bored pile wall

NORTH BLYTH BIOMASS PROJECT

RIVER WALL REPAIR WORKS - REMEDIAL SECANT PILE WALL

METHOD STATEMENT

14 June 2012

Document Ref: 02377-003093 Issue: 01

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NORTH BLYTH BIOMASS PROJECT

RIVER WALL REPAIR WORKS - REMEDIAL SECANT PILE WALL

METHOD STATEMENT 14 June 2012

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North Blyth Renewable Energy Plant

SKM Enviros

I:\JEIA\Projects\JE30584\Deliverables\Reports\River Wall MS\North Blyth R E Plant - New Pile River Wall - Method Statement_Final 03.07.12.docx PAGE i

Contents 1. Introduction 1

1.1. Background to the Works 1 1.2. Scope of this Outline Method Statement 2

2. Location of the Works 3 3. Topography, Ground Conditions, Hydrogeology and Hydrology 4

3.1. Data Source 4 3.2. Topography 4 3.3. Ground Conditions 4 3.4. Hydrogeology 5 3.5. Hydrology 5

4. Existing River Walls 6 5. The Selected Design Options 9

5.1. Summary of Selected Options 9 5.2. The Secant Pile Wall 9 5.3. River Wall Reinstatement and Tie-back 10

6. Sequence and Methods of Construction 11 6.1. Considerations for Piling Activities 11 6.2. Overall Sequence of Construction 11 6.3. Construction Methods for the Bored Secant Pile Wall 12 6.4. Construction Methods for the Remediation of the Old River Walls 12 6.5. Contractor’s Compound 13

7. Mitigation Measures to be employed during the works 14 Drawings 16 Appendix A: Options Report – Riverwall Remedial Works, D/I/D/81354/10517 Appendix B: Preliminary Outline Method Statement for other Remedial Options- Riverwall Remedial Works, D/I/D/81354/10 18



SKM Enviros I:\JEIA\Projects\JE30584\Deliverables\Reports\River Wall MS\North Blyth R E Plant - New Pile River Wall - Method Statement_Final 03.07.12.docx PAGE ii

Document history and status Revision Date issued Reviewed by Approved by Date approved Revision type

1 25/06/2012 DB MDF 25/06/2012 Initial Draft

2 02/07/2012 DB MDF 02/07/2012 Final Draft

3 03/07/2012 RLT MDF 03/07/2012 FINAL

Distribution of copies Revision Copy no Quantity Issued to

1 1 1 RES

2 1 1 RES

3 1 1 RES

Printed: 3 July 2012

Last saved: 3 July 2012 09:27 AM

File name: I:\JEIA\Projects\JE30584\Deliverables\Reports\River Wall MS\North Blyth R E Plant - New Pile River Wall - Method Statement_Final 03.07.12.docx

Author: Richard Thomas

Project manager: Matt Faulkner

Name of organisation: RES

Name of project: North Blyth Biomass Project

Name of document: River Wall Method Statement

Document version: Final

Project number: JW30086



SKM Enviros I:\JEIA\Projects\JE30584\Deliverables\Reports\River Wall MS\North Blyth R E Plant - New Pile River Wall - Method Statement_Final 03.07.12.docx

1. Introduction 1.1. Background to the Works

North Blyth Energy Ltd (NBEL), a wholly owned subsidiary of RES UK & Ireland (RES) is seeking consent under the Planning Act 2008 (referred to hereafter as the PA 2008) to construct and operate the North Blyth Renewable Energy Project (referred to hereafter as the Project), which has an electrical export of approximately 99.9 Megawatts of Electricity (MWe) and is located at Battleship Wharf, Port of Blyth, Northumberland. NBEL has submitted an application under Section 31 of the PA 2008 for a Development Consent Order (DCO) to the Planning Inspectorate (PINS) for the Project.

This document provides a method statement describing improvement works that will be required at the existing Port of Blyth site entrance to ensure vehicles from the Project do not impede Port of Blyth vehicles or other road users. This will include widening of the existing access road which leads from the adjacent, North Blyth roundabout that will in turn require repair to the existing river wall. The widening will take place on the riverward side of the road.

The adjacent river walls are in a poor state of repair and therefore associated remedial support measures are required to support the ground adjacent to the new road alignment and, thus, to enable road widening to take place.

It was initially proposed to seal the walls with concrete, to prevent further washout, and to place rock armour against them to prevent collapse. This option provides the quickest and lowest cost construction option for the works, low noise disturbance and would not require any excavation within the potentially contaminated fill material behind the existing river wall. Discussions with Natural England, the Environment Agency and the MMO highlighted that the use of rock armour would result in the loss of Biodiversity Action Plan habitat (mudflat) from the River Blyth Estuary. As a result of feedback from consultees, a number of alternative construction techniques were considered for the works.

The selected solution is a bored secant (interlocking) pile wall, constructed on the landward side of, and generally parallel to the existing river wall, at a distance of the order of 4.0 to 4.5m from it. The “interlocking” is achieved by drilling and forming the piles in an appropriate overlapping sequence.

Further background is provided in the report to RES, D/I/D/81354/105 “Options Report – Riverwall Remedial Works” of February 2012 that was prepared by Fairhurst to inform the selection of a construction technique that would not result in unacceptable loss of intertidal mudflats. This report is provided within Appendix A of this report.



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1.2. Scope of this Outline Method Statement

This outline method statement relates to the secant (interlocking) pile wall installation, together with related works to reduce the potential for further instability in the existing river walls and to minimise wash out of material from the existing river wall. The method statement has been prepared in consideration of the Options Report provided in Appendix A and the requirement to cause no loss of mudflat habitat.

No reference is made to Health and Safety requirements, design, construction quality assurance, regulatory liaison and liaison with third parties.




2. Location of the Works The location and layout of the works are shown on Figure 1 with an aerial photograph of the works area provided in Figure 2. The remedial works location is at the southern end of Battleship Wharf, North Blyth. The works will take place entirely within the Application Boundary, as defined within the DCO Application that has been submitted by NBEL.




3. Topography, Ground Conditions, Hydrogeology and Hydrology

3.1. Data Source

The following geological and related data are reproduced from the report, reference D/I/D/81354/105 “Options Report – Riverwall Remedial Works” of February 2012 that is provided in Appendix A.

3.2. Topography

Comparatively shallow slopes are present on the landward side of the existing river wall, beyond these the topography is reasonably flat. Beyond the line of the walls the slopes extend down to the channel of the River Blyth.

