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

Home Browse Search

City of Centerville Project SP-07-03738 November 29,2007 Page 7 Organic soils are compressible and may have adverse affects on pavement performance if more than a foot of cutting or filling will occur within the area. Should this be the case,provisions should be made to remove topsoil from within 3 feet of pavement subgrade where cuts are made,and remove it altogether where fills are required. Again,the boring logs indicate that the soil stratum across the site consists of layered alluvium and glacial soils ranging from poorly graded sand with silt to sand lean clay. The sandy lean clay and clayey sand portions of the on-site soils are considered to be compressible and will have adverse affects on pavements if present near the surface of pavement subgrade elevations The proposed 20-inch granular subbase layer placed under pavement materials will likely reduce negative affects these soils will have on pavement performance,and will likely improve the lifespan of the pavements and provide strength- Based on the results of the borings,it appears the there may be limited sources of material that could be used for a granular cushion. Given this,it is likely that appreciable amounts of granular material will have to be imported. Granular materials placed over slow-draining clayey sand and sandy lean clay are,however,apt to trap surface runoff,which may contribute to strength loss and/or frost heave of underlying materials. One way to reduce the likelihood of water becoming trapped is to install subdrains at the lower boundary of the granular cushion, and route them to nearby storm sewer utility or surface-drainage outlets. Draintile that will be included as part of the reconstruction of the roadways will be installed near the edge of the roadways situated at the base of the granular cushion. To help water to flow to the draintile,we recommend that the pavement subgrade be graded with a"crown"at the centerline of the roadway and gentle tapering down to the draintile invert. C.3. Utility Support Although plans indicating the extent of the utility construction were not available to us, it is our understanding that utilities will be installed in most of the roadways that were explored for this evaluation near the deeper borings. Excavations at these locations will likely temrinate in clayey sand or sandy lean clay. Based on penetration resistances,it is our opinion that these glacial soils are generally competent and suitable for utility support. The Borings Logs,however,indicate that wet soils will likely be encountered during installation;it is also possible that perched groundwater will be encountered. Excavation bottoms where groundwater or wet soils are encountered,or that are allowed to become wet may be difficult to keep stable during utility installation;it may therefore be necessary to remove even some of the otherwise competent soils and replace them with a stabilizing aggregate.