Width of Blocks. The maximum width of blocks is controlled by the size of horses' hoofs. To afford good foothold to horses draw ing heavy loads, it is necessary that the width of each block, measured along the street, shall be the least possible consistent with stability.
If the width be great, a horse drawing a heavy load, attempting to find a joint, slips back, and requires an exceptionally wide joint to pull him up. It is therefore desirable that the width of a block shall not exceed 3 inches; or that four blocks, taken at random and placed side by side, shall not measure more than 14 inches.
Length of Blocks. The length, measured across the street, must be sufficient to break joints properly, for two or more joints in line lead to the formation of grooves. For this purpose the length of the block should be not less than 9 inches nor more than 12 inches.
Form of Blocks. The blocks should be well squared, and must not taper in any direction; sides and ends should be free from irregular projections. Blocks that taper from the surface downwards (wedge shaped) should not be permitted in the work; but if any are allowed, they should be set with the widest side down.
Manner of Laying Blocks. The blocks should be laid in parallel courses, with their longest side at right angles to the axis of the street, and the longitudinal joints broken by a lap of at least two inches (see Figs. 54 and 55). The reason for this is to prevent the formation of longitudinal ruts, which would happen if the blocks were laid length wise. Laying blocks obliquely and "herring-bone" fashion has been tried in several cities, with the idea that the wear and formation of ruts would be reduced by having the vehicle cross the blocks diagonally. The method has failed to give satisfactory results; the wear was ir regular and the foothold defective; the difficulty of construction was increased by reason of labor required to form the triangular joints; and the method was wasteful of material.
The gutters should be formed by three or more courses of block, laid with their length parallel to the curb.
At junctions or intersections of streets, the blocks should be laid diagonally from the center, as shown in Fig. 56. The reasons for this are: (1) To prevent the traffic crossing the intersection from following the longitudinal joints and thus forming depressions and ruts; (2) Laid in this manner, the blocks afford a more secure foot hold for horses turning the corners. The ends of the diagonal blocks
where they abut against the straight blocks, must be cut to the re quired bevel.
The blocks forming each course must be of the same depth, and no deviation greater than one-quarter of an inch should be permitted. The blocks should be assorted as they are delivered, and only those corresponding in depth and width should be used in the same course. The better method would be to gauge the blocks at the quarry. This would lessen the cost considerably; it would also avoid the in convenience to the public clue to the stopping of travel because of the rejection of defective material on the ground. This method would undoubtedly be preferable to the contractor, who would be saved the expense of handling unsatisfactory material; and it would also leave the inspectors free to pay more attention to the manner in which the work of paving is performed.
The accurate gauging of the blocks is a matter of much impor tance. If good work is to be executed, the blocks, when laid, must be in parallel and even courses; and if the blocks be not accurately gauged to one uniform size, the result will he a badly paved street, with the courses running unevenly. The cost of assorting blocks into Tots of uniform width, after delivery on the street, is far in excess of any ad ditional price which would have to be paid for accurate gauging at the quarry.
Foundation. The foundation of the blocks must be solid and unyielding. A bed of hydraulic-cement concrete is the most suitable, the thickness of which must be regulated according to the traffic; the thickness, however, should not be less than 4 inches, and need not be more than 9 inches. A thickness of 6 inches will sustain traffic of 600 tons per foot of width.
Cushion Coat. Between the surface of the concrete and the base of the blocks, there must be placed a cushion-coat formed of an incom pressible but mobile material, the particles of which will readily adjust themselves to the irregularities of the bases of the blocks and transfer the pressure of the traffic uniformly to the concrete below. A laver of dry, clean sand 1 to 2 inches thick forms an excellent cushion-coat.