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Evolving techniques for monitoring geotechnical risk on the Antrim Coast Road

by Kaine Lynch, MEng. (DRD, Roads Service/Part-Time PhD Student, QUB), David Hughes, PhD, MICE FCIHT CEng , (Senior Lecturer, Civil Engineering, QUB), Jennifer McKinley, PhD,PGCE, C.Geol., FGS, (Senior Lecturer, Geography, Archaeology and Palaeoecology, QUB) , Andrew Bell, BSc, MSc, and Ruth Harley, MEng. (PhD Students, QUB), Conor Graham, BSc, PGCE, (Geographical Information Systems (GIS) Research Officer, QUB)

AbstractThe interaction between problematic geology and environmental variables along the Antrim Coast Road results in frequent instances of geotechnical instability. During such instances of instability, mudslide debris encroaches on the carriageway posing a hazard to motorists, causing lengthily tailbacks. This paper examines some of the geotechnical and spatial analysis techniques currently being implemented to monitor slope stability on this key transport route.

IntroductionA team of researchers from the School of Civil Engineering and the School of Geography, Archaeology and Palaeoecology at Queen's University, Belfast are working together with Roads Service to investigate new techniques for monitoring landslide movement on sections of the Antrim Coast Road. The research is sponsored by Roads Service, with Clive Robinson, the Roads Service Section Engineer, working closely with the team from QUB to improve the understanding of landslide trigger mechanisms along this beautiful section of coastal road.

The Antrim Coast Road stretching from the seaport of Larne in the East to the famous Giant’s Causeway in the North has a well-deserved reputation for being one of the most spectacular roads in Europe (Day, 2006). Despite this beauty, since construction in the 1830’s there have been a number of locations along the road that have experienced geotechnical instability, including rock falls at Garron Point and landslides at Minnis and Straidkilly Point. This article examines both the geotechnical and spatial analysis techniques currently being implemented to monitor instability on soil slopes along this key transport route.

Effect of rainfall on slope stabilityOne key consideration when analysing slope instability is the interaction between rainfall, soil moisture and pore water pressure. Large-scale failure events where the failure plane is located below the phreatic surface are initiated when an increase in pore water pressure causes a decrease in shear strength of the soil at the failure plane. Luckily this type of deep-seated failure mechanism is exceptional on the Antrim Coast. The most common mechanism is a progressive failure of the near surface zone, with a maximum failure plane depth of 4m below ground level (Hutchinson et al, 1974). This near surface failure is caused by fluctuations in soil moisture and the dissipation of soil suction, which ultimately results in a decrease in the soil’s shear strength. For example,

in some soils the undrained shear strength at the plastic limit (Cu; PL) is approximately 170 kPa, compared to only 1.7 kPa at the liquid limit (Cu; LL) (Atkinson, 2005).

At Straidkilly, one geotechnically active site on the Antrim Coast Road, piezometers and a weather station have been installed to measure deep pore water pressure and provide weather data. The weather data has been further manipulated to provide a Soil Moisture Deficit (SMD) value by analysing the inputs and outputs to the ground water balance model. SMD is a measure of how much rainfall it would take to fully saturate the ground with high values indicating dry soil conditions. The considered variables in this water balance model are infiltration, drainage, evapotranspiration and surface run-off (Schulte, 2005). The

SMD and piezometer data provide an indication of the strength of the soil above and below the phreatic surface, respectively. Figure 1 illustrates the correlation between rainfall, SMD and pore water pressure at Straidkilly between 1st December 2011 and 31st May 2012. This data generally confirms what would be expected; in the winter months the SMD value is low and the pore water pressure is high due to the increase in precipitation and the decrease in evapotranspiration. The data also illustrates that the inverse is true in the spring and early summer months.

