Francesco di Giorgio Martini sketched what is believed to be the first mechanically accurate drawing of a pile driver in his 1475 treatise Trattato di Architectura. While it is certainly more rudimentary than modern devices, the basic goal is the same: use machinery to install deep foundations for supporting structures. Over the past 545 years, Martini’s machinery has been left in the dark ages to be replaced by state-of-the-art drilling rigs. Now, FDOT is continuing to improve upon Martini’s idea with an innovative geotechnical practice: Measuring While Drilling.
Measuring While Drilling, or MWD, is the process of obtaining real-time data from drill rig sensors to record the drilling process and assess characteristics of underground geological formations. This information can then be used to assess the strength and resistance of materials. Drilling is necessary for almost anything geotechnical engineers do, from site investigation work to the construction of bridge foundations. When designing bridge foundations, having more data is a geotechnical engineer’s dream; however, conventional soil testing methods are expensive, so typically less than one percent of the soil in the bridge foundation area is tested.
For State Geotechnical Materials Engineer David Horhota, this is a persistent challenge – how to acquire more geotechnical data in a cost-effective way. FDOT has pursued many different avenues, including methods like Full Waveform Inversion Testing. FDOT sees an additional opportunity with MWD to harness the power of modern drill rig sensing capabilities to continue to improve geotechnical site characterization for bridge foundation design. FDOT has conducted a series of research projects to develop and test MWD practices, including on a current construction project for Interstate 395 in Miami.
Through these research projects, MWD is already paying off by providing more data in crucial locations. As FDOT Project Manager David Horhota says, “The cool part about using MWD is that as foundations are being constructed it gives information about the soils exactly where the foundation is going. This is an easy way to get more data for the site and confirm or refine the design.”
Drill Rig Data Paints a Clearer Picture
Before getting data into geotechnical engineers’ hands, researchers first had to identify what real-time information was needed from the drill rig. This involved a thorough laboratory study in which researchers defined five drilling parameters to be measured: torque, crowd (downward thrust), rotational speed, penetration rate, and the area of the excavation which is defined by the bit diameter and does not change. By measuring the five drilling parameters, the energy required to drill through rock can be derived, which was found to be directly related to the rock’s unconfined compression strength. As foundations and structures supported by rock commonly use unconfined compression strength in design, this finding showed there could be significant benefits in using MWD to provide geotechnical engineers with more data.
This propelled the FDOT State Materials Office to the next phase of the research project, “Measuring While Drilling for Florida Site Investigation.” The MWD principles discovered in the first project were also found to be true for site investigation drilling applications. Building off the information gained from the first two MWD projects, the next stage of the research focused on implementing MWD on projects that utilize drilled shafts, a type of deep foundation system. Drilled shafts are constructed by drilling a hole, placing a steel reinforcing frame in the hole, then filling the hole with concrete from the bottom of the hole up to the surface. Test sites for drilled shaft projects included the Selmon Expressway in Tampa and the CR-250 bridge over the Suwannee River. Through a combination of traditional geotechnical tests and MWD readings, the research team hoped to refine the method for relating MWD data to other forms of rock strength such as the side shear capacity of drilled shafts.
From testing at these sites, researchers were able to make these refinements and identify additional benefits from MWD. First, MWD provides direct feedback of the materials encountered in the exact location of the drilled shaft, at full-scale, which provides more confidence in the as-built foundations. Second, the centimeter-scale resolution of MWD allows for more accurate foundation designs, which optimizes the cost per foundation. Lastly, the research team found that MWD can lead to more efficient drilling practices, which optimizes drilling production rates and reduces the time of completion.
Dr. Michael Rodgers at the University of Florida was one of the principal investigators for the research tests at Selmon Parkway and CR-250. He says, “With current conventional methods, we get strength assessments every 2.5-5 feet. But with MWD, we’re getting strength assessments every 1-2 centimeters. We get a much better image of what’s going on underground.” FDOT is now getting a clearer picture, and the Department is not the only one who likes what it is seeing.
Innovations in Auger Cast Piles
Like Full Waveform Inversion Testing, MWD has caught the eye of the Federal Highway Administration (FHWA). FHWA highlights proven innovations through its Every Day Counts program, where MWD has been included in FHWA’s Advanced Geotechnical Methods in Exploration (A-GaME) innovation. While Every Day Counts primarily showcases MWD’s site exploration benefits, FDOT’s latest research project further leverages MWD to assist in bridge construction supported by auger cast piles.
Auger cast piles are similar to drilled shafts, in that both are a type of deep foundation. The method for constructing these foundations are slightly different, as an auger cast pile uses an auger to drill the borehole and the steel reinforcing frame is added after the concrete or grout is injected. In the past, auger cast piles have only been used for non-structural elements. MWD makes it possible to use auger cast piles for structural elements because the technology allows for the required quality assurance/control measurements to be collected, which provides more confidence in the as-built product.
The most recent research project using MWD to help in the construction of auger cast piles is currently underway, with the team expecting to complete work by May 2021. A spreadsheet developed by the researchers is already being used by FDOT geotechnical engineers to monitor the production of auger cast piles, assess the final as-built pile, and identify the need for any verification testing. FDOT is using auger cast piles to construct bridge piers at I-395 in Miami and Seminole Drive Bridge in West Palm-Boca Raton. For the I-395 site in Miami, auger cast piles are being used to support the Signature Bridge.
Francesco di Giorgio Martini’s medieval machinery is almost unrecognizable from the modern drill rigs of today. The data MWD can provide geotechnical engineers is enlightening, an advancement over 500 years in the making.
Phase 1: BDV25-977-37 Load and Resistance Factors Design (LRFD) Resistance Factors for Tip Grouted Drilled Shafts
Final Report | Summary
Phase 2: BDV31-977-91 Implementation of Measuring While Drilling Shafts in Florida (FLMWDS)
Final Report | Summary
Phase 3: BDV31-977-125 Assessing Axial Capacities of Auger Cast Piles from Measuring While Drilling
Research In Progress Listing