Geotechnical and Phase 11 Contamination Report
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Suite 7, Westway Farm Business Park Wick Road, Bishop Sutton, Somerset BS39 5XP United Kingdom Tel: 01275 333036 www.integrale.uk.com Proposed Redevelopment Windmills Phase 4 Alm Place Portland DT5 1NH GE OTECHNICAL AND PHASE 11 CONTAMINATION RE PORT REPORT NO. 20065/A, February 2021 GEOLOGICAL • GEOTECHNICAL • ENVIRONMENTAL • ENGINEERING Intégrale is a trading name of Integrale Limited Registered Office: The Granary, Chewton Fields, Ston Easton, Somerset, BA3 4BX, United Kingdom Company Registration No. 2855366 England VAT Reg. No. 609 7402 37 Geotechnical and Phase II Contamination Report Proposed Redevelopment Windmills Phase 4 Alm Place Portland DT5 1NH Client: Betterment Properties [Weymouth] Limited Intégrale Report No. 20065/A, February 2021 Signature/Date Project Co-ordinator Phil Cresswell & Report Preparation: Mentor Consultant & Andrew Harris & Advice: Gareth Thomas Technical Director & Kay Boreland Report Approved: Final Check by: Phil Cresswell CONFIDENTIALITY STATEMENT This report is addressed to and may be relied upon by the following: Betterment Properties [Weymouth] Limited Unit 1, 2 Curtis Way Weymouth Dorset DT4 0TR Integrale Limited has prepared this report solely for the use of the client named above. Should any other parties wish to use or rely upon the contents of this report, written approval must be sought from Integrale Limited. An assignment fee may then be charged. GEOLOGICAL • GEOTECHNICAL • ENVIRONMENTAL • ENGINEERING Integrale Limited, Suite 7, Westway Farm Business Park, Wick Road, Bishop Sutton, Somerset, BS39 5XP, United Kingdom Tel: 01275 333036 www.integrale.uk.com Registered Office: The Granary, Chewton Fields, Ston Easton, Somerset, BA3 4BX, United Kingdom Company Registration No. 2855366 England LIST OF CONTENTS - EXECUTIVE SUMMARY – 1.0 INTRODUCTION 1 1.1 General & Background 1 1.2 Current Scope 1 2.0 THE SITE 2 2.1 Location and Description 2 2.2 Published Geology and Mining 2 2.2.1 British Geological Survey Mapping 2 2.2.2 BGS Previous Investigation Records 3 2.3 Previous Investigations 3 2.3.1 Previous Mining/Quarrying 3 2.4 Outline History 3 2.5 Hydrogeology, Hydrology & Groundwater Vulnerability 3 2.6 Environmental Information 4 2.6.1 Pollution Information and Licencing 4 2.6.2 Geological Information 5 2.6.3 Background Soils Chemistry 5 2.6.4 Contemporary Trade Directories 5 2.6.5 Radon Risk Information 5 2.7 Conceptual Exposure Model 5 2.7.1 Proposed Redevelopment 5 2.7.2 Potential Sources of Contamination 6 2.7.3 Potential Pathways 7 2.7.4 Potential Receptors 7 3.0 GROUND INVESTIGATION 8 3.1.1 Mechanical Trial Pitting 8 3.2 Soils Borings 8 3.3 Groundwater and Soils Gas Standpipe Installations and Monitoring 8 3.4 Geotechnical Laboratory Testing 8 3.5 Contamination Analyses 8 3.6 Referencing 9 4.0 GROUND & GROUNDWATER CONDITIONS 10 4.1 Summary of Strata Encountered 10 4.2 Strata Properties 10 4.2.1 Made Ground 10 4.2.2 Portland Freestone Member 10 4.3 Groundwater 10 4.4 Ground Gas 10 5.0 GEOTECHNICAL CONSIDERATIONS 12 5.1 Scheme Details & Structural Loadings 12 5.2 Site Preparation and Earthworks 12 5.3 Foundations and Ground Floor Slabs 12 5.3.1 Typical Ground Conditions 12 5.3.2 Other Shallow Reinforced Spread Foundations 12 5.3.3 Ground Improvement 13 5.3.4 Piles 13 5.3.5 Ground Floor Slabs 13 5.3.6 Formations 13 5.4 Pavement Design 13 5.5 Earth Pressures and Retaining Walls 13 5.6 Protection of Buried Concrete 14 6.0 GENERIC QUANTITATIVE CONTAMINATION ASSESSMENT 15 6.1 Summary of Soils Results with Respect to Human Health 15 6.1.1 Generic Human Health Assessment 15 6.2 Summary of Soils Results with Respect to Phytotoxicity 15 6.3 Summary of Soils Results with Respect to WRAS 15 6.4 Leachate Testing 16 6.5 Gas Mitigation 16 6.6 Conceptual Exposure Model & Risk Assessment 16 6.7 Recommendations 16 6.7.1 For Protection of Human Health 16 6.7.2 For Protection of Building Materials & Services 17 6.7.3 Reuse and Disposal of Surplus Spoil 17 6.7.4 Recommended Further Assessment 17 FIGURES 1. Site Plan 2. Tentative Geological Cross-Section APPENDICES A. Site Location B. Site Description / Photographs C. Desk Study Information D. Trial Pits E. Boreholes F. Gas & Groundwater Monitoring G. Results of Geotechnical Laboratory Testing H. Results of Contamination Analyses I. Proposed Redevelopment J. Acceptability Criteria Suite 7, Westway Farm Business Park Wick Road, Bishop Sutton, Somerset, BS39 5XP, United Kingdom Tel: 01275 333036 www.integrale.uk.com EXECUTIVE SUMMARY 20065- Windmills Phase 4 – Geotechnical and Phase II Contamination Report Betterment Properties propose to redevelop this former quarry site with 5 No single storey residential dwellings. The geology map reports artificial ground [Quarry Backfill] over Portland Freestone Member Limestone. Old maps show the site to be quarried from 1902-1929. It is not clear when the quarry was backfilled but must have occurred between 1929 and the 1960s as the site is shown to be open land from the 1960s. Intrusive investigation by trial pitting and rotary