Just about any construction project requires digging holes and trenches, with much of the volume of those holes and trenches being filled after the project is complete. If you’re running lines in a trench, you dig it, then fill it with those pipes; but what do you use to fill in the rest of the hole?
You can use sand or gravel for backfill, as both are easy to use. They both discourage weeds from growing there, and they don’t take a lot of maintenance to look great. Backfilling is the technique of filling those holes all the way in, and you can use sand, gravel, or other materials for it.
While this article examines both of these potential backfill materials, please understand that this is not a how-to manual. There are specific techniques, layering, and things that you will need to know if you’re doing this yourself. Unless you’re a pro, hire someone for the work.
Why You Can’t Use Soil as Backfill
Your backfill material serves several purposes, but basically, it’s to fill holes. Many amateurs make the mistake of thinking, “I can just use the dirt I dug out of this hole, right?”
Think about when you’ve dug a hole and then went to fill it back in. Even if you didn’t put anything in the spot, not all the dirt fit neatly back in there, did it?
The soil in your yard has been there for a very long time, and as a result, it has compacted with rain, people walking over it, and the passage of time. When you dig it up, it gets, for lack of a better word, uncompacted.
This is why it won’t all go back in the hole–you added a bunch of air and space to it when you dug it up and disturbed it. When you shovel it back into the hole, it immediately begins settling and compacting again, but returning to its original compacted state will take time.
If you’ve built or set anything on top of that dirt you replaced, as the dirt shifts and compacts, whatever you put on top of it will shift, as well.
If you laid a length of pipe and then filled in the void with the soil you dug out from that trench, the future shifting of that dirt can be extreme enough to damage or break the pipe, and then you’ve got to do it all again.
Another reason you shouldn’t use the dirt you took out of the hole has to do with what, other than dirt, is in that pile. If there are roots or other vegetation, those things will die now that they’ve been dug up and will potentially end up covered with a ton of soil.
Rainwater, traffic on top of the soil, and organic activity will all do their work on them.
When those organic materials die and then begin to decay, the backfill around them will shift. We’ll discuss shifting and its potential for disaster in a bit, but for now, the takeaway should be that the dirt you dug out of the hole is not a good choice for backfill when it’s time to fill in the hole at the end of your project.
What To Use Instead Of Soil and Why You Should Compact It
So you need dedicated, proven backfill materials. You should be able to compact them properly so you can avoid any shifting or settling issues in the future. Sand and gravel are both excellent solutions, though each has its specific uses, and depending on the project, you don’t want to use the wrong one.
Whatever backfill material you use, you must remember to add it in layers.
You cannot just dump a bunch of sand in a hole, smooth out the top, and call it a day. You’ll need to add layers a few inches thick one at a time.
Once you’ve added your layers, you’ll need to use a compactor like the Multiquip MVC82VHW Plate Compactor, available through Amazon.com. However, these can be pricey, so you may want to rent one, or if your contractor is doing the work, he may already have his own.
Compactors mimic time and weather’s effects on the ground, essentially cramming all the small pieces of material together and forcing out as much air as possible to prevent future shifting and settling.
Compacting your backfill is vital, and if you screw up you could wind up with a costly rebuilding project.
Compact your fill too firmly, and you risk destabilizing the soil. (Basically, you can pound your gravel into coarse sand and your sand into crumbly brittle sandstone). But if you don’t compact it enough your backfill will settle further and lead to structural issues.
Sand Backfill: A Basic Overview
Sand is just what you think it is– the same stuff that gets everywhere when you go to the beach. At one time, a grain of sand was part of a more substantial rock. Over time, rocks break down until they finally become so small that they become grains of sand.
Sand compacts very well because the individual pieces are so small, meaning there is much less room between each grain of sand. That’s why it’s easier to keep the sand in place over time.
It also allows for drainage, so we often find it used in wetter areas such as swimming pools.
If you’ve ever visited a beach and picked up a handful of damp sand, you probably noticed the water dripping out of it as you held it. You probably also saw that the handful you had felt like one thing, a lump with high mass, and not like a handful of thousands of minuscule rocks.
This is due to the high compaction rating of sand, which again owes to the size of the grains. Sand backfill often accompanies in-ground pool and septic tank installations due to its draining properties.
Finding sand backfill presents few problems.
For use with small projects, you can find bags of it at your local hardware or home improvement store. Larger projects mean buying it by the ton, most likely, a need generally met by construction supply companies.
With a job requiring professionals, procuring the sand likely falls on their shoulders, which means one less thing for you to worry about while planning and executing your project.
Pros of Sand Backfill
- Sand compacts exceptionally well. Again, going back to the beach, you notice when you pick up sand– especially wet sand– how heavy it seems to be relative to its size. Because the grains fit so closely together, you can get a lot of mass into a small area. This means the grains are compacted together.
- Sand does not shift easily. When you have larger rocks together, there is less surface area of each stone held against other ones and less friction binding them together, which is not an issue with sand and its tiny grains. When you need a backfill less likely to shift at all, sand should top your list.
- Sand holds its shape well. Laying down sand as a backfill requires water, as you need to keep it wet to help it compact properly. Once it dries out, it becomes very solid. Not that you could build a skyscraper with it, but once sand dries in place as backfill, it offers a hard mass that won’t move around much at all.
Cons of Sand Backfill
- Sand can hold water if it gets saturated enough. While sand allows water to drain, it can only do so to a certain point, and once sand gets enough moisture in it, it will stop draining. Because the grains are small and so closely fit together, only so much water can collect there before your backfill reaches a saturation level. You could end up with a flooded mess if you get a lot of rain at one time.
