The following section will explain this.
In many cases the foundation is simply formed by shaping the soil to the required contour, and covering this with 3 or 4 inches of gravel or cinders, which is afterwards either rammed or consolidated by the traffic; upon this the setts are placed as closely as possible, the joints are then filled with fine gravel well worked in with a “cramming iron,” the whole surface being then covered with a grouting of lime and sand, which is brushed into the joints with a stumpy broom.[62]
In Leeds, Manchester, Salford, and many other important cities, I believe the foundations are formed in the manner just described, but of greater depth, the grouting also is a bituminous mixture, which I will presently describe, instead of the ordinary lime grouting.
The paved streets of Manchester are proverbial for their excellence, which is attributable to the manner in which the foundations of the streets are consolidated by the traffic before any setts are placed on them; in many cases the old macadamised surface of a street being utilised as a foundation, this process being almost identical with that recommended by Sir Henry Parnell fifty years ago.[63]
Where the traffic is heavy, however, a firmer foundation even than this is necessary, and up to the present time no better foundation has been introduced than that of good Portland cement concrete. This should be at least 9 inches in thickness, and be composed of one part of Portland cement, two parts of clean sharp river sand, and four parts of clean river ballast, or broken stones, or other suitable material. The surface of the concrete, after having been placed in position, should be smoothed over with the shovel, so as to present the proper convexity and have an even surface for the granite setts to be bedded upon.
Another description of foundation now very extensively used where the traffic is heavy, is that known as “Bituminous Concrete,” which is made as follows:
The ground being excavated to the proper depth and contour, broken stone as for macadam is spread for a depth of 6 or 9 inches; this is then levelled and thoroughly rolled with a light roller, a boiling mixture of pitch and tar, or creosote oil is then poured over the whole surface until every interstice is filled, when a thin layer of small broken stone is spread upon it, and then well rolled until it consolidates.
It may be well to observe here that in all works involving concrete foundations and paving in streets, the traffic should be entirely stopped if at all possible. Streets paved half at a time are never quite satisfactory, and the concrete should have at least a week to set before the pavement is placed upon it.
Upon a foundation of either Portland cement or bituminous concrete, the granite setts themselves should be grouted with a bituminous mixture instead of cement or lime grouting. This renders the pavement more impervious to moisture, makes it less noisy, and adds considerably to its strength; the mode of applying it is nearly similar to that of ordinary grouting. The setts are placed on about an inch of sand and well rammed, the boiling mixture is then poured over the whole surface, which is then covered with a thin coating of small, sharp gravel.