Comparative Analysis of Different Methods for Interpreting MWD Profiles

Abstract

To determine a robust geotechnical model from measurements of drilling parameters is one of the great desires of geotechnical investigations. Drilling parameters have the advantage of presenting very granular data (data-points every centimetre) much like the CPT. They allow for the identification of soil samples during the drilling process and can traverse all terrain types, from soft clays to hard rocks. However, interpreting each parameter, like penetration rate or thrust, in isolation can be challenging, as these parameters can vary greatly within each soil layer due to natural heterogeneity, the drill rig’s configuration and the noise introduced by the discontinuous interaction between soil and drill bit. Various authors propose using compound parameters such as specific energy, penetration resistance and alteration index alongside cautious filtering allows for a better interpretation, giving physical meaning to the measurements. These compound parameters have been developed from simple correlations, aiming to normalise parameters heavily influenced by drilling conditions or even aiming to evaluate the work or the energy spent in excavating the soil. Beyond that, many authors have devised algorithms to automate or standardize the interpretation of drilling logs by identifying homogenous zones or the probability that a given point belongs to a certain layer. This paper presents a comparison of such methodologies for identifying soil layers based on MWD profiles proposed in the literature. Assessment of the geotechnical structure may be made through different analytic and advanced statistical methods. MWD profiles from worksites throughout France will be used to compare and qualify these methods.

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