377. There are other methods of trenchless cable laying, namely guided augers and micro-tunnelling. These, however, require drilling and reception pits to be sunk either side of the obstacle, and then a duct is drilled between the two. Although there is no need for a separation distance between the drilling site and obstacle boundary, these methods take longer to complete because of the need to excavate and shore up the drill and reception points.
5.6.4.8.1 Open Cut Watercourse Crossings 378. The majority of watercourse crossings would be constructed using conventional open cut methodologies as outlined below. The exceptions to this would be the major river crossings at the Deben and Martlesham Creek, which would be constructed using HDD techniques as described in the next section.
379. Stream and drain crossings can be identified along the route as those that permanently contain flowing waters throughout the year. The Environment Agency would be consulted to help determine the detailed method statement governing each crossing.
380. Where possible, spoil storage would be set back 5m from water courses, in order to minimise potential for silt run off from the working width.
381. A number of factors would affect the choice of crossing method, including depth of water, available space, duration of works, bed conditions, accessibility and potential ingress of water. The default crossing method of watercourses would be trenching.
382. The cable route haul road would traverse across the watercourse using a temporary bridge, which would lie within the working width.
383. The exact methodology to achieve an open trench across each watercourse and the temporary bridge arrangements required would be decided by the works contractor. The profile of the trench running through the stream would be determined in consultation with, and with the approval of, the Environment Agency. Determining engineering factors would be the required cover underneath the stream bed, the surrounding stream bank profiles and the minimum bend radii of the ducting.
384. Open cut crossings can either be wet or dry. One dry technique involves damming the watercourse upstream and downstream of the crossing, thus creating a dry area where the cable crosses. Water is then pumped from where it has been impounded upstream and discharged downstream of the crossing area.
385. In the wet open cut technique, construction takes place within flowing water. The cable trench is typically constructed across the watercourse by equipment operating
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 82
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