~ '01. 6, No.2 FlELDNOTES Page 5

Slope Form and Stabilityin the Northwest portion of tbe Mount Lemmon Quadrangle, Pima County

by Bruce J. MurphyAssistant Field Geologist

Introduction

During the past two decades, thepublic has become increasingly aware ofcatastrophic events involving the massmovement of rock and soil within urbanareas. The problem of landslides has beenparticularly severe in the Greater LosAngeles region. In one period in 1952,two people were killed and over 100houses were damaged there by land­slides; the monetary loss was almost 7.5million dollars. Up until that time, land­slides had constituted only a minorhazard, but with the advent of denserhousing on steeper slopes, massive slides

. of rock and soil were triggered by anabnormally rainy season.

The study of landslides in urban areashas been gravely neglected for years byboth professional and public groups alike.As one of the deans of engineeringgeology, Charles P. Berkey, wrote pro­phetically in 1937:

"I am convinced that the question oflandslides is a matter of much largerimportance than is usually assumed.Recent experience lends to the beliefthat it is of special significance inconnection with many practical prob­lems, particularly those connectedwith engineering projects. In myown case, some of these featureswere for a long time overlooked, andit is clear that a better understand­ing of them would have been useful."(Personal communication to C.F.Sharpe, March 31, 1937).Yet it wasn't until after the devastating

landslides of 1952 that responsible electedofficials acknowledged the need to limitconstruction on unstable ground by enact­ing restrictive legislation. Special gradingcodes, designed to withhold buildingpermits from hillside lots until conditionswere shown to be safe by the geologistand civil engineer, were initiated. Theserestrictions have since resulted in amarked reduction in damage and in mostcases a decrease in surface erosion.

Following the example made in pastyears by officials from the West Coast,individuals and groups in other parts ofthe country have forseen the need todelineate potentially unstable slopes.Arizona, witnessing the fastest growthrate in the nation and a decrease insuitable land for development withinurban locations, is no exception. Futuredevelopments on steeper slopes and ridgelines necessitate the need for a thorough

site examination to maximize safety andeliminate destructive erosion. The Ari­zona Bureau of Mines is currently explor­ing this topic of potentially unstableslopes with a grant provided by the U.S.Geological Survey. The project is de­signed to study a group of relatedgeologic hazards, including slope stabil­ity, whose impact on Arizonans must beexamined. .

Area of Study

A portion of the Mount Lemmon15-minute quadrangle was one such areachosen for a slope stability analysis (seeFigure 6). The growing communities ofOro Valley and Vista Catalina are locatedwithin this quadrangle. Construction ofsingle dwellings and large, tract-typehousing developments is progressing rap­idly. The area covers approximately 61square miles, and is bounded by theCoronado National Forest on the east,the Pinal County line to the north, andIna Road to the south. Physiographically,the region encompasses a wide range ofgeomorphic features, including the flood­plain of the Canada del Oro and thesteep, precipitous cliffs of the SantaCatalina forerange. Altitudes range from2480 feet to 4400 feet above sea level, andlocal relief is highly variable. Home sitescan be found on various terrain features,ranging from major floodplains to steepmountain ridges in the Tortolita Moun­tains. The general area is expected tohave a large population influx by the year2000.

The study region thus represents aunique situation whereby the dependencyof home sites upon the geologic environ­ment can be analyzed for future safetyand design criteria.

Geology of the Area

The geology of the study area iscomprised almost wholly of unconsoli­dated sediments, although some crystal­line bedrock crops out locally. An under­standing of the relationships between thetypes of materials present and thestability of the natural slopes in the areais critical in order to objectively assessthe potential for hazardous conditions. Abrief review of the rock and soil unitsrecognized in the area follows.

Tinaja beds. The Tinaja beds are onlyexposed along the margins of the SantaCatalina front where erosion has removedthe overlying coarse gravels. The maxi­mum thickness of the beds is unknown

but is thought to be as great as 5,000 feetin some areas (Davidson, 1973). Thesebeds unconformably overlie the PantanoFormation and in turn are unconformablyoverlain by the Fort Lowell Formation.Correlation of the Tinaja beds with theRillito 2,3 formation of Pashley (1966)seems probable.

Generally, the Tinaja beds consist ofsand and gravel, gypsiferous clayey silt,and mudstone. Basaltic andesite flowsand dacite tuffs also occur within theunit. In the Mount Lemmon quadranglethe beds exposed are thought to be theuppermost part of this formation and arecollaborative with Rillito 3 of Pashley.These beds contain abundant graniticfragments in a feldspathic, clay-sandmatrix. The unit is poorly bedded, butlocally dips up to 28° toward themountain front have been observed. Nostructural deformation was found in theMt. Lemmon quadrangle although thetypical section, found in the TucsonBasin, contains numerous faults.

These grayish-white beds contain alarge amount of montmorillonite clay andare only poorly consolidated and weaklycemented. The average slope stands at 15percent and is easily eroded, as evidencedby extensive rilling and gullying.

