Severely Settling Sidewalk 2

[Hnewton]

I am designing a repair for a severely settled sidewalk. In some locations the sidewlak has settled in excess of 4". There is a sanitary sewer that runs undernesth the sidewalk for its full length. We have sent a robot through the line but have not detected any signs of leakage. My approach has been to propose demolishing the existing sidewalk, installing a Geotextile membrane, a QP base then pouring 4" of new sidewalk.

My thinking is that the membrane will help to stabilize the underlying soil while the QP will provide a firm base.

Any thoughts on this approach?

 

[JAE]

Sounds like the backfill for the sewer wasn't properly compacted. If that is the ultimate cause of the settlement, then you should first determine if all the potential settlement is "out" of the soil below - if not, partial excavation and recompaction is in order.

 

[GeoPaveTraffic]

I agree with JAE.

 

[Hnewton]

The sewer was constructed circa 1926. The sidewalk was built much later. Is it likely that the backfill is fully consolidated?

 

[JAE]

Well... after that long - I would say yes, but sometimes buried pipes provide a sort of least resistance track for water to flow along - this could be the root cause.

But still, I'd say that settlement most always means poor subgrade - fix that first before you put another sacrificial lamb-slab on top of it.

 

[GeoPaveTraffic]

With the sewer built that long ago and movement of the soil over the pipe, I would expect that there are separated joints in the pipe. I know that you said there is no evidence of leakage, however, that is still the most likely cause. Have you talked to the person who actually conducted the test? Ask them if they could see the joints in the pipe and if they were tight? It is possible that they were looking at other areas of the pipe and did not see the separations. Another thing that we have seen is that the lift holes for the pipe may not have been properly plugged.

Let us know about the pipe exploration.

 

[Hnewton]

We hired a consultant to perform the test. They videotaped the probe and reported to us that the joints were tight. They did not mention anything about "lift holes" The pipe is VC, would it have holes in the perimeter?

 

[GeoPaveTraffic]

It could depending on the size of the pipe. In larger pipes there is generally one lift hole near the center of the pipe where a cable can be inserted, then a bar is placed through the cable on the inside of the pipe allowing the pipe to be lifted.

If the joints are tight, then I don't know what the problem is. It could be settlement, though that is unlikly on this old of a pipe. You might want to post this question in the Geotechnical engineering other topics form and see if anyone has any better answers. At this point I would say that recompacting 3 or 4 feet of the sewer backfill directly under the pipe would be purdent.

 

[Prepakt1]

Dont forget the sewer pipe itself might be settling. If there is any leaking it will erode below the pipe first.

 

[sc]

At 100mm its not settlement its collapse, well when its under a footpath anyway.

If the failed area is relatively short (<200m, average block length where I am)

Demolish the footpath (its buggered anyway), ecavate the trench, over the sewer and backfill it with a compacted stable material and rebuild the footpath. Problem solved.


regards

sc