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June 2014

Deadline V Appendix 21 – Watercourse Crossing Outline Method Statement

DOGGER BANK CREYKE BECK

Deadline V Appendix 21 © 2014 Forewind Page 1

Document Title Dogger Bank Creyke Beck

Deadline V Appendix 21:

Watercourse Crossing Outline Method Statement

Forewind Document Reference F-EXC-RW-DV-App21

Issue Number 1

Date 13/06/2014

Drafted by Tamsyn Rowe

Checked by Mark Thomas

Date / initials check MT 13-June-2014



Introduction

Purpose of this document 1 This outline method statement provides a framework for a future watercourse crossing

construction method statement, to be completed as part of future detailed construction planning.



Outline Method Statement

Introduction 2 This document contains an outline method statement. The detailed method statement

will be completed as part of the detailed construction planning phase, prior to commencing construction of the high voltage direct current (HVDC) and high voltage alternating current (HVAC) cable routes. The detailed method statement will be defined post-consent once the relevant contractors have been appointed.

3 All sections of this method statement will be produced in line with best practice and relevant legislation, and have regard to mitigation measures and commitments made as part of the consents and licenses for the project, including all those referenced as part of the Code of Construction Practice (CoCP) and Construction Environmental Management Plan (CEMP).

Watercourse Crossing Options 4 The HVDC and HVAC cable routes intersect a number of large watercourses such as

the Leven Canal and the River Hull, in addition to numerous field drains of varying widths and depths.

5 The options for watercourse crossing methods were defined within the Environmental Statement submitted with the application. Identification of the most appropriate crossing method for each watercourse will be confirmed during the detailed design process, prior to construction commencing.

6 There are two main crossing methods for installing the cables across watercourses – these are horizontal directional drill (HDD) and open-cut trenching. In addition, to provide a continuous access route along the length of the cable route, it will be necessary to install temporary vehicle bridges at some locations, where the cable route intersects ditches and small to medium watercourses.

7 Further details of these main watercourse crossing options are described below.

Horizontal Directional Drill 8 HDD is the preferred method of cable installation to cross large watercourses. It is a

steerable trenchless method for installing underground ducts and cables in a shallow arc along a prescribed bore path by using a surface launched drilling rig.

9 HDD has the advantage of minimising impacts to surface water bodies, and reducing the levels of reinstatement requirement. It is a well-established and appropriate technique for crossing sensitive water features.

10 The HDD technique used is described fully in Chapter 5, Project Description (Ref: 6.5) of the Environmental Statement (paragraphs 4.4.26 to 4.4.59 inclusive). Information is provided on:



• preparation of the site for HDD;

• typical launch site – including layout, processes, plant and vehicles required;

• typical reception – including site layout and activities undertaken;

• HDD drilling operation; and

• demobilisation.

11 A full method statement for HDD operations will be completed in due course once the relevant crossings have been identified and defined. It will be based upon the information within Chapter 5 of the ES, and will include consideration of:

• Access arrangements – including provision of necessary personnel and equipment, and any necessary restrictions to public access;

• Site safe working procedures – including consideration of HDD-specific activities;

• Management of drill fluids if relevant; and

• Relevant mitigation measures detailed in the final CEMP, e.g. appropriate pollution prevention.

Open Cut Trenching 12 The method and sequencing of work associated with open trench cable installation was

described in Chapter 5, Project Description (Ref: 6.5) of the Environmental Statement (paragraphs 4.4.5 to 4.4.25 inclusive), and the likely methodology for crossing watercourses (including drains) is outlined below. The details are indicative to provide an overview of the likely works required; the specific details of which will be determined during detailed design pre-construction.

13 Throughout the works, the measures outlined in the final CEMP will be implemented to ensure that all relevant mitigation measures outlined in the ES for works in proximity to watercourses are fully adhered to. This will include pollution prevention, habitat manipulation/restoration, and water vole dispersal etc. Details of these measures are not repeated again here.

Work Phase 1 - Setting Out of the Works and Erection of Fencing 14 Prior to commencing works, it will be necessary for the boundary of that section of the

cable corridor to be set out by a survey team. 15 The cable corridor will be fenced off to segregate the construction activities from the

adjacent parcels of land and clearly define the extent of the works. The fence type to be installed will be dictated by the adjacent land use and through consultation with the landowner.

Work Phase 2 – Construction of Dam, and Pump Installation 16 Construction of the soil dam will follow the arrangement as shown in Figure 1.



17 It is likely that some imported material for sidewall construction will be required. The exact requirements for the soil dam will be determined at the construction stage.

18 Once the soil dam is in place, a diesel powered pump will be used to pump water round and bypass the completed dam. If the watercourse is deep pumping may commence before completion of the dam.

Work Phase 3 - Excavation of Trenches 19 The excavation of the trenches should be undertaken under the supervision of a

banksman at all times.

20 The cable trench excavation will be checked for line and level by the contractor’s engineers. Once satisfied, the base of the cable trench will receive a layer of cement bound sand (CBS) or subsoil. The use of CBS will be dependent upon the subsoil’s thermal resistivity properties.

21 It may be practical to install cable ducts to cross the watercourse in which case the trench can be prepared and reinstated and the cable pulled through later. If cable ducts are used it is essential that the specified separation distance between the ducts is achieved and maintained during the trench reinstatement works.

Work Phase 4 – Cable Installation 22 The cabling will be installed within the trench across the watercourse from adjacent

designated joint bay positions. In the eventuality that ducts are used, cables will be pulled through the ducts using a winch and powered trailer.

23 Once the cables (or ducts, if used) are in place, the trench will be backfilled with CBS or subsoil (where soil properties permit). The backfill will be installed to a level 100mm above the soffit of the duct. Once the backfill is levelled, protective cable tiles and warning marker tape will be put in place.

24 The reinstatement of the cable trench above the marker tape (up to topsoil level) will utilise the subsoil that was previously excavated from the trench. Excess spoil will be removed from site in a tipper wagon to an appropriately certified disposal site.

25 The base of the watercourse will then be fully reinstated. 26 Once the consolidated adjacent cables lengths are in place, the cables will be jointed

together, and layer of CBS will placed around the jointed cables, similar to the main duct runs.

27 On completion of the cable installation and jointing, the joint pits will be reinstated using the excavated subsoil. Future access to the joint bays will not be required and joint pits will not require inspection chambers or access covers.

Work Phase 5 – Reinstatement 28 Once the cable is laid and the trench reinstated and the watercourse bed consolidated,

the soil dams will be removed in a reverse procedure to construction.

29 Any remedial works will be carried out to insure integrity of the banks on either side of



the watercourse, and the water flow will be recommenced and the pump removed.

Temporary Vehicle Bridges 30 Temporary vehicle bridges will be installed across some small to medium sized water

courses to retain access either side of the water body for construction workers. Access across the remaining watercourse crossings will be maintained by using the existing road network.

31 The use of both temporary vehicle bridges and culverts to provide access across water bodies was discussed with the EA pre-application. As a result culverts were removed from the project and temporary vehicle bridges are the preferred solution to cross ditches where necessary.

32 The use of temporary vehicle bridges is described in Chapter 5 of the ES (paragraphs 4.5.18 – 4.5.19, including a photograph of a typical vehicle bridge).

33 A method statement for the installation and use of temporary vehicle bridges will be completed in due course once the relevant activities have been defined. It will be based upon the information within Chapter 5 of the ES, and will include consideration of:

• Access arrangements – including provision of necessary personnel and equipment, and any necessary restrictions to public access; and

• Site safe working procedures – including specific consideration of works near to watercourses.



Figure 1. Typical Open Cut Across a Drain

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