Geophysical survey identifies taxiway voids
Missouri River flooding in 2011 created a high groundwater condition at the Charles B. Wheeler airport in Kansas City, Mo., for a period of several months. The high groundwater condition reportedly caused a sinkhole to form on the south side of the Taxiway G run-up pad in the north section of the airport.
Two geophysical methods – ground penetrating radar (GPR) and multi-channel analysis of surface waves (MASW) – were used to identify the location of potential voids beneath a reinforced concrete taxiway. Prior to conducting the geophysical surveys, airport personnel observed a failed joint within a 42-inch corrugated metal pipe situated approximately 22 feet beneath the taxiway during a CCTV inspection. Evidence of sand infiltration was observed within the pipe during the CCTV inspection, and the airport was concerned about voids forming beneath the taxiway slab above the pipe.
Exploration using GPR and MASW surveying was performed on the taxiway to assist in identifying the location of potential voids, and to help identify locations for invasive exploration methods such as drilling or slab removal. 3D GPR data were collected over the suspected void area using several closely spaced orthogonal 2D GPR survey lines. Following the 3D GPR survey, MASW data were collected over two orthogonal survey lines situated over the survey area.
A high-amplitude GPR anomaly was observed in the vicinity of the failed pipe joint. The presence of a void was interpreted immediately beneath the taxiway slab, and the bottom of the void was interpreted at approximately 9 feet beneath the slab. The 3D GPR data suggested the presence of a cone-shaped void above the failed pipe joint area. Additionally, a lower shear wave velocity zone was observed on the MASW data to a depth of approximately 25 feet in the area of the 3D GPR anomaly, suggesting the presence of less consolidated soil beneath the interpreted void.
Upon subsequent removal of the concrete taxiway to remediate the suspected void, a large void was discovered beneath the taxiway slab. The void was discovered to have the same approximate dimensions as indicated in the final geophysical interpretation.
The sewer was replaced, the void backfilled, and the taxiway repaved. As a result, the public was protected from a potential catastrophic failure of the taxiway run-up pad.
By Michael S. Roark, RG, LEED AP, Geotechnology Inc., St. Louis; Jeremy s. Strohmeyer, RG, Geotechnology Inc., Kansas City, Mo.; and Boston Fodor, Geotechnology Inc., St. Louis
Award-winning composite dam design
Schnabel Engineering Inc.’s Deep Creek Watershed Dam 5D was selected as recipient of the 2013 United States Society on Dams’ (USSD) Award of Excellence in the Constructed Project category. Schnabel is the principal designer and engineer of record for the new Deep Creek Watershed Dam 5D constructed in Yadkin County, N.C. This project was made possible through joint cooperation and funding made available by the U.S. Department of Agriculture Natural Resources Conservation Services (NRCS), the North Carolina State Conservation Commission, and the local sponsor, Yadkin County.
A composite dam design was selected to efficiently make use of the complex existing foundation conditions. The composite arrangement was selected as being the least costly of 12 alternatives evaluated by Schnabel and reviewed by Yadkin County and the NRCS. However, designing both a large high-hazard roller compacted concrete (RCC) gravity dam and zoned earth embankment on a variable foundation presented considerable challenges. Particular attention was needed at the connection between the two dam types where differential settlement and seepage may occur.
The Deep Creek project included the first use in the United States of grout-enriched roller compacted concrete (GERCC) as the sole
upstream barrier. This innovative process includes addition of a cement grout to the no-slump RCC at each lift along the upstream face, and then mixing the grout and RCC using hand-held vibrators to consolidate the material and provide a seamless lower permeability zone of concrete (see "Alternative concrete for dams," CE News, February 2012, page 32). GERCC resulted in considerable savings to the project and its successful use at Deep Creek Dam has gained considerable attention from the U.S. engineering community.
Information provided by Schnabel Engineering Inc.
Solar panel stormwater management
Throughout development of the Longwood Gardens Photovoltaic Facility, the main challenge was the stormwater management component of the solar panels. In addition to developing an ideal approach to managing the impacts of the stormwater runoff rate and volume, it was essential to Longwood Gardens to maintain the ground’s natural topography and perform the least amount of grading possible.
Working closely with Chester County Conservation District and EPC firm groSolar, Pennoni Associates proposed a stormwater management model in which meadow ground cover surrounding the solar panels provided the primary means of stormwater management rate and volume control.
The idea developed through discussion with the project team was to place the panels on driven piles to limit the disturbance to the natural environment of the meadow fields surrounding the panels and create an aesthetically pleasing and environmentally sensitive solar field. The meadow ground cover is the primary best management practice (BMP) for the site as it treats stormwater at the source, the solar panels. In addition to the meadow ground cover, a rain garden BMP was designed to control the runoff rates and volumes associated with the project, in particular the gravel driveway and impervious area for the electrical inverter.
