Pipeline Trench Backfill Specifications

[morgwreck243]

My question is can anyone point me to some previous specifications for backfilling the pipe's trench. I have not overseen many pipeline installations in my time, but the few that I have have mostly been for gravity / stormwater pipes (pond outlets / inlets).

I've taken over as the field QC for my company's project in southern Ohio. Part of the work is to install at 12-inch PVC raw water line and construct a sediment pond for the future landfill. I'm currently baby-sitting the water line installation and I believe the specifications are too restrictive / not producing the quality that they want to get. Currently, for the backfilling, we are required to backfill, with sand, in 6" lifts up to 9-in above the pipe. From there we will backfill with the excavated spoils (clay material) and compact in 4-in loose lifts and testing for 95% density test (ASTM D6938). The testing starts with the 2nd clay lift. Due to the depth of the pipe the clay fill will be approximately 3.5 ft to 4 ft thick.

This water line is not within any road ROWs, rather just out in the country-side.

One idea I have is to take a test area (roughly 50 LF) and try compaction at different thicknesses (8-in or 12-in) and test these areas to prove they will achieve the desired density.

My other is to try and present what others are doing / requiring for backfilling.

--morgwreck243

 

[cvg]

well, I doubt that 12 inch thick clay lifts is a good idea
8 inch thick might be acceptable, 4 inch thick loose lifts seems a bit excessive

95% seems high, but not unreasonable

so, are you working for the contractor?
suggest you request the engineer of record to review your proposal unless you wish to take responsibility for his design.

 

[EireChch]

If it's in the country side and not going to be loaded with vehicles etc, I fell 95% may be a bit high. What about trying an 8" lift then do a shear vane test. I would consider 75kPa relatively easy to achieve.

Is the pipe going to be adopted as public infrastructure ? In that case you may just have to stick to the local building authority codes.

 

[morgwreck243]

cvg - I am the contractor's QC. I agree 95% is high, but it is doable with the 4 passes required. I know that 12" is possible because I've done it before, but mainly I'm trying to convince them to go up to 8".

EireChch - This is part of the new landfill for the DOE, which I imagine will be under their control in perpetuity.

--morgwreck243

 

[cvg]

if you can demonstrate with a test fill that 8 inch loose lifts can be compacted to the required density, than that should be acceptable.

MINIMUM TRENCH COMPACTION DENSITIES

type I (Under any existing or proposed pavement, curb, gutter, attached sidewalk, roadway
shoulders, and other areas within right-of-way subject to vehicular traffic, or when any part of the trench excavation is within 2-feet of the existing pavement, curb, or gutter.) - 95%

type II (On any utility easement or right-of-way outside limits of Type I backfill.) - 85% and 90% for the bedding

type III (Around any structures (manholes, etc.) or exposed utilities outside limits of Type I
backfill.) - 95%

 

[Okiryu]

cvg, are those compaction grades based on standard or modified proctor? They are good references, so just wanted to confirm...

 

[cvg]

standard proctor

 

[Okiryu]

Thanks cvg. By the way, do you know if there is a correlation between standard and modified proctors?

 

[cvg]

just depends on the soil

 

[Ron]

I would look at two considerations.....

1. This is a trench, so nuclear methods must be checked for trench effects.

2. You might consider that the in-place density should be slightly higher than the natural density of the area (for bedding purposes), so you might check the actual density of the material adjacent to the trench (I would suggest a drive sleeve method for this) and then set the in-place density to be about 10% higher than the natural density....forget the Proctor value in this case.

 

[Okiryu]

Hi Ron, that is new to me. For trenches, is that the typical approach in the US? Do you have any literature reference about this approach?

 

[BigH]

If you google, you will find some information from various state DOTs (Dept of Transportation). See:

http://www.dot7.state.pa.us/BPR_pdf_files/Documents/LTAP/TechSheets/techSheet163_color.pdf

This seems to be, now, at odds with work done by Spangler back a half century ago . . . if you can find Leonards Foundation Engineering book, Spangler has an article on conduits and their backfill. He was addressing the deformation of the pipes. In his work, he stressed to leave a loose zone above the pipe - will reduce the weight of the soil on the pipe due to arching effects. PENNDOT as I referenced above wants high compaction to 4 ft above the pipe obvert and then 100% standard MDD in the upper 3 ft (presumably they are referring to where the drainage pipes cross the road.

I've seen contractors loosely backfill above conduits crossing the road and then place a bit extra for a "bump" - with time and with the traffic, the bump will settle a bit. Top it up if needed or after a season, cut it down. Not that I like this - but I saw one box culvert - about 3 m deep backfilled by a Malaysian contractor for a gov't road in less than a day . . . no layers/no tests and to date, after two monsoon seasons, there is no noticeable depression. Of course they could have just gotten lucky! There are plenty of other culverts crossing the same road (earlier construction) that definitely show depressions and growing each season.

 

[fattdad]

when water is flowing by gravity in a pipe that extends through an embankment (i.e., an outlet pipe for a stormwater pond), I would put the upper 2/3rds of the pipe run in a concrete cradle (concrete to the spring line) and then use ordinary fill (95 percent relative compaction) to completion. For the lower 1/3rd of the pipe run, I would only use open-graded backfill enveloped by separation geotextile. I'd want at least 12-inches of this open graded backfill in all directions about the pipe. I'd use ordinary fill for all other fill.

I've seen too many cases of piping failure.

f-d

ípapß gordo ainÆt no madre flaca!

 

[cvg]

well, if you are going to construct a pipe through a dam or a levee then you probably want a full filter diaphram and drain. If it is a pressurized line through a dam, then it needs to go through a casing. But I did not get the impression there was any embankment on this project, so a concrete cradle and drain fill would be overkill.

 

[fattdad]

sediment ponds for a future landfill imply (to me), there may well be some version of an embankment and some version of an outlet.

I guess I'll let the op let me know if I was helpful. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[cvg]

if this raw water line is under an embankment (or under the future landfill), than I would not recommend PVC.

 

[jgailla]

cvg,
I've used PVC in a steel carrier pipe in similar installations. If the embankment's not too high, I just use PVC with mechanical joints.
What would you use?

 

[morgwreck243]

Here's an update:

Finally, the client has accepted my proposal for the test section. We performed the test section (8-inch loose lifts) and the compaction tests passed with flying colors (99% relative compaction). I am now waiting for test results to be processed through the client's bureaucracy before changing the specifications.

fatdad: Some of the raw water line (estimated 500 LF) runs through the sediment pond's embankment, but the areas that I was speaking of was where the raw water line is running through undeveloped areas (no roads or structures near the pipe's location).

--morgwreck243