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Stubbins and Ramsbottom Flood Risk Management Scheme Ground Investigation Summary December 2011

GTG20110263/Geotechnical Memo 1

To: Russell Butcher (Atkins Rivers and Coastal)

From: Catherine Kenny (Atkins GE)

Caroline Jones (Atkins L&D)

Email: [email protected]

Phone: 01925 238481 Date: 16 December 2011

Ref: GTG20100937 cc: Paul Coney, Owen Mills

Subject: Ramsbottom Flood Risk Management Scheme Project Appraisal Report (PAR)

Ground Investigation Summary

Russell,

Following the completion of the recent ground investigation at Ramsbottom, we have pleasure in providing the following summary of our main findings and recommendations.

Introduction

Atkins Ground Engineering (GE) and Atkins Land and Development (L&D) were instructed by Atkins Rivers & Coastal (R&C) to provide a desk study memo for a section of the River Irwell at Ramsbottom, Lancashire. Subsequent to this, Atkins R&C instructed Atkins GE and L&D to specify and supervise a ground investigation and provide geotechnical and environmental input into a Project Appraisal Report (PAR). The site covers an approximate 2.5km length of the right bank of River Irwell located approximately between national grid references SD 791 182 in the north and SD 793 169 in the south. The site has been identified by the Environment Agency (EA) as susceptible to flooding in a 25 year event and the EA requires PAR for a preferred option to reduce the flood risk from the River Irwell in and to the north of Ramsbottom. The EA would like to consider introducing a series of raised flood defences to provide a fixed barrier between the river and ‘at risk’ properties, preventing flooding in events up to and including a 1 in 100 year flood event. Utilising the existing East Lancashire Railway (ELR) embankment as a flood defence is also to be considered. The ELR is currently operating as a steam railway.

A desk study was undertaken by Atkins and reported on 26th July 2011. The desk study made

recommendations for a ground investigation.

This memo summarises the available geotechnical and environmental information from the desk study phase and describes the ground investigation carried out. This memo also provides a summary of the ground conditions, a generic environmental assessment and an assessment of potential geotechnical risks to the scheme. Outline geotechnical design carried out is also discussed.

Available Information

Table 1: A summary of the information available

Information Source

Topography Walkover survey by Atkins Senior Geotechnical Engineer, conducted on 1st March

2011

Ordnance Survey Landranger Map 109 ‘Manchester’ (1:50,000 scale)

Geology British Geological Survey (BGS) Map Sheets – 76 ‘Rochdale’ (1:50,000 scale, Solid & Drift Editions)

BGS Lexicon of Named Rock Units

BGS Borehole Record Viewer

Coal & Brine Report provided by The Coal Authority (Ref: 00039480-11) dated 30 June 2011

Atkins Desk Study Memo (Ref: GTG20100121.132) dated 10th March 2010 –

HIRPS, River Irwell, Strongstry

Site Use & History Envirocheck Report supplied by Landmark Information Group (Ref: Ramsbottom PAR) dated 23

rd June 2011

Walkover survey by Atkins Geotechnical Engineer, conducted on 1st March 2011.

Continued….

GTG20110263Geotechnical Memo 2

……Table 1 (Continued)

Information Source

Ground Investigation Fugro Engineering Services Ltd: Ramsbottom Flood Risk Management Scheme:

Draft Factual Report on Ground Investigation Reference: CON113087 Dated November 2011

Site Description and History

For the purpose of the description, the site has been divided into two sections; Northern Extent and Southern Extent. The northern extent has been further subdivided into sections (STU-1 to STU-6) in accordance with the types of defence proposed. The locations of the sections (STU-1 to STU-6) are is described in the appended Figure 1 – Defence Height Overview Plan. Comments are provided below on the current proposals at each section. Northern Extent STU-1B: The existing East Lancashire Railway line runs on an embankment to the north of the site. Between the railway embankment and the River Irwell at this point is a low lying field (Pin Meadow). The meadow is bounded to the south east by residential properties, to the north east by the River Irwell and to the west by the railway line. It is proposed to construct a new wall across the south eastern end of this field. Historical maps indicate the present day housing to have existed since at least 1963. STU-2 The site then runs along the right bank of the River Irwell past a number of residential properties. In this area, residents have extended their gardens across a footpath and up to the river bank. It is understood that limited work is likely to be undertaken in this area, probably in the form of landscaping. Open grassland is present beyond the far (left) river bank. Historical maps show very little change in this area since the development of the site from open land to residential properties in the 1960s. STU-3: The River Irwell then passes beneath a road bridge carrying the A676 – Bolton Road North and at this point is bounded on the right bank by a combination of property gable walls and adjoining stone walls. To the east, the river is bounded by industrial buildings and wooded land. Historical maps show very little change in this area since the first edition map of 1893. STU-4-6 The site then follows the river as it runs adjacent to Dale Street in the north, curving round to sit adjacent the East Lancashire Railway embankment in the south. In the vicinity of Dale Street, the site is bounded by residential properties with commercial units (Cuba Industrial Estate) extending to the south towards the rail embankment. To the east, beyond the left bank of the river, open farm land is present. Current proposals are to construct a new flood defence wall along this section of the right bank of the river. Adjacent to Dale Street, it is understood that local ad hoc gravel extraction has taken place from within the river. Historical maps show a gas works at the southern end of this extent, until around 1963. Cotton mills are present within the Cuba Industrial Estate. By 1929, an area of filled ground is indicated in the Industrial Estate, with a unit constructed over it by 1983. Southern Extent The southern extent of the site runs from the East Lancashire Railway embankment in the north to the Bridge Street overbridge in the south. Current proposals are to utilise the existing railway embankment as a flood defence through this section. The section of land present between the right bank of the river and the railway embankment houses a waste water treatment works and an area of disused land. Open farmland is present beyond the left bank and river to the east of the study area.


