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11.5 Method Statement: Wet Open Cut 146


11.5.1 Plant and Vehicle Access 147


The following sections outline a generic method for wet open cut crossings of watercourses by the Onshore Cable Route.


In most instances, temporary flume pipe(s) or bridges are installed at watercourse crossings to enable plant and machinery to cross during construction, to minimise delay and impacts on local roads of construction plant and machinery. In all cases, vehicles will be prohibited from driving through watercourses.


148


Flume pipe bridges are constructed by installing one or more sections of pipe in the watercourse ramped over to allow uninterrupted flow of water within the watercourse and a continuous running track for construction vehicles. These pipes are surrounded with sand bags before a temporary road is laid over the top. Additional sand bags are installed each side of the road to prevent mud falling over the side and temporary safety fencing erected where necessary. Access crossings will have proper temporary headwalls at both ends to reduce the risk of hardcore material making up the access track from collapsing into the watercourse.


149


Flume pipes will be sized in accordance with expected flows and will be oversized to compensate for any unexpected high flows. Monitoring will take place on a regular basis to ensure that no material from the running track over the temporary flumes enters the watercourse. If this does occur the material will be removed immediately.


11.5.2 Construction Technique 150


The cable trench will be excavated across the watercourse by equipment operating from either the banks or from flume pipes laid in the river to maintain flow and provide an equipment crossover from one bank to the other.


151 Stockpiles of excavated topsoil and subsoil will be set back from watercourse banks 5m where possible, in order to minimise direct runoff in wet weather and to minimise the risk of excess loading on watercourse banks. Provision will be made to direct any stockpile runoff away from watercourses.


152 After excavation of the trench, ducting will be placed into the trench. The cable trench design for open-cut watercourse crossings will ensure that the cable is sufficiently buried beneath the watercourse bed allowing a natural substrate to redevelop, rather than causing localised scouring.


Outline Code of Construction Practice. Version 3.


Page 47


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