Backfill is the dirt that goes back in after the hole has done its job. Put it back wrong and it settles, holds water against a wall or pushes on a foundation. We backfill foundations, walls, trenches and tanks in lifts, with the right material in the right place, and leave the grade falling away from the house.
Backfilling in Boise and Nearby Towns
Backfilling looks like the easy part. The foundation is poured, the pipe passed inspection, the tank is set, and the dirt just has to go back in the hole. But a lot of the long-term problems around a new foundation or a repaired line trace back to how that dirt went back in.
We backfill for builders, homeowners and other trades in Boise, Meridian, Eagle, Nampa, Middleton and Star. Some jobs are ours from the first scoop to the final grade. Others start with a call from a plumber, a foundation crew or a homeowner who has an open hole and needs it closed up the right way.
Backfill Around Foundations
Foundation walls are built to carry a house, not to hold back a machine shoving dirt at them. Backfill goes in after the walls have cured and, on a basement, after the floor framing is on to brace the top of the wall. Before that, the dampproofing and any footing drain need to be in, and inspected if the job calls for it.
A few rules we follow:
- Keep heavy equipment back from the wall and use lighter compaction close to it, since a loaded machine pushes on the wall too
- Bring the fill up evenly, so one side of a stem wall isn't loaded far ahead of the other
- Place it in lifts instead of pushing it all in at once, so each layer gets compacted before the next goes on
- Keep rocks, concrete chunks, wood scraps and frozen clods out of the fill against the wall, since they leave voids and can damage the dampproofing
Backfill around a house still settles some, even when it's done right. Plan on the grade near the foundation needing a top-off after the first year. For the dig itself, see foundation excavation.
What Material Goes Back In
Native soil
Reusing the dirt that came out of the hole is usually the cheapest option, and often a fine one. Sandy or gravelly native soil makes good backfill. Heavy clay is another story. It holds water against walls and swells when it's wet, which adds pressure on the foundation.
Imported fill
When the native dirt is mostly clay, caliche chunks or big cobble, we may recommend hauling some off and bringing in granular fill for the zone right next to the wall. The native soil can often still go farther out and higher up. Common choices are pit run, sand or a clean crushed gravel, depending on the job and what your builder or engineer specifies. Extra native dirt that doesn't go back in can be spread on site or hauled off.
Topsoil on top
The last few inches should be soil that grass and plants can actually grow in, not subsoil or caliche. When there's room on site, we set the topsoil aside during the dig for this. If it wasn't saved, or the lot never had much, we can bring in topsoil for the finish layer.
Behind Retaining Walls
Retaining walls fail from water more often than from dirt. Saturated soil weighs more and pushes harder. That's why the zone right behind a block wall gets clean drainage rock, usually with a perforated pipe at the base that carries water out to daylight. Native soil goes in behind the rock, and a cap of compacted soil on top keeps surface water from pouring straight down the back of the wall. If water is the bigger issue on your lot, see French drain installation.
Utility Trenches, Septic Systems and Tanks
Pipe has its own backfill rules. It usually gets bedding sand or fine material around it first, so no rock sits against it, and the rest of the trench goes back in lifts. Nothing gets covered until the plumber's or electrician's inspection is done. See utility excavation.
Septic tanks and drainfields are backfilled the way the permit spells out, and only after the inspector from Central District Health or Southwest District Health has signed off. Tanks get careful, even backfill on all sides so they don't shift. Drainfield trenches don't get heavy compaction, because the soil there has to keep draining. More on that on our septic system installation page.
Timing Backfill Around Inspections and Weather
Backfill is the step everyone is waiting on, and it's also the one that shouldn't be rushed. Footing drains, dampproofing, pipe and conduit all have to be inspected before they're buried. Wet clay won't compact and frozen clods leave voids, so a stretch of rain or a hard freeze can push backfill back a few days. Tell us your inspection date and we'll plan around it, so an open hole next to the house isn't left sitting longer than it needs to.
Final Grade That Slopes Away From the House
Backfill ends with grade, not a pile that's been knocked flat. The code calls for at least 6 inches of fall in the first 10 feet away from the foundation. We shape the backfill to that, keep the dirt a few inches below the siding, and leave the downspouts somewhere to send their water.
On Boise lots with clay soil this matters a lot. In older neighborhoods like the Bench and Vista, the ground next to many houses has settled flat or even back toward the wall over the decades, and reworking that backfill zone is a common fix.
Why Hire Us for Backfilling
We're an Idaho registered contractor (RCE-65836) with a 5.0 rating on Google. One customer had us put in a stone block retaining wall and drainage around their home and said we "were timely, professional, and paid attention to all the small details." Call (208) 697-2429 or request a free estimate.
