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ARIZONA GEOLOGICAL SuRvEY

The Arizona Geological Survey (AZGS) became a stand-alone State agency onJuly 1, 1988, in accordance with Senate Bifi 1102, which was enacted in 1987. Thepurpose o the AZGS is to encourage and. assist stewardship of lands and mm-eral resources in Arizona by conducting scientific, and investigative research andproviding geologic information. Responsibility for regulating the drilling andproduction of oil, gas, geothermal resources, and helium was assigned to theAZGS on July 1, 1991.

The Office of the Territorial Geologist was established by the Territorial Legislature in 1881. Its primary duties were to collect and provide information aboutmineral resources. :From 1893 until statehood in 1912, Territorial Geologists wereaffiliated with the University of Arizona and its mineral-testing laboratory, knowninformally as the “Bureau of Mines.” A 1915 statute created the Arizona Bureauof Mines as a State agency administered by the University of Arizon.a, continuing, ësseiitially unchanged, the functions of the Territorial Geologist and “Bureauof Mines.” Data collection and research activities continued to be concentrated on.mineral resources. In 1977, the agency’s enabling legislation was modernized andits name was changed to the Arizona Bureau of Geology and Mineral Technology. It continued to be administered by the University of Arizona. The agencywas charged with investigating geologic hazards and limitations, as well as thegeologic framework and mineral resources of Arizona, in anticipation of population growth and increased competition for and conflict over land, water, mineral, and energy resources.

• AZGS geologists prepare geologic maps of Arizona; investigate the State’sgeologic framework; conduct research on Arizona’s geologic hazards and limitations, as well as its mineral and energy resources; compile data; and maintain a

• geologic library and a repository of rock cuttings and cores. AZGS geologistsregularly conduct cooperative projects with Federal, State, and local agencies and

• work closely with university faculty and graduate students on projects withinArizona. Advisory committees for environmental and engineering geology, mineral resources, and earth science edu1ation provide program guidance.

The Arizona Geologic Information System (AGIS) includes several databases:AZGS library holdings. AZMIN, which contains mining production data, minenames, and mineral-related references; AZAGE, a compilation of radiometric age

• determinations; and AZGEOBIB, a comprehensive bibliography of more than• 111000 references on Arizonageology. The AZGS publishs maps, reports, and-

Arizona Geology, a quarterly newsletter. A list of available publications may beobtained from the AZGS at the address listed below. The AZGS library is open

- to the pu.blic during normal working hours.•.

‘

To obtain copies of this publication,contact .the Arizona Geological Survey,845 N. Park Ave., Suite 100, Tucson, AZ 85719-4816; phone: (602) 882-4795.

——

-

LiD SUBSIDENCE

EAJuI-I FIssuRIs

iN ARIZONA

Steven Slaff

ARIZONA GEOLOGICAL SURVEYDown-to-Earth Series 3

1993

Copyright © 1993 Arizona Geological Survey

Permission is granted for individuals to make single copies for their personal use inresearch, study, or teaching, and to use short quotations or figures from this book forpublication in other books and journals, provided that the source of the h’iformafion isappropriately cited. This consent does not extend to other kinds of copying for generaldistribution, for advertising or promotional purposes, for creating new collective works,or for resale. The reproduction of multiple copies and the use of extracts for commercial purposes require specific permission from the Arizona Geological Survey.

Published byArizona Geological Survey845 N. Park Ave., Suite 100

Tucson, AZ 85719-4816(602) 882-4795

500CP893AZGS 1279

Printed on recycled paper

CATHY SCHULIEN

1956-1988

WELLENDORF

Dedicated to the memory of

PREFACE

_______

Land subsidence and earth fissuringhave occurred in large portions of southern Arizona, where they have caused avariety of structural damage and land-management problems. As Arizona’spopulation continues to increase, so wifithe demand for ground water. Subsidence and earth fissuring wifi continue,extend into new areas, and create additional problems. The purpose of thisreport is to describe, in terms that areunderstandable to persons who are nottrained in geology, what land subsidenceand earth fissures are, why they develop,where they occur, what kinds of problems they create, and what can be doneabout them.

This report is dedicated to thememory of Cathy Schulten Wellendorf.Cathy developed a deep interest in applied geology while working on abachelor’s degree in geology at the University of Dayton. She continued hereducation in applied geology by completing a master’s thesis, “EnvironmentalGeology of the Tempe Quadrangle, Mancopa County, Arizona,” at Arizona StateUniversity. Cathy subsequently was acoauthor of a report with the same titlethat was published by the Arizona Geological Survey as Geologic InvestigationFolio GI-2.

In 1980, Cathy Schulten Wellendorf began workingat the U.S. Bureau of Reclamation’s Arizona Projects Office, where sheplanned and conducted numerous applied geologic investigations, includingstudies of earth fissures in the ApacheJunction and Picacho areas. In addition,she served as lead geologist for theStewart Mountain Dam ModificationProject from 1984 until her death in 1988.In her memory, Cathy’s family established the Cathy Wellendorf MemorialFund with the Arizona Geological Survey. The fund is used to support engineering and environmental geologyprojects and activities. This report is dedicated to Cathy’s memory because of herexperience and strong interest in appliedgeology, including land subsidence andearth fissures.

Steven Slaff began working at theArizona Geological Survey in 1988 to investigate earth fissures in south-centralArizona. I asked him to prepare this report because of the understanding of earthfissures that he has developed. The CathyWellendorf Memorial Fund provided partof the financial support that was neededto prepare and publish this report.

Larry D. FellowsDirector and State GeologistApril 1993

ACKNOWLEDGMENTS

Preparation of this report was facifitated by a grant from the Cathy S. Wellendorf fund, administered by theArizona Geological Survey. I am honoredto be the first recipient of research support from this fund, which commemorates the work and interests of Ms.Wellendorf.

Assistance from several individualsenhanced this report. Discussions withDon Pool and Mike Carpenter of the U.S.Geological Survey and Don Helm of theNevada Bureau of Mines and Geologyclarified certain aspects of compaction,subsidence, and earth-fissure formation.Paul Lindberg, consulting geological engineer, provided a copy of his report on

Devil’s Kitchen sinkhole. Herb Schu- mann ofthe U.S. Geological Survey andRob Genualdi of the Arizona Departmentof Water Resources provided photographsthat were used as figures. Larry Fellowsand Phil Pearthree of the Arizona Geological Survey critically reviewed themanuscript. Evelyn VandenDolder andEmily Creigh carefully edited the text;Evelyn also designed the layout. PeteCorrao skillfully drafted the figures anddesigned the cover. I sincerely appreciatethe contributions of each individual.

Steven Slaff

COES

What Is Su5sence.1

Which Areas in Arizona Are Su5siding . 2

What Is Pumping Subsidence7 4

Where Does Pumping Subsidence Occur7 6

What Are the Effects of Pumping Subsidence7 $

Effects on Structures 8

Effects on Natural Systems 9

Can Pumping Subsidence Be Stopped7 10

What Is an Earth Fissure? 11

What Is the Life Cycle of Earth Fissures? 13

Where Do Earth Fissures Develop? 16

What Are the Effects of Earth Fissures7 17

Effects on Structures 17

Effects on Natural Systems 1$

Can Earth-Fissure Formation Be Stopped? 19

How Can the Hazards of Subsidence

and Earth Fissures Be Reduced7 20

Conclusion 22

Selected References 22

INTRODUCTIONDuring a recent summer thunder

storm in a rural part of Pinal County, aman heard a loud rambling that he couldnot identify. He walked in the directionof the sound, from his house to a smallartificial pond nearby. The pond wasusually dry, but that night it was full ofwater from previous storms. In the dimlight the man saw a crack in the groundat the edge of the pond. The roar he hadheard came from water pouring into thecrack. As he watched, the crack grew, extending across the pond toward him andthe back porch of his house. In less thana minute, more than 100,000 gallons ofwater disappeared into the crack, emptying the pond. At the same moment, thecrack quickly lengthened and damagedthe man’s house and driveway.

