RAILROAD LOCATION AND CONSTRUCTION — MACKENZIE. 117 



Double box culverts are now rarely used; the middle wall 

 collects driftwood and may cause trouble. Arches are used in- 

 stead. The cost of box culvert masonry may be estimated as 

 follows : — 



Coursed masonry laid in lime and pointed with cement, $8.00 p.c.y. 



Paving in bottom and at ends 5.00 



Riprap at ends (hand-laid) 2.00 



Excavation in earth 0.30 



Arch Culverts. — Arch culverts should be built, generally, with 

 splayed and stepped wings deflecting 30 degrees from the longi- 

 tudinal line of the culvert. Right-angled wings, with buttresses, 

 for upper end, and straight stepped wings for lower end, have 

 been much used in Canada, but splayed wings are now considered 

 the best practice. There should be no recess or shoulder where 

 the wings join the head wall, to collect drift-wood and cause 

 scour. When the ends are funnel-shaped, as above described, 

 the discharge is increased 100 ^ over the square-end culvert, 

 when discharging under a head. 



The following dimensions may be used for arcli culverts : 



