NORTH BLYTH BIOMASS PROJECT THE PROPOSED · PDF fileTHE PROPOSED NORTH BLYTH BIOMASS POWER ... • There would be less likelihood of requiring the use of bentonite in the process.

NORTH BLYTH BIOMASS PROJECT THE PROPOSED NORTH BLYTH BIOMASS POWER STATION ORDER APFP Regulation: 5(2)(a)

Document Title: Preliminary Outline Method Statement Intake Structure (Appendix 4.2 of Environmental Statement) Date: February 2012 Author: Fairhurst Ltd Document Number: 6.2.08

Document Ref: 02377-001224 Issue: 02

Revision History Document Title: Preliminary Outline Method Statement Intake Structure (Appendix 4.2 of Environmental Statement) Revision Date Author

01 February 2012 Fairhurst Ltd


Preliminary Outline Method Statement – Intake Structure

February 2012

RES North Blyth Renewable Energy Plant D/I/D/81354/101


North Blyth Renewable Energy Plant D/I/D/81354/101

CONTROL SHEET

CLIENT: RES

PROJECT TITLE: North Blyth Renewable Energy Plant

REPORT TITLE: Preliminary Outline Method Statement – Intake Structure

PROJECT REFERENCE: 81354 Issue and Approval Schedule:

ISSUE 1

DRAFT Name Signature Date

Prepared by A Lowes

Reviewed by D Ladkin

Approved by D Ladkin

Revision Record:

Issue Date Status Description By Chk App

2 25/07/11 Information Minor Amendments

3 7/09/11 Information Minor Amendments

4 2/02/12 Information Minor Amendments LT AJL AJL

5

This report has been prepared in accordance with procedure OP/P02 of Fairhurst’s Quality Assurance System.



CONTENTS 1.0 INTRODUCTION ......................................................................................................... 1 2.0 LOCATION OF WORKS ............................................................................................. 1 3.0 FORM OF STRUCTURE ............................................................................................ 1 4.0 EXPECTED GEOTECHNICAL AND HYDROLOGICAL CONDITIONS ..................... 1 5.0 TECHNICAL CONSTRAINTS TO BE OVERCOME ................................................... 2 6.0 POTENTIAL METHODS OF CONSTRUCTION ......................................................... 2 7.0 SEQUENCE OF CONSTRUCTION ............................................................................ 4 APPENDIX A DRAWINGS



Page 1 February 2012

1.0 INTRODUCTION As part of the proposal to develop a Biomass Power Station at Battleship Wharf, Port of Blyth, it will be necessary to construct a cooling water system with an intake abstracting water from the River Blyth. The intake structure will be located to the rear of the retaining wall, on the land-ward side of the existing Quay 4.

This preliminary outline method statement discusses the potential methods and sequence of construction for this structure based upon outline design information supplied by SKM, available geotechnical and hydrological information for the area and constraints imposed by constructing the intake within an operational port.

This method statement has been prepared to discuss the technical procedures and sequence of works for the installation of the intake structure. Health, safety and environmental requirements, design and construction quality assurance procedures, regulatory liaison and liaison with third parties are not included in this method statement.

2.0 LOCATION OF WORKS The preferred location for the intake structure is to the rear of the existing retaining wall at the southern end of Quay 4 at Battleship Wharf. As the intake is on the landward side of the existing cofferdam river wall no construction activity will take place within the River Blyth Estuary, other than when the existing cofferdam and sheet pile river wall are broken through for the installation of the header pipe work.

The intake structure will extend from the retaining wall to close to the south western corner of the Bulk Shed 2. The area in which the intake structure will be located is currently utilised as a hardstanding area with a circulation road running north – south through it. It is understood that the existing quay retaining wall is restrained by high level ground anchors.

3.0 FORM OF STRUCTURE The preliminary general arrangement of the intake structure is shown on sketch drawing dated 9th February 2010, Appendix A. It will comprise a chambered reinforced concrete structure with a reinforced concrete cover slab designed to allow the trafficking or port vehicles and plant over it. The section of the structure that is nearest to the river will consist of a settling chamber. The central section will contain a band screen. The inland section will consist of a dry well pump chamber and a manifold/valve pit. Water will be drawn into the intake via a transverse slotted pipe intake suspended below the deck of Quay 4. The pipe will penetrate through the existing retaining wall.

