Progress so far; understanding the risks; and how technology, planning and logistics help address the challenges of tunnelling through a highly fractured chalk aquifer.

given by David Eve, Align (Head of Engagement & Compliance for HS2)

Abstract:

Who are Align and how does the company fit within the HS2 structure? This leads on to an overview of HS2. The HS2 route is split into various sectors, so I need to give an overview of Sector C1. What progress have we made to date and what were the results of the Ground Investigation?

What are the risks?

Volume loss; frac in and out;
Loss of slurry to surface;
Loss of ground stability within excavation chamber;
Loss of air to surface (if intervention conducted under hyperbaric conditions);
Loss of grout to surface; and over-excavation within excavation chamber.
Leading to: Ground deformation; and excessive movement of structures.

How do we mitigate for these risks?

Overview of the ‘system’ – TBM to Slurry Treatment Plant;
Pre-treatment where required (shafts);
Technology of the TBM (selection) – speed, variable slurry density control, anulus Grouting to maintain <1% volume loss, continuous monitoring (MobyDic, borehole data, ground instrumentation) and adaption;
Logistics and processing of slurry;
Exclusion zones and planning of appropriate maintenance stops;
Securing of land in higher risk areas.