A geologic investigation of foundation failures in small buildings in Tucson, Arizona
Item Type text; Thesis-Reproduction (electronic)
Authors Crossley, Robert William, 1946-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/554059 A GEOLOGIC INVESTIGATION OF FOUNDATION FAILURES IN
SMALL BUILDINGS IN TUCSON, ARIZONA
by
Robert W. Cross!ey
A Thesis Submitted to the Faculty of the
DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE WITH A MAJOR IN GEOLOGICAL ENGINEERING
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 6 9 STATEMENT DY AUTHOR
This thesis has been submitted in partial fulfillm ent of require ments for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of th is m anuscript in whole or in p a rt may be granted by the head o f the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In a ll other instances, however, permission must be obtained from the author.
SIGNED: Lu~
APPROVAL BY THESIS DIRECTOR
This thesis has/been approved on the date shown below:
D ateDR. W ./JZ LACY DateDR. W Professorfessc of Mining and Geological Engineering ACKNOWLEDGMENTS
The writer is grateful to Doctor W. C. Lacy who suggested this
topic and served as thesis advisor, and to Doctor W. C. Peters and
Doctor 0. F. Abel who served on the committee. Data on subsurface
geology by Mr. R. W. D avis, Mr. R. S t r e it z , and Mr. E. F. Pashley was
essential to the analysis of results, as were the hydrologic maps
provided by The University of Arizona Department of Agricultural Engineer
ing. The w riter also wishes to acknowledge information and suggestions
by members of the City's Inspections Department and by several local
engineers and c o n tracto rs.
i i i TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... v i
ABSTRACT...... v i i i
1. INTRODUCTION...... 1
1.1 Purpose...... 1 MC C CO LO ID VO CO CM CM 1.2 Procedure...... 1.3 Previous Work......
2. GEOGRAPHY OF THE TUCSON BASIN......
2.1 The Basin and Range Province 2.2 The Tucson Basin...... 2.3 Geology of the Ranges...... 2 .4 The Basin F i l l ......
3. INVESTIGATION OF FOUNDATION PROBLEMS IN TUCSON...... 9
3.1 Explanation of Terms...... 9 3.2 Results of Investigation ...... 10 3.3 Possible Explanations...... 11
4. GEOLOGIC HISTORY OF THE TUCSON BASIN...... 14
4.1 The Paleozoic Era ...... 14 4 .2 The Mesozoic E ra ...... 14 4.3 The Early Tertiary Period ...... 15 4 .4 The Basin and Range Orogeny...... 16 4.5 Post-Orogenic Activities in the Tucson Basin...... 19 4.6 The Pleistocene Epoch...... 21 4 .7 Recent Geologic H is to ry ...... 22
5. INFLUENCE OF GEOLOGY AND HYDROLOGY ON FRACTURE PATTERNS.... 24
5.1 Role o f Terrace D eposits...... 24 5 .2 Role o f C ollapsing S o il...... 28 5 .3 Role o f C a lic h e ...... 30
iv V
TABLE OF COHTENTS-Continued
Page
5 .4 Role o f Ground Water D e p le tio n ...... 31 5.5 Role of Basement Geology and Tectonic Activity.... .43 5.6 Results of the Geologic Investigation ...... 44
6. ENGINEERING ASPECTS OF FOUNDATION FAILURE...... 47
6.1 Ground In s t a b ilit y and the P u b lic ...... 47 6.2 Recommendations for Zoning and Planning...... 49 6.3 Recommendations for Building Codes...... 50 6.4 Recommendations for U tilitie s ...... 52 6 .5 Future Foundation Problems in Tucson...... 53
7. SUMMARY AND CONCLUSIONS...... 56
REFERENCES 57 LIST OF ILLUSTRATIONS
Figure Page
la . Cross Section Showing T ra d itio n a l Basin and Range C o n fig u ra tio n ...... 4
lb . Map Showing Geologic Provinces o f the Southwest...... 4
2. Basement Depth Contour Map of the Tucson Basin (from R. W. D a v is )...... 8
3. Map Showing Areas o f Foundation F a ilu re s in Tucson...... 12
4. Map Showing Fracture Pattern Superimposed on Terraces in Tucson (Terraces by G. Smith, 1938)...... 2G
5. Isopacheous Map Showing C aliche D is trib u tio n in Tucson ( a f t e r R. S t r e i t z ) ...... 32
6. Ground Water Table Contour Map (Based on Data from Department o f A g ric u ltu ra l Engineering, 1 96 7)...... 35
7. Isopacheous Map of Sediments Dewatered in Tucson from 1947 to 1967 (from Department of Agricultural Engineer in g ) ...... 37
8. Cross Section Through Severely Fractured Area in Tucson (subsurface data by Robert S treitz) ...... 39
9. Cross Section Through Severely Fractured Area (subsur face data by Robert S t r e i t z ) ...... 40
10. Cross Section Through Trouble-Free Area in Tucson (subsurface data by Robert S treitz) ...... 41
11. Lithofacies Map of Dewatered Sediments (1947-1967)...... 42
12. S tru c tu ra l Contour Map Showing Contact Between R illito Beds and Quaternary Basin Fill (from Pashley).. 45 LIST OF ILLUSTRATIONS-Continued
Figure Page
13. Map Showing Zones L ik e ly to Experience Land Subsi dence in .th e F u tu re...... 54
14. Intensity Pattern of Building Foundation Failures in In Tucson, A rizo n a ...... Pocket ABSTRACT
Buildings in certain areas in Tucson are subject to damage in the form of cracked foundations, walls, and floors, The cause is presumed to be d if f e r e n t ia l movement re s u ltin g from ground in s t a b ilit y .
During the summer of 1968 the w riter inspected small buildings throughout Tucson, noting the location and severity of cracks in walls and foundations. Zones of greatest intensity of foundation failures are located on the Cemetery Terrace and can be correlated with layers of subsurface sand from which ground water has been withdrawn within the last decade.
Ground in s t a b ilit y is re la te d to regional and lo cal geology.
The two major causes of foundation problems in Tucson are believed to be (1) pockets of low density unstable soils which were deposited in a water deficient environment and which experience a great loss of volume under conditions of saturation and loading, and (2) land subsi dence caused by increased effective stress on subsurface strata which
have been dewatered.
v i i i 1. INTRODUCTION
Late one summer a family returned from vacation to their fashion able east Tucson home to find that their backyard patio had been reduced to a mass of concrete rubble, a side wall had completely collapsed, and a portion of the footings had settled more than a foot. The expensive home lay on a terrace above Pantano Hash. It was a one story structure, only a few years old, and built by a reputable contractor.
Many people have the mistaken impression that foundation failures must be associated w ith v is ib ly unstable te r r a in , such as swamps, peat bogs, land-shore interfaces, and regions subject to earthquakes and land slides. The soils of the semi-arid Southwest appear firm and cohesive, but they have caused havoc for contractors. At times desert soils seem static and inert, but under certain conditions they become dynamic materials. The surprising number of fractures in buildings throughout
Tucson will testify to this fact.
1.1 Purpose
Foundation failures are becoming an increasing problem in
Tucson. The purpose of this study is (1) to outline zones of foundation failures; that is to determine the location and severity of the problem,
(2) to discover the causes of the failures, especially their relation ship to the geological environment, and (3) to recommend engineering solutions to the problem.
1 2
1.2 Procedure
During the summer of 1968 the w riter inspected one-story buildings in a selected 38 square mile area of Tucson. All observations were made from streets and sidewalks, except in the case of a few vacant houses. Each time a cracked building was spotted, its location and degree of damage was noted on an enlarged section map carried on a clip board. The results of each day's field studies were analyzed in an attempt to discover a geographical pattern to the failures. No attempt was made to correlate the patterns with any other parameters until the entire survey was completed, thus insuring an unbiased picture of the problem as it occurs in existing structures.
1.3 Previous Work
A similar survey of foundation failures in Tucson was made by W. S. Platt in 1959. The building density was not as great then as now, nor did the c it y e n ta il as much land. 2. GEOGRAPHY OF THE TUCSON BASIN
The engineer treating an earth science problem must firs t be fam iliar with the physical geography of the area in which he is working.
He should also be aware of the events in the geologic past which have
produced the present landscape.
2.1 The Basin and Range Province
Tucson is located in a region characterized by large mountain
ranges separating broad, fla t basins. This physiographic province is
aptly named the Basin and Range, and it covers most of Nevada and Southern
Arizona, and parts of California, Utah, New Mexico, and Texas as seen in
Figure lb. Most of the region is arid or semi-arid.
