Pipe Bedding layer: sand vs gravel 5

[pelelo]

Hello,

I have seen Reinforced Concrete Pipes (RCP) which have had compacted sands as bedding layers. In other projects, I have seen the same situation using gravel material (stone no. 57), instead.

Does anyone know what would be the advantages or disadvantages of using sand vs gravel as bedding layers?. To me they would do the same, it would be a matter of what's is available at the site and which one would be cheaper.

 

[geotechguy1]

I have also seen both. I've also seen specifications calling for lightly compacted sand to requirements for gravel 'bedding' to be compacted to 100% modified proctor. There many factors and some depend on the specific grading of the sand and gravel and also how you compact it but, some general ides:

Heavily compacted sand or gravel, especially right under a concrete pipe results in much higher stresses within the pipe. Probably the worst case would be to place a well-graded crush gravel and "Compact the shit out of it" to use contractor speak, creating a perfectly level, flat surface.

A sand can be ideal in this sense because it allows for some stress redistribution and thus less stress in the pipe.

Some gravels, for example an open graded angular gravel can be bad because they also result in stress concentrations; to that a rounded stone might be better.

Open graded gravel can be a risk for material above and adjacent to it eroding into and filter compatability or protection like some form of geosynthetic between the gravel and native soil may be required.

Sand can be an erosion risk on it's own; especially once there are any cracks in the concrete pipe or openings at the joints where the sand can come in. Once again to use contractor speak, if the pipe has enough grade and water flowing into and there's a path for sand to get into the pipe and water to get out of the pipe into the sand, the sand might decide it would rather flow away in the pipe. Once again, geosynthetics or clay plugs can help.

 

[Ron]

The controlling factor is joint displacement, not pipe stresses. RCP can handle a lot of stress, though there is comparatively little.

The bedding can be either sand or aggregate...makes little or no difference. Just make sure that the pipes are firmly bedded with a low chance of joint displacement.

 

[dik]

I'm not into piping, but if there's a potential for bedding to wash away with a leak, I'd avoid sand.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SlideRuleEra]

During installation in areas with high water table (near ground surface elevation), gravel does not require much compaction. At this same time (during installation), gravel tends to act as a french drain allowing pumps (diaphragm pumps in engineer speak, called "mud hogs" in contractor speak) to temporarily dewater without bedding erosion. Using sand for bedding for these conditions is really not practical.

 

[dik]

My experience is that with a good gravel, I think it achieves 90% of the compaction just in placing it...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[geotechguy1]

Another factor is that with a bell at the joints means you need to make sure the bell is seated properly when installing it (which is easiest with loose material, usually sand, and hardest with well graded gravel that's heavily compacted). If you don't do this then the pipe ends up as a beam unsupported on the bottom.

Also for a rigid concrete pipe the pipe is much stiffer than the soil generally speaking so the pipe is going to change the in-situ stress distribution by picking up load from the soil above and concentrating it, ultimately in the soil underneath the pipe. For usually bedding thicknesses that I've seen I'd expect long-term differential pipe movements (Thus effect the joints) to be controlled by the soil beneath the bedding layer rather than the bedding layer itself.

 

[ATSE]

Just an aside and to be pedantic, I believe using the phrase "compaction to xx%" with respect to gravel is a bit confusing.
In the US, the most common guides for compaction are DOT specs (like Caltrans or Texas DOT) and the lab test for basis is ASTM D1557; these are all based on well graded materials.

If 50% or more of your "soil" is 3/4 inch or larger gravel, and/or your soil material doesn't have sufficient distribution of fines between #4 and #200 (usually 12% minimum), conventional compaction measurement via relative compaction or modified proctor does not apply. There's no range of moisture retention for gravels or clean sands.
That is, "compact gravel to 95% relative compaction" makes no sense, as it cannot be quantified in the lab or measured in the field.
Gravels can indeed be compacted, but we do not measure compaction against a compaction curve done via lab test. Usually it is just normative; i.e. 3 passes with a vibroplate in both directions.

 

[1503-44]

ATSE well said. It makes little sense to try to compact a medium which will retain large void spaces.

Petrol & gas pipelines do not use gravel; always sand, or at least a large percentage. Pipeline beds are sloped, especially gravity lines, so frankly I am surprised to see gravel being thought about here, as that would permit water to run down the full length of the trench, at least in a worst case scenario. When the water finally reaches a low evelation on the pipeline, its pressure increases and might easily, actually more easily, flow upwards out of the trench, rather than down into more dense soil below. That risks washing out all material from the trench. We try to avoid flows parallel to the trench at all locations, above the surface and below. Even with the use of well compacted and well graded materials, pipeline trenches are often washed out on steep slopes due to permeation of water into the trench. Once in the trench, water will often run along the pipe outside wall surface, washing bedding away in the process. That process has spelled doom for many leaves and dams where drain pipes, or other utility pipe had been laid within. To try to mitigate such flows, we will place permanent "ditch breakers" in the trench, which are sand cement sacks, poly foam, or other impermeable material, to act as water stops to try to dam the flows and avoid such continuous underground flow within the trench. Surface water attempting to flow parallel to the trench will often be directed away from the trench with chevrons plowed into the top soil's surface.