Fort Lowell Formation. The FortLowell Formation, referred to as "basinfill" by Pashley (1966), outcrops exten­sively along the margin of the SantaCatalina Mountains where up to 400 feethave been removed by post-Fort Lowellerosion (Davidson, 1973). The formationmay be as much as 1,200 feet in thevalley, and it thins toward the mountainsand headwaters of Canada del Oro. Thesimilarity in lithology between the baseof the Fort Lowell and the top of theTinaja beds makes delineation difficult inmost areas.

The Fort Lowell Formation can beidentified on the basis of its grain size,lithology, and to a certain extent itscolor. The deposit is generally flat-lying,crudely bedded, and reddish-brown incolor. The lithology of the basin fillreflects the nature of the surroundingbedrock outcrops, and consists primarilyof gneissic and granitic cobbles in a sandygravel matrix. The beds are generallymoderately to poorly cemented, weaklypacked, and very porous. Imbrecationdirections on a statistically significantnumber of samples indicate a direction ofdeposition in a closed basin environmentinto a series of low-lying lakes andplayas. Structurally, the beds are gentlydipping and relatively undeformed.

Page 6

---------------'-----' ------

ARIZONA BUREAU OF MINES

fN

K::Il5I=:EO:::::>O;;O::::]IiOC:i0......................""'"'=====:::::i2 miles

Scale

EXPLANATION

;::" ROCK FALL..f VERTICAL OR OVERHANGING SCARP

f. SMALL LANDSLIDE OR DEBRIS SLIDE

Figure 6. SLOPE STABILITY MAP

JUlie, 1976

Vol. 6, No.2

Map UnitZone 1

Zone 2

Zone 3

Zone 4

Zone 5

FJELDNOTES

RemarksHighest stability - near-flat to moderate slopes «25%) underlainby hard rock. Locally bare to thin cover of surficial material. Nosignificant downslope movement of material.

Hign stability -:- gentle to moderately gentle surficial alluvialslopes (5%-15%); moderate slopes (15%-25%) underlain by FortLowell Formation consisting of moderately cemented coarsegravel, sand, silt, and clay. Small probability of downhillmovement of material except in areas adjacent to Incised streamswhich have eroded a vertical erosional scarp. Undercutting of thescarp's toe will produce a total loss of stability on an intermittentbut continuing basis.

Moderate stability - very steep competent rock slopes up to100%; slopes in poorly-consolidated fine-grained alluvium up to45%; loose, well-rounded, surficial deposits overlying moderate(15%-25%) to steep (25%-45%) well-cemented Fort LowellFormation. Slopes subject to minor debris slides in well-roundedalluvial material; minor soil slumps in fine-grained deposits wherehighly saturated with water.

Generally low stability - moderate slopes (up to 25%) in poorlyconsolidated fine-grained alluvial deposits containing a highpercentage of clay material. Subject to slumping and high soilerosion during saturation. Very steep slopes (up to 100%) border­ing floodplain scarps or deeply incised highland drainages. Blockglide soil failure in moderately cemented alluvium; soil fall innonresistant soil; rotational block slump failure rare but may belocally present. Soil failure on slopes with vegetation, producingsmall terrace-like features or terracettes. Very steep rock slopescontaining a thin surficial deposit of taluvium (rock rubble andweathered soil-size particles).

Low stability - very steep to precipitous slopes in highlyfractured and weathered rock. Subject to rock falls which in someinstances may produce large volume of material. Debris flowscommon.

Page 7

The first to divide these surficialdeposits into definable groups was Smith(1938), who identified four primary se­quences as: University, Cemetery, andJaynes terraces, and bottomland. Theseunits, lying in an erosional trough onbasin fill or older sediments, can beidentified on the basis of their topo­graphic relationships. Generally, in thearea studied, the Cemetery terrace cov­ers the largest area. The eastern pedi­ment of the Tortolitas contains gravelstentatively correlated with this sequence.The University terrace is found in twoareas: the Cordonnes region, where it lieson top of the Fort Lowell Formation, andin an exposure on the high dissectedfoothills buttressing the west side of theSanta Catalina Mountains. Identificationwas based on a thick caliche bed that ischaracteristic of the University terrace,and a coarse, bouldery, highly dissectedsurface. The Jaynes terrace and bottom­land are usually found adjacent to themain tributary drainages, though one ortwo isolated exposures occur next toCanada del Oro.

The degree of cementation and packingof the deposits generally decreases fromoldest to youngest. While the Universityterrace is well cemented and packed, theCemetery and especially the Jaynes andyounger deposits are essentially uncon­solidated. The high porosity and perme­ability allows these gravels to be welldrained.

