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Stop 6 - Rafferty and Boundary Dams and Reservoirs, and Estevan/Bienfait Coal Mines: Geology, Hydrogeology, Geotechnology

The Estevan-Bienfait area is a paradise for geotechnical engineers, hydrogeologistsand geoscientists wanting to observe and explore Tertiary bedrock structure and Quaternary overburden stratigraphy, structure and associated earth processes and geotechnical problems.

Let's begin with Rafferty Dam, move east to view openpit coal mines, then on to the Shand Power Plant and its

Figure 36
Fig 36: Layout of Raffery Dam and appurtenant works.

associated native prairie flora research centre, winding up at the museum in Bienfait--all before we head back to Regina.

Rafferty Dam-Site Geology

Rafferty Dam site (Fig. 36) presented geotechnical and geological engineers

Figure 37a
Fig 37a: Massive jointed sandstone rock, south abutment, Rafferty Dam.

with some of the most challenging exploration, design and construction problems imaginable: thick compressible postglacial alluvial and lacustrine sediment in the valley bottom, including underconsolidated highly plastic clay strata that accumulated under water and was never exposed, along with exposed and desiccated intercalated intervals of waterlaid plastic clay--the entire valley bottom package of postglacial sediment reaching some 25 m (80 ft) in thickness.

Figure 37b
Fig 37b: Rills (some following systematic joints), in Rafferty Dam south abutment.

The south abutment consists of a thin bouldery till overlying jointed Ravenscrag Formation sandstone, predominantly blocky, with thinner beds of jointed siltstone and shale (Figs. 37a and Fig. 37b. An inclusion of till in bedrock occurs high in the south abutment (Fig. 38). Relatively fresh-looking road cuts leading to the north end of Rafferty Dam reveal unusually large concretions having the shape of a hospital bedpan (Fig. 39). Farther along our traverse, the north abutment and spillway and fill-borrow areas have all been affected by glacier thrusting. This is evident in a displaced, steeply dipping slab of displaced coal surrounded by silty Ravenscrag sediment in a borrow pit. Also note moisture seeping from the excavated abutment face (Fig. 40); an arched, anticlinally deformed, thin coal seam (Fig. 41); and shiny, freshly excavated slabs of waxy-looking slickensided bentonitic shale associated with "glacial shear" (Fig. 42).

Figure 38
Fig 38: Pebbles washed out of a till inclusion in Ravenscrag strata, south abutment, Rafferty Dam.
Figure 39
Fig 39: Large sandy concretion in shale, in road cut leading to the north end of Rafferty Dam.

Rafferty Dam project engineers Jim Dobson and Scott Manson will provide more detailed information on how the many and varied dam foundation problems were handled, including the innovative exploration, design and installation of a closelyspaced (about 5 m) grid of wick drains to remove water from the foundation clay and reduce pore-water pressure from building up during and following construction.

Aggregate sources for concrete manufacture were located nearby. Boulder sources for riprap were also located close by on the densely boulder-studded Souris Spillway upland. Note the boulder riprap placed on the dam's upstream face, as well as along the adjoining reservoir shoreline subject to wave and current erosion.

Figure 40
Fig 40: Coal layer in displaced Ravenscrag Formation strata, north abutment area, Rafferty Dam
Figure 41
Fig 41: Deformed coal seam and seepage patch (blackush area), north abutment area, Rafferty Dam.
Figure 42
Fig 42: slab of shiny, slickensided, waxy-looking shale, north abutment, Rafferty Dam.

Groundwater Development to Replace Evaporation Losses from Boundary Reservoir, the Source of Cooling Water for the Boundary Thermal Power Plant

Unusually high evaporation losses from Boundary Reservoir, estimated to be 7000 imperial gallons per minute (igpm), was a major concern at the Boundary Dam Thermal Power Station. To replace this water, which is used for cooling the coal-fired power plant, a major groundwater exploration and development program was initiated. It resulted in developing 4 water wells at about 120 m depth in the 70-km-long, 3- to 4-km wide preglacial Estevan Valley Aquifer, each producing 500 igpm; and another 500 igpm from each of 4 water wells in subtill Empress Formation sand overlying Ravenscrag Formation bedrock, at an average 30-m depth in the Tableland Aquifer (Fig. 43). However, the 500 igpm from 8 wells, or 4000 igpm in total, could not keep up with Boundary Reservoir evaporation losses. Both aquifer systems required a large amount of test drilling to discover, block out, and develop the 8 high-capacity water wells. Because the Estevan Valley Aquifer static water level was lowered a few tens of metres,it was necessary to deepen some existing farm wells and also install new wells for farmers whose water level in their wells had dropped significantly.

Figure 43
Fig 43: Major aquifers in preglacial valleys in the Estevan, Sk. area.

© J.D. Mollard and Associates Limited

   

    Last Modified: 2004-12-10