2.1 Overview of the Proposed Project - Keystone XL pipeline - US ...

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Draft Supplemental Environmental Impact StatementKeystone XL Projectenvironments are difficult to contain primarily because bentonite readily disperses in flowingwater and quickly settles in standing water. Further discussion of frac-outs is presented inSection 4.3, Water Resources; Section 4.7, Fisheries; and Section 4.8, Threatened andEndangered Species and Species of Conservation Concern.2.1.9.2 Wetland CrossingsConstruction across wetlands would be similar to typical conventional upland cross-countryconstruction, with modifications to reduce the potential for effects to wetland hydrology and soilstructure. The wetland crossing methods used would depend largely on the stability of the soils atthe crossing location at the time of construction. The 110-foot pipeline construction corridorwidth would be reduced to 85 feet for wetlands in Montana and Nebraska, and 75 feet forwetlands in South Dakota, unless conditions require a wider construction corridor. In instanceswhere the wetland is supersaturated or inundated, the corridor ROW may be increased to ensuresafe construction conditions. The operation ROW would be 30 feet in all three states, unlessconditions require a wider permanent ROW width.If wetland soils are not excessively saturated at the time of construction and can supportconstruction equipment without equipment mats, construction would occur in a manner similarto conventional upland cross-country construction techniques. Low ground pressure equipmentor conventional equipment supported by pre-fabricated matting would be used in all wetlands.Where wetlands are located at the base of slopes, permanent slope breakers would be constructedacross the ROW in upland areas adjacent to the wetland boundary. Temporary sediment barrierswould be installed where necessary until revegetation of adjacent upland areas is successful.Additional TWAs would be required on both sides of saturated or inundated wetlands to stageconstruction, fabricate the pipeline, and store materials. These additional TWAs would belocated in upland areas a minimum of 10 feet from the wetland edge. More information islocated in the Site-Specific Waterbody Crossing Plans located in the CMRP (Appendix G) withrecommended modifications to the CMRP provided in Section 4.4.4, Recommended AdditionalMitigation.Clearing of vegetation in wetlands within the construction corridor would be limited to trees andshrubs, which would be cut flush with the surface of the ground, leaving the root stock in place,and removed from the wetland. Scrub-shrub and forested communities would be allowed toregenerate outside of the maintained operations ROW, and in areas where HDD was used to spanopen water areas. To avoid excessive disruption of wetland soils and the native seed androotstock within the wetland soils, stump removal, grading, topsoil segregation, and excavationwould be limited to the area immediately over the trench line to the maximum extent practicable.Trench breakers would be installed where necessary to prevent the subsurface drainage of waterfrom wetlands. Trench width may vary in some wetlands to provide an even, safe work area,which depends upon topography, soil moisture content, and groundwater levels. Severetopography may require additional disturbance to create an even safe work area. More saturatedsoils usually require a wider trench in order to maintain a safe ditch and to avoid unstable trenchwalls. During clearing, sediment barriers, such as silt fence and staked straw bales, would beinstalled and maintained on down slopes adjacent to all wetlands and within additional TWAs, asnecessary, to minimize the potential for sediment runoff.Construction equipment working in all wetlands would be limited to the area essential forclearing the ROW, excavating the trench, fabricating and installing the pipeline, backfilling theProject Description 2.1-60 March 2013
Draft Supplemental Environmental Impact StatementKeystone XL Projecttrench, and restoring the ROW. In areas where there is no reasonable access to the ROW exceptthrough wetlands, non-essential equipment would be allowed to travel through wetlands only ifthe ground is firm enough or has been stabilized with approved pre-fabricated matting (nottimber slash) to avoid rutting. Where wetland soils are supersaturated or inundated, the pipelinecould be installed using the push-pull technique. The push-pull installation process wouldinvolve stringing and welding the pipeline outside of the wetland, and excavating and backfillingthe trench using a backhoe supported by equipment mats or timber riprap. Constructionequipment working in all wetlands would be limited to that area essential for clearing the ROW,excavating the trench, fabricating and installing the pipeline, backfilling the trench, and restoringthe ROW. In areas where there is no reasonable access to the ROW except through wetlands,non-essential equipment would be allowed to travel through wetlands only if the ground is firmenough or has been stabilized with approved pre-fabricated matting (not timber slash) to avoidrutting. The pipeline segment would be installed in the wetland by equipping it with floats andpushing or pulling it across the water-filled trench. After the pipeline is floated into place, thefloats would be removed and the pipeline would sink into place. Most pipes installed in saturatedwetlands would be coated with concrete or installed with set-on weights to provide negativebuoyancy. The criteria used to determine pipe selection are based on site-specific conditions,ease of implementation, and practicality of implementing the installation method. The preferenceis set-on weights.Following installation of the pipeline, subsoils (clays and gravel/cobbles) would be backfilledfirst followed by the topsoil. Restoration of contours would be accomplished during backfilling.No grading would occur in wetlands and the soil surface would be roughed to enhance seedgermination. Soil strata would be restored above the pipeline to replicate original conditions.Topsoil would be replaced to the original ground level leaving no crown over the trench line.Excess excavated material would be removed from the wetland and spread along the uplandROW, placed in a location as requested by a landowner, or disposed of at an existing authorizedlandfill. Equipment mats, gravel fill, geotextile fabric, and straw mats would be removed fromwetlands after backfilling except in the travel lane to allow continued, controlled access throughthe wetland until the completion of construction. Once revegetation is successful, sedimentbarriers would be removed from the ROW and disposed of at an existing authorized landfill. Inwetlands where no standing water is present, the construction ROW would be seeded tosupplement regenerated growth from root stock from original excavation of soils, in accordancewith the recommendations of the USACE, local soil conservation authorities, or landmanagement agency.2.1.9.3 Aboveground and Ancillary Facilities Construction ProceduresPump Station ConstructionConstruction at each new pump station and pigging facility would begin with installing sedimentcontrol measures, and then the clearing of vegetation and removal of topsoil will take place Afterthat, the site would be graded as necessary to create a level working surface for the movement ofconstruction vehicles and to prepare the area for building foundations. Each pump station wouldinclude one electrical equipment shelter (EES), and a variable frequency drive equipment shelter.The EES would include electrical systems, communication, and control equipment. Foundationswould be installed for the EES and the pump equipment shelter. The structures to support thepumps, manifolds, pig receiving and pig launching equipment, densitometers (where present),Project Description 2.1-61 March 2013
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