The deal builds on awards for the first two ventilation shafts on the project in August at Chalfont St Peter and Chalfont St Giles.
Taken together the two double sets of shaft excavation jobs are worth over £26m.
The next two ventilation shafts along HS2’s 10 mile tunnel drives through the Chiltern Hills are located at Amersham and Little Missenden.
Excavation of the shaft in Amersham will be to a depth of 55m.
Keltbray’s works include internal shaft construction works to build slip-formed concrete cores and lining walls, reinforced concrete works at the base of the shafts, to create a collar structure to enable the TBMs to pass through the base of the shaft.