above the water table and deep (water table lowering) drains.
In a very permeable soil, separate drainage may not be necessary, only provision of some capacity for immediate runoff and for snow and snowmelt.
Open channel drainage systems are favoured because of their low cost (compared with capacity), easy rehabilitation and maintenance. However, deep open ditches and steep inner road slopes can be dangerous for wandering vehicles, especially on a road with high traffic speeds. Steep slopes may also increase cracking along the centre of narrow roads, and, possibly, increase erosion and the need for channel cleaning.
Subsurface systems have advantages in special circumstances and can be efficient tools in road rehabilitation projects, especially when there is limited space available, limited support of slopes or a need to use deep drainage.
The correct depth for the drainage depends on the pavement thickness, the road layout (cutting or embankment), the type of subsoil and the climatic conditions (intensity of runoff, frost depth and snowmelt conditions). The drainage depth is usually the depth of all the layers of the road structure that contribute to the bearing capacity of the pavement.
Fig. 13.3 Drainage economics
The design of drainage is a part of the design of the whole road, so all aspects of the road can have an effect on it. Drainage design is, simply described, the action of finding an optimum balance between the total costs (investment and maintenance costs) compared to the possible advantages and disadvantages (pavement life, traffic safety, easy maintenance and wider environmental aspects) — Fig. 13.3.
Considering all these aspects, there are a number of steps which, in general terms can be considered fundamental. They are listed in the Portuguese Road Drainage manual as follows (IEP, 2001):
1st Step — Gather all the relevant information:
• Road importance (traffic flow values, status and whether a “lifeline” road);
• Geometric characteristics of the road (layout and profiles);
• Drainage areas and existing drainage systems;
• Geology;
• Meteorological data (precipitation, temperature, frost, etc.);
• Hydrological and hydrogeological conditions in the area surrounding the road (ground-water conditions); and
• Identification of specific constraints (technical, social, economic or environmental);
2nd Step — Identify critical/sensitive areas:
• Vulnerable areas with particular conditions, geological, environmental or ecological;
• Areas with a high frost formation probability;
• Specific areas of the road, such as high and low level points;
• Extreme gradient and cross fall situations; and
• Cutting/embankment transition areas.
3rd Step — Adopt standard/typical layouts, where possible, for road segments with similar characteristics.
4th Step — Define the basic data for the water flows in each layout, using existing methods, tables and software to perform necessary calculations.
5th Step — Analyze the possible and adoptable solutions, based on standard drawings and typical dimensions used in each region or country.
6th Step — Perform the hydraulic calculations so as to obtain drainage sizes. If the estimated amount of water is small, exact hydraulic calculations may not be needed.
7th Step — Consider the location of the discharge points, as well as the need to design retention and/or treatment basins, which may be associated with individual drainage systems.
Roads are normally constructed with two types of drainage systems, the surface and the subsurface drainage systems, each taking care of their separate sources of water and moisture.
