Scour around important foundation elements for bridges and other coastal infrastructure is the leading cause of failure and instability of those structures. Traditional scour mitigation methods, such as the placement of riprap, the use of collars or slots, embedding foundations deeper, or a combination thereof can be costly, require long-term maintenance, and can potentially have detrimental environmental effects downstream. These difficulties with traditional methods are potentially alleviated with the implementation of self-sustaining bivalve (e.g., mussel, oyster, scallop) farms that could act as mats of interconnected living barriers, protecting the seabed from scour. The mats would help to attract larval settlement by making the substrate a more suitable habitat, contributing to the sustainability of the bivalve farms. Colonies of bivalves are already being used as living shorelines for retreatment mitigation, embankment stabilization, and supporting habitat for other marine life. These applications are accomplished, in part, by bivalves' strong attachment capabilities from the bioadhesives they secrete that act as a strong underwater glue, adhering their shells to granular substrate. Some species of mussels have been shown to withstand water flow velocities greater than 6 m/s without detaching. For reference, riprap with a median grain size of about 655 mm has been shown to require a flow velocity of at least 1.7 m/s for incipient motion of the boulder-sized riprap. In addition to the contiguous living bivalve mat offering scour protection, the whole or fragmented shells (i.e., shell hash) that are left behind from dead bivalves are hypothesized to reduce erosion potential. Shell hash-laden sediments should be able to better withstand shearing, thereby increasing the critical shear stress required to erode material, compared to sediment without shell hash.
Habitat suitability for bivalve colonies is also an important consideration to evaluate what surface enhancements may be needed for a site to be selected for implementation of bivalve scour mats. Bed surfaces that consist of unconsolidated fine-grained sediment are unlikely to be able to support bivalve species as the organisms could sink into the sediment, not allowing solid anchoring points. In contrast, harder substrates typically found in granular sediments offer much more suitable habitats. Along with testing the influence of shell hash and bioadhesive on sediment behavior, this thesis aims to establish a methodology to evaluate whether a section of seafloor can support bivalves or enhancement materials (e.g., shell, shale, or slag fragments) without them sinking, thereby depriving them of oxygen.
Together, the examining of geotechnical aspects of bivalve habitat enhancement through seabed soil alteration and the influence of shell hash and bioadhesives on sediment shear behavior are part of a novel multidisciplinary approach toward this proposed bioengineered scour solution. Consequently, the research objectives explored in this thesis are as follows: (1) characterize morphology of existing bivalve colonies through acoustic and direct field measurements; (2) evaluate the spatial variation of the sediment shear strength in terms of proximity to bivalve colonies; (3) expand the domain of confining pressures and shell hash weight fractions used in sediment strength testing; (4) quantify the changes in shear strength and erodibility from laboratory tests on sampled material with and without the presence of bioadhesives, as well as shell fragments mixed in with the sediment; and, (5) develop a methodology ranking system for the suitability of a surficial sediments to support seeding material to improve benthic life habitat substrates.
Three exploratory field surveys were conducted where colonies of oysters and other benthic life were present: in the Piankatank River in Virginia, in the Northwest Arm of the Sydney Harbour in Nova Scotia, Canada, and at the Rachel Carson Reserve in North Carolina. Field sampling techniques included Ponar grab samples, hand-dug samples, X-ray rectangular prism cores, and cylindrical push cores, which were all pivotal to understanding sediment composition, size and shape of particle distributions, as well as in-situ depth profiles of shells. Remote sensing and intrusive instrumentation included a rotary scanning sonar, acoustic Doppler current profilers, CTD (Conductivity, Temperature, Depth) probes, underwater cameras, a portable free-fall penetrometer, and in-situ jet erosion testing which helped to characterize the morphology of the bivalve colonies and the spatial variability of sediment strength. Subsequent laboratory experiments included grain size distribution analyses, vacuum triaxial tests to measure changes in shear strength with and without shell hash, and miniature vane and pocket erodometer tests on bioadhesive-treated sediments. The results showed: (1) a significant increase in the standard deviation of the backscatter intensity where the oyster reef was located; (2) the in-situ sediment shear strength increased slightly closer to the oyster reef at the Piankatank River site; (3) samples with a higher oyster density exhibited less uniform particle size distributions; (4) the presence of less than approximately 4% (by weight) of shell fragments increased the secant friction angle by approximately 6° relative to samples with no shell fragments; and, (5) the harbor bed of the Northwest Arm of the Sydney Harbour is a suitable stiffness for enhancement with shell hash over about 23% of its area. Preliminary testing showed a subtle increase in the torsional shear resistance and a decrease in erodibility for bioadhesive-treated samples; however, further testing is needed for confidence to be achieved in the results due to bioadhesive supply issues. / Master of Science / Oysters and mussels are aquatic mollusks (i.e., bivalves) that are known to be able to withstand strong storm flows without detaching from rocks and other hard surfaces. Knowing this and the increasing need for environmental- and ecological-friendly solutions in engineering and construction further accelerated by climate change and sea level rise are the motivations for studying whether bivalves can be used in this capacity. Traditional methods to protect against bridge failures caused from individual piers that become unstable from sediment eroding away from their bases can be costly, require long-term maintenance efforts, and can potentially have detrimental environmental impacts. As an alternative to or supplement to traditional methods, bivalves could be laid down in mats near the base of piers to act as a protective interconnected layer, diverting strong water flows away from the otherwise exposed sediments susceptible to erosion while strengthening the seabed.
Much is known and has been investigated on the biology of bivalves but understanding how these organisms influence the sediments near them has not been studied extensively from a geotechnical engineering perspective. Specifically, within geotechnical engineering, this study is focused primarily on the influence of oyster shell fractures, naturally found in the vicinity of bivalve colonies, and the organic glue that bivalves use to attach themselves to rocks on the engineering behavior of nearby sediments. Secondary to that main objective is to establish a methodology to evaluate whether a section of seafloor can support bivalves without them sinking, thereby suffocating them. In summary, this thesis investigates methods to evaluate whether the seafloor is suitable for supporting bivalves and if their presence changes the way sediments behave after various forces are applied.
To accomplish these research goals, three exploratory field surveys were conducted for this thesis: in the Piankatank River in Virginia, in the Northwest Arm of the Sydney Harbour in Nova Scotia, Canada, and at the Rachel Carson Reserve in North Carolina where bivalves were present. Through field sediment sampling, underwater sonar imagery, penetrating probes, and subsequent geotechnical laboratory testing, shell-sediment interactions were characterized. The results showed: (1) an oyster reef in the Piankatank River could be observed in great detail with sonar imagery; (2) sediment strength increased slightly the closer to the oyster reef; samples with more oyster shells in them exhibited (3) a wider range of particle sizes and (4) an increase in sediment strength; and (5) less than a quarter of the harbor bed of the Northwest Arm of the Sydney Harbour is suitable for armoring the seafloor with pieces of shell, shale, and slag to support bivalve growth. Initial tests with the organic underwater glue from bivalves showed promising results with respect to improvements in sediment strength and decreased erodibility, however, further testing is needed as supply of the organic glue was limited.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/107088 |
| Date | 24 June 2020 |
| Creators | Consolvo, Samuel Thomas |
| Contributors | Civil and Environmental Engineering, Stark, Nina, Hatcher, Bruce Gordon, Castellanos, Bernardo Antonio |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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