Trench Plugs

[palmahouse]

Do you recommend placing trench plugs where utilities are long runs down slopes flatter than 2:1?

Bedding sand will have a sand equivalency of at least 30 and will have little percent fines.

If so, do you recommend them for 3:1 or steeper, 4:1 or steeper, etc.?

What minimum spacing for various slope inclinations do you recommend.

I have seen guidelines that seem overly conservative.

One quick thought: Perhaps if you keep gradients in the bedding sand below 0.5, that erosion should not occur, and you can't get gradients above 0.5 unless you are 2:1 or steeper. But, I would need to think more about this. Keeping gradients under 0.5 is a rule of thumb for the downslope toe of a levee to prevent piping. Of course, we should all sharpen are pencils on levees.

Any comments?

 

[cvg]

if the sand is a reasonably good filter match to the in-situ soil, than I'm not sure why you need plugs. However, you may need pipe anchors to anchor the pipe. These usually also function as a plug and are recommended at 3:1 or steeper.

 

[oldestguy]

CVQ is right on the filter aspect.

Then, think about the consequences of a "dam" within a trench carrying groundwater. Do you want water coming to the surface somewhere along the line,because it can't get past the plug?

Then, so there are no plugs. Can you infiltrate what you pick up along the line, somewhere down slope? I've see the situation where a perforated sub-drain is also placed in the trench to collect this water and do something with it not to cause grief.

Sewers somehow don't like to be in ground water, should there be cracks, etc., since infiltration into them is usually not wanted. Placing a plug can increase the hydrostatic pressure on the sewer can increase the inflow that otherwise might happen there.

 

[palmahouse]

Thank you for your input.

I thought about the buildup of hydrostatic behind the plugs, but did not include that in the question to keep the topic more focused.

Good point about water comming out of the line above the plugs. Not a concern in my case, but blowing the backfill out of the ground is (because we will have clay above bedding sand). I did a rough calculation. I estimate that you would need about 55 feet of head pressure in the bedding sand to blow out of the ground 2 feet of compacted clay backfill over (using an estimated fully saturated undrained shear strength of 800 psf at the interface between the backfill and the trench sidewall). That being said, and because we do not want to get too close to failure, I believe that drains behind the plugs are not required unless you have the plugs spaced more than about 40 vertical feet for the conditions I describe, which would usually not be the case anyway, and if having seepage at the ground surface is not a concern.

However, oldestguy got me thinking that I need to be sure the downstream ends of trenches that enter floor slabs are plugged and drained whether or not we place plugs along the line - Thank you oldestguy - I forgot about that very important detail.

Good comments about the sewer. Thank goodness I am not dealing with sewer. Boy, we have lots of details to consider!

Incidentally, the logic in my original post was somewhat flawed, because we should expect higher hydraulic gradients around the pipe perimeter that are higher than but related to the pipe inclination. However, at least I recognized that the erosing issue depends on gradients that cause erosion, and I learned from cvg that you need to consider filtering (which in my case, probably does not govern because the native is clay and claystone rock). I suppose you could to a detailed seepage analysis and figure out localized gradients - but you figure someone figured out that problem long ago. This all made me realize that pipe diameter should fit into the evaluation.

I got hold of a set of plans for a similar job where trench plugs were called out. This project was designed by a large national wet civil firm. They called for trench plugs on terrain 3:1 or steeper, every 10 vertical feet, regardless of the slope. Every 10 feet seems very close.

In the end, I believe I need to do more research. For my project, however, my gut feel is that plugs are not required along the line, and only at the ends where the utilities come into floor slabs.

I will post again if and when I get recommendations from the project Civil Engineer, which I requested.