King George V House

Chiltern Society meeting with HS2

Chiltern Society

John Gladwin, Keith Hoffmeister, Haydon Bailey, Jim Conboy


Luke Nipen (Local Engagement Manager),
Chris Casey (Traffic Manager, Central area)
David McCann (Engineer, section C1- Chilterns Tunnel)
Simon Dale Lace - Geologist

Tunnelling & Aquifer, Depth @ Ch St Giles  (DMcC)

Slurry tunnel boring MC doesn’t like dry conditions (TBM type to be specified by contractor)
Use a Bentonite - liquid mix (except in water SPZ)
Expect 450m/month, operating 7days/week
Lowest point of tunnel between Amersham VS and Shardeloes Walled Garden
Highest gradient from Walled Garden to South Heath (3%)

Misbourne & Shardeloes geology

HS2 have to return to Ch.St Giles for further Ground Investigation (GI) (access problems previously)

  • Fractures, High Water Table
  • Forward scan from TBM (using Ultra-sound ?) may detect conditions ahead
  • Phase 1 - more GI is required (for pricing contract)
  • Will need FOI to get GI results (one has been submitted, by ??)

Shaft & Cross passage construction –

Open face work; 380m spacing between passages. Need grouting for water control
Shafts 33m deep, diaphragm walls. Rectangular or Circular ? High water table may cause construction problems, delays

Geology (SDL)

Surveys are underway, monitoring water levels, ( too late – no water at present ?)
Plenty of tunnels have been bored in chalk


  • Turbidity prevents UV water purification; may need to swap sources (e.g. with Thames Water – different usage patterns)
  • Study effects of drilling the boreholes on turbidity at adjacent holes
  • A simple sand filter can remove turbidity
  • Balance supply between Thames & Affinity ; need infrastructure (a pipe ?)
  • May use different (temporary) extraction points


  • Only allow ‘safe’ materials in construction; approved by Environment Agency

Controlled release into Misbourne -

  • Must pre-treat water to satisfaction of EA; cheaper to tanker away
  • Route of pipelines to river safeguarded, filtration plant required (Can this be clarified in LEMP ? LN will investigate)
  • South tunnel portal potential problems with runoff from spoil ?

How do you know what the boreholes say ?(HB)

  • Logging
  • Geophysics
  • Interpretation - which has taken time.
  • Problems e.g. striking Flints

HB comments

  • Train engineers on Geology before they start digging
  • Jointed chalk creates the  Aquifer
  • Vent shafts, cross-passages - water entry will create problems
  • See Kensworth Quarry pictures showing pipes through the chalk..
  • Ch St Giles - 4m solid chalk, under 15m rubble
  • New borehole - 27m to chalk, unstable above


  • 28 days notice is required to stop up any PROW
  • Contractors consult with Local Traffic Liaison groups; 5 in Bucks/Central area
  • LTLGs coordinate planning for worksites etc
  • Temporary Traffic orders
    • 8-12 weeks notice
    • Resident notification letters
    • Signs on site

Misbourne roundabouts

       Traffic approaches too fast ?
ES flows are baseline/limits

A413 cycle way proposal

       & Deep Mill bridge - nothing planned yet; risks will be assessed.

Tunnel Safety…

       Not a construction issue; need to arrange another meeting with Mark Hard, Rubin McDonald

JEC 4/8/17