March 2006 - Laserfiche WebLink

Home Browse Search

The filter layer provides separation of the riprap from the underlying soil, thereby preventing migration of fine soil particles through the voids in the riprap and permitting relief of hydrostatic pressure. Above the waterline, the filter also prevents surface water from eroding beneath the riprap. 2.5 Descrintion of Proposed Action: Construction of the riprap slopes would require vegetation removal and some grading and fill activity to ensure a uniform, stable finished slope. The contractor would be required to minimize the number of trees removed and to protect all trees outside the immediate work area. No trees would be removed from the Leeper Park site. All trees within the riprap zone at the CSO and North Shore sites must be removed. Along the upper bank of the North Shore site in the bio -engineered erosion control zone, only trees less than six inches in stem diameter at breast height' would be removed. The roots of removed trees would be grubbed out of the ground. 2.6 Site preparation also includes (all three sites combined) excavation of approximately 2600 cubic yards (cy) of existing material, about 1,100 cy of which would be from below the water line. Approximately 1,200 cy of backfill would be placed into the riverbariks of the three sites, about 500 cy of which would be below the water line, in preparing the slope for the bank protection. Bank protection, both in -water and above the water line, includes 1,800 tons of gravel filter stone and 5,900 tons of riprap stone. 2.7 The fill beneath the riprap layer would consist of well -compacted granular material with good drainage characteristics—likely a compacted silty sand, or medium -fine sand soil. Coarse material such as sandy gravel, crushed gravel, and crushed concrete is more desirable for backfill below the water line because it requires less compaction to attain the required density. Once the slopes are cut and backfilled the respective filter layer (gravel or geo-textile) would be placed in preparation of placing the riprap protection. 2.8 The contractor must protect the city water intakes and must control sediment transport into the river during construction. Construction may be from on land or from the river. Land-based access must be achieved with minimal disruption of trees and other habitat that is to remain in place. A maximum load limit will be imposed on construction equipment along the upper bank at the North Shore Drive site until bank protection is in place because the existing slope already is considered overstressed. 2.9 Water-based construction access may be by floating plant or, if water levels are too low, by rubber -tired equipment in the riverbed. In -water access to the North Shore and CSO sites would likely be along the riverbed from Leeper Park. Any equipment operation or travel on the riverbed outside the excavation area shall be limited to a single path to limit the area directly disturbed. Riverbed travel shall be limited to that necessary for construction. 2 Trees with a stem diameter at breast height (DBH) of six inches or greater. The DBH is measured 4.5 feet above ground on the uphill side of the tree. -4-