Karst

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This giant sinkhole is the result of the dissolution of thick salt layershundreds of feet below the surface. The size of this sinkhole can bejudged by the truck in the background. (Photo by Raymond C. Harris)

WHAT Is KARST?

~rst is the name applied to landforms that

develop in areas underlain by comparatively sol­ble rocks such as limestone, gypsum, and salt.

Karst terrain is characterized by solution features suchas caves, sinkholes, depressions, enlarged joints andfractures, and internal drainage that can have a negativeimpact on use of the land. The name was derived fromthe Karst region of Slovenia (part of the formerYugoslavia), which is underlain by limestone.

The passage of water through soluble rocks resultsin the formation of cavities in the rock. If the ceiling ofa cavity collapses, a sinkhole may form at the groundsurface. Karst terrain is commonly characterized byhighly uneven depths to bedrock; residual red, clay-richsoil; and surface drainages that disappear underground.Voids in bedrock can capture surface-water flow anddisrupt the surface drainage system. Soil and other sur­ficial material may be washed into the undergroundnetwork of cavities.

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Hazards from karst include the formation of sink­holes or collapse pits, as well as cracking of walls, foun­dations, roads, and other structures. Less obvious butequally important are the impacts karst can have onwater quality. Networks of interconnected caverns andvoids allow contaminants such as sewage, landfillleachate, or hazardous chemicals to travel unimpededinto shallow aquifers that may supply drinking water.The possible presence of solution features must be care­fully considered when making land-management deci­sions, including protecting water supply, locating septicsystems, and siting of waste disposal facilities.

AREAS IN ARIZONA AFFECTED By KARST

Karst terrain is common on the Colorado Plateau ofnorthern Arizona. Because the Colorado Plateau hasextensive areas oflimestone at the surface and gypsum andsalt in the subsurface, there is potential for property dam­age and severe water-quality problems related to dissolu­tion of these soluble rocks. Karst features are particularlycommon south of Interstate 40, from the Springerville­Saint Johns area northwest to Winslow, and in the GrandCanyon region from Flagstaff to the Utah border.

Karst in the southern part of the Colorado Plateauresulted from the dissolution of salt and gypsum beds.Collapse propagated upward through the overlyingsandstone and limestone. Minor karst features are alsoforming at the surface in limestone. More than 300 sink­holes have been identified in the area betweenSpringerville and Winslow.

On topographic maps, sinks show up as smalldepressions that have internal drainage and commonlycontain small lakes. In limestone terrane in northernArizona, open caverns are encountered commonly whendrilling water wells. In the Sedona area sinkhole collapsehas occurred in historic times.

Karst features also are present in southern Arizonain areas underlain by limestone. Examples are ColossalCave near Tucson, Kartchner Caverns near Benson, andnumerous other caves in the mountains of southeastern

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The Colorado Plateau of northern Arizona has large areas of karst features. Most of the karst in the Holbrook region is related to dissolu­tion of thick layers of salt at depth. In other parts of the plateau, karst features result from dissolution of limestone exposed at the surface.

Arizona. Because exposures of limestone in southeasternArizona are not as extensive as on the Colorado Plateau,collapse features are not as common. Solution featuressuch as enlarged joints and small caves are numerous,however, and present the same concerns for water quali­ty as those on the Plateau.

WHAT To Do ABOUT KARST

Surface karstfeaturesIf you are planning to build in an area that has the

potential for karst, we strongly advise that you thorough­ly examine a property for signs of karst features beforeconstruction. When exposed at the surface, karst featuresare usually quite obvious. Depressions, holes, and fissuresare readily apparent on the ground and, if large enough,can be distinguished on aerial photographs or topo­graphic maps. Geologic maps show areas underlain bylimestone.

The simplest method to mitigate a sinkhole ordepression is to fill it in. However, this method works

only if the sinkhole is inactive and is already mostly filledin at depth. If a sinkhole is still open at depth and con­nected to other voids, surface material may continue towash into the voids. Filling an active sinkhole may beonly a temporary solution. In any case, surface drainageshould be directed away from karst features to avoid pip­ing or collapse.

Hidden karstfeaturesProblems can arise when solution features are hidden

or are not obvious at the surface. The additional weight ofa building may cause collapse if the roofof a cavern is closeto the surface. A septic system installed over unidentifiedvoids may result in water contamination.

To detect the presence of near-surface karst, severalgeophysical methods can be employed. These includeground-penetrating radar, electrical resistivity, sponta­neous potential, gravity, and magnetic surveys. Thesemethods rely on differences in physical propertiesbetween the caverns or their filling materials versus thesurrounding rock.

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Bending of rock layers above voids created by dissolution of saltat depth has caused giant cracks southwest of Holbrook. (Photoby Raymond C. Harris)

Differential settling is an indirect problem associatedwith building on karst. Because karst regions often havevariable depth to bedrock, a building may sit partly on soiland partly on solid bedrock. Settling of the soil may occur,causing cracking of foundations and walls. Compoundingthe potential for fill-related structural damage is the abun­dance of expansive clay in soils on the Colorado Plateau,which may cause its own problems. (Expansive clay is dis­cussed in the chapter on Problem Soils.)

WHERE To Go FOR MORE INFORMATION

U.S. Geological Survey topographic maps depictdepressions and sinkholes in numerous areas on theColorado Plateau. Topographic maps may be purchasedat outdoor and sporting goods stores, map stores, andArizona Geological Survey (AZGS). Geologic maps of

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karst areas are available from the AZGS and USGS.Links to sites having information about karst are avail­able on the AZGS website.

Consultants for karst problems may be found in theengineering (geotechnical or geological), or geologistsections of the yellow pages of the telephone directory.Information about building restrictions in karst terrainmay be available from county planning and zoningdepartments.

Standard homeowners insurance mayor may notcover damage from sinkholes. Check with your insuranceagent to confirm if karst-related damage is included.

ELECTED REFERENCES

Supai Salt Karst Features: Holbrook Basin, Arizona,by J.T. Neal, 1995: in B.E Beck, EM. Pearson, and PE.LaMoreaux, editors, Karst Geohazards: Engineering andEnvironmental Problems in Karst Termne: Proceedings ofthe Fifth Multidisciplinary Conference on Sinkholes and theEngineering and Envil'onmental Impact of Kant:

Gatlinburg, Tennessee, 2-5 April 1995: Rotterdam,Netherlands; Brookfield, VT: A.A. Balkema, pp. 53-59.

Evaporite Karst in the Western Part of the HolbrookBasin, Arizona, by J.T. Neal, and R.M. Colpitts, 1997:in B.E Beck, and J.B. Stephenson, editors, The

Engineering Geology and Hydrology of Karst Terranes:Proceedings of the Sixth Multidisciplina1Y C01iference onSinkholes and the Engineering and Environmental Impact

of Karst: Springfield, Missouri, 6-9 April, 1997:Rotterdam, Netherlands; Brookfield, VT: A.A. Balkema,pp.107-115.

Geotechnical Aspects of Karst Terrains: Exploration,Foundation Design and Performance, and RemedialMeasures, by Nicholas Sitar, editor, 1988: AmericanSociety of Civil Engineers Geotechnical SpecialPublication No. 14, 166 p.