2003-08-27 CC Packet - Laserfiche WebLink

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sand, and silty sand. Alluvial soils have been deposited above the till. Some mixed till soils and topsoil also appear above the Till, very likely as fill. Normally, a glacial till can provide adequate support for an elevated tank of this size. That is, Till can normally provide a bearing capacity of 3,000 psf with a total settlement of I" and a differential settlement of approximately ~". These are acceptable soil properties for an elevated water tank foundation. However, based on the labora,tory consolidation testing that was done on the standard penetration boring test samples, the data suggests that this particular (glacial) Till has been significantly over-consolidated by the glaciers as they retreated. Based on the laboratory testing, the total settlement was judged to be on the order of 5". Also based on this testing, the differential settlement was anticipated to be on the order of 2 W'. The differential settlement value is completely unacceptable for an elevated water tower. Beneath the (glacial) Till, the deep boring encountered Dolostone, which appears to be part of the Shakopee formation. The Dolostone was somewhat weathered and could be a gravity deposited rock, but bedrock maps published by the Minnesota Geological Survey indicate that contact with bedrock should be in the vicinity of the depth that the boring indicated. Because the standard penetration borings forced us to conclude that we had unacceptable settlement values, the use of one day's work of an electric cone penatrometer (CPT) was authorized. This allowed the data to include six additional tests to a depth of approximately 45 feet. The goal of the CPT use was to quantify the apparent uniformity of the soft soil in the hope of reducing both the total settlement prediction and more importantly the differential settlement prediction. The results of the CPT testing did, in fact, show that the glacial till at the anticipated foundation depth w~ in fact very uniform. However, it also verified that it was significantly over consolidated. The total settlement prediction was reduced from 5" to only 4" and the differential settlement prediction was reduced from 2 W' to approximately I". Unfortunately, even I" of differential settlement is more than the elevated tank manufacturers will allow. Therefore, it does not appear possible to support the elevated tank on a spread footing foundation. It is recommended that the tank be supported on a deep foundation, or more specifically an end bearing pile foundation. This site is conducive to a reasonable length pile Water Tower Feasibility Report 15