Res. #15-036 - Approving Plans & Specs. & Ordering Ad. For Bids

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City of Centerville Project SP-07-03738 November 29,2007 Page 9 D. Recommendations D.J. Pavement Subgrade Preparation Prior to placing pavement materials(granular subbase),we recommend removing existing pavement materials in their entirety to expose underlying existing fill,alluvial or glacial soils. Given the variable nature of those soils,we recommend—at a minimum—scarifying,blending and surface compacting the upper foot of exposed soils, Depending on construction conditions,excavations may have to be extended locally to remove wet,loose,soft or otherwise unstable soils that become disturbed during the excavation process and lose strength. We recommend removing organic soils from within 3 feet of pavement subgrade where cuts are made, and remove it altogether where fills greater than I foot are required. To provide lateral support to replacement backfill,additional required fill and the structural loads they will support,the excavations should be adequately oversized. We recommend oversizing(widening)the excavations at least 5 feet horizontally beyond the limits of the unstable area. Granular subbase materials placed over slow-draining clayey sand and sandy lean clay are apt to trap surface runoff,which may contribute to strength loss and/or,frost heave of underlying materials. As described to us,it is apparent that provisions have been made to install subdrains at the lower boundary of the granular subbase. We recommend routing the subdrains to a storm sewer utility or other suitable disposal site. Preferably the subdrains should consist of perforated pipes embedded in washed gravel, which in turn is wrapped in filter fabric. Perforated pipes encased in a filter"sock"and embedded in washed gravel,however,may also be considered. To help provide for adequate drainage characteristics, we recommend that the pavement subgrade be graded with a"crown"having a local highpoint near the centerline of the roadway and gently sloping down towards the draintile invert. D.2. Selecting Replacement Backfill and Additional Required Fill On-site materials free of organic soil and debris can be considered for reuse as backfill and fill. Note that the sandy lean clay,clayey sand,silt and silty sand portions of these soils,being clay-and silt-rich by nature,may be particularly difficult to compact if wet or allowed to become wet,or if spread and compacted over wet or marginally stable subgrades. For ease of compaction,imported material(below the granular subbase layer),if needed to replace unsuitable on-site materials or balance cut and fill quantities,may consist of material meeting Mn/DOT Specification 3149.2B for Granular Borrow