It is understood, from the information presented in the desk study provided in Appendix 15.1 of the accompanying Environmental Statement, that the required excavation depth for the construction of the intake is approximately -7.5mOD and that the overall width of the structure will be approximately 9m.

4.0 EXPECTED GEOTECHNICAL AND HYDROLOGICAL CONDITIONS Based upon historical site investigation information, presented in the desk study provided in Appendix 15.1 of the accompanying Environmental Statement, the following approximate geotechnical sequence can be expected at the proposed location for the intake structure.




Elevation Description

+4.00m - +3.50m OD Port hardstand pavement construction

+3.50m - -2.75m to -4.5mOD

Heterogeneous granular made ground consisting of sand, gravel including mudstone, demolition arisings and ash overlying natural wind blown loose sand

Below -2.75mOD to -4.5mOD

Middle Coal Measures consisting of sandstone overlying mudstone and coal.

The tidal range in the River Blyth is:

Highest recorded +3.61m OD

Mean High Water Springs +2.44m OD

Mean Low Water Springs -1.88m OD

Lowest Recorded -3.61m OD

5.0 TECHNICAL CONSTRAINTS To construct the intake the following technical challenges must be considered in the design of the temporary and permanent works and construction methodology.

1. Significant temporary works required to construct through heterogeneous and loose soils overlying rock.

2. Maintaining a dry working area adjacent to a river with a large tidal range which is significantly above the proposed excavation depth.

3. Minimising impact on port operations throughout construction.

6.0 POTENTIAL METHODS OF CONSTRUCTION There are three potential method of construction for this structure:

1. Construct intake structure within cofferdam constructed as a steel combi wall spigotted into the bedrock with rock excavation below rockhead.

2. Construct intake structure within a cofferdam constructed from concrete diaphragm walls. The diaphragm walls would form the external walls of the permanent structure.

3. Construct intake structure within a cofferdam constructed from concrete secant pile walls. The secant pile walls would form the external walls of the permanent structure.

6.1 Cofferdam Constructed as a Steel Combi Wall A conventional sheet pile wall would not penetrate into the bedrock sufficiently to provide toe restraint. To avoid the requirement to install a low level prop at rockhead, a modified form of sheet pile wall, a combi wall, which uses both steel tubes and sheet piles to form the wall could be used. The tubes would be driven to refusal into the rock and cored out. A spigot consisting of a steel dowel driven into the rock and restrained by a concrete plug would be installed into the bottom of the tubes. Up to three sheet piles would be installed as infill panels between the tubes. It is anticipated that a high level prop would be required to maintain stability.

Below rockhead excavation would be carried out directly through the bedrock with the cut face of the rock forming the extent of the excavation. It is anticipated that the




sandstone would be strong enough to form a near vertical face, however, if mudstone layers are encountered temporary shoring of the face using sprayed concrete may be required.

On completion of the excavation the intake structure would be constructed. The sequence of construction would require careful integration with the temporary works design to ensure that when temporary props are removed the partially constructed intake structure has sufficient stiffness to resist the lateral earth pressure loads previously resisted by the props.

There are significant disadvantages with this form of cofferdam installed where the bedrock horizon is higher than the base of the excavation:

• It is difficult to waterproof sheet pile and combi walls and as such seepages through the clutches and at the wall bedrock interface should be expected.

• If mudstone layers are encountered the excavation face should be supported and protected with sprayed concrete.

• Unless the sheet piles or combi wall is designed to form part of the structure the temporary works will have to be removed on completion of the structure

6.2 Cofferdam Constructed As A Diaphragm Wall In this form of construction a trench is excavated and usually supported by bentonite. When the trench reaches the design depth concrete is poured into it which displaces the bentonite. On completion of the pour a reinforcement cage is inserted. After the wall has been constructed the central area can be excavated and the base slab and internal compartment walls can be constructed.