This man was one of very few peopleto witness the formation of an earth fissure. Earth fissures, like the one picturedon the cover of this report, are related toland subsidence. Both phenomena are examined in the following pages.

The Earth’s surface may seem stableand unchanging, but it is actually subject

Subsidence is the downward movement or sinking of the Earth’s surfacecaused by removal of underlying support.The movement may be slow or fast, andit may affect a large region (thousands ofsquare miles), a medium-sized area, or alocal area (smaller than 1 acre). Tens ofthousands of square miles of the Earth’ssurface have subsided worldwide. Theeffects of this process are usually not assudden and spectacular as those of anearthquake or volcanic eruption, but theyare nonetheless significant. Subsidence

to many dis- turbances. Some occur so infrequently or slowly ‘‘ that theyare easily ignored. Fortunately, most geologic conditions have minor impacts onpeople and property. Normal geologicprocesses, however, can become geologichazards. The Glossary of Geology defines ageologic hazard as a “geologic conditionor phenomenon that presents a risk or isa potential danger to life and property.Examples include Iandsliding, flooding,earthquakes, [and] ground subsidence”(Bates and Jackson, 1987, p. 271).* Monetary losses caused by geologic hazardsamount to millions (and frequently billions) of dollars in the United Statesevery year.

*A name(s) and date in parentheses identify theauthor(s) and publication date of a book, article,or report that is the source of the informationjust presented. It is a method of giving anabbreviated citation. Complete bibliographicinformation is included in the Selected Referencessection beginning on page 22.

NEARELOY.AZ

causes annual economic losses of ap- 977 p r ox i -

mately $500 million in the UnitedStates, according to the National Research Council. Another $10 million peryear is spent on studying subsidence.Some areas subside naturally, whereasothers sink because of human activities.Dr. R.L. Ireland and his colleagues at theU.S. Geological Survey identified subsidence as one of the largest and mostimportant changes of the Earth’s surfaceever caused by human beings.

WI-TAT Is Su135IDENcE?

I

Which Areas in Arizona Are Subsiding?

2

More than 3,000 square miles havesubsided in Arizona. Slow, large-scalesubsidence is occurring in several portions of the State (Figure 1). In PinalCounty, between Phoenix and Tucson, anarea of more than 100 square miles sankat least 7 feet between 1952 and 1977(Figure 2). The region includes the townof Eloy, a 5-mile segment of InterstateHighway 10, more than 6 miles of StateHighway 87, and more than 5 miles ofthe Southern Pacific Railroad. Dr. DonaldR Pool, a hydrogeologist with the U.S.Geological Survey, measured approximately 2 inches of subsidence thatoccurred between October 1988 and Febmary 1989 at a site near Eloy. This is avery high rate, and the land is stifi sinking. Subsidence usually occurs so slowlythat it is undetectable unless careful landsurveys are made or until the cumulativeeffects become apparent.

Many natural processes and certainhuman activities cause subsidence. (Seethe box on page 5.) Most of the measurable subsidence currently taking place in

Arizona occurs when more water ispumped out of wells than is returned tothe natural underground water-storagearea that the wells tap. In this report,this process is informally referred to aspumping subsidence. Although this report focuses on pumping subsidence andrelated processes, two of the other causesof subsidence in Arizona are briefly discussed below.

One cause is sinkhole formation. Insome areas, underground water dissolvessoluble rocks, such as limestone, and creates subterranean cavities. When loss ofsupport becomes too great, the groundsurface collapses and the cavities becomesinkholes. (In some cases, the groundmerely sags, creating small closed depressions.) There are many sinkholes in central and northern Arizona. A sinkhole inFlagstaff called the Bottomless Pits isabout 50 feet in diameter and 25 feet deep.Devil’s Kitchen is a sinkhole near Sedonathat was studied by consulting geological engineer Mr. Paul A. Lindberg, at therequest of the U.S. Forest Service. It is 80to 160 feet wide at the ground surfaceand 40 to 60 feet deep. The cavity beneath Devil’s Kitchen is much deeper, butmost of it is filled with rubble. Cavitiesin Arizona’s soluble rocks formed thousands and even millions of years ago,perhaps when wetter climates providedmore underground water. Only a few of

Figure 1. Regions of Arizona known to haveundergone pumping subsidence. The regionsshown are ground-water areas. Only a portion ofeach area has subsided. In McMullen Valley,subsidence has not been measured but has probablyoccurred. AV = Avra Valley; GB = Gila BendBasin; HP = Harquahala Plain; HR = towerHassayampa River valley; LS = tower Santa CruzRiver basin; MV = McMullen Valley; SR = SaltRiver Valley; SS = San Simon Basin; US = upperSanta Cruz River basin; WB = Wilicox Basin.Modfied from Schumann and others (1986).

112° 00

Arizona’s sinkholes have formed or enlarged during historical times, probablybecause of the general lack of surfacewater and shallow underground water.

Hydrocompaction or near-surfacesubsidence, which is also common inArizona, is caused when water is addedto a certain type of soil at or near theground surface. This soil is very lightweight because it has a lot of air spacebetween the solid particles. The largestparticles are called gravel. As particle sizedecreases, the materials are called sand,silt, and clay. Individual clay particles arevisible only through a microscope. In soilthat is susceptible to hydrocompaction,the clay and silt grains form “bridges”that prevent the sand grains from touching each other. These bridges are strongwhen dry, allowing the soil to supportits own weight, along with that of a houseor other structure that may be built on

top of it. When the bridges are saturated,however, they collapse, allowing the sandgrains to move closer together. The soilcompacts, and the ground surface sinks.This type of subsidence usually affectssmall areas where large amounts of water accumulate. In some places, merelywatering plants in the yard around ahouse causes localized subsidence.Hydrocompaction is most common in thewide valleys in the western and southern parts of Arizona. It has occurred inPhoenix, Scottsdale, Tucson, other partsof Maricopa and Pima Counties, andYuma County.

112° 00

Figure 2. Areas in Pinal County known to have subsided 7 or more feet between 1952 and 1977. Shading showsapproximate extent of areas. Adapted from Laney and others (1978).

3

What Is Pumping Subsidence?

Pumping subsidence has been documented in Arizona since 194$. To understand how pumping subsidence works,you need to know something about theland surface in Arizona and what liesbeneath it.

Arizona may be divided into threemajor regions based on gross physicalcharacteristics of the land (Figure 3). Thesouthern, south-central, and westernregion, called the Basin and Range Province, is characterized by wide, gently sloping valleys (also called basins) separated

Figure 3. Arizona is divided into three regionsbased on physical characteristics of the land. Thesouthwestern area (white) is called the Basin andRange Province. It consists of wide, gentlysloping valleys (basins) and isolated, steep,narrow mountain ranges. The central region(black), the Transition Zone, includes ruggedterrain and steep slopes where mountain rangesare separated by narrow, moderately slopingvalleys. The Colorado Plateau is the northeasternarea (gray). It is characterized by wide, gentlysloping plateaus and mesas, deep canyons, andscattered mountain ranges.

by steep, narrow mountain ranges. Beneath the valleys are accumulations ofsand, silt, gravel, and clay that are hundreds or thousands of feet thick. The sand,silt, gravel, and clay are called sediment,and between each sediment grain are tinyopen spaces called pores. Some of therain that falls on the valleys and mountains seeps into the ground and flowsthrough the pores in the sediment. Below a certain depth, all of the pore spacesare full of water.

You can simulate a sediment-ifiledvalley by placing sand, silt, gravel, andclay in a glass bowl. Even though thebowl is full of sediment, it can stifi holdwater. As you pour in water, it seepsdownward until it reaches the bottom ofthe bowl; you can see it saturating thepores in the sediment from the bottomupward. When the water reaches 1/4 inchor so from the top of the sediment, thebowl is a scale model of a valley in southern Arizona.

