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IGSL Ltd _________________________ Boherkill Sand & Gravel Quarry ________________________ Slope Stability Assessment

Project No. 22517 May 2020

M7 Business Park Naas Co. Kildare Ireland T: +353 (45) 846176 E: info@igsl.ie W: www.igsl.ie

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Boherkill Sand & Gravel Quarry Slope Stability Assessment

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DOCUMENT ISSUE REGISTER

Distribution Copies Rev. Date of Issue

Prepared By: Approved By:

Kildare Sand & Gravel

PDF by email 0 28-5-2020

P. Quigley BEng CEng MIEI MICE FGS

D. Green BAI CEng MIEI FGS

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Boherkill Sand & Gravel Quarry Slope Stability Assessment

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TABLE OF CONTENTS

Foreword

1. Introduction 2. Site Inspection

3. Ground Assessment

4. Conclusions & Recommendations References Appendices Appendix 1 - Site Location / Plan Figures Figure 1 - Boherkill Quarry Site Location Figure 2 - Boherkill Site Development Figure 3 - Quaternary geological map Map for Site Figure 4 - Bedrock geological map for the site Figure 5 - Image showing sand and gravel slope at north / north-east corner of facility Figure 6 - Image showing localized undercutting of sand and gravel Figure 7 - Image showing side slope and berm at north-eastern corner of facility Figure 8 - Image showing side slope adjacent to access road Figure 9 - Image showing existing edge protection along access road

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FOREWORD The following conditions and notes on the slope stability assessment should be read in conjunction with this report.

Standards The site inspection and slope assessment was conducted by IGSL in accordance with BS 5930 (2015) and Eurocode 7. Reporting This report has been prepared for Kildare Sand and Gravel and the information should not be used without their prior written permission. IGSL Ltd accepts no responsibility or liability for this document being used other than for the purposes for which it was intended.

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1. INTRODUCTION At the request of Kildare Sand & Gravel, IGSL has undertaken an assessment of the stability of the side slopes or batters within Boherkill Quarry, Rathangan, Co. KIldare. The assessment was carried out to evaluate the stability of the side slopes at the facility which receives Construction and Demolition (C&D) inert waste from the building and construction sector. The site is located at Boherkill off the R4011 and lies approximately 3km south west of Rathangan (refer to Figure 1). Sand and gravel was formerly extracted from the facility (quarry) and a screening plant is positioned at the southern end of the site. The adjoining lands are agricultural (tillage) and the quarry is sited above groundwater level. The historical extraction or production faces to the north and north-east extend upwards of 10 to 12m in places. This report presents an evaluation of the ground conditions and stability assessment of the side slopes at the facility. Figure 1 – Boherkill Quarry Site Location

22517 – Boherkill Quarry : Site Location Plan (Aerial imagery obtained from Google Maps 2020)

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2. SITE INSPECTION A site visit was conducted by an IGSL geotechnical engineer on 21 May 2020. The inspection entailed a walk-over of the facility to examine the exposed side slopes. Particular attention was given to the steeper slopes at the northern and north-eastern end of the quarry and those adjacent to the access road from the wheel wash to the weighbridge. The development of the site can be seen from Google maps dating from 2000 to 2020 (refer to Figure 2) Figure 2 – Boherkill site development

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3. GROUND ASSESSMENT The sand and gravel exposures were examined in accordance with BS 5930 (2015), I.S. EN 932-3 and I.S. EN 14689-1:2018. The walk-over and geotechnical inspection shows that the exposed pit faces consist of a variable sequence of sand and gravel with localised zones or horizons of fine to medium sand. The gravel and cobble size clasts are dominated by grey fine-grained limestone with infrequent sandstone. A wide range of particle sizes were viewed with rare cobble beds (mainly in the NE corner of the pit). Boulders were occasionally viewed in exposure, ordinarily measuring up to c300mm. Reference to the GSI records (Figure 4) shows the underlying bedrock consists of the Boston Hill Formation (BN). This is characterized by Carboniferous limestone (argillaceous) and shale. No bedrock outcrops or exposures were evident in any of the exposed side slopes. Figure 3 – Quaternary Geological Map for the Site

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Figure 4 – Bedrock Geological Map for the Site

