By: Josh Pruett, PE, Geotechnical Engineer, GeoEngineers, Inc.
Have you ever taken an art class where you made a ceramic bowl, bud vase, or mug? Remember that soft, sticky, gray stuff you lumped onto the potting wheel to shape it? While many people think of “coast” and think white sand beaches, most of what lies under coastal Louisiana is closer to the clay you used in your art class than the sugar sand of Florida beaches. Coastal Louisiana formed as the result of thousands of years of seasonal flood cycles from the Mississippi River, its distributary channels, and neighboring streams. During each flood, the rivers carried sand, silt, and clay mud over their banks, covering the land on either side of the river. Larger, heavier sediment particles fell out of the mud first, leaving mounds of sand and silt near the edge of the river, while clay particles flowed farther out before finally coming to rest. Large floods sent more material over the banks to greater distances than small floods, leading to thin layers of silt and fine sand occasionally interbedded with the generally clay substrate. Generations of marsh vegetation growing, seeding, laying down, and dying—with the next generation growing through the matted fibers of its parent plants—led to an organic profile that transitions from grass, to peat, to organic material intermixed with clay, to inorganic clay several meters below the surface.
For years, Louisiana marshes grew in rich beds of freedraining, organic soils augmented by a seasonal boost of mineral soil and nutrients, which largely ceased after the levees were built to protect the growing human and commercial population along the banks of the Mississippi and other major streams. Without mineral soil to sustain the marshes, geological processes like subsidence and faulting, already in play prior to the levees, have outpaced biological accretion of material, leading to decreased resiliency and loss of coastal lands occurring at a now legendary rate. What does this have to do with geotechnical engineering for marsh creation? As it turns out—everything.
For the layperson, geotechnical engineering is branch of civil engineering dealing with ascertaining the ability of the soil/ground to hold a structure. General applications include building and bridge foundations, earth retention (walls and excavations), embankment design and monitoring, geologic hazard mitigation, and a number of other elements. Geotechnical investigation and design is especially critical in areas with soft, cohesive soil—like the modeling clay described earlier— and, as mentioned previously, coastal Louisiana is covered with the stuff. Marsh creation projects are generally built by first building an earthen embankment ring (generally called a containment dike) around the designated marsh fill area, then pumping mud dredged from a nearby water bottom into the fill area. Stability and settlement of the containment dike, any special containment provisions (say, to close off flowing channels), behavior of the ground under the marsh fill, and behavior of the marsh fill are all geotechnical elements. As with any project, a few things need to happen before we can establish geotechnical design guidelines, namely field exploration, laboratory testing, and engineering evaluation.
A dredge barge cuts soil from a nearby water bottom and pumps the slurry into the marsh creation area. Due to settlement, equipment traffic, and desiccation, active containment dike maintenance during the marsh filling phase is an important operational consideration. Photo credir: Sellers & Associates, Inc.
Field Exploration
Coastal Louisiana soils vary widely depending on proximity to a river or the gulf. Before it made the delta as we now know it, the Mississippi River has carved out a wide swath of stiff, relatively dry, compacted clay and clay mixtures from the Pleistocene geologic age. These Pleistocene soils are still present near the ground surface for coastal environments on the North Shore of Lake Pontchartrain and west of the Atchafalaya basin and make for relatively good foundation support. In the intermediate areas, however, relatively fresh soils (Holocene age) dominate the top 100 ft of the soil profile. In the marshes, regardless of the soil age at depth, the top 10 ft is often dominated by very soft organic clays and peat.
Specially adapted drill rigs, such as this one mounted on a three-engine airboat, are used to collect soil samples from proposed marsh creation project areas in the field exploration/data collection phase of marsh creation geotechnical investigations.
There are no roads and no drivable surfaces in marsh creation areas, and most of the land where marsh creation projects are needed is private property. Extensive coordination with landowners and lease holders is required before we can access the site. Specialized drill rigs mounted on airboats, amphibious tracked buggies, and pontoon barges are commonly used to collect samples. Collecting usable near-surface marsh soil samples is difficult, and the additional seal and suction provided by a piston sampler goes a long way in sample retrieval and maintaining specimen quality. Clay, peat, and organic clay samples are only partially inspected in the field; in order to preserve sample quality, it is best to keep the sample in a sealed sampling tube and transport it gently to the testing laboratory where it can be removed from the tube and evaluated in a more controlled, stable environment.
Cone Penetration Tests (CPTs) from an airboat or marsh buggy rig have proved a valuable resource for containment dike foundation analysis by providing additional soil information between widely spaced soil borings. Because CPT collects electronic data without a physical sample, CPTs need to occasionally be paired with soil borings in order to calibrate the results. CPT information can be evaluated without the cost and time for traditional soil sample testing. This can help save time and money during the field and laboratory portions of a project.
