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CHAPTER TWO
Types of Damage Caused by Water in Pavements
2.1 HISTORICAL BACKGROUND
Road builders from ancient times have known that water is the greatest enemy of a stable, long-lasting pavement. The ancient Romans, who started building the 50,000-mile Imperial Roman road network in 312 B.C., knew of the damaging effects of water andtried 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 top of the subgrade and below the first course of flat stones that were generally cemented together. The durability of those highways is proved by the fact that many of them still exist.
Little basic progress was made during the 20centuries after the road building of the Roman Empire, until Tresaguet, Metcalf, Telford, and McAdam in the first half of the nineteenth century introduced important improvements in the then-prevailing road building methods (O’Flaherty, 1967). One of the most important principles they “rediscovered” was the need of keeping roadbeds dry. In 1820 John L. McAdam made his oft-quoted statement to theLondon Board of Agriculture (McAdam, 1820) 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; that whilst it is preserved in a dry state it will carry any weight without sinking . .. that if water pass through a road and fill the nativesoil, the road whatever may be its thickness loses support and goes to pieces.
He goes on to say,
The erroneous opinion so long acted upon, and so tenaciously adhered to, that (by) placing a large quantity of stone under the roads, a remedy will be found

for the sinking into wet clay, or other soft soils, or in other words, that a road may be made sufficiently strong, artificially, to carryheavy carriages though the sub-soil be in a wet state and by any such means to avert the inconvenience of the natural soil receiving water from rain, or other causes, has produced most of the defects of the roads of Great Britain.
Later in this chapter somewhat detailed descriptions are given of the kinds of damage that are caused by the dynamic pounding of heavy wheel loads on pavements in whichfree water fills the pore spaces and is trapped between the various layers of pavements, bases, and subbases. It will be brought out that modern design methods are not fully taking into account these dynamic actions. Ironically, they could not take place at all if McAdam’s advice were being followed, and there were no free water to be acted on. These damaging actions are not being stopped by theuse of artificial methods for strengthening pavements. Thus, McAdam’s complaint (made in 1820) is just as valid today, more than 150 years later.
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 latter half of the nineteenthcentury. In 1910, however, the merits of drainage were clearly brought out in The Art of Road- making (H. Frost, 1910) in statements 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 about 1940, pavementengineers widely proclaimed the need for drainage of roadbeds (and railroad beds), and nearly every textbook on road building or railroad construction 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 the rational methods for...