The Northern Line Extension (NLE) between Kennington and Battersea was proposed to help regenerate the Vauxhall, Nine Elms, and Battersea areas and was scheduled to be completed by the end of 2021. Figure 4 (a) shows the displacement velocity map obtained from Sentinel-1 images between October 2016 and October 2020. It highlights distinct subsidence and uplift pattern near Battersea Power Station and Kennington, respectively. To help analyze these patterns, the time series of surface deformation for both the uplifting and subsiding areas were also extracted (Figure 4 (c) and 4 (d)).
The main tunnelling work of the NLE consisted of creating two tunnels between Battersea and Kennington Park. The construction for the NLE began in July 2016 and required dewatering of the ‘deep’ aquifer, which includes the lower part of the Lambeth group. The dewatering shafts for the NLE are located on the northern edge of Kennington Park, and the location of the two main shafts required for placing the tunnel boring machines are shown in Figure 4 (a) (black triangles). The geology of the area is relatively complex, with several faults, buried hollows and laterally discontinuous superficial strata. The underground construction work, tunnelling shafts, and groundwater extraction contributed to the land motion pattern observed in this area.
In Kennington, the average land motion trend during the observed time-period is that of uplift (Figure 4 (a)), which is concurrent with the change in groundwater level (Figure 4 (b)). However, the deformation time series exhibits phases of both subsidence and uplift during this period (Figure 4(c)). Specifically, the ground subsided during 2016-2017, before continuously uplifting since November 2017. This motion corresponds to subsidence due to dewatering during the construction of the tunnels, followed by groundwater rebound (heave) once the dewatering ceased in November 2017.
Around Battersea Power Station, the time series (Figure 4(d)) shows a linear trend of land subsidence between 2016 and 2020. The surface displacement here is consistent with the decrease in the groundwater level during this period (Figure 4(b)). It is most likely due to the groundwater abstraction that was undertaken to dewater the locality for the NLE tunnelling. The construction activities around Battersea Power Station are still ongoing; hence, the groundwater extraction and associated ground deformation can be seen to continue beyond the observed time-period.