GEOSTRATA Magazine

Every day in the United States, subsurface investigations use various boring machines and penetration rigs to advance boreholes in conditions varying from soft ground to hard rock. It's conceivable that, in the U.S., drilling contractors, consulting firms, and government agencies drill more than a million boreholes between 12 and 45 m deep annually, resulting in at least 32,000 km drilled each year (more than 7 times the width of the contiguous U.S.). For example, drilling-specialized companies using a 20-rig fleet will advance about 400 borings per month (7,300 m), totaling approximately 70 drilled km annually for geotechnical purposes alone. Unfortunately, very little useful information is retrieved from many of these borings if they are used for water wells, monitoring wells, and installation of various types of instrumentation. For soil profiling, boreholes typically yield discrete information at 1.5-m-depth intervals. In rock, however, stratigraphic profiling often relies on core cuttings that rarely extend more than 1.5 to 3 m into bedrock, except for cases where drilled shafts are planned for foundation design or for tunneling investigations.

Nearly a century ago, the field of geotechnical engineering took shape, and formal studies of soil mechanics and quantification of earth material properties to solve engineering challenges began in earnest. In 1948, during his opening address to the 2nd International Conference on Soil Mechanics and Foundation Engineering Rotterdam, Netherlands, Karl Terzaghi stated: “Further progress depends chiefly on the improvement of our methods for measurement in the field, on the scope and quality of the field observations, and on the adaption of our methods of subsoil exploration to practical requirements.” We have adapted methods from other industries and developed new ones, and every step continues this betterment. Among them is Measurement While Drilling, or MWD.

The best outcome of a geotechnical drilling program is a cogent package of qualitative and quantitative information describing what's under our feet. Getting this result requires a team effort between the drill crew, the geologist, and the end user of the data. While the driller operates the rig to optimize sample recovery and drilling efficiency, the geologist details the lithology, geologic environment, and other diagnostic features of the material, and the end-user provides context for the data needs with an outcome-based approach to inform an analysis or design. An experienced team will not only sufficiently characterize material using traditional methods, but also observe the subtleties in the drill rig response to changing conditions in the subsurface and note how those may affect later design considerations.

The purpose of the first generation of Eurocodes developed in the European Union was to facilitate free access to markets in other member states for construction companies and engineering consultants. Eurocode 7, dedicated to geotechnical design, was produced in 1990 by the Technical Committee for Structural Eurocodes, CEN/TC250 SC7, “Geotechnical Design.” At the same time, it was complemented by a set of testing standards covering all areas of geotechnics, earthworks, and geosynthetics, the first one drafted under the auspices of the Technical Committee for Geotechnical Testing and Investigation, CEN/TC341, “Geotechnical Investigation and Testing.”

Measurement While Drilling, or MWD, is the practice of recording physical measurements from the sensors installed on drill rigs during the drilling process. The overall goal is to quantify the response of soil or rock to infer engineering properties about the ground that's being drilled. MWD is used to complement traditional site investigation tools by providing correlations of measured drilling parameters to well-known geotechnical engineering properties. It can also help to optimize drilling performance by identifying and mitigating risks, improving borehole quality, enhancing the borehole stability, and boosting production drilling rates, along with assisting with maintenance scheduling. As it becomes more established in geotechnical practice, geoprofessionals will explore more capabilities of MWD that are beneficial to the profession.