SubsidenceLearning ObjectivesSubsidence, the sinking of theland, is an important geologicprocess capable of causing exten-sive damage in some areas of theworld. Your goals in reading thischapter should be to:

■ Understand the causes andeffects of subsidence

■ Know the geographic regionsat risk for subsidence

■ Understand the hazards associated with karst regions

■ Recognize linkages betweensubsidence and other hazards,as well as natural servicefunctions of karst

■ Understand how humans interact with the subsidencehazard

■ Know what can be done tominimize the hazard from subsidence

Flooding in Piazza San Marco, Venice, Italy An exceptional 137 cm (54 in.) high tide on this day, November 1, 2004, caused the

flooding of 80% of Venice in northern Italy. Flooding is becoming morecommon as Venice subsides and sea level rises.

(Associated Press)

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Venice is SinkingItaly’s beautiful and famous city of Venice faces a serious geologicproblem. The city is sinking, or subsiding, up to 2 mm (0.08 in.) peryear in some areas. Venice is built on 17 small islands connected byover 400 bridges. While its coastal location and numerous canals arepart of Venice’s draw as a tourist destination, the presence of so muchwater surrounding a subsiding city makes Venice extremely prone toflooding.

The land on which Venice is built is often only a few centimetersabove sea level, and many buildings are flooded repeatedly. Althoughsubsidence has been occurring naturally for millions of years, the over-pumping of groundwater from the 1930s to the 1960s significantlyincreased the rate at which it is sinking. This subsidence has resultedin numerous floods from the sea, the response to which has been toraise buildings and streets above the flood level. The overpumping andthus the human contribution to this natural hazard ended in the1970s. Unfortunately, natural subsidence is still occurring and floodsare common. As world-wide sea level continues to rise, the future ofVenice is uncertain.

6.1 Introduction to SubsidenceSubsidence is a type of ground failure characterized by nearly verticaldeformation, or the downward sinking of earth materials (Figure 6.1).This type of ground failure may occur on slopes or on flat ground. Itoften produces circular surface pits, but it may produce linear or irreg-ular patterns of failure.

Subsidence is commonly associated with the dissolution of solublerocks, such as limestone, beneath the surface. The resultant landscapehas closed depressions and is known as karst topography. Other majorcauses of subsidence include the thawing of frozen ground, compactionof recently deposited sediment, and the shrinking of expansive soils.Toa lesser degree, earthquakes and the deflation of magma chambers arealso responsible for causing subsidence. Human-induced subsidence,discussed in Section 6.6, can result from the withdrawal of fluids fromsubsurface reservoirs; from the collapse of soil and rock over subsur-face holes, such as those left by underground mining; and from thedraining of wetlands.

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174 Chapter 6 Subsidence

Subsidence

FIGURE 6.1 SUBSIDENCE Subsidence is the sinking of a mass ofearth material below the level of the surrounding material. Subsidencemay be caused by dissolution and collapse of rock, removal of fluids andcompaction of sediment, removal of rock, or faulting.

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(a)

(b)

(c)

Sinkholes

Sinkholes

Sinkholes

Disappearingstreams

Groundwatertable

Groundwater table

Groundwater table

Disappearing streamRiver

Cave

Solution valley

r

FIGURE 6.2 DEVELOPMENT OF KARST TOPOGRAPHY (a) In the earlystage of karst formation in a limestone terrain, water from the surfaceseeps through fractures and along layering in the soluble rock. Weak acidin the water then dissolves the rock. (b) As a river erodes more deeply intothe land surface the groundwater table drops. Caves begin to form andcollapse to become sinkholes. Some surface streams disappear under-ground to become groundwater. (c) In later stages of karst development, adownward-eroding river continues to lower the groundwater table. Largecaverns and sinkholes develop and eventually merge to form solutionvalleys which form without surface streams. In humid tropical climates,intense dissolution removes nearly all of the rock leaving behind pillars oflimestone referred to as karst towers. (Modified from illustration by D. Tasa in

Tarbuck, E. J. and F. K. Lutgens. 2005. Earth: An introduction to physical geology, 8th

ed. Upper Saddle River, Prentice Hall)

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KarstIn many areas, subsidence results from dissolution ofrocks beneath the land surface. Dissolution occurs aspercolating surface water or groundwater movesthrough rock that is easily dissolved. Some rock types,such as rock salt and rock gypsum, will dissolve directlyas fresh surface-water flows through the ground. Otherrock types, such as limestone and dolostone, will dis-solve if the percolating water is acidic. Overall, rock saltis approximately 7,500 times more soluble and rockgypsum 150 times more soluble than limestone.

Percolating water may become acidic when carbondioxide is dissolved in it.This acidification generally occursin the soil where carbon dioxide is produced by bacterialdecomposition.The respiration of most soil bacteria is likerespiration in humans—they take in oxygen and releasecarbon dioxide. Dissolving carbon dioxide in water pro-duces carbonic acid, the same weak acid that is present incarbonated soft drinks like Coke and Pepsi. Carbonic aciddissolves limestone more readily than dolostone; thus landunderlain by limestone is more susceptible to dissolutionand more likely to become karst topography.

Areas underlain by dense, thin-bedded, fractured,or well-jointed crystalline limestone are especially vul-nerable to dissolution. In such areas, surface waters areeasily diverted to subterranean routes along fracturesor to planar cracks between sedimentary layers. Wherethe percolating surface water has become acidic it

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enlarges fractures by dissolving rock (Figure 6.2). Suchdissolution results in empty spaces of various sizes,sometimes called voids, forming beneath the landsurface. Where the void space is relatively close to thesurface, pits known as sinkholes may develop.

Sinkholes may exist individually (Figure 6.3), ordevelop in large numbers to form a pockmarked surface

Introduction to Subsidence 175

known as a karst plain.The Mitchell Plateau in southernIndiana is an example a karst plain (Figure 6.4).

In addition to sinkholes, many karst areas have otherfeatures developed by the chemical weathering ofbedrock (Figure 6.2). In humid temperate climates, karstareas are characterized by beautiful rolling hills withalternating areas of subsidence and undisturbed land.These areas may be underlain by extensive cave systems ifthe voids excavated by dissolution are at sufficient depthbelow the surface. Cave openings can be the site of disap-pearing streams where surface water goes underground,or the place where groundwater comes out at the surfaceto form springs. In humid tropical regions, extensive disso-lution removes most of the soluble bedrock leavingbehind a landscape of steep hills known as tower karst.

Sinkholes In karst areas, sinkholes vary in size fromone to several hundred meters in diameter and canopen up extremely rapidly (Figure 6.5). There are twobasic types of sinkholes:

1. Solutional sinkholes

These pits form by dissolution on the top of a buriedbedrock surface. Dissolution occurs where the down-ward infiltration of acidic groundwater becomes con-centrated in holes created by joints and fractures. Inthe formation of these sinkholes, groundwater is typ-ically drawn into a cone above a hole in the lime-stone, like water being drawn into a sink drain.

2. Collapse sinkholes

These are the most common type of sinkhole. Theydevelop by the collapse of surface or near-surfacematerial into part of an underground cavern system.As subsidence features, these sinkholes can developinto spectacular collapse structures (Figure 6.5).

