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 information 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

500CP893AZGS1279

*Printed on recycled paper

Dedicated to the memory of

CATHY SCHULTEN WELLENDORF

1956-1988

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PREFACE

Land subsidence and earth fissuringhave occurred in large portions of south­ern Arizona, where they have caused avariety of structural damage and land­management problems. As Arizona'spopulation continues to increase, so willthe demand for ground water. Subsi­dence and earth fissuring will 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, Mari­copa 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, Cathyten Wellendorf beganat 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-eentralArizona. 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 facili­tated 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 sink-hole. 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 Siaff

CONTENTS

Introduction . 1

What Is Subsidence? .. 1

Which Areas in Arizona Are Subsiding? 2

What Is Pumping Subsidence? 4

Where Does Pumping Subsidence Occur? 6

What Are the Effects of Pumping Subsidence? 8

Effects on Structures 8

Effects on Natural Systems 9

Can Pumping Subsidence Be Stopped? 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 Fissures? 17

Effects on Structures 17

Effects on Natural Systems 18

Can Earth-Fissure Formation Be Stopped? 19

How Can the Hazards of Subsidence

and Earth Fissures Be Reduced? 20

Conclusion 22

Selected References 22

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to many dis- tur-bances. Some occur so in-frequently 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 /Igeologic conditionor phenomenon that presents a risk or isa potential danger to life and property.Examples include landsliding, 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.

INTRODUCTION ---<

During a recent summer thunder­storm in a rural part of Pinal County, aman heard a loud rumbling 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

WHAT Is SUBSIDENCE?Subsidence is the downward move­

ment 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

causes annual eco-nomic losses of ap- pro x 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. 1

...Which Areas in Arizona Are Subsiding?

&II

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 Feb­ruary 1989 at a site near Eloy. This is avery high rate, and the land is still 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

37°-r----'------l.-_...L-__'--_...,

o 25 50 milesL.----!---J

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 = Avm Valley; GB = Gila BendBasin; HP = Harquahala Plain; HR = lowerHassayampa River valley; LS = lower Santa CruzRiver basin; MV =McMullen Valley; SR =SaltRiver Valley; SS =San Simon Basin; US =upperSanta Cruz River basin; WB = Willcox Basin.Modified from Schumann and others (986).

32° 45'

112°00'

I I i I I Io 1 2 3 4 5 miles

112°00'

f------~"'O'---+-----\287r_---_()La Palma

32°45'

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).

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.

3

What Is Pumping Subsidence?

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.

Pumping subsidence has been docu­mented in Arizona since 1948. 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

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. Be­Iowa certain depth, all of the pore spacesare full of water.

You can simulate a sediment-filledvalley by placing sand, silt, gravel, andclay in a glass bowl. Even though thebowl is full of sediment, it can still 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 1/4inchor 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 equilibrium. When a lot ofwater is removed over a short period,however, the pores are drained and thewater table drops.

.Safford

Bisbee.Nogales

,.q A, •, vD Florence

f?,.qYGTucson 12"

4

Objects are more buoyant inwater than they are in air. It is easierto lift a heavy object in a swim­ming pool 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-

not reexpand the pores. The water­storage 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 com­pactible 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

Figure 4. Dates on pole dramatize the amount ofpumping subsidence that occurred at a site nearEloy between 1952 and 1985. In 1952 the groundswface 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.

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 maybe larger thansuspected. Subsidence had been docu­mented in nine ground-water areas by1983. In most of these localities, the landis probably still 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

7

Phoenix..

I3 miles

.'

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-eyell 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.

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-

Cactus Rd.

Thunderbird Rd.

ti5"CN

Shea Blvd. M

~~;~O~:~~;;,~~~:::S::::::····~·,···, - :"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 andNwe (1986).

Bell Rd. -e1------4.------1,-------+--->+----

iiiE;;j

CiiI-

Figure 6. Protruding wellhead near Stanfield, Arizona. Darkcylinder in center of photo is top ofcasing. Compaction ofsedimentat depth caused ground-suiface subsidence that broke the concreteslab. Compaction also crushed the casing at depth. Photo by RobertB. Genualdi.

•

8

WHAT ARE THE EFFECTS OF

PUMPING 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.

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 ali, which may cause ponding,overflowing, or overloading of check­points and distribution systems. Inextreme cases, subsidence can cause flu­ids to flow backward through force-of­gravity 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 subsidence­monitoring 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 isstill 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 re-

....

placed with new wells drilled 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 well­head protrusion. A wellhead 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 wellhead does not. Because thewellhead 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 l-mile-wide band of water that left the main flood path and flowed northover subsided land near Eloy. Modified from Roeske and others (1989).

