Highway culvert settlement (frost heave)

This is a bit of a strange one!

I have two thoughts which occur to me on this, both on a simiar vane:

1) If the road is a number of winters old then the entire road pavement would be susceptible to frost heaving prior to your arrival. Unless your construction work has altered the moisture content of the subgrade then you would have noticed problems with frost heave damage at the outset. If not then you have a robust wearing surface. The solution in this case may be to backfill over the culvert with original as-dug material to limit differential subgrade movement.

2) The counter argument (and probably the more sensible) is that if frost heave is now evident as a result of moisture content change, then year upon year the frost heave will affect the road. In this case you would either live with increasing repair costs (re-planing and resurfacing regularly) or excavate the frost susceptible materials and reconstruct with suitable fill. Andy Machon

This problem is evident in Maine and I would assume other parts of the US with cold long winters. The only way to avoid the problem is to backfill the trench with native soil. However, if you don't get good compaction, you could very easily get settlement problems.

The other option I can think of to minimize the problem would be to slope the sides of the trench so that there is not a sudden change in subgrade soils. And thus,while the native soils will likely still heave more than the granular materials in the trench, the effect shouldn't be as noticable. You'll have a larger excavation, but if your problem is severe, it may be worthwhile.