Historical maps indicate many historical railway sidings between the Cuba Industrial Estate and the southern end of the site at Bridge Street. Cotton mills have been variously present adjacent to the site. By 1994, a Waste Water Treatment Facility comprising numerous tanks is apparent adjacent to the river bank.

Geology

The published geological maps indicate recent drift deposits of Alluvium within the River Irwell floodplain. River Terrace Deposits are shown to the west of the East Lancashire Railway in the north of the site. The recent deposits are shown to be underlain by Glacial Till, referred to as “Ribblesdale Drift” in the geological memoir. This drift comprises stiff plastic clay of blue-grey colour with limestone and chert boulders. Patches of Glacial Sand & Gravel are shown on the geological map to the west of the site area. The solid geology is shown to be Fletcher Bank Grit in the north and Haslingden Flags in the far south, both of which are Upper Carboniferous in age. A northwest-southeast trending fault separates the two geologies. It crosses the River Irwell approximately 350m upstream of the Bridge Street over bridge. The fault is downthrown to the southwest. The Fletcher Bank Grit is described as “sandstone and pebbly sandstone, coarse grained with angular grains, with quartz and quartzite pebbles, massive and current-bedded, and subordinate beds of mudstone and coal”. There is no description provided for the Haslingden Flags however, they are part of the Rossendale Formation which is described as “fine to very coarse-grained and pebbly, feldspathic sandstone, interbedded with grey siltstone and mudstone, and subordinate marine black shales, thin coals and seatearths”.

Hydrogeology

The majority of superficial deposits and bedrock have been classified as Secondary A aquifers. Some superficial deposits in the eastern most meanders of the river are indicated as unproductive strata.

The EA website indicates that the site is not within a groundwater protection zone. There are three

groundwater and two surface water abstractions within 100m of the site. The three groundwater abstractions

are used for industrial/ commercial ‘Process Water’.

Hydrology

The River Irwell flows in a southerly direction. The EA website indicates that the River in the area of the site has chemistry water quality classification of Grade B – good (data from 2009).

Outline Conceptual Site Model

Table 2 provides a preliminary conceptual site model (CSM) for the Northern Extent of the site, which has been developed based on currently available information. The CSM only includes identified sources on the right (western) bank of the river as it is considered unlikely that land uses on the opposite side of the river would have caused a significant impact on land quality.


Table 2 Conceptual Site Model for the Northern Extent of the Site

Sources Pathway Receptor

Potential contaminants in soil / groundwater originating from:

On-site:

Possible Gas Works Related Structures

Unspecified Depot

Cotton Mill

Fill material

Marshy ground Off-site

Gas Works

Cotton Mills

Woollen Mill

Various commercial premises

Pollution incidents / illegal waste

Railway land

Inhalation, ingestion and dermal contact with contaminants in soil and soil-derived dust from areas where soil is exposed at the surface.

Humans (on & off site current/future users)

Migration & inhalation of ground gas / vapours.

Humans and property (on & off site current/future users & buildings/services)

Lateral migration of free and dissolved phase contamination & entrained dust within surface runoff.

Leaching from unsaturated soils to groundwater, then lateral migration of dissolved and free phase contamination within groundwater

River Irwell

Vertical migration of leachable contaminants

Groundwater within the Secondary A aquifer.

It is understood that no flood defence works will be carried out on the Southern Extent of the site, with the aim being to utilise the existing railway embankment as the flood defence for the area. Table 3 provides a preliminary conceptual site model (CSM) for the Southern Extent of the site.

If flood waters cover the land in between the river and the railway embankment, there is a possibility for contaminants to be leached from the railway embankment material and from the land that has been flooded (formerly railway sidings and now a sewage works).

Table 3 Conceptual Site Model for the Southern Extent of the Site


Potential contaminants in soil / groundwater originating from:

Proposed flood embankment:

Railway sidings Between railway and river:

Railway sidings

Sewage works Off site to west:

Cotton Mills

Engineering Works

Various commercial premises

Landfill *

Petrol station

Inhalation, ingestion and dermal contact with contaminants in soil and soil-derived dust from areas where soil is exposed at the surface.

Humans (on & off site current/future users)

Migration & inhalation of landfill / ground gas.

Humans and property (on & off site only current/future users & buildings/services)

Continued…..


…..Table 3 (continued)


Migration of dissolved and free phase contamination & entrained dust within surface runoff.