I recommend using a well graded materials that precludes the possibility of allowing water to flow through void spaces.

https://duckduckgo.com/?q=pipeline+ditch+breakers&t=brave&iax=images&ia=images

Ideally the bedding should be well compacted below and around the side of the pipe to a height of 30% of its diameter to allow good bearing stress on the lower circumference, inspected, then filling the sides and top layers with more well graded, compacted fill. I've even wrapped bedding in certain problem areas with geotextile to ensure it remains in place. The last thing we want is a French drain inside the trench.

 

[geotechguy1]

There are plenty of soils classified as gravel for which you can use % compaction as a specification, likewise for sand as long it's not uniformly graded. For civil construction (i.e. wastewater / stormwater pipes) I have seen 'clay plugs' or similar devices used as what you call a 'ditch breaker'. I'm not sure I see the point about gravel vs sand acting as a french drain; it all depends on the gradation of the material and a sand could act as a drain just as easily as some well graded gravels. In either case water can still move in a well graded material just at a slower rate.

1503-44 - how many petroleum pipelines are made out of concrete?

 

[1503-44]

Obviously not as many as I have, but that is why I qualified my comment to apply specifically to those pipelines.
However I fail to see why anyone would purposely put a material potentially containing high void ratios under a load bearing structure of any kind. How many petroleum pipelines does the USACOE construct. They actually do short ones, but what's the point. Better to address the concerns I listed.

USACOE (from the Unified Specifications) recommends the following grades Type I and II for utility bedding material. This would not preclude the use of gravel in specific locations for particular engineering intent, however the OP question is general, not for any specific concern, so I believe this is relevant and applicable to the question on general utility bedding requirements.

2.3 UTILITY BEDDING MATERIAL
Except as specified otherwise in the individual piping section, provide
bedding for buried piping in accordance with AWWA C600, Type 4, except as
specified herein. Backfill to top of pipe shall be compacted to 95
percent of ASTM D698 maximum density. Plastic piping shall have bedding
to spring line of pipe. Provide ASTM D2321 materials as follows:

a. Class I: Angular, 6 to 40 mm 0.25 to 1.5 inches, graded stone,
including a number of fill materials that have regional significance
such as coral, slag, cinders, crushed stone, and crushed shells.
b. Class II: Coarse sands and gravels with maximum particle size of 40 mm
1.5 inches, including various graded sands and gravels containing
small percentages of fines, generally granular and noncohesive, either
wet or dry. Soil Types GW, GP, SW, and SP are included in this class
as specified in ASTM D2487.
**************************************************************************

TABLE 1
[Type I
[Gradation E
11
ASTM C33/C33M]]
[Type II
[Gradation 57
ASTM C33/C33M]]

[ASTM D422 Sieve Size]
[Percent Passing] Type I Type II
[37.5 mm] [1.5 inches] [--] [100]
[25.0 mm] [1 inch] [--] [90 - 100]
[9.5 mm] [3/8 inch] [100] [25 - 60]
[4.75 mm] [No. 4] [95 - 100] [5 - 40]
[2.36 mm] [No. 8] [--] [0 - 20]
[1.18 mm] [No. 16] [45 - 80] [--]
[300 micrometers] [No. 50] [10 - 30] [--]
[150 micrometers] [No. 100] [0 - 10] [--]

 

[FacEngrPE]

RCP is insensitive to scratches from bedding material.

Most Petrol & gas pipelines pipelines are constructed from steel (or other metal) with some sort of coating. Keeping the coating sound (electrically isolating) is important. The use of sand bedding is therefore more important, perhaps vital to a successful project.

If your pipe is laid at a pitch I can see why you would want to put periodic "ditch breakers" so the bedding does not become a conduit.

 

[1503-44]

Indeed, concrete pipe is a different material, but pipe material is only one of many factors of a pipe system design.

Groundwater is virtually never stagnant and where it is brings on other considerations as well, one of which is that sand, gravel, or stones by themself, even in significant quantities, are unlikely to greatly improve the bearing situation, if at all. Gravel beds under roadways in high water table areas certainly prove that point. The addition of a hydraulic gradient only makes it that much worse. In a dry area, you can pretty much do as you will without any further thoughts, but we like further thought.