percussiuon boreholes has established Quarry Backfill, comprising typically loosely to moderately compact silty sandy Gravel of limestone with cobbles and rare boulders. The base of this stratum is inferred at c. 15-20m depth. Groundwater was not proven. The Quarry Backfill is not capable of supporting spread foundations. Consideration may be given to ground improvement of the quarry backfill by vibro replacement. Piled foundations seem to be the most appropriate option, and it is understood that rotary drilled high capacity piles have been adopted on the Windmills Phase 3 site. Suspended ground slabs are recommended. Design CBR values of at least 3-4% onto the Quarry Backfill are feasible. Effective design friction (Ø’) of 32° is appropriate for the Quarry Backfill. Contamination assessment concludes that, following site strip: - External garden and soft landscaped areas to be capped with a minimum of 600mm of certified clean/imported topsoil. Topsoil should be placed above a suitable geotextile warning membrane to prevent mixing of topsoil and underlying Quarry Backfill. - 1 No. sample of topsoil should be provided from each garden and tested for a generic contaminant suite, CWG TPH and asbestos screen. - Design Sulphate Class of DS-1 and ACEC Class of AC-1 are recommended for buried concrete. - New water pipes require protection. - Full Radon protection measures are required. - Surplus spoil should be removed to a suitably licenced tip. GEOLOGICAL • GEOTECHNICAL • ENVIRONMENTAL • ENGINEERING Intégrale is a trading name of Integrale Limited. Registered Office: The Granary, Chewton Fields, Ston Easton, Somerset, BA3 4BX United Kingdom Company Registration No. 2855366 England VAT Reg. No. 609 7402 37 1.0 INTRODUCTION 1.1 General & Background Betterment Properties (Weymouth) Limited are proposing to redevelop this site with 5 No. Residential single storey houses with conventional gardens and short cul-de-sac approach. The project architects are Western Design Architects Limited. Contaminated land assessment is required in planning conditions imposed by Dorset Council. The following condition (No. 10) applies to the proposed development: Before the commencement of development…the developer shall submit for the written approval of the Local Planning Authority: (a) a 'desk study' report documenting the site history; (b) a site investigation report documenting the ground conditions of the site, and incorporating a ‘conceptual model’ of all potential pollutant linkages, detailing the identified sources, pathways and receptors and basis of risk assessment; (c) a detailed scheme for remedial works and measures to be taken to avoid risk from contaminants/or gases when the site is developed; (d) a detailed phasing scheme for the development and remedial works. The remediation scheme, as agreed in writing by the Local Planning Authority, shall be fully implemented before the development is occupied. Any variation to the scheme shall be agreed in writing with the Local Planning Authority in advance of works being undertaken. On completion of the works the developer shall provide written confirmation that all works were completed in accordance with the agreed details. SOCOTEC UK Limited have completed a Phase 1 Baseline Desk Study and Preliminary Risk Assessment, aimed at addressing Part (a) of the above condition. It is assumed that has been submitted to and approved by the Local Authority. 1.2 Current Scope Integrale Limited (Intégrale) are commissioned to undertake a ground investigation and complete a Geotechnical and Phase II Contamination Report. This assessment is to provide information for geotechnical aspects as well as addressing Part (b) of the contamination condition. The investigation scope was determined by Betterment Properties in liaison with Intégrale. This interpretative report therefore summarises the previous desk study completed by others, describes the scope of current fieldworks, laboratory investigations and monitoring, discusses the ground and groundwater conditions encountered, and gives advice on foundations and other geotechnical aspects. The results of contamination analyses and generic quantitative risk assessment are reported and used to establish a conceptual model of pollutant linkages. Potential implications for the development are discussed and recommendations for further investigation or potential remedial works or design measures given. A further 3 No. gas and groundwater monitoring visits are planned and this report will be updated, reissued on completion, although based on the findings to date, it is not anticipated that recommendations will change significantly. Windmills Phase 4, Portland, DT5 1NH, 20065/A, January 2021 1 2.0 THE SITE 2.1 Location and Description As shown in Appendix A, the site is located within the eastern most extents of the former Bottom Coombe Quarry, Park Road, Easton, Portland. It has a central Ordnance Survey Grid Reference of 369298 071484 and postcode DT5 1NH. Notes describing the site were prepared during the site visit and are included as Appendix B, together with typical photographs.