Ground Improvement Measures in Advance of Drive G TBM Arrival at Victoria Dock Portal
Summary
Shortest of all C305 running tunnel drives, Drive G had the most complex geology, steepest vertical gradient, shallowest cover at Victoria Dock Portal (VDP) and the most complicated with Third Party assets on C305. At VDP, as the tunnel alignment rose toward the portal, the crown rose into superficial deposits above the London Clay. Advance works were therefore required in advance of TBM arrival, including:
Construction of an anti-flotation slab at the surface immediately to the west of the portal headwall; and
Ground treatment of the deposits above the London Clay to maintain stability of the TBM face and the tunneling works.
Ground treatment works at VDP were significantly challenging, mainly due to the position of utility services (including an NGG IP gas main) and DLR assets severely constraining the access on site and consequently the design.
To overcome these constraints the design went through an iterative process with all parties involved until a scheme was produced that satisfied all requirements and with minimal impact to Third Parties. The result of this collaborative approach was a bespoke detailed design, consisting of permeation grouting with tubes-Ã -manchette (TAMs), soil-mixing, micropile wall and anti-flotation slab. The main focus was to ensure differential movements on utilities and DLR assets was kept to a minimum and that volume loss of the running tunnels remained within the specified limits.
In order to assess the efficiency of these measures, a series of different scenarios and methodologies were developed, such as empirical Gaussian curves correlations, a Plaxis 2D non-linear finite element model, and FLAC3D numerical models to evaluate the different scenarios that were anticipated. As an additional measure this exercise prompted a detailed review of TBM parameters approaching VDP to provide greater control over the excavation works.
The outcome was a successful scheme that ensured safe tunnel excavation and breakthrough, mitigating the impacts on third parties assets to ALARP level during both the execution of the ground improvement works and the excavation of the running tunnels.