Construction
The site chosen was on a stretch of the Missouri River flowing from south to north. The river bed at the site consisted of approximately 160 feet (49 m) of alluvial deposits, varying from coarse, pervious sands and gravels to impermeable clays. Beneath these deposits lay a thick (approximately 1,000 feet (300 m)) deposit of Bear Paw shale. This shale is classified as a firm shale and contains thin (<1 to 6 in (2.5 to 15 cm) layers of bentonite. The topmost layer of soft clay was removed from the alluvium in order to found the dam on the stable sandy deposits beneath, at an elevation of approximately 2,050 feet (620 m). The remaining deposits consisted of the alluvial materials mentioned above. These deposits had many interconnecting layers of coarse sands and gravels, necessitating the installation of a steel sheet pile wall down to the firm shale, from the left to the right abutment.
The dam was planned to extend to an elevation of 2,275 feet (693 m), for a total height of 225 feet (69 m) from the cleared river bed and had a length from the left to the right abutment of approximately 10,500 feet (3,200 m). The upstream face was designed with an average slope of one vertical on four horizontal and included three horizontal shelves built into the slope. A flatter (1 on 7.5) berm was to be placed between stations 30+00 and 75+00 (approximately the center half of the length of the dam).
Since the construction method of hydraulic fill was chosen, four electric dredges needed to be built. Due to the distance of the site from the nearest shoreline, a shipyard was started on the site, affectionately dubbed “The Fort Peck Navy” and “The Biggest Shipyard in Montana” by the workers. These dredges would pump material from nearby borrow pits to the dam site where it was discharged by pipes along the outside edges of the fill. The coarser material settled out quickly, while the fines were carried downhill toward what would eventually become the core of the dam. Samples were take from all zones regularly to ensure that the material had the gradation and consolidation characteristics specified by the design.
Read more about this topic: Fort Peck Dam
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