slope stabilty assessment dunsink landfill fingal … · 2016-06-28 · (overgrown) as the area is...

SLOPE STABILTY ASSESSMENT DUNSINK LANDFILL FINGAL COUNTY COUNCIL

APRIL 2016

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P0013

SLOPE STABILTY ASSESSMENT DUNSINK LANDFILL

User is Responsible for Checking the Revision Status of This Document Rev. Nr.

Description of Changes

Prepared by: Checked by: Approved by: Date:

1 Issue to Client JON/MG CJC BG 26-04-2016 Client: Fingal County Council Keywords: Slope, Stability, Assessment, Dunsink, Fingal Abstract: Fingal County Council retained Fehily Timoney & Company (FT) to carry out a slope

stability analysis of the landfill side slopes in order to comply with Condition 8.9.1 of Waste Licence W0127-01.

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TABLE OF CONTENTS PAGE

1 INTRODUCTION ...................................................................................................... 1

1.1 BACKGROUND ........................................................................................................................................................... 1

1.2 PURPOSE ................................................................................................................................................................... 1

1.3 SITE DESCRIPTION .................................................................................................................................................. 1

1.4 SITE WALKOVER ...................................................................................................................................................... 1

2 SLOPE STABILITY OBSERVATION ........................................................................... 3

2.1 SLOPE 1 .................................................................................................................................................................... 3 2.2 SLOPE 2 .................................................................................................................................................................... 4 2.3 SLOPE 3 .................................................................................................................................................................... 5 2.4 SLOPE 4 .................................................................................................................................................................... 5 2.5 SLOPE 5 .................................................................................................................................................................... 7 2.6 SLOPE 7 .................................................................................................................................................................... 9 2.7 SLOPE 8 .................................................................................................................................................................. 10 2.8 SLOPE 6 .................................................................................................................................................................. 11 2.9 SLOPE 9 .................................................................................................................................................................. 12 2.10 SLOPE 10 ............................................................................................................................................................... 13 2.11 SLOPE 11 ............................................................................................................................................................... 15 2.12 SLOPE 12 ............................................................................................................................................................... 17 2.13 SLOPE 12A ............................................................................................................................................................ 18 2.14 SLOPE 13 ............................................................................................................................................................... 18

3 SUMMARY AND CONCLUSIONS ............................................................................. 20

LIST OF APPENDICES Appendix 1: Slope Location Plan

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LIST OF FIGURES PAGE

FIGURE 2.1: SLOPE 1: VIEW TO NW FROM TOP OF SLOPE 1 (1) ..................................................... 3 FIGURE 2.2: SLOPE 2: MINOR ANIMAL POACHING SLOPE 2 (3&4) .................................................. 4 FIGURE 2.3: SLOPE 3 TOE OF SLOPE LOOKING NE (1), VIEW NE FROM TOP OF SLOPE (2), VEHICULAR RUTTING

ALONG FACE (3) ............................................................................................. 5 FIGURE 2.4: SLOPE 4: SIGNIFICANT LOCALISED POACHING (3), HEAVILY VEGETATED STREAM REQUIRING

REMEDIAL WORKS (4) ....................................................................................... 6 FIGURE 2.5: SIGNIFICANT EROSION GULLIES AND CHANNELS (3) (4) (5), VIEW FROM SLOPE APEX LOOKING N8 FIGURE 2.6: SLOPE 7: NORTH WEST FACING SLOPE CREST (1) TOE VIEWING TO SW (2) NORTH EAST FACING

SLOPE VIEWING TO SE (4) ................................................................................. 9 FIGURE 2.7: SLOPE 8: SLOPE FROM CREST FACING NE (1), SLOPE FROM CREST FACING SOUTH WEST (2) .. 10 FIGURE 2.8: SLOPE 6: SE FACING SLOPE PORTION FROM CREST (1), SW FACING SLOPE PORTION FROM TOE

(2) ........................................................................................................... 11 FIGURE 2.9: SLOPE 9: EXPOSED LEACHATE DRAIN (2) ............................................................... 12 FIGURE 2.10: SLOPE 10: SECURITY FENCING WITH GAS VENT STANDPIPES FOREGROUND (1, 2), MINOR