The grid-tied installation is providing approximately 18 percent of the facility’s electricity consumption and will reduce Longwood’s annual carbon dioxide emissions by 1,367 tons. Pennoni Associates’ design work was recognized with a Diamond Award Certificate in the Water and Stormwater Category by the American Council of Engineering Companies’ Pennsylvania Chapter. The project previously was recognized with an Honorable Mention by the Philadelphia Chapter of the Pennsylvania Society of Professional Engineers.
Information provided by Pennoni Associates.
Tracking sediments in record dam removal
Salmon are beginning to swim up the Elwha River for the first time in more than a century. But University of Washington (UW) marine geologists are watching what’s beginning to flow downstream – sediments from the largest dam-removal project ever undertaken. The 108-foot Elwha Dam was built in 1910, and after decades of debate it was finally dismantled last year. Roughly a third of the 210-foot Glines Canyon Dam still stands, holding back a mountain of silt, sand, and gravel.
Removal of the upper dam was halted in January while crews repaired a water-treatment plant near Port Angeles that got clogged with leaves and other debris. For engineers, this phase may be the trickiest part of the dam-removal project. For oceanographers, "the best is yet to come," said Charles Nittrouer, a UW professor of oceanography and of earth and space sciences.
It turns out there is even more sediment than originally thought – about 34 million cubic yards. That’s more than 3 million truck loads, enough to bury all of Seattle in a layer almost 3 inches thick.
"One of the risks of just looking at these beautiful plume pictures is that you really don’t know the extent of where that sediment actually ends up," said Andrea Ogston, a UW associate professor of oceanography. "Our focus is looking at what’s happening very close to the seabed – how it’s going to move, where it’s going to get to, what’s its ultimate fate."
For the last five years, Ogston and Nittrouer and their students have been studying the sediment around the river mouth, initially with the support of Washington Sea Grant, to understand the condition before the dams’ removal. Their current project, funded by the National Science Foundation, is looking for events that could act like a hundred-year storm and bury the sediment deep in the ocean.
The UW researchers have instruments to track particles in the water and record them accumulating on the ocean floor. They are on high alert for a rapid response when the river floods and dislodges the sediment. When that happens, they want to be onsite to record as much data as possible – and perhaps be the first to witness a rare geologic event.
In nature, deep-sea sediment flows triggered by earthquakes or extreme storms can be important for creating oil reserves and other geologic deposits, as a component of the global carbon cycle, and for burying communication cables.
Computer models and the geologic record suggest that when the sediment is in high-enough concentrations, it goes directly to the ocean floor. Instead of the fresh river water floating on top of the seawater, the river water becomes denser than the sea, and the sediment-laden river water plunges below the ocean water. For the Elwha, that path would take much of the sediment away from the coastline and deep into the Strait of Juan de Fuca.
"A surface plume is very much at the whim of the winds and tides, whereas these underflows are just going down the steepest gradient," Ogston said. "These are two very different mechanisms that would create very different impacts to the seabed."
"There is an understanding of the general type of flow, and people have predicted that it occurs in rivers, but no one has seen the smoking gun yet," Nittrouer said. "This is a chance to document a 100-year storm. It’s really somewhat new territory."
So far there have been dramatic changes to the seabed in the shallows, but few changes below about 20 feet, Ogston said.
Where the sediment ends up is of practical interest. Sediment can make the water murky, creating conditions that make it difficult for salmon to lay eggs, or block light from reaching algae and other life on the ocean floor. On the other hand, the sediment also has positive impacts. Many people hope that removing the dam will help with erosion along the Olympic Coast. The new sediment could accumulate and restore natural beaches on the bluffs near Port Angeles. A better understanding of sediment transport could also help determine the timing of future dam removals.
"One of the arguments is that rather than having a river that’s unacceptable to salmon for many years, you can accelerate the erosion to flush the system. That way you have two or three really bad years instead of two or three pretty bad decades," Nittrouer said. Future projects might be trickier, he added, if the sediments contain pesticides or other chemicals.
Nobody knows when the Elwha’s sediment mother lode will begin to shift. A heavy rainfall combined with spring melt could dislodge the heap; if not, next fall and early winter rains will do the job. Either way, the UW marine geologists will be ready to hop in their van, hitch up a boat, and race out to see what happens.
Information provided by University of Washington/Hannah Hickey
Mining waste byproduct can help clean water
A byproduct resulting from the treatment of acid mine drainage may have a second life in helping clean waters coming from agricultural and wastewater discharges, according to a recent study by scientists from the U.S. Geological Survey (USGS) Leetown Science Center.
