Anchored Sorbent Cap for Bond Brook Steep Slopes: Former Augusta Gas Works, Augusta, Maine

摘要

Background/Objectives. The former Augusta Gas Works (a former manufactured gasplant [MGP]) is located adjacent to Bond Brook, approximately 600 feet upstream of itsconfluence with the Kennebec River in Augusta. The site is located on partially-filledland that forms a flat terrace along the western bank of Bond Brook. The fill soils of thesite contain MGP residuals and hardened tar deposits that periodically contributed tosheens observed in the brook. These residuals were identified within a steep (1:1) embankment.Gravity and pressure sewer lines within the lower portion of the slope, along alocal utility easement, further complicated remedial design and construction. The endangeredAtlantic Salmon is present in the brook and regulators wanted to reduce possibleexposure to impacted sediment in the brook.Approach/Activities. An anchored sorbent cap (a combined armor mattress and reactivecore mat), sediment cover, and temporary hydraulic bypass was successfully designedand permitted. Implementation of the remedy occurred in summer-autumn 2012. The anchoredsorbent cap design reduces the risk of erosion of bank soils and surficial soilinstability of the steep slope. Site preparation prepared the subgrade soils for placementof the organoclay reactive core mat and armor mattresses. Soil anchors were supplementedby a shallow toe trench to support the armor mattresses, while embedding the sorbentlayers needed for sheen control. The design has enabled future (partial) access formaintenance and monitoring by the selection of materials and anchor details. Riffles andhabitat structures (naturally functioning weirs) are incorporated into a sediment cover tocontrol flow velocities and elevation through the tidally affected study area and providefor fish passage. To facilitate construction of the anchored cap and sediment cover in asetting where water levels in the brook are expected to vary considerably through theconstruction season, several bypass options (open channel diversion, piped bypass, andopen channel/access road) were designed, and the specifications structured to give thecontractor greater flexibility in managing the uncertainty of brook conditions. Given relativelynarrow brook dimensions and steep slopes, access challenges were a major factor