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Railway track formations generally consist essentially of a layer of coarse aggregate, or ballast, in which the sleepers are embedded (see Fig. 12.1). The ballast may rest directly on the subgrade or, depending on the bearing capacity, on a layer of blanketing sand. The layer of ballast is intended to provide a free draining base which is stable enough to maintain the track aUgnment with the minimum of maintenance. The function of the blanketing sand is primarily to provide a filter to prevent contamination of the ballast by fine particles derived from ascending waters (see Fig. 12.2).

The fundamental engineering is reasonably well understood. It concerns the transmission of the load from the base of the sleeper on to an area of closely packed ballast beneath. The sleeper bears on to the points of the angular ballast which are, in consequence, liable to fracture or abrade under load, a problem more severe with concrete than with wooden sleepers. If the layer of ballast is not close-packed and of sufficient depth there is a tendency for the sleepers to move up and down as the wheels of the traffic pass in succession, eventually causing the sub-grade to be disturbed.

Consequently the ballast layer must be thick enough to hold the track in position and prevent traffic loads from distorting the subgrade, while the aggregate under the sleepers must be tough enough to resist the abrasion and degradation caused by the intermittent traffic loads. The main requirement, confirmed

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