Roadway Settlement

[GODsRebel]

Hey yall, I've got a project where the roadway has settled where the water mains run perpendicular under the road. I did some research and found that the road has some organics deep in the subgrade, however, a couple years ago the subgrade was injection cemented. The utility pipes are laid with granular coarse aggregate material and the raod is still failing at these locations. We are set to repair these roads, any suggestions? I am yet to see what the existing conditions are, and will witnees when they cut out the road at these locations.

 

[LCruiser]

What is the native material at and above the elevations of the pipes? Where is the water table?

 

[BigInch]

This is typical of inadequately compacted backfill into an open cut excavation when installing a utility and repairing the pavement above. Excavate as much of the backfill over the utility as you possibly can and replace that with well compacted and controlled backfill. Compacting to maximum Procter density is paramount. Also reroute all possible surface water runoff away from that area. It is critical that surface water must not be allowed to enter the soil in and around any utility trench under a road. If you don't do it exactly like I told you, this problem will never go away. Even if you do exactly as I told you, it still might not.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[LCruiser]

BigInch is right in that when you have inadequately compacted backfill that will happen. However, there are other reasons for this problem as well.

 

[BigInch]

True, but 95% of the time its backfill not compacted to 95%.
The other problem is the subgrade under the road actually has less compaction than the trench fill, leaving a bump in the road after awhile. Difficult to get it balanced right.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[LCruiser]

There can also be migration of fines into bedding material if it has high permeability and there is low cohesion material adjacent to or overlying the pipe bedding, or if it's subject to tidal action or vibration (e.g. heavy truck traffic). You are correct that the single highest probability cause is poor compaction - (watch especially if it gets backfilled while the superintendent is taking the inspector to lunch or something). Time to accomplish the backfill is the key.