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Constructing diaphragm walls in fractured chalks for HS2 - lessons learned

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11 July 2024

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VSL UK completed 4 diaphragm walls within the wider ground engineering package on HS2 section C1 awarded to KVJV ( A joint venture between VSL UK and Keller). The ground engineering contract was awarded as an early set of works ahead of the main C1 ground engineering contract due to its importance on the critical path - ahead of the main tunnelling works to construct the Chiltern tunnels – the longest tunnels on the HS2 project.

Due to this contract being let as early works within the overall C1 contract many details of the works were not finalised. VSL collaborated with the Align JV team (including their supporting design JV) in commercial, contractual, and regulatory discussions with HS2 (the Client), planning authorities, regulators and other external stakeholders including the Local Authority Environmental Health teams in particular regarding noise, vibration, dust and extended work hours matters for the section 61 applications.

Join the webinar to improve your knowledge of these contractual issues and how the challenges were overcome. 

The original design of the D walls was undertaken by Align JV’s design team. VSL provided input to the design team on reinforcement cage design (bar sizes, spacing, coupler locations), and joint milling methodologies.

Additionally, VSL value engineered the guide wall for the D wall and increased the efficiency of temporary works such as amending designs of stiffeners, lifting bars, etc to increase the integrity of the RF cages during handling and installation. BIM is being using extensively within the HS2 project.

VSL have collaborated with Align to provide data obtained from the ‘real time’ monitoring and survey files to allow Align’s BIM team to produce the as built models. 

The ground conditions for the construction of the diaphragm walls shaft were primarily in heavily fractured chalk formations with variable rock strengths. The chalk also included karst zones, dissolution features and very large flint inclusions. Due to the highly variable chalk including the flint banding, and working within aquifer protection zones, a heavily modified bentonite support fluid was required. 

In addition to the D Walls VSL also constructed 132 barrettes to support the tunnelling works. These critical barrettes were installed to high accuracy to prevent ovalisation of the C1 tunnels as they exit the ground at the Northern Portal and also to provide safe stopping zones for the TBM’s for essential maintenance ahead of breaking through the portal.


Neil Taylor

Neil Taylor


Project Director

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Neil Taylor

Neil Taylor is a geotechnical engineer and is currently VSL's Project Director for Ground Engineering on HS2 section C1.

Prior to becoming project director, Neil was VSL's project manager responsible for the construction of the diaphragm walls and barrettes on section C1.

Neil has over 20 years’ experience working on major infrastructure projects both in the UK and internationally including major road, rail and tunnelling projects.