The Application of Base Exchange and Soil Physics to Problems of Highway Construction

Highways are-built on soil, and often soil materials are used in their surface structure. The most important purpose which this surface structure serves consists in spreading the stresses exerted by the traffic over a. volume of soil which is sufficient for their absorption without a damaging deformation. The natural stability which a soil possesses under varying weather conditions influences, therefore, the type and design of the pavement to be used on it. The stability of soil materials is governed by such physical phenomena as friction, cohesion, adhesion and surface tension. For the excavation of cuts, and the construction of fills, soil materials have to be loosened and moved. . In this type of work, just as ,in agricultural plowing, the cohesional and frictlonal properties of the soil play an important role. In cuts, various soil horizons may come in contact with the pavement and furnish unequal support to the pavement because of differing physical properties, thus hazarding the quality and the permanency of its service. The physical and physico-chemical forces cooperating in the erosion of soils attack the slopes on highway fills and ditches, making necessary the purchase of additional right-of-way. Therefore, the study of erosion is also one of great interest to the highway engineer. This very incomplete enumeration shows that the highway and the agricultural worker have many soil-physical problems in common. An evaluation of all of these problems from the point of view of a highway engineer would by far transgress the time allotted to this paper as well as the ability of the writer. Therefore, this. paper is limited to the description and analysis of a few typical cases of applied soil physics which, though limited in scope, throw some light on the activities of the soil physicist in highway engineering in general and in the rapidly developing field of low-cost construction in particular. One of the most important single substances in natural soil systems is water. It conditions the cohesional, adhesional and frictional properties of the soil, affects the reaction of bituminous coatings on soil particles, and is present as the everready enemy of bituminous and similar soil treatments. Its movement in the soil may lead to moisture and ice accumulation at certain places of the roadbed, resulting in an often dangerous decrease of stability. For these reasons, it may be .interesting to discuss here the effect of water on the binder properties of soil fines and on the factors influencing the accumulation of water in certain zones, especially in frost heaving. The corresponding phenomena in bituminous soil stabilization, as waterproofing and cementing by asphaltic materials, and the factors influencing the flow of bituminous oils in soil systems shall also be treated.