The top of the saturated sediment,typically tens to hundreds of feet belowthe ground surface, is called the watertable. The water in the saturated sediment is called ground water. This groundwater is tapped when a well is drilledinto the sediment. Water flows from thepores into the well and is pumped to theland surface.

A tremendous quantity of waterexists below most of southern and western Arizona’s valleys. This water accumulated over thousands of years. It alsotakes a long time for new water to seepdownward from the ground surface andoutward from the mountains to the basins to replace what is pumped out. If asmall amount is pumped out, it can bereplaced by new water seeping in, thusmaintaining equffibrium. When a lot ofwater is removed over a short period,however, the pores are drained and thewater table drops.4

Objects are more buoyant inwater than they are in air. It is easierto lift a heavy object in a swimming pooi than on the ground because the water supports part ofthe object’s weight. In the sameway, ground water supports partof the weight of sediment withinand above it. Over thousands ofyears, as large amounts of sedimentwere eroded from surroundingmountains and deposited in thevalleys of southern and westernArizona, ground water also accumulated in large quantities, helping to support the tremendousweight of the sediment.

When the water table movesdeeper because of excessive withdrawal of ground water, the buoyant support that the water gives thesediment decreases. The overlyingparticles press down harder, causing the sediment in the newlydrained zone to compact. Compaction occurs when sediment grainsmove closer to each other. The volume of space occupied by the sediment decreases, as does the size ofthe pores. Thus, there is less spacein which to store water.

Imagine standing on top of anopen aluminum can full of soda. Ifthe soda were drained slowly, thecan would crumple beneath yourweight. The soda represents theground water, and the can is likethe sediment. Compaction of thecoarser grained sediment, the sandand gravel, may be reversed in manycases if more water under sufficient pressure moves into the pores and expandsthem (e.g., see Lofgren and Klausing,1969, p. 74-76). If you could force sodaback into the crushed can under highenough pressure, the can would expandto its original shape. Intense compactionof most finer grained sediment (clayand silt), however, is irreversible. Evenif additional water is available, it can-

CAUSES OF SUBSIDENCE

Any one of several processes can cause the removal of underlyingsupport that leads to subsidence. Some of these operate naturally,whereas others result from human activities. The list below is divided into natural and human causes of subsidence. Not all of theseprocesses occur in Arizona.

Natural Processes

• Dissolving of soluble rocks and sediment, such as limestone• Forces within the Earth that cause earthquakes and pull

some areas downward• Earthquake shaking that causes some deposits to compact

and settle• Decay of organic matter in organic-rich sediment and soils,

such as peat• Thawing of “permanently” frozen ground (permafrost)• Certain types of volcanic activity• Erosion and weathering that operate beneath the ground

surface— Accumulation of a heavy load on the ground surface, such

as the filling of a lake, growth of a glacier, flowing of lava,or deposition of a thick mass of sediment

• Long-term climatic change, which may result in lowering ofthe water table, drying out of soil and sediment, growth of aglacier, or filling of a lake

Human Activities

• Withdrawing subsurface fluids, such as water, petroleum,natural gas, or brine

— Thawing “permanently” frozen ground (permafrost)• Saturating near-surface, low-density, collapsible sediment• Mining by certain methods— Draining or reclaiming land that causes clay to dry out, peat

or other organic-rich sediments to decompose, etc.• Dissolving soluble rocks and sediment, such as limestone— Placing a heavy load on the ground surface, such as forming

a reservoir by damming a river— Manipulating certain surface-water and ground-water

systems on a large scale

not reexpand the pores. The water-storage capacity of the material is permanently reduced. Using the above analogy,once the soda can is crushed, it remainscrushed.

As the soda can collapses, you movedown with it, and so it is with Arizona’svalleys. Compacted sediment occupies asmaller volume at depth, and the groundsurface subsides.

5

6

WHERE DOES PUMPING

SUBSIDENCE OCCUR?

Pumping subsidence can occurwherever a fluid is removed from a cornpactible deposit. The fluid does not haveto be water. Significant land settlementhas resulted from the extraction of petroleum, natural gas, or brine in LongBeach, California; along the Gulf Coastof Louisiana and Texas; and around Lake

Maracaibo, Venezuela, to name a fewplaces. This report, however, focuses onsubsidence caused by ground-waterwithdrawal. Cities that have suffereddamage from this type of subsidenceinclude Houston, Texas; Las Vegas, Nevada; San Jose, California; Mexico City,Mexico; Venice, Italy; Bangkok, Thailand;and many more worldwide.

In the United States, more than11,750 square miles of land had beenaffected by pumping subsidence by 1981,as reported by Dr. Joseph F. Poland ofthe U.S. Geological Survey. Californiahad the largest area of subsidence, followed by Texas. Arizona was third, withmore than 1,040 square miles affected bysubsidence, including parts of Tucsonand Phoenix (Figure 1). A much largerportion of Arizona, more than 3,000square miles, had subsided by 1983,according to Dr. William E. Strange ofthe National Oceanic and AtmosphericAdministration. Strange studied Arizonain detail and compiled all the subsidenceinformation that was available at thetime. Because land surveys and othermeans of verifying subsidence have notbeen undertaken in many parts of Arizona, additional areas may be affectedand known areas may be larger thansuspected. Subsidence had been documented in nine ground-water areas by1983. In most of these localities, the landis probably stifi subsiding.

The maximum measured pumpingsubsidence in the United States by 1981was 29.6 feet at a site in the San JoaquinValley of central California. The maximum amount of subsidence measured inArizona was approximately 15.4 feet atthe time of this writing. It occurred about3 miles south of Eloy between 1952 and1985 (Figure 4). More subsidence hasprobably occurred there since the lastmeasurement was made. Mr. Herbert H.Schumann, a hydrologist with the U.S.Geological Survey who has studied subsidence and earth fissures in Arizona formany years, suspects that a similarly

Figure 4. Dates on pole dramatize the amount ofpumping subsidence that occurred at a site nearEloy between 1952 and 1985. In 1952 the groundsuiface was where the sign is now, high on thepole. By 1985 the land had sunk more than 15feet. Herbert Schumann of the U.S. GeologicalSurvey, who is 6’2” tall, is included for scale.Photo by the U.S. Geological Survey.

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Figure 5.Location, size, anddepth of pumping-subsidence“bowl” in northeastern Phoenix. Heavylines outline the amounts of subsidence, in feet,that occurred between 1962 and 1982. The landbetween lines 2 and 3, for example, sank morethan 2 feet but less than 3 feet. From Larson andPéwé (1986).

large (and possibly larger) amount ofsubsidence may have occurred near LukeAir Force Base in Maricopa County. Researchers are surveying the area to determine the magnitude.

Pumping subsidence occurs only inareas where water-saturated unconsolidated or semiconsolidated sediment exists underground, and where much morewater has been removed than replaced.As mentioned above, most places withthese conditions are broad valleys in thewestern and southern parts of the Statethat are devoted to farming or urban use.According to Mr. Mason R. Bolitho ofthe Arizona Department of Water Resources, in 1990 (the most recent yearfor which preliminary figures are available), 79 percent of all the water used inArizona was for agriculture, 19 percentwas for municipal and industrial pur

poses,and 2 percent was forpower generationand mining.

Pumping subsidence affects relatively large areas, andin most cases the greatest amount ofsettlement occurs approximately wherethe most ground water has been removed.The result is a large, bowl-shaped depression that can be represented on a mapwith a “bull’s-eye” pattern of concentric“circles” (Figure 5). Monitoring programsdesigned to measure the amounts andrates of subsidence are being carried outin parts of Arizona, mostly in urban areas and where highways and expensivefacilities are located.

