«PUBLISHED PROJECT REPORT PPR636 A83 Rest and be Thankful: Ecological and Related Landslide Mitigation Options M G Winter (TRL) and A Corby (Transport ...»
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PUBLISHED PROJECT REPORT PPR636
A83 Rest and be Thankful: Ecological and Related Landslide
M G Winter (TRL) and A Corby (Transport Scotland)
Prepared for: Transport Scotland, Trunk Road and Bus Operations
Project Ref: TRTC 2000-097
B Shearer R Woodward
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Contents amendment record
This report has been amended and issued as follows:
Version Date Description Editor Technical Referee 1 01/11/2012 Draft 1c MGW 2 02/11/2012 Draft 1d MGW BS 3 08/11/2012 Draft 1g MGW RW M G Winter & A Corby i PPR 636 A83 Landslide Mitigation Contents
2 1 Introduction 3 2 Use of Explosives 5 3 Livestock 9 3.1 Reducing the Height of Grass 9 3.2 Un-grazed Grass 9 3.3 Sheep and Runoff 9 3.4 Damage from Animal Hooves 11 4 Vegetation Planting 15 4.1 Benefits and Dis-benefits of Vegetation 15 4.2 Potential for Planting 18 5 Summary and Conclusions 22 Acknowledgements 23 References 24 M G Winter and A Corby 1 PPR 636 A83 Landslide Mitigation Abstract In recent years a number of debris flow events have closed the A83 trunk road in the vicinity of the Rest and be Thankful. The damage caused has been both difficult and costly to repair, and the associated closures have caused traffic delays with attendant socio-economic impacts. This report examines the merits of a number of ecological and related landslide mitigation options for the southwest facing slopes of Beinn Luibhean in Glen Croe above the A83 trunk road. The use of explosives, and issues related to livestock and vegetation planting are discussed and an outline scheme for planting, developed by Forestry Commission Scotland and others in collaboration with the Authors, is briefly described. The planting scheme has the potential to reduce instability in the long-term but must be considered as part of a broader strategy that incorporates other aspects of land management, including stock control, and appropriate engineering measures.
M G Winter and A Corby 2 PPR 636 A83 Landslide Mitigation 1 Introduction The Transport Scotland Scottish Road Network Landslides Study (Winter et al., 2005, 2009) undertook a hazard identification and ranking exercise and also set out the approach to the management and mitigation of debris flow events. While the primary approach was one of management, or exposure reduction, provision was also made for a more active mitigation, or hazard reduction, approach to be applied for the highest ranked sites such as the A83 Rest and be Thankful.
Debris flows are a regular occurrence in the general area of the A83 at the Rest and be Thankful. For over 20 years the first Author has observed such events on a regular, approximately annual, basis as they affect the area between Ardgarten and west of Cairndow. In recent years, broadly since 2007, these events have occurred in a somewhat more concentrated area, affecting the westbound approach to the Rest and be Thankful (Figure 1). These events are sourced on the south-westerly facing slopes of Beinn Luibhean (Figure 2).
Figure 1. Map showing the notional geographical limits of debris flow occurrence affecting the A83 trunk road in the general area (blue lines) of the Rest and be Thankful and the approximate limit for events since 2007 (red lines).
Reproduced by permission of Ordnance Survey, on behalf of HMSO, © Crown copyright and database rights, 2012. All rights reserved.
Ordnance Survey Licence number 100046668.
M G Winter and A Corby 3 PPR 636 A83 Landslide Mitigation Figure 2. The A83 on the approach to the Rest and be Thankful showing the south-west facing slopes of Beinn Lubhean (image dated October 2007).
Transport Scotland has initiated a major study to examine longer term solutions to the A83 Rest and Be Thankful landslide problem and to address broader route issues. Jacobs were appointed to examine engineering options and have been advised by TRL, drawing on earlier involvement in the Scottish Road Network Landslides Study (SRNLS).
As part of the aforementioned A83 study, Transport Scotland gave an undertaking to examine potential ecological solutions to the problem based around tree-planting and other forms of re-vegetation of the south-westerly facing slopes of Beinn Luibhean. It was also envisaged that this part of the work would examine some of the alternative solutions that have been proposed including those that advocate the use of explosives or livestock as part of the solution.
This report addresses these proposed solutions and suggests ways forward where appropriate.
M G Winter and A Corby 4 PPR 636 A83 Landslide Mitigation 2 Use of Explosives It has been suggested that Transport Scotland should “… just fix the … hillside dynamite it! Like they do in the rest of the world …”.
It is not entirely clear what inspired the correspondent to suggest this approach but a degree of observational experience of rock blasting in either a quarry or infrastructure context seems most likely. Certainly there is a variety of forms of blasting that is used for varying purposes in the context of rock bodies and slopes. These include, in the quarrying context, blasting to break apart large bodies of rock to allow further processing to produce aggregate, for example.
The detailed explosive charges can be configured to better approximate the target aggregate size range required. In an infrastructure construction context blasting is used to form cuttings (Figure 3) either using bulk blasting techniques or pre-split techniques, the latter producing a smoother and lower maintenance final face, although one that is not necessarily in keeping with adjacent naturally weathered rock slopes and outcrops (Figure 4). Blasting is also sometimes used to scale loose material from existing rock slopes.
Figure 3. Rock blasting adjacent at the A83 Artilligan (image dated May 1997).
The hillside at the Rest and be Thankful comprises a layer of soft soil that overlies the rock strata. It is the soil material that forms the debris flows that create such a problem when they block and otherwise affect the road. As a consequence it is this material, the product of weathering and previous slope movements, which would need to be blasted. This presents a somewhat different picture compared to blasting rock.
