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For: Taylor Wimpey Manchester Coopers Consulting Engineers

Ground Gas Risk Assessment Report for land at Dilworth Lane, Longridge

Ref: 6258gra-A Page 1 of 9 14 December 2015

Document Verification

Project Title Dilworth Lane, Longridge

Project Number 6258

Document Title Ground Gas Risk Assessment Report for land at Dilworth Lane, Longridge

Document Number 6258gra-A

This document is not to be used for contractual or engineering purposes unless the document verification sheet is signed where indicated by the approver of the document.

Prepared Checked and Approved by

JA Lewis P R Sykes BSc(Hons), MSc, MCIWEM C.WEM, C.Sci, C.Env. BSc (Hons), MSc (Eng), CGeol, FGS Document Revision

Report Reference

Date Description Prepared Checked and Approved

6258si 18 September 2015 Site Investigation Report JA Lewis P R Sykes 6258gra 9 December 2015 Ground Gas Risk Assessment

Report J A Lewis P R Sykes

6258gra-A 14 December 2015 Ground Gas Risk Assessment Report

J A Lewis P R Sykes

This report has been prepared for and on behalf of our client, in accordance with the terms and conditions of the appointment agreement with Coopers. Other than where specifically allowed for in the said appointment agreement, any other party using this report for any purpose whatsoever does so at their own risk and any duty of care to that party is specifically excluded.




Contents

Section Page No.

Document Verification ........................................................................................................................................................................ 1

Contents ................................................................................................................................................................................................ 2

List of Appendices................................................................................................................................................................................ 2

List of Tables ........................................................................................................................................................................................ 2

1.0 Introduction .......................................................................................................................................................................... 3

2.0 Conceptual Site Model......................................................................................................................................................... 3

3.0 Ground Gas Risk Assessment............................................................................................................................................. 6

4.0 Conclusions............................................................................................................................................................................ 9

List of Appendices

Appendix 1 - Trial Pit Logs Appendix 2 - Small Diameter Borehole Logs Appendix 3 - Ground Gas Monitoring Results Appendix 4 - Drawings

List of Tables

Page No.

Table 1 BS8485 (2015) Points System for Gas Mitigation Measures for Residential Houses (not

apartments) 9




1.0 Introduction

It is proposed to develop land at Dilworth Lane, Longridge with 191 No. plots comprising semi-detached and detached residential properties with associated private gardens, car parking and public open space. The site, located approximately 10km to the north east of Preston at National Grid Reference SD 613 374, is illustrated on Drawing No. 6258/L1 in Appendix 4. Coopers undertook a site investigation, on behalf of Taylor Wimpey Manchester, from 3 to 6 August 2015, which incorporated 54 No. trial pits across the site and 8 No. small diameter boreholes approximately 30m apart along the north eastern site boundary. Small diameter boreholes WS1A, WS2 to WS5 were installed with monitoring wells, to enable gas monitoring. Drawing No. 6258/01, in Appendix 4, illustrates the location of the exploratory holes, with the exploratory hole logs presented in Appendices 1 and 2. This report should be read in conjunction with Coopers Site Investigation Report for Land at Dilworth Lane, Longridge, Report Reference No. 6258si. The site investigation included a preliminary risk assessment with regards to the potential for ground gas to impact on the proposed residential properties. This was based on one round of gas monitoring undertaken on 17 August 2015. This report reassesses the risk based on a further five rounds of ground gas monitoring undertaken on 16 September, 6 and 15 October and 5 and 18 November 2015.

