Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1

4.1.2. 4.1.2. Foundations

The wheelways of roads and streets are prepared for traffic by placing upon the natural soil a covering of some suitable material which will furnish a comparatively smooth surface on which the resistance to traction will be reduced to the least possible amount, and over which all classes of vehicles may pass with safety and expedition at all seasons of the year.

The covering usually consists of two parts: a foundation, and a wearing surface. The functions of the foundation are as follows:

1) to protect the soil from disturbance and the injurious effects of surface water;

2) to transmit to and distribute over a sufficiently large area of the soil the weight of the loads imposed upon the wearing coat;

3) to support unyieldingly the wearing surface and the loads coming upon it.

The efficiency of the wearing surface depends entirely upon the quality of the foundation. If the foundation be weak, the wearing surface will be disrupted speedily, no matter how well constructed.

The foundation, when once constructed, should not require to be disturbed nor reconstructed. The materials employed in its construction may be the cheapest available, such as local rock. The important point in the design being to provide sufficient thickness, so that when consolidated it will maintain its form under the heaviest traffic liable to come upon it. The preparation of the foundation involves two distinct operations: 1. preparation of the natural soil; and 2. placing an artificial foundation upon the drepared natural soil. The essentials necessary to the preparation of the natural soil are:

1) the entire removal of perishable vegetable and yielding matter;

2) the drainage of the soil where necessary;

3) the improving of the bearing power of the soil where required; and

4) compacting the soil.

All soils are improved by rolling, and weak spots, which otherwise would pass unnoticed, are discovered. The essential requisite in the construction of the artificial foundation is that it be a dense mass, and the type of foundation to be employed varies with the character of the wearing surface. The foundation may be composed of broken stone, gravel, or furnace slag so graded that the voids will be reduced to the smallest possible amount. The voids may be filled with stone dust; a mixture of sand and clay; a mortar and grout composed of hydraulic cement and sand; bituminous cement; or hydraulic-cement concrete, mixed and placed upon the soil bed (Pict. 4.6.).