Slope movement measurements using terrestrial and airborne LiDARTo spatially analyse the movement of the slope at Straidkilly and to investigate the effects of varying pore water pressures and SMD values, periodic terrestrial LiDAR scanning was carried out. The LiDAR scanner sweeps the slope with a laser beam, recording the topography with an approximate accuracy of 6mm over 50m distance. The measurements are line of sight and therefore can be adversely affected by vegetation and require post-processing in order to remove this false signal. Two monitoring positions were chosen to survey the full extent of the study area. Laser scanning targets were set out and surveyed by the scanner for the registration to a unified point cloud. The post-processed data was exported to ArcGIS and a Digital Terrain Model (DTM) of the site was generated. Figure 2 illustrates a comparison between DTMs generated from subsequent scans to provide time referenced slope deformation data. Red illustrates areas of erosion and blue illustrates areas of

Kaine Lynch

Figure 1 - Rainfall and Soil Moisture Deficit values compared with Pore Water Pressure between 1st Dec 2011 and 31st May 2012 at Straidkilly. (Source: Lynch et al. 2012)

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deposition. When comparing the slope deformation data with the previously obtained geotechnical and weather data, it can be seen that the magnitude of the slope deformation is dependent upon the pore water pressure, but more closely associated with values of SMD. As the SMD value increases, the soil moisture decreases and the shear

strength of the soil increases resulting in less deformation.

In addition to the site specific terrestrial LiDAR scanning carried out at Straidkilly, Garron Point and Minnis, an aerial LiDAR survey of the site has been

flown. A specialist Dornier 228 aircraft (Figure 3) from the National Environmental Research Council (NERC) has carried out an aerial LiDAR survey along this section of the North Antrim Coast.

This aerial data was georeferenced to the Irish Grid using a ground based Leica RTK Global Navigation Satellite Systems (GNSS) receiver (Figure 4). The post-processed data provides highly accurate spatial information, from which large-scale DTMs can be generated for numerical modeling purposes. The advantage of the airborne survey is that the LiDAR scanner is elevated at 5000ft. and can have an uninterrupted bird's eye view of the landscape. Figure 5 has been generated through interpretation of this airborne data and clearly distinguishes landforms, relict landslides and debris flows.

Real Time GPS and remote (satellite) assessment of landslidesThe LiDAR analysis techniques currently being implemented have the disadvantage of the operator physically having to visit the site to carry out the survey. Future work will seek to obtain data of a similar accuracy level remotely, using Global Navigation Satellite Systems (GNSS) and Synthetic Aperture Radar (SAR) interferometer from satellite data obtained by the European Space Agency (ESA).

Roads Service in conjunction with Queen’s University, Belfast, has recently agreed to install a GNSS receiver at Straidkilly Point. This study aims to provide a ‘warning system’ if the magnitude of slope deformation exceeds predetermined site-specific thresholds. The system itself will provide remotely accessible deformation data in the x,y,z plane in real time to an accuracy of 10mm-20mm, with sub-centimetre accuracy at hourly intervals based on archive data automatic post-processing analysis. The GNSS receiver movement is automatically calculated relative to the closest OSNI/LPS fixed Continually Operating Reference System negating the requirement for an on-site reference system, yet still providing the required accuracy. Using GNSS Roads Service will be able to make better-informed decisions regarding the monitoring regime

of soil slopes in the area during periods of intense precipitation.

One further method under consideration to spatially analyse geotechnical instability on the North Antrim Coast is Synthetic Aperture Radar (SAR) interferometer. Recent advances in Differential Synthetic Aperture Radar (DInSAR) and an increasing number of earth observation satellites have facilitated the generation of wide-area maps, which measure ground surface deformation with millimeter precision. Depending on the revisiting period of the satellite being used for data acquisition, this method provides regular comparable scans which can penetrate cloud cover and vegetation (Colesanti et al. 2006). Additional benefits of this method of analysis are that numerous sites can be encompassed by a single scan and even small landslide movements can be observed remotely.

ConclusionThe work described will aid in the understanding of landslide trigger mechanisms and determine the key environmental thresholds, which will facilitate a more informed management regime by Roads Service.

