Excavation & Site Preparation · Duncan
Perimeter Drains and Ditching in Duncan
Perimeter drains, ditching and service trenches — dug to a depth that accounts for where the water actually sits.
Duncan Excavating installs perimeter drains, ditching and service trenches across the Cowichan Valley. Trench depth drives the work: under the OHS Regulation any excavation over 1.2 metres that a worker enters must be sloped, benched or shored, and excavated material has to stay 60 centimetres back from the edge.
Drainage is the quietest line item on a build and the one most likely to be regretted. A drain that works is a matter of depth, fall and where the water is being sent, and all three are decided by ground that varies from one end of this valley to the other.
Trenching is commonly priced by the metre rather than by the job, and no Vancouver Island contractor publishes a rate. What moves the number is depth, whether the trench passes the 1.2 metre line and needs shoring, how much of the run is rock, the distance to a legal outlet, and whether existing landscaping has to come out and go 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
An outlet is a regulatory question before it is a technical one. 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
- 1
Find the fall — Levels shot from the outlet backwards, because the outlet is the constraint and everything upstream has to work to it.
- 2
Locates before the trench — BC 1 Call requests are free, with member companies responding in three working days, or five for large projects.
- 3
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.
- 4
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. On the valley floor the ground is unconfined sand and gravel and drains freely, but the water table is at a median of 2.4 metres and one provincial observation well swings about 3.1 metres between its winter high in November to January and its summer low. Cowichan Bay is the opposite case: the CVRD's own community plan for that area describes much of it as underlain by highly impervious clay soils, and notes that high water volumes combined with vegetation removal on steep slopes create conditions conducive to accelerated erosion and landslide.
In practice
- Situation
- A 1970s house above Cowichan Bay taking water into a crawlspace every winter, with no working perimeter drain.
- Complication
- The lot is clay, the fall to the nearest legal outlet was shallow, and a section of the run passed within the 30 metre riparian assessment area of a watercourse.
- Resolution
- The run inside that strip was assessed by a Qualified Environmental Professional before any soil moved, and the trench was cut deeper than the original to buy fall the clay would not give up on its own.
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 costsTODO: cost guide blurb.
- Permits and setbacksTODO: permits guide blurb.
- Footings and FoundationsExcavation and backfill for footings and foundation walls across the Cowichan Valley, cut to surveyor's levels and planned around a winter water table.
- Driveways and CulvertsRural driveways, turnarounds and culvert crossings across the Cowichan Valley, including the access permits that apply on provincial and municipal roads.
- Duncan Excavating home
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Duncan Excavating quotes excavation across the Cowichan Valley after a site visit.