Coopers have appraised the potential risk of ground gases to provide an initial conceptual model in terms of source-pathway-receptor in accordance with CIRIA C665: ‘Assessing risks posed by hazardous ground gases to buildings.’, NHBC Report No. 10627-R01 (04) “Guidance On Evaluation Of Development Proposals On Sites Where Methane And Carbon Dioxide Are Present,” British Standards BS 8485:2007: ‘Code of Practice for the Characterization and Remediation from Ground Gas in Affected Developments’, CL:AIRE Research Bulletin RB17, ‘A Pragmatic Approach to Ground Gas Risk Assessment’, November 2012 and BS 8576: 2013 ‘Guidance on Investigations for Ground Gas – Permanent Gases and Volatile Organic Compounds (VOCS)’. It should be noted that the characteristic design models within the NHBC report assumes all properties have a pre-cast suspended slab. We have been advised by the report authors that for other types of slab construction reference should be made to the design and assessment criteria within CIRIA C665.

All information and advice regarding requirements for gas precaution measures are considered to be provisional until approval from statutory bodies has been granted.

The conceptual model below demonstrates the potential sources of ground gases, the pathways available to the proposed development and the receptors.

2.0 Conceptual Site Model

2.1 Potential Sources

General: - The potential sources of hazardous ground gas are appraised within the conceptual model which identifies the potential sources that may impact future receptors.

Landfills (off site): - The Envirocheck report, presented in Report No. 6258si, did not identify any current landfill sites located within 250m of the site boundary but did identify two former landfill




sites at 135m and 162m to the north of the site boundary. There is no detail available concerning the type of waste received by the landfill 135m from the site or licence/permit information. Due to the lack of data available, it is considered that this landfill was active prior to 1974 when the Control of Pollution Act 1974 introduced the requirement for landfills to be licensed. Such landfills would have been undertaken within sites where dispersal systems rather than containment systems were employed. Any organic waste which may have been present within the landfill would be almost completely biodegraded due to the age of the waste and as such the generation potential of methane concentrations would be considered to be low and carbon dioxide concentrations low to moderate. If only inert materials had been deposited within this landfill then ground gas generation would be considered to be negligible. The former landfill located 162m from the site received both inert and domestic waste between 1984 and 1993. It is likely that the site is still generating elevated concentrations of ground gases including methane and carbon dioxide. However, this site was licensed and as such the site should have been engineered to mitigate the migration of ground gases from the site.

Coal Mining: - The site is not underlain by coal measures and as such does not lie within an area potentially affected by coal mine gas.

Deep madeground (on site): - Madeground (similar to topsoil) and madeground was localised to a

few areas of the site where generally it was no deeper than 0.4m. These materials would not impact on the foundations of the proposed development. The greatest depth of madeground was noted only in trial pit TP40 to 1.3m. This was limited to a width of approximately 2m, following the path of the land drain. This comprised a 1m thickness of granular materials overlying reworked clay with no organic matter content observed. The total organic carbon (TOC) content of the granular materials was noted in the sample taken from TP40 at 0.6m bgl to be very low at 0.5%. Based on the very low TOC and the relatively shallow depth of the materials, these would have a very low potential to give rise to ground gases. Organic Rich Strata (on site): - Peat was encountered within trial pit TP19 at between 1.2m and 1.5m bgl. This was recorded beneath natural clay. The TOC for this material was recorded at 14%. This may have a potential to give rise to ground gases associated with the degradation of organic matter. As the peat was found to be fibrous, there is still potential for future ground gas generation. As the peat was found in only a thin lens, the potential volume of ground gases which may be generated will be highly limited. Much of the gases are anticipated to be restricted to the pond area, with the peat being bound within the natural clay, which would be relatively impermeable to ground gases. Based on the current planning layout presented in Taylor Wimpey Manchester’s Sketch Layout 5, Drawing No. 01 (see Appendix 4), the southern pond and the area to the south of the pond is proposed to be public open space. Providing peat does not extend beneath the footprint of proposed houses or within 10m of the footprint, it is considered that the peat would pose a very low risk to the proposed development. Should the planning layout be altered such that properties would be within the 10m zone of the underlying peat, and depending on the finished levels ground gases from this source may pose a low to moderate risk, if left in situ or remain within 10m of the building footprint. Former ponds: - One former pond was noted on the earliest historical OS map, dated 1847 but not on later maps. This was located in the northern area of the site. It should be noted that this pond may be ephemeral, with it having been recently surveyed as a water feature. No madeground was noted in the trial pits in this vicinity. If any organic waste materials had been deposited in the pond, this