POACHING (4) .............................................................................................. 14 FIGURE 2.11: SLOPE 11: VEGETATED SURFACE WATER DRAINAGE CHANNEL (2, 3, 5) ............................ 16 FIGURE 2.12: SLOPE 12: VIEW TOWARDS LANDFILL GAS TO ENERGY PLANT (4) .................................. 17 FIGURE 2.13: SLOPE 12A: ACCESS ROAD TO LANDFILL (1), DUNSINK HORSE CLUB BUILDINGS (2) ............ 18 FIGURE 2.14: SLOPE 13: MINOR POACHING (1, 2), TREE PLANTING (3, 4) ......................................... 19

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Section 1 Fingal County Council Dunsink Landfill: Slope Stability Assessment

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1 INTRODUCTION 1.1 Background Fehily Timoney and Company (FT) was appointed by Fingal County Council, in November 2015, to undertake a slope stability assessment of Dunsink Landfill, Dunsink Lane, Finglas, Co. Dublin. Previous slope stability assessments were carried out for the landfill by:

FT in December 2014 (Ref: Q:/2014/LW14/164/07/Reports/Rpt001-1.doc) RPS in December 2012 and 2013. BMA GeoServices Ltd (BMA) in May 2004, August 2005, November 2006, September 2007, November

2008, August 2009, July 2010 and December 2011. All of the above reports have been used to facilitate this current slope stability assessment. FT completed a site walkover and visual slope stability assessment of Dunsink Landfill on the 21st December 2015. The results of the visual assessment indicate that all slopes are stable. Some minor issues (poaching and areas of bare vegetation) will require simple remediation works typically re-grading and reseeding. Monitoring and maintenance of over vegetated and silted drainage channels and ditches will also be required. 1.2 Purpose This assessment is in accordance with Condition 8.9.1 of the EPA waste licence issued to the site (Current Licence No: W0127-01) which states:

“Within three months of the date of grant of this licence, the licensee shall carry out a stability assessment of the side slopes of the facility. It shall be repeated annually thereafter.”

1.3 Site Description Dunsink Landfill is located approximately 5 km northwest of Dublin city centre, adjacent to the M50. The total site is approximately 76 ha. The site was previously developed as a municipal landfill facility with the southern part of the landfill being the oldest. Domestic waste acceptance ceased at the site in 1996 with construction fill type being accepted thereafter. Domestic waste has been deposited predominantly to the south and centre of the site. All waste acceptance ceased in approximately 2002 and capping works were completed between 2002 and 2006. The site is currently vegetated with grass mainly, with some tree planting on the North West section of the site, adjacent to the M50 and minor planting locally within the site. 1.4 Site Walkover FT undertook a complete site walkover of the site on the 16th December 2015 in the presence of Mr. Alain Kerveillant of Fingal County Council. All slopes within the site were inspected, the assessment was a visual assessment with written and photographic notes being taken. All slopes were inspected on foot from toe to crest across the entire length of the slope. All were assessed for signs of instability or identification of potential factors which could affect the future stability of the landfill slopes.

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All slopes were identified as per previous studies to ensure continuity. Slopes were identified as Slope 1 – Slope 13 inclusive and Slope 12A Appendix A to this documents contains a Slope Location Plan1 previously prepared by RPS showing the location of each slope.

1 Slope Location Plan by RPS, File Ref MDT0657DG0003.dwg

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2 SLOPE STABILITY OBSERVATION 2.1 Slope 1 Slope 1 is located to the NW of the site. The slope is vegetated with minor scrub and grass. Minor animal poaching is evident. A vegetated stone drain runs diagonally across the length of the slope to a drainage channel at the bases. No issues that may indicate slope instability were noted. The slope is considered stable.

