Agenda item
PONT LLANNERCH
To consider a joint report by the Council’s Corporate Director: Environment & Economy, the Head of Highways & Environmental Services, and the Risk & Asset Manager (copy attached) which seeks the Committee’s feedback to help inform the next steps in relation to the project to replace Pont Llannerch.
11am – 12pm
Minutes:
The Lead Member for Environment & Transport alongside
the Corporate Director: Environment & Economy, the Head of Highways &
Environmental Services, the Risk & Asset Manager
and the Senior Engineer - Bridges & Construction presented the Committee
with a report (previously circulated) updating members on the Pont Llannerch
project.
Due to the complexities involved with this specific subject
the Head of Highways & Environmental Services guided members through the
report's contents. He also welcomed Dŵr Cymru Welsh Water’s Head of Water
Production to the meeting to answer the Committee’s questions on their
operations in the bridge’s vicinity and how any proposals may impact on those
operations.
Following the collapse of Pont Llannerch during Storm
Christoph in January 2021, Highways & Environmental Services has been
working on a project to build a replacement bridge. The project was split into
three stages: Optioneering, Detailed Design, and Construction. It had now
reached the end of the Detailed Design stage. The detailed design stage had
been complicated and lengthy and has raised significant challenges. The main
challenge being the consideration of the foundations required for a new bridge.
This discussion had been complicated because Pont Llannerch was located above a
freshwater aquifer, which was within a layer of weathered sandstone, and
because Dŵr Cymru Welsh Water (DCWW) had a freshwater abstraction site
situated directly next to where the old bridge was located. This aquifer and
freshwater abstraction site provided water to around 85,000 homes in the
region, therefore, the freshwater aquifer and water abstraction assets were
extremely important to many DCWW customers. Consequently, the Council would
need to ensure that it did not compromise these assets when building a new bridge.
A preferred design using raft foundations had been chosen
because raft foundations worked by distributing the load over large areas of
ground. However, significant risks still existed because construction would
require sheet piling – essentially drilling into the rock below where the
foundations would sit. This drilling created a risk of compromising the assets.
Provisional ground investigations at the pioneering stage determined that this
weathered sandstone sat between 12 and 36 metres below ground level and
extended as far upstream as Ruthin. Further ground investigations during the
detailed design stage showed the water level to be as close as 10 metres
beneath ground level. There was a standard for assessing the predicted scour
depth of a bridge, and this showed that all 10m of the riverbed gravel in this
location would likely erode in the future. Therefore, to protect a new bridge
and to ensure it was not undermined, the newly constructed sheet pile curtain
containing the new raft foundation would need to be drilled/driven into the
area beneath where the foundations were placed, and this would penetrate the
weathered section of sandstone in which the aquifer sits. Drilling into the
aquifer could potentially compromise water quality at the DCWW abstraction
point at Llannerch Park. Compromising the site would present a risk of supply
loss to 85,000 DCWW customers. The project team had been working closely with
DCWW throughout the Detailed Design Stage, and the DCWW view was that all risk
assessments had failed to provide suitable evidence that the drilling work
would not introduce risk to this critical asset.
DCWW has stated that drilling into the aquifer would
ultimately create a pathway for the risk of water supply contamination,
compromising the aquifer local to the abstraction point could lead to several
factors related to risk and safeguarding for their customers. An introduction
of significant risk associated with the water supply had the potential to
introduce a public health risk with wide-reaching consequences. DCWW has also
stated that rectifying an issue created by drilling into the ground would not
be straightforward and would be extremely costly to resolve. DCWW had also
indicated that it may not even be feasible to repair it if a physical pathway
was driven into the aquifer. For these reasons, DCWW had assessed the bridge's
construction to be a high-risk activity.
Members were also guided through a technical presentation
illustrating further details regarding the project's design stage and all of the proposed designs for the bridge.
Responding to Members questions the Lead Member, Officers
and DCWW’s representative –
- clarified that due to the existence of the aquifer building a
bridge of any size to accommodate vehicle traffic was not feasible without
risk.
- confirmed that the concerns were not solely confined to the
weight capacity of the bridge but to the potential velocity and power of
the flood waters running underneath the bridge. Due to these factors, a strong foundation
would be needed regardless of the size and weight capacity of the bridge,
it was the foundations that would cause risk for the aquifer.
- confirmed that the Council had a 10-year bridge maintenance
plan in place with a view to safeguarding such structures. As part of this plan restrictions had
been placed on the stone bridge over the River
Clwyd in to Rhuddlan. However, this
bridge was built on a rock foundation, which was not the same as the
Llannerch bridge.
- confirmed that the potential impact on the aquifer had not
been fully assessed until the detailed design stage.
- advised that it was hard to quantify the impact of the loss
of the bridge on local communities, however, roughly 1,600 vehicles were
thought to use the road daily before the bridge was swept away.
- confirmed that the Council had strived to carry out as much
correspondence with residents as possible throughout the whole design
process for a potential replacement structure.
- emphasised that they fully understood the value the bridge
had to residents and the potential of erecting a temporary bridge had been
explored, but the risks would remain the same as it would for a permanent
structure as both would require strong foundations. The crux of the problem was the foundation
and the potential risks that posed to the aquifer.
- confirmed that it would not be possible to erect a
replacement bridge either a few miles up or downstream because the issue
would remain as the aquifer ran along approximately 22km below the
riverbed.
- highlighted the advice received from the Council’s insurers
(paragraph 10 in the covering report) if it proceeded to build a
replacement bridge which resulted in damage caused to the aquifer.
- provided assurances that, with a view to improving
connectivity between the communities effected, improvement work on the
current diversion routes was being considered with funding from the Welsh
Government.
The Committee, following a comprehensive discussion:
Resolved:
to communicate to the Project Team and Cabinet -
(i)
that having considered the report
and received the presentation summarising the detailed design stage, attached
at Appendix A to the report, emphasised that its desire was similar
to that of local residents to have a replacement bridge built to connect
both communities and ease travel between them.
Nevertheless, based on the expert engineering and risk management advice
received the delivery of this project did not seem viable as its construction
would likely pose significant risk to critical infrastructure in the area which
supplies water for circa 85,000 homes;
(ii) acknowledging
the impact the loss of the bridge had on local
communities and their residents that the Council should make every effort to
reduce the impact on connectivity between the communities affected by improving
other local transport links and highway routes in the area; and
(iii)
to confirm that, as part of its
consideration of this matter, it had read, understood
and taken account of the Well-being Impact Assessment, Appendix B to the
report.
Supporting documents:
-
Pont Llannerch Report 030425, item 6.
PDF 232 KB
-
Pont Llannerch Report 030425 - Appendix A, item 6.
PDF 2 MB
-
Pont Llannerch Report 030425 - Appendix B.pptx, item 6.
PDF 108 KB