The authors would like to thank Roads Service for their continued support and dedicate this article to the memory of Bob McKay from Roads Service who was closely involved in this project, and gave freely of his time and knowledge to support the work.

ReferencesAtkinson. J (2005) Touring BGA lecture.

C. Colesanti, J. Wasowski, (2006). Investigating landslides with space-borne Synthetic Aperture Radar (SAR) interferometry. Engineering Geology. 88, p173-199.

C. Day, (2006). Ireland. 6th ed. London: Cadogan Guides. p375.

J.N. Hutchinson, D.B. Prior, N. Stephens. (1974). Potentially Dangerous Surges in an Antrim mudslide. Quarterly Journal of Engineering Geology. 7, p363-376.

K. Lynch, A. Bell, D. Hughes, J. McKinley. (2012) Combining Terrestrial LiDAR Monitoring and Geotechnical Monitoring: application to Straidkilly Point, Northern Ireland. (Unpublished).

R.P.O Schulte, J. Diamond, K. Finkele, N.M. Holden, A.J. Brereton. (2005). Predicting the soil moisture condition of Irish grasslands. Irish Journal of Agricultural and Food Research. 44, p95-110.

Figure 3 - Dornier 228 Aircraft carrying out aerial LiDAR scanning. (Source: NERC, 2012)

Figure 2 - Comparison of subsequent LiDAR scans at Straidkilly between Sep 2011 and May 2012. (Source: Bell, 2012)

Figure 4 - Leica RTK GNSS reference station (Source: Lynch 2012)

Figure 5 - DTM of Minnis from NERC aerial LiDAR data. (Graham, 2012)

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Belfast Streets AheadFive years ago the public realm in Belfast city centre was an uncoordinated patchwork, which lacked quality and cohesion. The appropriate linkages were not in place to encourage pedestrians to explore the historic and cultural areas of the city.by Andy Patterson of AECOM

Following an international design competition, the department of Social Development (DSD) appointed AECOM to develop a public realm masterplan for the whole of the city centre. The purpose of the masterplan was to define a vision for the City Centre’s public realm, which through its realisation would lift the quality and profile of the city as a European destination. The vision was to be delivered

over multiple phases. Through their work on the masterplan, AECOM identified opportunities and defined the priority projects for all phases. Phase One of the Streets Ahead project in central Belfast has been completed for nearly a year and , at £24 million, the investment has been the most significant in the city’s public realm to date.

The Phase One scheme was centred on the heart of the

city centre, with the strategy being to create a unified core of high quality public realm that has brought a completely new character and style to the streets; improving the appearance and quality of numerous streets, spaces and squares; incorporating public art. The project established a strong civic spine through the city centre, creating key linkages between the existing high footfall areas of Donegall Square, Donegall Place and Royal Avenue in the retail district, with formerly disparate areas of the city centre at Laganside and Cathedral Quarter, the cultural core of the city centre. As further phases are undertaken, this unified core will spread out to include other parts of the city centre, further enhancing

linkages; and it is hoped that the regeneration effect could be advanced into other areas of need beyond the city centre, through unique catalyst projects.

The design remit was to convey a strong sense of cohesion across the city, consolidate the city’s primary assets, retain the historic fabric of Belfast’s streets, and to use standardised street furniture, paving, road surface material and street lighting to give the city a distinctive unified character and aid ongoing maintenance. AECOM achieved this through the use of high quality natural stone paving materials, enhanced street lighting and furniture. Significant time and thought went into the reordering of the street elements to remove unnecessary clutter, ensuring that the redesigned streets have a natural order, with clear lines and simple alignments that has reduced obstructions, made street cleaning easier and given the pedestrian an altogether more enjoyable experience.

