At Silver Islet, Lake Superior, mining was successfully carried on for years under the protection of a coffer dam and an arch of rich silver ore less than 20 ft.
The experiment was repeated year after year till 1867, when the barrage cracked right across from foundation to top. A massive coffer-dam was then erected, covering the eleven arches nearest the crack; but the work was never trusted again, nor the water-surface raised more than about 3 ft.
The work included a massive new concrete foundation, hence the necessity to build the coffer dam.