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WHAT ARE THE EFFECTS OFPUMPING SUBSIDENCE?

Lowering the land surface causesseveral things to happen; some occur immediately, whereas others take years todevelop. The impact of subsidence onstructures is discussed below, followedby its impact on natural systems.

Figure 6. Protruding welihead near Stanfleld, Arizona. Darkcylinder in center of photo is top of casing. Compaction of sedimentat depth caused ground-surface subsidence that broke the concreteslab. Compaction also crushed the casing at depth. Photo by RobertB. Genualdi.

Effects on Structures

In most cases, pumping subsidenceaffects broad areas, decreasing in severity from the centers to the edges. Thismeans that a structure such as a factoryor house normally sinks uniformly withthe ground and is not damaged. Harm ismore likely to occur where differentialsubsidence lowers one side of a buildingmore than another. The facilities thatsuffer most commonly from pumpingsubsidence are long ones, such as canalsand pipelines, that cross all or a largepart of a subsidence “bowl.” Canals,aqueducts, sewers, and drains are builtwith very precise slopes so that the liquids flow under the force of gravity orare pumped at a fixed pressure. Subsidence, however, changes the slope and

causes liquids to flow too slowly, too fast,or not at all, which may cause ponding,overflowing, or overloading of checkpoints and distribution systems. Inextreme cases, subsidence can cause fluids to flow backward through force-of-gravity systems.

The Central Arizona Project (CAP)structures were located, designed, andbuilt taking subsidence predictions intoaccount. These measures resulted inhigher costs, the use of more materials,and the need for an ongoing subsidence-monitoring program. Considerablemoney and time were probably saved inthe long run, however, by addressing thehazard before rather than after the projectwas completed.

In northeastern Phoenix, pumpingsubsidence has decreased the slopes ofsewers, thereby reducing their capacities.This could lead to the generation ofexcessive sewer gases, which wouldrequire treatment with chemicals or installation of pumps. Remedial action hasnot been required yet, but subsidence isstifi occurring in the area and is beingclosely monitored.

On some farms in Arizona, irrigationcanals and drains had to be repaired after subsidence rendered them useless.Agricultural fields had to be regradedafter subsidence interfered with irrigationand drainage in the Salt River Valley, thelower Santa Cruz River basin, and probably other areas.

Other facilities commonly damagedby subsidence are water wells. Most wellsare cased; that is, after the hole is drilled,it is fitted with cylindrical steel or plasticpipe called casing. The casing is loweredinto the hole in sections that are attachedto each other end-to-end. The casing hasholes or slits in its walls at the appropriate depths, allowing water to flow intothe well while keeping sediment out. Thetremendous compressional force of sinking land causes some well casings tobend, collapse, or break. Those wells musteither be repaired or abandoned and re8

placed with new wells drified nearby.Casing damage at depth indicates, insome cases, that subsidence is occurringwhere it has not been measured yet.

Wells used for municipal water supply and irrigation have been damagedby bent or broken casings and by well-head protrusion. A weithead is the uppermost portion of a well. A concrete slabis normally constructed at the groundsurface at the top of a well and attachedto the casing. The slab serves as a foundation for a pump and other hardware.In many wells, the casing extends all theway to the bottom of the hole, deepenough so that when the land subsides,most of the compaction occurs above thecasing bottom. The ground surface sinks,but the welihead does not. Because thewelthead is left protruding from theground, the pump may become difficultor impossible to use (Figure 6).

Effects on Natural Systems

Streams are the primary natural features affected by subsidence. Most of

Arizona’s streams and rivers flow onlyafter considerable rainfall or snowmelt,so it is not obvious that they can be justas effective in causing erosion and deposition as streams that flow year-round.In fact, most of Arizona’s landscape hasbeen produced or modified by streams.

The slope of a stream bed is called itsgradient. Gradient is delicately adjustedto the amount of water flowing in thestream, the amount of sediment in thewater, the grain size of the sediment, andother factors. The gradient of a streamthat crosses a subsiding valley becomessteeper where the stream enters thesunken zone and gentler where thestream crosses the zone’s center andwhere it leaves the zone. The steepeningcauses the stream to erode more sedimentupstream from the subsidence zone, andthe decrease in gradient causes moredeposition of sediment in the subsidencezone. Increased erosion and gullyingcause loss of topsoil and dissection of theland. Increased sediment depositionraises the land surface and buries preexisting features. These effects on natural

Figure 7. Approximate area inundated along part of the lower Santa Cruz River during the flood of October 2-4, 1983.Arrows show the direction offlow. Notice the 1-mile-wide band of water that left the main flood path and flowed northover subsided land near Eloy. Mothfied from Roeske and others (1989).

112°00 33°00’ 111°30

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9

systems also have an impact on facilitiesthat are built in subsiding areas.

Where subsidence forms a closed depression of the land surface, water thatflows into the area is trapped. With nowhere lower to drain, the water stands inthe depression until it soaks into the soilor evaporates. This problem is especiallyapparent where the sea has invaded sinking coasts, such as in parts of Californiaand Texas.

The danger of flooding, which affectsboth natural systems and structures, alsoincreases as land sinks. In 1983, after aweek of abundant rainfall, flooding occurred on the Santa Crnz, San Francisco,San Pedro, and Gila Rivers and smallerstreams in southern Arizona. Most of thefloodwater carried by the Santa CrnzRiver followed the river’s usual routethrough the southern and western portions of the lower Santa Cmz River basinto the Gila River (Figure 7). Because thebasin had subsided, however, a 1.5-mile-wide band of water flowed northwardalong ditches, roads, and a remnant ofan old Santa Crnz River channel, flooding the eastern part of Eloy. The waterflowed into an area that had sunk morethan 7 feet between 1952 and 1977 (Figures 2 and 4) and covered it to recorddepths. The flooding caused more than$50 million worth of damage in the Eloyarea (Roeske and others, 1989).

CAN PUMPING SUBSIDENCEBE STOPPED?

Pumping subsidence can be stoppedeither by suspending all withdrawal ofground water or by allowing only limited pumping. The first option is obviously unfeasible at present. Pumping canbe limited, however. One way is for farmers to adopt irrigation techniques andselect crops that consume less water. Inurban areas, low-flow plumbing devicescould be installed, native plants could beused for landscaping, and greater usecould be made of treated effluent.

Hydrologists can estimate the amountof ground water that may be safely removed during a year without appreciablylowering the water table. They do this bycalculating the average annual quantity ofwater that seeps downward and laterallyto join the ground water beneath a valley.If more water than usual reaches the saturated sediment during a particular year,more is available for withdrawal. The additional water may come from unusuallyheavy precipitation or from human manipulation of natural systems. An exampleof the latter is Colorado River water diverted into the CAP system and then intowashes or ponds, where it seeps downward to join the ground water at depth.

Many options are available to counteract the effects of pumping subsidence.Water-use programs may be tailored toeach subsiding valley depending on local hydrologic, geologic, and economicconditions. In some areas it may be advantageous to distribute the pumpingamong more wells or to use some wellsonly during certain years. In other areasit may be feasible to pump water mostlyfrom sediment of low compressibility.This procedure was tried in North LasVegas, Nevada, and may have slowed orstopped subsidence there during the mid-1960’s. Sediment of low compressibilityis more likely to reexpand when additional water enters it, reversing compaction and preventing a permanent decreasein its water-storage capacity. In someparts of Arizona, Colorado River watersupplied by the CAP system may be usedinstead of ground water.

Unfortunately, subsidence will notstop as soon as excessive pumping ceases,just as a bicycle wifi not stop rolling theinstant its rider ceases pedaling. Excessive withdrawal of ground water removesthe buoyant support that helps hold upthe overlying sediment — remember thecan of soda? The sediment presses downharder, squeezing more water out of thepores as it compacts. Fine-grained sediment (clay and silt) tenaciously holds10

water in its pores and contains little spacefor water movement. Thus, water drainsslowly from this sediment, and compaction and subsidence occur gradually.Even if excessive pumping were to ceasetomorrow, its effects might continue formonths or even years. Subsidence wifislow down and eventually stop, however,after excessive pumping is curtailed.

