BTS | HS2
South Geology – Cross Passage Frozen Sand
Advance 1 full face: very stiff frozen ground Frozen Lambeth SAND
sand. Various methods of ground stabilisation were considered, including dewatering, grouting and freezing. On the basis of safety and programme certainty, SCS elected to freeze the most onerous conditions, a decision endorsed by the HS2 Independent Tunnelling Expert Panel. Of the 21No. CPs in Lambeth Group, 12No. were frozen, 5No. dewatered and 4No. were untreated.
Central The Chiltern Tunnels are bored through most of the different formations of the Upper Cretaceous Chalk Group, from the older firm, blocky Zig Zag Chalk to the younger moderately dense Newhaven Chalk. The water table varies from almost 80m below
ground level, to being at ground level at the location of the River Misbourne.
Frozen Lambeth mottled CLAY
continues for at least four months and goes through a robust close-out process. Ground movement above these tunnels was minimal, even given the potential for settlement above zones of unravelling sand - the volume loss maintained to 0.4% or less. The ground conditions were also challenging
Slice through Bromford Tunnel geology
for some CPs, with chalk in the inverts and sand channels in the face and above the crown. These 4m-diameter, 11m-long SCL tunnels faced risks of water ingress from these beds and collapse of the
Challenges Challenges tunnelling in chalk include: ● Variation in the geology; ● High permeability and groundwater pressure; and, ● Soluble rock.
In the case of the Chiltern Tunnels, there is quite
a variation in the chalk materials through which the tunnels were bored, with varying density of chalk, numerous layers of marls, hardgrounds and flints, including tabular flints, which can have implication for wear on TBM cutterhead tools. Discontinuities in the chalk give the rock a high
bulk permeability and in the Chiltern Tunnels, where the tunnels are deep, the associated water pressures are high. Align reported that they were unable to undertake some of their planned cutterhead interventions in the chalk, due to high groundwater ingress, and all CPs required grouting. A further challenge of tunnelling in chalk is the
soluble nature of the rock, which can be subject to the process of dissolution. The resultant solution features are a risk to tunnelling, including in the Chilterns, providing an additional dimension of variable ground for these HS2 tunnels. Align used a VD TBM to deal with the variable
ground conditions. The presence of a screw and crusher was considered invaluable to deal with the hard flints encountered, which sometimes made up over 10% of the face.
North The Bromford and Long Itchington Wood tunnels are built through the Sidmouth, Tarporley and Branscombe Formations of the Mercia Mudstone Group. The water table is generally at ground level. In engineering terms, the mudstone is categorised
as a weak rock and has variable material properties, which is a function of its state of weathering. Grading schemes, such as that found in CIRIA C570 ‘Engineering in Mercia Mudstone’, are used to distinguish engineering properties - Grade I & II are structured
18 | March 2026
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