Laboratory Testing
Samples fall into two main categories for marsh creation projects: borrow samples (or samples from water bottom borrow areas) and foundation samples (from under design containment feature alignments and marsh creation fill areas). Geotechnical engineers approach testing differently for the two soil categories. For foundation samples, we are looking for properties that will help us know if the soil can hold structures, such as earthen embankments or sheet pile walls, how much settlement to expect under loaded areas, and how quickly to expect it, along with ancillary issues like fill shrinkage over time and construction sequencing based on material quality. For borrow samples, we want to quantify settlement properties of hydraulic fill materials and establish fill placement criteria. We begin to identify these properties in our laboratory testing program.
Foundation Properties
Stability and settlement are the primary concerns for foundation soils in the marsh area. Soil strength tests before and after sample extrusion (removal from the sampling tube via steady push by a hydraulic ram) help to set the stage for the engineering analysis. Consolidation tests help inform our analysis about settlement quantities and rates. We assess classification and general behavioral traits through a variety of physical property determination tests, including grain size, plasticity, moisture content, unit weight, organic content, fiber content of organic soils, and specific gravity (the ratio of the density of solid soil particles to the mass of clean, fresh water).
Hydraulic Fill Properties
Soil type and slurry behavior are important considerations for hydraulic fill. Borrow area soils are tested for natural moisture content and other physical properties, then mixed together into one or more composite samples to run additional testing. Testing on the composite samples includes physical property tests (grain size, specific gravity, plasticity, etc.) and specialized settlement testing on a slurry made by watering down the composite sample, specifically column settling tests and self-weight consolidation (or low-pressure) consolidation tests.
ENGINEERING EVALUATION
Data Reduction
In order to use laboratory testing results in a marsh creation project design, geotechnical engineers first organize the results into logical groupings based on boring location and depth. Sometimes it makes sense to combine several borings and CPTs to form a single profile. Other times, explorations are best evaluated individually. Specific properties are plotted corresponding to sample depth, including moisture content, unit weight, and shear strength. An engineer will review the plots, identify layers, and assign layer properties based on trends in the data.
Design
After design profiles have been developed, engineers will complete design analyses to produce stable containment dike geometries, settlement estimates, and other important recommendations. Containment dikes are generally low earthen levees that engineers will design conservatively, which entails establishing stable height, width, and side slopes for the tallest feasible dike with the lowest foundation. Design containment dike sections are typically partly underwater. Slope stability, bearing capacity, and settlement are key containment dike design parameters.
Marsh creation settlement gets a little trickier. In the 1990s, the U.S. Army Corps of Engineers sponsored the creation of a dredged material settlement computer program called Primary consolidation, Secondary compression, and Desiccation of Dredged Fill (PSDDF). Settlement within hydraulic fill can be evaluated through PSDDF. However, PSDDF has a quirky way of handling foundation settlement, and GeoEngineers, for one, elects to use other software to calculate foundation settlement and ties them together in a spreadsheet to adjust for fill compression and settlement of the fill below the static water table over time. This allows us to move the analysis forward with a more intimate understanding of the processes involved. It is an iterative process that combines empirical and theoretical information with engineering judgement to produce a final outcome.
Containment dike construction is an important part of defining marsh creation areas and retaining hydraulically dredged and pumped slurry in order to form a new marsh platform. Excavators mounted on amphibious, tracked buggies are common tools for containment dike construction. Photo credir: Sellers & Associates, Inc.
The agencies requesting the design generally ask for estimates of cut-to-fill ratios for containment dike construction and marsh creation fill. While there are publications and geometries that we use to provide those estimates, data from constructed projects provides better insight into the cut-to-fill issue. Sediment losses during dredging, settlement during construction, mud-waving of foundation soils during fill placement, and other reallife variables are difficult to predict with any degree of certainty from the comfort of an air-conditioned office. A database of real as-built information will go farther in helping design engineers and planners than the numbers generated based on theory.
Results
The end result of a geotechnical marsh creation study is a report cataloging field exploration results, lab testing results, and engineering results and recommendations. The report should provide insights into the project constructability and potential red flags for using the site as a containment dike foundation. Alternative ways to build containment dikes, such as using geotextile fabric reinforcement under the dike, building the dike in several stages with a prescribed amount of time between each stage, or reducing the fill load by using fill alternatives (e.g., hay bales) should be discussed. Deep gaps with significant tidal flow or site drainage may need to be closed with an earth retention structure like a sheet pile wall or cellular cofferdam—the report should describe design considerations for those too, if needed. Construction recommendations about site access and use and other considerations to improve project implementation and chances for success should close out the report.
Conclusion
In the end, the agency or organization that receives the report should be able to use the results to inform their go/no-go decision and get them well on their way to a successful marsh creation project.
So the next time you take a sip from your favorite ceramic mug, think of how Louisiana and its consulting engineers are remolding and replacing Louisiana’s eroding coast, one chunk of clay at a time.