Figure 2.1: Slope 1: View to NW from top of Slope 1 (12)

2 Photographs Labelled from Left to Right, Top to Bottom and denoted (#)

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2.2 Slope 2 Slope 2 is located to the NW and runs general in parallel to the M50. It is well vegetated with grass (overgrown) as the area is securely fenced and un-grazed. Minor poaching was noted in areas across the slope. A vegetated drainage channel runs SE to NW at the base of the slope. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.2: Slope 2: Minor animal poaching Slope 2 (3&4)

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2.3 Slope 3 Slope 3 is located on the most northern aspect of the site. The slope runs NE turning about and continuing in a due S direction. The slope is well vegetated with grass and furze. Minor variations in topography across the slope were noted, possibly indicative of differential settlement across the slope. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.3: Slope 3 toe of slope looking NE (1), view NE from top of slope (2), vehicular rutting along face (3)

2.4 Slope 4 Slope 4 is located to the south of Slope 3 running N to S before turning towards the East. An area of significant poaching was noted along the slope as photographed. Minor waterlogging or flooding of neighbouring lands adjacent to the access track running along the toe of Slope 4 was noted at the time of the walkover as per previous walkovers. The culverted Scribblestown Stream runs along the toe of a portion of Slope 4 in a W-E direction. Minor poaching and rutting were observed along the face of the slope in some areas.

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The stream was seen to be heavily vegetated in places. It is recommended that this stream should be suitably cleaned to ensure continuous drainage and minimise waterlogging in the area. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.4: Slope 4: Significant localised poaching (3), heavily vegetated stream

requiring remedial works (4)

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2.5 Slope 5 Slope 5 is located to the south of the surface water attenuation pond. The slope runs in a NW-SE direction before turning sharply in a narrowing valley and continuing in a S-N orientation. A number of rills and gullies are evident in the southernmost aspect of the slope at the attenuation pond drainage outfall. The rills and gullies have a significant depth (See Figure 2.5 (3), (4) & (5)). This riling was also noted as part of the previous slope stability assessment in December 2014. The rills and gullies when referenced to historical photographic assessments appear to have deepened over time. The cause of this problem is significant uncontrolled drainage run off from an inadequately drained area to the top of the slope. See Figure 2.5 (6). Significant ponding was noted in this area. Previously identified remedial works comprising of the installation of a stone lined drainage channel have yet to be undertaken. A drainage channel is planned to be installed on the western portion of slope 5 to intercept the uncontrolled flow and carrying the surface water to the attenuation pond to the North. These works should significantly reduce uncontrolled drainage down the slope. Notwithstanding that stated previously, no issues that may indicate instability were noted. The slope is considered stable. It is suggested that the planned remedial works are undertaken in a timely manner to prevent further riling and gully development which may if left unchecked precipitate translational instability or damage to the cap.

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Figure 2.5: Significant erosion gullies and channels (3) (4) (5), View from slope apex looking N

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2.6 Slope 7 Slope 7 consists of perpendicular slope faces, one slope facing to the North West and one to the North East. The North West facing slope is generally well vegetated with grass. The North East Facing section of the slope is constructed in two tiers and heavily vegetated with scrub grasses, thistles etc. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.6: Slope 7: North West facing slope crest (1) Toe viewing to SW (2) North East facing Slope viewing to SE (4)

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2.7 Slope 8 Slope 8 is a continuation of Slope 7 running to the South East. The slope is well vegetated with scrub grasses, thistles and some small trees. An open drain runs along the toe of the slope. The drain is heavily vegetated. Minor cleaning works are recommended to ensure drainage is maintained to prevent waterlogging. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.7: Slope 8: Slope from crest facing NE (1), Slope from Crest facing South West (2)

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2.8 Slope 6 Slope 6 is located to the North West of the main leachate lagoon. The South West facing portion of the slope is irregularly graded and heavily vegetated with grass, thistles and furze. The South East facing portion is regularly graded and vegetated by grass only. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.8: Slope 6: SE facing slope portion from crest (1), SW facing slope portion from

Toe (2)

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2.9 Slope 9 Slope 9 is a continuation of Slope 6 around the leachate storage lagoon. The slope is divided into two sections; a South West facing slope and a North East facing slope. Both slopes are heavily vegetated. An exposed leachate drainage line exists at the toe of the south west facing slope which displays red deposited material of unknown character (See Figure 2.9 (2)). No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.9: Slope 9: Exposed leachate drain (2)

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2.10 Slope 10 Slope 10 runs along the boundary of an unopened sports field area located in the south east portion of the site. A palisade type security fence runs along the face of the slope. The toe of the slope is generally delineated by a number of gas vent stand pipes inserted at regular intervals parallel to the slope along the boundary of the sports fields. Some minor poaching of the ground was noted in some areas. No issues that may indicate instability were noted. The slope is considered stable.