3.3. Ground Conditions

The area retained by the river walls historically has been reclaimed from the estuary. Site investigation information for this area is not available, but based on geotechnical information from other locations in the Battleship Wharf area, ground conditions to the landward site of the river walls have been inferred and are as tabulated below:-

Ground level to 2 – 3m below ground level

Made Ground consisting of heterogeneous colliery spoil

2 – 3m to 4 – 5m below ground level Wind-blown Sand

Greater than 4 – 5m below ground level

Bedrock comprising moderately strong to strong sandstone

Colliery spoil is likely to contain contaminants including heavy metals, sulphate, combustible materials, high pH, Polycyclic Aromatic Hydrocarbon (PAH) and Total Petroleum Hydrocarbon (TPH), however these contaminants tend to be present in low concentrations with low solubility. From historical records of the area it is considered that the fill has been in place for over 100 years and that any leachable concentrations of contaminants are likely to have been leached from the soil in the intertidal groundwater zone close to the wall by tidally influence groundwater movement.

Land immediately north of this location is known to have been contaminated by past uses as railway sidings and for ship breaking. The affected area was remediated by Northumberland County Council in 1993, hence any contamination which may remain is likely to be localised and in low concentrations. (Chapter 15 Hydrogeology, Geology, Ground Conditions and Soils of the Environmental Statement (ES), that has been submitted to PINS alongside the DCO application provides further information on contamination).




On the riverward side of the river walls, sand overlying bedrock can be observed. As outcrops of rock are visible, it is likely that beach sand deposits in this location are typically less than the order of 1m thick.

3.4. Hydrogeology

Groundwater levels behind the existing river walls and in the vicinity of the proposed secant pile wall are in a zone which is influenced by tidal conditions.

3.5. Hydrology

The tidal range in the River Blyth is:-

Highest recorded +3.61m OD

Mean High Water Spring Tides +2.44m OD

Mean Low Water Spring Tides -1.88m OD

Lowest Recorded -3.61m OD




4. Existing River Walls The nature of the existing river walls was described in the Fairhurst report “Preliminary Outline Method Statement - Riverwall Remedial Works”, Ref, D/I/D/81354/10 dated September 2011 and provided in Appendix B, as follows. The river edge is made up of old concrete and masonry walls which vary in distance between about 5m and 20m from the kerb line of the existing road. An inspection of the river walls was carried out by Fairhurst during the low tide period on 8th Feb 2011.

Working from the Port entrance gate, the first section of wall (labelled Wall Section 1 on Figure 1) is approximately 20m long, is constructed from stone masonry and gradually increases in height to about 2m where it meets and old timber jetty. A section of wall has collapsed and an area behind has been washed out by the river/sea, as shown in Photograph 1. This area has been roughly infilled with whinstone rocks to prevent further washout.

Photograph 1: View looking north west showing area of collapse within Wall Section 1 on right hand side of the frame

The next section (labelled Wall Section 2 on Figure 1) of wall is approximately 20m long, is constructed in mass concrete and is approximately 3m high. The wall is founded on timber piles but there are gaps between the timbers, thus allowing material to wash out. Consequently, the ground behind the wall is subsiding.




Photograph 2: View looking west showing Wall Section 2

The third section of wall (labelled Wall Section 3 on Figure 1) leading up to the vicinity of the roundabout is another stone masonry wall approximately 3m high but reducing in height towards the roundabout. This wall is in reasonable condition but there are some areas where washout is occurring through open joints.




Photograph 3: View looking east showing Wall Section 3

Photograph 3: View looking west showing Wall Section 3




5. The Selected Design Options 5.1. Summary of Selected Options

The selected option to provide support to the widened road and immediately adjacent land, without causing a loss of mudflat habitat, is the installation of a bored secant (interlocking) pile wall, constructed on the landward side of and generally parallel to the existing river wall, at a distance of the order of 4.0 to 4.5m from it. Whilst being more expensive than the original proposals involving the use of rock armour, this option does not require excavation of potentially contaminated materials, does not result in any loss of mudflat habitat and the use of bored piles reduces the potential for noise and vibration impacts to occur. Chapter 3 of the ES that has been submitted to PINS alongside the DCO application provides further detail regarding the choice of this option over other methods described within Appendix A.

Associated work will include the provision of measures which will reduce washout through the existing wall, together with the tying back of the existing wall to the secant pile wall.

5.2. The Secant Pile Wall

The secant wall option requires the minimum excavation of in situ soils, since the volume excavated corresponds to the pile diameter and length and no additional excavation is required to facilitate the construction of this remedial support measure for the access road widening. The proposed Secant Pile Wall will be approximately 55m in length, located approximately 4.0 – 4.5m on the landward side of the existing river walls and is anticipated to employ piles up to 500mm in diameter. Figure 3 provides an indicative cross section of the proposed secant pile wall.

The piles will be bored through the superficial deposits into the sandstone bedrock to sufficient depth to provide an adequate factor of safety against movement, should any further erosion or local collapse of part of the existing river walls occur. Subject to detail design, it is anticipated that the piles would need to extend 2 to 4m below the bedrock horizon, depending on the depth of surface weathering in the rock. With the expected ground conditions summarised in Section 3.3, this would provide an overall depth below the existing ground level of 7 to 9m, subject to detailed design.

Subject to detail design, local provision will be made to prevent the build-up of groundwater behind the wall, where appropriate.

Assuming the use of 500mm piles with a 10% overlap, an overall wall length of 55m and depth of 8m below ground level, it is estimated that the works will generate approximately 190 m3 of soils and rock arisings (Section 7.1 describes how arisings will be managed).

The use of bored piles will significantly reduce the potential noise and vibration levels arising from the works when compared with sheet piling or other percussive piling techniques. The piling will conform to the restrictions on piling described in Chapter 4, Section 4.5.80 of the ES that has been submitted to PINS alongside the DCO application.




5.3. River Wall Reinstatement and Tie-back

The western section (Wall Section 1 as shown on Figure 1) of the existing river wall constructed from stone masonry between Battleship Wharf and the timber jetty will be reinstated against the new bored pile wall following its installation.

The remaining sections of mass concrete wall and stone masonry wall between the timber jetties and the staithes will be tied back to the new bored pile wall to prevent the existing wall collapsing into the river. The form of the tie-back will be determined after inspection of the rear face of the river walls to establish their condition. However, it is considered the most likely options to achieve tie-back would include mass concrete infill between the walls together with either a tie at crest level or the use of bolts drilled through selected blocks to tie back the existing wall into the propose secant pile wall.

Local mass concrete works will be carried out immediately against the timber piles of the second section of wall to minimise washout of material.

Subject to detail design, local provision such as weep holes containing or backed by a filter medium, will be made in walls to prevent the build-up of groundwater behind walls, where appropriate.