112°00' 33°00' 111°30'\

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Sacaton /\ /\ Mts/\ /\

MIs /\ /\/\/\/\ /\/\

Casa Grande0

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8

33"00' 112°00' 32°30' 111°30'

I I I '\ 0I'1'~0 5 10 15 miles

9

10

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 Cruz, San Francisco,San Pedro, and Gila Rivers and smallerstreams in southern Arizona. Most of thefloodwater carried by the Santa CruzRiver followed the river's usual routethrough the southern and western por­tions of the lower Santa Cruz River basinto the Gila River (Figure 7). Because thebasin had subsided, however, a 1.5-mile­wide band of water flowed northwardalong ditches, roads, and a remnant ofan old Santa Cruz 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 SUBSIDENCE

BE 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.Ifmore 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 ofnatural 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 mid­1960'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 will 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 holds

...

WHAT Is AN EARTH

FISSURE?

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 willslow 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 dem­onstrated in California's San Joaquin Val­ley (Lofgren and Klausing, 1969). The

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 natural 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-easa 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 simplycannot 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 shak-ing from a distant ear t h-quake. 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

': :.0.' 0 : :.Q' 0'.. :. sand: .... : .

. ', and 0 .. '. 'gravel .'. '.: O· .. '. : O·.'. 0 ' ... 0

earthfissure

silt and clay

a

d

. .' .. O· ..... O· .----=.-....--=--~-.-- .....~.=....----=--= 5~·:~"7-:-~..~D..:.~:.:~"7.:.~-.~~

- -dewatered and compac~ed zone:: '~'. ".: .

current water table ---I~~~~~~~~~"-;;~~""~""

current water table ---.. I------------------=~...-:-~_.,..:...,.__...:....,_'_:-+:_I

original land surface --==~:6~:rr."'"':"'!:lJ.T'"-~=;'TT.'~Lr.:"~~ffi~;Y~current land surfaceoriginal water table

original water table ---..

:::::111::::~;:::~ --~-_ _-------------_ __ _----._-_._[.~::~-~~~-~~:.~---~-------~--

12

Figure 8 (opposite page). Underground conditions that influence where earth fissures fonn. (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) modified 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 instru­ments 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 1 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 ofthat ofmanyearth fissures when they firstbecome visible. The lens capis 2 l/S inches in diameter.Photo by Steven Slaff

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.

E

14

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 makesuch an estimate arethat the shape of afissure 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

the same ones that change the shapes ofmountains 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 gullies, 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-

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

,

Figure 12. Aerial photographtaken in August 1987 of areasoutheast of Picacho in PinalCounty. 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 pullingforce that opened theminitially. In some cases,a fresh crack forms in thefloor of a fissure (Figure 15). At othersites, 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 filledand 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.

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

Figure 14. Earth fissure (bare "trail" through center of photo)nearly completely filled with sediment. Eventually, no surfacetrace of the feature will remain. Note the shovel for scale. Photoby Steven Slaff

Figure 15. Reactivated earth fissure. The originalground surface is near the top of the photo on theright and left. The fissure formed and then filledwith sediment up to the original floor level shownby the light-colored, smooth surface to the left ofthe shovel. Renewed pulling split the floor open,forming the cleft in which the shovel rests. Photoby Steven Slaff

Where Do Earth Fissures Develop?

16

Since the 1950's, the number of fis­sures in Arizona has increased dramati­cally. Hundreds have been identified inthe valleys of northwestern Cochise,western Pima, western Pinal, and south­ern Maricopa Counties (Figure 16). Somemay have formed from natural causes,but most are related to excessive with­drawal of ground water. Earth fissureshave also been noted in New Mexico,Nevada, California, Utah, Idaho, Wyo­ming, Mexico, China, and Japan. They

Figure 16. Regions ofArizona where earth fissuresexist. Excessive use of ground water has causedmoderate to large increases in the depth of thewater table in each of these areas. The largestconcentration of fissures is in the lower SantaCruz River basin, where the most land subsidenceand deepening of the water table have occurred.The regions shown are ground-water areas. Earthfissures have formed in only a portion of eachregion. AV = Avra Valley; HP = HarquahalaPlain; LS = lower Santa Cruz River basin; MV= McMullen Valley; SR = Salt River Valley; SS= San Simon Basin; WB = Willcox Basin.Modified from Schumann and Genualdi (1986b)and Schumann and others (1986).

have probably formed in other locationsas well, but Arizona appears to have morethan any other region of comparable sizein the United States.

