Delivery of a new road tunnel under the River Thames in London presented various engineering challenges. Working with RiverLinx, on this Transport for London (TfL) project, we were able to inform and derisk the design and construction phases of the project by completing complex ground investigations, reducing design and construction cost and optimising the overall construction schedule.
The Silvertown Tunnel project consists of a 1.4-kilometre twin bore road tunnel under the River Thames in East London together with 600 m of approach ramps and roads. Passing through alluvium, London Clay and Lambeth Group, the two 12.10 m diameter bores follow an S-shape to link the Greenwich Peninsula with the Royal Docks area of Silvertown.
Ensuring project efficiencies and tight control of the project schedule was a priority for TfL, as well as the need to minimise and carefully manage local impacts of the construction programme.
Pivotal to the success of these outcomes, the site investigation (SI) needed to deliver accurate Geo-data, including the quality of hydrogeological data from a tidal river zone, to fully characterise ground risk and accurately inform the tunnel design.
In addition to the traditional challenges of geotechnical work in the Thames Basin geology, the SI also required sensitive and efficient implementation on land and overwater at a busy urban river crossing, while later adapting to Covid-19 restrictions.
Fugro's Aran 120 jack-up mobilised to its first borehole position as part of the Silvertown Tunnel project.
Our comprehensive in-house ground investigation capabilities, supported by the real-time data expediency of our Virgeo® portal, played a central role and delivered benefits during distinct phases of the project life cycle:
- Project initiation
- Construction ground investigation
- On-going live monitoring
During the feasibility phase of the Silvertown project, we implemented a broad grid of 125 geotechnical boreholes across the site including 31 within the River Thames providing all tendering parties with a robust characterisation of general ground risk. The SI also included geophysics surveys, pumping trials with river wall monitoring, pavement inspections and CCTV surveys.
Construction ground investigation
The design and build ground investigation for RiverLinx focused on obtaining outstanding ground data for the chosen tunnel design and final engineered construction. Here we were able to finetune borehole locations to reinforce data quality in the difficult ground conditions, adjusting positions by as little as 20 m to fill gaps and target data collection.
On-going live monitoring
Fugro provided water pressure and pumping data to aid safe tunnel boring machine operation and tunnel construction during the feasibility phase of the project. Due to the success and value derived from this real-time water testing via the Virgeo® portal, we were asked by RiverLinx to not only keep hardware in place, but add additional monitoring points to our network, and maintain our live data feed to help with tunnel construction. The project team were able to watch live data, processed and fast-tracked from the pressure pumping stations to their computer screens, saving time, reducing costs, and guiding tunnelling design.
The availability of this real-time data feed bolstered engineering confidence and decision making during the construction phase. This type of on-going service is a best value approach being fostered by our Geo-risk Management Framework (GRMF), which furnishes the client with appropriate Geo-data to manage underground risk across the asset life cycle.
Real-time data from the response to two pump tests targeting upper and lower aquifers in a borehole in the centre of the River Thames.
Fugro pushed the boundaries in both the quality and ‘live’ reporting of pump testing and water state measurements to achieve the level of clarity and certainty on ground risk. This was done through the application of novel techniques including the design, installation, and maintenance of an intricate system consisting of vibrating wire piezometers (VWP) below the bed of the Thames to determine the effects of water table drawdown in the ground layers.
Working closely with the Port of London Authority, we utilised proven marine drilling technology to reach target depth ahead of installing the VWPs and overcame significant environmental challenges through proven and tested expertise in positioning and drilling to place the sensors within an extremely small margin of error.
Weighted armoured marine cable running along the riverbed linked the VWPs to a proprietary data box on the river wall. Powered by a solar cell, the box enabled data from the sensors to be linked in real-time to our online Virgeo® portal, which was available 24/7 to key stakeholders.
We also trialled Deep Line®, our new onshore wireline cone penetration test system, to gather better and more continuous data through the challenging London geology. At this stage it was a demonstration consisting of one borehole, but we will be using this technology in the future to acquire faster, safer data in complex strata and boreholes.
Our ground investigation strategy provided deeper insight, and therefore greater control of risk and cost throughout the project life cycle, from feasibility and tendering, through to design and construction.
- On time delivery:
We delivered a diverse range of investigation and monitoring techniques including both marine and onshore intrusive and non-destructive surveys and despite significant additions to the scope and various onshore access constraints the final report was delivered in-line with the program schedule. A huge achievement considering the complexity of the project.
- Real-time reporting:
We maximised the use of real-time reporting of critical data to support risk management and design decisions across the procurement and construction stages.
The planning and logistics of the SI work involved close collaboration with numerous stakeholders to maintain transparent, positive relationships and meet compliance requirements.