Some sinkholes open into a subterranean passage, allow-ing water to escape during a rainstorm. Most, however, arefilled with rubble that blocks any passage to the subsur-face. Blocked sinkholes usually fill up with water, formingsmall lakes. Most of these lakes eventually drain when thewater is able to filter through the debris. Similar drainageproblems can result when artificial ponds and lakes areconstructed over sinkholes (see Survivor Story 6.1).

Cave Systems As solutional pits enlarge and watermoves downward through limestone, a series of caves orlarger caverns may be produced. Mammoth Cave, Ken-tucky, and Carlsbad Caverns, New Mexico, are two ofthe famous caverns, or cave systems, in the UnitedStates. The primary mechanism for forming caves isgroundwater moving through rock. Cave systems tendto develop at or near the present groundwater tablewhere there is a continuous replenishment of water thatis not saturated with the weathering products of thelimestone. Many cave systems have passages and under-ground rooms on a number of levels. In these systems,each level may represent a different time of cave forma-

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FIGURE 6.3 SMALL SINKHOLE Not all sinkholes are largesubsidence features. Small collapse sinkholes, such as thisone in Boyle County, Kentucky, are common. (Kentucky Geolog-

ical Survey)▼

FIGURE 6.4 KARST TOPOGRAPHY Thisrolling landscape of the Mitchell Plateau insouthern Indiana is typical of karst topographyin a humid temperate climate. (Samuel S.

Frushour, Indiana Geological Survey)

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176 Chapter 6 Subsidence

FIGURE 6.5 LARGE COLLAPSE SINKHOLE Thissinkhole near Montevallo in central Alabama wasdubbed the “December Giant” after it measuredclose to 120 m (400 ft.) in diameter and 45 m(150 ft.) in depth. (U.S. Geological Survey)

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FIGURE 6.6 CAVE FORMATIONS Carlsbad Caverns, NewMexico, contains stalactites, which hang from the ceiling,stalagmites, which grow up from the ground, and flowstone,which forms as water flows slowly down the walls or across anincline. Recall that a stalactite hangs “tight” to the ceiling andstalagmite has a “g” because it “grows” up from the ground.(Bruce Roberts/Photo Researchers, Inc.)

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Flowstone

Stalagmite

Stalactite

Column

tion that is related to a fluctuating groundwater table.Fundamentally, caves are enlarged as groundwatermoves through limestone along fractures or alongplanes between sedimentary layers, eventually forminga cavern. Later, if the groundwater table moves to alower level, water seeping into the cave will deposit cal-cium carbonate on the sides, floor and ceiling. Thesedeposits form beautiful cave formations such asflowstone, stalagmites and stalactites (Figure 6.6).

Tower Karst Large, steep limestone “towers” risingabove the surrounding landscape are known as towerkarst. Most common in humid tropical regions, karst tow-ers are residual landforms of a highly eroded karst land-scape. Cuba and Puerto Rico exhibit tower karst, as doesmuch of Southeast Asia. The most spectacular examplescome from Guilin, China where they have been paintedby artists since the First Century (Figure 6.7).

Disappearing Streams Karst regions are oftenunderlain by a complex groundwater network thatoccasionally intersects with the land surface. In these

areas, surface streams may suddenly disappear into caveopenings. Such disappearing streams do not actuallydisappear; rather, they flow directly into the groundwa-ter system and continue along a subterranean route.

Springs Areas where groundwater naturally dis-charges at the land surface are known as springs. Mostsprings in karst areas are highly productive, especiallyduring rainy periods. Karst springs are an importantresource, but many are drying up as a result of over-pumping of groundwater.

ThermokarstSome karst terrains develop from processes other thanthe groundwater dissolution of rock. In polar regions, andat high altitude, much of the soil and underlying sedimentremain frozen throughout the year. This natural condi-tion, called permafrost, may be continuous throughoutthe soil or, in slightly warmer climates, may exist as dis-continuous patches or thin layers. Permafrost consists ofparticles of soil or sediment that remain cemented with

Introduction to Subsidence 177

SinkholesLake Chesterfield disappeared just as if someone had“pulled the plug”

Lake Chesterfield was a pleasant 23-acre manmade lake ina quiet suburb of St. Louis where residents could fish fromtheir small paddleboats. Until it disappeared. Residents ofthe Wildwood, Missouri, community say the entire lakedrained in the course of three days in early June 2004.

“It was like someone pulled the plug,” said DonnaRipp, who lives across the street from the site of lake,which became a giant mud hole (Figure 6.A). Ripp saidshe began to notice the water level sinking, and that bythe second day the lake was half empty. A day later itwas gone entirely.

The culprit is clear: at the north end of the lake liesa gaping sinkhole that Ripp’s husband, Eric, estimatedto be about 70 feet in diameter. What geologists are nowinvestigating is what the larger subterranean networklooks like.

This region of Missouri, which Ripp described as “abeautiful area where we still have a lot of deer,” is rifewith underground caves and cavities, including manythat are expansive enough for humans to explore. Thelarger network of cavities, which form when undergroundwater dissolves the limestone bedrock, is known as karsttopography.

Geologist David Taylor, who inspected the lake short-ly after the water drained into the ground, said the sink-hole itself actually consisted of two long chimneys andlooks deceptively large because of the large amount ofsilt at the bottom of the lake.

“It’s really not that big of a sinkhole,” he said. But itdoesn’t take a very large sinkhole to knock out an entire

lake. Taylor said a hole one foot in diameter can drain atleast a thousand gallons a minute.

Taylor is the head of a St. Charles-based companycalled Strata Services, Inc., that specializes in repairinglakes that are draining into subterranean cavities. “In mybusiness I have fixed hundreds of leaky lakes,” he said.

But before Taylor can consider repairing LakeChesterfield, he and his colleagues first must get a senseof the network of cavities under the lake—a task that hesaid is exceedingly difficult.

“There’s all kinds of crazy stuff going on downthere,” he said. “This is all subsurface work. It’s veryunpredictable and very difficult.”

Taylor found that the subsurface cavity responsiblefor the sinkhole under Lake Chesterfield runs laterallyunderground for several miles. A tracing dye placed nearthe sinkhole reemerges in a spring three and a half milesfrom the lake.

In order to develop a better picture of the cavities,Taylor drilled five test holes at 12-m (40-ft.) intervals,two of which revealed empty cavities below. But heestimated that it would require 600 holes in a 12-m(40-ft.) grid to even begin to understand the regioncompletely.

Once that picture emerges, Taylor’s company thenfills the cavities with a cement-like substance so thatother sinkholes don’t open and create a similar problem.“If we just put a Band-Aid over the hole and fill the lakeback up, it will happen again,” he said.

In the meantime, the residents of Wildwood are get-ting a crash-course lesson on karst topography. “I didn’teven know there were underground caves here until allthis happened,” Donna Ripp said.

—Chris Wilson

6.1 SURVIVOR STORY

FIGURE 6.A LAKE DRAINED BY SINKHOLEResidents of Wildwood, Missouri examine a mudflatthat was the site of Lake Chesterfield, a 23-acre man-made lake in suburban St. Louis, Missouri, thatdrained in three days when a sinkhole opened beneathit. An estimated 75 million gallons of water funneledthrough the sinkhole in the lake bottom.

