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Ground investigation: Shore thing

Loch Lomond may be a beautiful spot for a holiday but carrying out a ground investigation on a major road along the shores of the loch presented some engineering challenges. 

With a route that passes close to some of Scotland’s best known landmarks, the A82 is a key route for tourists as well as being an important trunk road between Glasgow and Inverness. The numbers of tourists flocking to see Loch Lomond, Loch Ness, Glencoe, and Ben Nevis and business growth means that traffic volumes on the route are growing.

Transport Scotland has earmarked the A82 route – first built in the 1930s – for improvement to increase capacity and equip it to handle today’s heavier traffic. That plan has just moved a step closer to reality with successful completion of a major ground investigation on a 17km section of the Loch Lomond section by ESG.

The work undertaken by ESG focused on the section of the A82 between the villages of Tarbet and Inverarnan. The geotechnical investigation aimed to define the depth to groundwater and the ground conditions including the depth of the local bedrock to aid design of the road upgrade. Running next to the shoreline of Loch Lomond for much of its length, the site presented ESG with a host of challenges that had to be tackled sensitively to deliver the required results.

The location in the heart of the Trossachs National Park is an area of outstanding natural beauty and is a site of special scientific interest due to the presence of sessile oak wood, an otter population, and other rare flora and fauna. Part of the route also ran past Pulpit Rock, a local landmark of historical and archaeological value.

All of this meant that any work carried out by ESG had to be done with great sensitivity to the area, with remediation afterwards to return the site to its original state.

As a major link between the Highlands of Scotland and the cities of the Central Belt, the A82 is used by a high volume of private cars and tourist coaches, as well as lorries and trucks. This made it important to carefully manage work and transport of equipment to and from the site to minimise the impact on traffic.

Undertaking the work during the autumn and winter months, in a region with an unusually wet microclimate, meant that ESG also had to contend with frequent torrential rain. A number of test sites were affected by flooding by the loch’s rising water levels and there were also a number of points along the banks of the loch where subsidence was an issue.

Overcoming obstacles

In order to successfully tackle these challenges and minimise disruption to the ground investigation project, a number of protocols were adopted.

As part of the work an otter mitigation licence had to be obtained. ESG worked closely with ecologists to carry out impact assessments and plan site activity to minimise the effect on the local otter population. ESG liaised with an ecologist throughout the project to ensure it did everything possible to safeguard wildlife in the area.

Archaeological experts were also recruited to oversee trial pits at specific points across the section of road under investigation, including those in the vicinity of Pulpit Rock, in order to ensure any items of historical value discovered were catalogued and preserved.

To mitigate the impact of work on motorists, traffic management procedures were put in place across the entire 17km length of the road. These were carefully planned to take into account the number of large vehicles using the route, as well as daily variations in traffic flow. By doing this, potential bottlenecks were eliminated and it also eased site access to vehicles for the investigation work.

As part of the ground investigation, 1,165 peat probes were carried out to determine the depth of the peat beds on the site. These were used to guide where boreholes would be sunk and trial pits dug to investigate the strength of the underlying peat and rock layers.

The main investigation involved 32 cable percussion boreholes sunk into the peat and a further 66 rotary boreholes undertaken in areas where the bedrock was found to be very close to the surface, with cores recovered for later analysis. Standard penetration tests were also carried out at intervals during borehole drilling to determine rock strength.

Drainage rates

Surface and groundwater levels across the site were monitored throughout the investigation to gauge drainage rates and establish the average depth to groundwater along the route. This monitoring is still ongoing on a monthly basis and this phase of work is not due for completion until October this year.

The results of the preliminary A82 ground investigation have been described as fascinating so far but the testing phase is still underway.

The trial pits found that in a number of locations on the site the peat was more than 4m deep with no sign of underlying bedrock. Initial geotechnical assessment of the site had suggested that the rockhead would have been encountered before reaching 4m below ground level. Nonetheless, pits in other areas revealed only a shallow layer of surface deposits above rock, suggesting a highly undulating bedrock surface.

Cores recovered from the boreholes gave a wealth of information about the structure of the local bedrock. Results show the deposits are indicative of the Southern Highland Group – a complex collection of strata dating to the Neoproterozoic era, between 600 and 900M years old. The British Geological Survey describes this group as low grade metamorphosed psammite and pelite greywackes.

ESG expects to present its findings and recommendations regarding the road and its final design to Transport Scotland.

 

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