The traditional concept of the Basin and Range Province holds
that huge mountain blocks were transected by high angle border faults
along which they have been uplifted with respect to the basins. As a
result, fronts of basin ranges are very long and straight and reflect the
position of the fault rather than any internal structure. Flat erosional
surfaces extending out into the basin from the mountains are called
pediments. Gently sloping alluvial fans fill the basins. This simpli
fied concept has been challenged by Pashley (1966) with regard to the
Tucson Basin as w ill be discussed in Chapter 4.
3 4
Figure la . Cross Section Showing T ra d itio n a l Basin and Range Configura tio n .
: w Columbia V.*.". *.y * Plateau
Basin and Range Province Rocky \ Mountains \\";Y
.Colorado Plateau
OTUCSON
Figure lb. Map Showing Geologic Provinces of the Southwest. 5
2.2 The Tucson Basin
The city of Tucson occupies the northern part of a typical basin about 30 miles long and 13 miles wide. It is bordered on the north by the Santa Catalina Mountains, on the west by the Tucson Mountains, on the east by Tanque Verde Ridge and the Rincon Mountains, and on the south by the Santa Rita and Empire Mountains. Its principle drainage is the
Santa Cruz River which rises in Mexico, crosses the border five miles east of Nogales, and flows north 65 miles to Tucson. From Tucson the
Santa Cruz flows northwest until it meets the Gila River southwest of
Phoenix. Drainage within the basin is provided by the intermittent
Ri11ito Creek and its two tributaries, Tanque Verde Hash and Pantano
Wash. These streams closely parallel the basin fronts to collect runoff from the ranges. Annual precipitation is about 10 inches on the basin floor and about 30 inches near the top of the Catalinas. Within the
basin, stream development is poor due to scanty rain fall, small gradients,
and the high permeability of the upper basin f ill.
2.3 Geology o f the Ranges
The Catalina, Tanque Verde, and Rincon Mountains are com
posed mainly of a foliated gneiss, previously believed to have had a
Precambrian beginning. The gneiss is banded and gives evidence that its
source was cnce a stratified sequence at the surface of the earth.
Another strong argument for a sedimentary origin of the rock is layers
of marble within the gneiss, which are interpreted as remnants of lime
stone beds which survived those metamorphic processes from which the 6 gneiss was created. The age of the original sediments is not known, but potassium-argon dating indicates that u p lift had proceeded to a point that metamorphism ceased by Upper Oligocene or Lower Miocene time.
An important structural feature of the Catalina Gneiss is the folding. Pashley identified a series of "large parallel, west trending, doubly plunging anticlines and synclines" and suggested that these folds played an important part in the physiography of the range. At depth the gneiss displays lineation, foliation, and folds. At shallower depths folding took place but it was not conducive to lineations.
The Tucson Mountains are composed mostly of Mesozoic and Paleo zoic sedimentary rocks capped by Tertiary volcanic units. Basalt, andesite, rhyolite, and tuff lie conformably above a complex of blocky boulders in a massive sandstone matrix called the Chaos Formation. This unit was believed by Horatio Brown to be the result of an overthrust, but more recent investigators believe that volcanic activity in the
Early Tertiary deposited the Chaos Member over the Mesozoic sediments.
2.4 The Basin Fill
The so-called "basement" in the Tucson Basin refers to the contact between the very old igneous and metamorphic rocks and the
Tertiary sediments which fille d the basin during and after the Basin and Range orogeny. Melton describes three sedimentary units common to nearly all basins in Southern Arizona. The oldest unit lies unconform- ably above the pre-Tertiary basement. In the Tucson Basin, these beds are called the Ri11ito Group (also the Pantano Formation on the east 7 slopes) and consist of a thick section of clastic continental material which has been highly deformed. Above the R illito Formation are the
Tertiary and Quaternary Basin F ill sediments. These undeformed alluvial fan deposits are the products of rapid mechanical weathering during the cold stages associated with the last two periods of glaciation. The third unit is the late Pleistocene and recent alluvium found as inner valley f ill along streams or as alluvial fans along the present mountain fro n ts .
Beneath the city of Tucson sedimentary deposits are on the order of 2000 feet thick. Magnetic and gravity geophysical work compiled by
R. K. Davis (1967) indicates that the basement depth reaches 7000 feet in locations south of the city (see Figure 2). South of the city is a basement scarp trending northeast and crossing Black Mountain and Davis-
Monthan Air Force Base. Below the scarp alluvium thickness increases to over 5000 feet. This depth prevails for about 15 miles to the vicinity o f H e lv e tia Road where the Sawmill Canyon f a u lt zone probably cuts the
basin floor. South of this fault the basin floor rises to meet the
Santa Rita Mountains.
Broad basement slopes are interpreted as pediments cut on the
igneous or metamorphic material. On the east side of the Tucson Mount
a in s , a smooth g ra v ity g ra d ie n t has been found, but ra th e r than a
pediment, it has been attributed to a spreading volcanic pile inter-
bedded w ith more and more a llu v ia l m a te ria ls as one proceeds toward the
center of the basin. South of Black Mountain is a graben which may
also be filled with volcanics. 8
iu C S O N
4 Mite*
’ 11 i - ______(from R» ^ Basement Depth Contour Map of the Tucson Basin Figure 2 D a v is ). 3. INVESTIGATION OF FOUNDATION PROBLEMS IN TUCSON
It is common knowledge among engineers and contractors in Tucson that houses and small buildings in certain sections of the city seem invariably to develop foundation failures resulting in cracked walls, broken plaster, and warped doors and windows. In other areas a lesser degree of damage is reported, while s till other areas seem to present no problems at a ll. In order to begin analyzing possible causes and cures for the problem it is necessary to know both the locations and the degree of foundation failures in the small buildings of Tucson.
3.1 Explanation o f Terms
Three classifications are used to denote intensity of
foundation failures: strong, moderate, and light. These terms reflect
a value judgment of the author and represent both the number of build
ings with foundation problems in an area and the magnitude of those
problems. Since building density varies within the city, percentages
rather than absolute numbers of failures become the criteria for deter
mining the classification to which an area belongs.
"Light" intensity characterizes an area of noticeable but minor
building cracks. Less than half of the structures in the area show any
problem at a ll. Adobe buildings exhibit small cracks adjacent to door
and window sills. Brick buildings show hairline cracks in the mortar
joints. In all buildings one, or at most two, cracks are visible in
9 10 three outside walls. Architectural damage inside these homes might in clude small plaster cracks, but these could be tolerated or cheaply re p a ire d .
"Moderate" intensity describes an area where more than half the structures show foundation problems. It may be that nearly all of the buildings have the small problems described by the "light" intensity term or that a fewer number show a more severe problem.
"Strong" intensity denotes the areas of the city with the worst foundation problem. In these neighborhoods virtually all of the struc tures are damaged and many to an unsightly degree. In adobe buildings, cracks run from the ground level to the roof, even in walls which are not complicated by doors or windows. In brick buildings, cracks occur across the bricks themselves, or if along mortar joints, wide displace ments result. Damage to the buildings is severe. Doors and windows are warped so that closing them is difficult. Floors are flexed and may even collapse. Plaster and bathroom tile are broken and displace ment cracks in walls are d ifficu lt and expensive to repair. Wooden roof
beams yield and roofing material cracks, resulting in leakage. In ex
treme cases pipes may be bent or even broken. The maintenance of entire
buildings becomes an expensive nuisance.
3.2 Results of Investigation
The area examined includes that part of the city east of
the Southern Pacific Railroad tracks, south of Wetmore Road, the R illito
River, and Wrightstown Road, north of the city lim its, and west of n
Harrison Road. The results arc shown in detail on Figure 14 and on a smaller scale on Figure 3. The pattern shows that the longest trend roughly parallels the R illito along Fort Lowell Road. Near A1vernon Way the pattern shifts slightly to the south and becomes more severe. East of Swan Road is a large but disconnected pattern. Another area of ex tensive foundation damage occurs between 22nd Street and Davis-Monthan
Air Force Base. Between Randolph Park and the railroad, patches of moderate and severe failure occur.
Virtually no section of the city is entirely free from trouble.
Even in new suburbs east of Pantano Wash a few cracks have been noted.
In many places in the city, a handful! of severely cracked buildings are surrounded by vast undisturbed areas. The results of the investigation raise many questions as to the meaning of the patterns.
