Trench Backfill with Native Material

[BigMoo99]

Hi all, I'm working on a project including install of a sanitary line to a depth of 2.5m in a winter climate (frost depth ~1.5m). We're doing a full roadway, boulevard and sidewalk reconstruction after the deep utility install. We're pretty lucky in that most native material in the area is well graded gravel, but in this case the native material is silt with some clay with relatively high moisture content on site. We'd be looking at a significant change for waste of native material offsite and borrow of acceptable material if that's the route we were to head. Would you typically hesitate to backfill with the native material, and if so, why (settlement, frost action, etc.) Am I right in assuming that we'd be at low risk of settlement, provided the silt/clay material is successfully compacted to spec (97% SPMDD, +-3%OMC).

I understand that we'll have to be careful with this material close to the surface due to concerns about frost action. Pavement structure is 700mm, and we would look to do 300mm subex, gravel and bi-axial geogrid in silt areas where proof roll doesn't pass. Our thought is that money for improvements against settlement would be better spent closer to the surface, rather than throwing money into the pit and replacing borrow with gravel.

 

[1503-44]

It doesn't sound like a problem for the pipeline. It will be below the ftost line.

What is the standard method of practice for road construction in the area? Do they replace the native soil in similar situations? What did you discover when you removed the existing soil, was it native, or borrowed fill?

Is the native soil compactable to 95% resulting in adequate bearing capacity? If so, why not use it? If it is not useable, discuss the situation with your client. Having to replace soil that was apparently adequately supporting the road pavement prior to your utility work could be cause for extra work payment. Presumably repairing an inadequate road infrastructure was not part of your utility construction work. Supposedly you would only be expected to repair or replace what was damaged during your utility installation at least to the level of fitness existing prior to your work. Are you expected to do actual road improvement too? Normal roadwork would tipically include 95% compaction under roadways, as that is the typical requirement for road subgrades, so you should provide the same. But it also means that was what was already existing there under the road before you began your work, if the road was built to customary standards. If it was not compacted to 95% before, than that IMO would be a road improvement. If the existing road subgrade was inadequate for a road, replacing it now with select fill would also be road improvement work. As for high moisture content, that is apparently not affecting use of the road. It is also something that existed prior to your arrival. Typically utility works that disturbs drainage is only required to reconstruct exactly what was disturbed to its prior condition. Its not a problem for the pipeline and supposedly was not a problem for the road before, so simple reconstruction to the existing condition should be enough. Road drainage improvements are beyond your scope. So all of those things could be items for which extra payment could be asked.

It leads me to believe that you can reuse all materials you encountered at the site. It sounds like all were suitable for their prior use with no reason to suspect that they suddenly become unusable, except for REcompaction needs, due to utility construction. If you disturbed some materials, like destroying any of the 95% level of compaction that was there, you could simply recompact that same material to 95%. Providing additional work and materials to bring a road up to customary standards is road improvement work and is most likely outside your utility construction contract.

The general rule is repair or replace anything changed, damaged or destroyed to the condition that existed before your arrival. Anything else is subject to extra payment, unless specific improvements are required by your contract. Does your contract address the issue?

Items required for your pipeline, such as special fill, are your responsibility. I think your pipeline does not need special fill. I think it does not need special drainage. Use existing native soil, if you can. Recompact existing materials only under the road or wherever you encountered it. Replace the damaged parts of the road surface. All to at least their condition prior to your arrival. Provide road crossing signs, if appropriate. Bury the pipe.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[geotechguy1]

I've had alot of bad experiences in the past with getting ~1-2% of the fill height in settlement out of silty clay fill in pipe trenches under roads regardless of hitting spec, and also differentially along a pipe trench alignment. I started recommending / advising clients to time projects so that the pavement above would be a patch that they expect to settle and come back in a year or two and fix up, and delay any full road surface re-construction for a year or two while things settle out.

YMMV. I suspect part of the issue in my case is that piping contractors have a huge incentive to cheat on trench compaction and QC doesn't really have control of the site / isn't there full time, so you end up with layers here and there that are properly tested or one stretch of a trench properly tested and the rest pretty much end dumped in.

The other thing to be mindful of is sometimes pipe designers intentionally design in soft / less compacted layers above a pipe.

 

[1503-44]

Actually we do not design soft layers over the pipe at all and never compact it to any %, except for enough to maintain existing ground profile or drainage. We usually do not design the backfill in any manner at all, other than separating topsoil, putting the topsoil back on top, and making sure that there are no sharp rocks against the pipe coating, which sometimes requires a sand or small crushed rock as pipe bedding. Except when we are installing a new pipeline under a road crossing.

As for the material to use in a new pipeline crossing, native soil should be checked for compaction issues, or replaced with select fill under road crossings whenever it is found to be unsatisfactory. The last thing we want is to go back and redo it correctly tomorrow, next month, or next year. Existing materials under the road should usually be adequate, unless the road surface is already failing. If it is failing prior to crossing, then that is road improvement.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[geotechguy1]

Another thing to think about is health and safety requirements. Often if you have a trench deeper than a meter or so the entire depth of the trench beneath that will be backfilled without testing and often using something like a plate tamper attached to an excavator arm which is likely to result in non-uniform compaction and be ineffective for clayey soils. Or simply end dumped and tamped down with the bucket. Time = money, doubly so for the contractors doing jobs like this. They will always cheat if someone isn't standing there watching them (and often if the testing consultant is the only one there they will cheat anyway).

I've seen soft layers used semi-commonly over concrete pipes and culverts to reduce the pipe load, even things like straw bales or soft tires.

 

[dik]

Silt is very bad for frost heave susceptibility, and maybe should be avoided.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik