Forest Road Engineering Guidebook - Ministry of Forests

Add crushed rock
on approaches
as needed
Road fill only
partially removed
Figure 31. Example of an armoured swale.
Insloping/outsloping road surface
Forest Road Engineering Guidebook
Road surface
Culvert pipe (bed in clean rock)
for modified armoured swale
Coarse rock to
prevent erosion at
outlet (as necessary)
The purpose of insloping or outsloping the road surface is to control water
without using ditches or cross-ditches. Insloping directs the water into the
road cut, while outsloping directs the water across the road to the fill slope
(or road shoulder) (Figure 32). This technique works to disperse water where
the road grade is no steeper than about 6%. If used on steeper road grades,
recognize that most of the water will tend to run down the road grade rather
than flow into the ditch for insloped roads, or off the shoulder for outsloped
roads.
This technique may be effective on roads where there is very little vehicle
traffic. But where roads receive regular traffic, the insloping or outsloping
will disappear with grading and development of wheel ruts. Outsloping also
can be a driving hazard and is not recommended for roads travelled by heavy
vehicles.
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