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

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 desirable due to the significant noise in the drilling speed. This paper re-examines the original MWD data published in the literature and addresses associated noise issue by transferring the data in depth-series into the data in time-series. The curve of 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 constant drilling speed zone for the London site and is compared with those at four other sites, which 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.