On drilling speed of London Clay from MWD data with time-series algorithm for ground characterisation

The ground conditions around an old tunnel in London Clay strata were extensively studied. The correlation between ‘measurement while drilling’ (MWD) drilling parameters and drillhole investigation results, however, was not satisfactory due to the significant noise in the drilling speed. In this paper the original MWD data published in the literature are re-examined and the associated noise issue is addressed by transferring the data in depth series into the data in time series. The curve of the drill-bit advancement depth with net drilling time can be visually divided into many linear segments. Each linear segment has a gradient or constant drilling speed and represents a homogeneous geomaterial zone with constant resistance to drill-bit loading. The depth variation of the constant drilling speed zones is further compared and well verified with the depth variation of lithological strata units identified from rotary-cored samples. A strength grade system is also proposed based on the linear zoning results. A new rock quality designation is calculated from the thickness of the zone of constant drilling speed for the London site and is compared with those at four other sites; this further enhances the understanding and confidence in the linear zoning result and the constant drilling speeds as resistant strengths associated with the lithological strata units in London Clay.