Backfilling corrugated pipe {caisson}

[caissondriller]

In loose sand and gravel {5-8-5 blows per six} we are advancing 42” steel pipe to 15 or 16 feet. After steel pipe is set permanent 36” corrugated pipe {CMP} is set. After 36” CMP is backfilled with existing drilled spoils, the 42” casing is extracted. A steel plate is set on the 36” CMP and vibrated with a plate tamper for approx 5 min. After vibrating any annular space is filled and CMP vibrated again. I have used this method on other projects with similar conditions, but I need to convince the field engineer this is a viable alternative to flow able fill. Any comments or suggestions appreciated.

 

[Zambo]

I guess the field engineer will say compaction depends on the grading of the drilled spoil and may be cautious as testing is not going to be possible. I am sure he would have no objection to you using a properly graded sand as backfill. What about rigging up a manual grading screen and removing the larger gravel sizes? But first agree an acceptable grading with the engineer. You will also need water to achieve your compaction.

Zambo

 

[Focht3]

What is this unique "caisson" to be used for? Will a floor slab be cast around the completed pipe? Why all the effort to save 1.5 yd³ of flowable fill? What does the flowable fill cost - $60/ yd³?

Hypothetically, your technique should be able to achieve the desired result under certain conditions. But I suspect that the amount of "brain damage" to the field engineer - over a fairly small (trivial?) cost - doesn't seem worth it to him. After all, if he approves it then he could be expected to help pay for any repairs should it be unsuccessful. I don't blame him - in most situations I'd probably do the same thing.

 

[caissondriller]

Thanks Zambo, that may be solution to the problem. Focht3 the reason flow able fill is not a practical method of backfilling is cure time and placement. The engineer on this project requires 24 hours of time previous to pouring concrete. There are about 800 total holes on this project. The ability to set pipe and receive concrete upon completion will triple production. The cost of flow able fill is insignificant to the project the real savings will be realized in labor. As far as the engineer on site’s brain power, when I was fist invited to this project, 42” steel pipe was set to depth. 36” Sono-tube {card board pipe} was set in side to depth. Flow able fill was placed in the annular space between the 36” and 42” and the 42” extracted leaving the sono-tube in place permanently. I am not a engineer, but I don’t think card board is adequate

 

[clayR]

Could you demonstrate to the engineer your method is acceptable?
Perhaps you could do a SPT in the backfill and compare to adjacent material?

 

[Focht3]

Sono-tube? What was he thinking? Or was he at all?


That's a lot of 36" CMP! Are these footings? Are they filled with concrete?

Anyway, back to your problem. There are flowable fill purveyors that can provide a mix that's completely set in only 3 or 4 hours, and is "hard" in an hour. They custom mix on site in whatever quantity you need for that location, so there's no waiting for a truck from the batch plant. (It's a self-contained batch plant on wheels.) And the material will achieve a minimum strength of over 100 psi, which will be as strong (or stronger) than a granular backfill. Stronger mixes are available if you need the extra strength.

Give the engineer notice that you intend to begin work on a date certain, with placements every working day until {whenever you think you will be finished}. This really will be faster than your compaction technique - and cheaper in the long run.

 

[Zambo]

Not much space for an SPT test. Seems caisson driller really doesn't want to use flowable fill, probably a sound production reason for this. As you've carried this out successfully before why not have the discussion with the Field Engineer see how he feels about it - probably doesn't fancy 800 inspections. As a back-up you can always buy sand and use that. Plenty of water and the vibrating plate on the caisson will compact the sand adequetly. Then a couple of layers of excavated material for the last 300mm

Interested to know what these are. Access chambers?

Zambo