Excavation & Site Preparation · Duncan
Perimeter Drains, Foundation Drainage & Service Trenching in Duncan
Perimeter drains, ditching and service trenches — dug to a depth that accounts for where the water actually sits.
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The more detail about the property and the work, the closer the first number will be. We send a written quote after we visit your site.
Duncan Excavating installs perimeter drains, foundation drainage repair, ditching, and water and sewer service trenching across Duncan, North Cowichan and Cowichan Bay. Under the OHS Regulation, we slope, bench or shore any trench over 1.2 metres that a worker enters, and we keep all excavated spoil at least 60 cm back from the edge.
Proper drainage depends on correct depth, slope fall and a clear outlet. Because soil conditions change across the valley, we design your system for your specific lot.
Trenching is commonly priced by the metre rather than by the job. Five things change your price: how deep we dig, whether the trench passes the 1.2 metre line and needs shoring, how much of the run is rock, how far it is to a legal outlet, and whether we have to lift landscaping out and put it back.
Depth, shoring and spoil
The OHS Regulation sets the thresholds that shape a drainage quote. Any excavation over 1.2 metres that a worker enters has to be sloped to a specified angle, benched, or supported by shoring or a trench box. Excavated material must be kept at least 60 centimetres back from the edge of a trench and 1.2 metres from any other excavation, and where a spoil pile beside a supported trench averages more than 60 centimetres deep, the shoring has to be selected for the extra lateral pressure. Past 6 metres, or where a structure sits next to the dig, written instructions from a qualified registered professional are required.
Where the water is allowed to go
We confirm where your drainage water can legally discharge before we plan the trench line. Under the Riparian Areas Protection Regulation, which applies across the Cowichan Valley Regional District, the riparian assessment area is a 30 metre strip on each side of a stream measured from the stream boundary, and the regulation defines development to include the addition, removal or alteration of soil — which is exactly what a trench is. Work inside that strip needs an assessment by a Qualified Environmental Professional, and the CVRD building permit application asks for that declaration up front.
What a drainage scope usually covers
Perimeter drain installation
Drain tile, filter cloth and drain rock laid to continuous fall around a foundation.
Ditching and roadside drainage
Open ditching and reprofiling where surface water needs somewhere to run.
Service trenching
Trenches for water, sewer and utility services, cut to the depths those services require.
Outlet and connection work
Getting the run to a legal outlet, including the setbacks that apply near a watercourse.
How a drainage job runs
- 01
Find the fall — Levels shot from the outlet backwards, because the outlet is the constraint and everything upstream has to work to it.
- 02
Locates before the trench — BC 1 Call requests are free, with member companies responding in three working days, or five for large projects.
- 03
Dig, protect, lay — Trench cut to depth with spoil set back from the edge, sloped or shored where it passes 1.2 metres, then pipe and rock laid to the fall.
- 04
Backfill and reinstate — Backfilled in lifts and the surface put back, which on an established property is often the visible half of the job.
Local conditions
Two different drainage problems sit within twenty minutes of each other here. Valley-floor sand and gravel drains freely, but groundwater sits at a median depth of 2.4 metres. Local observation wells show water levels swinging about 3.1 metres up to their winter peak between November and January. In Cowichan Bay, heavy clay soils do not absorb water quickly. The CVRD community plan warns that clearing slope vegetation allows heavy rainfall to cause rapid erosion and landslides.
People also ask
How deep does a perimeter drain need to be?
Below the footing it is protecting, with continuous fall to an outlet — the depth is set by the building, not by a standard figure. What is fixed is the safety threshold: past 1.2 metres the trench must be sloped, benched or shored before anyone enters it.
Does a drainage trench need a permit?
Often not on its own, but the surrounding rules still apply. Within 30 metres of a stream boundary or the top of a ravine bank, the Riparian Areas Protection Regulation requires an assessment by a Qualified Environmental Professional, and in the Agricultural Land Reserve drainage works are exempt from a Notice of Intent only up to 320 cubic metres per 16 hectares a year.
Can a new drain tie into an existing one?
Only if the existing line has the capacity and the fall, and on a house of any age that is worth checking rather than assuming. A camera inspection answers it before a trench is opened rather than after.
Serving nearby communities
Keep exploring
- Compare excavation costsOur hourly machine rate, typical project ranges, and the tipping and permit fees for your job.
- Permits and setbacksWhich authority governs a parcel, the setbacks and depth thresholds that apply, and the notices that have to be filed first.
- Footings and FoundationsWe handle foundation digging and footings in Duncan, cutting strictly to surveyor's levels and planning around the winter water table on every new build.
- Driveways and CulvertsDriveways, access road grading and culverts in the Cowichan Valley. We handle the access permits on provincial and municipal roads for you.
- Duncan Excavating home
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We quote excavation across the Cowichan Valley after a site visit.