would have biodegraded over the intervening period such that ground gases from these materials would be considered to be negligible. Three ponds were located within 150m and 250m of the eastern site boundary and one pond was located 110m to the south eastern boundary. One of the three ponds was enveloped by the construction of the reservoir to the east of the site. The other two ponds to the east of the site may have been filled with unknown materials by 1967 as they were no longer evident on the historical maps. However, these were also lost when the reservoir was constructed. It is thought likely that as these ponds were within the vicinity of the newly constructed reservoir, that these ponds were filled with engineered materials, which would not have included biodegradable matter. One pond was located 110m to the south of the site. This appeared to become a depression within the ground which remained as such until 1994. The later available maps were small scale and did not show this detail. Therefore it is unclear as to whether this will now have been filled. The ponds may also include natural organic rich materials but these would likely be in relatively low volumes. The ponds appeared to have been between 10m and 20m wide and it is likely that they would have been relatively shallow. If the ponds had been filled with waste materials, given the age of the majority of these materials, they would likely have contained low volumes of biodegradable matter. As such, even if the ponds were filled with waste deposits, the total volumes would not have been significant and the biodegradable matter would have been limited. The materials would not have the potential to give rise to high volumes of ground gases. It is considered given the age of the potential fill materials, the relatively low volume within them, any biodegradation will have significantly reduced. The madeground and possible organic rich strata within the off site ponds are deemed to pose a very low risk to the proposed development.

Radon: - The Envirocheck report states that the proposed development lies within an area where less than 1% of properties are affected by radon concentrations above the action level. As such, no radon protection measures are required for the proposed properties.

Petroleum Hydrocarbons: - No visual or olfactory indication of petroleum hydrocarbon contamination was identified within the ground investigation on the site.

2.2 Potential Pathways

The geological maps and ground investigation have disclosed that cohesive ground comprising firm to very stiff, slightly gravelly clay was present across the majority of the site, which would significantly attenuate the migration of ground gases. However, sand and sand and gravel were noted on the northern, western and south eastern site boundaries, which would permit migration of ground gases laterally and vertically where there is no overlying clay. Similar ground conditions would be expected beyond the site boundary. If potential ground gases migrated to the foundations they may find pathways into the properties via cracks within the floor slabs and surrounding service entrances. When working in confined spaces (including trenches), contractors must assess the potential for hazardous gases to be present and health and safety provisions should be applied in accordance with HSE guidance.




2.3 Receptors

Future Occupants: The primary receptors are anticipated to be the future occupants of the residential houses via the build up and ingress of potential hazardous gases/vapours through the ground floor slabs of the properties. These are considered highly sensitive receptors for the purposes of the conceptual model.

Adjacent residents and workers: These can be exposed to hazardous gas if sources of gas are generated on site and migrate beyond the site boundary or preferential pathways exist for the horizontal migration of gases beyond the site boundary. The likelihood of this occurring is minimal due to the low potential for gas generation on site.

Construction/Maintenance Workers: Other receptors include construction workers, primarily through work within trenches or excavations where preferential pathways for the migration and collection of gases could occur. HSE guidance on safe work in confined space details potential dangers from gases within trenches, assessments required and precautions to take prior to entering a confined space. The appointment of a supervisor to ensure the necessary procedures and precautions are in place where construction workers intend to enter a confined space is advisable in accordance with HSE guidance.

