The Seymour Falls Dam holds a reservoir that supplies nearly 40 per cent of the drinking water for the Greater Vancouver Regional District (GVRD), so its ability to withstand an earthquake is critical. Located in the mountains above North Vancouver, the dam is in an area of high environmental value, close to a fish hatchery and within the Lower Seymour Conservation Reserve.
The original dam was completed in the early 1960s as a composite earthfill and concrete structure. It consists of a 220-m long, 30-m high embankment buttressed by a concrete gravity retaining wall. Also, a concrete transition block connects the earthfill dam to the 220-m x 30-m concrete slab and buttress portion of the dam.
As it no longer met the criteria for earthquake safety, the Greater Vancouver Water District (GVWD) carried out a seismic upgrade to ensure the structure could withstand a “maximum credible earthquake.”
Vancouver-based engineering consultants Klohn Crippen Berger were the prime consultant on the project and engineered the upgrade to the earthfill dam. Hatch Energy were subconsultants on the concrete dam portion. The Greater Vancouver Regional District provided design oversight.
Keeping the reservoir operational
Work on the concrete dam could be performed mostly from the dam crest or downstream face of the original structure, which presented minimal risk to the continuing operation of the reservoir and to the environment.
However, in order to keep the reservoir operational while upgrading the earthfill dam, it was decided to build a new earthfill dam downstream, on an improved foundation. The new earthfill dam acts as a buttress to stabilize the downstream slope of the old dam. The new dam is a standalone structure, but the impervious cores of the old and new dam are connected and provide a single continuous component.
The new earth fill dam’s base is wide enough to provide for the top of the dam to be raised in the future.
Explosive and dynamic compaction
The seismic design for the new earthfill dam required densification of the new dam foundation to depths of almost 30 metres below the original ground level. The depth is beyond the range of conventional dynamic compaction methods, and the abundance of boulders in the foundation precluded the use of low-impact methods such as vibro-compaction.
The solution was an innovative combination of dynamic compaction and explosive compaction, combined with some foundation excavation. A trial program was completed prior to tendering to demonstrate the approach’s viability and safety.
The design approach was to achieve ground densification up to the toe of the old earthfill dam.
Setting off 30,000 kilograms of high explosive charges in a small and environmentally sensitive area, next to an operating dam, and in an urban setting, is no small feat.
By continually modifying and improving the blast sequence and layout, and by strictly controlling the detonation of each charge, the blast program was designed so that the vibrations did not affect nearby structures. Within 6 to 8 metres were an existing concrete dam gravity wall, a steel pipeline connecting the reservoir to the Greater Vancouver Water District’s distribution network, a water treatment chlorination plant and the existing toe of the operating dam. It was also important to monitor nitrate levels following the blasts to ensure the water quality in the adjacent Seymour River and fish hatchery met the fisheries’ requirement.
The project involved excavating 190,000 square metres of earth. The excavated materials were processed and reused as dam fill.
The greatest difficulty with the new embankment was placing the impervious core using very wet materials in a wet environment. It required innovative methods of blending the clay/silt with sandier silt, or substituting local till materials. For each alternative, additional design, reviews and stability analysis was required.
Upgrading the concrete dam
Hatch Energy’s upgrade of the concrete dam used EcoSmart concrete, which reduces greenhouse gas emissions. Post-tensioned strand anchors were used to strengthen the spillway piers.
The requirements for the upgrade were determined by creating a series of complex 3D finite element models of the structure. A dynamic time history analysis was then done, which incorporated assessing slip displacement techniques to reduce the stabilization and upgrade requirements. The results allowed the engineers to significantly reduce the number and length of anchors compared to what would have been required using conventional modelling techniques.
Prior to construction, the Greater Vancouver Water District upgraded the fish hatchery infrastructure, and since the dam was completed in December 2006, the GVWD has been restoring the area, including planting 7,000 trees. It has also constructed a new viewing area and trail to the hatchery.CCE
Name of project: Seymour Falls Dam Seismic Upgrade, Vancouver
Award-winners: Klohn Crippen Berger, Vancouver (prime consultant) (Len Murray, P.Eng., Neil Singh, P.Eng.); Hatch Energy, Vancouver (subconsultant,concrete works) (David Daw, P.Eng., Wayne Lozinski, P.Eng.); GVRD (design oversight) (David Siu, P.Eng., Frank Huber, P.Eng.)
General and civil contractor: Peter Kiewit Sons Co.
Hatchery water supply: Kerr Wood Leidal Associates
Other key players: ECI Explosive Compaction (explosive compaction contractor); Geopac West (dynamic compaction contractor); Pacific Liaison Assoc. (project manager)
New earthfill dam and gravity wall extension at left, with the original earthfill dam showing as a grassy ridge along the top; the concrete slab and buttress dam is at right.
Early construction of the gravity wall extension that retains the new earth fill dam.