6. Sequence and Methods of Construction 6.1. Considerations for Piling Activities

In order to mitigate potential impacts from piling noise on nearby sensitive ornithological receptors, it is proposed that the noise from piling activities during the winter (October to March) is limited to 55 dB(A) LA Max or below at sensitive ornithological receptors unless otherwise agreed with Northumberland County Council and Natural England. It is proposed that no piling works be commenced until a Piling Method Statement has been submitted to and approved by Northumberland County Council in consultation with Natural England, the Environment Agency and the MMO.

The method statement will be informed by the detailed design stage and further geotechnical site investigation. It is intended that this will be a Requirement of the DCO and will inform the method of piling to be used during construction, the mitigation measures that will be employed to reduce the impact of piling upon aquatic and marine animals and the method of measuring piling noise at the Special Protection Area on the beach and Cambois coal staithes within the River Blyth Estuary to the northwest of the Project. The piling method statement will also consider any potential impacts upon groundwater. (Chapter 15 Hydrogeology, Geology, Ground Contamination and Soils of the ES that accompanies the DCO application, assesses existing Made Ground and superficial deposits to be granular in nature and therefore natural migration of contaminants is understood not to be restricted and that piling operations cannot introduce additional potential pathways to lower water bodies).

6.2. Overall Sequence of Construction

Installation of the bored secant pile wall will commence first and remedial works to the existing river walls,which require temporary excavation behind them, will commence later in stages as sections of the secant pile wall concrete gain sufficient strength. This is so that the secant pile wall will provide stability as sequential, limited lengths of excavation are carried out behind the existing river walls, to facilitate their stabilisation and tie-back, together with the construction of measures to reduce wash-out. Local mass concrete works in front of the second section of wall, necessary to reduce washout from between timber piles can be carried out at any time during the works.

These works will be carried out prior to the works required for the proposed new access to Battleship Wharf that will involve widening the existing road on the riverwards side. It is anticipated that the construction activities for the installation of the secant pile wall and remedial works to the existing river walls can be conducted without impeding the existing North Blyth roundabout. However, if this is not possible then a temporary road diversions, such as the temporary lane closure required for the site access realignment works, may be required during the installation of the bored piles.




6.3. Construction Methods for the Bored Secant Pile Wall

The bored piles are excavated by means of a mechanical auger. The auger is advanced through soils within a casing, to prevent collapse of soils during the drilling process. On completion of excavation, the reinforcement cage is installed in the bore and concrete is placed. Any casing used in the installation is withdrawn as the concrete is placed. The piles are constructed in an overlapping, sequential basis to form the secant pile wall.

Although the use of casing will reduce any seepage of groundwater into the bore, it is possible that potentially contaminated groundwater could collect within the bore prior to the concrete being poured. If the bore does fill with significant quantities of groundwater, suitable bunding (such as sandbags), will be placed around the work area to contain any water displaced during the pouring of concrete. Any contained, potentially contaminated groundwater will then be transferred to a tanker for treatment offsite at a suitable licensed facility. If it is sediment within the water that is suspected of being contaminated then settlement tanks will be used to separate this from the water, with the collected sediment sent for treatment offsite at a suitable licensed facility.

The soil and rock arisings from the bored piles will be handled and stored to ensure that no silt or sand will be washed into the river during rainfall events. Although the topography on the landward side of the wall is comparatively flat, should there be local variations which may concentrate rainfall runoff, temporary sandbag (or other style) bunds will be placed to control runoff which could otherwise mobilise silts and sands during their excavation and disposal.

Depending on the presence or absence of contamination, soil and rock arisings may be stockpiled for later use, rather than being taken off site. Such stockpiles are likely to be in a designated area away from bored pile wall construction and existing sea wall remediation and tie-back. This matter is addressed below in Section 7.

Although it is envisaged that the bores will be filled with concrete almost immediately so as not to leave open excavations for any length of time, if there are exceptional circumstances whereby bores need to remain unfilled overnight (such as severe machinery failure), mitigation to prevent entrapment of otters will be put in place. Suitable boarding or covers will be used to prevent otters from accessing any deep and steep sided excavations near to the River Blyth Estuary. If this is not practical a suitable means of escape will be provided within the excavation, to enable otters to climb out of the construction. Otters are only likely to try and access the structure outside normal working hours and so excavations will be covered or means of escape provided at the end of each working day.

6.4. Construction Methods for the Remediation of the Old River Walls

The western section of the existing river wall constructed from stone masonry between Battleship Wharf and the timber jetty will be reinstated against the new bored pile wall. Where tie-back is not necessary simple options for surface paving will be considered.




As determined appropriate, the remaining sections of mass concrete wall and stone masonry wall between the timber jetties and the staithes will be tied back to the new bored pile wall to prevent the existing wall collapsing into the river. The form of the tie-back will be determined after inspection of the rear face of the wall to determine its condition. However, it is considered that it will include mass concrete infill between the walls together with a tie at crest level.

Excavations behind the existing river walls will generate soil arisings. These will be handled with care to prevent wind-blow transmission of fine particles into the river. If necessary, for this part of the works, temporary sandbag (or other style) bunds will be placed to control runoff which could otherwise mobilise silts and sands during their excavation from behind the existing river walls. Depending on the presence or absence of contamination, soil arisings may be stockpiled for later use, rather than being taken off site. As indicated for arisings from bored pile wall construction, such stockpiles are likely to be in a designated area away from bored pile wall construction and existing sea wall remediation and tie-back.

Where mass concrete infill is required, this will be contained by formwork which will have all joints on the inside of the shuttering taped prior to concrete pouring. The concrete will press the tape onto the shutter when it is poured, preventing any leaks. An anti washout additive will be added to the concrete if it is considered there will be wave action before the concrete cures.

All heavy plant or machinery will be located on the landward side of the existing river wall and will not be allowed to access the mudflat area. Appropriate signage and fencing will be used to enforce this during the works. Only hand-held equipment, such tools used to erect formwork used to contain the poured concrete, will be allowed onto the mudflat area and will be removed immediately after use. No equipment or materials will be stored on the riverward side of the existing river wall, regular checks and inspections will be made at the end of each work period (such as the return of the tide), to make sure no materials are left in this area.

Any inspection activities and construction works on the River Blyth Estuary side of the existing river wall will take place at states of low tide to prevent the disturbance of sediments within the river, to allow access to the existing river wall and also to allow examination of the shuttering for leaks.

6.5. Contractor’s Compound

If a separate compound is provided for the contractor’s use, suitable measures will be applied to prevent run-off of silts and sands directly to the estuary, following rainfall events. Further related detail is provided below in Section 8.




7. Mitigation Measures to be employed during the works

Contractors for this work will be required to adhere to an Environmental Management System (EMS) to ISO140001 or similar for the construction and decommissioning works, within the quality assurance component of the Contract. Contractors will be required to agree to and develop the overall Application Site Construction Environment Management Plan (CEMP) that will be agreed with Northumberland County Council and the Environment Agency prior to commencement of the construction works.

