Totterdown towerblock proposal: Concerns about stability of riverbank, road and bridge

Arup has provided specialist land stability advice in relation to the Hadley planning proposal for a towerblock development at Totterdown Bridge (planning reference 18/04620/F), and has expressed concerns about potential problems for the river bank, the Bath Road and Totterdown Bridge. The full report is below. Extracts from the discussion and recommendations are here:

8 Discussion and Recommendations
There are three key concerns relating to land stability for the proposals for this site:
• Stability of the River Avon bank;
• Stability of the steep cliff to the Bath Road; and,
• Stability of the Totterdown Bridge.

The River Avon, Bath Road and Totterdown Bridge are key assets for various different reasons
(historically, ecologically, and as infrastructure). It is therefore vital to safeguard these features and the current proposal suggests that there is potential for damage to these features. The BuroHappold (2019) Bank Stability Design Note [3] does not provide enough detail around this issue at present. There are a few issues that have either not been identified or not been discussed as part of this note, which are summarised for each key feature below.

8.1 Stability of the River Avon Bank
Firstly, the associated BuroHappold reports indicate that instability is not possible as the overall line of the river bank will be left untouched. This is not considered likely as the construction process will disturb the current levels (especially given that two basements have been included as part of the proposal) and additionally the building loads are expected to be significantly higher than anything that has been experienced at the site previously. In addition, the temporary stability of the river bank could be compromised during construction due to uncontrolled excavations, surcharge from construction traffic, piling rigs, and other plant.

8.2 Stability of Bath Road
Bath Road is an important artery into Bristol, and Bristol Temple Meads railway station. It also has
historic value with Victorian housing present on its south side. It is therefore an important
infrastructure link that will require careful protection.
From the general arrangements drawings, it is surmised that the existing retaining wall will be
removed as part of the works. It is currently unclear what the retaining wall is protecting, if
anything, in addition to the concrete hardstanding. However, as shown in Figure 4, there are
significant steep slopes from the lower area of the site to the upper area (adjacent to Bath Road).
The retaining wall and its removal could be problematic if it is currently supporting the steep cliff.
It is hard to know what is being retained without a ground investigation.
In addition, to build the 2-level semi basement structure it is anticipated that an excavation will be
made into the existing steep slope. This will affect the stability of the slope supporting Bath Road.
The Applicant’s Engineer will therefore need to develop a clear solution for supporting the road in
the temporary and permanent case, but will also need to consider the impacts of future demolition
(i.e. should not allow for internal slabs to prop against and support the slope as future demolition
may exacerbate instability). It is anticipated that a secant/contiguous piled wall may be needed to
support the road an allow for construction.
It is recommended that thorough checks of the stability of the cliff at all stages of construction are
required ahead of any works being allowed. This will require de-vegetation, engineering geological
inspection and topographic survey. During any works, it is also recommended that a strategy is in
place to monitor the stability of the cliff in order to identify any unexpected movements or events.

8.3 Stability of the Totterdown Bridge
The Totterdown Bridge has been present next to the site since 1904, taking over from a ferry across the river at a similar location. It currently experiences heavy traffic during rush hour and is a key location for crossing the river. It therefore also requires protection, particularly its foundations,
which neighbour the site.
It will be necessary as part of the ground investigation to discover the location of the foundations
for the bridge. These foundations may spread underneath the site and no information has been
provided as to whether this has been investigated. The ground investigation and the eventual
foundation layout is a significant risk to the existing foundations.

8.4 Summary
A summary of the key recommendations is provided below:
• An understanding of the Totterdown Bridge foundations should be required ahead of any
ground investigations or other major intrusive works. If the foundations are shown to be close to
any of the proposals, analysis to confirm the stability of the bridge throughout all stages of
construction (including in-place and demolition) is recommended.
• An understanding of the existing retaining wall and its foundations should be required ahead of
any ground investigations or other major intrusive works. Analysis to confirm the stability of
the cliff throughout all stages of construction (including in-place and demolition) is
recommended.
• Analysis to confirm the effect of the loading on the bank through all stages of construction
(including in-place and demolition) is recommended.
• Any works near the bridge, cliff or bank should ensure that strategies are in place to ensure that
the River Avon, Bath Road and Totterdown Bridge features are protected against any
unforeseeable land stability issues or events.
• It is recommended that any company completing works on this site (in addition to completing
the above steps) is required to receive professional indemnity insurance that will allow (as a
worst-case) full repairs to the cliff, Bath Road, Totterdown Bridge and the River Avon should
collapse occur.