Importance of Efficient Drainage System for Long-lasting Pavements & Principles of Good Drainage

Road builders from ancient time have known that water is the greatest enemy of a stable long-lasting pavement. The ancient Romans, who started building the 50,000 miles Imperial Roman road network in 312 B.C knew of the damaging effects of water and tried to keep their roads above the level of the surrounding terrain. In addition to constructing these roads with thick sections, they often provided a sand layer on the top of the sub grade and below the first course of flat stones. The durability of those highways is provided by the fact that many of them still exist.

Little basic progress was made during the twenty centuries after the road building of the Roman Empire, until Tresaguet, Metcalf, Telford and Mc Adam in the first half of the nineteenth century introduced important improvements in the then prevailing road building methods. They provided a good camber to the road surface and took precautions to drain away the water. One of the most important principles they ‘rediscovered’ was the need of keeping roadbeds dry. In 1820, John L. Mc Adam made his oft-quoted statement to the London Board of Agriculture that,

“The roads can never be rendered thus perfectly secure until the following principles be fully understood, admitted and acted upon: namely that it is the native soil which really supports the weight of traffic; thus whilst pass through the road and fill the native soil, the road whatever may be its thickness loses support and goes to pieces”

Although the significance of good roadbed drainage has been acclaimed on and off through the centuries, it has often been unheeded by road designers and little attention was given to drainage of roadbeds during the later half of the nineteenth century. In 1910, however, the merits of drainage were clearly brought out in the ‘The Art of Road Marking (H. Frost, 1910) in statement such as, “A road on a wet, undrained bottom will always be troublesome and expensive to maintain and it will be economical in the long run to go to considerable expense in making the drainage of the subsoil as perfect as possible”.

From about 1910 to 1940, the pavement engineers widely proclaimed the need or drainage for roadbed and nearly every text book on road building and many published articles contained a statement to the effect that, “There are just three things that are necessary to get a good roadbed and they are drainage, drainage and more drainage”. But with the advent of rational methods for designing pavement and laboratory testing of saturated samples of subgrade and base course materials, the emphasis moved to density and stability rather than drainability. The practice has been to try to design sufficiently ‘stout’ pavement using sufficient thick erosion-resistance bases and subbases and stabilised subgrades, so that the pavement can carry the required loads in the presence of water. Also, there has probably been a tendency to underestimate the amount of water that is available to enter most pavement particularly from natural precipitation.

Leading up to the practice of designing pavement for saturated strength were many detailed studies of moisture conditions under pavement which disclosed that semi-arid and arid climates, water can accumulate under pavements.

One of the first design procedures using saturated strengths of subgrades and bases was the CBR test, which was originated in California by O.J. Porter. Porter (1938) explains the necessity for soaking test specimen as follows:

“Test results indicate that clay and other adverse soils usually have a good supporting value when in thoroughly compacted and relatively dray state. The ideal conditions, however, does not maintain over a period of year. Such material usually absorbs moisture from rainfall ground water or by capillarity, to cause expansion and as a consequence of the increase in water content, the soil often reaches a state of compaction and wetness comparable with the soaked specimen in our bearing value test”

When the design of a pavement is based on the testing of saturated specimen, there is a general expectation, that there is no need to make further allowances for environmental factors such as the amount of rail that falls on the pavement. The modern design techniques generally assure sufficient thickness of pavement and bases to prevent overstressing of subgrades; however as already noted, some of the most serious damage from water will occur regardless of the thickness or stability of a base course used, because it is caused by pore pressures and movement of free water contained in structural sections/layers.

Over the decades, it has been proved that during the time that pavement carry traffic while their sections contain free water, the rate of damage can be up to thousands of times greater than when no free water is present. So, if the total long-term cost of pavement is to be kept to a minimum, design standards must be modified to produce structural sections/layers capable of freeing themselves of free water rapidly after its entry rather than in days, weeks or months. Rapid drainage of pavement structural sections/layers can be accomplished best by providing a layer of highly permeable, macadam type of open graded material under the full width of the carriageway with collector pipes and outlet pipes to ensure constant gravity drainage. Such system not only protect pavements from large surface inflows, but from inflows from high ground water in wet period, spring inflows that do not show up during construction and virtually any anticipated or unexpected inflow into structural sections.

A good drainage system should:

• Remove water from the road surface
• Prevent ingress of water into the pavement
• Pass water across the road, either under or over
• Prevent scour and/or washout of the pavement, shoulders, batter slopes, water courses and drainage structures
• Remove sediments and pollutants from run off

Principles of Good Drainage

Since water is, beyond any doubt, one of the culprits causing failure of roads, the designer should aim at keeping the water away from the roadbed. This can be achieved by following principles of drainage design:

• The surface run off over the pavement surface and shoulders should be drained away as quickly as possible, preventing the water from finding entry into the pavement layers from the top and into the subgrade from the top and sides
• Precipitation over the open land adjoining the highway should be led away from highway through natural drainage channels or artificial drains. Suitable cross drainage channels should be provided to lead the water across the highway embankment which may be cutting across the natural courses
• Consideration should be given to deal with the precipitation on the embankment and cut slopes such that erosion is not caused
• Seepage and sub surface water is detrimental to the stability of cut slopes and bearing power of subgrades. Similarly, it can be of great importance in preventing frost action. An effective system of sub surface drainage is a guarantee against such failures
• The design of a subsurface drainage system should be on a rational basis using seepage principles to calculate probable inflow qualities and to determine required outflow conductivity
• If a pavement is to be drained fast enough to significantly reduce water-induced damage, a highly permeable macadam type of base drainage layer is needed under the full width of heavy traffic lanes, fitted with longitudinal collector pipes and outlet pipes to ensure free gravity drainage out of the system and thus prevent harmful accumulation of water within the structural section
• The special base drainage layer needed for rapid drainage of a roadbed generally can be substituted on an inch-for-inch basis for the non-draining bases and subbases being replaced by the drainage layer. Frequently the only added cost for a well-drained highway or airfield pavement will be the cost of the collector pipes and outlet pipes
• The fast draining layer should be constructed of strong, durable aggregates generally in the range of ? to 1-inch particle size.
• Crushed durable rock or gravel with high frictional strength and high interlocking properties will often be sufficiently stable without the use of additives or binders; however, if the aggregate are largely rounded, or if added cohesion is needed for the structural requirements, construction stability or permanent stability, it may be necessary to stabilise these materials by plant-mixing with the 2 to 4 percent hot paving grade asphalt of other suitable binders.
• Landslide-prone zones deserve special investigation for improving drainage
• Poor embankment soil can perform satisfactorily, if drainage is considered in design
• Water-logged and flood prone zones demand detailed consideration for improving the overall drainage pattern of the area through which the highway is aligned.