All water removed from excavations during construction will be treated where necessary and discharged in accordance with relevant consents to discharge agreed with the Environment Agency.

As part of the CEMP, mitigation to prevent the wash out of uncovered contaminated soils during construction works will be employed. Where appropriate, bunds shall be constructed around work areas where existing soils are exposed to prevent flood water inundating work areas and washing out potentially contaminated surface soils.

As part of the CEMP, areas for locating the storage of construction materials will be indentified outside the flood zone.

Specific guidance in dealing with silt contained in surface water runoff arising from construction activities is contained within PPG5. A number of measures can be put in place in order to minimise the actual transport of silt into surface water features, and will be agreed via the CEMP, including:

• Bunding – either earth bunds, cut-off ditches or other forms of diversion or sheeting of stockpiles will be used to minimise the potential for silt laden runoff. Where appropriate silt fences or straw / rock barriers at the toe of the stockpile would be put in place to mitigate runoff during rainfall events. A silt fence comprises a geotextile filter fabric, straw bales, or a combination of both, installed in the path of runoff and can filter out heavy sediments; and

• Wheel washing to prevent soils being spread onto roads outside of the construction area.

Appropriate measures will be put in place through the CEMP to mitigate potential effects of contamination during concrete pouring, for containment of fuelling operations, and during fire fighting events. Measures will be based on best practice (e.g. EA PPG Notes and CIRIA) and will include the following:

• designated facilities designed and used for oil and fuel storage and refuelling (incorporating appropriate bunds and drip trays);

• a site oil, chemical and product inventory;

• a list of emergency procedures, responsive to a risk assessment of areas of high sensitivity;

• site induction of all personnel on emergency spillage procedures and staff trained in emergency procedures;




• a contact list for the emergency services, the relevant environmental regulators, the local water supply and sewerage undertakers, the Health and Safety Executive and specialist clean up contractors, if required; and

• emergency response equipment available at appropriate locations.

Prior to the commencement of construction and as a proposed Requirement accompanying the DCO, a “Pollution Incident Response Plan” will be developed. In the event of an accidental spillage, the “Pollution Incident Response Plan” will become effective. This plan will be part of the contractor’s registered Quality Assurance programme to ISO 9001 and ISO 140001. This will include details for the relevant contact at the MMO, should a marine pollution incident occur. The Pollution Incident Response Plan will be agreed with consultees including the MMO and the Environment Agency prior to commencement of the works.

The storage of potentially polluting materials will be kept to a minimum where practicable and, where less hazardous or inert materials are available, these will be specified. In addition, absorbent mats/pads, absorbent granules and sand will be made available, and site operatives trained in their use, to deal with any spillages.

The positioning of fuel storage tanks and other potentially polluting materials and maintenance facilities will be on bunded areas of hard standing with dedicated drainage systems. Bunded areas will provide 110% storage volume to contain any possible leaks. Refuelling will also be undertaken only in designated areas, where small spills and leaks can be controlled. Drip trays and absorbent materials will be used to control spills.

Washing out of concrete wagons or other equipment used in concreting operations will be undertaken in designated contained washout areas or offsite.

To mitigate the low risk of impact to controlled waters by riverside construction activities, silt screens or silt curtains will be utilised as a precautionary measure. These will screen the working area preventing suspended sediments from being transported away from the Facility Site works. The silt curtain will be installed by an appropriate contractor prior to commencement of the works. The curtain will initially be laid out in sections that will be joined together on shore. It will be moved into place using a small boat and the skirt and chain ballast lowered from the floating boom. Depth measurements will be taken to confirm that the chain ballast rests on the bottom of the estuary and a provision is made for water depth during high tide. This will remain in place for a further period of time after the construction works to allow settlement of sediments. This will comply with the requirements of Pollution Prevention Guidance note 5 (PPG5)




Drawings Figure 1: Remedial Secant Pile Wall location

Figure 2: Aerial Photograph

Figure 3: Indicative Cross Section A – A’




Appendix A: Options Report – Riverwall Remedial Works, D/I/D/81354/105


Options Report – Riverwall Remedial Works

February 2012

RES North Blyth Renewable Energy Plant D/I/D/81354/105


North Blyth Renewable Energy Plant D/I/D/81354/105

CONTROL SHEET

CLIENT: RES

PROJECT TITLE: North Blyth Renewable Energy Plant

REPORT TITLE: Option Report – Riverwall Remedial Works

PROJECT REFERENCE: 81354 Issue and Approval Schedule:

ISSUE 1

DRAFT Name Signature Date

Prepared by A Lowes

Reviewed by I Gill

Approved by I Gill

Revision Record:

Issue Date Status Description By Chk App

2

3

4

5

This report has been prepared in accordance with procedure OP/P03 of W A Fairhurst & Partners' 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 ..................... 2 5.0 TECHNICAL CONSTRAINTS .................................................................................... 2 6.0 OPTIONS FOR REMEDIAL WORKS ......................................................................... 3 7.0 SEQUENCE OF CONSTRUCTION FOR EACH OPTION .......................................... 3 8.0 ESTIMATED BUDGETARY COSTS .......................................................................... 7 9.0 ADVANTAGES AND DISADVANTAGES FOR EACH OPTION ................................ 0 10.0 SUMMARY .................................................................................................................. 0



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 re-align the roundabout and access road to Battleship Wharf closer to the River Blyth. At this location the existing riverwalls are in a poor state of repair with a partial collapse in one section and gaps in the wall leading to the wash out of retained material in other sections.

Report D/I/D/81354/100 “Improvements to Access – Consideration of Proximity to Old River Walls” and the subsequent method statement D/I/D/81354/104 “Preliminary Method Statement – River Wall Remedial Works” recommended placing a rock berm against the wall to prevent it from collapsing into the intertidal mudflats on the basis that this is the most cost effective and simplest form of remedial works. This solution would extend into the intertidal mudflat and as such there is the potential for habitat loss. After discussion with RES, Natural England and The Environment Agency it has been agreed that other options for remedial works to the river wall which would not cause habitat loss should be considered.

This Option Report discusses the possible different forms of construction which could be implemented to repair the riverwall to minimise the potential for further instability and wash out of material which would lead to surface deformations in the area of the revised road layout. This method statement has been prepared based on the general arrangement shown in report D/I/D/81354/100 “Improvements to Access – Consideration of Proximity to Old River Walls”.

2.0 LOCATION OF WORKS The location of the road improvements and the remedial works to existing river walls is at the southern end of Battleship Wharf, North Blyth.

3.0 FORM OF STRUCTURE The proposed new access will involve widening the existing road on the riverwards (West) side. Allowing for a 2m wide verge and a small batter down to existing levels, the riverwards edge of the new road works will be approximately 9m closer to the river edge than the existing kerb line at the widest part of the new works.

