HIGHWAY CONSTKUCTION 



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. 



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. 



Gutter formed of 3 rows of 

 blocks, Set ongirciinalty 



Fig. 54. 



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. 



