With the offshore wind industry picking up speed on the east coast of the United States, there is much to celebrate with the progress being made. There are plans for offshore wind farms from Massachusetts to North and South Carolina to provide energy to the states. But in these waters are unique soils that pose potential issues for construction of these turbines.
Glauconite can be found on and offshore in Massachusetts, New York, and New Jersey. Glauconite is formed by three different processes. The first is precipitation of seawater, the second is modified illitic and biotic clay, and the third is by the fossilization of fecal pellets. Particles that formed where the soil and water interfaced millions of years ago became a green and black colored sand sized particles with tiny cracks and fractures that we know as glauconite today. These soils appear to behave like sand in their natural states but act more like cohesive soils when they crush. Because of their fractures they can easily break and can cause messy worksites on shore. Some of the documented problematic behavior include pile refusal, change in soil plasticity, and the crushability of the sand sized particles.
While there is very little published geotechnical engineering data on glauconite soils, some of the emerging research that is currently occurring is on soil crushing and degradation, pile driving, and strength and compressibility. As more offshore samples are being collected, it is important that we continue to investigate these behaviors from onshore samples and compare them to their offshore behaviors. One interesting feature of these soils due to their crushing capability is their change in plasticity. While most sands will not change their liquid and plastic limits due to the crushing of their particles, Glauconite soils increase in plasticity depending on how much crushing occurs. Also, the soil color can change dependent on how much crushing occurred. Other problematic features of this soil are its high acidity which can affect laboratory testing equipment and potentially cause corrosion to metal piles.
Here in the GeoTesting Express laboratory, we have been getting some high-quality intact offshore glauconite samples to be tested. We have tested samples in consolidation, triaxial compression and extension, static and cyclic direct simple shear, and resonant column equipment. It has been very exciting getting these bright green samples in house and seeing how they behave when they are tested.
The second type of problematic soil can be found in the waters off the southeast coast of the United States from North Carolina down through the Caribbean. In this region, calcareous soils line the sea floor. This soil is made up of the discarded shells of marine organisms such as foraminifera, coccolithophores, coral, mollusks, and algae. Calcareous soils can also be created from the erosion and fragments of coral reefs. While glauconite has internal cracks and fissures, calcareous soils can be hollow and almost bubble-like with interior void spaces. Their solid shells are thin and primarily made up of calcium carbonate making them brittle and easily crushable. This causes problems such as pile running, high compressibility, and reduction of strength when particles are constructed on.
These hollow and easily crushable soils have properties and issues of their own that differ from the problems posed by the crushability of glauconite. One of these unique issues is that the bulk void ratio of these soils will decrease during compression, while their intraparticle or skeletal void ratio has the potential to increase. This could explain the issues of pile running and frictional strength reduction. Additionally, the fine-grained particles of these soils are solid while their sand sized particles can have hollow particles leading to differing densities of the soils from coarse to fine fractions. This has implications on the transitional behavior and the transitional fines content, which is the number of fines that transitions a soil from coarse-grained to fine-grained behavior. A typical soil will exhibit a transition in behavior starting at 5% fines and 20% fines for hydraulic and mechanical properties. In calcareous soils with variable densities that fines content are estimated to be higher, transitioning at 14% fines for hydraulic properties and 75% for mechanical properties.
While both glauconite and calcareous soils are problematic, their uniqueness makes them exciting to study. We will be building more offshore windfarms, and we will keep encountering these special soils. One thing that I am personally excited about is to test the strength and compressibility of them under different conditions. As geotechnical engineers, we have barely scratched the surface when it comes to the geotechnical properties of glauconite and offshore calcareous soils, and we are excited about figuring out how and why they behave the way they do.
Post by: Danilo Zeppilli, Geotechnical Engineer, and Laboratory Specialist for GeoTesting Express. Danilo received his BS and MS in Civil Engineering from Rowan University where he worked as a Laboratory Technician and studied critical state mechanics. After his masters, Danilo spent time working as a field and Lab Engineer in Delaware before returning to start his PhD. He did a research fellowship on injection induced earthquakes and finite element modeling in France at École des ponts ParisTech. Danilo recently finished his PhD in Engineering and Applied Science from UMass Dartmouth where he has been studying problematic offshore soils such as calcareous and glauconite sands for offshore wind.
For further reading on glauconite and calcareous offshore soils check out:
Beemer, R. D., A. N. Bandini-Maeder, J. Shaw, U. Lebrec, and M. J. Cassidy. 2018. “The Granular Structure of Two Marine Carbonate Sediments.” Offshore Geotechnics; Honoring Symposium for Professor Bernard Molin on Marine and Offshore Hydrodynamics, 1–10. Madrid, Spain: American Society of Mechanical Engineers.
Beemer, R. D., A. Bandini-Maeder, J. Shaw, and M. J. Cassidy. 2020. “Volumetric Particle Size Distribution and Variable Granular Density Soils.” Geotechnical Testing Journal, 43 (2). https://doi.org/10.1520/GTJ20180286.
Beemer, R. D., A. Sadekov, U. Lebrec, J. Shaw, A. Bandini-Maeder, and M. J. Cassidy. 2019. “Impact of Biology on Particle Crushing in Offshore Calcareous Sediments.” Geo-Congress 2019, 640–650. Philadelphia, Pennsylvania: American Society of Civil Engineers.
Westgate, Z. J., D. J. DeGroot, C. McMullin, Y. Zou, D. Guo, S. Van Haren, R. D. Beemer, D. Zeppilli, K. G. Miller, and J. V. Browning. 2023. “Effect of Degradation on Geotechnical Behavior of Glauconite Sands from the U.S. Mid-Atlantic Coastal Plain.” Ocean Engineering, 283: 115081. https://doi.org/10.1016/j.oceaneng.2023.115081.
Westgate, Z. J., C. McMullin, D. Zeppilli, R. Beemer, and D. J. DeGroot. 2022. “Geological and Geotechnical Characteristics of Glauconitic Sands.” Geo-Congress 2022, 113–121. Charlotte, North Carolina: American Society of Civil Engineers.
Zeppilli, D., Beemer, R., Turner, B., Westgate, Z. 2023. “Invoking Carbonate Ramp Theory for Insights in U.S. South-Atlantic Margin Carbonate Sediment Behavior.” Offshore Site Investigation and Geotechnics 2023. London, England: Society for Underwater Technology.
Zeppilli, D., E. Dennis, Z. Westgate, G. Zhang, D. Degroot, K. Miller, J. Browning, R. Beemer. 2024. “Atterberg Limits of Two Crushed and Natural Glauconite Soils.” Geo-Congress 2024, 177-186. Vancouver, BC, Canada: American Society of Civil Engineers.
