2009-05-27 CC Packet Add.'l Handouts

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City of Centerville c/o Bonestroo May 27, 2009 Page 2 00 2.0 Meeting Recommendations A meeting was held at the site on May 26, 2009 for the purpose of reviewing the construction performed to date and the conditions observed, and arriving at appropriate recommendations for reducing future roadway and utility movements. Based on this, engineered approaches were discussed as presented below. . Please note that these discussed approaches are based on described conditions by field personnel which were observable. However, we have no deep soil boring in the area for specific evaluation of future movement. The intent is to reduce future movement as much as possible considering time and reasonable cost. If the City cannot accept the potential for future movements, then additional study, including more extensive field exploration and testing, must be performed. Elimination of movements would likely need to be associated with pile foundation systems. The soils which are supporting the reconstructed and new utilities have felt overburden load in the past. Some of the overburden load was due to the organic swamp deposits, which have less unit weight than the inorganic clay soils native to the site. We understand the swamp was previously overlain by about 3 feet of the inorganic clay materials, meaning there has been at least some pre-compression of the swamp soils. The organic swamp materials should not be reused as backfill. Because these soils have been disturbed from their previously "compressed" condition, the soils are uncompactible and have an extremely high settlement potential. If inorganic mineral soils were to be used as backfill, the mineral soils would have a higher unit weight than the peat soils they replaced. Therefore, load on the soils supporting the pipe would be increased and could induce additional settlements. Accordingly the intent of the backfilling operation should be to provide a lightweight material, similar in weight to the swamp soils for those areas of the profile where swamp previously existed. With the presence of the DNR wetland with standing water to the south of the street, it is obvious that the soils below about 3 feet will be saturated. Accordingly, the appropriate lightweight fill in this scenario is wood chips. The wood chips would have a unit weight similar to the organic swamp materials. Future earthwork should involve re-excavation of the street area down to the top of the sand fill which was placed in the watermain trench. This depth would be roughly I foot above the pipe, or 7 feet below proposed grade in the Deep Swamp Area. On either side of this, the subcut would then be on the order of 5 feet below grade, which is about 3 feet above the pipe. The excavation depth should then be carried laterally across the full width of the street, plus an additional I foot of oversize beyond the curb. Tapering in the longitudinal direction (parallel to centerline) should be somewhat gradual, preferably on the order of 10: I (H:V).