Hydraulic Barrier Wall at Common Creek Reservoir
The Common Creek Reservoir project included a hydraulic barrier wall between the reservoir storage and adjacent groundwater table.
Located approximately 1km southwest of Copper Cliff, Sudbury, Ontario, Common Creek comprises a nominally flat marshland bounded by granitic outcrops to the southeast. The Common Creek flows through the marshland and passes through a culvert beneath a Canadian Pacific Rail (CPR) railway, into a riprap lined outlet ditch.
The Common Creek Reservoir was designed by Klohn Crippen Berger Ltd (KCBL) for CVRD Inco (now Vale Canada Ltd) to reduce the normal flow from Common Creek up to a designated design flood event, to prevent overloading of CVRD Inco’s Copper Cliff Waste Water Treatment Plant.
The reservoir connects to the outlet ditch north of the railway tracks, through an outlet decant pipe running underneath the tracks. Flows beyond designed values are discharged through the CPR culvert, into the outlet ditch, via a spillway.
Sub-surface conditions at the site are varied with up to 10ft of peat and organic soils in the northeast of the site, and immediately adjacent to the CPR tracks. The peat is underlain by varying thickness of sandy silt which in turn, is underlain by soft to stiff clays.
Groundwater at the site is at or near surface with recorded low pH and high metal concentrations.
Due to the relative close proximity of the CPR track to the reservoir, and the presence of potentially compressible peat, it was determined that the effects of reservoir construction and operations on the adjacent groundwater table should be minimized, to prevent significant consolidation settlement beneath the CPR tracks.
As such, key design criteria for the reservoir included a low permeability hydraulic barrier wall between the reservoir storage and adjacent groundwater table.
Hydraulic Barrier Wall
Several options were considered including a HDPE liner, Geosynthetic Clay Liner (GCL) and a sheet pile wall. Evaluation was based on permeability requirements, longevity, cost and ease of construction.
The final reservoir design incorporated approximately 2350ft of vinyl sheet pile wall, comprising 10 and 20ft long sheet piles, supplied by Wolf Remediation Ltd.
This design decision was based primarily on;
• Achieving equivalent permeability of 10-9 m/s with a hydrophilic sealant
• Possibility of an operational life in excess of 50 years in low pH conditions
• Total cost, including installation was within an acceptable range of the other options
• The high existing groundwater level at the site meant installation of a HDPE liner or GCL would have required extensive water management works. These works could have potentially lowered the groundwater table beneath the CPR tracks, possibly resulting in consolidation settlement. The vinyl sheet pile wall could be installed prior to construction works, effectively isolating upstream works from the adjacent groundwater table
Installation of the sheet pile wall was performed by local contractors and took approximately 7 weeks. Overseen by Wolf Remediation Ltd, the sheet piles were installed through the peat and sandy silt, into the underlying clays. Approximately 5ft was left exposed above the existing ground level to form the low permeability component of the majority of the retention dyke.
To assess the performance of the sheet pile wall, and the effect of the reservoir operations in general, KCBL designed an instrumentation program to monitor pore pressures downstream and the phreatic surface across the wall respectively. Using piezometers adjacent to the CPR tracks, and observation wells on the upstream and downstream sides of the wall, data was recorded for a two year period following installation, showing positive results.
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