Ground Freezing and TBM Interface Management at the HS2 Project: Practical Experience from Tunnelling in London

SThe construction of the cross passages in the Northolt Tunnels West (NTW) section of the HS2 project in London/UK required artificial ground freezing under challenging geological, logistical and construction-related conditions. The freezing works for eleven cross passages were carried out by the RDMI Freezing JV (Redpath Deilmann GmbH, Dortmund/Germany and DMI Injektionstechnik GmbH, Berlin/Germany) and took place in parallel with the tunnel boring machines (TBMs) drives led by the Skanska-Costain-Strabag (SCS) JV.

This article outlines the interface challenges between active tunnelling operations and ground freezing works, including planning, drilling, freezing operations, monitoring and final sealing. It also highlights the challenges resulting from discrepancies between actual ground conditions and the original geotechnical reports, which led to scope adjustments for the last cross passages.

For the executed ground freezing works, key success factors included the precise integration of the freezing design based on up-to-date as-built data, a multi-layered communication and coordination system, dynamic personnel planning and training and flexible management of operational processes.

The experience gained from the NTW works provides valuable insights for future projects where ground improvement measures are required in parallel with active excavation of the main tunnel drives. Finally, recommendations are given for optimising planning processes, site organisation and personnel management in complex underground construction environments.

1  Introduction

The HS2 project is one of Europe’s most significant infrastructure undertakings and aims to create a high-speed rail connection between London and Birmingham. In the Northolt Tunnels West (NTW) section in West London, two tunnel boring machines (TBMs) named Caroline and Sushila (Figure 1) excavated two parallel tunnel tubes over a length of approximately 8 km through highly variable, water-bearing ground conditions consisting of sand, chalk and London Clay. TBM Sushila reached the ventilation shaft at Green Park Way in Ealing in December 2024, while TBM Caroline completed its drive in April 2025.

To safely construct the cross passages, i. e. the connecting adits between the main tunnel tubes, planned in critical geological zones, artificial ground freezing (AGF) was selected as the preferred method. This applied especially in areas with high groundwater pressure where conventional excavation methods were not safely feasible. Initially, 13 of the 19 cross passages in the NTW were planned to be secured using AGF. However, updated findings during construction revealed that the actual ground conditions in some cross passage areas did not fully align with the assumptions made in the original geological and hydrogeological reports. As a result, two of the originally planned 13 frozen cross passages were removed from the AGF scope. These passages were instead secured by alternative methods, including dewatering by third-party contractors or excavation without additional ground improvement measures.

The ground freezing works were carried out on behalf of the client, HS2 Ltd. The main contractor for the tunnelling works was the Skanska-Costain-Strabag joint venture (SCS JV). RDMI Freezing JV (comprising Redpath Deilmann GmbH, Dortmund/Germany, and DMI Injektionstechnik GmbH, Berlin/Germany) was subcontracted by SCS JV to execute the artificial ground freezing works, including the associated drilling, freezing operations, monitoring and final sealing of the freeze pipes. For the design of the freezing works, RDMI JV engaged the engineering firm CDM Smith as a sub-consultant. CDM Smith’s scope included the development of the drilling layouts, thermal calculations for the freeze bodies and the monitoring concepts for the freezing phases.

This article presents the specific interface challenges between artificial ground freezing operations and active TBM drives, outlines the solutions implemented on-site, and provides practical insights for future large-scale tunnelling projects involving concurrent ground improvement works.

2  Ground freezing and TBM tunnelling: Principles, planning and implementation

Artificial ground freezing is a well-established method for temporary ground stabilisation and groundwater sealing, which has been successfully applied for decades by both Redpath Deilmann and DMI Injektionstechnik, often in joint projects. The method involves freezing the pore water in the surrounding soil to transform unstable, water-bearing ground into strong, impermeable frozen bodies. At HS2, ground freezing was selected as the preferred method for constructing the cross passages, as it enables the creation of stable and predictable ground conditions even in highly heterogeneous soils.

2.1  Planning

Planning the ground freezing works in the NTW section in London presented several specific challenges. Only half of the tunnel cross-section was available for the works, which imposed geometric constraints on the arrangement of the freeze pipes and their accessibility with drilling equipment. In the cross passage areas, special and heavy segment rings were installed to ensure tunnel stability during cross passage excavation. These high-strength segments allowed only designated drilling corridors and defined starting points for the freeze pipes. As a result, planning became significantly more complex, as the constrained conditions strongly influenced the choice of drilling angles and entry points. While other projects typically allow drill entry points to be selected based on the ideal freeze body geometry, the freeze design at NTW had to be adapted to the structural and geotechnical constraints of the project.

