Piletec Geotechnical was entrusted by Amco Giffen to deliver a critical part of the infrastructure stabilisation for the Tottenham North Curve Tunnel project. This involved the comprehensive supply, installation, and testing of production ground anchors.
Background and initial challenges:
The project at Tottenham North Curve Tunnel was initiated in response to infrastructure concerns affecting the Kentish Town City Farm. A moving wall near the tunnel raised critical safety concerns and impacted local amenities, including the farm’s horse arena. The closure of the arena disrupted community activities, especially the pony club for children and young people with disabilities.
Piletec’s proactive approach through Early Contractor Involvement (ECI) was instrumental in addressing the unique challenges of the Tottenham North Curve project. Their early engagement allowed for the development of tailored solutions to overcome the site’s tight access constraints and ensure efficient construction.
We brought unmatched expertise and precision to the diamond drilling operations at (TNC), showcasing our proficiency in tackling complex geotechnical challenges. With years of experience and advanced techniques, Piletec delivered outstanding results, ensuring the success of this critical stage.
We undertook core drilling to depths of up to 6 metres through brick and concrete masonry, requiring a flexible, well-planned approach. Alongside the expected masonry, we encountered a steel Armco barrier fixed to the structure and encased in concrete, demanding equipment adjustments to maintain precision. Fractured brickwork caused intermittent loss of flush, and a redundant cable trough further complicated the process all navigated while sustaining momentum and protecting the surrounding structure.
Drilling
The project’s success hinged on the early installation of ground anchors, strategically executed ahead of wall construction. Engineered to provide long-term stability, these anchors were designed to restrain the future concrete retaining wall by transferring tensile loads deep into the ground. This preemptive anchoring approach ensured the new structure would be effectively supported from the outset, countering potential movement and safeguarding its integrity over time.
We utilised the Klemm Bohrtechnik KR 802-3 drilling rig equipped with the KH 16 drill head, selected for its compact footprint and outstanding power-to-size ratio. This setup enabled us to auger to impressive depths of up to 55 metres, meeting the demands of deep anchoring in challenging ground conditions.Despite operating in a restricted environment, the KR 802-3 handled the works with precision and reliability. Its high-torque drill head and advanced hydraulic system provided the consistent performance needed to maintain verticality and stability throughout the drilling process.To accommodate project specific requirements, safety gates were removed, and a comprehensive Safe System of work was implemented. Strict safety protocols were followed at all stages to ensure the wellbeing of personnel while maintaining productivity and control on-site.
we encountered unforeseen obstacles, including: – A steel Armco barrier, which required adjustments to our equipment and techniques to maintain accuracy without compromising progress. Interception of a previously installed test anchor, necessitating careful navigation to prevent delays or structural damage.
As part of a complex anchor installation project in restricted access zones, four anchors were installed using the Klemm KR 702 rig, working from a heavy-duty scaffold platform. These anchors were located in particularly challenging positions where conventional equipment could not be safely or effectively deployed
Surveying
The IMDEXHUB-IQ™platform played a crucial role in streamlining data management for the boreholes in this project. By providing real-time access to validated field data, it ensured greater accuracy and operational efficiency. Its secure cloud-based storage allowed seamless monitoring and analysis, enabling informed decision-making and reducing downtime. Integration with subsurface instrumentation offered comprehensive insights, further enhancing the precision and performance of the drilling operations. This innovative tool contributed significantly to overcoming challenges and delivering successful outcomes on this project.
Permanent Multi-Stage Anchor
This system is a form of post-tensioned pre-stressing commonly used in bridge construction and other large-span concrete structures. In this configuration, up to seven 15.2 mm diameter, 7-wire pre-stressing steel strands are housed within a single duct, each anchored independently at the anchorage zone. What distinguishes this system from traditional bundled tendons is the unequal lengths of each strand.
Key Advantages and Engineering Rationale:
- Reduced Wedge Penetration Losses: Traditional systems experience uneven anchorage behaviour due to wedge slip. This system minimises these losses by engaging strands sequentially and more uniformly.
- Optimized Stress Distribution: The variation in strand length allows force to be transferred more gradually along the tendon’s length. This reduces peak stresses near the anchorage and helps distribute loads more evenly within the concrete member.
- Improved Redundancy and Monitoring: Since strands are stressed and locked off individually, any issues (e.g., strand slippage, stress losses) can be detected and corrected per strand. It also improves post-installation monitoring.
- Enhanced Durability: With separate anchoring and the ability to use protective sheathing or greasing per strand, the system offers better corrosion protection and maintains higher service life in aggressive environments.
Grouting
Grouting Operations and Specification Compliance
Grouting for all anchors was executed in full compliance with BS 8081 clause 12.1, ensuring consistent quality and performance. The grout mix consisted of a Ordinary Portland Cement (CEM I), grade 42.5N, and water, maintaining a maximum water-cement ratio of 0.4 to deliver optimal strength and bond characteristics.To mix and pump the grout, we utilised the CX 410 a high-performance, twin-compartment mixing and pumping unit known for its reliability in specialist grouting applications. This unit enabled precise batching, continuous flow, and efficient delivery under pressure, ensuring consistent material quality throughout the injection process.For encapsulation, grouting conformed to clause 12.1.5.3 of BS 8081, meeting requirements for long-term protection of the tendon system. Grout placement was carefully controlled to avoid voids and to fully encapsulate anchor components, ensuring both structural reliability and corrosion resistance.
Anchor Testing
Anchor Stressing and Verification
Testing DYWIDAG carried out the stressing and testing of all ground anchors in accordance with the project specification and relevant standards, namely BS 8081 clause 14.6, as well as clauses 8.4 and 9 of BS EN 1537. The first three anchors installed underwent site suitability stress testing in accordance with BS EN ISO 22477-5:2018 clause 8.3, using Test Method 1 and the Table 2 loading sequence applicable for permanent anchors in fine soil. These initial tests confirmed compatibility of the design assumptions with actual ground behaviour.
All subsequent anchors were subjected to acceptance stress tests, following BS EN ISO 22477-5:2018 clause 8.4, again using Test Method 1, this time applying the Table 4 loading sequence for anchors in fine soil. The proof load (Pp) and lock-off load (Po) for each anchor were taken from the approved anchor schedule. Every stress test concluded with a lift-off check, during which the gripping wedges were raised 1 mm from their seats to confirm residual load transfer through the system. Anchors were then securely locked off at their designated lock-off loads. Additionally, three anchors were selected for supplementary lift-off checks once all anchors were installed, tested, and locked off. These checks were proposed by the client and accepted in line with specification guidance, further verifying the long-term anchorage performance across the array.
What our client had to say
What impressed us most wasn’t just the quality of the anchor installations, but the way the team carried themselves throughout the project. They were proactive, communicative, and always one step ahead. The site was kept safe, clean, and organized, and the work was completed without a single issue.