3.3 Possible Explanations
I t must be remembered th a t the re s u lts shown on Figure 3 are strictly a representation of what is showing in structures above ground level. Any attempt to read soil or geological parameters on the map must be done in d ir e c tly . The age o f b u ild in g s , the m a te ria ls used, and the types of foundations varies from one neighborhood to another, and obviously these factors would have important influence on the degree of foundation failure observed.
The firs t reaction of a homeowner to foundation trouble is to blame the contractor. He is likely to assume that poor materials were used and improper building procedures implemented in order to cut costs. Prince Rd \.\*
Gront R d"^
I Brpqdwoy <
intensity pattern of building fractures
STRONG MODERATE LIGHT
Figure 3. Map Showing Areas of Foundation Failures in Tucson. 13
In individual cases this may be true, but the extent of foundation damage in Tucson is so great that this can be ruled out as the major cause. An excellent example is provided by the severe area east of
Grant Road and A1vernon Way. These sections have been developed lot by lot on a random basis for at least 30 years. There are brick, adobe, and concrete block structures built by scores of contractors and yet almost all buildings of every age and type show some stages of founda tion failure.
The engineer w ill immediately answer that the structures have settled or have suffered some other sort of differential movement. Thus the soil beneath the buildings becomes the focal point of investigation.
Too often, however, the civil engineer confines his investigations only to the s ite in question. Because the w eight o f a one sto ry b u ild in g is so small, he considers only the top few feet in the soil profile. His view of the earth and its soil cover is that of a static body which acts only when acted upon. For a widespread problem such as in Tucson, knowledge of the physical geography of the area is not enough. The natural processes which have acted and are s till acting upon the earth must be understood. To this a thoughtful consideration of the effect of man's activities must be added in order to suggest explanations and solutions for geological engineering problems. 4. GEOLOGIC HISTORY OF THE TUCSON BASIN
The writer who would attempt an historical narrative is usually confronted with the problem of determining the point in time his story began. The geologic history of Southern Arizona and of the Tucson Basin began with that of the earth itself. Obviously, the farther back in
time one goes, the sketchier is the evidence of what took place. For
this reason, little is factually known of the earth before the beginning
of the Paleozoic Era.
4.1 The Paleozoic Era
The geography of North America and of the earth during the
Paleozoic bore no resemblance to that of today. The Paleozoic geology
of Arizona is open to many interpretations, and as might be expected,
there is little certainty or agreement even among experts. Geological
events o f th is era have been overshadowed by more recent happenings in
the Basin and Range Province and w ill not be discussed here. Interested
students are referred to Wilson's (1962) resume of Arizona geology.
4.2 The Mesozoic Era
The Mesozoic geology of Arizona has been summarized by
Wilson. During the Triassic and Jurassic, seas invaded Northwestern
Arizona and later possibly the southwestern part of the state. South
eastern Arizona was the scene of crustal deformation and the invasion
of intrusive magmas accompanied by some volcanic eruptions. Radioactive
14 15 dating by Damon, Erickson, and Livingston (1962) indicates that deep- seated metamorphism began early in this era to produce the Catalina Gneiss.
During the Jurassic Period the Nevadan Orogeny began w ith the uplift of the Cordilleran Geosynclinal belt. This trough, filled mostly with volcanic debris, extended from Canada through Washington, Oregon,
California, and Mexico.
Crustal instability in Southern Arizona continued into the
Cretaceous Period, as evidenced by the widespread occurrence of igneous rocks of this age. Potassium-argon dating indicates that the earliest folding of the Catalina Gneiss began at this time. The area around
Tucson must have been a shallow basin as a succession o f non-marine and near shore sediments are found in the Santa Rita Mountains. These
units may be equivalent to Cretaceous sedimentary rocks in the Tucson
Mountains.
It was during Late Cretaceous time that the Larimide Revolution
signaled the rise of the Rocky Mountains. Longwell (1950) insists that
faulting which shaped the Basin and Range began at this time, although
i t is commonly held th a t major movements began in mid-Cenozoic tim e.
4.3 The Early Tertiary Period
The early Tertiary period was a time of maximum warmth and
humidity. Paleobotanical work compiled by Barghoorn (1951) indicates
there was a worldwide distribution of tropical and subtropical flora.
Fossils in Miocene sediments on the Sierra Nevada give evidence of a
deciduous fo re s t whose maximum e le v a tio n could not have been more than 16
2500 feet. Rainfall must have been plentiful in Southern Arizona, as the S ie rra Nevada was s t i l l low enough to admit abundant m oisture from the Pacific. After the Pal eocene Epoch there was a gradual lowering of temperature on the earth which continued into the ice ages of the
Pleistocene. This caused tropical flora to retreat southward as the polar region became cooler. As the mountains of the Far West slowly rose and deposition of coastal plain sediments moved the shores of the
Gulf of Mexico further outward, rainfall in the Southwest became in creasingly less abundant.
4.4 The Basin and Range Orogeny
By the beginning of the Oligocene Epoch the Rocky Mountain
Orogeny had practically died out. The low-lying Colorado Plateau was at approximately the same elevation as regions to the west and south.
During the late Oligocene or early Miocene (20-30 m illion years before present) a series of dynamic geological events began to occur. The southern end of the S ie rra Nevada began to r is e , causing the clim ate in
Southern Arizona to change from subhumid to sem i-a rid by the mid-Miocene.
By this time the faulting which would shape the Basin and Range Province was in full operation. In the late Miocene the uplift of the Colorado
Plateau began, signaling the start of the cutting of the Grand Canyon.
During the late Oligocene the oldest post-Tertiary sedimentary beds were deposited in the Tucson Basin. This pre-orogenic unit, like the oldest beds in many basins, is a highly deformed thickness of con
tinental sediments and volcanics deposited during a time of crustal 17 in s t a b ilit y and a subhumid c lim a te . In the Tucson Basin th is u n it is loosely called the R illito Beds. Pashley divides the R illito Beds into three units. R illito Bed I is the oldest and lies unconformably above the igneous and metamorphic basement. It contains almost no material derived from the Catalina Gneiss; presumably it is the remnant of those units lying above the gneiss. R illito Bed I is a firmly cemented inter- bedded conglomerate, sandstone, and mudstone w ith occasional beds o f gypsum and volcanic ash. The pebbles include volcanic rock, pink granite, limestone and quartzite. The absence of Catalina Gneiss pebbles is characteristic of R illito Bed I.
By the middle Miocene the Basin and Range orogeny has started.
Great outpourings of lava and tuff took place in the north and west parts of the province. The metamorphism of the Catalina Gneiss was complete and folding was well underway when block faulting and u p lift began. The Sierras were nearly 3000 feet high and soon the Pacific
Coast ranges would begin rising.
During the orogeny the other members of the R illito Group were deposited. R illito Bed II is a moderately cemented conglomerate with lesser amounts of interbedded sandstone. The percentage o f C a ta lin a
Gneiss is greater. R illito Bed III contains pinkish-gray sandy con glomerate but is dominated by Catalina Gneiss pebbles. The increase in gneiss pebbles in each successive member has been cited as evidence of slow u p lift of the ranges and gradual uncovering of the Catalina Gneiss.
R illito Bed I is highly deformed and faulted, reflecting crustal move ment after deposition. R illito Beds II and III are but moderately 18 tilted and deformed and show evidence of rapid deposition under a hot semi-arid climate. By the time bed III was deposited the cover of volcanics and other rocks above the Catalina Gneiss was removed. Pebble composition and distribution of sediments in bed III indicates that the streams were originating in the Catalina, Tanque Verde, and Rincon
Mountains.
The mechanism causing the Catalina, Rincon, and Tanque Verde
Mountains to rise is in doubt. The established concept holds that the
Catalinas were uplifted along a series of east-west trending normal border faults. McCullough (1963) believes these mountains were formed as the result of a complex vertical movement. The structure in the mountains is described as a series of lobes which are the result of doming caused by re lease o f heat along fra c tu re in te rs e c tio n s . This doming began not e a r lie r than Late P aleo zo ic, but not la t e r than Lower
Miocene.
Pashley does not consider the idea of doming, but insists that the physiography of the ranges is related to the folds in the gneiss.
