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 |
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 |
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. |
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). |
Fig 38: Pebbles washed out of a till inclusion in Ravenscrag strata, south abutment, Rafferty Dam. |
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. |
Fig 40: Coal layer in displaced Ravenscrag Formation strata, north abutment area, Rafferty Dam |
Fig 41: Deformed coal seam and seepage patch (blackush area), north abutment area, Rafferty Dam. |
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. |
Fig 43: Major aquifers in preglacial valleys in the Estevan, Sk. area. |
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