2.4 Conceptual Model Summary The conceptual model has demonstrated that generally, there is a very low risk posed by ground gases and vapours to be present on site, but that there is a low to moderate risk posed by the migration of carbon dioxide from a landfill site to the north. Pathways are present via possible off-site granular materials and those within the northern site boundary. In accordance with current guidance, Coopers considered that gas monitoring was required to assess the risk posed by the off-site landfill to proposed properties within the northern area of the site, but that gas monitoring was not required for the remainder of the site.

3.0 Ground Gas Risk Assessment

3.1 Risk assessment where no gas monitoring required

For properties across the site excluding the northern area, Coopers have appraised the gas generation potential of the ground conditions in accordance with the CL:AIRE Research Bulletin RB17 and BS8576:2013, which outline an approach for pragmatic assessments of hazardous ground gas on low risk sites without in situ monitoring. The approach for this site has been based upon the presence of localised peat, within natural ground and localised madeground. Consideration has been given to the TOC content of the peat and the nature of the peat which is fibrous. As discussed earlier this may have the potential for ground gas generation, but as the peat layer is relatively thin, the volume of gases likely to be present/generated would be limited. The Research Bulletin RB17 states that such ground conditions would not require gas monitoring but consideration needs to be given to relevant ground gas protection measures. The peat in the southern area of the site may pose a low to moderate risk to proposed properties, if they were constructed over or within 10m of the peat. However, as this area is currently in a no build zone and the closest properties will not be within 10m of the peat, these will not require gas precaution measures. They would be considered in accordance with CIRIA C665 to be Characteristic Situation 1.




Consideration was given to the TOC content of the madeground and the depth of the materials. As previously discussed, based on the very low TOC and the relatively shallow depth of the materials, the madeground would have a very low potential to give rise to ground gases. Areas of the site with madeground would be considered to be designated as Green in accordance with NHBC Report No. 10627-R01(04) (Characteristic Situation 1 in accordance with CIRIA C665).

3.2 Risk assessment where gas monitoring required Gas monitoring wells were installed within small diameter boreholes WS1A and WS2 to WS5. These were positioned along the section of the northern site boundary, which intersects with the possible gas migration pathway from the landfill site to the north, to the proposed development site. The gas monitoring wells were installed to enable the assessment of the presence of potential ground gas. Gas monitoring was undertaken on six occasions, using a calibrated Geotechnics GA5000 infra-red gas analyser to monitor methane, carbon dioxide, oxygen, carbon monoxide and hydrogen sulphide concentrations and flow rates. The results have been presented in Appendix 3.

Fresh air readings were undertaken before and after the monitoring within the small diameter boreholes. Methane and hydrogen sulphide concentrations were recorded generally below the level of detection but a methane reading of 0.1% v/v was recorded on one occasion. Carbon monoxide was recorded below the level of detection prior to the monitoring but at 1ppm on one occasion. Carbon dioxide concentrations were recorded up to 0.2% v/v before and after the monitoring. Oxygen concentrations were recorded between 20.3% and 22.2% v/v. All methane and hydrogen sulphide concentrations to date were recorded to be below the detection limit, apart from one methane concentration, which was marginally elevated at 0.1% v/v. Carbon dioxide was recorded in low concentrations in small diameter borehole WS3 between 0.32% v/v and 1.7% v/v. Slightly elevated concentrations of carbon dioxide of up to 9.6% v/v recorded in the remaining monitoring wells. Marginal concentrations of carbon monoxide were recorded to 2ppm compared with the Health and Safety Executive EH40/2005 (second edition) Workplace Exposure Limits document, which states a long term exposure limit of 30ppm and short term exposure limit of 200ppm. Low flow rates were recorded between 0.0 l/hour and 0.5 l/hour.

The source of the carbon dioxide may either be from the landfill to the north of the site or it may be generated within the natural ground. Carbon dioxide may be generated within natural ground from the degradation of carbonaceous materials.

