Old Rectory Court High Street Burwash East Sussex Geotechnical Assessment Report Report No. R17-12510 November 2017 Report prepared for the benefit of: Ink Development Company Ltd Grosvenor House 125 High Street Croydon Surrey CR0 9XP Document Control Report Section Prepared By Approved By Factual Section Ryan Jones Stephen Atkinson BSc (Hons) FGS BSc FGS Geotechnical Assessment Stephen Atkinson Rebecca Webb BSc FGS BSc FGS Revisions Revision No. Revision Date Notes Limitations This report was prepared specifically for the Client’s project and may not be appropriate to alternative schemes. The copyright for the report and licence for its use shall remain vested in Ashdown Site Investigation Limited (the Company) who disclaim all responsibility or liability (whether at common law or under the express or implied terms of the Contract between the Company and the Client) for any loss or damage of whatever nature in the event that this report is relied on by a third party, or is issued in circumstances or for projects for which it was not originally commissioned. R17-12510 EXECUTIVE SUMMARY The following presents a summary of the main findings of the ground investigation. It is emphasised that no reliance should be placed on any individual point until the whole of the report has been read as other sections of the report may put into context the information contained herein. The proposed development is understood to comprise of the demolition of the existing property, ‘Old Rectory Court’, and the construction of four new 2-bedroom dwellings and a new apartment building comprising fifteen 1-bedroom flats, along with associated garden areas, soft landscaping, car parking and vehicular access. The site currently comprises of an existing two storey vacant residential building ‘Old Rectory Court’ with an associated hard surfaced car parking area. The southern part of the site comprises soft landscaping. Reference to geological datasets indicates that the site is expected to be underlain by the Tunbridge Wells Sand Formation (beneath the northern part of the site) and Wadhurst Clay Formation (beneath the southern part of the site). The ground investigation confirmed the underlying soils to comprise a variable thickness of made ground, overlying Tunbridge Wells Sand Formation deposits. The Tunbridge Wells Sand Formation is classed as a Secondary A Aquifer, whilst the Wadhurst Clay Formation is classed as an Unproductive Strata. The site does not lie within an Environment Agency Source Protection Zone with regard to the protection of the quality of groundwater that is abstracted for potable supply. Groundwater was encountered at depths of between 2.20m and 2.88m below ground level during the intrusive works and subsequent monitoring period. The design of precautions against shrinkage and heave for any new foundation system should assume a low volume change potential for the fine grained Tunbridge Wells Sand Formation soils. A net allowable bearing capacity of 175kN/m2 may be assumed for pad or strip foundations up to 1.0m in breadth that are constructed directly onto Tunbridge Wells Sand Formation soils of at least stiff consistency. Whilst in generally stiff consistency soils were encountered below depths of around 1.0m, locally foundations may need to be deepened to bear below any localised made ground or soft/firm soils such as those encountered at boreholes WS03 and WS05 to depths of 2.4m and 2.0m below ground level, respectively. A DS-1 Design Sulfate Class and an AC-1 ACEC classification should be assumed as a minimum for the design of concrete in contact with the ground. In view of the variable thickness of made ground and the presence of soils of up to low volume change potential, it is recommended that ground floors should be suspended. The results of the infiltration tests indicate that the Tunbridge Wells Sand Formation deposits possess poor to moderate infiltration characteristics. i R17-12510 TABLE OF CONTENTS 1. INTRODUCTION 1 2. SITE DESCRIPTION 2 3. GEOLOGICAL AND HYDROGEOLOGICAL INFORMATION 2 4. GROUND INVESTIGATION 3 4.1 Introduction 3 4.2 Exploratory Holes 3 4.3 Sampling 3 4.4 In Situ Testing 4 4.5 Installations 4 4.6 Laboratory Testing 5 5. GROUND CONDITIONS 6 5.1 Stratigraphy 6 5.2 Stability 6 5.3 Groundwater Conditions 6 6. GEOTECHNICAL ASSESSMENT 8 6.1 Foundations 8 6.2 Groundwater 10 6.3 Stability of Excavations 10 6.4 Aggressivity to Concrete 11 6.5 Ground Floors 11 6.6 Stormwater Infiltration Systems 11 FIGURES AND APPENDICES FIGURES Figure 1 Site Location Plan Figure 2 Site Plan APPENDIX A Proposed Development Layout APPENDIX B Exploratory Hole Notes In Situ Testing Notes Exploratory Hole Records DPSH-B Dynamic Probe Records Summary of Borehole Falling Head Soakage Test Results Summary of Trial Pit Falling Head Soakage Test Results Groundwater Monitoring Results APPENDIX C Geotechnical Laboratory Testing Notes Geotechnical Test Results R17-12510 1. INTRODUCTION The proposed development is understood to comprise of the demolition of the existing property, ‘Old Rectory Court’, and