One of the most distinctive and notable elements to emerge in the city centre public realm are the 16m high iconic “feature lighting masts” on the east side of Donegall Place that were designed and built in Northern Ireland. These were conceived to honour Belfast’s industrial heritage and its maritime traditions. The copper masts are enhanced with LED lighting, sail-like banners and branded with the name of significant White Star Line ships that were built in Belfast by Harland and Wolff: Olympic, Oceanic, Britannic, Laurentic, Celtic, Nomadic, Traffic and inevitably Titanic. They serve to frame the vista to City Hall, whilst having a distinctive presence on the street.

Inclusive design and maintenance were given particular attention. Representatives from the major mobility groups were involved in the design process from the early stages of the design, right through to completion. The project achieved the key

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The A32 is a trunk road that links the two main towns of Omagh and Enniskillen in the west of Northern Ireland. The route consists of 26 miles of primarily rural single carriageway but it also passes through the towns of Irvinestown and Dromore. A number of improvement projects have been carried out, however many sections of sub-standard alignment remain. Traffic volumes along the route vary, with 13,700 vehicles per day (vpd) recorded on the outskirts of Enniskillen and 8,300 vpd close to Omagh.

In 2007 Roads Service, Western Division carried out a comprehensive review of standards along the route and developed a document entitled “A32 Omagh to Enniskillen Improvement Strategy”. This strategy aimed to provide a route with consistent geometric standards; improved overtaking opportunities; improved journey times, and improved road safety. A total of 15 potential schemes were identified, 8 of which were major off-line realignments and 7 of which were on-line

works. A project to carry out a number of improvements to the route was subsequently included within the Investment Delivery Plan for Roads (2008).

Coinciding with the development of the Strategy the Department

for Health in Northern Ireland (DHSSPS) were finalising plans for the construction of a new acute hospital in Enniskillen. With acute facilities being withdrawn from the existing Tyrone County Hospital in Omagh, a greater emphasis was being placed on

the need for a high quality route linking the two towns, to meet the needs of blue light services in particular. The Strategy was then given a boost when the DHSSPS, in anticipation of the new hospital opening in 2012, offered a £5 million financial contribution towards road improvements along the A32 Omagh to Enniskillen route.

During 2008 the Western Division Strategic Road Improvement Team carried out a review of the A32 Strategy. This identified 8 schemes which were prioritised in terms of meeting the Strategy objectives together with affordability and deliverability within a short timescale. A rolling programme for delivery was developed as design work got underway with some schemes allocated to the Roads Service internal consultancy (RSC) and others commissioned to external consultants, Amey and AECOM.

With the ensuing curtailment of expenditure in roads infrastructure two schemes were selected for early delivery; A32 Drumskinny and A32 Shannaragh.

Setting the Road StraightThe opening of a new acute services hospital in Enniskillen has increased the significance of the A32 route between Omagh and Enniskillen. As the road passes through the undulating countryside it has significant stretches of substandard alignment which can make the 26 mile journey feel like a marathon. Despite being one of the provinces lighter trafficked trunk roads, the necessity for Emergency Vehicles to travel quickly between the towns has meant that Roads Service is going about setting the road straight!

by Seamus Keenan IEng MCIHT MCMI. Senior Professional &Technical Officer, Strategic Roads Improvement Team, Western Division (MCMI is Member of the Chartered Management Institute)

A32 Scheme Locations

aims for the blind and partially sighted; to reduce obstructions and aid navigation through the introduction of visibility bands on furniture and improved street lighting, as well as removing building threshold obstructions and steps wherever possible. To address the legacy of ongoing maintenance various initiatives were adopted. Services were upgraded through the project and by utility companies to reduce the likelihood of later disturbance.

The project also mapped

out the entire existing utility infrastructure to allow future maintenance of this infrastructure to be more targeted and precise. A maintenance package was developed through the project that includes full method statements on the deconstruction and reconstruction of the surface, as well as the design drawings. This, combined with a materials stockpile, allows for future maintenance works to be completed to match in with the original works, so the new

street environment can be enjoyed for years to come.

