=Foundation.=--The foundation for brick roads is usually of Portland cement concrete, the thickness varying with the nature of the traffic and the kind of soil upon which the pavement is built. For well drained soils and normal highway traffic, 5 inches is the ordinary thickness of foundation. Under favorable conditions such as locations with sandy soils or in semi-arid or arid regions where the soil is always stable, the foundation may be four inches thick, and a considerable mileage of brick road has been built with concrete foundations less than four inches thick. In other locations the soil and traffic conditions require a base six inches or more in thickness, and the proper thickness can be determined only after all of the factors involved are known and have been analyzed. It is impractical to adopt a standard thickness of foundation that will be equally economical for all locations and all kinds of traffic. As the brick pavement is essentially a heavy traffic type of surface, the design cannot be varied greatly with similar foundation conditions because the weight of individual loads is the significant factor and this does not vary so much as the volume of traffic. A variation in volume of traffic may be compensated for by a variation in the quality of the brick as already set forth. The mixtures for the concrete foundation vary widely because of the variation in the aggregates employed. If the fine and coarse aggregate for the concrete are of good quality a mixture of one part cement, two and one-half parts sand and five parts of coarse aggregate would insure concrete of adequate strength. A somewhat leaner mixture is sometimes employed and would be satisfactory if the aggregates were of exceptional concrete making quality. Mixtures of sand and pebbles (unscreened gravel) may also be used if care is exercised to secure a mixture of adequate strength. The proportion will of necessity vary with each particular material and the discussion of the various considerations involved may be obtained from various standard works on concrete and concrete materials. Broken stone macadam is sometimes utilized for the foundation course of the brick pavement and such foundations are constructed as water-bound, which is described in a previous chapter. The thickness, like that of the concrete foundation, varies with the soil conditions and the weight of the loads that are expected to use the road. The macadam is placed in a single layer and is rolled and bonded with screenings as described in the chapter dealing with water-bound macadam. Six inches is a common thickness for the macadam base. This type of foundation should be employed only where the soil is quite stable and where material costs are such as to insure that the macadam base is materially cheaper than one of concrete. This would usually be in locations where the cost of cement is high because of long hauls and where suitable macadam materials may be obtained close at hand.