A comparison of rotary and sonic borehole drilling techniques in the Lambeth Group and Chalk Group on the HS2 alignment in Northwest London

Sonic drilling is a relatively new technique in the UK ground investigation sector. The method has been praised for its continuous core recovery, making it an attractive technology for use in variable stratigraphy, as encountered below London.

A comparison of sonic drilled and rotary drilled cores from the Lambeth Group and the underlying Chalk Group was conducted using data from HS2 on the ground investigation stages of the Northolt Tunnel project. A comparison has been made between core recovery, sample quality and laboratory testing data within the encountered strata.

Sonic drilling performed well in providing near-complete core recovery throughout the Sand Channels, Mottled Clay and the Chalk Group relative to the rotary boreholes. Differences were observed in the recovered fabrics of the Lambeth Group, with rotary drilling preserving more of the finer geological structures. In the Chalk Group, the sonic drilling technique remoulded the chalk to a putty, and therefore solid core recovery and sample quality were far higher in the rotary-drilled boreholes.

This comparison highlights that the strengths and limitations of drilling technologies must be understood to maximise their potential and drilling methods should be tailored to the ground conditions and the geotechnical objectives to optimise the data received.