. Text-book on roads & pavements . may give satis-factory results, but when the culvert is likely to flowfull at certain times it should be laid up in hydrauliccement mortar, and in any case the greater stabilitygiven by the mortar would be well worth the smalladditional cost. Fig. 8 shows a section of the ordi-nary form of box culvert. The use of head walls andpaving the waterway for a short distance is necessaryfor these, as for pipe culverts. Where suitable stone is available, box culverts areeasily constructed and economical. They are com- 42 A TEXT-BOOK. ON ROADS AND PAVEMENTS. raonly use
. Text-book on roads & pavements . may give satis-factory results, but when the culvert is likely to flowfull at certain times it should be laid up in hydrauliccement mortar, and in any case the greater stabilitygiven by the mortar would be well worth the smalladditional cost. Fig. 8 shows a section of the ordi-nary form of box culvert. The use of head walls andpaving the waterway for a short distance is necessaryfor these, as for pipe culverts. Where suitable stone is available, box culverts areeasily constructed and economical. They are com- 42 A TEXT-BOOK. ON ROADS AND PAVEMENTS. raonly used for openings 2 to 4 feet in width and2 to 5 feet in height. The width that may be useddepends upon the available cover stones. Where theallowable width is not sufficient to give the neededarea of waterway, a double culvert may sometimes beused to advantage. This consists of two openingswith a middle wall to support the covers. The culverts opening should always be large enoughto admit of a man passing through it for the purpose. W////////M W//M V////M m UUAl ? Fig. 8. of cleaning it — at least 18 by 24 inches. The side wallsshould extend downward below the bottom of theculvert sufficiently to obtain a good foundation, andthe thickness required for the side walls usually variesfrom one-half to three-fourths the height, dependingupon the pressure likely to come against them. In many cases for small work the side walls, insteadof extending downward, rest upon the paving which isextended imder them. This gives a somewhat lessexpensive construction, and is often satisfactory ongood ground. The cover stones may be from \ to \ the span inthickness, and should be long ienough to have a bear- DRAINAGE OF ROADS AND STREETS. 43 ing upon each side wall of at least one-half the thick-ness of the wall. Pipe Culverts. Pipe culverts may be constructedeither of salt-glazed vitrified sewer pipe, or of ironwater pipe. For culverts of sizes up to about 30 inchesdiameter, vitrified pipe is ofte
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