Trenching & Backfill for Landscape Lighting: Sand vs Gravel vs Screened Soil in Winnipeg
- Ditchfield Soils
- Jan 2
- 5 min read
Here's a conversation we've had about a hundred times:
Homeowner: "My landscape lights are all crooked after one winter. I guess I bought cheap fixtures."
Us: "What did you backfill the trenches with?"
Homeowner: "The dirt I dug out. Why?"
That's why. And that's what this article is actually about.
Most landscape lighting problems blamed on "poor quality fixtures" actually come from what's happening underground. Winnipeg's frost goes down 1.8-2.4 meters every winter, among the deepest in Canada. Your landscape lighting wires sit maybe 15-30 cm deep.
That puts them right in the zone where soil freezes, expands, heaves upward, then settles back down. Repeat this 8-12 times per winter (thanks, Manitoba freeze-thaw cycles), and you've got tilted fixtures and stressed wiring.
University of Manitoba engineering research and regional studies suggest a majority of premature landscape infrastructure failures in Winnipeg are frost-related.
The fix isn't better fixtures. It's better trenching and backfill materials.
Why Depth and Materials Actually Matter
When water in soil freezes, it expands. That expansion pushes everything upward with enough force to crack concrete and lift buildings. A little landscape lighting fixture? No contest.
By spring, your level in October installation leans at 15 degrees. Wiring that was slack is now pulled taut. Connections stressed from repeated movement start failing. You blame the fixture. The fixture's fine, it just can't overcome physics.
Three things cause this:
Frost heave from freezing soil pushes fixtures up and tilting them.
Poor drainage creates ice pockets that expand and damage mounting hardware.
Settling from inadequate compaction that creates voids where water collects and freezes.
Manitoba Infrastructure research shows proper drainage alone reduces winter damage by 78%. Add correct materials and adequate depth, and you're looking at a 15-20 year fixture life instead of 3-5 years.
How Deep Should You Actually Go?
The
Canadian Electrical Code says 15 cm minimum for low-voltage landscape lighting. That's code-legal. It's also inadequate for Manitoba conditions.
We typically trench 30-45 cm for main runs, 20-30 cm for branch lines to individual fixtures. Yes, it's more work. But you're getting your wires below the zone of most active frost movement and protecting them from surface disturbance (snow removal, cultivation, kids playing).
For width, 15-20 cm works for most single-conduit runs. Gives you room for proper bedding material around the conduit without making trenches unnecessarily wide.
And speaking of conduit: use Schedule 40 PVC rigid conduit, not that flexible stuff. Rigid conduit protects wiring from frost heave damage and makes future maintenance actually possible. Canadian Standards Association guidelines and best practices for frost-prone regions strongly favour rigid conduit over flexible options.
What to Put Back in the Hole
This is where most DIY installations go wrong. Here's what actually works:
Sand
Good for: Bedding layer directly around conduit (5-8 cm)
Why it works: Flows easily around pipes, no sharp edges to damage insulation, stays workable in cold weather
Why not to use it for everything: Poor compaction leads to settling, holds moisture through capillary action, frost-susceptible when wet
Clear Stone / ¾-inch Gravel
Good for: Primary backfill layer (15-25 cm) over sand bedding
Why it works: Excellent drainage preventing water accumulation, resists frost heave because minimal fine particles, provides structural support
Why it costs more but matters: This layer is what actually protects against frost heave. Clean washed stone with less than 3% fines content (the really small particles) is key.
Dirty stone or crusher run with lots of fines doesn't drain and will heave.
Screened Topsoil
Good for: Final 5-10 cm surface layer where you need grass to grow back
Why it works: Supports vegetation, looks natural, compacts reasonably when moisture-conditioned
Why only on top: Organic content causes long-term settling. Variable quality. Can be frost-susceptible depending on clay/silt content. Use it to restore appearance, not as structural backfill.
The Four-Layer System That Actually Works
Professional installations don't just dig a hole and fill it back in. We layer materials to optimize each one's strengths:
Bottom layer (5-8 cm): Clean sand Creates cushioned support for conduit. Ensures complete contact so there are no voids where stress concentrates.
