Pavement Subgrade Problem

[SteveGregory]

I need to remove some old fill and replace it under a road adjacent to an abutment for a one lane bridge. I want to start at the abutment and excavate in 4 foot-long sections so that if an emergency vehicle needs to use the road, it could be filled "rapidly" and some 4'x8' steel plates could be placed on top to let the vehicle pass.

Could I use rip-rap (specs?) for emergency fill and then use a mixture of rip-rap, crushed stone (specs?) and flowable fill (2500 psi) for my permanent fill? I plan to excavate 3' deep (minimum) to as much as 5'. Once the flowable fill cured for (?) days, another 4' long section would be opened up. I will need to do this for about 12'-16' on each side of the bridge.

I am planning to use concrete pavement on top. The pavement is asphalt now.

 

[oldestguy]

If you are looking for hints or approval of your plan, maybe say why you are replacing "old fill'. Apparently this will cost more than simple full removal and then replace, so perhaps some "fixing" of what is there might also be considered. What sort of abutments are there? Piling?

 

[SteveGregory]

The abutments are 6' tall concrete walls including a footing. The fill has some low blow counts and the pavement is settling adjacent to the abutments.

 

[oldestguy]

OK, that gives us a little idea. If the backfill is loose non-organic earth that is one thing. If it is peat, that is another. Settled fill is common, but replacement under the duress of emergency use, is another. I'd bet there are mud-jacker contractors in your area that would gladly pump in a cementeous material to raise the grade. Grouting contractors regularly compact low blow count material without removing anything. Others have been known to break up the pavement, apply compaction, add fill and then pave. You might be surprised how effective dynamic compaction is on thick layers. The risk you have then is how the work is affecting the abutment. Of course that also can be taken into effect by where and how you do these jobs. I'd first check out compaction grouting.

I only bring these up since apparently the aspect of full time passage of emergency vehicles seems to control what to do. These treatments would likely allow that. I suspect your sequences of trenches may turn out not to work if the trench sides cave in due to weak soil. Would you trench brace every excavation step? Maybe blow the budget.

Question: When you do your rigid pavement job with concrete, how does the fire truck traverse the wet or undercured concrete?

 

[Ron]

Stay away from rip-rap for structural fill. Too many voids and other material will flow in and cause problems above it. I like OG's grouting suggestion.