How do I backfill a trench beneath a sidewalk?

Submitted by Kalarix t3_126uabk in DIY

Hello everyone, one of the spring projects I have on my plate is to get a downspout to drain away from my foundation. However, it is between the house and a sidewalk walkway to my front door. I plan to trench under the sidewalk for a drainage line, but I'm concerned about undercutting the sidewalk. I've got all of my drainage parts sorted out and the digging and drainage pipe is straightforward.

How do I backfill to get the necessary support beneath the sidewalk? Looking online has mostly given information around jacking, rather than just supporting, but I'm concerned about using those materials as the sidewalk is perfectly even and level right now. I've seen some responses about hand packing as best you can, but that seems not great.

When I find videos on youtube, they all cover methods for digging the trench, but not how to backfill it, that's typically right when the videos end.

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allangee t1_jeazn3l wrote

It sounds like you have a poured sidewalk (otherwise you would have just lifted a block). Very likely it's reinforced with rebar and/or steel mesh. That means it'll bridge gaps well.

I'm guessing you're going to put in a 4", maybe 6" pipe to accommodate the downspout outflow. If you keep your excavation close to the width of the pipe, the concrete will bridge that without back filling so that should ease your mind a bit.

But I know where you're coming from... I feel the same way about these kind of projects.

Depending on your access, and how wide the sidewalk is, you could mix a "soupy" batch of Quikcrete and let it free flow in, in layers. If one end is higher than the other, you'd obviously start there and block the other end as the concrete rises.

If that's not possible, poke in as much coarse gravel as you can (jagged stuff, not pebbly). Then run water through with sand to fill up the gaps.

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Kalarix OP t1_jeazvi0 wrote

Correct, it it's a poured sidewalk and a 4" drainage line. Thanks for the reassurance and recommendation.

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allangee t1_jeb0a6k wrote

Depending on your soil conditions, you can possibly notch the end of 4" pipe and twist/drill it through under the walk, sucking sand/gravel out with a shop vac.

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Kalarix OP t1_jeb1xnv wrote

I can hope they put a decent base down under it, but other areas I've hit clay pretty early unfortunately

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Most-Region8151 t1_jeb04t7 wrote

OK, you are going to have a trench under the walk....keep it as narrow as you can. When you are done try and block off one end with a plank..on the low side. After the plank, backfill the trench to give it support. Then from the open end start filling and tamping with whatever fits. Ram it right against that board you put in. Pack it in...repeat until you get to the top. Then pull the board and backfill.

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Kalarix OP t1_jeb0c6f wrote

I've got a trenching shovel to keep it narrow from last years drainage project, so I'm hoping to keep it small width. I like the plank idea for tamping, thank you.

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sjg97 t1_jebgbfs wrote

You could hire someone to bore the line for you which wouldn't require any backfilling but if you don't want to spend the money keep the trench as narrow as feasible and mix up a sand bedding mix real wet so it flow fills the gap around the pipe and supports your walkway

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xBeamer t1_jebr0wq wrote

Pack it as best you can with dry concrete. It will take slightly long but enough water will get in from the soil for it to set.

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Kesshh t1_jec74xk wrote

Is it a public sidewalk or your own private one?

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Kalarix OP t1_jecacza wrote

It's my sidewalk, it goes to my front door.

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Kesshh t1_jecb3o1 wrote

We did our drain line by digging up everything and trenched down. Our run was almost 40’ (forgot) so there was quite a bit of grading we needed to do. The grade was held up by gravel, 4” drain pipe (ours have holes to drain along the way, not just the down spout), fabric wrap to prevent clogging, gravel topped, then soil back fill, then everything went back on top.

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ybonepike t1_jecmdua wrote

I put in downspout drainage under a sidewalk at my house last fall.

4" corrugated drainage tile which is actually 4.75" OD so the tunnel under the sidewalk was much larger.
I just backfilled and soaked the dirt with a hose until it settled, backfilled again and soaked it again.

Another method of you're worried about supporting it is use expanding foam. It's a method used to Jack up sunken concrete driveway slabs, and a newer way to support fence posts. It will work just fine.

Personally I wouldn't really worry about it much. It's such a tiny area

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CanadianBaconMTL t1_jedc3d5 wrote

The best way would be no trench. Dig on both sides and slam the pipe in the dirt so it's perfectly in the dirt with no void. Use water to clear the pipe after

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usedTP t1_jecay09 wrote

It's going to crack if you dig it or trench it, whether you backfill it with sand, gravel, or atomic space dust. If cracks will bother you keep the water on top.

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