In some cases, subsidence may bereversed. As pumping air into a flat tireraises a car, pumping sufficient waterdown into wells and then out into thesurrounding sediment may raise theground surface. Merely curtailing removal of ground water and allowingsurface water to seep down naturally canraise the land surface. This has been demonsfrated in California’s San Joaquin Valley (Lofgren and Klausing, 1969). The

WHAT Is AN EARTH

FIssuRI?

________

A fissure is a crack or opening that iscaused, in most cases, by somethingbreaking or pulling apart. Most fissuresare long, deep, and narrow. An earth fissure is a crack at or near the Earth’s surface that is caused by subsidence. Manyother processes can cause cracks to format the Earth’s surface, but the term “earthfissure” is usually reserved for crackscaused by pumping subsidence or subsidence due to natuial lowering of thewater table.

The first documented earth fissure inArizona was discovered 3 miles northeastof Picacho on September 12, 1927, themorning after an intense rainstorm. Thefissure was approximately 1,000 feet long,up to 15 feet deep, and up to 3 feet widewhere eroded. It crossed the railroad andthe Tucson-Casa Grande Highway (nowInterstate Highway 10). The geologist whostudied the fissure in the late 1920’s

technique tends to work better wherecoarse-grained sediment (sand andgravel) has been compacted. In mostcases, though, the land surface does notrise to its original elevation.

Obviously, water is a rare and valuable resource in most of the semiaridand arid Southwest. Its scarcity has traditionally limited plant and animal populations, including human habitation.Modem technology has allowed the discovery and rapid exploitation of vastamounts of water hidden beneath thedesert soil. Current rates of use simpiycannot be maintained unless alternatesources are discovered. In lieu of suchdiscoveries, conservation and recyclingare the simplest ways to ensure a prosperous and long-lasting human presencein the Southwest.

>—

believed that it wascaused by ground shaking from a distant “ earthquake. Most geologists now thinkthat it was caused by subsidence interacting with underground conditions at thesite. In the 65 years following its discovery, the fissure has become filled withsediment and overgrown with plants,making it difficult to recognize.

As mentioned previously and shownin Figure 5, land does not sink uniformlywithin a subsidence zone. Sediment thathas sunk more pulls adjacent sedimentthat has sunk less, and the latter pullsback. Where the pull is strong enough,the land splits open into a fissure. Imagine an apple pie that has just been takenout of the oven. As it cools, the filling inthe center sinks more than the fillingaround the edges. The crust stretches untilit breaks open between the center and the 11

aearthfissure

original land surface /. I,, \ 4 1/ \currentland surface u3. N \NNoriginal water table - / /\ bedrock- / /\

/ \ -_ 1/ \ 1/ \_

ly \ _ I

current water table 3. / / / /\ / /\ / /

/ / / fault, arrows show relative. I— / -— I”... -— ‘ N \direction of movementSN//\bedrockN//\N/

original land surface b earth fissure

current land surface

________________ __________

original water table dewatered and compacted zone5current water table

-

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original land surface C re&t fissure

.

— —— dewatered and compacted zone;

current water table — :—— —

—

- — — — L__.s. sand—

silt and clay —. and C ..

— . gravel .

d earthfissure

original land surface

____

current land surface , /1/\ z_’/\ _‘/

original water table NN \ NN N-dewatered and compacted zone — / /\ _— / /\ / /\ _-

currentwatertable OOODZO _//\ çì/\ ç//0oedent / bedrock>/

-O-O-/\ çì/\ _I/\ _//\ _I/\ _I/\ _I/

12 /\

Figure 8 (opposite page). Underground conditions that influence where earthfissuresfonn. (a) Fissureover buried inactive fault. Thicker sediment accumulation to the left of the fault allows more compactionthere as the water table drops. (b) Fissure over buried bedrock ridge. Thicker sediment accumulation oneither side of the ridge allows more subsidence there. (c) Fissure at boundary between coarse- and finegrained sediment. The silt and clay compact and subside more than the sand and gravel as the watertable drops. (d) Fissure at edge of subsiding area. The region to the right of the fissure is stable becausethe original water table was deeper than the top of the bedrock. The region to the left of the fissuresubsides as the water table drops, exerting the strongest lateral pull on sediment near the fissure. (b)and (d) modfled from Larson (1982).

edges. The cracks form curved lines parallel to the edge of the pie. Similarly, earthfissures often develop close to mountainsthat border valleys, and they parallel thetrends of the mountain ranges.

Certain underground conditions influence where earth fissures form andincrease the force of the pull. Some ofthese conditions are shown in Figure 8.If geologists can determine where suchsubsurface conditions exist, they can identify general areas in which fissures arelikely to form. The precise location of afuture fissure cannot be predicted, butareas where fissures are likely to formmay be identified if sophisticated instmments that measure extremely smallchanges in pulling force are installed inthe right places.

Along some of Arizona’s earth fissures, the ground on one side is higher

than on the other, which resembles theappearance of some earthquake faults.(Fissures cannot, however, generate earthquakes.) A height difference across somefissures is present when the cracks form.For example, one that formed south ofMarana in 1988 and damaged the CAPaqueduct (see Figure 17) was 2 incheshigher on one side when the crack firstappeared. In contrast, both sides of somefissures are the same height when thecracks form, but a height difference develops slowly over time. The ground wasthe same height on both sides of a fissure east of Picacho when the crackformed. Thirty-four years later, one sidewas I to 2 inches higher than the otherside. During the next 20 or so years, theheight difference increased by 22 inches.This is the largest known height difference across an Arizona fissure.

What Is the Life Cycle of Earth Fissures?

When a fissure first forms at theground surface, it is a thin crack 3 inchesor less wide (Figure 9). Many are lessthan 1 inch wide, and some are not cracksbut a line of small pits or depressions inthe soil. These young fissures may bemore than 1,500 feet long and hundredsof feet deep. (It is difficult to measuredepths when fissures are narrow.) Theirwalls are steep or vertical.

Some geologists believe that certainfissures may initially extend from theground surface to the original water tablethat was present before it was lowered by

excessive pumping fromwells. The huge quantities of water and sediment that can move intofissures suggest thatsome of the cracks are

Figure 9. Narrow crack at theground surface; its appearance is typical of that ofmanyearth fissures when theyfirstbecome visible. The tens capis 2

‘8inches in diameter.

Photo by Steven Slaff

“.‘. quite deep. Thedepths of some fissures have been estimated by makinga simple calculation.If a fissure’s lengthand width areknown and if thevolume of waterand sediment needed to fill it aremeasured, then itsapproximate depthcan be calculated.(The assumptionsrequired to make

- -- such an estimate are-. that the shape of a

fissure is regularalong its height andthat no large cavities exist at depth.)Using this method,University of Arizona hydrologistscalculated depths of175 to 1,500 feet forsome of Arizona’sfissures!

The processesthat change theshapes and sizes ofearth fissures are

change the shapes ofthe same ones thatmountains and valleys: erosion and deposition. Many fissures cut across gulliesand washes. When enough rain falls, thewashes fill with water carrying sediment.As the water and sediment flow into thefissures, more sediment is eroded fromthe fissure walls. The wall erosion widens the cracks (Figure 10), and the deposition of sediment at the bottom of thefissures makes them shallower. Erosionalso connects lines of surface pits into continuous cracks as sediment caves intoopen spaces underground. Plants growlarger and closer together along fissuresbecause fissures receive more water andretain it longer than the adjacent desert.