He described a series of "large parallel west-southwest trending, doubly plunging anticlines and synclines" and showed that topographic highs were in the axes of the anticlines and topographic lows coincided with the axes of the synclines. Of greater controversy is the mode of dis placement. Pashley claims that there were two movements. The firs t, in mid-Tertiary time, consisted mainly of folding and thrust faulting which elevated the Catalina, Tanque Verde, and Rincon Mountains to give the basin its present configuration. This was followed by erosion and 19 deposition of the R illito Beds. A second movement in the late Tertiary or early Quaternary resulted in normal faulting and was responsible for the slight amount of folding and faulting in the R illito Beds II and
III.
Some writers believe that thrust faulting is impossible because the regional stress pattern was that of tension rather than compression.
Thrusting during the mid-Tertiary was previously used to explain the
Tucson Mountain Chaos Member in which boulders of Paleozoic rock in a sandstone matrix lie above the Late Cretaceous Amole Arkose. It is now believed by Mayo (1966) that the chaos was emplaced by masses of rising andesitic magma. Positive proof of thrust faulting in the Tucson Basin
is lacking, and most geologists accept normal faulting as the mechanism for uplift of the ranges in the area.
4.5 Post-Orogenic Activities in the Tucson Basin
Immediately after the u p lift of the mountain blocks pedi- mentation began on the slopes of the Catalina, Tanque Verde, and Rincon
Mountains. This poorly understood process occurs in areas of scant
rainfall on slopes that yield easily to weathering. Pediments are ero
sion slopes cut across bedrock and are usually veneered with alluvial
and residual gravel. The continuous apron of alluvial fans which, along
with the pediment, make up the piedmont slope of a range is called a
bajada. Pediments meet the mountain slope at a distinct angle rather
than gently sloping as in the case of alluvial fans. The pediment on
the Catalina Mountains truncates both the gneiss and the deformed 20
Tertiary sediments. The manner in which pediments develop has not been clearly determined but they are described as bedrock surfaces over which products from retreating mountain fronts are transported to the basins.
The Middle Pliocene saw a climate of greater aridity but milder temperatures than exist today. During this period the ancestral drain age in the Tucson Basin seemed to be blocked, possibly as a r e s u lt o f volcanic outpourings near the northwest corner of the basin. In the central basin a body of sand, s ilt, and gravel were deposited in angular unconformity over the recently deformed R illito Beds. These sediments are called the basin fill deposits and were derived to a great extent from the R illito Formation.
During the middle and Late Pliocene and Early Pleistocene nearly
all of the basins in the eastern part of the province were the scenes
of large standing bodies of water. It is certain that lakes occupied
basins around Safford and Hi11 cox, but it is believed by Melton that
lakes were not present in the Eloy, Salt River, and other Western Arizona
basins. It is not certain where the westernmost edge of the lake belt
lay. Although it has not been established that there were any large
lakes in the Tucson Basin, some deposits of uniform sand and s ilt in
the basin fill seem to have a lacustrine character. Lakes were plenti
ful in the Basin and Range until the middle Pliocene. By this time the
climate in Southern Arizona was very dry, but colder than now. By late
Pliocene the water available for lakes became very scarce and evapora
tion began to exceed replenishment. Today salt layers in basin fills
give evidence of these ancient lake bottoms. 21
4.6 The Pleistocene Epoch
The salient feature of the Pleistocene Epoch, which includes the present day, is the ic e ages. The P leistocene began between one and two m illio n years ago as a r e s u lt o f worldwide cooling clim ates and the accompanying formation of large ice sheets. Four times glaciation ad vanced and retreated across North America. Although none of the glacia tions ever reached as far south as Arizona, the extreme climates played a part in the physiography of the Basin and Range.
During the times of maximum glacial advance so much of the world's water was locked up in the ice sheets that sea level was about 400 feet lower than it is today. Thus, the distance from the Tucson Basin to the source of water for precipitation (the Pacific Ocean and the Gulf of
Mexico) were greatly increased. Thus, the ice ages in Arizona were very cold and very dry. The lakes in the eastern basins became increasingly intermittent and before the beginning of the third glacial advance, they became extinct. The third and fourth glacial stages (the Illinoian and
Wisconsin Glaciations) were times of frigid temperatures, frost action, and rapid mechanical weathering in the Basin and Range. It was during this time that the Quaternary terrace deposits were lain disconformably over the alluvial basin fill deposits. At some time during this period the darning of the ancestral drainage in the Tucson Basin was ended, probably as a result of the floods caused by melting ice and snow on the higher mountains. At the close of the Wisconsin Age (possibly
20,000 years ago) the sediment supply increased greatly, and the maximum height of the basin fill was reached. Rainfall was cyclic and during the very dry spells, the three or four caliche zones were developed beneath the topsoil.
4.7 Recent Geologic History
The last two or three thousand years have been a time of erosion and degradation in the Tucson Basin. Four cycles of erosion and deposition are recognized in the basin and they are expressed in the form of terraces cut in the Quaternary f ill. The oldest and highest terrace is the University Terrace whose surface may represent the top of the original valley f ill. The deposits on the younger terraces (the
Cemetery Terrace, Jaynes Bench, and the Bottomland) consist of recent f ill, as they were built by excavation followed by deposition. Each trough is narrower and shallower than the one before.
At the beginning of the erosional period R illito Creek was 1-1/4 mile south of its present position. During the firs t cycle the basin streams, which must have carried a greater runoff than now, carved the
Cemetery Terrace and deposited the soil which caps it. Following the
Cemetery deposit, the streams cut Jaynes Bench in the deeper f ill, as the
R illito migrated northward. This same cycle was repeated to form the floodplain in which the R illito and Santa Cruz Rivers flow today.
The soils deposited on the terraces, particularly on the two most recent ones, deserve special attention. They are often composed of uniform s ilt which is heavily organic. These soils were deposited
in such a way th a t they have an unusually low d en sity and t h e ir unique s tru c tu re allows them to decrease in volume by as much as one th ir d 23 under certain conditions of loading and saturation. These soils belong to a group loosely classified as "collapsing soils."
The last chapter in the geologic history of the Basin and Range was written by man. In the mid-nineteenth century ranchers arriving from the east found Southern Arizona covered with sacaton grass. This plant with its thick intertwining root system kept the top foot of soil moist most of the year. Realizing that the sacaton grass is superb grazing material for cattle, the ranchers brought thousands of stock to the unfenced wilderness. By the turn of the century the sacaton grass was v ir t u a lly gone as a r e s u lt o f overgrazing and m esquite, black brush, and creosote bush had taken its place. These new plants do not have the root systems needed to protect the soil from erosion, thus gullying has been a problem in the Southwest ever since. These gullies or arroyos are especially apt to form in the recent beds where the collapsing char acter of the soil aids the process; but they cut into older valley fill as well, possibly following fractures in caliche layers.
Today man continues to tamper with his geologic environment.
The results are not always pleasant and are seldom expected. The wide spread occurrence of foundation failures in Tucson is a case in point. 5. INFLUENCE OF GEOLOGY AMD HYDROLOGY ON FRACTURE PATTERNS
What causes foundation failures? It is certain that sub-surface conditions must play a role. Geologic factors contributing to the prob lem may be collapsing s o il a t the su rface , te c to n ic a c t iv it y , and basin subsidence due to groundwater depletion. Thus, the engineering prop erties of the soil and rock in the Tucson Basin should be considered in an attempt to correlate them with the foundation fracture patterns.
5.1 Role of Terrace Deposits
The weight o f a one sto ry b u ild in g has l i t t l e e ffe c t on most soils, and the influence of this extra load diminishes rapidly with depth. For example, assume a house has dimensions o f 40 by 60 fe e t and weighs 100 tons. If the footings are 18 inches wide, the load is trans mitted to the soil over 290 square feet. This is equivalent to 2000 X 100 690 pounds per square fo o t or 4 .8 pounds per square 290 inch. Using a Boussinesq analysis for a long foundation in a sem i-infi
nite homogeneous isotropic elastic solid (which is an inexact but
acceptable substitute in the case of soil) it can be calculated that
the increase in stress three feet directly below the surface is 1/3 the
uniform foundation pressure or 1.6 psi. Twelve feet beneath the surface
the effect of the surcharge is to increase the pressure by but 0.08
times the surcharge load or 0.4 psi.
24 25
It is obvious that the weight of a one story building causes significant stress increase in only the top few feet of the soil. Thus, the firs t suspect in seeking an explanation for foundation failures is the surficial soil. Unfortunately, a thorough study of the engineering properties of the surface soils in Tucson does not exist; indeed, such an undertaking would re q u ire enormous time and la b o r. The only s o il map of Tucson which is currently in use is Young's (1931) soil map. Young separated soil units on the basis of their agricultural characteristics and these may or may not be related to the engineering properties.