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Drained lake floor

178 Chapter 6 Subsidence

Karst towers

FIGURE 6.7 TOWER KARST (a) Painting of Chinese tower karst, “Peach Gar-den Land of Immortals” by Qiu Ying (photo credit), and (b) tower karst south ofGuilin in South China’s Guangxi Zhuang Autonomous Region. (photo credit)

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ice for at least two years. Melting this frozen earth mater-ial can produce subsidence, especially if it contains alarge amount of ice. Land subsidence of several meters ormore is possible by the melting of permafrost.

If undisturbed by human activity, the thawing ofpermafrost is generally limited to the upper few metersof earth material. It thaws during summer months andthe thawed zone refreezes in the winter. However, moreextensive thawing of permafrost can produce an irregu-lar land surface known as thermokarst. Climatic warm-ing over the past four decades has thawed large areas ofArctic permafrost and formed thermokarst. In someareas the uppermost layers of permafrost are thawing atrates of close to 20 cm (8 in.) per year.

Sediment and Soil CompactionRapidly deposited fine sediment and organic-rich soil areboth susceptible to subsidence.This subsidence may occuras the sediment compacts or, in organic soils, as water isdrained from the soil. Compaction of sediment and soilcan occur naturally or as the result of human activities.

Fine Sediment Rapidly deposited fine sand and mudcontain a great deal of water-filled space between thesediment particles. With time the amount of water

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between the particles is reduced and the sediment com-pacts. Rapid deposition and compaction is especiallycommon in river deltas. In deltas, natural subsidence hasto be balanced by additional sedimentation to keep theland surface of the delta, called the delta plain, fromsinking below sea level. Episodic events such as floodsand earthquakes can cause deltaic sediment to remobi-lize and subside. Historically this subsidence has sub-merged coastal cities.

The natural tendency for deltas to subside can beamplified if sedimentation on the delta plain is slowedor stopped. Sedimentation has been sharply reduced onboth the Mississippi and Nile Deltas during the past 125years. On the Mississippi, sedimentation on the deltaplain was slowed and stopped by levees built on bothsides of the river by the U.S. Army Corps of Engineers.These levees have protected communities like NewOrleans from flooding, but they have also kept newsediment from being added to the delta plain. In thecase of the Nile Delta, construction of the Aswan damsupstream and the diversion of two thirds of the riverwater into canals have stopped sediment from reachingmuch of the delta plain. Part of the subsidence inboth the Mississippi and Nile Deltas is also from thecompaction of organic soils.

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(a) (b)

Introduction to Subsidence 179

Organic Soils Some wetland soils forming in marsh-es, bogs and swamps contain large amounts of organicmatter. Called organic soils, these earth materials con-sist of partially decayed leaves, stems, roots, and in cold-er regions, moss, which soak up water like a sponge.When water is drained from these soils they dry out,compact and are exposed to processes that cause themto disappear.

Bacterial decomposition is the primary processcausing subsidence in drained organic soil. This processconverts organic carbon compounds to carbon dioxidegas and water. Other destructive processes includewater and wind erosion and the combustion of peat inprescribed burns and wildfires. The decomposition, ero-sion, and burning of organic soils cause the irreversiblesubsidence of drained wetlands. Part of the subsidenceof the City of New Orleans can be attributed to com-paction of organic soil.8

One of the most dramatic examples of subsidencein organic soils has occurred in the Florida Everglades.Land drainage, primarily for agriculture and urbandevelopment, has combined with droughts to causeover half of the freshwater Everglades to subside from0.3 to 3 m (1 to 9 ft.) during the twentieth century.

Expansive SoilsChanges in moisture conditions can also produce subsi-dence in some clay-rich soils. Referred to as expansivesoils, these soils shrink significantly during dry periodsand expand or swell during wet periods. Most of theswelling is caused by the chemical attraction of watermolecules to the surface of very fine particles of clay(Figure 6.8a). Swelling can also be caused by thechemical attraction of water molecules to layers withinthe crystal structure of some clay minerals.

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SwellShrink

Beforeexpansion

Afterexpansion

Clay plates

Water

(c) (d)

(b)(a)

CracksDrying cracks

FIGURE 6.8 EXPANSIVE SOILS (a) Smectite clay expands as water molecules are added onto and within the clayparticles. (b) Effects of soil’s shrinking and swelling at a home site. (After Mathewson, C. C., and J. P. Castleberry, II. Expan-

sive soils: Their engineering geology. Texas A&M University) (c) Drying of an expansive soil produces this popcorn-like surfaceand a network of polygonal desiccation cracks. Pen in lower left for scale. (U.S. Geological Survey) (d) Shrinking andswelling of expansive soil cracked the concrete in this driveway. (Edward A. Keller)

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180 Chapter 6 Subsidence

The smectite group of clay minerals, including themineral montmorillonite, typically has the smallest claycrystals and thus in bulk has the greatest surface area toattract water molecules. Smectites are abundant inmany clay and shale deposits derived from the weather-ing of volcanic rock, and they are the primary mineral inbentonite, a rock that forms from the alteration of vol-canic ash. Clay, shale, and clay-rich soil containing smec-tite have the greatest potential for shrinking andswelling.

The presence of shrink-swell clays can often be rec-ognized from land surface features or structures. Thesefeatures include deep cracks produced by the drying ofthe soil (Figure 6.8c); a popcorn-like weathering textureon bare patches of clay (Figure 6.8c); an alternating pat-tern of small mounds and depressions in the land sur-face; a series of wavy bumps in asphalt pavement(Figure 6.9); the tilting and cracking of blocks of con-crete in sidewalks and foundations (Figure 6.8d); andthe random tilting of utility poles and gravestones(Figure 6.10).

Structural damage to houses and other buildings onexpansive soil is caused by volume changes in the soil.These volume changes are a response to changes in thesoil’s moisture content. Factors that affect the moisturecontent of an expansive soil include climate, vegetation,topography, and drainage. Regions that have a pro-nounced wet season followed by a dry season oftenhave shrink-swell soils. These regions, such as the south-western United States, are more likely to experience anexpansive soil problem than regions where precipita-tion is more evenly distributed throughout the year.

Vegetation can also cause changes in the moisturecontent of a soil. Large trees draw and use a lot of localsoil moisture, especially during a dry season. Thiswithdrawal of water may produce soil shrinkage(Figure 6.8b).

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EarthquakesAlthough we commonly think of earthquakes as associ-ated with uplift of the ground surface, they may alsocause subsidence. As mentioned in Chapter 2, the 1964Alaskan earthquake (M 9.2) caused extensive subsi-dence resulting in flooding of some communities. In thePacific Northwest, the geologic record contains evi-dence for repeated episodes of subsidence along thecoast of British Columbia, Washington, and Oregon.These subsidence episodes are believed to indicateepisodic great earthquakes along the Cascadia subduc-tion zone. The hypothesis proposed to explain this rela-tionship states that between great earthquakes, strainbuilds up along “locked” segments of the subductionzone. Through time this strain causes the western edgeof the North American tectonic plate to buckle. Thisbuckling drags the underwater seaward edge of the

FIGURE 6.9 DAMAGE FROMEXPANSIVE SOIL Uneven shrinkingand swelling of expansive clays inlayers of steeply dipping bedrockproduced the rolling surface of thisroad and sidewalk in Colorado.(David C. Noe, Colorado Geological

Survey)

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Depressionsin road andsidewalk

FIGURE 6.10 SURFACE MOVEMENT IN EXPANSIVE SOIL Repeatedshrinking and swelling of a clay-rich soil caused the tilting of thesegravestones in a cemetery in Manor, Texas. (Robert H. Blodgett)

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Regions at Risk for Subsidence Related Hazards 181

continent downward and produces an upward bulgealong the coast (Figure 6.11). When strain is releasedduring a great earthquake, the edge of the North Amer-ican plate bounces back to its undeformed position.Thisrealignment results in the underwater uplift of the pre-viously downwarped area and subsidence of the coastalbulge area. A similar cycle has been observed along asubduction zone in Japan.