The river edge is made up of old concrete or masonry walls which vary in distance between approximately 5 and 20metres from the kerb line of the existing road.

Working from the Port entrance gate the first section of wall is approximately 20m in length, is made from stone masonry and gradually increases in height to approximately 2metres where it meets an old timber jetty. A section of this wall has collapsed and an area behind has been washed out by the river/sea. This area has been roughly infilled with whinstone rocks to prevent further washout.

The next section of wall is approximately 20metres in length, is made from mass concrete and is approximately 3metres in height. This wall is founded on timber piles but there are gaps between the timbers and thus allowing material to wash out and the ground behind the wall is subsiding.

The third section of wall is approximately 35m in length leading up to the roundabout is another stone masonry wall approximately 2 metres high but reducing in height towards the roundabout. This wall is in reasonable condition but there are some areas where washout is occurring through open joints.




4.0 EXPECTED GEOTECHNICAL AND HYDROLOGICAL CONDITIONS On the riverward side of the river walls, beach sand overlying bedrock has been observed. Outcrops of rock are visible, as such, the beach sand deposit is expected to be thin, less than 1m thick.

The area retained by the river walls has historically been reclaimed from the estuary. Site investigation information for this area is not available, however based upon geotechnical information from other locations within the Battleship Wharf area, the following ground conditions to the landward side of the river walls are inferred:

Ground level to 2-3m below ground level

Made Ground consisting of heterogeneous colliery spoil

2-3m to 4-5m below ground level

Wind Blown Sand

Below 4-5m below ground level

Bedrock consisting of moderately strong to strong sandstone

Colliery spoil is likely to contain contaminants including heavy metals, sulphate, combustible materials, high pH, Polycyclic Aromatic Hydrocarbon (PAH) and Total Petroleum Hydrocarbon (TPH), however these contaminants tend to be present in low concentrations with low solubility. From historical records for the area it is considered that the fill has been in place for over 100 years and as such any leachable concentrations of contaminants are likely to have been leached from the soil in the intertidal groundwater zone close to the wall by tidally influenced groundwater movement.

Land immediately north of this location is known to have been historically contaminated by past uses as railway sidings and for ship breaking activities. The affected area was remediated by Northumberland County Council in 1993 and as such any contamination which may remain is likely to be localised, isolated and in low concentrations.

The tidal range in the River Blyth is:


Mean High Water Springs +2.44m OD

Mean Low Water Springs -1.88m OD


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

1. Existing riverwalls which potentially are unstable.

2. Working in the intertidal zone.

3. Presence of made ground which may contain low concentrations of contamination.

4. Shallow competent bedrock.

5. Presence of wading and sea birds in the surrounding area.




6.0 OPTIONS FOR REMEDIAL WORKS The following options for remedial works have been considered:

1. Repair to gaps in the wall and construction of a rock revetment on the riverward side of the existing river wall.

2. Construction of a new riverwall using sheet piles immediately in front of the existing wall.

3. Construction of a new retaining wall using sheet piles immediately to the rear of the existing wall.

4. Construction of a new retaining wall using bored piles immediately to the rear of the existing wall.

5. Construction of a new gravity retaining wall immediately to the rear of the existing wall.

The form of construction and the advantages and disadvantages of each form of construction are discussed in the sections below.

7.0 SEQUENCE OF CONSTRUCTION FOR EACH OPTION Presented below is a brief summary of the sequence of construction for each option considered.

7.1 Option 1 Repairs to the Existing Wall And Rock Revetment The sequence of construction for the remedial works for the river wall is discussed below and split into the three sections of wall identified in report D/I/D/81354/100 and summarised in Section 3 above.

For all three sections, work will take place at low tide to prevent the disturbance of sediments within the river, to allow access to the existing river wall to carry out the works and also to examine the formwork for leaks.

Any joints on the inside of formwork will be taped prior to concrete pouring. The concrete will press the tape onto the shutter when it is poured, preventing any leaks.

An anti washout additive will be added to the concrete if it is considered there will be wave action before the concrete cures.

The final timescale will be determined during detailed engineering design for the works.

7.1.1 Section 1 – Stone Masonry Wall between Battleship Wharf and the Timber Jetty For this section of the wall, in the area where the collapsed section has been infilled, the following works should be undertaken:

1. Select a position for a long reach excavator to stand and work as far back from the existing wall and area affected from wash out as possible.

2. The existing stone infill should be removed using a long reach excavator and the stone sorted and laid out for possible reuse in the works.

3. Remove beach sand from the area in front of the collapsed wall to expose bedrock.

4. After removal of existing infill and beach sand a filter cloth should be laid over the exposed face of material retained behind the wall. The filter cloth should extend vertically over all exposed retained material and horizontally across the area of river bed which will receive armour stone. The filter cloth shall be securely fixed into position.




5. A rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective layer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm. As shown on Drawing 81354/1003 the armour will extend to the full height of the wall, less an allowance for wave break.

7.1.2 Section 2 – Mass Concrete Wall between Timber Jetties 1. Remove sand from in front of the riverwall.

2. In locations where there are gaps in the timbering or the wall, erect formwork and place concrete.


4. After concrete has cured a rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective layer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm.

7.1.3 Section 3 – Stone Masonry Wall between the Timber Jetty and the Staithe 1. Remove sand from in front of the riverwall.



4. After concrete has cured a rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective layer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm.

7.2 Option 2 Construction of a New Sheet Pile Wall In Front of the Existing Wall The sequence of construction for the remedial works for the river wall for this option is discussed below.

Due to the presence of shallow bedrock, an adequate penetration depth for the piles to prevent instability of the wall cannot be achieved using conventional pile driving methods. It will be necessary to cut a trench into the bedrock into which the sheet piles will be inserted and set in place using concrete. It is anticipated that the depth of the trench would be in the order of 2-4m below the bedrock horizon, depending on the depth of surface weathering in the rock.

Works to excavate the trench could only be carried out during periods of low water.

Prior to breaking out of the rock to form the trench the overlying superficial deposits would have to be removed and where necessary the existing wall would be shored to allow excavation plant to be positioned close to the edge.

The trench would be excavated using conventional hydraulic breaker equipment mounted on a long reach excavator. Arisings from the trench would be removed using a long reach excavator.




After the trench is formed the sheet piles would be set into the trench and fixed in place using mass concrete. Concrete will only be placed at low water when the top of the trench is above the water level. An anti washout additive will be added to the concrete to prevent migration of the concrete into the watercourse as the tide rises.

After the concrete has cured the small gap between the existing wall and the new sheet pile wall will be infilled with either clean processed granular arisings generated from the construction of the pile toe trench or mass concrete.