The sand and gravel material is deemed to represent a very heavily over-consolidated outwash glacial deposit. Figures 5 to 8 illustrate the make-up and current condition of the highest slopes (estimated at 9 to 10m) at the north and north-eastern portion of the facility. A berm is present at the north east corner and has a slope or batter of the order of 1V to 2H (26°). No information is available on the background to the berm but it is suspected that this may be formed to mitigate the risk of instability during the extraction operations. The very close packing of the sand and clast constituents demonstrates the surcharging effects of ice loading (most likely repeated phases) which is responsible for the very dense nature of the granular deposits. The high degree of over-consolidation is reflected by the embedment of sub-rounded and sub-angular clasts in the sand matrix. These types of deposits produce very high angles of shearing resistance or friction (∅). Large shear box tests on heterogeneous sand and gravel from similar deposits have produced friction angles of the order of 45 to 55°. Such high internal angles of friction in conjunction with a high degree of over-consolidation means that sand and gravel (granular deposits) has the capability to remain stable at

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very steep (i.e. sub-vertical) angles. The material is what can be termed free draining and therefore cannot have effective cohesion (C’). Sand and gravel deposits do not lose strength with time (unlike cohesive soils) and the key to stability in such deposits is their cemented nature, free or well draining characteristics and lack of groundwater. Groundwater was not observed within any of the exposed faces or slopes during the course of the site inspection. This was not unexpected given the very dry period from late March through to May 2020.

From a geotechnical aspect, the examination showed no evidence of over-stressing (i.e. slope failures or instability) in the side slopes. Pockets of undercutting of the slopes can be viewed in Figure 6 (northern side slopes) with localized slumping noted within the base of the slope from the wheel wash to the weighbridge. These are considered to be historical features and most likely occurred after the access road was formed. Surface wash-out was visible on some of the side slopes but generally very moderate with no prominent scouring or undermining observed. The stability of granular deposits is governed by friction angle and presence or absence of groundwater. Even taking a friction angle of 45 to 50° and dry conditions (i.e. very low phreatic surface), the very steep (sub-vertical) slopes should not be stable (from a theoretical soil mechanics perspective). However, the very high degree of over-consolidation and lack of water within the deposit means that the sand and gravel has the ability to stand or remain safe at steep angles (i.e. 75 to 80°). This is borne out by the condition of the side slopes within the facility and satisfactory historical performance over a number of years.

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Figure 5 – Image showing sand & gravel slope at the north / north-east corner of the facility

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Figure 6 – Image showing localized undercutting of the sand and gravel material at the northern part of the facility

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Figure 7 – Image showing berm supporting side slope and berm at the north eastern corner of the facility

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Figure 8 – Image showing side slopes adjacent to the access road (wheel wash to site office / weighbridge)

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4. CONCLUSIONS & RECOMMENDATIONS The site inspection and examination of the side slopes at Boherkill Quarry shows that apart from very localized undermining and wash-out channels (due to rainfall run-off) the slopes are considered to be in a stable condition. There was no evidence of failures within the exposed side slopes. It is recommended that the side slopes (mainly at the north and eastern areas of the facility) be examined by a competent site representative of the Client on a regular basis (suggested 4 to 8 week intervals during receipt of waste). The purpose of this is to ensure a safe working environment for plant operators (dozer driver) especially during and after periods of heavy rainfall or snow (freeze-thaw). This would be regarded as ‘good house-keeping’ and a proactive approach to maintaining safety for field operatives. One area of concern is the access road from the wheel wash to the weighbridge. The existing edge protection is somewhat haphazard (refer to Figure 9) and should be upgraded. This is important as the underlying quarried side slope is sub-vertical and presents a key risk or hazard in the event that a tipper lorry veers towards the edge of the access road (left hand side approaching the weigh bridge). It is therefore advised that the berm (minimum height of 1.5m, crest width of 1m and side slopes of 1V to 2H) be upgraded with sand and gravel (uncompacted to dissipate action of a lorry on impact) to form adequate edge protection. Figure 9 - Image showing existing edge protection along access road

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References

1. Geological Survey of Ireland, Geology of Kildare – Wicklow (1:100,000 series)

2. Kildare Architects and Design Ltd, EIS on Boherkill Quarry

3. I.S. EN 14689-1-2018: Geotechnical Investigation & Testing – Identification and Classification of Soil

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Boherkill Sand & Gravel Quarry Slope Stability Assessment

Appendix 1

Site Location Plan

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22517 – Boherkill Quarry : Site Location Plan (Aerial imagery obtained from Google Maps 2020)

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