Deflation of Magma ChambersSubsidence can also occur from volcanic activity. Asmagma moves upward into underground chambersbelow a volcano, the surface of the volcano may beforced upward. When it erupts, the volume of magma inthe underground chamber is reduced and the land ini-tially uplifted by the magma will subside. As mentionedin Chapter 3, cycles of uplift and subsidence are usefulin predicting volcanic eruptions.

6.2 Regions at Risk for Subsidence Related HazardsAs previously mentioned, dissolution of soluble rocksand permafrost, compaction of sediment and soil, andshrinkage of expansive soils are common causes of sub-sidence. In particular, the dissolution of limestone pro-duces distinctive karst topography. It is estimated thatkarst landscapes compose up to 10 percent of Earth’ssurface, and approximately 25 percent of the land sur-face in the United States is underlain by limestone(Figure 6.12). Major belts of karst topography in theUnited States include (1) a region extending throughthe states of Tennessee, Virginia, Maryland, and Penn-sylvania; (2) south-central Indiana and west-central

Kentucky; (3) the Salem-Springfield plateaus ofMissouri; (4) the Edwards Plateau of central Texas;(5) most of central Florida; and (6) Puerto Rico.Subsidence and other karst-related phenomena are amajor problem in these areas.

Permafrost covers more than 20 percent of theworld’s land surface. Most of Alaska and more thanhalf of Canada and Russia are underlain by permafrost.Hazard maps show that many towns and smaller settle-ments are threatened by thawing permafrost: Barrow,Alaska; Inuvik, Northwest Territories, Canada; andYakutsk, Norilsk, and Vorkuta in Russia.

Subsidence caused by the compaction of sedimentis most pronounced in areas where it was rapidlydeposited or where it contains abundant organic matter.These areas include many of the world’s marine deltas,such as those of the Mississippi River, the Sacramento-San Joaquin Rivers in California, and the Nile River inEgypt.

Organic-rich soils susceptible to subsidence arecommon in cold-region wetlands of the Upper Midwest,Washington State, Alaska, and Canada, where thewetlands are variously called “bogs,” “fens,” “moors,”and “muskeg.” Coastal wetlands underlain by peat ormuck deposits are also susceptible to subsidence in theFlorida Everglades, the Sacramento-San Joaquin Deltaof California, and coastal Louisiana and North Carolina.

In North America, expansive soils are a problemprimarily in the western United States and Canada.Their distribution results from a combination of geolo-gy and climate. Clay-rich earth materials in this regioncontain fine smectite clays that are especially prone toshrinking and swelling. Furthermore, marked seasonalchanges in precipitation produce alternating wet anddry conditions in the soil, which in turn, increases theamount of shrinking and swelling.

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Coastline

UpliftBetween Earthquakes

Locked

Coastline

Subsidence

Earthquake

Rupture

FIGURE 6.11 SUBSIDENCE CAUSED BY EARTHQUAKES A proposed model for the cycle of uplift and subsidenceassociated with subduction along the Pacific Northwest coast. The Juan de Fuca oceanic plate on the left is being sub-ducted below the North American continental plate on the right. (a) Between earthquakes, the two tectonic plates arelocked together along the subduction zone and elastic strain builds up. The seaward edge of the North American plate isthen dragged downward, causing the coastline to be uplifted. (b) When a great earthquake occurs, similar to the 2005M 9 earthquake in Indonesia, strain is released and the deformed seaward edge of the North American plate “snaps”back into place. This may cause uplift of the seafloor and subsidence along the coastline. (Modified after Geological Survey

of Canada, Sidney Subdivision figure on http://www.pgc.nrcan.gc.ca/geodyn/eq_cycle)

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(a) (b)

182 Chapter 6 Subsidence

Large-scale seismic-related subsidence is a risk forareas of the Pacific Northwest in both Canada and theUnited States, and deflation of a magma chamber cancause subsidence in any volcanic area.

6.3 Effects of SubsidenceTwo of the most common types of subsidence, karst andexpansive soils, cause significant economic damage eachyear. Karst regions are home to a host of problems suchas sinkhole collapse, groundwater pollution, and vari-able water supply. Subsidence in expansive soils oftendamages highways, buildings, pipelines, and other struc-tures.

Additional damage from subsidence occurs inmarine deltas, drained wetlands, and in many areasunderlain by permafrost.

Sinkhole CollapseSinkholes have caused considerable damage to high-ways, homes, sewage facilities, and other structures.Both natural and artificial fluctuations in the watertable are probably the trigger mechanism for sinkholecollapse. High groundwater table conditions enlargecaverns closer to the surface of Earth by dissolving theroof and side of the caves. As long as a cavern remainsfilled with water, the buoyancy of the water helps sup-port the weight of the overlying earth material. Lower-ing of the groundwater table eliminates some of thebuoyant support, and the cave roof may collapse. Thissituation was dramatically illustrated in Winter Park,Florida, on May 8, 1981, when a large collapse sinkholebegan developing.The sinkhole grew rapidly and within24 hours had swallowed a house, part of a communityswimming pool, half of a six-lane highway, parts of three

businesses, and parking lots containing several Porschesand a pick-up camper(Figure 6.13). Damage causedby this sinkhole exceeded $2 million.

Sinkholes form nearly every year in central Floridawhen the groundwater level is lowest. Although exactpositions cannot be predicted, more sinkholes form dur-ing droughts. The Winter Park sinkhole and severalsmaller sinkholes developed during the 1981 droughtwhen groundwater levels were at a record low.

In urban and some rural areas the evidence of asubsidence hazard is sometimes masked by humanactivities. For example, a sinkhole in the Lehigh Valleynear Allentown in eastern Pennsylvania, identified inthe 1940s by a 65-m (210 ft.)-diameter pond, was subse-quently filled with dead stumps, blocks of asphalt, andother trash. By 1969, the sinkhole had been completelyfilled and covered with a cornfield. Although no longerrecognizable as a sinkhole, the filled depression contin-ued to collect urban runoff from nearby pavement andbuildings. The runoff loosened the plug of soil and trashand most likely led to further dissolution of the underly-ing limestone. An increased demand for groundwater inthe area also contributed to a lowering of the ground-water table.

These factors combined to cause a catastrophic col-lapse of the land surface on June 23, 1986. Within only afew minutes, the collapse produced a pit approximately30 m (100 ft.) in diameter and 14 m (46 ft.) deep.Although the damage was confined to a street, parkinglots, sidewalks, sewer lines, water lines, and other utili-ties, the subsequent stabilization and repair costs werenearly one-half million dollars.

