Geotechnical Investigation and Contamination Assessment Report St. Mary’s Garage, Fore Street, Kingskerswell, Newton Abbot, Devon Mrs S Wells March 2010 SR/SB/DT/10127/GICAR St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd REPORT CONTROL SHEET SITE ADDRESS: ST. MARY’S GARAGE FORE STREET KINGSKERSWELL NEWTON ABBOT DEVON CLIENT: MRS S WELLS REPORT TITLE: GEOTECHNICAL INVESTIGATION AND CONTAMINATION ASSESSMENT REPORT ISSUE DATE: 31 MARCH 2010 REPORT NO.: SR/SB/DT/10127/GICAR PREPARED BY: STEVE BISHOP SIGNATURE: POSITION: ENGINEERING GEOLOGIST QUALIFICATIONS: BSc (Hons). MSc. FGS. ISSUED BY: SIMON RUDDLESDEN SIGNATURE: POSITION: DIRECTOR QUALIFICATIONS: BSc (Hons). MSc. DIC. FGS. RUDDLESDEN GEOTECHNICAL LTD 65 LANGATON LANE PINHOE EXETER EX1 3SP TEL: 01392 678082 FAX: 01392 678083 e-mail: [email protected] web: www.ruddlesden.co.uk Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR ii St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd CONTENTS 1.0 INTRODUCTION 1 1.1 General 1 1.2 Scope of Investigation 1 1.3 Scope of Report 2 2.0 THE SITE 3 2.1 Site Location 3 2.2 Site Description 3 3.0 DESK STUDY 5 3.1 General 5 3.2 Site History 5 3.3 Site Geology 6 3.4 Environmental Information 6 3.5 Underground Tank Information 7 3.6 Initial Conceptual Model 8 3.7 Sampling and Analysis Plan 10 4.0 FIELDWORK 12 4.1 General 12 4.2 Window Sample Boreholes 12 5.0 LABORATORY TESTING 13 5.1 General 13 5.2 Geotechnical Testing 13 5.3 Contamination Testing 13 6.0 RESULTS OF THE INVESTIGATION 15 6.1 General 15 6.2 Ground Conditions Encountered 15 6.3 In-Situ Testing 16 6.4 Groundwater 16 Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR iii St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd 6.5 Geotechnical Laboratory Testing 17 6.6 Contamination Laboratory Testing 17 7.0 GEOTECHNICAL ASSESSMENT 18 7.1 Proposals 18 7.2 Ground Profile 18 7.3 Foundations 19 7.4 Retaining Walls 21 7.5 Roads 22 7.6 Groundwater and Excavations 22 7.7 Soakaways 23 8.0 CONTAMINATION ASSESSMENT 24 8.1 General 24 8.2 Human Health Risk Assessment 24 8.3 Controlled Waters Risk Assessment 26 8.4 Ground Gas Assessment 28 8.5 Revised Conceptual Model 28 8.6 Discussion and Recommendations 29 8.7 Off-Site Disposal of Excavated Soils 33 9.0 CONCLUSIONS 34 10.0 REFERENCES 35 11.0 TERMS AND CONDITIONS 36 Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR iv St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd APPENDICES APPENDIX A WINDOW SAMPLE BOREHOLE LOGS (6 pages) APPENDIX B PHOTOGRAPHS (4 pages) APPENDIX C LABORATORY TESTING RESULTS Geotechnical Laboratory Testing (2 pages) Contamination Laboratory Testing (9 pages) APPENDIX D DESK STUDY INFORMATION Landmark Envirocheck Report (66 pages) Underground Tank Information (5 pages) APPENDIX E SITE PLANS Site Location Plan (1 page) Aerial Photograph (1 page) Window Sample Borehole Location Plan (1 page) Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR v St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd EXECUTIVE SUMMARY It is proposed to redevelop land at St. Mary’s Garage, Fore Street, Kingskerswell, Newton Abbot, Devon, for residential purposes. Old Ordnance Survey maps showed the site to have been largely undeveloped until at least 1955, by which time a “garage” building is present at the location of the main building today, near the centre of the site. Since that time, various additional buildings were built until 1980, when the site resembled its present day layout. The site was used as a petrol filling station until 1998. Six window sample boreholes generally encountered ground conditions of topsoil or made ground, underlain by stiff to very stiff reddish brown silty gravelly clay, underlain by very dense grey silty gravel. Foundation recommendations are strip or trench-fill foundations at a minimum depth of 0.90m below existing or proposed ground levels, whichever is deepest. Foundations should be deepened below any made ground and in accordance with NHBC Standards where building near trees for soils of low volume change potential. Foundations might need to be re-designed, e.g. deep trench-fill or reinforced beam, if proposed properties are built over the infilled excavations of the former tanks. Elevated levels of contamination were recorded within the ground, particularly in the area of underground fuel tanks, which represent an unacceptable risk of causing harm to human health, given the proposed end-use. Proposed remedial measures include providing 600mm of inert subsoil and topsoil underlain by a geo-textile membrane in garden areas and the excavation and off-site removal of all underground fuel and surrounding contaminated soil. No radon protection measures are required and ground gas protective measures are not considered to be necessary. Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR vi St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd 1.0 INTRODUCTION 1.1 General In March 2010, a combined Phase 1 and Phase 2: Geotechnical Investigation and Contamination Assessment was undertaken by Ruddlesden geotechnical ltd on behalf of Mrs S Wells, for the proposed residential re-development of land at St. Mary’s Garage, Fore Street, Kingskerswell, Newton Abbot, Devon. The investigation was undertaken to determine subsurface ground conditions, to provide recommendations for foundations and associated structures, and to assess the extent of any contamination at the site. The investigation comprised a desk study and walkover survey followed by the formation of six window sample boreholes with in-situ and laboratory testing. 1.2 Scope of Investigation The investigation covers geotechnical and contamination aspects relating to the development. The brief was understood to comprise the following: Carry out desk study and walkover survey. Undertake exploratory holes. Schedule geotechnical and contamination laboratory testing. Establish the ground conditions across the site. Make recommendations for foundation design. Make recommendations covering other geotechnical aspects, including drainage. Undertake a contamination risk assessment. Provide details of any contamination remedial measure requirements. Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR 1 St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd 1.3 Scope of Report The report is presented as a description of the procedures employed and the data obtained. This is followed by a thorough description of the ground and groundwater conditions, together with an assessment of material and mass ground parameters. The final part of the report comprises analysis, recommendations, and conclusions, which are provided in two separate parts: geotechnical and contamination. Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR 2 St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd 2.0 THE SITE 2.1 Site Location The site is located at St. Mary’s Garage, Fore Street, Kingskerswell, Newton Abbot, Devon, see Appendix E (Dwg. No.’s 10127/01 and 10127/AP). The British National Grid Reference of the site is 287900, 67770. The site is situated in a predominantly residential area of the village of Kingskerswell. The surrounding topography is hilly. Access to the site is gained from Fore Street, to the north. 2.2 Site Description The site is irregular in shape, measuring approximately 50m x 40m (0.20Ha), and is generally level with a gentle slope down to the southwest for the most part, although the most southern part of the site slopes very steeply down towards the southwest. The site comprises a garage (St. Mary’s Garage). A car sales forecourt, with numerous cars, is present in the northern part of the site. An underground workshop, with a southerly facing downward slope leading to it, is present in the western part of the site, above which is situated on open yard area. The existing garage building, comprising a sales showroom and repair centre, is present in the central area of the site. A single-storey residential dwelling is present in the southern part of the site (Lyndhurst) and another single storey bungalow (St Mary’s Bungalow) and small domestic garage building is present in the eastern part of the site. The site is covered by a mix of bitmac, concrete and grass. Several large manhole covers were noted during the walkover survey, notably in the northern and western parts of the site. A retaining wall, approximately 2m in height, runs along the southern boundary of the site. The site is surrounded to the north by a road (Fore Street) before residential housing and shops, to the east by residential dwellings with associated gardens, to the south by a road (Daccabridge Road), approximately 7m below Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR 3 St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd general site ground level before residential dwellings with associated gardens, and to the west by a commercial building (Datel Electricals) and residential dwelling (Little Gara). Photographs of the site are presented in Appendix B of this report and the main site features are shown on the borehole location plan (Appendix E, Dwg. No. 10127/02). Geotechnical Investigation and Contamination Assessment Report Report Ref: SR/SB/DT/10127/GICAR 4 St. Mary’s Garage, Kingskerswell Ruddlesden geotechnical ltd 3.0 DESK STUDY 3.1 General A desk study was undertaken to provide background information, comprising the consultation of: Old Ordnance Survey maps. Geological maps and memoirs. Environmental information. Underground tank information This information was used to produce a “conceptual model” of the site so that an appropriate intrusive investigation could be carried out. 3.2 Site History A full set of old Ordnance Survey maps of the site was obtained as part of the Landmark Envirocheck report (Appendix D of this report).