Understanding history to engineer the future
The A82 in Scotland is one of the deadliest roads in the UK. Over 24 months, road accidents on the 65-mile stretch between Tarbet and Fort William claimed 22 lives and injured more than 450 people. At Pulpit Rock, the danger increases because of a narrow, twisting road around Loch Lomond less than 5 m wide that allows only one car to pass. In 2008, Transport Scotland, under public pressure, proposed two remedies: expand the breadth of the road to run over Loch Lomond, supported by a retaining wall or engineer a silted carriageway. Developer Scott Wilson contracted RSK company Structural Soils to help select the most viable option. Northern Scotland’s geological features are complex and deeply revealing about our planet’s history. Scotland is dotted with fracture zones, which make ground investigations, particularly drilling, arduous.
Initiative defies challenges
Structural Soils assembled an expert and responsive team and set up base on the A82 near Pulpit Rock. Although the scope of work required only 17 boreholes and 7 trial pits in Phase 1 and 16 boreholes and 6 trial pits in phase 2, the team anticipated challenges.
To access the ground conditions below the loch, the team used a jack-up barge to drill eight rotary boreholes about 10 m from the shoreline. Later, the crew drilled nine boreholes, two inclined at 30-degree angles, into the schist bedrock. During phase 2, a specialist rope-access subcontractor drilled five boreholes into the slopes. Simultaneously, the team began a two-stage geophysics programme to create a comprehensive model of the principal subsurface geological features. The data identified 12 m of sand and gravel above the schist bedrock. Moreover, their surveys helped to identify fracture zones and other weaknesses in the bedrock that saved time and resources later in the project.
Drilling into history
The geological nature of northern Scotland was always going to provide the team with challenges but they saw this as a chance to showcase their ingenuity and expertise. Drilling into igneous and metamorphic rocks with traditional equipment will quickly become redundant: drill bits will weaken and break, and the depths needed to extract high-quality samples cannot be reached. The team quickly encountered large boulders within the glacial deposits, initially thought to be outcrops, so they used a combination of dynamic sampling, coring and open holing using a down-hole hammer to reach the depths required. This enabled the team to test material types in situ while maintaining the borehole annulus to install casing. Overcoming the underlying bedrock, however, would require the team to form innovative solutions.
Ingenuity and expertise
The bedrock, comprising schist with abundant quartz veining and foliations, posed two challenges: seating the drill casing into the bedrock to allow rotary coring, especially into inclined holes, and drill bits quickly wearing and failing while drilling the bedrock. The team needed to carefully select casing shoes and coring bits, so its engineers collaborated with bit and rig manufacturers and in-house specialists to select several casing shoes and coring bits to suit the diverse needs of different drilling rigs and to consider bit weight, rotation speed and on-site torques. By tailoring the bits to the rock, the team maintained even borehole drilling and core recovery, essential to the project, which was verified by down-hole, geophysical logging.
Structural Soils completed this complex project in 2010. Moreover, the resultant data helped Transport Scotland to choose the most viable option: building a viaduct running parallel with Loch Lomond and widening the carriageway to two lanes. The enormous engineering feat was completed in 2015.