Decision details

Reason for the decision

The decision is required to authorise capital funding to replace the ageing, life-safety critical ventilation system in Chestfield Tunnel, following previous failures and the risk of further unplanned tunnel closures and consequential disruption to road users and additional cost.

Background

-      In June 2024, a safety critical failure with the ventilation system fans was uncovered during a routine inspection. This led to the Coastbound closure of the Whitstable to Herne Bay section of the A299 Thanet Way, whilst emergency works were undertaken to repair some of the fans.

-      The ventilation fans are specialist pieces of equipment which are manufactured and reconditioned to order, and specialist teams are required to deliver and install them.

-      Chestfield Tunnel relies on 32 jet fans (16 per bore), 5 of these were refurbished following the 2024 failures and of the remaining fans, 22 are 27 years old. Whilst the aged system has an increased maintenance and testing regime, the fans have exceeded their service life and are at risk of failure.

-      The proposed solution is a fan replacement with deflectors and Fan relocation. This will improve the effectiveness of airflow and smoke control. In addition, Fan relocation will improve access for future maintenance.

-      It is more expensive than the basic like-for-like and deflector-only options, at £2.78m, but avoids the significantly higher cost, technical complexity, construction risk and prolonged closures associated with the larger fan or passive portal deflector options. It therefore represents the most proportionate, deliverable and future-proofed option.

-       

Options (other options considered but discarded)

-      A longlist of 39 options was developed and evaluated in a scoring workshop, from which four were shortlisted for safety risk assessment by WSP (Designers)

-      Option 1 – like-for-like replacement. This is the do-minimum option. It offers no improvement in smoke control over the current situation, and the constraint on maintaining the lighting driver near Fan 2 persists. £2.31m

-      Option 2 – deflectors. As Option 1, but with angled louvre deflectors fitted to each fan to increase the effectiveness of the airflow. This improves smoke control relative to the baseline and to Option 1, with minimal additional construction complexity. £2.39m. although less costly than the preferred option, it does not address future maintenance access constraints.

-      Option 3 – larger fans. Requires significant electrical and control system redesign, structural works, and a long 24-hour bore closure of around 19 weeks. Between £5.78m - £6.41m

-      Option 4 – passive portal deflectors and wind walls. Like-for-like fans supplemented by passive deflectors and wind walls at the portals, requires extensive computational fluid dynamics analysis with an uncertain outcome, and introduces new hazards. £7.71m

-      Both option 3 and 4 are more costly than the preferred option

How the proposed decision supports the Council’s Strategic Statement

-      The decision supports Aim 2 by focusing on practical outcomes rather than a like-for-like replacement. A specialist team has assessed the options and selected a proportionate, deliverable solution that improves tunnel safety, performance and long-term maintenance.

-      It supports Aim 4 by helping to improve road quality and reduce disruption. The preferred option will improve smoke control and ventilation reliability, while planned traffic management will minimise the impact of works on residents, businesses and road users.

-      It also supports economic growth and quality of life by maintaining the A299 Thanet Way as a critical transport link and reducing the risk of future unplanned tunnel closures.

-      If this scheme does not proceed it is very likely by 2030 that there will need to be unplanned closure of the tunnel until the funding is available and the scheme can progress.

As Cabinet Member for Highways and Transport, I agree to

PROCEED with the replacement of the Chestfield Tunnel Ventilation System.

AUTHORISE the associated financial commitment, subject to the completion and approval of a satisfactory commercial business case demonstrating affordability, value for money and compliance with KCC procurement requirements.

DELEGATE authority to the Corporate Director of Growth, Environment and Transport, in consultation with the Cabinet Member for Highways and Transport, to take all necessary actions to implement the decision, including the procurement and award of contracts, subject to approval of the commercial business case.