Belfast Streets Ahead Phase One has transformed the city centre creating a more vibrant city centre; a safer, cleaner, more attractive environment in which to shop, visit and work. The success of the project goes beyond the aesthetic improvement; it has inspired positive change in the city, bolstering the city centre in a time of recession, attracting new inward investment and increased footfall to the retail

environment, delivering on key objectives of the project.

AECOM led the Client Team that developed the initial designs to form the Phase One works, developing these to detail design stage; a Design and Construct team led by Farrans Construction were then appointed through an NEC option C contract to deliver the Phase One works. The project teams worked in partnership to deliver this historic transformation to help Belfast compete on the European stage.

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A32 DrumskinnyThe existing road at Drumskinny, just west of Dromore, had a substandard vertical alignment and the poor ground conditions meant that the surface was undulating giving rise to a continuous maintenance burden. The scheme proposed on-line widening over a 1.1 kilometre stretch, with much of the centre line of the new carriageway being offset by a lane width to avoid the worst ground conditions, and to simplify traffic management during construction. Peat excavation to a depth of 3.5 metres over a considerable length of the scheme would nonetheless still be required. The new carriageway would be 7.3 metres wide with 1 metre hard strips, replacing the old 6.5 metre carriageway with no meaningful verge width. Forward visibility and thus overtaking opportunity would also be improved significantly.

Given the nature of the scheme and the fact that only 5 landowners were involved,

many of the time consuming statutory procedures associated with larger schemes could be avoided. In January 2010 RSC awarded an advance contract for site clearance and some preliminary earthworks to Patrick Bradley Ltd, who held the term contract for minor improvements to roads and car parks. This paved the way for the main contract, awarded under open tender in August 2011 to Fox Building & Engineering Ltd with a value in the region of £1 million. Work commenced on site in September 2011 and was substantially completed in March 2012.

A32 ShannaraghThe existing road at A32 Shannaragh is characterised by a particularly poor horizontal and vertical alignment through a series of sharp bends. There are also 3 substandard minor road junctions in the vicinity.

In 2009 Roads Service commissioned AECOM to design and deliver an improvement scheme which would be consistent with

the aims of the Strategy. The proposed new road line was taken through the necessary statutory procedures. Extensive consultations with affected landowners throughout reduced the number of objections to the proposals and a public inquiry was not therefore deemed necessary. The procurement process got underway in April 2011 culminating in the award of contract to a joint venture between Coffey Construction Ltd and Whitemountain Quarries Ltd in January 2012.

The improvement scheme involves the construction of 2.2 kilometres of 7.3 metre wide single carriageway, with 1.0m hard strips and 2.5m grass verges. It includes the realignment of some local roads to form 2 new junctions with the main line. Also within the contract are the construction of a 21 metre span bridge over the Owenreagh River; 2 reinforced concrete box culverts; a strengthened earthwork retaining structure; soft ground improvements and substantial earthworks.

This is the largest of the schemes listed within the A32 Strategy with a Project value of £7.5 million. Work commenced on site in March 2012, is progressing well and is expected to be complete by March 2013.

RSC in partnership with Amey have also been taking forward the design of other priority schemes within the Strategy with two further schemes at an advanced stage of preparation. These are at A32 Cornamuck and A32 Esker Bog.

The scheme at Cornamuck involves 1.4 kms of off-line realignment with a significant volume of earthworks. This scheme when combined with the newly constructed Drumskinny scheme will provide a significant improvement in overtaking opportunities just west of Dromore. The estimated construction cost of this scheme is £4 million.

The scheme at Esker Bog, Dromore involves 1.1 kms of off-line carriageway realignment and the realignment of a crossroads junction. The estimated construction cost is £3 million.

Site Clearance works underway at A32 ShannaraghFuture Priority Schemes along the route: Cornamuck and Esker Bog, Dromore

The scheme at A32 Drumskinny nearing completion

Beams for the new bridge being placed over the Owenreagh River