Leaching from unsaturated soils to groundwater, then lateral migration of dissolved and free phase contamination within groundwater

River Irwell

Vertical migration of leachable contaminants

Groundwater within the Secondary A aquifer (solid and drift strata).

* The landfill is located to the south of the site and it is anticipated that groundwater flow in the area would not transport any leachate towards the site. Landfill gas might be able to migrate through unsaturated soils towards the site area.

Potential risks posed to construction workers have not been considered in this CSM. Temporary risks to work forces are normally assessed by the contractor and included in their Health & Safety documentation. The potential risks posed to buried foundations are considered in accordance with BRE Special Digest 1:2005 (see below).

Recent Ground Investigation

The ground investigation was carried out between 3rd

and 17th October 2011 by Fugro Engineering Services

under part time supervision from Atkins. The ground investigation comprised the following elements:

7 no. cable percussion boreholes to a maximum depth of 10.50m,

8 no. windowless sample holes to a maximum depth of 6.45m,

1 no. hand dug pit to a depth of 1.45m,

Groundwater monitoring installations in all exploratory holes. These installations were

monitored by Fugro following the site works.

The aims and objectives of the investigation were:

Geotechnical:

STU-1B: Pin Meadow: Confirm ground conditions along length of proposed flood defence wall.

STU-3: Dale Street: Confirm ground conditions, investigate existing wall foundations.

STU4-6 Cuba Industrial Estate: Confirm ground conditions along length of proposed flood defence wall.

Railway Embankment: Confirm ground conditions and assess suitability for use as flood defence.

Land Quality

Collect chemical data to allow generic quantitative risk assessment (GQRA) to inform material re-use

potential and preliminary waste characterisation.

Details of the ground investigation are provided in the Factual Report supplied by Fugro and included as

Appendix A. Both environmental and geotechnical laboratory testing were undertaken as part of this

investigation and are discussed in the following sections.


Geotechnical Laboratory Testing

The following geotechnical laboratory tests were scheduled by Atkins and undertaken by Fugro or their sub-contractors:

36 No. Atterberg Limits

36 No Moisture Content Determinations

13 No. Particle size distributions (by sieving and pipette, where appropriate)

8 No. Unconsolidated Undrained Triaxial Compressions

1 No. Laboratory hand vane

40 No. BRE Special digest 1 sulphate and pH suites (plus 26 No. total sulphate and pH

tests included within the environmental testing)

The results of the laboratory testing are included in Fugro’s Factual Report in Appendix A.

Environmental Sampling and Laboratory Analysis

Environmental soil samples were recovered from exploratory holes by Fugro. All soil samples were placed into clearly labelled jars and tubs provided by the testing laboratory, Derwentside Environmental Testing Services Ltd (DETS), a UKAS and MCERTS accredited laboratory, to undertake chemical analysis. Sample jars were temporarily secured in cool boxes prior to daily transfer to the laboratory, in accordance with chain of custody procedures.

27 no. soil samples, 12 no. soil-derived leachate samples and one water sample from the River Irwell were submitted for analysis. Full details of the suite of determinands and the results of the laboratory analysis are presented in full in Fugro’s Factual report in Appendix A.

Ground & Groundwater Conditions

The ground conditions are summarised below. For detailed descriptions of the site specific geology, please refer to the logs Fugro’s Factual Report in Appendix A. STU-1B: Pin Meadow 4 no. windowless sample holes were undertaken:

WS1T and WS2T at the northern extent of the scheme at the location of a proposed flood defence bund. (It is understood that the bund is no longer being considered).

WS4T and WS5T adjacent to the residential properties at the southern edge of the pin meadow, at the location of a proposed flood defence wall.


Table 4: Summary of Ground Conditions : STU 1B

Stratum Description

Range of Depths of Stratum

Encountered (m bgl)

Proven Thickness

Range (m)

SPT N Values

Plasticity Index

Sieve Analysis (%)

Topsoil Topsoil 0.0-0.3 0.3 - -

Alluvial Clay 1

Soft sandy clay with occasional cobbles

0.3-3.0

1.7-2.7 0,0,1,1,10,0 20, 13, 18

-

Organic Silt (WS2T only)

Very sandy silt with organic fragments

3.0-3.4 0.4 3 Non

plastic -

Sand and Gravel

Very clayey fine to coarse sand and gravel

2.0->5.45*

1.8-3.0 0,17,11, 16, 10, 38, 7, 41, 33

- Clay - -

Silt 15 4

Sand 32 12

Gravel 51 79

cobbles 2 5

Clay 2 (WS4T and 5T only)

Soft brown silty clay

4.0->6.45 >0.45 - >2.45 5,5,10,11 39,27 -

*Base not proven

Groundwater was not encountered.

STU-3: Dale Street: Foundations to existing wall

Hand dug pit HDTP1T was excavated to locate the foundations to an existing wall at the end of Dale Street. The pit was excavated on the river side of the wall.

The pit found that the stone wall continued to 1.35m bgl. No concrete footings were located. The wall was founded upon natural sand and gravel. Groundwater was not encountered.