Young did, however, associate soil types with the various ter races in the city. Figure 4 shows the fracture pattern superimposed on a map of the terraces. It is clear that most of the damage, especially the severe damage, occurs on the Cemetery Terrace. Failures are present on the limited portion of Jaynes Bench covered by the survey, but the
University Terrace is free of all but scattered light cracking.
It is not possible to state in the absence of tests that soils on the University Terrace provide stronger foundation material than those on the other te rra c e s . The Cemetery Terrace encompasses a g re a te r por tion of the city and includes more of the recent small construction.
Moreover many areas on the Cemetery Terrace are not plagued with founda tion failures.
The soils capping the younger terraces do, however, fa ll into an engineering classification different from that of the University Terrace soils. The University Terrace contains, for the most part, pre-consoli- dated soils while those on the three depositional terraces are normally 36 th St th 36 — — o 22nd St 22nd Broadway < Broadway Speedway Figure 4. Map Showing Fracture Pattern Superimposed on Terraces in Tucson (Terraces by G. Smith, 1938). 27 consolidated soils. The term pre-consolidated refers to a soil which once bore a heavier load than it does under its present overburden pres sure. It is a soil which has been unloaded, possibly as the result of erosion, glacial action, or man-made excavation. In the case of the University Terrace, it is likely that several feet of soil have been removed by sheet ero sio n , because larg e numbers of boulders and cobbles seem to have been l e f t behind as the sm aller p a rtic le s washed away. I f s ix fe e t o f s o il weighing 130 pounds per cubic fo o t have been removed, the surface soil has been pre-consolidated to a load of 690 psf, which was the load on our hypothetical footing. Thus, we would not expect the building to settle much since the soil was once consolidated under an equivalent load. (The soil would have experienced partial re bound following the erosion.) Although it is not known whether any soil in Tucson has experienced such a heavy pre-consolidation load (this could be determined experimentally by correcting consolidation curves by the Schmertmann Method), any amount o f p re -c o n s o lid a tio n would act as a hedge against settlement. Thus, we should not be surprised to find fewer foundation problems on the University Terrace. Most of the soils on the other terraces are normally consolidated; that is, they have never been under a greater load. Many of these soils were deposited in a water deficient environment and have uncommon structures. Cohesionless silts and fine sands which settle out of s till water o fte n have lower d e n s itie s due to an unstable honeycombed s tru c tu re . Soils such as these are likely to experience consolidation when a load is placed on them. Structures placed on these terraces are more susceptible to settlement. 28 In reality, the correlation between terraces and soil conditions is more complicated than has been presented here. During the process of development in Tucson there has been a great deal of cut and f ill in order to provide the gentler slopes desired in an urban area. Man-made f ill cannot always be recognized, and since its degree of consolidation is unknown (it is probably small), fill presents a problem. On the other hand, cut surfaces are more stable because their soils are pre-consolidated. 5.2 Role of Collapsing Soil Many soils in the Southwest exhibit such appreciable loss of volume upon wetting and loading that they are called "collapsing s o ils ." They are a common phenomena among recent deposits in a rid and semi-arid climates. Collapsing soils can have various structures, but are generally described as consisting of loosely packed grains held together by clay binders, cementing agents, surface tension, and possibly electrolytic bonds. The processes by which these soils are deposited are poorly understood, but transport in the water deficient environment must play a major part. Sophisticated experiments are currently being conducted to deter mine the microscopic structure of some collapsing soils. In the field it is not always possible to know whether a soil is of the collapsing variety or not, even after soil tests have been made. Two clues, how ever, are density and Atterberg Limits. Because a collapsing soil must have a high void ratio, its dry density is lower than normal. Soil den s itie s have been recorded as low as 80 pounds per cubic fo o t, but any figure below 120 pcf should arouse suspicion. 29 Atterberg Limits are values of moisture content of a soil as it passes from one physical stage to another. The liquid lim it (LL) gives an indication of the water content above which the soil becomes a viscous liquid. The plastic lim it is an indication of the water content at which a soil w ill deform but not crack when stressed. The plasticity index gives the range of water contents through which a soil is plastic; it is the difference between the LL and the PL. From these parameters it is possible to deduce many things about a soil. A collapsing soil will often have a low LL and a small PI. This suggests that a small increase in water content can cause a soil to go from its normal state to a viscous liquid, practically bypassing the plastic state. A slight drop in the water content then causes the soil mass to become r ig id again. All soils consolidate when loaded; some more than others. The term "collapsing soil" is reserved for those soils which do so most spectacularly. Many very compressible soils are found in the arid Southwest. A general rule seems to be that the younger the deposit, the more likely the soil is of the collapsing type. The most vivid example in Tucson is a subdivision east of Silverbell Road and north of El Rio Country Club. This development, which is outside the region of the survey, is on the Bottomland, and contains the most disastrous founda tion failures in the city. Many homes have cracked so badly that they have had to be abandoned. Soil tests (Atterberg lim its, consolidation, field density, and dispersion) run by geological engineering students have shown that these houses were built on a sandy s ilt which appears 30 very firm when dry, but experiences tremendous collapse under a combina tion of wetting and loading. Apparently, the addition of water acts to break the bonds which hold the loosely packed grains together. If the soil is stressed or vibrated at this time the structure may be destroyed by collapse. Soils of the collapsing variety are likely to be found anywhere in the city, although not all may exhibit such extreme consolidation as the one just described. Undoubtedly, some of the foundation failures are re la te d to th is phenomena. I t is th is w r it e r 's b e lie f , however, th a t the longer a s u r fic ia l s o il has been exposed, the more lik e ly i t has become stable. This is because longer exposure to the elements of nature would allow either more opportunities for the soil structure to density or greater time for the bonds which hold the unstable grains together to strengthen. On the basis of empirical observations then, a general rule might state that the younger the soil deposit, the greater the danger of foundation problems. 5.3 Role of Caliche Caliche is a common occurrence in the soil profiles of the sem i-arid Southwest. I t consists o f sand and s i l t cemented w ith calcium carbonate which has been deposited by the p e rc o la tio n o f subsurface water. Studies show that caliche forms at the bottom of the capillary zone and proceeds upward. It is believed by some investigators that the 10 inches of annual rainfall which the basin now receives is not conducive to the development of caliche; that the present layers must have formed 31 during more arid periods. The depth and thickness of caliche varies widely in the Tucson Basin. In attempting to unravel the basin's stratigraphy, Robert Streitz examined hundreds of well logs kept by the University of Arizona's Department of Agricultural Engineering. From these he made an isopachous map of the caliche layer which is shown on Figure 5. The fracture pattern is superimposed on this map. It would seem that since caliche is as hard as many types of bedrock (ask any contractor who must excavate it) that its presence would reduce the probability of differential settlement, especially if that settlement is the result of deep-seated conditions. Figure 5 does not bear this out. The severe area southwest of 22nd Street and Craycroft lies above thick caliche, while that near Grant and A1vernon has little . The light cracking on the University Terrace corresponds with thin caliche. Apparently foundation failures are in no way related to the caliche. 