Pipe Settlement during construction - Advise Wanted

[dec4trax]

Contractor is trenching approx 8 ft thru random fill, placing bedding, 3 ft re-concrete pipe, and compacted granular backfill (#8).
Invert of pipe is critical. We have now encountered some very soft soil (STP=woh) along the trench alignment below the random fill at the bottom of the trench. Contractor reports that pipe in this area settled 3 inches overnight based on a lazer sighting. Before continuing the trench line, we want to adjust the installation procedures to minimize (prevent) settlement. Does anyone know any tricks of the trade that pipe contractors use when they encounter this situation? Our contractor has suggested using timber/cribbing or concrete in the trench to spread the load ?? Any suggestions are welcome. Thanks !!

 

[VAD]

You may wish to look at using lightweight concrete - flowable fill , elastizell. Timber cribbing -raft, courduroy can work as well.

Investigate first the depth of soft soil. Perhaps soft soil can be removed if depth is not uneconomical.

If you cannot remove soft soil try constructing a raft using screw piles embedded into hard ground to support timber cribbing. There are also combinations of lightweight backfill etc that can be thought of depending on situation.

The above are some thoughts. Others may have additional or differing solutions.

 

[Focht3]

VAD gave useful suggestions; just be aware that the sections that are not supported on cribbing, raft, etc. may behave differently - screwing up your invert grade.

 

[jdmm]

The pipe in the ground once it is backfilled should weigh less that the soil prior to installation provided that the fill over the pipe does not weigh more than the trench fill excavated. If this is the case then consolidation settlement should not be an issue and the pipe grade should remain constant after construction. It sounds like the problem arises leaving the pipe in the open trench overnight. If this is the case then only install that pipe that can be backfilled in the day.

One common situation that I see is settlement of manholes in weak soil. Comparing the weight of the manhole to the weight of the soil excavated suggests that the pressure under the manhole base is neutral (ie. no increase in pressure from the original condition) so that settlement should not be an issue. However, the contractor installs all of the rings of the manhole before backfilling so that the stresses beneath the manhole exceed the bearing pressure and the manhole settles before there is time to backfill. This could drag your pipe down. If the manhole is backfilled as each ring is installed the manhole won't settle. Compare the bearing capacity of a footing at grade vs. a deep footing.

We frequently have to install storm and sanitary pipe in weak soil such as peat. We normally over-excavate 2 to 3 feet not only to provide support for the pipe but to allow for ease of construction.

I hope some of this might shed some light on your problem.

regards

 

[Grouser]

3trax
I've seen concrete cradles used to support RCP in gelatinous marl. The cradles were about 2 feet wide by 4 feet long and were about 3 feet deep. They had 6 or 8 #10 or 12 bars extending longitudinally 3 to 4 feet beyond each cradle. The cradles were set and plastic concrete was placed between the cradles to construct a 'bridge' for the pipe's support. Sorry, I do not recall how deep the marl was below the invert. Cradle tops were concave to accept the RCP. The cradle concrete may have been lightweight, I don't know. I was the density tech and not involved with the engineering but too dumb to ask questions.
The only stupid question is the one that isn't asked!
Hope this helps.

jdmm
I see your theory but, could it be that the removal of overburden allowed the trench bottom soil to expand above the excavation bottom elevation thus requiring its removal(over-excavation?)? When the backfill was placed the resulting stress increase resulted in elastic settlement of the excavation support soil to below the elevation the over-excavated soil would have attained? Or does the entire pipe/manhole system settle similarly assuming uniform support and backfill soil conditions?