7.3 Option 3 Construction of a New Sheet Pile Wall to the Rear of the Existing Wall The sequence of construction for the remedial works for the river wall for this option is discussed below.

Due to the presence of shallow bedrock, an adequate penetration depth for the piles to prevent instability of the wall cannot be achieved using conventional pile driving methods. It will be necessary to cut a trench into the bedrock into which the sheet piles will be inserted and set in place using concrete. It is anticipated that the depth of the trench would be in the order of 2-4m below the bedrock horizon, depending on the depth of surface weathering in the rock.

Works will be carried out in the intertidal zone and groundwater fluctuation matching the state of tide can be expected.

Prior to remedial works being undertaken it will be necessary to undertake site investigation to determine the profile of the rear of the wall, founding stratum and chemical characteristics of the made ground to confirm:

• Any shoring works to maintain wall stability during the works. • The potential to liberate contamination in the made ground into the water column

during the time the excavation is open. • Determine the mitigation measures required to prevent short term contamination

of the river during construction works.

It will be necessary to construct a trench through the made ground and natural blown sand deposits to expose the bedrock surface. The existing wall will be shored appropriately to prevent collapse while the trench is open. On the landward side the trench side would be battered back to a safe angle. This will expose potentially contaminated materials at the surface of the slope which may have an increased leachable potential due to exposure in the face of the excavation. If necessary mitigation measures identified during the site investigation phase will be implemented to prevent contamination. It is anticipated that, if required, this would take the form of covering the cut face with an impermeable membrane.

Works to excavate the trench could only be carried out during periods of low water.

The trench would be excavated using conventional hydraulic breaker equipment mounted on a long reach excavator. Arisings from the trench would be removed using a long reach excavator.

After the trench is formed the sheet piles would be set into the trench and fixed in place using mass concrete. Concrete will only be placed at low water when the top of the trench is above the water level. An anti washout additive will be added to the concrete to prevent migration of the concrete into the watercourse as the tide rises.

In Section 1 of the existing wall as identified in report D/I/D/81354/100, the existing rock revetment would be reinstated against the new sheet pile wall.




In Sections 2 and 3 the existing wall would be tie back to the new sheet pile wall to prevent the existing wall collapsing into the river. The form of the tied back would be determined after inspection of the rear face of the wall to determine its condition, however it is considered that it will include mass concrete infill between the walls together with a tie at crest level.

7.4 Option 4 Construction of a New Bored Pile Wall to the Rear of the Existing Wall The sequence of construction for the remedial works for the river wall for this option is discussed below.

As an alternative to using sheet piles interlocking bored concrete piles could be utilised to form a new retaining wall to the rear of the existing river wall. This method of construction does not require the excavation of the superficial deposits or the construction of a trench in the bedrock. The piles would be bored through the superficial deposits into the rock to sufficient depth to provide an adequate factor of safety against collapse. It is anticipated that this would be to a similar depth to as required for the sheet piles.

In Section 1 of the existing wall as identified in report D/I/D/81354/100, the existing rock revetment would be reinstated against the new bored pile wall.

In Sections 2 and 3 the existing wall would be tied back to the new bored pile wall to prevent the existing wall collapsing into the river. The form of the tie back would be determined after inspection of the rear face of the wall to determine its condition, however it is considered that it will include mass concrete infill between the walls together with a tie at crest level.

7.5 Option 5 Construction of a Gravity Wall to the Rear of the Existing Wall The sequence of construction for the remedial works for the river wall for this option is discussed below.

As an alternative to an embedded sheet pile or bored pile wall a gravity retaining wall, constructed in mass concrete could be constructed to the rear of the existing river wall.

Based upon an assumed height of wall of 5m it is anticipated that the base of the wall would be approximately 3m wide. The rear face would be tapered or stepped to give an assumed wall thickness at the crest of the wall of 1m.

Works will be carried out in the intertidal zone and groundwater fluctuation matching the state of tide can be expected.

Prior to remedial works being undertaken it will be necessary to undertake site investigation to determine the profile of the rear of the wall, founding stratum and chemical characteristics of the made ground to confirm:

• Any shoring works to maintain wall stability during the works. • The potential to liberate contamination in the made ground into the water column

during the time the excavation is open. • Determine the mitigation measures required to prevent short term contamination

of the river during construction works.

It will be necessary to excavate through the made ground and natural blown sand deposits to expose the bedrock surface. The existing wall will be shored appropriately to prevent collapse while the trench is open. On the landward side the trench side would be battered back to a safe angle. This will expose potentially contaminated materials at the surface of the slope which may have an increased leachable potential




due to exposure in the face of the excavation. If necessary, mitigation measures identified during the site investigation phase will be implemented to prevent contamination. It is anticipated that, if required, this would take the form of covering the cut face with an impermeable membrane.

Works to excavate the work area could only be carried out during periods of low water.

After the rock surface has been exposed localised breaking out will take place to level the surface and remove any loose materials. Once the surface has been prepared, formwork will be erected into which the concrete will be placed. Any joints on the inside of formwork will be taped prior to concrete pouring. The concrete will press the tape onto the shutter when it is poured, preventing any leaks. As the concrete will be contained within the formwork it is not considered that an anti washout additive will be required.

After the concrete has been cured the formwork will be removed. And the excavated area will be infilled with the original arisings. Surplus arisings will either be reused within the main works or disposed of off site.

In Section 1 of the existing wall as identified in report D/I/D/81354/100, the existing rock revetment would be reinstated against the new gravity wall.

In Sections 2 and 3 the existing wall would be tied back to the new gravity wall to prevent the existing wall collapsing into the river. The form of the tied back would be determined after inspection of the rear face of the wall to determine its condition, however it is considered that it will include mass concrete infill between the walls together with a tie at crest level.

8.0 ESTIMATED BUDGETARY COSTS For the options considered, the estimated costs are presented below:

8.1 Option 1 - Repair to gaps in the wall and construction of a rock revetment on the riverward side of the existing river wall For this costing the following assumptions have been made.

• The length of wall to be treated is 75m.

• The average cross section of the revetment is 3m high x 9m wide with a cross sectional area of 13.5m2.

• Revetment will be constructed from locally sourced Whinstone.

• Method Related Charges and Preliminaries include for intertidal working and minor operations.

• A provisional sum of £10,000 has been allowed for localised repairs to wall.




Principal Items Quantity Rate Cost Supply Rock for revetment 1015m3 £50/m3 £50,750

Supply and place filter cloth 2025m2 £5/m2 £10,125

Repairs to existing wall - Prov. Sum. £10,000

Lay revetment 1015m3 £10/m3 £10,150

Sub total £81,025

Method Related Charges and Preliminaries (25%)

£20,275

Total £101,300

8.2 Option 2 - Construction of a new riverwall using sheet piles immediately in front of the existing wall For this costing the following assumptions have been made.


