The Project
The Castlerigg tunnel forms an important part of the 36km long Thirlmere link Raw Water Aqueduct. The project involves the construction of a 1300lin/m tunnel drive through mixed geology at depths of up to 60m. Passing under the 5000-year-old Castlerigg stone circle the tunnel will remove the need to construct around 7.5km of conventional open cut main and reduce overall project programme by around 6 months.
Location
The project is located just outside Keswick, within the boundaries of the Lake District National Park, in itself presenting unique challenges with regard environmental protection and access.
Challenges
From early in the project it was identified that Castlerigg and its surrounds formed a natural topographic and cultural obstacle to a gravity pipeline and a tunnelled solution formed part of the initial optioneering, though further review at desk study stage identified that the most economic option would be to divert the main around Castlerigg.
The aftermath of storm Desmond in December 2015 however meant that the planned route would now require major stabilisation works within both the River Greta SSSI and the steep slopes above. The costs associated with these additional works combined with the continued long-term risk to a strategic asset meant the preferred alternative was no longer considered viable and a tunnelled solution was developed.
Set against the back drop of the aftermath of storm Desmond and the closure of the A595 the project team were required to design, scope, procure and deliver an extensive ground investigation whilst adhering to significant programme and stakeholder constraints.
Solution
Overall project control was maintained by United Utilities Engineering who worked closely with an extensive list of stakeholders including: Environment Agency, Natural England, Highways England, and Cumbria County Council. Working with the tunnelling contractor the UU engineering team were responsible for the geotechnical design, scoping, procurement and implementation of the works.
Geotechnics Ltd, acting as Principal Contractor, were responsible for the delivery of the intrusive and nonintrusive phases of the site investigation. Managing all the site-based activities including the laying of 3.5Km of aluminium trackway, construction of a new highway access, access bridge and embankment.
Past experience of working in this area meant Geotechnics Ltd were acutely cognisant of the stakeholder challenges surrounding the overall scheme and we’re able utilise personnel and specialist sub- contractors, such as Drilcorp and Terradat, who had built up good working relationships with land owners over previous elements of the project.
The Initial non-intrusive Seismic refraction survey was completed by Terradat to produce an initial bedrock profile. Using their proprietary tractor towed seismic generator they were able to rapidly asses overburden thickness, rock strength and consistency along 3 possible tunnel alignments. This early stage works proved invaluable in the design and implementation of the intrusive phases whilst allowing on-going solution refinement.
The original proposals called for an investigation to cover 3 route options estimated at £1.3M. Early geophysical works (Phase 3) allowed the development of a preliminary ground model enabling the discounting of one option. The remaining 2 options were then further investigated with the completion of an initial 125m pilot hole (Phase 3.1). Completion of a pilot drill prior to the main works allowed both further confirmation of the geophysical ground model and provided feedback on the likely drilling conditions to enable mobilization of appropriate plant and equipment.
Once mobilised, Geotechnics acted as Principal Contractor and undertook:
• Rotary drilling - 2634lin/m
• Seismic profiling - 1700lin/m
• Downhole acoustic imaging
• High pressure lugeon testing
• Specialist rock testing
Benefits
Early use of geophysics and the pilot bore allowed plant and equipment to be pre-ordered and cached at work sites. Prepositioning of plant and equipment allowed drilling crews to achieve utilisation in excess of 95% during the main works, with only 100hrs non-
operational across a total of 1615rig/hrs. This optimization of the main (Phase 3.2) works meant significant reductions in preliminary costs such as temporary trackway, site accommodation, security, etc resulting in a reduced overall cost.
As further data became available it was apparent that high, and in places artesian groundwater levels allowed the elimination of a 2nd option meaning the remaining Ground Investigation scope could be further amended to allow additional costs savings resulting in a final outturn cost of £881k.
The phased approach and continual review meant it was possible to complete the ground investigation as part of the optioneering process allow optimization of both solution and ground investigation.
Outcome
The Castlerigg scheme represented a unique set of challenges. From a technical perspective the depth of tunnel drive combined with a high groundwater table meant conventional testing and monitoring equipment and processes were unsuitable.
As highlighted, a section of the investigation required works within the A595 a main trunk road closed due to storm Desmond. These works had the potential to cause additional delays and disruption to both the repair efforts and the reopening of the road.
Ensuring the work was achieved within the closure required close cooperation with Highways England and Cumbria County council along with the various contractors involved. UU engineers attended Bi-weekly progress meetings ensuring that operations on both projects did not adversely impact the other.
The longstanding working relationship and trust built up between United Utilities and Geotechnics meant early contractor involvement from the outset, and an excellent ongoing working relationship throughout the project.
The use of a phased approach combined with hold points within the program allowed the solution design to be developed in tandem with the investigation, recovered data fed into solution refinement which in turn allowed a more focused and targeted approach to the next GI phase. This incremental approach not only led significant cost savings, the “heads up” knowledge it generated contributed to significant increases in productivity allowing the delivery of a unique project to a challenging programme whilst reducing cost.
Inclusion of the tunnelling and drilling contractor in the design and scoping process allowed an open exchange of requirements and ideas leading to full confidence that the main targets for the site investigation were suitable and achievable.
Failure of the team to deliver adequate and quality information in the timeframe required would have resulted in considerable risk to the main construction program with the potential to miss key targets set by the regulator with associated financial penalties. However, through carefully planning, a phased approach and an integrated delivery team it was possible to deliver the works to a program originally considered unfeasible.
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