5 .4 Role o f Ground Water D epletion Soil is a three phase material; it contains matter in the solid, liquid, and gaseous states. The volume of a soil mass consists of the solid mineral particles and the interstices (or voids) which con tain air or water or both. The pore water has a great influence on the strength properties of a soil. If the voids are but partially filled with water, as in the soil zone above the water table, the molecular attraction at the boundaries of the three phases give rise to surface tension. This force acts parallel 1 FORT LOWELL RD VUiN 1 VUr< UN1 c.r\YAL lUhtfcLi C" — ^ 20/ ^ io- / ------' <— 1 MILE-^ C ^ - O - GRANT RD rw w h z^T) Z^X - x l0 5 ^ y / ( C > \ r--j l t \\ U / / »- BROADWAY V LJ TO" \^5 0 K / 1 V 22 ND ST ^ ^ ~ 30kj 1 -To-- ^ . ^ _____ 40 In a saturated soil the neutral stress (u) is equal to the product of the unit weight of water times the height (h) of the water column. The effective stress (o) is that part of the total stress (o) which is carried by the soil grains. Thus, o = a + u. This is the most important re la tio n s h ip in s o il mechanics. I t must be remembered th a t w ater can offer no shearing resistance, thus all the shear strength parameters of a s o il depend upon the e ffe c tiv e s tre s s . Assume the water table is 10 feet below the surface of a soil weighing 100 pounds per cubic fo o t dry and 120 pounds per cubic fo o t saturated. At point A 10 feet below the water table the neutral stress is 10 X 62.4 = 624 psf, and the total stress is 10 X 100 + 10 X 120 = 2200 psf. The soil must then carry an effective stress of 1576 psf. If the groundwater table is drawn down 10 feet, the neutral stress at point A is zero, the total stress is 20 X 100 = 2000 psf, thus the effective stress is 2000 psf. The column of dewatered soil is now subjected to a g re a te r e ffe c tiv e stress than before drawdown. In most s o ils th is added load w ill cause consolidation. 34 Subsurface aquifers arc the only source of water in the Tucson Basin and in most other basins in the Southwest. Ground water with drawal has boon greater than the natural recharge in most cases. As a result, dewatered layers have consolidated and ground surfaces have subsided. A most striking example is the heavily irrigated Picacho-Eloy Basin between Tucson and Phoenix. Here, earth fissures were reported as early as 1927. U. S. Coast and Geodetic Survey leveling showed the basin subsided 3.6 feet between 1905 and 1960 and 7 feet between 1905 and 1968. D. E. Peterson studied the area and concluded that the subsidence was due to increased effective loading on clay lenses. U. S. Geological Survey and U. S. Coast and Geodetic Survey leveling data indicates that there has been land subsidence in the Tucson Basin. In the course of the city-wide survey, the writer observed a few small fissures in vacant lots near areas of foundation failures, but they may have been due to desiccation rather than subsidence. It would be in te re s tin g to know how the amount o f subsidence in severely fractured areas compares with that in trouble-free areas. The available survey lines, however, do not provide reliable data since they are usually not tied to stationary points. Lines run by the University of Arizona Department of Agricultural Engineering between wells in the city indicate elevation changes in the order of 0.2 to 0.6 inches over about 10 years. Differential settlement of this magnitude could cause local foundation problems. Figure 6 shows the ground water contours in 1967 superimposed on the fracture pattern. The water table drops gently across the city 35 Flowing V/ells Rd O r o c I e Campbell Avc Country Club 'Alvernon v/oy Swan* Rd Croycro ft Rd VVilmont R Figure 6. Ground Water Table Contour Hap (Based on Data from Department of Agricultural Engineering, 1967). 36 toward the northwest, but shows no correlation with the foundation failures.. One would expect the subsurface strata to have a wide range of permeabilities, but they must not be so extreme as to cause abrupt deviations in the flow lines, except possibly east of Alvernon and south of Broadway. One might expect a relationship to exist between thicknesses of the dewatered aquifers and the intensity pattern. It is reasonable that greater drawdown should increase the likelihood of subsidence. Figure 7 is an isopachous map of sediments dewatered from 1947 to 1967. The map shows that there has been greater removal in the south and east portions of the city during the past few years. Whether there is a relationship between withdrawal and fracture patterns is open to interpretation, but the writer believes that a subtle correlation is possible. The role of ground water depletion involves not only the thick ness of sediments dewatered, but the lithology of these units. Never theless, we should expect that areas of heavy withdrawal would be more likely to have foundation problems. Figure 7 shows greater withdrawal near the general vicinity of Grant and Alvernon (the area of most severe failures) and also heavy withdrawal around 22nd and Wilmot, which is also experiencing difficulty. The question arises as to the role of subsurface stratigraphy. The best work th a t has been done on th is su b ject is Robert S t r e it z 's thesis (1962) in which he examined hundreds of well logs in the city, and utilizing the most revealing ones, prepared a fence diagram of the Wotr Flowing Wells Rd Oracle Rd Compbol I Compbol Swan Rd Country Club l.mont ‘V/l Kolb Kolb Rd Croycroft Alvernon to 1967 (from Department of Agricultural Engineering). Contour Interval 10 F© Figure 7. Isopachcous Hep of Sediments Dewatered in Tucson from 1947 38 city's subsurface. Three sections from Streitz's work have been selected by the author and are shown in Figures 8, 9, and 10. Figures 8 and 9 show soil profiles through the two most troublesome areas of the city, while Figure 10 shows a line across the University Terrace where foundations are stable. In Figures 8 and 9, the predominant unit being dewatered is a sand, while that in Figure 10 is a sandy clay. A d rille r described (by personal communication) a well near a damaged home on Hi!mot Road as penetrating the water table in a unit of fine well- rounded quartz sand. It is possible that these subsurface sand layers have a lower than normal density and the removal of the in terstitial water could cause settlement greater than would be experienced by a com pressible clay layer. Also, the consolidation of the sand would be much more rapid than in the case of clay. Using Streitz's fence diagrams and ground water level data from the Department of Agricultural Engineering, the writer prepared a litho- facies map of sediments dewatered from 1947 to 1967. The map is shown on Figure 11. This lithofacies map must not be interpreted too literally because it was necessary to extrapolate between lines on the fence diagram and also because more than one unit may have been dewatered during that time period (in which case the predominant unit is shown). Figure 11 seems to indicate, however, that a sizeable portion of the area affected by foundation failures lies above layers of dewatered sand. 39 Fort lov.ell Rd cracking Heavy Moderate Light Grant Rd U*eO LEGEND 1 1 1 Co t i c he Clav G rqve 1 Conglomer ote — — — Water level 1948 Sand s i ® ---- Water level 1967 Figure 8. Cross Section Through Severely Fractured Area in Tucson (subsurface data by Robert S treitz). 40 CRACKING Heavy Moderate Golf-Links Rd. 0 feet LEGEND Caliche i H Grovel Sand C lay Water level 1948 SCALE Water leve I Vertical Exaggeration 14.3 I I Figure 9. Cross Section Through Severely Fractured Area (subsurface data by Robert S treitz). 41 Grant Rd ----Av LEGEND Col i c he Clay G rave I Water level 1948 Sand Water Level 1967 Figure 10. Cross Section Through Trouble-Free Area in Tucson (subsur face data by Robert S treitz). leg end v>. ~ - p :::::: Grovel m G ra n t: Rd E H ] Sand m • • • • * • . •••-*, 111Ik C lay a 1 ® ■ m SiE m Campbol 1 m m Broadway ■■ s x 2 2 nd St ^ !;mm m I* 6 9 l mm « m Figure 11. Lithofacies Map of Dewatered Sediments (1947-1967). 43 5.5 Role of Basement Geology and Tectonic Activity Tectonic activity refers to any process or mechanism which results in a deformation of the earth's crust. Common examples are faulting, folding, doming, and sinking. Tectonic activities may be gradual, requiring great stretches of geologic time, or they may be sudden and catastrophic, resulting in alteration of the landscape and local property damage. If tectonic activity be the cause of any founda tion failures in Tucson, then one must expect that the activity should have been recorded immediately proceeding the failure. Platt suggested that a slow tectonic process might be responsible fo r d if f e r e n t ia l movements around the periphery o f the basin. He be lieves that the entire basin is subsiding (due to consolidation) and that the greatest subsidence is in the center of the basin where sediments are th ic k e s t. V e rtic a l movements would be sm aller close to the mountain slopes, but stress concentrations would be greater, resulting in more planes of slippage. The theory is reasonable, but this writer does not feel that there is enough evidence (in the form of earth cracks) to affirm this position. Several m illion years ago the Basin and Range Province was an active regional tectonic belt. Pashley believes that even after the mountains surrounding Tucson were lifted into their present configura tio n , movements (in c lu d in g a m ajor one) took place along the C a ta lin a Fault. The folding and faulting in the R illito Beds gives evidence that there was significant tectonic activity as recently as the early Quaternary. 44 The basin f ill above the R ill 1 to Beds has not been disturbed (this is the significant distinction between the two units) suggesting that tec tonic activity in the basin has greatly diminished in recent time. Figure 12 is a structural contour map showing where Pashley believed to be the contact between the basin fill and the top of the R illito Beds. The gradient is gentle and roughly parallels the basin periphery. There seems to be no correlation with the fracture pattern. Sensitive seismic instruments at the Tucson Magnetic Observatory record about twenty tremors a month. These are called Tucson locals, and it has not been established whether any of these shocks have an epicenter in the basin. Faults are a major structure in the mountain blocks surrounding Tucson, but they are not believed to have been active re cently. A major shock was recorded in Tucson around 1880. Thus, the possibility of an occasional local tremor in the Basin and Range should not be completely dismissed. Any shock, whether from a small seismic tremor, thunder, je t airplanes, or operating machinery could contribute to settlement. Energy may be transmitted from grain to grain and cause rearrangement of the soil structure resulting in lesser volume. This situation would be most likely to occur in the low density soils during times of saturation. 