As erosion and deposition of sedimentcontinue, fissures mature, becoming evenwider and shallower. They may become50 feet wide and 16 feet deep or evenlarger (Figure 11). At this stage they looklike gu]Jies, except that most of their floorsare uneven and do not slope steadilydownward in one direction. Plants growalong them in such profusion that thesecracks appear as distinct dark lines onphotographs taken from airplanes (Figure 12). The fissure on the left side ofFigure 12 that extends along the entirephotograph is almost 10 miles long. It isthe longest known fissure in Arizona.

With continued filling by sediment,fissures become shallower (approximately1 to 2 feet deep) and have smoother floorsand more gently sloping walls (Figure 13).Some of the preexisting stream channelsthat were cut off by the fissures reestab

I.

r%.

1•

1I__

•-‘‘-:

-.-‘- ..

:

. :__i..

Figure 10. Erosion has widened thisearth fissure into a more hazardoussize than that of the crack shown inFigure 9. This fissure rendered a roadimpassable. Photo by Steven Slaff

Figure 11. This earth fissure is 20 to 30 feetwide and 10 to 13 feet deep. Erosion anddeposition have made the fissure resemble a gully.Note the person standing in the fissure. Photo bySteven Slaff

-I_I,

“L

-

14

Figure 12. Aerial photographtaken in August 1987 of areasoutheast of Picacho in PinatCounty. Dense vegetationgrowing along earth fissuresmakes them appear as darklines. Ef = earth fissure;PM = southern end ofPicacho Mountains; CAP =

Central Arizona Project aqueduct; SPRR = SouthernPacific Railroad; 1-10 =

Interstate Highway 10. Photoby Arizona Department ofTransportation.

lish their courses rightacross the cracks. Ultimately, fissures becomevery difficult to recognize because they arecompletely or nearlyfilled with sediment (Figure 14). If no vague furrow or linear strip ofplants remains, old fissures are invisible.

Some earth fissuresmay be split open againafter they first formedbecause of a continuationor renewal of the puffingforce that opened theminitially. In some cases,a fresh crack forms in thefloor of a fissure (Figuresites, a new crack opens beside an existing fissure. Either change slightly modifies the pattern of erosion and filling thatthe feature undergoes.

Researchers do not know preciselyhow long it takes from the time a fissurefirst appears until it is completely fifiedand invisible, but it probably ranges froma few years for some fissures to morethan 50 years for others. Many factorsinfluence the rate of fissure development,including soil type, climate, and location.Some fissures may undergo a differentdevelopment pattern from the one shownin Figures 9 through 14.

V

- fr-L-

4i•%Ir . 4..:. . . . 1

-:w

.1 -

áal -t —• - ..-J- -

•: .. .-— ••‘

2.

-. 0 0.25 0.5 0.75 1.0 mile- . -, I

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15). At other

- • - ..;::‘ .. —‘I.- — - .. .‘-

— - ‘

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Figure 13. As sediment continues to fill a fissure, it becomesshallower and more rounded. Photo by Steven Slaff

before the aqueduct was built. This planning probably prevented the aqueductfrom breaking, but repairs were still required. According to Mr. Gary A. Ditty, acivil engineer with the U.S. Bureau of Reclamation, the repairs cost approximately$50,000. Before certain other portions ofthe CAP were constructed, their proposedlocations were changed because the hazard posed by existing and potential earthfissures was considered too great. Thisdelayed the project and added to its cost.U.S. Bureau of Reclamation geologist Mr.John P. Sandoval estimated that by 1989,$120,000 had been spent to monitor subsidence and earth fissures along the CAP.Since then, approximately $30,000 per yearhas been spent on monitoring.

Interstate Highway 10 has been repaired several times where a fissurecrosses it near Picacho. The same fissuredamaged a railroad and natural-gas pipeline, necessitating costly repairs. A trainderailment is believed to have resultedfrom misalignment of the track causedby another fissure. Fissures have alsoseverely cracked house foundations andwalls, undermined and exposed buriedutility lines, and made dirt roads impassable. At least one person was injuredwhen a motor vehicle was accidentallydriven into an open fissure.

The impact of fissures on land use issubstantial. The damage they cause is notcovered by some insurance policies. Thepresence of one or more earth fissureshas driven down property values dramatically in some areas. A fissure in anundeveloped area near Mesa affected landuse when the city would not issue building permits sought by developers.

Other fissure-related problems occurred in Paradise Valley, an area innortheastern Phoenix. Since approximately 1950, large amounts of groundwater have been pumped from wells inthe area. The depth to the water tableincreased by as much as 550 feet betweenthe mid-1950’s and 1980. During the1960’s and 1970’s, the land surface in

southern Paradise Valley sank at least 3feet. A 425-foot-long earth fissure formedthere in 1980, in a housing subdivisionthat was under construction. Dr. Troy L.Péwé, a faculty member in the GeologyDepartment at Arizona State University,reported that this was the first fissureknown to have occurred in an Arizonacity. (As Figure 16 shows, most fissureshave formed in rural parts of the State.)The developer of the subdivision estimates that the cost of the project wasincreased by approximately $500,000 because of the fissure. Construction was delayed, consultants were hired, and planswere modified to reposition all buildingsaway from the crack.

EFFECTS ON NATURAL SYSTEMS

As mentioned above, many fissurescut across washes and stream courses.When the streams flow, they end abruptly by depositing all of their waterand sediment into the fissures. Not onlydoes this interfere with the normal patterns of erosion and sedimentation, butit also allows for potentially seriousground-water pollution.

Fissures trap sediment and cause newgullies to form nearby. Their great depthsmake them the lowest places in an area,so water flows into them, carrying withit sediment eroded from the surroundingland. The upslope sides of fissures areespecially susceptible to gullying and lossof topsoil. As fissures gradually fill withsediment, “tributary” gullies, which maybe 12 or more feet deep, also fill. Openfissures and deep guffies are dangerousto people and livestock. Domestic animalshave died after falling into or gettingtrapped in fissures or gullies.

Streams intercepted by fissures markedly deepen their channels just upsiopefrom the cracks. As new drainage patterns form, erosion can chisel away thesoil that supports crops and structures,thereby changing the slope of the landsurface. Farming may become impossible18

in the affected areas, and structures maybe damaged.

As mentioned above, many fissuresmay initially be very deep, possibly extending to or close to the original watertable. Surface water flows rapidly downthe crack instead of seeping slowlythrough the thick layers of sediment thatnormally help purify it before it joinsthe ground water. Some of the water entering earth fissures may have flowedover agricultural fields or cattle feedlots,picking up chemical fertilizers, pesticides,herbicides, or animal wastes. Waterdraining from roads and highways maycontain petroleum products, antifreeze,brake fluid, or other toxic compounds.

Fissures are popular sites for illegaldumping of garbage (Figure 18). Becausemost landfills are excavations in theground, many people believe that a preexisting trench, such as an earth fissure,is the perfect place to dump garbage. Theydo not realize that the next rainstorm maycarry pollutants straight down to theirown (or someone else’s) water supply.

Despite the dangers that fissurespresent, they do have some beneficial effects. They may provide paths along whichunpolluted surface water can descend

rapidly to join the ground water, insteadof flowing away or evaporating. Wildlifeconcentrates in and along the cracks because fissures are cool, shady, protectedplaces with abundant vegetation. Afterrainstorms, water remains in some fissuresfor up to several weeks after it has evaporated from the ground nearby. Fissuresappear to be a desirable habitat for a variety of birds, reptiles, and mammals.

Can Earth-Fissure Formation Be Stopped?

As already stated, most fissures inArizona are caused by excessive withdrawal of ground water. Reducing thisdepletion — at least to an amount thatequals the amount of ground water replenished each year by natural and artificial sources — will stop or delay theformation of most new fissures. It wifialso slow or stop the reopening, lengthening, and branching of existing fissures.Fissure formation, however, wifi not endthe instant pumping is reduced to a sustainable level. There is a delayed reaction, as with subsidence; fissures maycontinue to form or grow for months oryears after pumping is reduced. Because

fissures go hand-in-hand with pumpingsubsidence, the techniques listed abovefor stopping subsidence will also reduceor eliminate fissure development. Methods for reducing the hazards of existingfissures are discussed in the next section.