2.2  Site preparation

Providing the necessary working space for the drilling equipment and setting up freezing units within the actively driven main tunnel tubes required extremely careful engineering planning. At each cross passage location designated for freezing, platforms (Figure 2) were constructed above the tunnel invert to increase the available clearance and allow logistics operations from other tunnel works to pass the freezing area. Compact equipment was deliberately selected to minimise the footprint and ensure that the required clearance for TBM-related logistics (material and personnel transport) was maintained at all times.

2.3  Drilling under constrained conditions

As outlined in Section 2.1, only a limited working space was available for execution. With an internal tunnel diameter of 9.60 m, operating an 18 t drilling rig was only possible under tight conditions. A specially adapted drill carriage system was therefore used to reach areas in both the crown and the invert. Additionally, drill string lengths of 100 cm, 50 cm and in some cases as short as 30 cm were used, significantly increasing the challenge in terms of drilling precision and sealing due to the number of pipe connections. By comparison, freeze pipe lengths of 2 to 3 m are typical for similar projects. The combination of confined space and the required drilling accuracy (maximum deviation of 3 %) posed a significant challenge for the drilling crews. Figure 2 shows the cross passage platform during drilling operations at the upper levels.

At each cross passage, between 28 and 38 freeze pipes, four temperature monitoring pipes and two pressure relief boreholes were drilled in a predefined and calculated pattern. For this purpose, core drillings with a diameter of 200 mm were first made up to 50 mm from the outer edge of the segmental lining. To protect against water ingress, standpipes with blow-out preventers (BOP) were then installed in the segment bores. This was followed by core drilling of the remaining 50 mm segment wall using a 130 mm crown under BOP protection. After switching to the freezing installations, the freeze pipes were installed at the specified angle and depth and the annulus was sealed with cement grout. Finally, the pipes were surveyed with a dual gyroscope and checked for tightness.

2.4  Installation and operation of freezing units

After the completion of drilling, the freezing units and brine circuit were set up. PE inliners and freeze heads were installed and connected to the brine circulation system. The equipment, comprising containerised freezing units and cooling systems, was installed on racks close to the work areas to save space and maintain the required clearance for tunnel traffic.

Brine solution cooled to an average of −33 °C was circulated through the system. Temperature sensors embedded in the monitoring pipes and the segmental lining allowed continuous tracking of freeze body formation. The goal was to achieve the required conditions for hydraulic sealing toward the opposite tunnel segment, as well as the structural strength necessary to allow safe cross passage excavation. Where needed, thermal insulation was installed around the cross passage areas to shield the freeze body from thermal influences caused by the tunnel climate and operational traffic. The freezing progress was continuously compared against thermal calculation models. Excavation of the cross passages (Figure 3) only began once the defined thermal and mechanical criteria had been met.

Throughout the entire period, TBM tunnelling continued under significant schedule pressure. This required maintaining face stability, segment installation, spoil removal and compliance with ventilation standards. In addition, tunnel logistics were further intensified, e. g., by the regular transport of fresh concrete using mixer trucks for the construction of the tunnel invert.

Carrying out ground freezing works in the environment of active TBM operations under these specific conditions significantly increased the complexity of execution and placed high demands on technical precision, construction quality, coordination of activities and adherence to schedule.

3  Key interface challenges and mitigation measures

The parallel execution of ground freezing works and TBM tunnelling posed several specific challenges.

3.1  Spatial constraints and interface coordination

The live tunnel environment offered only limited space for setting up the drilling and freezing equipment (Figure 4). To maintain logistic routes for TBM operations, platforms were erected on the tunnel invert and compact equipment solutions were implemented. A multi-level coordination system was introduced to manage the complex interfaces:

• Daily start-of-shift (SOS) meetings enabled detailed planning of work areas and safety measures.
• Weekly interface meetings supported proactive coordination of schedules, logistics and technical adjustments between all involved parties.
• Direct and open digital communication channels allowed for immediate exchange of information with the main contractor, enabling fast decision-making and on-site execution. These channels proved especially useful and effective in managing disruptions and design changes.
• Project-specific online tools such as Datascope, EnableMyTeam, and CEMAR were also used to structure and document progress tracking and interface management transparently and contract-compliantly.