STU-4 to 6: Cuba Industrial Estate

3 no. windowless samples (WS6AT, WS8T, WS10T) and one cable percussive borehole (BH3T) were undertaken adjacent to the river. WS7T was terminated at 0.60m where it was unable to penetrate a cobble and boulder horizon. (WS6T was not undertaken due to access restrictions and was moved to position WS6AT).

Table5: Summary of Ground Conditions : STU 4 to 6

Stratum Description


Encountered (m bgl)

Proven Thickness

Range (m)

SPT N Values

Plasticity Index

Sieve Analysis (%) and

Cu (kPa) (Triaxial test)

Made Ground*

Clayey sand with gravel and cobbles, wood fragments

0.0-2.0 0.6 – 2.0

10, 8

Mixed Alluvial Materials (Absent in WS10T)*

Soft organic clay, soft gravelly clay, sand, gravel,

0.6-4.0

1.1-2.0 27, 16, 27, 23 Clay -

Silt 18

Sand 33

Gravel 47

cobbles 2

Lower Alluvial Clay

Soft gravelly clay

1.8-4.60 0.45-1.8 38, 6, 8,

11, 28, 10 28, 8, 18, 25, 10

Continued…..


…..Table 5 (continued)

Stratum Description


Encountered (m bgl)

Proven Thickness

Range (m)

SPT N Values

Plasticity Index



Sand and Gravel

Very clayey fine to coarse sand and gravel, occasional cobbles

2.55-7.8

>0.55- 3.2 38,16, 11, 13, 33, 26, 13

Clay - -

Silt 2 12

Sand 7 48

Gravel 86 39

cobbles 5 1

Glacial Clay (WS6T BH3T only)

Firm grey clay/ soft slightly gravelly clay

4.55->10.5 (base unproven)

>0.45- >2.7 (thickness unproven)

13, 20 19, 22 Cu = 40 kPa

Groundwater was encountered at 3.3 m rising to 3.0m after 20 minutes in BH3T only.

*The mixed alluvial materials contained discontinuous layers of clay, sand and silt, which were sometimes organic. At the bottom of these materials, a clay layer (lower alluvial clay) was present on top of the layer of sands and gravels. In WS10-T, the deeper deposits of made ground in this location were present directly on top of a clay layer (lower alluvial clay). The mixed alluvial materials and made ground are sometimes similar hence the boundaries between these materials are not clearly defined.

Existing Railway Embankment

BHs 1T, 5T, 6T, 7T, 8T and 9T were undertaken from the top of the existing railway embankment. LIDAR survey data shows the embankment to be approximately 4m in height at the southern end of the scheme, which is approximately co-incident with the depth of made ground in this location.

Table6: Summary of Ground Conditions Railway Embankment

Stratum Description


Encountered (m bgl)

Proven Thickness

Range (m)

SPT N Values

Plasticity Index



Made Ground

Ballast and ash over sandy gravelly clay

0.00-4.30 3.0 - 4.30

6,1,8, 8, 25, 14, 19, 5, 9

15, 20 Cu = 24 kPa

Organic Layers (Absent in BH8T)

Soft sandy gravelly clay , clayey very gravelly sand, very soft organic silt

3.0 – 7.6

0.9- 4.6 25, 33, 19, 10, 32, 13, 8, 29, 9

17, 12, 13, 18, 20, 9, 9, 17

Clay - -

Silt 17 14

Sand 31 29

Gravel 51 57

cobbles 1 0

Cu = 15, 46, 9 kPa

Sand and Gravel (Absent in BH9T and BH6T)

Very clayey sandy gravel, gravelly cobbles,

3.80-9.10

1.2- 4.0 39, 34, 30,38, 30, 48, 25, 50/175, 16, 50/270, 27, 40

Clay - - - - - -

Silt 1 3 3 4 8 3

Sand 7 4 4 21 22 34

Gravel 67 55 55 64 68 30

cobbles 25 38 38 11 2 33

Glacial Boulder Clay

Soft to firm / Firm sandy slightly gravelly clay, laminated clay

7.2 - >10.5 (base unproven)

>1.4->2.9 (thickness unproven)

17, 30, 13, 18

29, 27,33, 29, 26, 23, 26

Clay 44

Silt 42

Sand 6

Gravel 8

cobbles 0

Cu = 67, 98 kPa


Groundwater was encountered within the sand and gravel layer at depths of between 5.3m and 8.6m bgl. Rises of between 0.2m and 0.75m were noted after a 20 minute observation, suggesting that the groundwater is under sub-artesian pressures. An additional strike was noted in one horizon of made ground at 1.8m bgl, rising to 1.4m after 20 minutes observation.

The sand and gravel layer required chiselling in order to advance the boreholes in BH1T, BH5T, BH7T, BH8T and BH9T. BH7T and BH9T also required chiselling in the made ground from 2.8 to 3.0m and from 2.1 to 2.3m bgl.

Visual and Olfactory Indicators of Soil Contamination

Ash and slag was noted in Made Ground from across the site area. In addition, an oily sheen was noted in Made Ground (ballast) recovered from BH6T. In WS8T a slight creosote odour was noted from 0.5m to 2.95mbgl and a slight hydrocarbon odour was recorded at 6mbgl in WS10T. The latter two being located close to the former gas works.