The report, published in the journal Water, Air, and Soil Pollution, shows that dried acid mine drainage sludge, or residuals, that result from treating acid mine drainage discharges can be used as a low-cost adsorbent elsewhere to efficiently remove phosphorus from agricultural and municipal wastewaters. The phosphorus that has been adsorbed by the mine drainage residuals can later be stripped from the residuals and recycled into fertilizer. The mine drainage residuals can be regenerated and reused for a number of additional treatment cycles. Application of this novel, patented technology has the potential to simultaneously help decrease acid mine drainage treatment costs, prevent degradation of aquatic ecosystems, and recycle valuable nutrients.
Acid mine drainage is produced whenever sulfide minerals associated with coal and metal deposits are exposed to air and moisture. The resulting acid and dissolved metals are toxic to most forms of aquatic life, and untreated acid mine drainage has impacted more than 5,000 miles of streams in the Appalachian region, with associated economic impacts of millions of lost dollars in the tourism and sport fishing industries.
When acid mine drainage is remediated, it is neutralized with a base, such as limestone or lime, and an iron-rich sludge is formed that must be disposed of, sometimes at considerable cost. The new process of using the sludge to filter wastewaters has the potential to reduce the need to dispose of the sludge, while providing an added and previously unknown benefit of using the residuals to effectively reduce phosphorus from wastewater discharges wherever needed.
Current technology for the removal of phosphorus from wastewater consists of addition of aluminum or iron salts to precipitate and adsorb phosphorus, but this is too expensive for the low concentrations and high volumes often encountered in many wastewaters. This new technology provides a more efficient and cost effective option.
Information provided by U.S. Geological Survey
Energy Department invests in offshore wind projects
The U.S. Department of Energy made seven offshore wind awards for projects in Maine, New Jersey, Ohio, Oregon, Texas, and Virginia. These engineering, design, and deployment projects will support innovative offshore installations in state and federal waters for commercial operation by 2017.
In the initial phase, each project will receive as much as $4 million to complete the engineering, design, and permitting phase of this award. The Energy Department will select as many as three of these projects for follow-on phases that focus on siting, construction, and installation and aim to achieve commercial operation by 2017. These projects will receive up to $47 million each over four years, subject to Congressional appropriations.
The seven projects selected for the first phase of this six-year initiative are:
- Baryonyx Corporation, based in Austin, Texas, plans to install three, 6-megawatt direct-drive wind turbines in state waters near Port Isabel, Texas. The project will demonstrate an advanced jacket foundation design and integrate lessons learned from the oil and gas sector on hurricane-resistant facility design, installation procedures, and personnel safety.
- Fishermen’s Atlantic City Windfarm plans to install up to six direct-drive turbines in state waters three miles off the coast of Atlantic City, N.J. The project will result in an advanced bottom-mounted foundation design and innovative installation procedures to mitigate potential environmental impacts. The company expects this project to achieve commercial operation by 2015.
- Lake Erie Development Corporation, a regional public-private partnership based in Cleveland, plans to install nine, 3-megawatt direct-drive wind turbines on "ice breaker" monopile foundations designed to reduce ice loading. The project will be installed on Lake Erie, seven miles off the coast of Cleveland.
- Seattle, Wash.-based Principle Power plans to install five semi-submersible floating foundations outfitted with 6-megawatt direct-drive offshore wind turbines. The project will be sited in deep water 10 to 15 miles from Coos Bay, Ore. Principle Power’s semi-submersible foundations will be assembled near the project site in Oregon, helping to reduce installation costs.
- Statoil North America of Stamford, Conn., plans to deploy four, 3-megawatt wind turbines on floating spar buoy structures in the Gulf of Maine off Boothbay Harbor at a water depth of approximately 460 feet. These spar buoys will be assembled in harbor to reduce installation costs and then towed to the installation site to access the Gulf of Maine’s extensive deep water offshore wind resources.
- The University of Maine plans to install a pilot floating offshore wind farm with two, 6-megawatt direct-drive turbines on concrete semi-submersible foundations near Monhegan Island. These concrete foundations could result in improvements in commercial-scale production and provide offshore wind projects with a cost-effective alternative to traditional steel foundations.
- Dominion Virginia Power of Richmond plans to design, develop, and install two, 6-megawatt direct-drive turbines off the coast of Virginia Beach on innovative "twisted jacket" foundations that offer the strength of traditional jacket or space-frame structures but use substantially less steel.
Information provided by U.S. Department of Energy
Find more information about these projects at http://www1.eere.energy.gov/wind/offshore_wind.html.
View an interactive map displaying the announced wind projects and U.S. offshore wind resource potential at http://energy.gov/maps/accelerating-offshore-wind-development.
Submit news and photos of planned, ongoing, or recently completed projects and research to Bob Drake firstname.lastname@example.org.
In June, "Project Notes" will focus on stormwater and site planning; the July section will highlight water/wastewater and transportation projects.