Use of rota-sonic drilling to investigate deep London Basin geology: a case study for an urban East London environment, including comparison to a conventionally drilled borehole

The Thames Tideway tunnel is a 25km long sewer which is being constructed to increase the capacity of the London network. CVB JV (a Joint-Venture of Costain, Vinci Grands Projets and Bachy Soletanche) are constructing the eastern section of the tunnel within the White Chalk Subgroup between Bermondsey and Stratford and during construction, low chalk cover is expected below the Limehouse Cut canal. Ground investigation was required to examine the interface between the Thanet Sand Formation and the Chalk to inform the design of a planned tunnel intervention. This paper presents a case study of a borehole successfully completed using rota-sonic techniques specified by the Authors following previous experience with the technique on the Lee Tunnel project. A rota-sonic drilling rig was mobilised utilising sonic drilling through the superficial deposits and top of Chalk, with conventional rotary drilling ensuring a high quality chalk sample was achieved for the remainder of the core, to a final depth of 75m. This was undertaken within a limited time period in a dense urban area where ensuring minimal disturbance to surrounding residents was of paramount importance. Previous ground investigation completed at depth in London have tended to avoid the use of sonic drilling due to reservations over the quality of the recovered core sample. Sonic boreholes drilled as part of the Lee Tunnel project were advanced in structureless grade D chalk using the sonic method only and cannot be directly compared with the investigation at the Limehouse Cut. Further case studies are therefore required to assess the viability of the sonic technique in higher quality CIRIA grades of chalk.