Environmental samples were screened on site using a photo-ionisation detector (PID). The PID did not indicate the presence of significant volatile contamination; the majority of readings being below the PID limit of detection with only a few samples recording values between this and the maximum value of 14ppm.

Laboratory analysis including a screen for asbestos was undertaken on selected samples and this is discussed in greater detail later in the memo.

Groundwater

Groundwater strikes during the ground investigation have been discussed in the section above.

Details of the installations are given in Fugro’s factual report. Generally, the installations were within the

granular horizons and around water strikes. The installations were monitored on two occasions by Fugro

after the completion of fieldworks. Monitored levels were between 124.4 and 130.3 mAOD.

Chart 1: Water Strikes and Monitored Water Levels

(Level m AOD v graphical distribution of borehole locations, not to scale)

120

122

124

126

128

130

132

134

136

Leve

l (m

AO

D)

1.11.2011

8.11.2011

Ground Level

Water Strike

Second water strike

BH1

BH3

BH5

BH6

BH7

BH8 BH9

WS1

WS2 WS

4

WS5 WS

6 WS8

WS10


Engineering Assessment

Outline geotechnical design has been carried out at STU-1B and STU-4-6 to allow the possibility of walls to be considered as flood defences. The ground conditions and geometry at STU-5 was used for the analysis at STU4-6 as a worst case section. The analysis included an assessment of the wall type, depth, height of defence possible and consideration of seepage issues.

The design of the wall was carried out in general accordance with the Eurocodes. At this outline design stage, only Design Approach 1 Combination 2 was considered as this will give the worst case scenario. The method of analysis for the different wall sections is summarised below:

STU-1B

Preferred wall type:

Embedded wall using steel or plastic sheet piles

Gravity wall.

Proposed flood defence height up to 1000mm

Method of analysis

The analysis was undertaken using programs from Oasys Ltd; Stawal (Stability of Retaining Walls) and Frew (Flexible Retaining Walls) and also Geosolve’s Wallap. All analyses were carried out under drained condition using effective stress soil parameters.

The analysis model considered the flood event with a 1m differential in water levels (i.e. the water level in front of the wall is at the top of the wall and the water level is at ground level behind the wall).

Ground model derived based on ground data given by WS4T and WS5T (see Fugro’s ground investigation factual report).

Design earth and water pressures derived as per EC7. The design considered wall friction ≤ ⅔ ’ and a surcharge of 5 and 10kPa behind the wall where applicable.

Results Poor ground conditions with SPT=0 to substantial depth were indicated by the ground investigation rendering the construction of shallow foundations impractical.

An embedded wall is feasible with a minimum indicative total pile length of 5.7m for a 1.0m flood defence wall, assuming a small pile such as an Arcelor AZ12 or 14, to be confirmed at detailed design.

A plastic sheet pile is not suitable due to insufficient structural capacity in bending and excessive deflections. A steel sheet pile will provide the required capacity and stiffness therefore will be suitable for use, however we understand this may not be considered viable on the basis of cost.

Seepage Seepage was not considered at this location

Conclusions An embedded wall option is feasible if steel sheet piles are used, however this option has been discounted due to cost. Plastic piles were found to be unfeasible. Recent communication established that the water level in front of the wall could be less than 250mm. We understand that the possibility of a gravity wall is currently being considered elsewhere, beyond the Pin Meadow. Further ground investigation should be undertaken to assess ground conditions at alternative sites.

STU-5

Preferred wall type:

Gravity wall

Proposed flood defence height is 600mm to 1500mm.

Method of analysis

The geotechnical analysis included a check for global stability (overturning, sliding, bearing, uplift) using effective stress soil parameters. Consideration of seepage was also carried out.

The analysis model checked for two flooding scenarios (Scenario 1 – water in


front of the wall is at the top of wall and at ground level behind wall. Scenario 2 – water has overtopped the wall with a 1m lag between the water levels).

Design earth and water pressures were derived as per EC7. The design considered wall friction as ≤ ⅔ ’ and a surcharge of 5 and 10kPa behind the wall where applicable.

Assumptions It is assumed that the proposed structure will not worsen the stability of the slope and that the existing slope is stable. It is also assumed that foundation settlement is not significant for this low height wall, but this will need to be confirmed at detailed design stage.

Results The stability checks indicate the following wall dimensions will be required as a minimum for the gravity wall:

Table 7: Proposed gravity wall dimensions

Flood defence height (mm)

Total wall height

(mm)

Wall thickness

(mm)

Total Base width

(mm)

600 1200 300 1500

(1000 front heel, 200 back heel)

1500 2700 300 4000

(3400 front heel, 300 back heel)

Seepage

Granular material is present from surface. Layers of clay are present within the made ground and underlying mixed alluvial materials but these appear discontinuous. A layer of clay between 0.45 and 1.8m in thickness is consistently present between 1.8 and 4.6m bgl, capping a gravel layer.