5.6 Results of the Geologic Investigation It would appear that the unstable surface soils and ground water depletion are equally responsible for the foundation failures in Tucson. The case for unstable surface soil is strengthened by the fact • v: v s ..v;.r?^'"•.’’r •’ 'V5,v*f-v'-i > 'PrTncV Rd Fort Cowett ^Rd 2200 Grant Rd ..Speedway 2000 .* : : Broadway CONTOUR INTERVAL 100 FEET 22nd St SCALE 3 MILES '36tS S* Figure 12. Structural Contour Map Showing Contact Between R illito Beds and Quaternary Basin F ill (from Pashley). 46 that problems occur more frequently on the recent depositional terraces. If surface soils were the complete answer, however, we would expect buildings to settle soon after they are built since the soils are mainly sands and silts. Comparison of the writer's work with that of Platt in 1959 indicates that new patterns are forming in established areas previously free of failures. It is the writer's belief that subsidence is occurring in the Tucson Basin as a result of ground water withdrawal. Units presently being dewatered are Late Tertiary and Early Quaternary basin f ill deposits. The units causing settlement are sands and silts; they are probably the remnants of small lakes v/hich fille d with wind blown sedi ment during the firs t two ice ages. These lakes may s till have con tained water while they were fillin g , thus the soils have abnormally low densities. They were covered during the rapid deposition of the Illin o ia n and Wisconsin Ages. The la c u s trin e beds have probably been submerged from the tim e of d ep o sitio n . The removal o f th is w ater by man has caused rapid co n so lid atio n o f the sand (sand and s i l t w ill experience most of their consolidation within a very short time, while a clay re quires years). This settlement not only causes differential displace ments o f surface s o ils , but may generate shock waves which cause the soil aggregates above the water table to occupy a lesser volume. Thus, subsidence may trigger partial collapse of low density soils. 6. ENGINEERING ASPECTS OF FOUNDATION FAILURES An engineer uses knowledge gained from scientific investigations to solve problems confronting mankind. The two most important param eters in any engineering project are people and money. The engineer must always keep in mind that the ultimate goal of his work is to improve the health, welfare, and safety of the public, but that his ideas will not be successful unless they are economically feasible. Foundation failures, land subsidence, and unstable soil could affect all the people in Tucson. While cracked walls and broken plaster may seem minor annoyances in light of other problems confronting the public today, the ground instability problem in Tucson is serious and should be of interest to all persons engaged in planning and building in th is c ity . 6.1 Ground In s t a b ilit y and the Public Many people, including public officials, are reluctant to believe that the phenomena described in the previous chapters constitute a hazard to the public. While foundation failures in one-story build ings are extremely unlikely to result in injuries or loss of life , ground instability could give rise to other dangers. In 1963 a disastrous explosion occurred in the basement of a cleaning establishment in Tucson, resulting in several injuries and deaths. The accident was found to be the result of several leaks in the gas line serving the establishment. 47 48 In court, the u tility company was found to be negligent, but it is not unreasonable to speculate that differential settlement may have played a part in causing the leaks. If a heavy building such as this were to experience differential settlement, the u tility lines (which are prob ably embedded in the concrete) would be stressed and could crack. Underground u tility lines of all types would be in danger in zones of stress concentration caused by basin subsidence. Besides gas leaks, the greatest threat to the public would come as a result of fractured sewer lines. Raw sewage could sink through earth fissures and pollute the city's water supply. The public is virtually unaware that ground instability is a problem in Tucson. When severe cracking develops in their homes, indi vidual owners become upset; when a tragedy such as at the cleaning plant occurs, there is a demand for investigation; and should the city ever be forced to announce that the basin's water supply needs additional treat ment because of pollution, the public would be dismayed. Since there is obviously no alternative to the continuation of ground water withdrawal in Tucson, land subsidence will continue in the future. In addition, low density soils which cause foundation problems in even one-story, single family dwellings are being increasingly discovered in areas of new construction in the city. For these reasons, it is important that engineers, architects, city planners, and public officials become aware of the extent of the problem and that they anticipate future develop ments. 49 6.2 Recommendations for Zoning and Planning The Tucson Zoning and Planning Commission oversees expan sion of the city by delimiting certain areas for specific types of urban and suburban development. The commission centers its attention on the social and environmental aspects of building locations. At the present time it makes no attempt to include site suitability (based on ground conditions) in its considerations. This should change; a specific a tti tude toward building on each of the four terraces should be adopted. building of permanent structures should be prohibited on the Bottomland Terrace, not only because of the probability of finding col lapsing soil, but also because this terrace is subject to flooding. The small percentage of land covered by such a restriction could be used for parks, golf courses, farming and ranching, and mobile home sites. Jaynes Bench includes only a small percentage of the present city, but it is more prevalent on the southeast side where many new subdivisions are being built. The local governments should work in cooperation with the University of Arizona and with local engineering firms to make a detailed soil study of the area south of Tanque Verde Wash between Pantano Wash and Old Spanish T ra il. The soils should be tested to deter mine their behavior under saturation and loading. Plate-bearing tests on dry soils in the field are insufficient; soils must be loaded while saturated. Zones found to contain collapsing soils could be delimited and future builders would be advised of their nature. Areas of except io n a lly weak s o il should be proscribed. 50 Zoning on the Cemetery Terrace presents a problem. Most of the failures occur here, but subsidence may be the major cause. In the absence of conclusive soil tests it is not recommended that zoning restrictions be made at this time. If information concerning present failure patterns is made available to responsible contracting and en gineering firms, many of them might take steps to prevent problems. The U n iv e rs ity Terrace has few foundation problems and no zoning restrictions are needed. Local failures may occur on this terrace if buildings are placed on uncompacted f ill. For all major buildings, a s o il re p o rt should be made. 6 .3 Recommendations fo r B uilding Codes The Inspections Department of the City of Tucson demands builders comply with the 1964 Uniform Building Code of the International Conference of Building O fficials, Pasedena, California. It is expected that the 1967 edition of this same code w ill be adopted. This national ly used code is probably sufficient for most phases of construction, but because of its widespread use it cannot take into account special problems of the various localities. A few minor addenda are occasionally added by the city to meet local problems, but with regard to foundations, a thorough revamping is needed. For a one-story, single-family dwelling, the code requires that footings be 16“ wide, 8" thick, and 12" below grade. In a severe area south of Tucson, the city has made the arbitrary requirement that footings be 24" wide and 18" below grade. If land subsidence be the cause of this 51 particular problem, then increasing the size and depth of the footings serves no purpose. O fficials in the city Inspections Department insist that founda tion failures are not so severe a problem in Tucson as to justify stricter building regulations. If the problem should become so severe in the future (a not unlikely occurrence) that action was demanded, the following policies should be considered. In areas of low density or "collapsing soil", the foundation material should be stabilized. It is not satisfactory simply to warn owners not to water close to the footings, because the soil rather than the water is the villain. While it is true that saturation is the catalyst which causes low density soils to collapse, a homeowner has the right to expect that he can land scape his property without having his house crack. The most practical method o f ground s ta b iliz a tio n is to p re-lo ad the s o il and a t the same time saturate it, thus drastically reducing the void ratio before erect ing the footings. In areas where subsidence is causing failures stronger founda tions arc needed. This problem originates at depth, so treatment of the surface soils is of no value. The builder must instead seek a structure which can absorb d if f e r e n t ia l movements w ith o u t being damaged. Frame buildings meet this need as do buildings paneled with aluminum siding. These materials are not popular, however, as the public demands brick and concrete structures. But brick and concrete cannot carry tensional stress and offer no yield. Therefore, the masonry buildings must be so constructed as to settle without cracking. 