- Backfill jobs are not generally DIY projects. If your project is small, you’ll have no real issues. But a big job like a pool? That’s not something you want to risk screwing up. The construction crew is already there. Let them do it, though it will cost you more.
Gravel Backfill: A Basic Overview
Technically, gravel can be made up of many sizes of rocks, up to and including boulders, but most of us think of something along the lines of pea gravel when we think of this material.
We often see pea gravel used as walkways in gardens, and some people use it as mulch, although this can be actively bad for your plants in some cases. Of course, in a rock garden, gravel serves as a terrific ground cover option.
One of the best uses for gravel revolves around soil erosion, and mixing gravel with your soil helps curb this problem.
But when it comes to backfill, we don’t worry too much about erosion but rather drainage. As referenced above, of the sizes and shapes of the individual pebbles in gravel, water drains very well through the myriad voids you get when you fill a hole with gravel.
This means gravel gets used as backfill around foundations, where you don’t want water pooling at all, especially if you’ve got a basement. In this case, when water fails to drain from the sides of your basement, you get leaks.
Gravel is widely available, so you should have no trouble getting gravel for your project.
Most cities have at least one vendor selling gravel. If you require a specific type of gravel, that may complicate things, but specialty gravel isn’t usually called for when using it as a backfill.
Pros of Gravel Backfill
- Gravel compacts very well, provided you use a compactor. When putting down sand as a backfill, you want to keep it wet– easy to do and not a project requiring extra machinery. However, you need a compactor with gravel, which slams a heavy plate down on the rocks to pack them together as closely as possible.
- Gravel provides excellent drainage. Unlike sand, only a remote possibility exists for it to get waterlogged. The comparatively large voids between individual rocks allow more water to course through them and away from structural elements.
Cons of Gravel Backfill
- Gravel can settle significantly over time. If you’ve got gravel down at the bottom of your retaining wall and it settles, an easy fix is to add more dirt on the top because you likely just have grass or landscaping on the top of it.
- Shifting can damage property and materials nearby. If you have gravel backfill under a structure, the movement presents a problem. Backfill shifting of any kind in construction can cause disaster.
Which One Should You Choose?
Sand backfill is usually best in wetter areas, while the drainage capacity of gravel is better for areas that need to remain relatively dry. Cost may also factor into your decision. Neither choice is likely to need a replacement job in the future unless you have a faulty initial installation.
Replacement of gravel or sand most often occurs in situations where they are exposed, as in your gravel driveway or your kids’ sandbox.
Gravel washes out with heavy weather, and most people with gravel driveways know to replenish the rocks every few seasons. Sand gets used by cats, but it can also wash away. None of that will apply to your backfill projects, because almost without exception, other materials cover the backfill and shield it from weather, animals, and erosion.
For a retaining wall, gravel is the better choice.
Water collecting at the bottom of the retaining wall will eventually contribute to the failure of that wall, so you need something that drains well. Gravel fits that bill. Once you’ve put in a significant amount of gravel backfill, you can then top it off with topsoil and landscaping, or at the very least, grass.
But for an in-ground swimming pool or a septic tank, sand is best.
Both projects have water associated with them, but they also require something that will hold its shape well. If you use a backfill material that begins to shift, your pool will develop problems quickly, ranging from cracks in the tile to catastrophic leaks.
With a pool or a septic tank, there are also pipes involved.
Any backfill that shifts around these pipes can damage them or break them apart completely. Sand around these items will stay put, and even if the dirt under the sand shifts, the sand backfill itself is unlikely to move with it to any appreciable degree.
Using the Wrong Backfill
We’ve briefly touched on this in a couple of spots already, but specifically, using the wrong backfill can completely sink your project and necessitate a costly redo. And some projects will hurt worse than others to do over again.
If you’ve got a retaining wall with lots of water running down it and in the ground, using sand will cause your wall to hold a great deal of water. At best, you’ll have a steady stream of water coming out of it. At worst, the wall could fail.
If you use a sand backfill in a trench where you have run some pipes, but you’ve done so on unstable ground, you can face serious problems.
Since sand holds its shape well, if the ground around it shifts enough, the sand can snap your pipes quickly and easily. If those are water pipes, you’ve got a leak to deal with. If they’re gas lines, you’ve got a visit from the local utility company in your very near future.
While prices have risen over the past decade, sand and gravel still cost roughly the same. Gravel (depending on the type) runs between $10 and $50 per ton, while sand goes for between $5 and $30 per ton. Again, you might find sand cheaper or more expensive than gravel, but not by significant amounts depending on each type.
Specific Uses for Sand and Gravel
You may need a straight-up list of things to use sand or gravel with. Let’s take a look at when you should use sand or gravel for your project.
|Use Sand||Use Gravel|
|A retaining wall with no groundwater issues.||A retaining wall where groundwater presence directly interacts with the wall.|
|A garden pathway with paving stones.||A garden pathway without paving stones.|
|Installing a swimming pool.||Installing a basement (gravel will put less weight on the basement walls).|
|Filling a trench in the hard ground (ground unlikely to shift)||Filling a trench in soft soil, as in a yard (gravel will more likely shift with the ground and not break your pipes).|
Your choice of backfill, then, will be dictated by your project.
Gravel backfill works well when you need something that will drain well, and it also stands as a lower-cost option. Sand backfill works best when you need something less likely to shift.
Whichever backfill material you choose, be sure your decision is based less on cost or availability of materials (though those issues do warrant consideration) and more on the nature of your project and the area in which it resides.
Consider drainage needs, groundwater levels, and the weights involved if the backfill will sit against something like a wall.
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