As water levels rise in the adjacent river, water is likely to seep under the wall because of the granular layers. A cut off, such as a clay filled trench, dug down into the natural clay layers could provide a solution. However, this will necessitate excavation to between 1.8 and 4.0m bgl and different excavation depths will be required in different areas of the site because of the variable nature of the ground. The trench could be formed of a bentonite grout or natural clay, or perhaps by utilising soil mixing to minimise spoil volumes.

At WS6T and BH3T, a cut off trench to 1.8m bgl will provide an adequate factor of safety against uplift. At WS10T, excavation to 2.0m would be required. However, between these two areas, WS8T indicates that there are no cohesive layers until 4.0m bgl. There are practical difficulties in excavating to this depth and a sheet piled cut off may be more appropriate. However, initial excavation will still be required to penetrate the layer of cobbles and boulders at the surface, and this solution may prove expensive. .

It is considered likely that a cut off trench will provide adequate protection against uplift and piping failure for this scheme, however, because of the ground conditions at WS8T, further consideration of the near surface materials will be required at detailed design stage.

Conclusions A gravity wall is found to be feasible and the proposed minimum wall dimensions are summarised in Table 7, pending consideration of some form of cut off below the wall to protect against seepage. As a worst case, this may need to be a sheet pile in some locations. It is worth noting that the base width of the gravity wall is high. This is mainly due to the water pressure and the low overburden weight.

If overtopping is considered possible, protection, probably by utilising a concrete slab, should be provided at the land side of the wall to protect against scour.


Existing Railway Embankment

Defence Type Existing railway embankment.

Seepage Groundwater will flow beneath the embankment in the granular layers. The boreholes undertaken from the top of the embankment suggest that the granular layers occur at or close to the ground surface at the base of the embankment. Further ground investigation would be required to fully assess the ground conditions beyond the toe of the embankment on the dry side (for example, to assess the depth of the gravel layer more accurately). The granular layers have been assumed to be present from the ground surface as a worst case for the seepage analysis.

A simple hand drawn flow net was constructed to analyse the potential for piping and an adequate factor of safety was calculated. This risk of piping is therefore considered generally low due to the low head differentials, and the width of the embankment (around 30m).

Although the risk of piping is low, due to the high permeability of the granular material, seepage will occur under the embankment which could cause flooding on the land side of the embankment. It is understood that this is acceptable providing that significant failure (e.g. piping or uplift) does not result. The amount of seepage will depend upon the duration of a flood event and should be considered further during detailed design.

Other Considerations

It is likely that the use of the embankment as a flood defence will result in continued wetting and drying and erosion of the historical made ground fill that forms the embankment. This may cause localised instability of the embankment. This could be mitigated by an engineered clay face on the river side of the embankment, or perhaps by using a protective liner. This should be further considered at detailed design.

Conclusions Seepage will occur due to the granular horizons below the embankment, which may lead to flooding on the land side of the embankment.

Concrete Design

Water soluble sulphate and pH testing was undertaken on samples from all boreholes. Results indicate that

in accordance with BRE Special Digest 1:2005 concrete should be classified as Design Sulphate Class DS-

1, with the site allocated an ACEC of AC-1.

Generic Quantitative Risk Assessment (Re-use of Site Won Materials Assessment)

Introduction

Atkins has used the information obtained during the ground investigation to assess the potential risk from the contaminant concentrations detected to the receptors identified in the CSM. Contaminant concentrations in soil and soil leachate from the exploratory holes have been screened against appropriate generic assessment criteria (GAC).

The GQRA will enable the preliminary risk posed by the contamination to be identified and provide an indication of re-use potential. The results of the GQRA screen are attached to this memo in Appendix B.

Controlled Water Receptors GQRA

The primary receptor is the River Irwell with consideration also given to the underlying Secondary A aquifers.

Concentrations of contaminants were measured in leachate taken from soil samples collected during the ground investigation. These give an indication of the concentration at which contaminants might leach from


the Made Ground. The GACs selected are Environmental Quality Standards (EQS) freshwater/inland applicable under the Water Framework Directive (WFD) to allow assessment of risk posed to the River Irwell. In the absence of an EQS WFD value, a Drinking Water Standard (DWS) has been selected. Hardness was tested in a water sample taken from the River Irwell to inform the GAC selection (as some water standards are hardness dependent). The resulting hardness was 112mg/l CaCO3.

The following contaminants were recorded at concentrations that exceeded their respective GAC:

Table 8 – Leachate concentrations in Excess of GAC

BH3T BH8T WS7T WS8T

Determinand Units GAC 0.5m 0.5m 0.2m 1.2m

Nickel µg/l 20 not

detected not

detected not

detected 190

Cyanide (Total) µg/l 50 not

detected not

detected 120 not

detected

Benzo[a]pyrene µg/l 0.05 0.07 0.07 0.29 0.06

Benzo[b]fluoranthene µg/l 0.03

0.07 0.05 0.43 0.12

Benzo[k]fluoranthene µg/l 0.04 0.03 0.19 0.04

Fluoranthene µg/l 0.1 0.12 0.08 0.58 0.16

Indeno[1,2,3-cd]pyrene µg/l 0.002 0.13 0.12 0.74 0.08

Benzo[g,h,i]perylene µg/l 0.05 0.04 0.31 0.03

Sum of 4 PAHs µg/l 0.1 0.29 0.24 1.67 0.27

Samples which have leachate exceedences are from boreholes located in the Northern Extent of the site along the river bank where the former mill/depot and gas works were located (BH3T, WS7T and WS8T) and in BH8T which is on the railway embankment in the Southern Extent of the site. However samples collected from other parts of the railway embankment (BH1T, BH5T, BH6T and BH9T) and Pin Meadow (WS4T) did not have any GAC exceedences.