This structured yet flexible coordination strategy ensured that drilling and freezing activities could be carried out without disrupting TBM logistics.

3.2  Personnel planning and flexibility

Drilling teams could only commence their work once the TBM had completely passed the respective cross passage position, the invert concrete had been poured in the working area, and the access platform had been installed to increase tunnel clearance. Fluctuations in TBM advance rates required dynamic reallocation of personnel to minimise idle times.

3.3  Thermal management

Ongoing logistical operations in the tunnel contributed significantly to the thermal load in the surrounding environment. Various interfaces and process dependencies influenced the start times of cross passage excavation. This led to prolonged freezing periods, leading to the freeze bodies becoming excessively solidified in some areas. Consequently, the operation of the freezing systems had to be continuously adjusted to maintain ground strength at a level that still allowed for safe and efficient excavation.

3.4  Scope changes due to ground conditions

Three cross passages originally designated for ground freezing were removed from the scope based on updated geological findings from investigations carried out directly from the tunnel. These cross passages were instead constructed using dewatering or direct excavation without ground improvement. Additionally, one cross passage initially not planned for ground freezing was included in the freezing scope based on the same investigative methods. Through flexible resource deployment and close contractual coordination, the impacts of these late scope changes were effectively minimised.

4  Lessons learned from Caroline and Sushila tunnels

The ground freezing works for the cross passages on the HS2 project provided several important insights into managing complex, parallel ground improvement operations in combination with active, concurrent tunnel excavation.

4.1  Importance of early and accurate survey data

Accurate as-built survey data provided by the client’s design team was essential for the planning team of CDM Smith’s precise development of the drilling layouts. Based on this data, CDM Smith developed detailed drilling layouts and thermal calculations for the ground freezing works. Data acquisition or handover inaccuracies would have significantly increased the risk of drilling and freezing issues. Close coordination and maximum precision, both in data collection and design, were critical for successfully executing of drilling and freezing operations.

4.2  Value of structured and flexible coordination and communication

The combination of daily detailed coordination, weekly strategic interface meetings and fast digital communication proved crucial for efficiently coordinating workflows, proactively avoiding conflicts and maintaining safe working conditions within the confined tunnel environment.

4.3  Workforce management, training and technical expertise

Experienced specialists and local (UK-based) personnel, many of whom had little or no prior experience in ground freezing, were employed for the drilling and freezing works. Identifying suitable workers, providing targeted hands-on training by experienced team members and retaining the most capable individuals throughout the project were key success factors for ensuring both the quality and continuity of the works. During unavoidable downtimes, it was essential to flexibly reassign personnel to other tasks to maintain motivation and preserve the acquired technical skills within the project team.

4.4  Dynamic personnel planning

As mentioned in 3.2, drilling and freezing operations could only commence after the TBMs had passed the cross passage position, the invert had been completed and the working platform for maintaining tunnel clearance had been erected. Fluctuations in TBM advance rates and changing operational priorities, such as short-notice reassignment of personnel to support TBM operations, demanded highly flexible resource planning. Maintaining personnel readiness, responding quickly to shifting conditions and avoiding unnecessary idle costs required continuously agile and forward-looking staff management.

4.5  Managing scope changes

The late descoping of works, i. e., the partial cancellation of AGF at selected cross passages, highlighted the importance of both contract and operational flexibility. The ability to absorb such changes without major impacts on budget or schedule is critical in large infrastructure projects where ground conditions remain uncertain.

5  Conclusion

The ground freezing works carried out for the construction of cross passages as part of the HS2 project demonstrated that concurrent execution alongside mechanised tunnel boring is fundamentally technically feasible. However, this parallel execution poses significant construction logistics challenges. In particular, the confined space within the tunnel, the limited availability of transport vehicles and construction machinery and the need for precisely coordinated material logistics, e. g. brine, freeze pipes and lining materials, require a highly synchronised planning approach. Delays in vehicle allocation, bottlenecks in tunnel material delivery or unforeseen interactions between work packages can lead to significant disruptions in construction progress. Successful implementation therefore depends on flexible construction scheduling, redundant logistics concepts and close coordination between the involved technical disciplines (Figure 5).