There are no known cases of blasting such soil materials to affect their removal in the technical literature. There are a number of instances in which blasting of
rock has triggered slides in soft, soil material and blasting has been used on one occasion to attempt to trigger a sub-lacustrine slide in Canada, in the 1980s, albeit with rather unpredictable results.
Figure 4. Pre-split rock cut slope on the A830 near Arisaig.
Notwithstanding the foregoing blasting, involving near-shore soft sediments, has been used in a very particular scenario in Norway. When a large amount of fill is to be placed on the sloping, soft sediments just offshore (for example in connection with road construction along the fjords), the soft soil is replaced with rockfill down to bedrock by intentionally inducing failure. As much rockfill as possible (i.e. safety factor just in excess of 1.0) is placed on the clay, and the foot of the rockfill is blasted to induce failure. The disturbance caused by blasting and the weight of the fill mobilizes the soft sediments such that they flow downslope; the rockfill above the clay then sinks replacing the clay. This process is repeated until the desired geometry is reached. Of course, once one starts a submarine slide in this way, the effects can be quite unpredictable. A similar effect has been induced in soft quick clay sediments by rock blasting on the foreshore, albeit with somewhat less intent.
The concept of blasting of slopes with soft material prone to debris flow is new to both the Authors and those experts in this field who have been consulted during the preparation of this report. A number of concerns have been expressed
regarding this proposed approach, as follows:
That blasting may merely increase the area without vegetation cover (i.e.
bare soil will be exposed) allowing water to infiltrate and erode the remaining soil cover more effectively causing further instability.
M G Winter and A Corby 6 PPR 636 A83 Landslide Mitigation That further marginally stable areas would be created as the soil is partially disturbed.
That, in places, the rock would be fractured creating material that is more easily weathered and that will later become unstable.
That the soft wet soils, including peat, present on the hillside will simply absorb the blast energy.
That the process will be only very partially effective but, in the process of creating more unstable material as described above, will destroy the natural appearance of the hillside.
It should also be noted that even if, as seems highly unlikely, blasting were effective in removing all of the soft soils, this would leave a very unsightly hillside comprising exposed rock that would itself be highly vulnerable to weathering that would then create more loose and unstable material and likely have a detrimental impact on the appearance of the attractive character of the wider Glen Croe area.
Indeed, when one considers the amount of potential soft material on the southwest slopes of Beinn Luibhean this could be of the order of a few million cubic metres. Typically the largest (rock) landslide volumes that have been treated in Norway, for example, are of the order of 20,000m3 as at Fatlaberget (20,000m3 in two stages) and 10,000m3 at Skjerping (Figure 5). Certainly the general view is that there is little experience of blasting larger (rock) landslide volumes in the order of 100,000m3, let alone several million cubic metres.
Figure 5. Dust cloud caused by blasting at Skjerping, Norway in 2010.
The cloud travelled around 7km down the valley and fjord of Nærøydalen. (Photograph by Paulsen, courtesy of Ulrik Domaas, Norwegian Geotechnical Institute.)
Other issues that must be considered include the removal of the material resulting from the blast and the safety of workers involved in the operations.
Perhaps the greatest concerns with this type of operation, apart from the relative unlikelihood of success, are the unpredictability of the movements of the materials during the process, particularly with respect to size of any landslide event triggered and its likely runout distance, and the means by which the very large amounts of blasted material may be removed from site and disposed. In addition, blasting on such a scale is likely to cause a significant dust cloud.
Figure 5 shows the dust cloud from the Skjerping blasting operation in Norway in 2010 – in this case the dust cloud travelled approximately 7km down the valley and fjord of Nærøydalen. This raises the prospect of a dust cloud that could travel from the Rest and be Thankful as far as Lochgoilhead, Arrochar, the far shores of Loch Fynne or Loch Long, or even Loch Lomond depending on the prevailing wind direction at the time of blasting.
Clearly it is essential that each case of instability should be evaluated on its own merits, but an approach founded in proven technology seems most appropriate.
3 Livestock The introduction of sheep to graze the hillside has been suggested as having the
potential to lessen instability of the hillside from three perspectives:
To keep the height of the grass down to help reduce the build-up of snow in the winter (Section 3.1).
To prevent un-grazed grass dying off and weighing down the slope (Section 3.2).
Sheep forming compacted paths as they traverse the hillside thus creating drainage run off (Section 3.3).
A further issue relating to the potential damage that animals can do to the slope surface is also addressed in this section (Section 3.4).
3.1 Reducing the Height of Grass Certainly snow melt is a recognised cause of landslide activity. This concept that short grass will either hold or maintain snow pack is an interesting one. Certainly ground roughness (in the form of boulders, logs and low level, woody shrubs) is recognised as having an anchoring effect on snow pack, reducing avalanche risk (e.g. http://www.avalanche.ca/cac/library/glossary/a-z?index=E) at least for relatively shallow depths of snow. However, there appears to be little evidence that short grass will have a similar effect.
Notwithstanding that, it does seem likely that the action of water and gravity on the slope will tend to lay long grass downslope. During the winter months this grass will, most likely, be wet and could well help in moving snow downslope;
this is likely to be a function of snow depth and slope gradient and whether this will occur for the relatively shallow depths of snow normally encountered in this area is not entirely certain.
It is also pertinent to note that the actual amounts of snow at the Rest and be Thankful generally tend to be relatively small, amounting to a few centimetres to a few tens of centimetres at any one time. It is by no means clear that any of the events in recent years at the Rest and be Thankful have been caused by a build-up of snow and its subsequent melting.
3.2 Un-grazed Grass