• The depth of embedment will be a worst case of 5m.

• Sheet piles will be AZ36 section (170kg/m2).

• The average retained height is 3m. As such, the area of piles required is 600m2 and the tonnage is 102 tonnes.

• The gap between the piles and the wall will be filled with marine grade concrete containing an anti segregation admixture.

• A provisional sum of £25,000 has been allowed for localised stabilisation of the wall whilst the trench to receive the sheet piles is excavated.

• The piles will be set into the trench and concreted in place using marine grade concrete containing an anti segregation admixture.


• Arisings from trench excavation can be disposed of as inert material.

Principal Items Quantity Rate Cost

Temporary works to stabilise wall during trench excavation

Prov sum £25,000

Excavate trench 5m deep x 0.5m wide 200m3 £25/m3 £5,000

Supply and pitch piles 102 tonnes £1000/tonne. £102,000

Concrete infill to trench 200m3 £100/m3 £20,000 Concrete infill between piles and wall 80m3 £100/m3 £8,000

Dispose of arisings from trench excavation

200/m3 £40/m3 £8,000

Sub total £168,000





£42,000

Total £210,000

8.3 Option 3 - Construction of a new retaining wall using sheet piles immediately to the rear of the existing wall For this costing the following assumptions have been made.



• Sheet piles will be AZ36 section (170kg/m2).

• The average retained height is 3m. As such, the area of piles required is 600m2 and the tonnage is 102 tonnes.

• A provisional sum of £25,000 has been allowed for localised stabilisation of the wall whilst the trench to receive the sheet piles is excavated.

• A provisional sum of £10,000 has been allowed for slope protection to the rear of the wall.

• The piles will be set into the trench and concreted in place using marine grade concrete containing an anti segregation admixture.


• Arisings from trench excavation can be disposed of as inert material. There will be no arisings from the excavations from the rear of the wall.



Prov sum £25,000

Excavate working area to rear of wall, 5m wide with a back slope of 1 in 1.

£1,500m3 £2/m3 £3,000

Excavate trench 5m deep x 0.5m wide 200m3 £25/m3 £5,000

Supply and pitch piles 102 tonnes £1000/tonne. £102,000

Concrete infill to trench 200m3 £100/m3 £20,000

Tie wall to piles 75m £25/m £1,875

Dispose of arisings from trench excavation

200/m3 £40/m3 £8,000

Reinstate fill to rear of wall £1,500m3 £3.50/m3 £5,250

Sub total £170,125


£42,575

Total £212,700




8.4 Option 4 - Construction of a new retaining wall using bored piles immediately to the rear of the existing wall. For this costing the following assumptions have been made.



• The piles will be bored and 300mm in diameter at 450mm spacing. The concrete will be marine grade. No of piles required 167 with an average length of 8m.

• A piling platform will not be required.

• Arisings from superficial deposits can be reused to level site.


• Arisings from sandstone excavation can be disposed of as inert material. The arisings from superficial soils will be disposed of as non-hazardous material.


Install interlocking bored piles 167 nr £750 / pile. £125,250

Tie wall to piles 75m £25/m £1,875 Construct Capping Beam 20m3 £250/m3 £5,000

Dispose of arisings as inert fill 180/m3 £40/m3 £7,200

Sub total £139,325


£34,875

Total £174,200

8.5 Option 5 - Construction of a new gravity retaining wall immediately to the rear of the existing wall. For this costing the following assumptions have been made.


• The average retained height is 4m including foundation depth. As such, the base of the wall will be approximately 3m wide stepped to 1m wide at top. Cross sectional are of wall is 7.8m2.

• A provisional sum of £25,000 has been allowed for localised stabilisation of the wall whilst excavation at rear is open.

• A provisional sum of £10,000 has been allowed for slope protection to the rear of the wall.

• The wall will be constructed fusing marine grade concrete.





• Arisings from excavation can be disposed of as non-hazardous material.



Prov sum £25,000

Excavate working area to rear of wall, 5m wide with a back slope of 1 in 1.

£1,500m3 £2/m3 £3,000

Construct mass gravity wall 585m3 £130/m3. £76,050

Tie wall to piles 75m £25/m £1,875

Dispose of arisings as inert fill 585m3 £40/m3 £23,400

Reinstate fill to rear of wall £915m3 £3.50/m3 £3,200

Sub total £132,525


£33,175

Total £165,700

8.6 Cost Summary

The preliminary budget costing presented above is summarised as follows: Option Estimated Cost Repair to gaps in the wall and construction of a rock revetment on the riverward side of the existing river wall.

£101,300

Construction of a new riverwall using sheet piles immediately in front of the existing wall.

£210,000

Construction of a new retaining wall using sheet piles immediately to the rear of the existing wall.

£212,700

Construction of a new retaining wall using bored piles immediately to the rear of the existing wall.

£174,200

Construction of a new gravity retaining wall immediately to the rear of the existing wall.

£165,700




9.0 ADVANTAGES AND DISADVANTAGES FOR EACH OPTION Presented in the table below are the disadvantages and advantages of each option considered in relation to each other.

Option Advantage Disadvantage

1 Repairs to the Existing Wall And Rock Revetment

Lowest cost solution

Quick to Install

Lowest potential noise disturbance

Will not require excavation into potential contaminated fill

Requires land take in intertidal mudflats

2 Construction of a New Sheet Pile Wall In Front of the Existing Wall

Will provide new river wall in front of existing as such, no tie back arrangements needed.

Will not require excavation into potential contaminated fill.

Requires land take in intertidal mudflats

Disturbance of river sediments in vicinity of the works required.

Excavation could have destabilising effect on wall and shoring required.

Excavation of trench will require hydraulic breaker. Noise is likely to disturb birds and vibration may disturb aquatic life




Option Advantage Disadvantage

3 Construction of a New Sheet Pile Wall to the Rear of the Existing Wall

Does not require works in the river. Excavation work has potential to expose contaminated materials at surface with surface water contamination mitigation required.


Excavation of trench will require hydraulic breaker. Noise is likely to disturb birds and vibration may disturb aquatic life.

Tie back to existing wall could present difficult details

4 Construction of a New Bored Pile Wall to the Rear of the Existing Wall

Does not require works in the river.

Does not require excavation works through potentially contaminated fill.

Low noise and vibration operation.

Tie back to existing wall could present.

Potential contaminated arisings will require disposal.

5 Construction of a Gravity Wall to the Rear of the Existing Wall

Does not require works in the river.

Low noise and vibration operation.

Excavation work has potential to expose contaminated materials at surface with surface water contamination mitigation required.


