Drainage Culvert

A culvert is a cutting that allows water to flow under a road or other pathway, either for cross-ditch relief or to allow the road to cross a natural stream or drainage area.

Typically, culverts are built when roads are first constructed and take the form of one or more pipes or other conduits, fully embedded in soil below the road bed. Given the right attachments, motor graders can play a role in constructing, repairing and unblocking culverts.

Both culverts and ditches play an important role in keeping roads safe and in usable condition. Without them, even fairly small or seasonal flows of water will damage road surfaces. Pooled water on a road can also cause vehicle tyres to lose their grip on the road surface, resulting in hydroplaning, or hide potentially dangerous potholes or obstructions.

To be effective, a culvert has to be the appropriate size, given the quantity of water it may have to drain. It also has to be laid in the correct position, and the surrounding slopes and drainage area may have to be graded and reinforced.

Positioning a culvert

Culvert grading tips

For a large culvert or a culvert for a major road, proper site analysis and engineering is required. However, these are some general tips for smaller culvert construction and grading, which is commonly required on forest and other dirt roads:

  • aim to lay a bed of gravel or soil that’s at least one-quarter the diameter of the culvert pipe; the bed and fill material should be free of large rocks
  • ensure that the bed isn’t below the water runoff channel; positioning a culvert too low will make it prone to accumulating soil and getting blocked
  • avoid laying corrugated metal pipe directly onto rock, which doesn’t have the required flexibility and often results in damage to the centre of the pipe, which takes the greatest load (when there’s no other option, use concrete pipe instead)
  • incorporating a camber at the centre of the pipe can help protect it from the pressure exerted by a fill or embankment
  • generally slope a culvert using about a 3% grade, to help prevent soil and other debris from accumulating; however, aim to use a grade that won’t significantly change the speed of the water’s natural flow
  • in the case of a culvert used for drain relief, position the culvert at the same grade as the ditch to be drained
  • skew culverts used for ditch relief to no more than 45 degrees to the centre line of the road, so water isn’t forced too far off its natural course
  • cover the culvert with soil to a depth of at least 30 centimetres for corrugated metal pipe (CMP), at least 60 centimetres for concrete pipe or a minimum of one-third of the pipe diameter for large culverts.

Reopening a blocked culvert

It’s possible to use a motor grader with crab steer to reopen a culvert that has become blocked.

Provided the bottom of the ditch is solid, position the grader’s front wheels in the ditch, on the side of the blockage. Keep the rear frame of the grader on the road surface.

Next turn the leading edge of the moldboard square to the mainframe and angle it to approximately two-thirds of the maximum forward tip.

Shift the moldboard to the side and extend the blade’s cutting edge into the ditch, taking care that you don’t damage the culvert. Then set the angle of the blade according to the slope of the culvert’s shoulder.

Slowly and carefully, move forward to remove the blockage. Then rotate the blade to pull the material up onto the side of the road. It may take multiple passes before you hit the bottom surface of the culvert.

At KH Plant, we specialise in reconditioning Caterpillar motor graders, restoring them to as-new condition. Contact us for more information or a quotation.

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