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Purpose of Foundation

load, sand and base

PURPOSE OF FOUNDATION.

The chief object of the foundation or base of a pavement is to distribute the concentrated loads which come upon the surface of the road over a greater area of the usually softer and weaker road-bed, in order that these loads may not produce indentations in the surface.

In a foundation composed of independent blocks extending through its thickness, as in the case of a stone-block pavement in which the blocks rest directly upon the road-bed or upon a thin layer of sand, the load which comes upon the of any block will be dis tributed over the area covered by the base of the block.

Where the foundation is composed of small independ ent particles, like sand or loose rounded gravel, with no cohesion through the mass, the pressure is distrib uted over the base of a cone whose vertex is in the point of application of the load, and the inclination of whose elements depends upon the friction of the par ticles of the material upon each other. In this case

the area over which the load is distributed varies directly as the square of the thickness of the founda tion. Sand, it is to be observed, has also the property, when confined as in a foundation, on account of its incompressible nature, of adjusting itself to a uniform pressure and resisting the deformation of the road-bed. If the small pieces composing the foundation are cemented together, or held as in masses of angular fragments by the interlocking of the angles, the foun dation may act more or less as a whole, causing a distri bution of the load over a considerable area, the extent of which will depend upon the resistance of the mass to bending.

The bases most commonly employed for pavements are sand, broken stone, and concrete. Foundations of brick and wood are also sometimes employed for pave ments of the same materials.