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Ground Investigation: Steeper, deeper, narrower

A82 GI

The £3.4M ground investigation for the A82 upgrade in Scotland has had to contend with steeper slopes, deeper bogs and narrower access than many other projects of this scale.

Transport Scotland is aiming to upgrade the A82 betweenTarbet and Inverarnan with wider carriageways, space for verges and a dedicated pedestrian and cycle path.

The A82 is a key route from Glasgow to Fort William. The aim sounds simple but the steep topography and the West Coast Mainline to the west and the tranquil water of Loch Lomond to the east mean that the upgrade is far from as traightforward widening project.

The engineering solution for this 17km section of the A82 has yet to be confirmed and hinges on the findings of a complex ground investigation that is now close to completion. Delivery of the work on the ground by Soil Engineering Geoservices has called for input from specialist subcontractors aswell as close collaboration with Transport Scotland and its consulting joint venture of CH2M and Fairhurst.

A82 GI

A82 GI

Work on land has called for a slope climbing rig

“The upgrade is vital project as it is restricted in terms of width and is substandard in both its horizontal and vertical alignments,” says Transport Scotland project manager Dominick Cafferkey. “The aim is to widen the route to remove these issues in orde rto reduce journey times and improve safety.“

At the moment it can be difficult for lorries and buses to pass as the existing carriageway is 6m wide – or less in places and there are no verges.

“The upgrade will widen the route to 7.3m and create verges, a shared cycle and pedestrian route next to the loch and add new stopping places.

”The wider route will also make it easier to undertake future maintenance.”

In terms of the solution, CH2M/Fairhurst engineering representative Chris Ravey says: “We are looking at cutting into the slope with both rock and soil cuttings.” The solution is also expected to involve construction of a number of embankments, bridges and culverts so gaining detailed ground investigation data is vital to progressing the project.

Soil Engineering started work on the £3.4M ground investigation in March, and is expected to complete the site work as this issue of GE went to press.

Soil Engineering says is it one of the largest ground investigations it has undertaken in Scotland. It is also one of the most varied with a large range of techniques used.

A82 GI

A82 GI

Soil Engineering undertook 60 over water holes

At the peak of work, Soil Engineering had 35 people on site with two overwater pontoons undertaking rotary and shell and auger drilling and a land-based team undertaking rotary and shell and auger drilling. Subcontractors Geosonic undertook sonic drilling and Geotechnical Engineering delivered drilling using slope climbing rigs. The contract also undertook trial pitting, carriageway coring and peat probing. Geophysics was also a key part of the work with overland work undertaken by Apex Geoservices and overwater down hole geophysics delivered by ClydesideSurveys.

“The ground investigation combined land and water work with 240 boreholes - 60 overwater boreholes and the rest on land,” says Ravey.

However, these numbers belie the logistical, environmental and traffic management challenges of delivering them.

“Some of the on land boreholes were located within the road, but the narrow carriageway made this complicated,” says CH2M/Fairhurst project geotechnical lead Corran McArthur.

“Previous ground investigation focused on the road and this was the first to look at the wider area and fill in the gaps up slope and over the water. There were strict limitations on lane closure and traffic management needs so Soil Engineering had to stay in continuous dialogue with Transport Scotland and area term maintenance contractor Bear Scotland.

“The area we were trying to work in on land is constrained by the loch on one side and an old stone retaining wall and steep slopes on the other,” says Soil Engineering UK operations manager Tom Walton.

A82 GI

A82 GI

Sonic drilling was used to ensure core recovery

“Getting to some of the positions has involved lifting rigs over the wall during overnight operations to minimise the disruption.”

Once over the wall, peat bogs also presented access challenges and widespread use of bog mats was needed to get to the hole locations without damaging the bogs. Ground conditions were also very variable – hence the need for a comprehensive ground investigation – but the variation encountered was more than that expected.

“The geology is very mixed with dense coarse fluvial material, glacialtills – although there was less of these than anticipated – and also historic landslide deposits,” says McArthur. “It was the experience of encountering the fluvial material on a previous phase of ground investigation that led to the use of sonic drilling on this project.“

The rock is mostly metamorphicschists but with quartz veining and igneous intrusions of dolerite and felsite.

“There have been no real surprises in terms of the ground conditions themselves but there is a definite contrast in ground conditions over a short distance.”

Walton adds: “Previous ground investigations at the site had suggested that the granular overburden above the rockhead was 10m to 15m thick. What we actually found was that the overburden varied from nothing t o40m.”

Walton says that the site team liaised closely with Transport Scotland and CH2M/Fairhurst to ensure the results achieved matched what was needed for design.

“We were aiming to get 12m of rockcore from the pontoon work but with 40m of overburden, it was challenging as it was difficult to bring in additional equipment due to the location,” says Walton.

Some of the early work resulted in an increase in scope with additional boreholes added, locations moved and drilling depths increased.

Ravey adds: “The depth to rockhead at a local level needed to be more clearly understood to define the ground model as the depth of overburden will have a major impacton the slope design.

“The ground investigation also focused on gathering the data needed for designing the rock cuttings and also used down hole televiewers to look at discontinuities and supplement the surface mapping of fissures in rock outcrops and core logging.”

The good weather over the summer has benefited the scheme and Walton says that there were very few days where the swell on the loch prevented the pontoons from operating which helped keep the work on programme. While delivering an efficient programme on a project with such a wide range of techniques and different equipment has been a challenge, the work has been hugely beneficial to the experience of Soil Engineering’s staff.

“We have used the scheme as a learning opportunity for our younger engineers to learn about all the different techniques,” says Walton. “Normally it might take engineers twot o three years to see all of these different techniques in action but here they have been able to see it all in one place, plus they got involved in thec omplex practical aspects of the work too.”

Work on the final, more challenging boreholes is now being completed and the final report is expected to be delivered later this autumn. No dates have yet been linked to the upgrade but it is clear that delivering the ground investigation will have given some insight into more than just the geotechnical challenges ahead.

“We’ve all worked on jobs of this scale before but what makes this one unique is the number of constraints,”concludes Cafferkey.

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