Hydro Quebec

The Sainte-Marguerite-3 (SM-3) Hydroelectric Project is located 90 km northwest of Sept-Iles City in the northeastern part of Quebec, Canada. The project includes a 160 m high rockfill main dam and 882 MW underground powerhouse and is the third project on the river, upstream of the St. Lawrence River. A section of the river are shown below. SM-3 is a Hydro-Québec project while the two powerhouses downstream are privately owned.

The reservoir has a surface of 253 km2 and retains a total volume of 12 634 hm3 with a live storage of 3272 hm3.

Longitudinal Section of Powerhouse Area

The underground powerhouse is 27 m wide, 39 m high and 106 m long. It will be equipped with 2 Francis turbines with room for the installation of a 3rd unit if required. The 2 units produce 870 MW, under a net head of 330 m. The average annual amount of energy produced will be 2,73 Twh with a utilization factor of 37,5 %. The rock in the powerhouse area is an anorthosite and the 8,3 km long power tunnel is generally in anorthosite or migmatic gneisses, except for some gabbro intrusions. Local sections of the migmatic gneisses present weaknesses. The tunnel is 11,5 m by 16,5 m and unlined, except for a certain distance upstream of the powerhouse. Wiremesh, rock bolts or shotcrete are the only reinforcement used in the powerhouse, the galleries or the outside excavations. Areas of pattern bolting and spot bolting are specified based on anticipated geological conditions, the geometry of the openings and the experience with a particular type of rock. Bolts of 4 m length on a square grid of 2,0 x 2,0 m or 2,3 x 2,3 m are most often used in pattern bolting while 6 m long bolts are used in areas of larger spans such as in the machine hall crown and tunnel intersections.

In open cut excavations, pattern rock bolting is specified on portal walls as well as on the sidewalls for a distance of 30 to 50 m from the portals.

In underground openings, pattern rock bolting is specified near portals, at tunnel intersections, at the collar of the surge and bus bar shafts and in the crown of all tunnels in the vicinity of the powerhouse, such as the main access tunnels, the gate chamber, the draft tubes, the penstocks and the collector.

The proportion of pattern bolting represents less than 2 % of the total length of the various tunnels. Elsewhere, spot bolting is specified. The density of bolts in the crown in spot bolted areas corresponds to 30 % of the density of bolts for pattern bolted areas. In small tunnels and shafts, spot bolting is generally specified with lengths of 2 and 3 m.

The spillway, at elevation 407 m, is located in a side valley and is designed to handle a flood of 1 in 10 000 years of 1875 m3/s.

The dam is located at kilometer 90 in a valley with relatively steep abutments and is founded on good rock. A portion of the downstream shoulder is built over a certain thickness of slide material considered dense enough to avoid deformation. The crystalline, igneous and metamorphic rocks which underlie the SM-3 area, belong to the Grenville geological province and form part of the Canadian Shield. Large exfoliation joints, due to glacial rebound, are located mostly near the surface and are parallel to the topography: these are often opened and filled with sand and clay. Most of the other joint systems are steeply dipping to sub-vertical and are generally water tight. The rockfill dam has a legth of 360 m and a crest width of 10 m.

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