the construction of four new 2-bedroom dwellings and a new apartment building comprising fifteen 1-bedroom flats. The development proposals also allow for private gardens, areas of communal soft landscaping, car parking and vehicular access. A copy of the proposed development layout is presented in Appendix A. Ashdown Site Investigation Limited was requested to provide an estimate for carrying out a ground investigation and geotechnical assessment of the site by Lenka Muir of OPTIVO, Building 1060, Cornforth Drive, Kent Science Park, ME9 8PX. The scope of the works allowed for and the terms and conditions under which the works were to be undertaken were set out within the offer letter Q17-6430, dated 3rd October 2017. The instruction to proceed was received in an email from the client’s agent, OPTIVO, on behalf of the client, Ink Development Company Ltd, dated 6th October 2017. The objectives of the works were to: a) Establish the expected geology and hydrogeology at the site; b) Investigate the shallow ground and groundwater conditions across the site; and c) Provide advice to assist others in undertaking design of spread foundations, ground floors and soakaways. Old Rectory Court, High Street, Burwash, East Sussex Page 1 R17-12510 2. SITE DESCRIPTION The site comprises an irregular shaped plot of land located at Old Rectory Court, High Street, Burwash, East Sussex, and is centred on the approximate Ordnance Survey national grid reference TQ 6753 2470. A site location plan and site plan are presented as Figure 1 and Figure 2, respectively. The site is bound by High Street to the north, residential properties to the east and west and open agricultural land to the south. The site currently comprises a vacant two storey residential apartment building of brick construction, ‘Old Rectory Court’, which occupies the majority of the northern and central parts of the site. To the east of the building lies a hard surfacing car parking area, which is accessed via the site entrance off High Street to the north. The southern part of the site comprises a landscaped garden area, primarily laid to grass and bound by mature hedges to the east and west. A number of mature trees are also present adjacent to the eastern site boundary. Levels across the site fall towards the south. 3. GEOLOGICAL AND HYDROGEOLOGICAL INFORMATION 3.1.1 Expected Geology, Aquifer Designation & Groundwater Vulnerability The stratigraphic unit that may be expected to underlie the site is presented in the following table. Table 1. Expected Strata, Aquifer Designation and Description Type Stratum Aquifer Designation Source Protection Zone The site does not lie within an Tunbridge Wells Sand Formation Secondary A Aquifer Environment Agency Source (beneath north of site) (Tunbridge Wells Sand Protection Zone with regard to Bedrock Formation) the protection of the quality of Wadhurst Clay Formation Unproductive Stratum groundwater that is abstracted (beneath south of site) (Wadhurst Clay Formation) for potable supply. The Lower Tunbridge Wells Sand and the Upper Tunbridge Wells Sand are separated by the Grinstead Clay Member and all three belong to the parent unit Tunbridge Wells Sand Formation. Where the Grinstead Clay Member thins and disappears, the Upper and Lower Tunbridge Wells Sand often cannot be separated. In maps and memoirs the undifferentiated sand unit has been called Tunbridge Wells Sand Formation. The succession commences with rhythmically bedded sandstones, siltstones and mudstones which pass up into massive sandstones. These are overlain by a generally more argillaceous rhythmic succession, including mudstones, siltstones and silty sandstones. The Tunbridge Wells Sand Formation is commonly differentially weathered to form dense sand or silt and stiff clay. The mudstones commonly weather to red clay and the siltstones and sandstones to mottled grey and orange silts and sands. The Wadhurst Clay Formation typically comprises dark grey thinly bedded mudstones (“shales”) and mudstones with pale grey siltstone, fine grained sandstone (locally calcareous where it is known as Tilgate Stone or colloquially "Hastings Granite"), shelly limestone and red Old Rectory Court, High Street, Burwash, East Sussex Page 2 R17-12510 clay ironstone. The top metre or so of the Wadhurst Clay often comprises stiff clay stained red by weathering. Nodular clay-ironstone occurs particularly in the lower part of the formation, but also near the top. 4. GROUND INVESTIGATION 4.1 Introduction The ground investigation comprised the excavation of a series of hand dug trial pits and dynamic sampler boreholes with accompanying in situ geotechnical testing. Groundwater monitoring standpipes were installed in selected exploratory holes and monitored using data loggers. The fieldwork was carried out on 13th October 2017. The exploratory hole locations are shown on Figure 2. Descriptions of the strata encountered and comments on groundwater conditions are shown in the exploratory hole records given in Appendix B, together with notes to assist in their interpretation.