Curt R. Peterson 2013-07-17 00:14:38
What Are Earth Fissures? Curt Peterson is the founding engineer of Peterson Geotechnical Group, a forensic geotechnical engineering consulting firm. He holds a bachelor’s and master’s degree in civil and geotechnical engineering and has observed thousands of structures throughout his career. As principal engineer and with over 16 years’ experience in the field of engineering each project is given the upmost attention with focus on accurate analysis and meeting the client’s deadlines. The firm provides expert services including deposition and trial testimony and mediation attendance on construction defect lawsuits. For more information call (480) 219-0014 or visit the website, www.pgg-eng.com. Recently, there have been several news articles about large earth fissures that had developed in Queen Creek, Ariz. These fissures are large cracks in the earth up to 10 feet wide and a few miles long that have caused significant damage to property, roadways and utilities. There was even a report about a horse falling into one of these fissures and dying. This information begs several questions: 1) Did anyone know about the existence of earth fissures in Arizona; 2) What is an earth fissure and how does it occur; and 3) why didn’t the civil or geotechnical engineer for the development know that these fissures existed prior to designing the improvements at the site? First, earth fissures have been occurring in Arizona for nearly 100 years and are located all over Arizona. The Arizona Geological Survey has studied the cause and locations of known earth fissures and recently published online maps which show the location of documented fissures across Arizona. In the Phoenix metro, the most prominent fissures are located just north of the San Tan Mountains in South Queen Creek and around Luke Air force base in the West Valley. There are smaller fissures located near the edge of the McDowell Mountains in Scottsdale, near Cholla Cove Park in Scottsdale, near the 202 Freeway/Main street exit in Mesa, and in Apache Junction just south of the US-60 Freeway. There are also numerous fissures near Picacho, Eloy and south of Casa Grande. Second, earth fissures are associated with land subsidence due to the lowering of groundwater levels. In some parts of Arizona, the ground water levels have lowered several hundred feet due to groundwater pumping for agricultural purposes and municipal use. As groundwater exits the soil, the tiny open pore spaces within the soil are no longer held open by the buoyancy forces from the water. Over time, these tiny pore spaces collapse and the soils compress. Near Luke Air Force base, the ground water levels have dropped more than 300 feet and the land has subsided nearly 20 feet. Where the subsidence is uniform, the effects at the surface go unnoticed. Where the subsidence is not uniform, the subsidence can cause irrigation canals and sewer lines to stop flowing or even reverse direction. Third, most of the soils in the Phoenix valley are alluvial soils, which are fine grained soils deposited by flowing water. On the edges of the valley are mountains composed of various types of rock. In situations where the alluvial soils in the valley experience land subsidence and the rocks on the edges of the valley do not, an area of differential soil movement may occur. As a result, large cracks develop in the soil near the junction of the soil and bedrock. Over time, a vertical offset can develop across this crack as well as erosion along the edges, thus creating a fissure. These fissures can be several feet wide and several feet deep; it doesn’t take much of an imagination to see why fissures can cause lots of damage to any structure built over it. Repairs can also be very difficult, especially if the land subsidence is active and the fissure continues to widen due to erosion. During the design phase of a project, whether it is a subdivision, roadway or infrastructure improvement, geotechnical engineers will typically perform a study to evaluate whether any known fissures or land subsidence has been documented in the area. This study may include a site visit to look for evidence of fissures, review of aerial photographs and/or review of published literature. Given the problems associated with earth fissures, the state of Arizona has funded several studies to map out existing earth fissures. These maps are great resources, are available on the Arizona Geological Survey website and are easy to read and understand. If the preliminary geotechnical analysis concludes that an earth fissure is present or may be present at a proposed development site, a more detail study is necessary. Such a study is typically performed by land subsidence experts and includes track-hoe excavations onsite to expose any potential earth fissures. In conclusion, earth fissures, while common in certain parts of Arizona, occur due to known causes that can be recognized by geotechnical engineers during the design phase of a new project. If an earth fissure is identified, the site design may need to be reconfigured and special considerations employed. If you have a client with earth fissure problems, please give us a call at 480-219-0014.
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