wet sand as backfill?

[dirtsqueezer]

I saw a project today where the contractor was backfilling his 6-8' steam line trench with wet sand out of the back of a concrete truck! I walked over it after it was filled- not compacted. The sand was well graded, saturated, and less than 2000pcf (yielded to foot pressure). My question is, with that much of a profile, won't this trench settle over time? Even with the 'jetting' effect? All areas are for existing city streets.

 

[civilperson]

Jetting, ponding, flooding and other names for saturated sand compaction by adding excess water works in soils that are free draining. I know this technique works in Tucson,AZ where no caliche is found. Also works in storm water trenches that outlet to daylight, the down stream portion of the trench is free draining. Does NOT work in silty clays where permeability is less than 10<-3 cm/sec.

 

[Rjeffery]

Here in northern VA a product called "flowable fill" is used. It looks like wet sand but it may have up to 1/2 sack (~47 lbs0 of cement per yard along with some times fly-ash. It will develop 150 - 500 psi. Seems to work without settelment issues.

 

[cvg]

sand will bulk unless it is completely saturated, not just very wet. wet sand out of the back of the truck is completely unacceptable unless he is planning to flood / jet the sand in the trench to consolidate it.

 

[Kealoha]

The fill will work if it is compacted and confined.

 

[msucog]

my understanding from associates is that there must be some compaction with wet sand fill that thick. once vibratory compaction is applied, the pore water pressure will get jacked way up, the sand will consolidate and the water will drain out of the top. sort of like a bucket of dry sand after water is added and then you beat on the side. i personally have not seen much of this in the field so you might call my description presumptive.

 

[dirtsqueezer]

Yeah, that fill wasn't compacted at all. I think they were counting on some sort of static-jetting effect (if you can imagine that oxymoron). Apparently it's standard for the city, to avoid the hassle of dump trucks and stockpiles. I just wonder about the eventual settling effects. I'll keep my eye on it in the coming years.

 

[GeoPaveTraffic]

It will settle. The local sewer district has used the method for years. It makes it easy to find the sewer pipes, just look for the settlement trench.

Jetting only works if the material is clean AND if the water has an exist. It has to be able to flow out of the trench.

I don't like jetting, never have and never will. It is just an excus to leave a problem for someone else to deal with after a year or two.

 

[cvg]

the problem isn't just jetting, it is just as much to blame on the sand used for shading the pipe. Compaction of clean sand can only be achieved with close attention to a very high water content. Water content too low will result in bulking of the material and eventual settlement.