Tie back to existing wall could present difficult details




10.0 SUMMARY It will be necessary to re-align the roundabout and access road to Battleship Wharf closer to the River Blyth. At this location the existing riverwalls are in a poor state of repair with a partial collapse in one section and gaps in the wall leading to the wash out of retained material in other sections.

The following options for remedial works have been considered:

1. Repair to gaps in the wall and construction of a rock revetment on the riverward side of the existing river wall.

2. Construction of a new riverwall using sheet piles immediately in front of the existing wall.

3. Construction of a new retaining wall using sheet piles immediately to the rear of the existing wall.

4. Construction of a new retaining wall using bored piles immediately to the rear of the existing wall.

5. Construction of a new gravity retaining wall immediately to the rear of the existing wall.

To summarise the advantages and disadvantages of each option:

• Of the options considered, Option 1 Repairs to the Existing Wall And Rock Revetment would present the lowest cost and simplest solution. However there is the significant disadvantage that land take in the intertidal mudflats would be required.

• Option 4 Construction of a New Bored Pile Wall to the Rear of the Existing Wall would provide a simple solution which has the lowest potential environmental impact in terms of risk of exposing potential contaminated materials, noise, vibration.

• Option 5 present the simplest form of construction for a new wall located behind the existing river wall. However, it does have the disadvantage that it will be necessary to expose potentially contaminated materials and there will be a requirement to dispose of potentially contaminated arisings.

As such, if work within the intertidal mudflats were to be permitted then the option to construct a rock revetment would present the simplest and most cost effective solution. However, if this will not be permitted then the construction of a concrete wall to the rear of the existing wall would present the most cost effective solution. Although slightly more expensive the bored pile wall would be the least risk option in terms of risk of pollution to the River Blyth as it would not require a large surface area of potentially contaminated material to be opened up to intertidal river water


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Appendix B: Preliminary Outline Method Statement for other Remedial Options- Riverwall Remedial Works, D/I/D/81354/10


Preliminary Outline Method Statement – Riverwall Remedial Works

September 2011

RES


D/I/D/81354/104



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 ................................................................................... 2

6.0 SEQUENCE OF CONSTRUCTION........................................................................... 2

APPENDIX A DRAWINGS




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 re-align the roundabout and access road to Battleship Wharf closer to the River Blyth. At this location the existing riverwalls are in a poor state of repair with a partial collapse in one section and gaps in the wall leading to the wash out of retained material in other sections.

This preliminary outline method statement discusses the works required to repair the riverwall to minimise the potential for further instability, wash out of material which would lead to surface deformations in the area of the revised road layout in this area. This method statement has been prepared based on the recommendations of report D/I/D/81354/100 “Improvements to Access – Consideration of Proximity to Old River Walls”.

This method statement has been prepared to discuss the technical procedures and sequence of works for the installation of the river wall remedial works. 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 location of the road improvements and the remedial works to existing river walls is at the southern end of Battleship Wharf, North Blyth.

3.0 FORM OF STRUCTURE

The proposed new access will involve widening the existing road on the riverwards (West) side. Allowing for a 2m wide verge and a small batter down to existing levels ,the riverwards edge of the new road works will be approximately 9m closer to the river edge than the existing kerb line at the widest part of the new works.

The river edge is made up of old concrete or masonry walls which vary in distance between about 5 and 20metres from the kerb line of the existing road.

Working from the Port entrance gate the first section of wall is approximately 20m in length, is made from stone masonry and gradually increases in height to about 2metres where it meets an old timber jetty. A section of this wall has collapsed and an area behind has been washed out by the river/sea. This area has been roughly infilled with whinstone rocks to prevent further washout.

The next section of wall is approximately 20metres in length, is made from mass concrete and is approximately 3metres in height. This wall is founded on timber piles but there are gaps between the timbers and thus allowing material to wash out and the ground behind the wall is subsiding.

The third section of wall leading up to the roundabout is another stone masonry wall approximately 2 metres high but reducing in height towards the roundabout. This wall is in reasonable condition but there are some areas where washout is occurring through open joints.

4.0 EXPECTED GEOTECHNICAL AND HYDROLOGICAL CONDITIONS

On the riverward side of the river walls beach sand overlying bedrock has been observed. Outcrops of rock are visible, as such, the beach sand deposit is expected to be thin, less than 1m thick.




The tidal range in the River Blyth is:


Mean High Water Springs +2.44m OD

Mean Low Water Springs -1.88m 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. Existing riverwalls which potentially are unstable.

2. Working in the intertidal zone.

6.0 SEQUENCE OF CONSTRUCTION

The sequence of construction for the remedial works for the river wall is discussed below and split into the three sections of wall identified in report D/I/D/81354/100 and summarised in Section 3 above. Proposals are shown on Drawing 81354/1003.

For all three sections work will take place at states of low tide to prevent the disturbance of sediments within the river, to allow access to the existing river wall to carry out the works and also to examine the shuttering for leaks.

Any joints on the inside of shuttering will be taped prior to concrete pouring. The concrete will press the tape onto the shutter when it is poured, preventing any leaks.

An anti washout additive will be added to the concrete if it is considered there will be wave action before the concrete cures.

The final timescale will be determined during detailed engineering design for the works.

6.1 Section 1 – Stone Masonry Wall between Battleship Wharf and the Timber Jetty

For this section of the wall, in the area where the collapsed section has been infilled, the following works should be undertaken:

1. Select a position for a long reach excavator to stand and work which is as far back from the existing wall and area affected from wash out as possible.

2. The existing stone infill should be removed using a long reach excavator and the stone sorted and laid out for possible reuse in the works.

3. Remove beach sand from the area in front of the collapsed wall to expose bedrock.

4. After removal of existing infill and beach sand a filter cloth should be laid over the exposed face of material retained behind the wall. The filter cloth should extend vertically over all exposed retained material and horizontally across the area of river bed which will receive armour stone. The filter cloth shall be securely fixed into position.

5. A rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective layer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm. As shown on Drawing




81354/1003 the armour will extend to the full height of the wall, less an allowance for wave break.

6.2 Section 2 – Mass Concrete Wall between Timber Jetties

1. Remove sand from in front of the riverwall.



4. After concrete has cured a rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective nayer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm..

6.3 Section 3 – Stone Masonry Wall between the Timber Jetty and the Staithe

1. Remove sand from in front of the riverwall.



4. After concrete has cured a rock armoured revetment shall be placed in the section of collapsed wall. A core of well graded rock shall be placed against the wall extending out at a gradient of 30˚ down to the riverbed. A protective nayer of 250mm to 500mm armour stone shall be laid to a thickness of 750mm..



February 2011

APPENDIX A

DRAWINGS


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bored pile wall

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