Filling cavity under road with concrete?

[psychedomination]

Hi there,

I am working on a project, where the bottom of a retaining wall eroded away and washed out some of the backfill that was supporting the road. Surprisingly the road hasn’t collapsed yet, but in this specific location the asphalt is not resting on anything. See images below :

I have instructed that the road be closed until this is repaired.

I was thinking the simplest approach would be to pour concrete in the hole to fill it (it's about an 8’ high hole). I just wanted to get your opinion on this and the approach?

Step 1 . Sawcut or knockout a hole in the road top surface with a machine. I plan to note in the drawing that at no time throughout the entire works should machinery or personnel be on the road surface that is not supported. (Is there a safe standard set back for the machinery from the unsupported road surface?)

Step 2. Place sandbags at the foot of the wall where the opening is, to temporarily close the gap and contain the wet concrete.

Step 3. Pour the concrete in 4’ high lifts. Wait 3 days after each lift for the concrete to full harden. (I plan to go this route because I am not sure of the existing retaining wall capacity, and I don’t want to overload it with the wet weight pressure from the concrete).

Step 4. Resurface the road

Step 5. Place small boulders or riprap at the base of the wall to help prevent future undermining. (Is there a typical detail for something like this or can the contractor just place small boulders at the toe?)

I do have one concern that the road surface collapses when they try to cut a hole at the top. It wouldn’t be the end of the world if this does happen as they can just continue to fill it up with concrete but from a health and safety point of view do you have any tips or advice?

 

[miningman]

Generally accepted concrete placement techniques do not allow for the airmailing of wet concrete much more than about 1 metre. Also how do you intend vibrating the wet concrete??

 

[JStephen]

Flowable fill might be another option.

 

[JoelTXCive]

We have used flowable fill for scour + erosion problems at bridge abutments before......but in a freshwater environment; and in a region that only gets wet during large storms.

Judging from the photos, you seem to have a much more aggressive environment and possibly even wave action.

Maybe you could do flowable fill to fill the void, and then a reinforced concrete 'face' of some sort. You could chip out a key around the perimeter of your hole and then dowel + key the concrete face in?

Here is TxDOT's spec on flowable fill.

It's really just weak grout. It's just a ~200psi cement mix with no large aggregate. I think it would do well for filling the void, but the low compressive strength makes me worry about durability issues. Hence, then need for a real concrete cap of some sort as protection.

https://ftp.txdot.gov/pub/txdot-info/cmd/cserve/specs/2014/standard/s401.pdf

 

[dik]

yes... CLSM stuff...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[miningman]

What is the lateral extent of the cavity , specifically in the X and Y directions ??? You kinda need this info if you plan to use flowable fill

 

[EireChch]

Does it it really need to be vibrated ? any low strength site concrete would be ok, Unvibrated concrete is a hell of a lot stronger that the road build up that was there.

I would really need a rough 3D sketch of the cavity to see what the best way to fill it is.

I was thinking filling it from the front might be easiest but it looks like the top of the cavity is higher than its face opening?

The most conservative and robust solution would be to deconstruct and re-construct the road. You would need to overlap your filling with the existing road filling to ensure a good "fix" together and reduce the potential for differential.

Placing rip rap is a good idea, however you need to size it based on the expected wave size. No point in placing a load of 250mm boulders when it needs 500mm dia. Look at the CIRIA Rock Manual

 

[psychedomination]

Hello everyone, thanks for the responses so far!

@miningman Thanks, what is meant by the term airmailing? I was intending to use a lean mix concrete and not vibrate it. Is vibrating required?

@JStephen & JoelTXCive Thanks, I was planning to use a lean mix concrete to allow it to flow within the void, which I think is similar/the same as the flowable fill that you mention. As for the front protection, I was thinking more of a riprap solution to dissipate the wave action. Also thanks for providing that information on flowable fill, it is very helpful.

@mining man Please see the cavity dimensions below

@EireChch I was thinking the same thing with regard to the vibrating. Yes, the top of cavity is a fair bit higher than the face opening. See dimensions below.

The client sent me this sketch; disregard their unit error in the cross-section. It should be 6.5' wide and 7.5' tall.

 

[EireChch]

Ok so filling from the road surface is the only option.

I would not fill directly to beneath the road surface as you will create a massive hard spot. Overtime the rest of the road may settle and this will creating a raised/proud portion.

I would board up the lower face opening and backfill with lean concrete to 4ft and then do a typical road build up above. IE layers of compacted aggregate and your asphalt or what ever your road finish is ontop.

If someone was able to map that cavity with such detail means that they were likely inside it which is not good. I would set all equipment back at least 10ft from edge (7.5ft really based on a 45 deg angle form base but lets call it 10).

You could place a long platform/deck or planks to span across to opening to allow someone to supervise the filling and make sure you hit the 4ft.

After that the road build up could be constructed with a small excavator and plate compactor. The excavator to step the sides of the 3.5ft remaining face. Could be done by hand either as its so small.

Stepping the sides will allow you to stitch your road build up into the existing road build up which will help them to perform as one.

On a side not, it seems as if you should do some geophysics to check for other cavities along the road in that specific area. MASW or GPR would be ideal for this.

EDIT - Sorry just realized you are in a rock cut from the sketch and photos. My previous recommendations are somewhat not applicable now.
Filling to nearly full height of the cavity is likely ok since theres not much difference in stiffness between concrete and rock.

 

[LittleInch]

I really think you need to work on fixing the hole at the base of this wall first. Looks to me like a coastal environment?

Get the area cleared away and back to something solid and then maybe drill in a few dowels for some starter bars. Then add some bars or mesh to give the C40 something to grip on.

If you've got breaking waves then some large rocks work well or implant some rocks into a sloping wall of concrete. Just go and look at any sea defence.

Get the base secure then you do a few trial holes in the road to find the edge of the hole and saw cut it and let it collapse into the hole. too dangerous to do anything else. Maybe 6-8ft back from the edge you could probably place your heavy equipment. Then compact down in layers of 300-500mm?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[miningman]

Littleinches last post make a lot of sense especially if you chose incorporate a pumping flange such that you are able to pump , at pressure behind the formwork. As concrete rises , it will displace the air volume and provide 100% filling of the void. Where you park the pump is a detail., best decided on site.