The City of Richmond has a population of approximately 180,000 and is located on a flood plain at approximately sea level. These physical attributes present numerous challenges with underground utility design, including sanitary sewers.
On January 21, 2005 the City declared a local state of emergency near the intersection of No. 3 Road at Cook Road due to the development of a large sinkhole (10 x 8 x 1.5 metres deep). A commercial building had partially collapsed and the City’s sanitary gravity main located at a depth of 4.5 metres, at an extreme flat grade of 0.27% and 2.0 metres off the face of a 100 metre long building, was no longer functional. City staff employed a unique methodology to return the sewer main to service and installed a City invented manhole pump/SCADA/bypass system to allow continuous sanitary service. All options to restore the collapsed sewer were exhausted starting with the seemingly least expensive methodology to repair the sewer at the collapse point leading to the adopted solution of re-routing the main. Construction of the re-routed main required a 5-metre deep excavation immediately adjacent to a 32-year old commercial building with substandard footings bearing on soft soils. With the ground water table at a depth of approximately 1.0 metre, uncontrolled dewatering would result in considerable building structural damage. Dewatering was further complicated by tidal fluctuations of the groundwater table. A grout curtain which is new technology not previously used by the City was constructed, leading to successful project completion.
BACKGROUND
The City of Richmond is part of the Greater Vancouver Regional District in British Columbia, Canada and has a population of approximately 180,000 in a region of overall population of approximately 2.2 million. The City is located on a flood plain with elevations ranging from approximately 0.6 metres to 2.5 metres above sea level. These physical attributes present numerous challenges and opportunities with underground utility design and construction. Design and construction of gravity sewers provides particular challenges given the consistent grades required between manholes and the considerable depths required to provide pipe slopes that achieve the required hydraulic capacities.
Past practice sanitary sewer design dictated that sewers be constructed within right-of-ways at the rear of properties or through right-of-ways secured from property owners. The philosophy of the time was to minimize potential utility conflicts due to congestion, to maximize separation from the potable water main system, to connect to existing septic fields and to minimize construction restoration costs. Accordingly, approximately 500 kilometres of sanitary sewer mainlines were installed in this manner over several years.
On January 21, 2005 the City of Richmond Mayor declared a local state of emergency near the intersection of No. 3 Road and Cook Road in the City Centre area. A considerable sinkhole with dimensions approximately 10 metres long x 8 metres wide x 1.5 metres deep developed adjacent to a strip of commercial buildings.
Incidental to the sinkhole was the partial collapse of a commercial building and damage to the City’s sanitary gravity main located at a depth of 4.5 metres (Photo1). This 200 mm diameter sewer design, following the design philosophy of the time, was at a very flat grade of 0.27%. Shortly after this sewer was built 45 years ago, an approximately 100 metre long strip of commercial building was constructed 2.0 metres from the sewer centerline. The sewer was located in a narrow 6.0 metre right-of-way and was near the end of the system that serves one of the more densely populated areas of the City and accordingly, carried considerable flow.