Trench for a new sewer connection keeps caving in Fredericton, why?

Why a Sewer Trench Keeps Caving In

A trench that collapses repeatedly is a serious safety situation. Everyone should stay out of it and back from the edges until the cause is identified and the walls are properly stabilised. That is not a judgment call: WorkSafeNB's General Regulation 91-191 requires that any trench 1.2 m (about 4 ft) or deeper that workers need to enter must be shored, sloped or benched, or cut in solid rock, and any shoring must be certified as adequate by an engineer. Ask the contractor how the walls are being supported.

What causes caving has to be judged on site by the contractor or an engineer, because ground conditions vary from lot to lot and cannot be assessed from a description. Factors a contractor will look at include how the walls were cut, whether the trench is deep enough to require support under the WorkSafeNB rule, whether water is entering the trench (the regulation requires it to be kept reasonably free of water), and whether excavated material has been kept at least 1.2 m from the edge as the regulation requires. Regulation 91-191 also requires that loose material be removed before anyone enters, and that a ladder extending at least 1 m above the top be within 15 m of where a worker is.

On the permit and inspection side, the City of Fredericton requires a building permit before new work begins, and the City's Engineering Division reviews water and sewer services at the permit stage. New water and sewer services in Fredericton are installed by the City's Water and Sewer Division, so ask the City directly how the connection is being arranged and who is responsible for which part of the trench. Regulation 2021-2 under the Building Code Administration Act requires a footing and foundation inspection before any backfilling begins on a new build; ask the City's building department how inspections are booked for your project.

Before any digging continues, confirm that both locate requests were made: Info-Excavation through Click Before You Dig (three working days' notice, valid until the end date on the request) and NB Power's separate underground locate service (five business days' notice, valid for 60 days from the date on the locate form, covering only NB Power-owned lines). Any private lines, drains or tanks a past owner installed are the owner's to find and mark. Under Regulation 91-191, the employer must determine the location of underground utilities before beginning an excavation, and must notify the operating authority and follow an adequate procedure within 600 mm of any line.

Ask the contractor to show you the engineer's certification for any shoring being used, and ask how they plan to keep the trench free of water while work continues. The contractor should also carry a WorkSafeNB clearance certificate confirming their account is in good standing. Under the Construction Remedies Act, you must retain a holdback of 10% of the contract price for 60 days after substantial performance.

Because the trench is actively failing, the right first call is to the contractor to stop work and assess the walls, and then to the City of Fredericton's building and engineering offices to confirm what permits, inspections and City-side responsibilities apply to your connection.