Some fissures may not be caused byexcessive pumping but may be related tonatural lowering of the water table ordrying of soil and sediment. These conditions could be brought about by natural,long-term changes in climate. Fissuresrelated to natural causes probably makeup a small fraction of the fissures that haveformed in Arizona during this century.

figure 18. Aerial view of tires and other garbage illegally dumpedin an earth fissure near the town of Queen Creek in MaricopaCounty. The fissure extends from the lower left to the upper rightof the photo. Note the narrower fissure to the right of the maincrack. Photo by Herbert Schumann.

19

How CAI\I THE HAZARDS OF

As with other geologic hazards, thereare three principal ways to deal with subsidence and earth fissures: (1) avoid living or building in the affected areas; (2)p]an ahead and construct facilities that canwithstand the damaging effects; or (3)repair and replace facilities as necessary,and abandon them if the damage becomestoo extensive. The first option is the safest; the third involves the most risk. Eachoption is discussed in more detail below.

To avoid living or building in a problem area (option 1), the hazard must beidentified and its extent determined. Although not every zone of subsidence andearth fissures in Arizona has been identified and mapped, most of the areas areprobably known or suspected. The Arizona Geological Survey can provide information about locations and specificsof hazardous areas. Many publicationsare available that describe these hazardsin more detail than is possible here. (Consult the list at the end of this report.)

Option 2 is to plan ahead and construct facilities that are expendable or thatcan withstand the effects of subsidenceand earth fissures. Materials and designsare available to strengthen and improvealmost every structure, but most of themincrease the cost or time required for construction. A property owner must determine the appropriate level of safety basedon an assessment of the expected usefullifetime of the structure, the available budget, the effect that damage would haveon the structure’s intended use, and otherfactors. For example, a school must besafer than a storage shed; a homeownermay be willing to risk having cracks inthe garage, but not in the living room.

In addition to sinking, subsidence may ‘S./ c a u S ethe ground to tilt, stretch, andcompress. In a structure such as a house,these conditions may result in crackingor separation of the foundation, floors,walls, or ceilings; sloping or buckling ofthe foundation or floors; broken utilitylines; doors and windows that will notmove properly; and nerve-racking soundsof groaning or tearing as the housemoves. Subsidence can also sever utilitylines where they attach to the trunk lineor where they enter a building. Flexiblelines with built-in slack usually solve thisproblem. Flexible lines are especiallyimportant for potentially dangerous utifities, such as electricity and natural gas.

Effective designs and materials areavailable for each part of a building tomake it more resistant to damage frommovement, but the foundation is probably the most important component.Many problems may be avoided by using specially reinforced foundations.These concrete slabs are similar to conventional foundations, but they arethicker and contain more reinforcing steelbars. They do not crack, even when oneend is lowered 1 to 2 feet below the other.

Sometimes subsidence begins or worsens markedly after facilities are in place.In these cases, only the third option remains. Known subsidence and fissurezones may be chosen for developmentanyway because of certain advantages,such as proximity to existing facilities.Option 3 is to repair, replace, or abandonfacilities that are damaged by subsidenceor fissures. This is the option that mostpeople choose or are left with because they

SUBSIDENCE AND EARTH

Fissuiis BE REDUCED?

20

were not aware of the problem in the firstplace, or they decided to ignore it andhope for the best. If damage does notoccur or is not extensive, this approachmay be the least expensive. If considerable damage does occur, however, repairsmay be costly, and the property ownermay suffer proportionately.

Earth fissures present additional dangers that may be divided into two majorgroups: (1) hazards of the open cracksthemselves, and (2) hazards associatedwith fissure-caused changes in stream-drainage patterns that result in erosionand sedimentation.

One problem with open fissures is thatpeople and livestock may fall into orbecome trapped in those that are largeand steep sided. This danger may bereduced by keeping people and animalsout of the general area or by fencing offthe fissures. More fence may have to beadded as fissures lengthen or branch. Thisapproach may not be practical for largetracts of land.

Another problem is that open fissures,especially deep, narrow ones, may provide paths for pollutants to reach theground water. To prevent possible contamination, trash should never be placedin or near a fissure. If polluted water flowing over the land surface could reach anopen fissure, the crack should be filledwith sediment. Clay-rich sediment is idealbecause of its low permeability. Packingsediment tightly into a narrow and deepfissure is difficult, however, so the sealmay not be completely impermeable. Theground water may be further protectedby constructing a berm (a long, lowmound of soil) immediately upslope fromthe filled crack, which would reroutesurface water away from the fissure.Vegetation planted along the fissure mayalso protect the ground water by drawing moisture out of the soil and into itsroots. The site and any protective devicesshould be inspected regularly in case thefissure widens or lengthens.

Open cracks reduce the support ofany structure under which they pass.Many structures can be designed andbuilt to withstand some loss of support,but most are more expensive than equivalent unreinforced structures. The highercost, however, is well spent when a reinforced structure is damaged very littleor not at all by an earth fissure.

The same remedial measures recommended for open fissures may be usedto slow or stop erosion and sedimentation caused by fissures. Flowing watermust be kept from entering fissures.Streams that have been intercepted byfissures cannot reoccupy their channelsdownslope from the cracks until the fissures have been filled with sediment. Aslong as streams flow outside their channels, the natural balance between erosionand deposition is upset.

For any hazard-reduction program tobe successful, the current status of fissures (and of any modifications made tothem) must be known. Periodic inspection is important, especially after intenserainstorms, when changes are most likelyto occur. Young, active fissures shouldbe examined every 2 months. One inspection per year is probably sufficient formost old, less active fissures, unless newcracks have formed nearby or heavy rainhas fallen. it is useful to place markers,such as stakes pounded into the ground,and to take photographs from knownvantage points to record changes overtime. Property owners can make measurements and keep written records if theydesire a high degree of accuracy in monitoring fissure development.

21

CoNcLusIoN

___

Significant rates of ground-waterwithdrawal began in Arizona in approximately 1910. Since the late 1930’s, theserates have greatly exceeded the replenishment rates in some areas. The highestwithdrawal rates occurred mostly duringthe 1950’s, 1960’s, and 1970’s. Excessiveground-water pumping causes unconsolidated and semiconsolidated water-bearing sediment to compact at depth andleads to land subsidence and earth-fissureformation. Compaction of some of thissediment is irreversible and permanentlyreduces its water-storage capacity.

Subsidence and earth fissures are significant geologic hazards in Arizona.Approximately 9 percent of the area affected by pumping subsidence in theUnited States is in Arizona (Poland, 1981).Hundreds of earth fissures have formedwithin the State just during the secondhali of the 20th century. Arizona may

The following reports, some of whichwere used to prepare this booklet, contain additional information about subsidence and earth fissures. Those markedwith an asterisk (*) are published by andavailable from the Arizona GeologicalSurvey (formerly called the Arizona Bureau of Geology and Mineral Technology).Other useful publications that are notmentioned here contain more technicalinformation and details about specific siteconditions. Most of these are identified inthe comprehensive bibliography compiledby Slaff (1990), which is listed below.

Anderson, S.R, 1988, Potential for aquifercompaction, land subsidence, andearth fissures in Avra Valley, Pima and

have more fis- suresthan any other area of comparable size in the U n it e dStates. As urban areas expand, especiallyat the expense of adjacent agriculturalland, subsidence and fissures wifi havean increasing impact on residents andfacilities.

