An End to That Sinking Feeling
The dramatic way in which sinkholes develop often makes for an interesting visual on the news. You might see aerial footage of a slowly collapsing soil pit that consumes an ever-larger slice of freeway somewhere in the United States. Or perhaps you see still photos of something that seems impossible, such as the 39 ft. sinkhole that developed beneath a woman s bed in Guatemala in July 2011. (It s hard to imagine one sleeping comfortably anywhere after that.) Most of the sinkholes we see on the news are the rarest events. The common occurrences are much smaller (e.g., as small as a 3 ft. depression). They occur slowly, and they occur suddenly. Often, the erosive or karst processes that precipitate their formation is caused by water, either flowing underground or infiltrating/ seeping into weakened soil or carbonate rock; but other times it may simply be the result of a natural or manmade cavity (e.g., an abandoned mine) collapsing. However great or small sinkholes may become, the dynamic processes that cause them make stopping them generally impossible; and our ability to accurately assess the risk of a sinkhole s development is limited. The damage they can inflict on infrastructure can be tremendous, but mitigation of that impact and reviving infrastructure more quickly can be engineered. Reinforcement geosynthetics are increasingly being used to stabilize soils in areas that have experienced sinkholes. Two brief case studies illustrate some important ways in which these reinforcement geosynthetics are being utilized today in sinkhole mitigation strategies. The first example (Union Bridge, Maryland) comes from Alan Dinges of Maccaferri Inc. The second (Schmalkalden, Thuringen, Germany) comes from NAUE Geosynthetics. Bridging the Divide in Union Bridge Sinkholes are a common problem in many parts of the world. Although not much can be done to prevent them from collapsing, catastrophic loss can be prevented. Geogrids have been used successfully for sinkhole mitigation and site remediation designs around the world. Work in the historic town of Union Bridge, Maryland, exemplifies this geosynthetic reinforcement approach. New development in Union Bridge is at times difficult for two reasons. First, the GEOSYNTHETICS town is listed on the United States National Register of Historic Places, so a premium is placed on preserving the city s traditional character and protecting the older structures and landscapes. Second, karst terrain is present, meaning that there is some surface instability and a heightened risk of sinkhole development. Sinkholes can develop in these soils gradually or suddenly, and are often caused by chemical dissolution of underlying carbonate rocks. New building and infrastructure projects proceed with understandable caution. Reinforcement is frequently preferred. One of the most important economic forces in Union Bridge is a major North American cement and construction materials company. To secure its future productivity in the town, the company sought to expand its limestone quarry; but to do so, it had to tackle the risks that quarry would cause for the town itself. An existing roadway would have to be realigned, and that meant that the roadway would now have to pass over a potential sinkhole zone. A solution was needed to mitigate potential damage. Maccaferri was approached by the engineering consulting firm Hydro-Geo More Than Just Gabions As part of a worldwide industrial group, Maccaferri has more than 130 years of experience in engineered environmental solutions. Traditionally known for our double twist gabions and Reno mattresses, we have extended our product range significantly over the last decade, allowing us to offer an unrivalled range of geosynthetic, wire and natural fiber products to the construction industry. When the job calls for more than just gabions, remember, Maccaferri is more than just gabions. Paralink 1000 Geogrid, Sinkhole Remediation, Union Bridge, MD Engineering a better solution www.maccaferri-usa.com 800-638-7744 www.landandwater.com 21
To pave the way for continued economic growth, the town of Union Bridge used geosynthetic reinforcement to ensure roadway strength near an expanding limestone quarry. Services, Inc. to help solve the problem. With many years of international experience in sinkhole mitigation, the geogrid manufacturer worked with the project team to identify what would be the best solution for effective, economical results and safe, long-term performance. The embankment design of the new roadway leg needed to account for the possible development of a 15-ft.