The generic screen indicates the reuse of site won materials (Made Ground) may pose an unacceptable risk to controlled waters, with the exception of soils recovered from Pin Meadow which do not pose an unacceptable risk to the River.

Once the design of the scheme has been finalised we would recommend re-visiting the assessment and considering the likely migration pathway and pollutant linkages of each area in detail. For example if material that has leachate exceedences is re-used under hardstanding and in the unsaturated zone then it is unlikely that the contaminants would be able to leach from the materials at concentrations likely to cause significant risk to the river or groundwater.

If however the detailed consideration of the migration pathway still indicates a unacceptable risk then further investigation/assessment such as undertaking a detailed quantitative risk assessment (DQRA) might be necessary.

Human Receptors GQRA

The GAC selected are the Atkins’-derived Soil Screening Values (SSVs) based on availability of land uses and a conservative soil organic matter content of 1% in the absence of analytical data. SSVs have been selected based on the use of the land adjacent to the proposed flood defence works. Land use along the route is a mixture of residential, commercial and open space. In order to be suitably conservative, the chemical results have initially been screened against the SSVs for ‘Residential Without Consumption of Homegrown Produce’ scenario.. Concentrations have also been screened against ‘Commercial’ landuse SSVs which represent a less stringent screen and give an indication as to the potential for site won material to be re-used in the less sensitive areas along the route (e.g. along the depot/former mill area of the site).


Potential risks posed to construction workers have not been considered in this human health GQRA, as the screening values are based on long term exposure.

Of the samples scheduled for laboratory testing several samples reported CoCs above the residential criteria as summarised in Table 9.

Table 9 –Soil concentrations in Excess of Residential Without Uptake GAC

BH3T

BH7T

BH8T WS4T

WS7T

WS8T

WS8T

WS10T

Test Units GAC 0.5m 0.5m 0.5m 0m 0.2m 0.6m 2.45

m 1.20-

1.40m

Arsenic mg/kg 35 9.1 41 26 54 27 45 27 13

Benzo(a)anthracene mg/kg 5.42 1.9 0.5 16 not

detected 14 3.9 11 0.7

Benzo(a)pyrene mg/kg 0.989 2.5 0.6 23 not

detected 12 3.1 13 1

Benzo(b)fluoranthene mg/kg 9.68 1.8 0.6 27 not

detected 15 5.2 8.9 1.1

Dibenzo(a,h)anthracene mg/kg 0.949 0.4 0.2 3.9

not detected 1.7 0.3 1.5 0.2

Indeno(1,2,3-c,d)pyrene mg/kg 9.53 2.1 0.5 19

not detected 15 3 9.4 1.1

Naphthalene mg/kg 0.598 0.1 0.2 0.03 not

detected 1.6 0.07 1 5.3

Only the WS4T exceedence is located adjacent to residential housing (in Pin Meadow), all the remaining exceedences are located either along the depot/former mill/gas works area of the Northern Extent or on the railway embankment. The open spaces SSV for arsenic is 138mg/kg and as only one of the three Pin Meadow samples exceed the residential SSV and this exceedence is much lower than the open spaces SSV, it is unlikely that re-use of soils sourced from Pins Meadow would pose an unacceptable risk to future site users or residents.

When compared to the commercial SSVs; only the reported concentration of benzo(a)pyrene in BH8T (23mg/kg) was in excess of the GAC (14.3mg/kg). However, even the commercial GAC are very conservative for the railway embankment as only railway maintenance workers and possibly trespassers are likely to access the embankment.

Several compounds were identified above the MDL for which there is no GAC available to screen against in samples from the depot/former mill/gas works area of the Northern Extent and railway embankment. These include:

Methylene chloride.

2-methylnaphthalene.

Dibenzofuran.

Carbazole.

The recorded concentrations of these semi-volatile compounds are only marginally higher than the MDL. Considering the relatively non-sensitive nature of the land uses in the vicinity of these recorded concentrations (commercial/railway) it is unlikely that a significant risk to human health is present.

Asbestos

Sixteen samples of Made Ground were submitted for asbestos screening. Three of the samples (located in the depot/former mill/gas works area and railway embankment) identified asbestos fibres as summarised in Table 10.


Table 10 – Asbestos Analysis

Sample Ref Depth (mbgl)

Result & Lab Comment

BH3-T 0.50 Chrysotile – White Asbestos – loose fibre bundles

BH8-T 0.50 Chrysotile – White Asbestos – loose fibre bundles

WS7-T 0.20 Amosite – Brown Asbestos – loose fibre bundles

The presence of asbestos within Made Ground at these locations has potential implications to human health of site users (and construction workers) and the re-use of materials. We recommend that advice is sort from asbestos specialists .

