Mildred Lake Settling Basin (MLSB)
The Mildred Lake Settling Basin is located on the north side of the Mildred Lake lease area. It is a tailings pond that serves three purposes. Firstly, the embankment was planned as storage for a substantial volume of sand. Secondly, the basin acts as a storage basin for process water, which is recycled, with a planned ultimate storage capacity of 350×106 m3. Thirdly, the fines that are not captured elsewhere settle and compact in the basin, and are later pumped out for long term storage. This means that the MLSB is a true dam in the sense that it is filled with water in the long term, rather than being quickly filled by solids as in many other tailings dams. The embankment has a circumference of about 18 km, an average height of about 40 m and a maximum height of about 88 m.
Two starter dams were constructed during 1976 to 1978 and were required until sufficient sand was available for building the embankments. The north starter dam had a crest elevation of 312 m. The original ground surface varied from 294 m to 305 m while up to 1.5 m of original ground was stripped for a trench above which was a compacted clay core. The crest width was 30 metres.
The main embankment was taken to a final elevation of 352 m for more than half of its length by 1994 and completed in 1995. For construction purposes the embankment was considered to be in a collection of 30 “cells”, each with a crest length of about 600 metres. Acceptable side slopes were determined on a cell-by-cell basis, based on the strength of available materials and foundation movement. The slope of the outer part of the embankment is much smaller than that of the inner part, in a ratio of about 4:1.
As this dam is frequently appears in lists as the largest dam structure in the world, it is necessary to estimate the accuracy of the quoted volume of construction material. The quoted the length of the embankment of 18 km is reliable. The average height of the embankment is quoted to be 40 m, and a check on this using four cross sections yields 45 m, which is of the same order of magnitude. The average base width of the embankment is variously 1,800 m, 800 m from Google Earth and 660 m. So whereas one report gives an embankment volume of 720×106 m3, calculations based on the width of the embankment base from these three sources give embankment volumes of 660, 290 and 240×106 m3 respectively. So there's some uncertainty as to the total volume of construction material
Read more about this topic: Syncrude Tailings Dam
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