Groundwater ConditionsKarst development creates a geologic environment inwhich groundwater is intensively used by humans andwildlife and is also easily polluted. Sinkholes, caves, and

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FIGURE 6.12 KARST MAP Distributionof karst topography in the contiguousUnited States. Approximately 40 percent ofthe land in the eastern half of this map iskarst topography. Alaska, Puerto Rico,Hawaii, and Canadian provinces and terri-tories also have areas of karst. (After

White, W. B., D. C. Culver, J. S. Herman, T. C. Kane,

and J. E. Mylroie. 1995. Karst lands. American

Scientist 83:450–459)

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Effects of Subsidence 183

related karst features can form direct connectionsbetween surface water and groundwater (Figure 6.14).Such connections make the groundwater vulnerable topollution and to groundwater table fluctuation duringdroughts. These vulnerabilities are a major concern forboth public and private sources of drinking water.

One common source of pollution comes fromsinkholes that have been used for waste disposal,especially where the bottom of the depression is nearthe groundwater table. Groundwater can also be con-taminated where polluted water from surface streamsflows into caves and fractures. This water can reach

the groundwater table without natural filtration bysoil or sand.

Groundwater-table fluctuations in karst areas affecthumans, plants, and wildlife. For example, groundwaterfrom karst limestone is heavily used along the EdwardsPlateau in central Texas. In this area, frequent droughtand heavy groundwater use can rapidly lower thegroundwater table and cause springs to reduce or stoptheir flow. Diminished groundwater flow threatens com-munities that rely on it as a source of water, such as SanAntonio, and the unique plants and animals that arefound only in the springs.

FIGURE 6.13 SINKHOLE SWAL-LOWS PART OF TOWN This sinkholein Winter Park, Florida, grew rapidlyfor three days in 1981, swallowingpart of a community swimming poolas well as several businesses,houses, and vehicles. The sides ofthe sinkhole have since been stabi-lized and landscaped, and it is nowa park with a small lake. (Leif

Skoogfors/Woodfin Camp and Associates)

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Municipal swimming pool

CarsCamperpickup truck

4-lanestreet

FIGURE 6.14 WATER FROM A SUBTERRANEANSTREAM In karst topography, groundwater and sur-face water may be directly connected. This waterfalldevelops from Falling Spring northwest of West Union,Iowa. (Greg Brick)

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184 Chapter 6 Subsidence

Damage Caused by Melting PermafrostEarly settlers in the Arctic built their homes directly onpermafrost only to find that heat radiated from thebuildings and thawed the soil. By the middle of thetwentieth century most structures were built on pilingssunk into the permafrost. This technique elevated thefloor of the buildings above the land surface and kepttheir heat from melting the soil. However, this use ofpilings assumed that the permafrost would remainfrozen. For the past several decades that assumption hasnot always been the case.

Recent thawing of permafrost has caused roads tocave in, airport runways to fracture, and buildings tocrack, tilt, or collapse (Figure 6.15). In two Siberiancities alone, an estimated 300 apartment buildings havebeen damaged. The State of Alaska now spendsaround 4 percent of its annual budget repairing per-mafrost damage.

Coastal Flooding and Loss of WetlandsThe flooding of coastal areas and destruction of wetlandsare two major effects of subsidence in marine deltas andbays. Subsidence of the Mississippi Delta during the pastcentury has contributed to wetland loss and the sinkingof New Orleans. Marshes on the delta plain are beingsubmerged at a rate of 65 to peryear. This area of submergence is about the size of Man-hattan. These wetlands protect the city and its suburbsfrom ocean waves and storms. Without intervention, themarsh will disappear by 2090 and New Orleans will bedirectly on the sea. Much of New Orleans is now near orbelow the level of both the Gulf of Mexico and adjacentLake Pontchartrain. Only a ring of levees surroundingthe city keeps it from being flooded by the river, lake, andGulf of Mexico. New Orleans is literally a “disaster wait-ing to happen.”

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In September 2004, Hurricane Ivan nearly madethis disaster a reality. The storm fluctuated between aCategory 4 and Category 5 hurricane as it headednorth through the Gulf of Mexico. Forecasters predict-ed that there was a one-in-four chance that it woulddirectly hit New Orleans. Government officials orderedthe evacuation of the metropolitan area of 1.2 millionpeople. However, after days only an estimated600,000 people had left. If Hurricane Ivan had hitNew Orleans those who stayed would have witnessed a5-m (17-ft.)-high surge of water into Lake Pontchar-train. This surge would have topped New Orleans’ lev-ees and flooded the city with water up to 6 m (20 ft.)deep. Ivan would have also flooded the inhabitedareas on the delta near the city and suburbs along thelake. In places the storm surge would have pushedwater as much as seven miles inland. An estimated40,000 to 60,000 people in the metropolitan area wouldhave perished.

Damage Caused by Expansive SoilsExpansive soils cause significant environmental prob-lems. As one of our most costly natural hazards,expansive soils are responsible for several billions ofdollars in damages annually to highways, buildings,and other structures. In many years this cost exceedsthe cost for all other natural hazards combined. Everyyear more than 250,000 new houses are constructedon expansive soils. Of these, about 60 percent willexperience some minor damage, such as cracks in thefoundation, walls, driveway, or walkway, and 10 per-cent will be seriously damaged, some beyond repair(Figures 6.98d and 6.9). Underground water linesin expansive soils may rupture when there is a signifi-cant change in soil moisture. This rupture can result ina loss of water pressure and require customers to boiltheir water before using it.

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FIGURE 6.15 MELTING PERMAFROST DESTROYSBUILDING The foundation of this apartment buildingin Cherskii in eastern Siberia was undercut by thawingpermafrost. Structural damage from melting per-mafrost is becoming common in Russia, Alaska, andCanada. (Reproduced with permission from Goldman, E.

2002. Even in the high Arctic nothing is permanent. Science

297:1493–1494)

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Natural Service Functions of Subsidence 185

6.4 Linkages betweenSubsidence and Other Natural HazardsAs mentioned previously, subsidence can be a sideeffect of earthquakes, volcanoes, and climate change.However, subsidence may also cause other natural haz-ards to occur. As described above for the MississippiDelta, the link between subsidence and flooding is acommon one.

In areas undergoing rapid subsidence, especiallythose that are also subsiding because of the overpump-ing of groundwater, flooding can be a major problem.As the land is lowered relative to surrounding bodies ofwater, the incidence of flooding increases (see openingCase History). In many growing cities, there is a highdemand for clean drinking water. Unfortunately, thisdemand leads to the mining of groundwater, oftendepleting the resource faster than it can be replenished.Because of groundwater mining, coastal and river floodshave become much more common and severe in low-lying coastal cities such as Bangkok, Thailand.

Subsidence also has direct linkages to climatechange. In arid areas, drought conditions commonlylower the groundwater table. The withdrawal of ground-water can further compact and shrink unconsolidatedearth materials. In the desert of the southwestern

United States, the drying of earth materials is contribut-ing to regional subsidence and to the formation of large,polygonal desiccation cracks (Figure 6.16). These cracksare similar in shape to the cracks that you see in mudafter it has dried out. Except for their polygonal form,the large desiccation cracks are the same size and depthas the large linear cracks, called earth fissures, which areproduced by the overpumping of groundwater.