Imported material

This assessment considers risks to controlled waters and human health from potential excavation and re-use of the Made Ground/soils currently on site. Should the scheme require additional materials to be imported, it would be necessary to ensure that such imported materials do not introduce new risks to identified receptors.

Waste Characterisation

Working in partnership with the McArdle Group, Atkins has developed the Waste Soils Characterisation Assessment Tool (CAT-WASTE

SOIL). The tool follows current regulations / guidance and provides an

indication of whether or not soils are likely to be considered hazardous.

Reported concentrations of contaminants from twenty six soil samples were entered into the CAT-WASTE

SOIL tool. The output of the CAT-WASTE

SOIL tool indicates the samples tested would not be classified

as hazardous. However, CAT-WASTESOIL

does not consider asbestos and therefore the potential implications of the recorded asbestos on waste classification has not been assessed. We recommend that advice is sort from asbestos specialists.

It should be noted that the above assessment only provides an indication of the likely classification of soils for off-site disposal. Further testing and assessment of spoil generated during the excavation phase of the construction works, including waste acceptance criteria WAC tests, will be required to fully characterise the soils in accordance with waste management procedures.

The results of the CAT-WASTESOIL

assessment are provided in Appendix C.

Ground Gas

The only potential sources of ground gas recorded by the investigation was Made Ground and some evidence of hydrocarbon contamination in three locations (BH6T, WS8T and WS10T). Currently the scheme proposes utilising the existing railway embankment as a flood defence and replacing existing wall/installing sheet piling along the depot/former gas works/mill part of the Northern Extent.

A gas risk assessment (i.e. screening the recorded concentrations) has not been undertaken as the CSM indicates it is unlikely viable gas pollutant linkages are present. Furthermore, the design of the flood defences does not include for any new buildings/confined spaces and the walls/embankments proposed in the scheme are unlikely to create new migration pathways.


Summary and Recommendations

Geotechnical

The key findings of the investigation are:

STU-1B

Sheet piling would be a suitable solution and outline design indicates an indicative pile length of

around 5.7m, assuming a small pile such as an Arcelor AZ12 or 14.

Recent communication suggests that the flood level may be much lower than considered in the

outline design and a gravity wall solution coupled with moving the wall to a different location is

currently being considered.

STU-3

The existing wall is founded at 1.35m depth upon natural sands and gravels.

STU4-6

Mixed near surface deposits of variable and poor strength

Near surface obstructions (cobbles and boulders)

Outline design has been carried out assuming a gravity wall constructed upon a clay filled cut-off

trench where the depth to low permeability strata is excessive, a sheet pile wall may be required to

form a cut off. Protection on the land side of the wall should be provided to prevent erosion resulting

from overtopping.

Railway Embankment

The embankment is founded on granular materials which will allow seepage under the embankment

and a possibility of flooding on the land side, but the risk of failure due to piping is low.

A protective layer of e.g. clay may be required on the river side of the embankment to protect the

embankment from localised instability and erosion.

Environmental

The key findings of the preliminary materials re-use assessment and waste characterisation are:

ASBESTOS:

The presence of asbestos (within the Made Ground in three boreholes) is a potential constraint on the

scheme. We recommend the EA seeks advice from a specialist asbestos consultant.

RE-USE:

o Soils sourced from the Pin Meadow area are unlikely to pose an unacceptable risk to human health/controlled waters and can thus be re-used anywhere within the scheme, subject to satisfying geotechnical suitability criteria.

o Further assessment/analysis is required before the Made Ground elsewhere on site (beside depot/former mill/gasworks and rail embankment) can be reused within commercial/railway embankment areas, as it might pose an unacceptable risk to controlled waters (and asbestos has been recorded in three locations).

DISPOSAL:

o CAT-WASTESOIL

indicates the material tested is unlikely to be hazardous waste, with the possible exception of the samples which recorded asbestos. Depending on the percentage of asbestos present in relation to the soil matrix, the material might be considered hazardous waste. Additional sampling/analysis to determine the percentage of asbestos within the Made Ground is required.


o Notwithstanding the recorded asbestos, further sampling and analysis for metals/hydrocarbons will be required once materials have been excavated/stockpiled to confirm their waste classification (this could include WAC tests). The WAC analysis might indicate the materials as inert, which would reduce disposal costs.

As with all ground investigations, we can only sample a small proportion of the ground and conditions can

vary greatly between exploratory hole locations. If when conditions other than those recorded are

encountered then further testing/assessment may be required.

We trust that this meets with your current requirements. If you have any queries, please do not hesitate to

contact us.

Originator Checker Reviewer

Catherine Kenny Senior Geotechnical Engineer Philip Liew Geotechnical Engineer

Joyce Brady Chief Engineer

Paul Coney Operations Director

Matt Lomas

Geo-Environmental Scientist

Caroline Jones

Senior Environmental Consultant

Owen Mills

Associate Director

Enc: Figure 1 – Defence Height Overview Plan

Appendix A: Fugro Factual Report

Appendix B: GQRA screen

Appendix C: CAT-WASTESOIL

assessment

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