Earth fissures form in sediment, notbedrock, so they are found in valleys, notmountains. They form in areas that are

37°-r----L-_----l__--L-__J----,

o 25 50 miles'---'----J

subsidillg or have subsided, where thepull on sediment is strong. This happenswhere sediment thickness changesabruptly, because a thick layer of sedi­ment can compact more than a thin layer.Sediment layers are much thinner nextto mountains and over buried bedrockhills, so fissures form where these areasflank the thicker layers of valley sediment(Figures 8a and 8b). Clay-rich sedimentcan compact more than sand or gravel,

so fissures may form where layers changelaterally from clay to sand (Figure 8c).Another factor that localizes earth fissuresnear the edge of a subsidence zone is thelack of subsidence outside the zone (Fig­ure 8d). This situation creates a strongpulling force on the sediment. A subsi­dence zone may end close to the edge ofan overused well field or close to theboundary between a ground-water areaand a dry area.

What Are the Effects of Earth Fissures?Fissures have a significant impact on

the areas where they develop. As withsubsidence, some effects occur immedi­ately, and some occur long after the fis­sures have formed. In certain cases, it isdifficult to separate the effects of fissuresfrom those of subsidence because bothphenomena occur in the same areas andalter some systems in similar ways. Fis­sures are likely to have an even greaterimpact as ground water is more rapidlydepleted and more facilities are devel­oped in high-risk areas. Ways to lessensome of these damaging effects are dis­cussed on pages 20 and 21.

EFFECTS ON STRUCTURES

Fissures that develop beneath struc­tures cause a fundamental loss of sup­port, which results in cracking, separa­tion, or weakening of the foundation andoverlying framework. In Arizona, facili­ties that have been damaged by fissuresinclude highways, roads, railroads, rec­reational facilities, houses, other build­ings, sewage-disposal facilities, irrigationcanals, water-storage systems, agricul­tural fields, buried pipelines, and water­distribution systems, including the CAP.

Unlined canals, ditches, and drainsmay be broken and completely emptiedby fissures. Once a break occurs, the wa­ter from the canal will quickly erode thefissure into a wider (and, in some cases,

longer) crack, which may damage struc­tures nearby. Isolated poles and towers,which have very narrow bases of sup­port, may lean or fall. Poles that holdutility lines, however, such as electrictransmission wires, may be preventedfrom falling or leaning too far by the sup­port of adjacent poles and lines.

The CAP was damaged by a fissurein 1988 (Figure 17). The fissure developedwhere the aqueduct was already rein­forced because the area had been identi­fied as a potential fissure-hazard zone

Figure 17. Earth fissure that formed in 1988 and damaged theCAP aqueduct in Pima County. The aqueduct (behind theembankment in the background) was cracked but not emptied. Thewhite area on the embankment is plastic placed there to keep rainfrom eroding the crack caused by thefissure. Photo by Steven Slaff

18

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.Pewe, 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 gullies 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 upslopefrom 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 impossible

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

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.

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 in

Arizona 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 willalso slow or stop the reopening, length­ening, and branching of existing fissures.Fissure formation, however, will 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.

19

In addition to sink-ing, subsidence may c a usethe 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 utili­ties, 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

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)plan 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.

How CAN THE AZARDS OF

SUBSIDENCE AND EARTH

FISSURES 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 stream­drainage 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

SELECTED REFERENCES

p

22

ONCLUSION

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 secondhalf 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 com-parable size in the Un i tedStates. As urban areas expand, especiallyat the expense of adjacent agriculturalland, subsidence and fissures will 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.

Pinal Coun- tie s ,Arizona: U.S. Geologi-cal 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, R.L., and Jackson, J.A., 1987, Glos­sary of geology: American GeologicalInstitute, 788 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.

Costa, J.E., and Baker, Y.R, 1981, Chap­ter 10: Exogenetic geologic hazards:Subsidence, in Surficial geology­Building 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-D92-ARIZ Report,44 p.

* Harmon, D.B., 1982, Subsidence innortheast Phoenix: A new problem forengineers: Arizona Bureau of Geol­ogy and Mineral Technology Field­notes, 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., andWinikka, 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 P€we, 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, RJ., 1929, An earth fissure insouthern Arizona: Journal of Geology,v. 37, no. 8, p. 765-774.

Lofgren, RE., and Klausing, RL., 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, HW., 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, RE., 1975, Geologic hazards in aportion of east Flagstaff, CoconinoCounty, Arizona: Flagstaff, NorthernArizona University, M.S. thesis,120 p.

Roeske, RH, 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, HH, and Genualdi, RR,1986a, 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, HH, Laney, RL., and Cripe,L.S., 1986, Land subsidence and earthfissures caused by ground-waterdepletion in southern Arizona, inAnderson, T.W., and Johnson, A.L,eds., Regional aquifer systems of theUnited States-Southwest alluvialbasins of Arizona: American Water 23

24

Resources Association MonographSeries 7, p. 81-91.

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

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

* __ 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.

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

*__ 1993b, Gravity and magnetic sur­veys at Brady earth fissure, PicachoBasin, Pinal County, Arizona: Rawdata: Arizona Geological SurveyOpen-File Report 93-1b, 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.