In accordance with CIRIA C665 and NHBC Report No. 10627-R01(04), the ground gas mitigation measures are primarily assessed by generating gas screening values by multiplying the gas concentration by the gas flow. A degree of professional judgement is also required to determine the proportionality of the concentration to the gas screening value. The NHBC document relates only to low rise properties (2 to 3 storey) with a pre-cast concrete floor slab and a clear vented sub floor void. All other properties are to be assessed using CIRIA C665.

Assuming the worst case scenario for this preliminary assessment, the maximum carbon dioxide concentration encountered in the north of the site was 9.6% v/v within small diameter borehole WS4, with concentrations above 5% v/v on at least one monitoring visit, in all other monitoring wells except WS3. One negligible concentration of methane was recorded at 0.1% v/v. To calculate the gas screening value, the concentration of ground gas is divided by 100, then multiplied by the




flow rate. With highest flow rate of 0.5 l/hour the GSV would be 0.048 l/hour. However, consideration must also be given to the typical maximum concentration. As the carbon dioxide concentration exceeded 5% v/v, based on the current gas monitoring, it is recommended that the northern area of the site is characterised as Amber 1 in accordance with the NHBC Report or Characteristic Situation 2 (CS2) in accordance with CIRIA C665. The preliminary ground gas risk assessment stated that this may apply to the properties within 115m of the far northern site boundary based on landfill sites posing a risk within 250m and the particular landfill being located 135m to the north. However, the prevailing flow rates, the ground conditions on site and the source of the ground gases were taken into consideration following the completion of the gas monitoring. It is not possible to confirm whether the slight elevations in carbon dioxide concentrations are sourced from the landfill site to the north or from the natural materials on site. Carbon dioxide can be generated up to concentrations of 9%, derived from the degradation of carbonaceous materials within the clay and sand. As discussed in the conceptual site model, the landfill site located 135m from the site boundary is likely to be at least 40 years old, with no details available of the landfill materials. Due to the age of the landfill, biodegradation of any organic matter, if present at all, would be complete or almost complete. As such, the risk of carbon dioxide still being generated at the landfill would be low to moderate, assuming biodegradable materials were deposited but would be negligible if only inert materials were deposited. Generally the ground comprised firm to stiff and stiff clays across the site including within 115m with the exception of the far northern area of the site along the boundary, where either clay was overlying sand or interbedded with sand. It was considered that ground gas migration was limited due to the negligible flow rates recorded and that any migration which may take place would be attenuated laterally by the clay to the south of the sand strata. It is recommended that precautionary measures to meet Amber 1 (CS2) are taken for Plots 56 to 63 with gas protection measures installed within the floor slab construction, as these plots are overlying the sand strata or within the near vicinity of these strata. Plots 43, 55, 64/65 and 71 overly clay strata but lie within approximately 20m of the sand strata. As a conservative measure, we would recommend these plots also have gas protection measures installed. The site is within an area where no radon protection measures are required for the properties. Where gas protection measures are required to meet CS2, the gas mitigation measures would be incorporated into the floor slab construction with design calculations based upon the points system defined within BS8485 (2015). Based on the developer’s preferred option, the proposed development is likely to be constructed with jetfloor (pre-cast suspended) floor slabs.

For residential houses on sites within a CS2 risk, a score of 3.5 points is required in accordance with BS8485 (2015). These are designed based around the developer’s preferred floor slab construction and approaches for verification, as illustrated in Table 1 below.

For: Taylor Wimpey Manchester Limited Cooper Consulting Engineers

Ground Gas Risk Assessment Report for land at Dilworth Lane, Longridge _______________________________________________________________________

___________________________________________________________________________ Ref: 6258gra-A 14 December 2015

Appendix 1

Trial Pits

For: Taylor Wimpey Manchester Limited Cooper Consulting Engineers

Ground Gas Risk Assessment Report for land at Dilworth Lane, Longridge _______________________________________________________________________

___________________________________________________________________________ Ref: 6258gra-A 14 December 2015

Appendix 2

Window Sampling Holes

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