Design and construction outlined of the Cannon mine tailings impoundment

Introduction The Cannon mine at Wenatchee, WA has recently been brought into operation to recover gold from the sandstones of the Swauk formation. The mine is a joint venture between Asamera Minerals (US) Inc. and Breakwater Resources. A tailings impoundment was constructed to impound 2.7 to 3.6 Mt (3 to 4 million st) of tailings at 1.8 kt/a (2000 stpd) over about five years. This paper describes the design and construction of the impoundment. In particular, it describes the 100 m (300 ft) high earthfill embankment that is the main feature of the facility. Project environment The project is located on the outskirts of Wenatchee, a town of about 40,000 people. The town is situated on the flat alluvial terraces beside the Columbia River. The Cascade Mountains rise from the terraces in a series of ridges and valleys. The mine is at the foot of the Cascades. The tailings impoundment is in one of the steep valleys. The climate is mild and dry (Table 1). Site selection Twelve alternative sites within a 16-km (10-mile) radius of the mine were examined. Decision making theory was used to select the most suitable site. Advantages of the chosen site include its proximity to the mine and its low visibility from points of common public access. Also, the area was already heavily impacted by an existing silica quarry. The major disadvantage of the site is the need to pump tailings over a distance of 1.5 km (1 mile) and against a static head of 200 m (655 ft). Also, it is located in a gulch just upstream from the town of Wenatchee. Site geology The impoundment is generally underlain by interbedded sandstones and siltstones. The siltstones are often shattered. The sandstones are often friable. Both rock types are extensively jointed. Some isolated faulting of the sediments occurred before the area was covered with basalt flows. About 10,000 years ago, at the end of the ice age, the basalts were covered by 3 to 10 m (10 to 33 ft) of wind blown loess - a fine sandy clay and silt. River deposits filled the base of the valley to depths between 5 and 25 m (16 and 82 ft). Under the main part of the embankment, these materials were removed to expose bedrock. Ground water flow is generally through the more permeable layers. The permanent water table is about 5 m (16 ft) beneath the ground surface. Project and impoundment layout Figure 1 shows the layout of the mine, the mill, and the tailings impoundment. The tailings deliv-