-diameter sinkhole. A double-layer system design was selected, one that utilized Maccaferri s Paralink 1000 kn geogrid in longitudinal and transversal layers for embankment reinforcement. Concrete blocks were added along the perimeter to provide a uniform wrap and to increase friction between the bottom grid and the underlying soil. The grid was covered by a one foot layer of crushed aggregate. To ensure the integrity of the overall design, measures needed to be prescribed to prevent an embankment-weaking mixing of fines and the primary aggregate layer. A woven geotextile separation fabric was installed over the crushed aggregate to prevent this soil migration. TBH/White Pine Construction installed more than 20,800 sq. yd. of Paralink 1000, the project was executed in a timely fashion, and the roadway opened (in early 2011). The quarry works can continue to flourish, and with them so too does Union Bridge. The town s physical and economic infrastructures have both been reinforced. A New Face for Stability An extraordinary crater in Schmalkalden, Thuringen became Germany s most prominent sinkhole after its sudden development on 1 November 2010. What made this sinkhole particular- Protect your Channel The PRESTO GEOWEB System The ultimate solution for channels and erosion control challenges. FREE Preliminary Design Genuine GEOWEB Made in the USA for 30 years AP-6679 PRESTO GEOSYSTEMS Appleton, Wisconsin, USA 22 November/December 2011
ly alarming was its development within the middle of a residential area. Geologists suspected that natural cavities were the cause and established test wells to monitor the situation. For three days the earth shifted until finally it settled, leaving For three days the earth shifted until finally it settled, leaving behind a 20 m (65 ft.) deep crater. behind a 20 m (65 ft.) deep crater. For days, gravel trucks rolled in, delivering approximately 30,000 tons of 8/16 gravel which was poured into the crater s mouth. Securing the edge, however, was more critical to provide greater protection against immediate and future shifts. Right next to the massive hole was a partially slipped embankment, the remains of which were extremely unstable. This threatened a nearby apartment building. The emergency installation of a geotextile/geogrid composite product helped secure this site in Germany, where a 65 ft. crater had developed in less than 72 hours. Geosynthetic reinforcement was requested. On 8 November, rolls of NAUE Combigrid 30/30 Q1 151/3 were delivered to stabilize the slope. This particular product is a unique geogrid/geotextile composite material that delivers reinforcement, filtration and separation in a single layer material. (The geotextile component is bonded between the flat geogrid bars during the manufacturing ENVIROSCAPE Erosion Control Materials Manufacturer of blanket 4 to 16 wide and many sizes in between 16 feet wide blanket Product videos on our website: Self Propelled Stapler Circle Top Pounder Blanket Bracket up to 16. Visit our website for a list of distributors or to become a distributor Circle top pounder Manufacturer of: Straw Blanket Straw /Coconut Blanket Coconut Blanket Excelsior Blanket Polypropylene Blanket Short Term & Biodegradable 9, 12, & 18 Wattles Reseller of: Metal Staples Biodegradable Staples Wood Stakes Silt Fence Staple Pounders Get Rolls up to 1,000 square yards! www.strawblanket.com 888-550-1999 www.landandwater.com 23
process.)these geogrid/geotextile composites are used in conjunction with soft and low CBR soils for applications such as base reinforcement, embankment reinforcement and pile cap platforms. From the former garage forecourt (now the area of the sinkhole) to the adjacent residential buildings, a wrapped-face reinforced wall was constructed. Five tiers of the composite geogrid were installed using an envelope method along the front of the embankment to provide permanent stability. Roughly 6,500 m² (69,965 ft. ²) of geosynthetic reinforcement were installed. The site remains stable today. L&W by Chris Kelsey Water for a sustainable future { Environmental Passive Integrated Chamber } RESPONSIBLE WATER MANAGEMENT Our total system approach expands beyond traditional stormwater systems, which historically only store and/ or treat stormwater pollutants. The Firestone Environmental Passive Integrated Chamber (EPIC Chamber ) is an onsite water management and reuse system designed to collect, filter, retain and distribute water below ground at its source. GREEN SPACE ATHLETIC FIELDS IRRIGATION PARKING LOTS STORMWATER RUN-OFF (888) 264-1735 www.firestonesp.com/lw10 info@firestonesp.com Chris Kelsey is the editor of www. geosynthetica.net and a regular contributor to Land and Water. RESOURCES Alan Dinges is an Area Manager for Maccaferri Inc. He can be reached at adinges@maccaferri-usa.com or by phone at +1 301 223 6910. Visit www.maccaferri-usa. com for more information. Kent von Maubeuge (kvmaubeuge@ naue.com) is available to handle inquiries about Combigrid composite reinforcement materials and other geosynthetics for NAUE America and NAUE GmbH & Co. KG. Visit www.naue.com for additional information. Other geosynthetics manufacturers who may be consulted for more information on how reinforcement geosynthetics are being used in sinkhole mitigation strategies include (but are not limited to): BOSTD Geosynthetics www.newgrids.com Colbond www.colbond.us HUESKER inc. www.huesker.com/usa Strata www.geogrid.com Tensar International Corporation www.tensar-international.com 24 November/December 2011