Upgrades and improvements are essential to enhance the infrastructure of London, a city of over 8 million people. The London Infrastructure plan 2050 aims to improve the quality of life while London grows by creating ‘a greener and more productive city that is environmentally, financially, socially and economically sustainable and remains at the forefront of new technology.’ Much of the infrastructure in this dense urban area is underground, with developments becoming deeper and thus interacting with the groundwater in some parts of the city. Deep construction since 1990 includes a new railway, extensions to the London underground, water, road and cable tunnels. The deepest project is the Thames Tideway Tunnel, running for 25km along the River Thames to intercept overflows from the combined sewerage system, thus stopping discharge of raw sewage into the river when it rains. When fully operational in 2023, the tunnel will normally be partially full, surcharged only a few times a year, for a period of 1-2 days. The Lee tunnel at the eastern end opened in 2016.
The tunnel follows the axis of the plunging syncline across the centre of the London Basin. In general, there are two aquifers: a shallow superficial aquifer and the lower aquifer comprising the sandy Thanet Formation and the Cretaceous Chalk. The two aquifers are hydraulically separated by the aquitard of the London Clay and Lambeth Group. As the tunnel depth increases from 30m in the west to 80m in the east, it passes from low permeability strata into the principal Chalk aquifer. In east London, this presents a potential risk to groundwater quality, but also has provides an opportunity for improved understanding of the aquifer system through investigation and long-term monitoring of water levels and quality.
Over a 10-year period, the project team undertook an extensive investigation programme along the route and at the 22 shaft sites, geological characterisation, permeability and pumping tests, groundwater level monitoring and water quality testing in the Chalk and overlying strata. The extensive dataset collected has informed the design, engineering, groundwater risk assessments, and evaluation and mitigation of the impacts of construction and operation.
Pre-construction groundwater data was used to establish baseline conditions for both water levels and quality. Comprehensive risk assessment was required to satisfy the environmental regulators that the mitigation measures ensure that the construction of the tunnel and shafts did not pose an unacceptable risk to groundwater.
For the operational phase, risks during surcharging events were assessed through a series of simplified fate and transport models.
Since the easternmost 7 km of tunnel was completed in 2015, the monitoring results from commissioning and operation provide a reality check on the modelling and have allowed refinement of the risk assessments.