Backfill material and concrete encasement for Piping Trenches

[sowhatso]

Dear All ,

While we are excavating the trenches for 12" Ductile Iron water main transmission line , we faced a very hard type of rocks ( like granite stones) and it was very difficult to dig deeper to reach the required depth of excavation for the trench ( 140 cm ). As per the design drawings details ,The trench depth should be about 140cm ,and to be backfilled with 65cm soft bedding msterial ,55 cm of single size aggregate ,and 20 cm of compacted top basecourse layer. Due to the problem in axcavation and the difficulty to reach deeper than 80 cm , the contrcator proposed to fill the 80 cm depth trench and to encase the pipe with B-300 PLAIN CONCRETE . I believe the concrete is very strong comparing to the backfill material , is it OK to go for this choice or you have different ideas ?????

My concern is that the datasheets for the pipe manufacturer call for " minimum allowable depth of cover is 3ft (0.9m)" , But concrete is very strong ..... so if you think it is OK please let me know ....

Thanks in advance

 

[zdas04]

I had a similar situation where we simply could not go deeper than about 60 cm on a 15 cm fiberglass pipe next to a road. We kicked it around a bunch (this was before eng-tips.com) and finally decided to backfill with 1200 psi (8400 kPa) compressive strength concrete (also called flowable fill, it is really soupy) that was died red. The road grader hit the concrete a couple of years later and it was no problem (he saw the red and stopped and called, everything was OK). It has been in the ground 16 years without incident.

This flowable fill is just barely concrete (but you cannot stick a shovel into it), I think of it as "engineered dirt" and it doesn't generate much heat in curing so you don't have to worry about heat like you would with high-strength concrete. I would definitely use the flowable fill and definitely dye it a bright non-natural color.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Remember, being ignorant isn't your fault; staying ignorant is.

 

[BigInch]

zdas, not bad, but I'll go a little further.

Its normally not required to fill the entire trench with this concrete. Ask if it is acceptable for him to fill the trench with the backfill material to 12" above the top of pipe, then a layer of only 4 to 6" thickness for placement of the red concrete slurry, then top off the remainder of the trench with the surface layer backfill materials. I think he will go for it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Note, wherever trench cover does not meet specified depth, you must be sure that no surface loads will be applied. If it could be exposed to surface loads, place the backfill to 12" above the pipe, then a red slab made of normal strength concrete with thickness and reinforcement as required for the expected loads, is placed 12" above the pipe and spanning the trench from side to side might be more appropriate.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Ron]

zdas04 gave a good solution...flowable fill. Make sure you carry it beyond any vertical load conditions, such as traffic, so that you don't have a sudden flexible/rigid transition under load.

 

[BigInch]

slurry is not suitable for traffic.

see #11 & #12

http://www.nbtexas.org/DocumentView.aspx?DID=406

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[zdas04]

Easy boys. None of this is suitable for traffic areas. In the example I used, the pipe was burried under the bottom of the bar ditch and had nearly zero probablity of ever seeing traffic (maybe someone could get run off the dirt lease road at 10 mph and come to rest on the pipe but that is it).

If you are somewhere that traffic at speed is a reasonable possibility then you get deep enough (even if you have to blast) and/or case the crossing. I was talking about normal ROW that may see a tractor, ATV, or a deer.

David

 

[cvg]

ductile iron pipe with a concrete blanket or structural concrete encasement can be installed with 2 feet of cover (or less) in street right of way. It is done all the time. Casing is another option and removal of the rock may still be a better way to go.

 

[BigInch]

zdas, you're a bit spooked today. You didn't mention traffic at all. If a road grader uncovered it, I assumed that was some off-street work, or maybe a new road being built, ie. not "traffic".

Use of any casing is not preferred by gas & petroleum PL companies, but may be installed in special circumstances, ie. very high traffic load (plant roads, etc.) & reduced clearances. Casings are often a condition of RR crossings due to the age-old wars between them.

Some amount of full strength concrete (filled in trench, or placed as preformed slab in top layers) is typically required under road and highway traffic, with any granular backfill material placed to 95% proctor density, if clear cover is reduced below 3, 4, 5, or 6 feet for subdivision street, county, state and interstate highways respectively (as a rule of thumb only). Check the traffic load and add full strength concrete (w/ wwf or bars) as needed.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rconner]

Just one comment for whatever it is worth involving the apparent premise of the inquiry that, "The datasheets for the pipe manufacturer call for "minimum allowable depth of cover is 3ft (0.9m)". This sounds a little strange (unless there is some sort of unusual extenuating or extreme circumstance of this particular project not explained in the original inquiry?) I say this in that of course the AWWA standards e.g. C150 and C151, and incidentally product manuals of at least many American DIP manufacturers have for many decades actually shown default tables with allowable depth of covers down to 2.5 feet or 0.75 m (including the transient effects of standard H20 truck loadings etc. on ground or transmitted through “flexible” pavement).
I have always been under the further impression that DIP is buried all over at least the southern sections of the USA at such depth of cover or maybe at least a little less, and may be actually preferred over other types of pipe due to their dependable, long-term strengths and moduli in such areas.
The only standard exception that I am aware of with regard to use at 2.5 feet cover e.g. is only for absolute minimum Pressure Class 350 thickness pipes (the very lightest available in the USA) installed with the poorest quality (meaning just "dumped" native soil) "Type 1" trench assumed by the AWWA standards/manuals. I suspect few if any Engineers would actually approve installation of a transmission pipe trench under a roadway with such installed as a sub-base! Of course also, four standard trench types better than Type 1 are also illustrated in the standards/manuals, and many Special Class thicknesses of 12” DIP thicker than minimum PC350 may also be available.
Beyond this, I believe DIPRA also has a "…Shallow Cover…" paper that talks about aspects, including examining loadings etc., at depth of cover situations even less than 2.5 feet or 0.75 m (that I believe you can obtain from them upon request.) [Of course also, there may have to be very good reason or justification before many Engineers would design (against any applicable codes or consensus guidelines?) the installation of any pipe materials extremely shallow in very cold regions, due to freezing etc.]

 

[Ron]

Flowable fill is acceptable below the base material for traffic applications. If you have a concern with rigidity, order a lower strength material. Instead of 1000-1200 psi, get 400 to 600 psi material.

 

[sowhatso]

All , thanks for all your inputs

I just want to know one thing .. as we are goint to encase the ductile pipes with concrete , do we need to wrap the ductile pipes with the polyethylene sheets , or no need !!

 

[rconner]

I believe ductile iron piping is often encased both ways. Obviously, if there are potentially corrosive soils outside the encasement, it would make sense to run the polyethylene wrap at least some distance into or completely through the encasement. Furthermore, if there for whatever reason might ever be a desire to remove the encasement from around the piping (and particularly if the piping in question contains any complex protruberances such as bolting etc.) loose polyethylene sheet between the pipe and any potential concrete bond would likely make such process easier.
DIPRA's Installation Guide for Ductile Iron Pipe does mention in the context at least of thrust blocks, "...a sheet of polyethylene film is sometimes placed between the fitting and the block to aid in later removal as desired."

 

[BigInch]

Or rockshield.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/