1994 - Geotechnical Services Report

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AET #94-1805 - Page 4 can be expected to settle in excess of 1' during a 50-year period. This amount of settlement is more than what is normally tolerable for construction of this type. We have considered surcharging the site to preconsolidate the existing fill and swamp deposits. Due to the highly organic nature of much of these materials, it is our judgment significant total and differential settlement of the structure could still occur even after surcharging. Soil Correction Method The building could be supported on footing foundations after soil correction. Based on the soil information at the boring location, it appears the unsuitable soils extend to a depth of about 141/2' below grade. Depending on the lateral extent of the organic deposits, it may be possible to replace the organic materials with compacted fill. For this method, the unsuitable existing fill and organic soils would have to be removed for their entire depth and laterally beyond the perimeter of the structure a minimum distance so adequate lateral support of replacement fill and the structure supported on that fill would be provided. For organic soils as soft and compressible as indicated in this boring, we recommend that the oversize be at least 11/2' laterally for each 1' of fill below normal depth foundations if the bottom of the organic soils is relatively level. If the surface of the underlying firm clay till materials are sloping downward, the lateral oversize should be increased to 2' horizontally for each 1' of fill required beneath foundations on the down slope side of the excavation bottom. The presence of ground water within the excavation, makes the excavation more difficult. In order to visually determine when the unsuitable soils are removed, you should plan on dewatering the excavation during the excavation operation. It may be difficult to dewater the excavation and maintain stability of the sideslopes without very flat backslopes or retention systems. Attempting to excavate under water is risky in that it is very difficult to determine if all unsuitable soils have been removed. Failure to excavate unsuitable would result in building settlement.