As discussed above, global warming is the primarycause of the melting of permafrost in the Arctic. It isalso the cause of the increased rate of sea-level rise. Incoastal areas, such as Venice, New Orleans, GalvestonBay near Houston, and the Nile Delta, subsidenceincreases the local rate of sea level rise and the resultingloss of land.

6.5 Natural Service Functionsof SubsidenceAlthough subsidence can cause many environmentaland economic problems, there are also benefits fromsome of the processes that cause subsidence, especiallykarst processes. Karst terrains are some of the world’smost productive sources of drinking water. Beautifulkarst formations such as cavern systems and tower karstare important aesthetic and scientific resources (Figure6.6). Lastly, the caves of karst areas are home to rare,specially adapted creatures, some of which are foundnowhere else. For example, caverns and other karst fea-tures in the Edwards Limestone in Texas are home toover forty unique species, eight of which are legally des-ignated as endangered (Figure 6.17). In fact, karstregions provide so many benefits that the Karst WatersInstitute maintains a “Top Ten List” of endangeredkarst ecosystems.

Water SupplyKarst regions contain the world’s most abundantgroundwater supply, thus providing a critical worldresource. About twenty-five percent of the world’s pop-ulation gets its drinking water from karst formations,and forty percent of the U.S. population relies on waterfrom karst terrains. For example, the Edwards Forma-tion, discussed above, provides drinking water for over2 million people.

Aesthetic and Scientific ResourcesAnyone who has visited a karst region knows that theyprovide an important aesthetic resource. Rolling hills,extensive cave systems, and beautiful formations oftower karst are among the landscape features found inkarst areas. Unique landscapes such as the tower karstregions of China are areas of unparalleled beauty thatoffer stunning vistas. Caves, too, have proved to be apopular destination for both spelunkers and tourists.

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FIGURE 6.16 LARGE DESICCATION CRACK This crack is part ofa polygonal network of large desiccation (drying) cracks in GrahamCounty, southeastern Arizona. Unlike similar earth fissures that resultfrom overpumping, these cracks appear to be the result of the naturallowering of the groundwater table in a drought. (Raymond C. Harris,

Arizona Geological Survey)

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186 Chapter 6 Subsidence

Mammoth Cave National Park, Kentucky, containsthe world’s longest cave system and attracts droves ofvisitors each year. Aesthetics aside, karst regions alsoprovide scientists with a natural laboratory in which tostudy the record of climate change contained in caveformations. The caves that form in karst regions alsoprovide an ideal environment for preserving animalremains, making them important resources for paleon-tologists and archaeologists.

Unique EcosystemsCaves are home to rare creatures, specially adapted tolive in the karst environment. Karst-dependent speciesknown as troglobites have evolved to live in the totaldarkness of caves. Such species include eyeless fish,shrimp, and salamanders; flatworms, and beetles (Figure6.17). Other species, such as bats, rely on caves to pro-vide shelter. Karst areas generally support a diversecross section of species; preserving these areas, andhence these organisms, for the future should be anobjective.

6.6 Human Interaction with SubsidenceAs discussed above, subsidence can both cause prob-lems and provide benefits. Commonly, when humanbeings live in areas underlain by karst, compacting sedi-ment and soil, permafrost, or expansive soil, previouslyexisting problems are exacerbated and new problemsarise. Human beings contribute to problems associated

with subsidence by withdrawing subsurface fluids,excavating underground mines, thawing frozen ground,restricting deltaic sedimentation, draining organic soils,and using poor landscaping practices on expansive soils.

Withdrawal of FluidsThe withdrawal of subsurface fluids, such as oil withassociated gas and water, groundwater, and mixtures ofsteam and water for geothermal power, have all causedsubsidence. In each case the high fluid pressure in thesediment or rock helped support the earth materialabove. For this reason a large rock at the bottom of aswimming pool seems lighter: buoyancy produced bythe liquid tends to lift the rock. Support or buoyancy byfluid pressure can be especially important in shallow orrapidly deposited earth material. In these instancespumping out the fluid reduces support and causes sur-face subsidence.

A classic example can be found in the centralvalley of California where thousands of square kilo-meters have subsided from overpumping groundwa-ter for irrigation and other uses (Figure 6.18a). Oneof the areas of greatest subsidence has been theLos Banos–Kettleman City area where more than

of land has subsided more than0.3 m (1 ft.), and the maximum subsidence has beenabout 9 m (30 ft.) (Figure 6.18b). As the water wasmined, fluid pressure was reduced, sedimentary grainscompacted and the surface subsided (Figure 6.19).Similar examples of subsidence caused by overpump-ing have been documented near Phoenix, Arizona;Las Vegas, Nevada; the Houston–Galveston areain Texas; San Jose, California; and Mexico City,Mexico. Such subsidence has reactivated geologic

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FIGURE 6.17 CAVES ARE UNIQUEECOSYSTEMS Unique species, such asthis endangered Texas blind salaman-der from Ezell’s Cave National NaturalLandmark in central Texas, haveevolved to live in the total darkness ofcaves. (Robert & Linda Mitchell)

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Human Interaction with Subsidence 187

faults and formed extremely long earth fissures inunconsolidated sediment (Figure 6.20).

Underground MiningSerious subsidence events have been associated withthe underground mining of coal and salt. Most subsi-dence in coal mining is caused by the failure of pillars ofcoal which have been left behind to support the mineroof. With time these pillars weather, weaken, and col-lapse. Then the roof caves in and the land surface abovethe mine subsides (see Professional Profile 6.2). In theUnited States more than of land,an area twice the size of Rhode Island, has subsidedbecause of underground coal mining. This subsidencecontinues today, long after mining terminated. In 1995, acoal mine that was last operated in the 1930s collapsedbeneath a 600-m (2000-ft.) length of I-70 in Ohio;repairs of the highway took 3 months. Although coal-mine subsidence most often affects farmland and range-land, it has also damaged buildings and other structuresin urban areas such as Scranton, Wilkes-Barre, andPittsburgh, Pennsylvania; Youngstown, Ohio; andFarmington, West Virginia.

In the case of salt, subsidence has taken place overboth solution and open-shaft mines. Solution mines, thesource for most of our table salt, use wells to inject fresh

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20water into salt deposits.The dissolved salt is then pumpedout of the well, leaving a cavity behind. Collapse of thiscavity and subsequent surface subsidence has taken placein solution mines in Kansas, Michigan, and Texas.

Open-shaft mining is used for extracting rock saltfrom the Earth and is taking place underground belowDetroit, Michigan, and Cleveland, Ohio. In the lastthree decades the catastrophic flooding of two under-ground salt mines has caused surface damage and subsi-dence. One, the Retsof Mine near Geneseo, New York,was the largest salt mine in the world. Collapse of itsroof in 1994 allowed groundwater to flood the mine.Flooding produced two large sinkholes and subsidencedamage to roads, utilities and buidings.

