HIGHWAY CONSTRUCTION 119 



dential and suburban streets of large cities, and also for the main 

 streets of smaller towns. The bricks should be a good quality of 

 paving brick (ordinary building brick are unsuitable, as they soon 

 wear out and are easily broken) . The bricks should be laid in parallel 

 rows on their edges, with their length at right angles to the axis of the 

 path. 



Curbstones. Curbstones are employed for the outer side of foot- 

 ways, to sustain the coverings and form the gutter. Their upper edges 

 are set flush with the footwalk pavement, so that the water can flow 

 over them into the gutters. 



The disturbing forces which the curb has to resist, are : (1) The 

 pressure of the earth behind it, which is frequently augmented by 

 piles of merchandise, building materials, etc. This pressure tends to 

 overturn it, break it transversely, or move it bodily on its base. (2) 

 The pressure due to the expansion of freezing earth behind and be- 

 neath it. This force is most frequent where the sidewalk is partly 

 sodded and the ground is accordingly moist. Successive freezing and 

 thawing of the earth behind the curb will occasion a succession of 

 thrusts forward, which, if the curb be of faulty design, will cause it to 

 incline several degrees from the vertical. (3) The concussions and 

 abrasions caused by traffic To withstand the destructive effect of 

 wheels, curbs are faced with iron ; and a concrete curb with a rounded 

 edge of steel has been patented and used to some extent. Fires built 

 in the gutters deface and seriously injure the curb. Posts and trees 

 set too near the curb, tend to break, displace, and destroy it. 



The use of drain tiles under the curb is a subject of much differ- 

 ence of opinion among engineers. Where the subsoil contains water 

 naturally, or is likely to receive it from outside the curb-lines, the use 

 of drains is of decided benefit ; but great care must be exercised in 

 jointing the drain-tiles, lest the soil shall be loosened and removed, 

 causing the curb to drop out of alignment. 



The materials employed for curbing are the natural stones, as 

 granite, sandstone (bluestone), etc., artificial stone, fire-clay, and cast 

 iron. 



The dimensions of curbstones vary considerably in different 

 localities and according to the width of the footpaths ; the wider the 

 path, the wider should be the curb. It should, however, never be less 

 than 8 inches deep, nor narrower than 4 inches. Depth is necessary 