Although earthquakes, volcanoes, andmost other geologic hazards cannot becontrolled, humans can stop or at leastreduce most pumping subsidence andearth-fissure formation. The key isground-water conservation. In some areas of Arizona, using water supplied bythe CAP aqueduct instead of groundwater may help. Solving the problem willrequire not only wise application of geologic and hydrologic knowledge, but alsotough decisions based on economic, social, and political factors.

PinalCoun- ties,Arizona: U.S. Geological Survey Open- File Report87-685, scale 1:250,000, 3 sheets.

* Arizona Bureau of Geology and Mineral Technology, 1987, Subsidenceareas and earth-fissure zones: Fieldnotes, v. 17, no. 1, p. 6-9.

Bates, RL., and Jackson, J.A., 1987, Glossary of geology: American GeologicalInstitute, 78$ p.

Carpenter, M. C., 1991, Earth-fissuremovements associated with fluctuations in ground-water levels near thePicacho Mountains, south-centralArizona, 1980-84: U.S. Geological Survey Open-File Report 90-561, 64 p.

SELECTED REFERENCES

____

22

Costa, J.E., and Baker, V.R, 1981, Chapter 10: Exogenetic geologic hazards:Subsidence, in Surficial geology—Building with the earth: New York,John Wiley & Sons, p. 284-307.

Davis, S.N., 1983, Measurement, prediction, and hazard evaluation of earthfissuring and subsidence due toground-water overdraft: University ofArizona, Department of Hydrologyand Water Resources, Office of Water Policy Project B-092-ARIZ Report,44 p.

* Harmon, D.B., 1982, Subsidence innortheast Phoenix: A new problem forengineers: Arizona Bureau of Geology and Mineral Technology Field-notes, v. 12, no. 3, p. 10-11.

Ireland, RL., Poland, J.F., and Riley, F.S.,1984, Land subsidence in the SanJoaquin Valley, California, as of 1980:U.S. Geological Survey ProfessionalPaper 437-I, 193 p., scale 1:126,720.

Kenny, Ray, 1992, Fissures: Legacy of adrought: Earth Magazine, v. 1, no. 3,

p. 34-41.Laney, R.L., Raymond, R.H., and

Winikka, C.C., 1978, Maps showingwater-level declines, land subsidence,and earth fissures in south-centralArizona: U.S. Geological SurveyWater-Resources Investigations Report 78-83, scale 1:125,000, 2 sheets.

Larson, M.K., 1982, Origin of land subsidence and earth fissures, northeastPhoenix, Arizona: Tempe, ArizonaState University, M.S. thesis, 151 p.

Larson, M.K., and Péwé, T.L., 1986, Origin of land subsidence and earth fissuring, northeast Phoenix, Arizona:Association of Engineering GeologistsBulletin, v. 23, no. 2, p. 139-165.

Leonard, R.J., 1929, An earth fissure insouthern Arizona: Journal of Geology,v. 37, no. 8, p. 765-774.

Lofgren, B.E., and Klausing, R.L., 1969,Land subsidence due to groundwater withdrawal, Tulare-Wasco area,California: U.S. Geological SurveyProfessional Paper 437-B, 101 p.

Mindling, A.L., 1971, A summary of datarelating to land subsidence in LasVegas Valley: Reno, University ofNevada, Desert Research Institute,Center for Water Resources Research,55 p.

National Research Council, 1985, Reducing losses from landsliding in theUnited States: Washington, D.C.,National Academy Press, 41 p.

* Peirce, H.W., 1979, Subsidence—Fissuresand faults in Arizona: Arizona Bureauof Geology and Mineral TechnologyFieldnotes, v. 9, no. 2, p. 1-2, 6.

Poland, J.F., 1981, Subsidence in UnitedStates due to ground-water withdrawal, in Proceedings of the American Society of Civil Engineers: Journal of the Irrigation and DrainageDivision, v. 107, no. 2, p. 115-135.

Reid, R.E., 1975, Geologic hazards in aportion of east Flagstaff, CoconinoCounty, Arizona: Flagstaff, NorthernArizona University, M.S. thesis,120 p.

Roeske, R.H., Garrett, J.M., and Eychaner,J.H., 1989, Floods of October 1983 insoutheastern Arizona: U.S. Geological Survey Water-Resources Investigations Report 85-4225-C, 77 p.

* Schumann, H.H., and Genualdi, R.B.,19$6a, Land subsidence, earth fissures,and water-level change in southernArizona: Arizona Bureau of Geologyand Mineral Technology Map 23, scale1:1,000,000.

*

_____

1986b, Land subsidence, earthfissures, and water-level change insouthern Arizona: Arizona Bureau ofGeology and Mineral TechnologyOpen-File Report 86-14, scale1:500,000.

Schumann, H.H., Laney, R.L., and Cripe,L.S., 1986, Land subsidence and earthfissures caused by ground-waterdepletion in southern Arizona, inAnderson, T.W., and Johnson, A.I.,eds., Regional aquifer systems of theUnited States—Southwest alluvialbasins of Arizona: American Water 23

Resources Association MonographSeries 7, p. 81-91.

* Slaff, Steven, 1989, Patterns of earth-fissure development: Examples fromPicatho Basin, Pinal County, Arizona:Arizona Geology, v. 19, no. 3, p. 4-5.

*

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1990, Bibliography on Arizonaearth fissures and related subsidence,with selected references for other areas: Arizona Geological Survey Open-File Report 90-7, 28 p.

*

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1991, Earth-fissure activity nearBrady and Picacho pumping plants,Tucson aqueduct, Central ArizonaProject, Pinal County, Arizona: Arizona Geological Survey Open-FileReport 91-1, 43 p., scale 1:24,000, 2sheets.

*

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1993a, Gravity and magnetic surveys at Brady earth fissure, PicachoBasin, Pinal County, Arizona: ArizonaGeological Survey Open-File Report93-la, 29 p., scale 1:24,000.

*

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1993b, Gravity and magnetic surveys at Brady earth fissure, PicachoBasin, Pinal County, Arizona: Rawdata: Arizona Geological SurveyOpen-File Report 93-Ib, 15 p.

* Slaff, Steven, Jackson, G.W., andPearthree, P.A., 1989, Development ofearth fissures in Picacho Basin, PinalCounty, Arizona from 1959 to 1989:Arizona Geological Survey Open-FileReport 89-10, 38 p., scale 1:24,000, 6sheets.

Strange, W.E., 1983, Subsidence monitoring for State of Arizona: NationalOceanic and Atmospheric Administration, National Geodetic Surveyreport, 74 p.

* Winikka, C.C., 1984, A view of subsidence: Arizona Bureau of Geologyand Mineral Technology Fieldnotes,v. 14, no. 3, p. 1-5.

24

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V;4

Director and State Geologist

ARIZONA GEOLOGICAL SuRvEY STAFF

April 1993

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Larry D. Fellows -

Geology and Mineral Resources ProgramThomas G. McGarvin, Geologist J V

Philip A. Pearthree, Research GeologistStephen M. V Richard, Research Geologist V

Jon E. Spencer, Research GeologistRichard A. Trapp, Geologist II

V Contracted Employees V

Raymond C. Harris, Project Geologist*P. Kyle House, Project Geologist*

Gary A. Huckleberry, Project Geologist*Michael H. Ort, Project Geologist*

V

Steven J. Skothicki, Project Geologist* V

Steven Slaff, Project Geologist*

V Oil and Gas Regulation ProgramSteven L. Rauzi, Oil and Gas Program Administrator

Pamela I. Lott7 Secretary

Publications and Administration ProgramRose Ellen McDonnell, Administrative Services Officer

Laurefte E. Colton, Publication Sales ManagerPeter F. Corrao, Graphics Supervisor

V

Emily C. Creigh, Editorial AssistantDenise M. Ingram, Accounting Technician J V

Marjorie Tiznado, Clerical Assistant*Evelyn M. VandenDolder, Geological Editor

Contracted EmployeeDiane Murtay, Librarian*

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