51 particular problem, then increasing the size and depth of the footings serves no purpose. O fficials in the city Inspections Department insist that founda tion failures are not so severe a problem in Tucson as to justify stricter building regulations. If the problem should become so severe in the future (a not unlikely occurrence) that action was demanded, the following policies should be considered. In areas of low density or "collapsing soil", the foundation material should be stabilized. It is not satisfactory simply to warn owners not to water close to the footings, because the soil rather than the water is the villain. While it is true that saturation is the catalyst which causes low density soils to collapse, a homeowner has the right to expect that he can land scape his property without having his house crack. The most practical method o f ground s ta b iliz a tio n is to p re-lo ad the s o il and a t the same time saturate it, thus drastically reducing the void ratio before erect ing the footings. In areas where subsidence is causing failures stronger founda tions are needed. This problem originates at depth, so treatment of the surface soils is of no value. The builder must instead seek a structure which can absorb d if f e r e n t ia l movements w ith o u t being damaged. Frame buildings meet th is need as do b uild ing s paneled w ith aluminum s id in g . These materials are not popular, however, as the public demands brick and concrete structures. But brick and concrete cannot carry tensional stress and offer no yield. Therefore, the masonry buildings must be so constructed as to settle without cracking. 52 Fractures in brick and masonry appear not because the structure has s e ttle d , but because the settlem en t has not been equal over the entire perimeter of the building. It is differential movement which causes stresses to develop in the foundation. If these stresses are greater than can be absorbed by the foundation material, they w ill seek release in the form of cracks which may spread from the footings to the walls and floors. To avoid this situation foundations must be strong enough to withstand stresses caused by differential movements. Steel reinforcing must be placed in the trench footings which serve as foundations for most one-story buildings in Tucson. The re inforcing in the footings must be tied to reinforcing in the floors and stem walls so that all parts of the structure which transmit load to the soil behave as a unit. In Tucson most homes have flat-slab concrete floors; a few inches of gravel are placed on grade and the concrete is poured over it. Builders should use hogwire or some other wire mesh to protect the slab from cracking, but it is most important that the mesh be tied to the steel in the footing. Otherwise, the floors might move separately from the footings and the outside walls. 6 .4 Recommendations fo r U t i l i t i e s Ground in s t a b ilit y poses a special hazard fo r buried u t i l i t y lines. While long steel pipes can absorb much stress without breaking, problems may occur at the interface between the building and the ground. I f the pipe is imbedded in the con crete, i t is l ik e ly to be sheared i f 53 the building experiences differential movement. For this reason, aper tures in the floor should be provided for all indoor plumbing and other u tilitie s should enter the house above the foundation. The effects of settlement on sewer lines is in doubt. Surely, large diameter clay pipes would crack if subjected to deep-seated earth cracks and land subsidence. The public can probably take some comforts, however. Small cracks would tend to clog with solid matter, preventing fu rth e r seepage. I f only a small amount o f sewage seeped in to the e a rth , it would move slowly and be reduced under anaerobic conditions before reaching the water table. The only suggestion that can be made is that if ever obvious earth cracks are observed (similar to those in the Picacho Basin) then sewer lines in that area should be unearthed and rep aired . 6.5 Future Foundation Problems in Tucson The ground instability problem in Tucson has been observed and documented for several years. There is no doubt that it w ill worsen as good building sites become scarcer and ground water continues to be withdrawn without replacement. The intensity pattern of building foundation failures w ill change in time. The writer expects that within the next few years many more cases of foundation failure w ill be reported in the new subdivisions east oi Pantano Wash, th is as a r e s u lt o f con solidation o f low d en sity s o ils . Figure 13 shows areas in which land subsidence is likely to continue. This prediction, based on Streitz's fence diagrams, shows where the sand iue 3 Mp hwn Zns kel o xeine ad usdne n te Future. the in Subsidence Land Experience to ly e ik L Zones Showing Map 13. Figure I s* Avo < o Cqmpbcll > x Speedway Broadwov at Rd rant G 'll! ot o/l Rd;. Lov/ell Fort „ i i 6 or -b c > o SI ' y ; H! HIiii ' ■ ' s% 1 22 d t w St nd 1 § Hiv? IBIS Iii -^1 #• e* * *• ##«• •e * l —» ile M 1 CZ TJ 0 . a: “O J c it s * .v*. JQ £ O P IN 55 u n it supposed to be one of the p rin c ip a l causes o f tro u b le remains below the water table. More problems may be expected in the vicinity of Kolb Road south of Broadway. D ifferential land subsidence and accompanying earth cracks are likely to occur where the water table begins cascading due to the sharp increase in the pediment slope (see Figures 6 and 12). It is not likely that the area surrounding downtown Tucson w ill experi ence significant foundation problems until the water table drops a hundred or so feet. 7. SUMMARY AND CONCLUSIONS Field investigations have provided detailed information on the location and intensity of foundation failures in.Tucson. A study of the geography and geologic history of the basin have convinced the w rite r th a t ground i n s t a b ilit y is the r e s u lt o f two separate phenomena; low density surface soils and deep-seated land subsidence aggravated by ground water removal. Fractures in walls and floors are an expensive nuisance to home- owners, while broken u tility lines present a hazard to the public. Stronger construction is needed to counter these problems, which w ill continue in the future. Extensive .fracture patterns have been correlated w ith a subsurface sand u n it which has been slow ly dewatered fo r the past decade. Newer homes in the southeast are being built on terraces with a history of unstable soil. Possibly, the most important step to be taken at this time is to inform the general public of the nature and ramifications of the problem, and to provide architects, engineers, and planners in Tucson with all the technical information available relating to local ground instability. 56 REFERENCES Anderson, R. Y ., Cenozoic climate in the arid Southwest; 1962 Arizona Geological Society Digest, Vol. 5, pp. 25-35. Barghoorn, E. S ., 1951, Age and environment: a survey of North American Tertiary floras in relation to Paleoecology; Journal of Paleont ology, Vol. 25, pp. 736-744. Brown, W. H ., 1939, Tucson Mountains: an Arizona Basin Range type; Geol. Soc. of Am. Bull., Vol. 50, pp. 697-759. Damon, P. E., Erickson, R. C., and Livingston, D. E., 1962, K-Ar dating of Basin and Range u p lift, Catalina Mountains, Arizona; Nuclear Geophysics, N.A.S.-N.R.C. Pub!. 1075, pp. 113-121. Davis, R. W., 1967, A geophysical investigation of hydrologic boundaries in the Tucson Basin, Pima County, Arizona; University of Arizona Ph.D. Thesis, 61 pp. Kidwai, Z. U ., The relationship of groundwater to alluvium in the Tucson area, Arizona; University of Arizona M.S. Thesis, 1957, 53 pp. Lacy, W. C ., 1964, Geological causes of foundation failure in the area of Tucson, A rizona; AIME Transactions. Longwel1, C. R ., 1950, Tectonic theory viewed from the Basin Ranges; Geol. Soc. Am. B ull., Vol. 61, pp. 413-433. McCullough, E. J ., 1963, A structural study of the Pusch Ridge-Romero Canyon area, Santa Catalina Mountains, Arizona; University of Arizona Ph.D. Thesis, 67 pp. Maddox, G. E ., 1960, Subsurface geology along northwest R illito Creek; University of Arizona M.S. Thesis, 232 pp. Mayo, E. B., 1966, Preliminary report on a structural study in the Museum Embayment, Tucson Mountains, A rizona; Arizona G eological S o ciety Digest, Vol. 8, pp. 1-32. Melton, M. A., 1965, The geomorphic and paleoclimatic significance of alluvial deposits in Southern Arizona; Journal of Geology, January, pp. 1-28. 57 58 Pashley, E. F., 1966, Structure and stratigraphy of the Central Northern and Eastern parts of the Tucson Basin, Arizona; University of Arizona Ph.D. Dissertation, 268 pp. Platt, W. S ., 1963, Land surface subsidence in the Tucson area; Univer sity of Arizona M.S. Thesis, 37 pp. Smith, G. E. P ., 1938, The physiography of Arizona valleys and the occurrence of ground water; University of Arizona Agricultural Experiment Station Bull. 77. Streitz, R., 1962, Subsurface stratigraphy and hydrology of the R illito Creek-Tanque Verde Wash area, Tucson, Arizona; University of Arizona M.S. Thesis, 57 pp. Wilson, E. D., 1962, A resume of the geology of Arizona; Arizona Bureau of Mines Bull. 171. Young, F. 0 . , e t al., 1931, Soil survey of the Tucson area, Arizona; U. S. Department of Agriculture, Series 1931, No. 19. north 5 : \ v 'r - : FIGURE 14 INTENSITY PATTERN OF BUILDING FOUNDATION FAILURES IN TUCSON, ARIZONA STRONG MODERATE I MILE LIGHT ROBERT CPOSSLEY Ji 1968 o ^ e m t I9 i> ?