TABLE 12. Specific Gravity, Weight, Resistance to Crushing, and Absorption Power of Stones.
Cobblestone Pavement. Cobblestones bedded in sand possess the merit of cheapness, and afford an excellent foothold for horses; but the roughness of such pavements requires the expenditure of a large amount of tractive energy to move a load over them. Aside from this, cobblestones are entirely wanting in the essential requisites of a good pavement. The stones being of irregular size, it is almost impos sible to form a bond or to hold them in place. Under the action of the affic and frost, the roadway soon becomes a mass of loose stones. Moreover, cobblestone pavements are difficult to keep clean, and very unpleasant to travel over.
Belgian Block Pavement.. Cobblestones were displaced by pave ments formed of small cubical blocks of stone. This type of pavement was first laid in Brussels, thence imported to Paris, and from there taken to the United States, where it has been widely known as the "Belgian block" pavement. It has been largely used in New York City, Brooklyn, and neighboring towns, the material being trap-rock obtained from the Palisades on the Hudson Kiver.
The stones, being of regular shape, remain in place better than cobblestones; but the cubical form (usually five inches in each dimen sion) is a mistake. The foothold is bad; the stones wear sound; and the number of joints is so great that ruts and hollows are quickly formed. This pavement offers less resistance to traction than cobble stones, but it is almost equally rough and noisy.
Granite Block Pavement. The Belgian block has been gradually displaced by the introduction of rectangular blocks of granite. Blocks of comparatively large dimensions were at first employed. They were from 6 to 4 inches in width on the surface, from 10 to 20 inches in length, with a depth of 9 inches. They were merely placed in rows on the subsoil, perfunctorily rammed, the joints filled with sand, and the street thrown open to traffic. The unequal settlement of the blocks, the insufficiency of the foothold, and the difficulty of cleansing the street, led to the gradual development of the latest type of stone block pavement, which consists of narrow, rectangular .blocks of
granite, properly proportioned, laid on an unyielding and impervious foundation, with the joints between the blocks filled with an imper meable cement.
Experience has proved beyond doubt that this latter type of pavement is the most enduring at d economical for roadways subjected to heavy and constant traffic. Its advantages are many, while its defects are few.
Advantages.
(1) Adaptability to all grades.
(2) Suits all classes of traffic.
(3) Exceedingly durable.
(4) Foothold, fair.
(5) Requires but little repair.
(6) Yields but little (lust or mud.
(7) Facility for cleansing, fair.
Defects.
(1) Under certain conditions of the atmosphere, the surface of the pavement becomes greasy and slippery.
(2) The incessant din and clatter occasioned by the movement of traffic is an intolerable nuisance; it is claimed by many physicians that the noise injuriously affects the nerves and health of persons who are obliged to live or do business in the vicinity of streets so paved.
(3) Horses constantly employed upon it soon suffer from the continual jarring produced in their legs and hoofs, and quickly wear out.
(4) The discomfort of persons riding over the pavement is very great, because of the continual jolting to which they are subjected.
(5) If stones of to unsuitable quality are used—for example, those that polish—the surface quickly becomes slippery and exceed ingly unsafe for travel.
Size and Shape of Blocks. The proper size of blocks for paving purposes has been a subject of much discussion, and a great variety of forms and dimensions are to be found in all cities.
For stability, a certain proportion must exist between the depth, the length, and the breadth. The depth must be such that when the wheel of a loaded vehicle passes over one edge of the upper surface of a block, the block will not tend to tip up. The resultant direction of the pressure of the load and adjoining blocks should always tend to depress the whole block vertically; where this does not happen, the maintenance of a uniform surface is impossible. To fulfil this require ment, it is not necessary to make the block more than six inches deep.