Due to the depth of the chamber required it is considered that as a minimum a temporary high level internal prop will be required to minimise the thickness of the diaphragm wall.

The advantage of this form of construction are

• The permanent works are also used as the temporary works.

• Seepages through the base the excavation can be controlled by sump pumping and general groundwater lowering will not be required.

• The reinforcement cages can easily be designed so that couplers project from the face of the wall to allow the connection of the reinforcement for the transverse walls without breaking out concrete.

• The diaphragm wall will have a rough surface from being cast against the ground, however this could easily be prepared, if required, to improve the hydraulic performance of the structure.

The disadvantages of this form of construction are:

• The installation requires the preparation and handling of large volumes of bentonite slurry which would have an adverse impact on the river if a spillage or seepage occurred. During concreting of the walls the displaced benonite would have to be carefully contained and stored prior to disposal.

• The bentonite would have to be disposed of as waste.




6.3 Cofferdam Constructed As A Secant Pile Wall An alternative form of top down construction to a diaphragm wall would be to construct the external walls using overlapping piles. This form of wall construction is known as a secant pile wall. After the piles have been constructed the intake chamber would be excavated and constructed to the same method as the diaphragm wall solution.

The advantages of this solution are:

• There would be less likelihood of requiring the use of bentonite in the process.

• The permanent works are also used as the temporary works.

• Seepages through the base the excavation can be controlled by sump pumping and general groundwater lowering will not be required.

The disadvantage of this solution are;

• The internal face of the wall will be a series of interlocking piles with a rough surface as the concrete was cast against the ground. This may not deliver the required hydraulic performance required for the structure and an internal non-structural facing of concrete may be required.

• It is very difficult to install a reinforcement cage so that couplers are correctly located at the face of a pile. As such, to form the tie between the external wall and internal walls extensive breaking out and drilling into the piles will be required.

6.4 Preferred Form of Construction The most practical form of construction for the intake would be top down construction using diaphragm walls. With this form of construction the excavation and construction of the internal walls of the intake can be constructed within a robust watertight cofferdam which then forms part of the permanent works. The drawback of this method of construction is that it will require the use of bentonite close to a controlled water. As such, rigorous surface run-off control measures to prevent bentonite entering the river will have to be designed, agreed with the Environment Agency and implemented.

7.0 SEQUENCE OF CONSTRUCTION The sequence of construction outlined below is based upon the construction of the intake within a cofferdam constructed from diaphragm walls. The schematic arrangement of the works is shown on Drawings 81354/SK003 and 81354/SK005, Appendix A.

1. Agree with the Port of Blyth and implement a traffic management system to allow port operations to continue around the work area.

2. Carry out trial trenching to locate ground anchors restraining existing quay retaining wall. Where anchors clash with location of intake, agree relocation of anchors with Port of Blyth.

3. Agree with Environment Agency mitigation measures required for batching, handling and disposing of bentonite. Works to be carried out in accordance with Environmental Permitting Regulations and a Construction Quality Assurance Plan.

4. Excavate trenches to design depth under bentonite support.

5. Place concrete and reinforcement. Contain and dispose of displaced bentonite.




6. After diaphragm walls have cured sufficiently, excavate to design formation level. Install props as required by the temporary works design.

7. Construct internal walls, remove props when required by the temporary works design.

8. Install intake header below existing quay.

9. Break through riverward wall and install header pipework into wall. The dimensions of the break through will be dictated by the mechanical design of the cooling water system, however it is anticipated that a break through for a pipe 1 to 2m diameter will be required.

There is only a risk of construction materials / demolition arisings falling into the intertidal mudflats during breaking through of the quay wall. Prior to this activity taking place, netting will be erected to contain falling materials. The break through will be designed so that saw cutting / coring through the wall will be utilised to allow materials to be removed in sections rather than broken up in situ using hydraulic breakers.

10. Install plant and commission. Remove traffic management system.

The final timescale will be determined during detailed engineering design of the works, though it is recommended that works on the wall and river side of the wall will be undertaken during the summer months to avoid impacting upon over wintering birds.



February 2011

APPENDIX A DRAWINGS




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