Supporting the refurbishment of one of London’s iconic structures; Hammersmith Bridge

Operating in a busy public environment

The site location in a busy central London borough on the River Thames meant working and collaborating with a range of stakeholders, including our client, engineering firm FM Conway, the London Borough of Hammersmith and Fulham, Port of London Authority (PLA), and the public. We also required permission from the Environment Agency in the form of a flood risk activity permit, which was obtained by Mott MacDonald on behalf of FM Conway. With the bridge and surrounding roads being open to pedestrians and traffic and the river being used by water vehicles, we had to plan our operations carefully.

We were required to address several health and safety risks, including the use of specialist site security equipment and the onsite security team to guard and contain a radioactive source required for the geophysical works. We also worked closely with the Royal National Lifeboat Institution (RNLI) Chiswick to ensure safety on the Skate 2D jack-up, and carried out an emergency evacuation drill to provide essential experience to the RNLI and our deck crew for projects involving river works.

Overcoming site challenges with the right equipment

The Hammersmith Bridge structure posed unique challenges, especially for the nearshore element, where the ground investigation had to be conducted over water in a tidal river. To address the challenges, we opted to utilise our jack-up barges, with the Skate 2D identified as the ideal solution due to its adaptability in the confined site and the ultra-low sulphur diesel that powers the barge.

One of the borehole locations was in very close proximity to the bridge and multiple Thames Water assets, which posed a significant challenge to the gathering of essential data. Working with FM Conway, the deck plan of the Skate 2D was rearranged to accommodate for this and allowed us to optimally position the jack-up to avoid damaging both the bridge and Thames Water assets.

The onshore scope of works, carried out on the riverbank around the bridge posed challenges as the bridge abutments had weight restrictions and there was ongoing structural works taking place. The Fraste piling and limited access rig was chosen as it could achieve the required depth of drilling whilst remaining below the weight limits, and the size of the rig also allowed us to work within the confines of the existing structural works.

Due to the geology of the site, all vertical positions were initially carried out using cable percussive drilling techniques to progress the borehole through any overlying gravels. The inclined boreholes and some of the deeper vertical holes were carried out using our Comacchio 602 rig which was able to drill to depths more than 50 m allowing us to prove and sample the Lambeth Group soil below the London Clay.

Bespoke cone penetration test (CPT) solutions

During the works, the Skate 2D utilised one of two purpose-built CPT innovations that were developed specifically for this project. One borehole required a new CPT deck mount to ensure efficient switches between boreholes and CPTs, and an easily switched incline moon pool with Oilon bearings to enhance precision and reduce friction to enable 30 degree inclined boreholes to be cored at this critical location.

The second cone penetration test innovation was a purpose-built CPT rig, which was developed in-house and attached to a floating barge and used to access the Seismic CPT hole under the bridge deck of the Barnes abutment.

The newly developed CPT solutions were a tremendous engineering achievement and were designed and installed specifically for this project and the client’s needs by our design and engineering department in Falmouth. The innovative solutions ensured an efficient and highly accurate delivery of works and data at this uniquely challenging site.

The solution for the Seismic CPTs carried out on the riverbed was to use a low ground pressure heavy crawler. The crawler can spread its 19 tonne weight with its LGP tracks, which, along with the use of track mats, prevented any riverbed scour and greatly reduced the environmental disturbance created by the works.

Due to strong stakeholder collaboration and the development of bespoke solutions we were able to overcome the challenges and considerations of this complex site and successfully deliver the project. This ensured that detailed data was provided to support our client with risk-proofing this iconic and vital structure.