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Figure 2.10: Slope 10: Security fencing with gas vent standpipes foreground (1, 2),

minor poaching (4)

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2.11 Slope 11 Slope 11 runs parallel to Dunsink lane which marks the southern boundary of the site. A vegetated surface water drain runs diagonally across the slope face. The drainage is noted to be shallow in portions due to possible siltation. Ongoing observations should be made to ensure sufficient depth is maintained i.e. deepening/cleaning. The slope is well vegetated by grass. A proportion of the lower portion of the slope is fenced off from the rest of the site and is heavily vegetated with scrub grasses and briars. No issues that may indicate instability were noted. The slope is considered stable.

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Figure 2.11: Slope 11: Vegetated surface water drainage channel (2, 3, 5)

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2.12 Slope 12 Slope 12 extends from Slope 11 continuing adjacent to the Dunsink lane boundary. Site access roads have been constructed across the slope including the main site access from Dunsink lane. The slope is generally well vegetated with minor areas of poaching evident in a number of areas due to animal grazing. Electricity pylons and poles run diagonally across the slope from the main Landfill Gas to Energy compound westward and eventually offsite. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.12: Slope 12: View towards Landfill Gas to Energy Plant (4)

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2.13 Slope 12A Slope 12A has been constructed within Slope 12 to facilitate access to the landfill and the gas utilisation compound. The slope is locally quite steep but well vegetated. No issues that may indicate instability were noted. The slope is considered stable.

Figure 2.13: Slope 12A: Access road to Landfill (1), Dunsink Horse Club buildings (2) 2.14 Slope 13 Slope 13 begins towards the South Eastern corner of the site and runs Northward then veering North Easterly parallel to the M50 and toward Slope 2. It was noted that a portion of the bottom section of Slope 13 has been planted with trees and fenced off. The trees were examined for tell-tale signs of slope slippage/settlement, i.e. a curving of the lower trunk, no curving was noted. Minor poaching was noted to areas across the slope. A vegetated surface channel runs diagonally across the slope to a drainage channel. The drainage channel runs parallel to the Northern site boundary. The channel flows SE-NW parallel to the M50. No issues that may indicate instability were noted. The slope is considered stable.

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Figure 2.14: Slope 13: minor poaching (1, 2), tree planting (3, 4)

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3 SUMMARY AND CONCLUSIONS FT completed a site walkover and visual slope stability assessment of Dunsink Landfill on the 21st December 2015. All slopes were walked in a toe to crest survey, photographed and examined for indications of instability. The results of this visual assessment indicate that all slopes are stable. Some minor issues were noted such as poaching and areas of bare vegetation which may be remediated by re-grading, reseeding or other minor works. The assessment also noted a significant number of erosion rills and gullies notably on Slope 5. A review of historic photographic records suggest that these rills and gullies have deepened since the previous FT site walkover in 2014. It is recommended as per previous FT reports that the remediation works identified by FCC to FT and previously suggested in the 2013 RPS study i.e. the installation of a stone lined drainage channel to the diagonal of the slope terminating in the site attenuation pond, be undertaken in order to ensure this erosion does not become more significant in this area. It is also recommended that ongoing maintenance of the identified vegetated drainage channels be undertaken to maintain drainage flow and prevent localised waterlogging. All slopes should be continually monitored by FCC and inspected annually as required by the conditions attached to Waste Licence No. W0127-01.

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APPENDIX 1

Slope Location Plan

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slope stabilty assessment dunsink landfill fingal … · 2016-06-28 · (overgrown) as the area is...

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