HS2 | BTS
during formation of rivers in the Tertiary Period. They were likely deposited in a flood plain environment in meandering and overlapping channels, which led to variable ground conditions.
Challenges In the first 2km, the NTW TBMs encountered challenges in the Lambeth Group, in particular, the sand channels in the face and above the crown had the potential to unravel above the TBM. The Contractor managed the tunnelling in this ground using their standard operations by running the NTW TBM in full EPB mode, constantly supporting the face, and using tail-skin grouting to fill the anulus around the tunnel lining. Data from the TBM and ground monitoring is
reviewed at the Shift Review Group (SRG), which includes spoil-grout reconciliation. Reconciliation compares the weight of the excavated material for a ring, to the theoretical weight and the volume of grout used in the anulus. Where data suggests more excavation than expected, checks are made for gaps in the grouting or for unravelled sand zones with probing and secondary grouting utilised where necessary. In these conditions, SCS used some additional
tunnelling measures including: ● having conditioners suited to granular ground available; ● applying bentonite under pressure around the TBM shield to support the ground above the crown; and
● longer probing was used at a higher frequency to check for zones of unravelled sand.
The reconciliation process is complemented by the surface ground movement monitoring, which
Group Formation
London Clay Formation
Harwich Fm
Lambeth Group
Reading Fm (Lower and Upper Mottled Clay prob
indistinguishable) Upnor Formation
Chalk Group
Unit B
A3 A2
Swanscombe Member Tilehurst Member
Sub-units A3i and A3ii Sandy silt/clay/cements Clay/silt/sand units/cements
?Upper Mottled Clay Mottled clays, silts, channel sands, ?cements
?Lower Mottled Clay Upnor/Mottled Upnor Ruislip Beds Formation
Sub Paleocene weathered Chalk
Seaford Chalk Lewes Nodular Chalk
March 2026 | 17
Mottled clays, laminated clays/silts/sands, channel sand ?cements
Mottled glauconitic clayey gravelly sands (if present) R2 Dark grey thinly interlaminated clay/fine sand
R1 Dark grey/black sandy clay with multicoloured fractured flint gravels
Above: South Geology – Cross Passage (CP) ground freezing
Left: South Geology Table
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