Main layer (15-25 cm): ¾-inch clear stone Handles drainage. Resists frost heave. Provides a stable structure that doesn't settle.
Transition layer (5-10 cm): Sand or fine gravel Prevents stone from migrating up into topsoil. Adds drainage capacity. Creates a good base for compacting the final layer.
Top layer (5-10 cm): Screened topsoil Restores appearance. Supports vegetation. Protects underlying layers.
Total depth ends up 30-50 cm, depending on conditions. Yes, that's more material cost
than just returning the dirt you dug out. For a typical fixture, we're talking $5-8 per meter of trench difference. Versus replacing failed fixtures every 3-5 years.
Compaction Matters (But Not Everywhere)
Sand bedding gets light compaction, just enough for complete conduit contact, not maximum density.
Stone backfill needs real compaction. Compact in 10-15 cm lifts with a plate compactor or hand tamper. Target 90-95% of maximum density. Skip this and you'll have settling within a year.
Topsoil gets light compaction to prevent excessive settling while staying workable for planting.
Trenches crossing driveways or walkways need heavier compaction (95-98% maximum density) to prevent future surface depression.
A Few Things Worth Mentioning
Warning tape: Electrical warning tape placed 15-20 cm below the surface warns future diggers that there's something down there. Manitoba code doesn't require it for low-voltage, but we include it. Prevents someone from putting a shovel through your lighting three years from now.
Geotextile fabric: Landscape fabric between soil layers prevents mixing while allowing water movement. Use it between native soil and stone backfill, and anywhere you're worried about contamination between layers.
Documentation: Take photos before backfilling. Make a sketch showing where things actually are. Future you will appreciate this.
Common Mistakes
Digging only the code minimum depth (15 cm). Going deeper costs effort now, saves replacement costs later.
Returning excavated soil to trenches. Creates identical conditions to what caused frost heave in the first place.
Poor or no compaction. Trenches settle 10-20%, creating depressions that pool water.
Treating trenches like they're just channels for wire. They're actually drainage systems that happen to contain wire; design them accordingly.
Working in frozen ground. Wait for thaw or plan installations for non-frozen periods. Proper excavation and compaction are impossible in frozen ground.
How Much Material You'll Need
For a typical trench (20 cm wide × 40 cm deep):
Sand bedding: 0.04 cubic meters per linear meter
Stone primary backfill: 0.05 cubic meters per linear meter
Topsoil restoration: 0.02 cubic meters per linear meter
Example: 30-meter lighting run needs about 1.2 cubic meters of sand, 1.5 cubic meters of stone, and 0.6 cubic meters of topsoil.
Ditchfield Soils supplies properly graded materials for landscape lighting installation, clean sand without clay contamination, washed clear stone in consistent ¾-inch sizing, and screened topsoil tested for appropriate composition. Our materials are spec'd for drainage, compaction, and frost resistance in Winnipeg conditions.
The cost difference between economy fill and proper materials is maybe $5-8 per fixture location. The performance difference is 5-year life versus 20-year life.
When to DIY vs. Hire Out
DIY can work if you're willing to do it right:
Trench to 30-45 cm depth minimum
Use proper layered backfill (not just returned dirt)
Compact in appropriate lifts
Use rigid conduit throughout
Include warning tape and documentation
Professional installation includes site assessment for drainage patterns and frost-susceptible areas, proper equipment for excavation and compaction, code-compliant electrical work, and warranty protection (most manufacturers require professional installation).
Cost difference: $50-150 per fixture, including labour and materials. But professional work typically delivers a 15–20-year service life versus 3-7 years for many DIY installations. Over the long term, the initial cost difference disappears.
Lawn 'N' Order landscape lighting installations include proper trenching and backfill as standard practice. We've been doing this in Winnipeg long enough to know what works and what fails after the first hard winter.
Related Resources:
National Research Council Canada: Frost Heave Research
Canadian Standards Association: Electrical Installation Standards
Trenching and backfill determine landscape lighting longevity more than fixture quality. Proper depth, appropriate materials, and correct layering transform lighting from recurring maintenance into reliable long-term infrastructure surviving Manitoba's challenging frost conditions.
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