The second flooding event occurred on the Jeffer-son Island salt dome in southern Louisiana. In 1980, anoil rig drilling for natural gas accidentally penetrated anunderground salt mine (Figure 6.21). The rig wasmounted on a floating barge in a small lake above thesalt dome. After drilling into the mine shaft, the rig top-pled over into the lake and disappeared with the tor-rents of water flooding the mine. Fortunately 50 minersand 7 people on the drilling rig escaped injury.The minewas a total loss, and buildings and gardens were dam-aged by subsidence on Jefferson Island. Within threehours the entire lake had drained and a 90 m (300 ft.)-deep and 0.8 km. (0.5 mi.)-wide subsidence crater

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Areas of major subsidence

Central Valley

Areas of lesser subsidenceCALIFORN

IA

SANFRANCISCO

Santa ClaraValley area

Los Banos–KettlemanCity area

PACIFIC OCEAN

FRESNO

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Land level 1975

Land level 1963Am

ount

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FIGURE 6.18 LAND SUBSIDENCE FROM GROUNDWATER EXTRACTION(a) Principal areas of land subsidence in California resulting from ground-water withdrawal. (After Bull, W. B. 1973. Geological Society of America Bulletin

84. Reprinted by permission.) (b) Photograph illustrating the amount of sub-sidence in the San Joaquin Valley, California. The marks on the telephonepole are the positions of the ground surface in recent decades. The photoshows nearly 8m (26 ft.) of subsidence. (Courtesy of Ray Kenny)

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188 Chapter 6 Subsidence

formed above the flooded mine. The structural integrityof salt mines is of particular concern because the U.S.Strategic Petroleum Reserve is stored in four GulfCoast salt mines. A fifth storage site below WeeksIsland, Louisiana, was emptied of oil in 1999 because ofgroundwater seepage through a sinkhole.

Melting PermafrostHumans have contributed to the thawing of permafrostthrough global warming and poor building practices.Shoddy construction practices, inadequate removal ofheat from beneath buildings, and burial of warm utility

FIGURE 6.19 PROCESS OF SUBSIDENCE Idealized diagram showing how surface subsidence results from pumpinggroundwater. The vadose zone is the area above the groundwater table where the holes (pores) between grains containboth air and water. The zone of saturation lies beneath the groundwater table. Pores in the zone of saturation are com-pletely filled with water. When groundwater is pumped out, the pores collapse and the surrounding earth material com-pacts. This compaction can cause the land surface to subside. (From Kenny, R. 1992. Fissures. Earth 2(3):34–41)

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Water table

Water table

Water-saturatedsediment

Compacted sediment

Vadose zone

Vadose zone

Zone of saturation

Zone of saturation

Bedrock

Bedrock

Ground surface

Ground surface Earth fissure

Ground level drops from subsidence

Human Interaction with Subsidence 189

lines have locally melted permafrost, broken pipelines,and damaged buildings.

Restricting Deltaic SedimentationMarine deltas require the continual addition of sedi-ment to their surface to remain at or above sea level.This sediment comes from the distributary channelsthat carry river water, sand, and mud to an ocean.Humans have stopped or reduced this sedimentation bythe construction of dams upstream on a river, by build-ing levees on both sides of distributary channels, and bydiverting sediment-laden river water into canals. All ofthese practices may contribute to subsidence of thedelta plain.

Draining Organic SoilsPeople have drained organic soils for agriculture andsettlement for centuries. Most of the western Nether-lands were drained for agriculture between the ninethand fourteenth centuries. In the United States, drain-ing organic soils has caused or increased subsidence inthe Florida Everglades and the Sacramento-SanJoaquin Delta.

Landscaping Expansive SoilsThe shrinking and swelling of expansive soils is oftenamplified by poor landscaping practices. Planting treesand large shrubs close to foundations may causedamage from soil shrinkage during dry periods as plant

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FIGURE 6.20 EARTH FISSURES DAMAGE ROAD This caution signwas erected after an earth fissure damaged this road in rural PimaCounty, Arizona, in 1981. Earth fissures have developed in the desert ofthe southwestern U.S. where overpumping has lowered the groundwatertable. (S. R. Anderson, U.S. Geological Survey)

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Lake Peigneur

Salt mine

Salt mine

Salt dome430 m

?

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Airshaft

Drill hole JEFFERSON ISLANDPr

oduc

tion

shaf

t

New Orleans

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Jefferson Islandsalt dome

Subsidence pit

Lake Peigneur

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FIGURE 6.21 SUBSIDENCE FROM SALT DISSOLUTION (a) Location of Lake Peigneur, Louisiana; and (b) idealized dia-gram showing the Jefferson Island Salt Dome collapse that caused a large subsidence pit to form in the bottom of the lake.An estimated of water flooded the salt mine after the natural gas well penetrated themine shaft.

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190 Chapter 6 Subsidence

Helen Delano, EnvironmentalGeologistFor anyone, looking to build a house in Pennsylvania,there are any number of natural hazards that he or shehas to consider. “We have something for everyone,” saysHelen Delano, from the Pennsylvania Bureau of Topo-graphic and Geologic Survey. “As a geologist working forthe State of Pennsylvania, one of our tasks is looking atwhere hazards may occur.”

While some of those dangers include deadly gases,such as methane and radon, which can migrate up fromburied organic or radioactive material, respectively, oneof the more common problems is the potential for sink-holes and subsidence, particularly given the many aban-doned coal mines in the area. Delano said thesouthwestern region of the state has been mined for coalsince the mid-1700s, and that the shallower mines areonly 50 to 100 feet below the surface. While the tech-nology does exist to empirically test an area, Delano saida lot of information can be gained simply by consultingcoal mining maps.

But when the maps do not allow for a complete pic-ture, relatively simple and inexpensive technology canhelp geologists determine whether there are any major

6.2 PROFESSIONAL PROFILE

FIGURE 6.B FIELD WORK IS REQUIRED FOR NATURAL HAZARD ASSESS-MENTS Environmental geologist Helen Delano is the Pennsylvania Geologi-cal Survey contact for local governments in southeastern Pennsylvania. Hereshe examines an exposure of the Gettysburg Formation in her area ofresponsibility. (Photo by James Shaulis, Pennsylvania Geological Survey)

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20p x 20p

roots pull moisture from the soil. At the otherextreme, planting a garden or grass that needs frequentwatering close to foundations can cause damage fromsoil swelling. Rather than keeping the soil at a constantmoisture level, irrigation systems commonly leaveexcess water in the soil. Excess water is considered themost significant cause of damage from swelling soil.

6.7 Minimizing the SubsidenceHazardMinimizing the hazards from and related to subsidencerequires an understanding of the landscape from a geo-logic perspective. Even with this understanding it is dif-ficult to prevent natural subsidence. However, there aresteps that can be taken to minimize the damage associ-ated with this hazard.

Artificial Fluid Withdrawal We will always beplagued with subsidence problems in areas where

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25 bedrock is aggressively being dissolved or wheregroundwater levels are continuing to fall because ofdrought. Natural sinkholes will continue to open uprepeatedly. We can, however, prevent some human-caused subsidence associated with the mining ofgroundwater or pumping of oil and gas.

Groundwater mining occurs where the amount ofwater coming to the surface in wells and springsexceeds the volume that is being replenished throughthe percolation of rain and surface water. Wheregroundwater, oil, or gas removal is causing the land sur-face to subside, it is possible to prevent or minimizefurther subsidence.

