storm drain settlement problem

[Chili91]

I am designing a fix for a 5,000 s.f. or so patio in a flood plain. The patio is built up 3-4 feet above the ground and is settling relative to the adj building (which is on piles) about 6-8 in.

The patio surface is brick pavers - which I can remove and regrade, but there are two problems: 1) The ground will likely continue to settle, and 2) There is a storm MH in the patio which has settled the same amount as the patio (the rim is even with the settling pavers) and I've got 'sinkholes' in line with the storm pipes from the MH. Looking in the MH, the pipes are becoming disconnected from the MH.

Any suggestions? Is there a flexible product I can re-connect the storm pipes with? Should I plan on re-building the patio every few years?

 

[oldestguy]

Is there any chance that you can minimize problems in the future by doing an overload now, then do your pipe and paver replacements?

Would "causing" more settlement now by this surcharge cause any nearby problem? Some extra effort now may be worth it, especially for possible future sale value of the property.

Some scoop on how deep the soft stuff is, etc. might help the discussion

 

[Chili91]

Hand auger borings in the patio consisted of a sand leveling course for the pavers underlain by geotextile fabric underlain by 2-4 feet of sand/clay/gravel. Underneath is old fill - the site is on reclaimed land. Even though the patio has sunk 6-8 inches, the patio walls, accessible sidewalk, curb are in good shape and show almost no differential settlement. It's as if the entire grounds are subsiding. The building sits on a campus on the bank of a large river. There is a 20' berm between the building and the river with a large stormwater pump house nearby. I probably wouldn't park there during a heavy storm.

 

[cvg]

so, you are right next to a levee...

is the fill mostly sand or clay? How stiff is the fill? Is the fill sitting on clay subgrade? How long did it take for the 6-8 inches of settlement to occur?

is there a flapgate on the end of your storm drain, or does water back up in the storm drain? Does the drain penetrate the levee embankment or just outfall at the pump station?

Repair the storm drain first and in my opinion, that means dig it up and replace it, then proper backfill and compaction. If possible, move it away from your patio.

 

[oldestguy]

It may be that settlement of paver area has stopped with it being caused only by shallow recent fill. Do you have any time-settlement information? Your description would indicate it is a shallow loose situation just at the pavers.

Are nearby walls on the same material or deeper on older fill?

It might even be that you could compact the full depth of loose recent fill stuff. I have seen one case where 4 feet of loose fill was suitably compacted full depth with a jumping jack type hand guided compactor. (Another one where pressure sensors on a wall showed increase in wall pressure by earth when compactors were working above that level, some 8 feet up.) If you try this, you may also cause nearby walk, curbs to settle some. You also should do at least some form of measurement to see if you are doing any good with compaction, such as stakes set in the top few feet on which elevation shots can be taken.

If access is there, a back-hoe mounted tamper pushed down on the fill for at least 20 seconds per location can help a lot. If it tends to bury itself, use less time, but then use several passes.

We don't like to tell contractors that deep compaction sometimes can be done, when specs call for thin layers of fill compacted

 

[Chili91]

The patio is only six years old - no settlement data is available other than observing the subsidence relative to the building - which is on piles. The exterior patio walls consist of planters, raised 2' or so above the paver surface, built of segmental blocks, and heavily landscaped. No information is available on the wall foundations.

The underlying material is 5-10' of cobblestones/gravel/clay/sand on 80' of silt...BTW the labor for this project is virtually free.

So far our plan is to:
1. remove and store the pavers, remove and dispose of the fill. patio walls/planters to remain.
2. repair/replace the failing storm pipes.
3. place geogrid + geotex fabric.
4. place new fill. compact with tamper!
5. replace the pavers. slope up to the existing building.
6. repeat in six years if necessary.

 

[oldestguy]

Why the geogrid? Did you see substantial differential settlement? Is driving on the "subgrade" not possible during the repair? Next time when you re-do the storm drain will that geogrid be in the way? Is there heavy traffic on the pavers requiring improving the "subgrade"? Is cost no object? You can see, my opinion is that if there is no obvious need for something, why do it.