For example, from the early 1900s to the mid-1970sgroundwater mining in the Houston-Galveston area ofTexas was the primary cause for up to 3 m (10 ft.) ofsubsidence over an area. Thisprompted the 1975 Texas Legislature to create a regula-tory district to issue well permits. Since creation of thisdistrict, subsidence has essentially stopped in areaswhere groundwater pumping has decreased. This hasnot been the case in parts of Florida, Arizona, and

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Minimizing the Subsidence Hazard 191

not radiate into the ground. With the recent wide-spread thawing of permafrost, new and more costlypractices are being developed, such as puttingbuildings on adjustable screw jacks or lattice-likefoundations to allow for the freezing and thawing ofpermafrost.

Reducing Damage From Deltaic SubsidenceCompletely stopping further subsidence of humansettlements on delta plains and restoring the deltas tonatural conditions is unrealistic. Levees must continueto be elevated to protect urbanized areas and adequatepumping systems must be maintained to remove excesssurface water from the levee-protected enclosures.However, in undeveloped areas, levees could bebreached to restore the sediment and freshwater supplynecessary to rebuild marshes. The restored marsheswould again help protect subsided urban areas fromstorms and rising sea level.

Stopping Drainage of Organic Soils Like mostgroundwater-induced subsidence, restoration of organicsoils is not possible. Only proper water management of

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Nevada, where increasing use of groundwater continuesto cause sinkholes or earth fissures.

Installation of injection wells is often suggested as away to minimize or stop subsidence from fluid with-drawal. This technique was used with some success inthe 1950s where water was injected at the same timethat oil was being pumped from the Long Beach, Cali-fornia, oil field. However, this method does not work formost groundwater mining because irreplaceable waterhas been the extracted from layers of fine earth materi-al. Once fine earth material is compacted it is not possi-ble to push the particles back apart.

Regulating Mining The best way to prevent damagefrom subsidence caused by mining activities is toprevent mining in urban areas. Although such laws arecurrently in place in many countries, the threat fromolder mines still exists.

Prevention of Damage From Thawing PermafrostMost existing engineering practices for building onpermafrost have assumed that the permafrost wouldremain frozen if heat from a building or pipeline did

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cavities in the ground. Delano said geologists sometimesuse what's called a “television borehole camera,” inwhich they literally drill a hole and lower a small camerainto the ground to examine the underground contents.This technology, however, only is available for relativelyshallow depths. Delano said geophysical methods fordeeper depths are far more expensive.

In addition to old coal mines, shallow caves, whichhave the potential to form sinkholes, can be detectedwith ground-penetrating radar—useful information foranyone considering a construction project in severalparts of the state. “In eastern Pennsylvania we get sink-holes from limestone subsidence,” Delano said.

Delano began her career examining trouble from nat-ural hazards in the Pittsburgh area. “When I first startedworking I would go out and look at people's backyardproblems and help them understand what was going onand collect data for our use, trying to keep track of thescope of the problems for the state,” she said.

Now she deals also with municipalities, advisingthem on when natural hazards, including subsidence,pose hazards to building projects.

“I have a folder on my desk right now for a ruralcommunity that has a proposal for a housing projectover eighteenth- and nineteenth-century iron mines,”Delano said.

When a major construction project is slated to bebuilt over a region where subsidence is a risk, Delanosaid there are several options. “Usually the cheapestthing to do is build your house somewhere else,” shesaid. But when the location is just too good to pass up,there are engineering solutions, such as digging a deepfoundation in firm bedrock.

The importance, Delano said, is in knowing the risksahead of time. “It's much, much cheaper to factor inthose costs at the beginning,” she said. But for the mostpart, a close study of geological hazards, such as subsi-dence, is not yet routine in Pennsylvania when it comesto small construction projects.

“Most residential construction doesn't take intoaccount geological hazards,” Delano said, adding that,as more and more of the valuable land is occupied, it willbecome increasingly necessary to consider such dangers.

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192 Chapter 6 Subsidence

permafrost and organic soils. The best we can do is toidentify high-risk areas in which construction should beprohibited or limited. Unfortunately, in areas of naturalsubsidence, such as karst regions, subsidence may be dif-ficult or impossible to predict.The groundwater system isconstantly altering the subsurface rock, and sinkholes arecommon. There are, however, some methods that canhelp identify areas of potential subsidence.

Geologic Mapping Detailed geologic maps can bemade to identify as accurately as possible the hazardspresent. An understanding of the geology, coupled withthe surface and groundwater systems in an area, willgreatly aid in subsidence prediction and avoidance.

Surface Features In areas underlain by limestone,surface features such as cracking of the land surfaceshould be noted. The appearance of cracks in theground may indicate that sinkhole collapse is imminent,and appropriate steps should be taken to avoid damageand injury. In the western United States, cracks in theground may indicate expansive soils or, in some desertareas, the falling of the groundwater table.

Subsurface Surveys When planners must makedecisions about where to build structures in karstregions, knowledge of the subsurface environment iscritical. Subsurface exploration with ground penetrat-ing radar (GPR) and drilling bore holes to examinethe subsurface geology is often desirable before con-struction begins. These techniques may help preventstructures from being built above shallow caves. Addi-tional geologic surveys may be needed to evaluatehigh-risk areas encountered during construction. Inareas of expansive soils or permafrost, geotechnicalborings and soil testing may be needed to properlydesign foundations.

Some states, such as Colorado, require disclosure ofthe presence of expansive soils when houses are sold.Disclosure requirements apply to new home builders,homeowners, and real estate brokers. Homeownerswho live in areas where subsidence hazards are presentshould check the hazard coverage in their insurancepolicies. For example, in many areas neither sinkholesnor mine subsidence are covered in standard homeown-ers’ policies and require additional coverage.

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existing drainage to marshes and swamps will minimizefuture subsidence from organic soils.

Prevention of Damage From Expansive SoilsProper design of subsurface drains, rain gutters, andfoundations can minimize the damage from expansivesoil. These techniques improve drainage and allow thefoundation to accommodate some shrinking andswelling of the soil. Another preventive method is toconstruct buildings on a layer of compacted fill that actsas a barrier between the structure above and the expan-sive soil below. This method helps control the moisturelevel in the soil and provides a stable base upon whichto build. For larger buildings, roads, and airports it maybe cost-effective to excavate and redeposit the upperpart of an expansive soil or to mix in quicklime to bindsoil particles together.

6.8 Perception of and Adjustment to the Subsidence Hazard

Perception of the Subsidence HazardSubsidence is a natural hazard that gets very littlemedia coverage. Few people living in the United Statesare concerned about subsidence hazards. However,people living in areas directly affected by subsidence,such as those in areas of expansive soils, permafrost, orrapid groundwater withdrawal, are more likely tounderstand the hazard. Furthermore, people living inregions where sinkholes commonly develop are gener-ally familiar with the hazard and perceive it to pose areal risk to property.

Adjustment to the Subsidence HazardThe most appropriate adjustment to the subsidence haz-ard is to avoid building in subsidence-prone regions.Clearly this is not always possible, because a significantportion of the eastern United States is underlain by karst,large areas of the western United States are underlain byswelling soil, and much of Alaska and Canada contain

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