Two Approaches to aSlope Stability Analysis

GEOLOGIC MAP UNITSLOPE MAP BEDROCK ALLUVIAL DEPOSITS

Metamorphic rocks Tinaja Ft. Lowell SurficialUXIT Tortolita Catalina

granodiorite gneiss/granite

less 5CC 1 1 2 1 2

5- 15CC 1 1 2 1 2

15- 25S{ 1 1 3 2 3

25- 45S{ 2 2 4 3 4

45-100~: 4 3 5 4 5

Surficial deposits. Surficial deposits(locally-derived alluvium deposited byfluvial processes) cover much of themapping area and can be differentiatedon the basis of this relationship to olderunits. Deposition of these units reflects achange in basin morphology from a closedbasin environment to a through-goingdrainage system. These deposits werelaid down on an erosion surface resultingfrom the lowering of the base level ofnearby streams (Smith, 1938). Surficialalluvium up to 50 feet in thickness can befound locally overlying all units in thearea; the alluvial deposits generally pinch

out toward the mountain fronts.The surficial deposits generally reflect

detritus being presently eroded from thenearby mountain fronts. These sedimentsare mainly coarse gravel and gravellysand with local lens of sandy silt. Cobblesup to 2 feet in diameter are sporadicallyencountered. Granite and gneiss frag­ments dominate, although minor amountsof volcanic and sedimentary fragmentsare also represented. The principal basisfor differentiating the surficial sedimentsfrom the older material is their character­istic weathered, yellow-brown stainedsurface.

The analysis of slope stability is ofinterest to both geologists and engineers,but from differing viewpoints. Whereasthe geologist examines slopes from thepoint of view of shape and the process bywhich they were formed, engineers takea quantitative approach and examine'Such problems as maximum angle ofexcavation, landslide potential and pre­vention, and methods of stabilizing exist­ing slopes. In order to reach a validconclusion regarding the stability of aslope, a combination of both approachesis needed. The quantitativE: determina­tions of the engineer must be based on athorough geologic examination of thestructure and form of the slope-formingmaterials, and the geologist will benefitfrom the results of physical tests made ofthe soil and rock materials by theengineer.

Geomorphological approach to slopes.The geomorphologist. a geologist whostudies the physical characteristics of thelandscape, is interested in the form andprocesses which control the shape of theterrain. The configuration of any slope iseventually determined by the relationship

Continued on page 10

Page 10 ARIZONA BUREAU OF MINES June, 1976

Figure 1. Idealized slope profile showing major components and processes likely tooccu r for that segment.

STRAIGHT SEGMENT:

Lol\dslides, tnld flowS

layers of an onion. Sheets of rock up tofour feet thick and tens of square feet inarea have been shed from the underlyingslopes. The second type involves thedifferential weathering of the rock intospherical boulders. These pieces of rockmay be attached to the underlyingsurface by only a few inches of material.Many have evolved to such an extent thatthey are no longer connected to theunderlying surface and any disruption ofthe block will set them into motion. Afew single residences have already beenbuilt below this zone.

On the eastern side of the TortolitaMountains, unstable rock slopes wherethe material falls into blocky rubble arewidespread and similar hazardous condi­tions exist (see photographs 2 and 3).

The straight segment. The straightsegment, situated below the cliff face, issimilar in appearance to a talus slope butdiffers in that the boulders on it usuallyform only a veneer on a slope whichotherwise consists of bedrock (Carson,1972) (see photograph 1). The gradient ofthis segment is slightly less than theangle of repose or angle at which thematerial will come to rest under a givenset of physical conditions. The physicalconditions responsible for maintainingthis angle are the angularity, composi­tion, production or detachment of frag­ments from the bedrock, and climaticconditions. Measured angles of debris­covered hillsides within the Tucson basinpeak at two values: 28.5 degrees and 34degrees (Melton, 1965). Both stabilityangles seem to be related to the frictionalproperties of the material. The lowervalue coresponds to the static frictionangle and the upper value to the slidingfriction angle. Thus, on the upper andsteeper slopes, only the mechanism ofgravity would be required to set debrisinto motion, whereas on the more gentlesegment, hydraulic forces predominate.As the slope angle flattens, the hydraulicfactor becomes more important in themass movement of surface material.Thus, straight-slope segments constitutepotentially unstable areas where massmovement of material would be morelikely to occur; construction on a straight­line slope usually requires cuts for roadsand housing pads. This tends to over­steepen the slope both above and belowthe site, resulting in accelerated erosionand landsliding.

Lower concavity. or break in slope. Inthe region surrounding Tucson, the lowerconcavity is the most important landformin terms of area involved. Concave desertplains may consist of thick, alluvialaccumulations or may have a nearlysmooth bedrock plane, called a pediment,close to the surface. These low gradientslopes are more affected by surface

BASAL CONCAVITY:

FloodillQ, .rosion, mvcI flows

convexity at the upper edge of a scarp.Due to the action of such weatheringprocesses as rain splash and sheetwash­ing, the rate of surface-lowering mustincrease toward the steep cliff face. Theprocess of rainsplash operating on theslope will thus remove the material at arate comparable to that of soil creep.Though mass movement on this particu­lar segment is not generally as prominentas on other segments (straight segments,for instance), the constant wearing backof this slope can increase the surfacegradient, thus affecting its stability.

Cliff face. Probably the most dominantand striking slope segment found insemi-arid climates is the cliff face (seephotograph 1). In terms of hillsidestability, this segment contributes muchdebris to lower slopes in the form of rockfalls. Weathering processes will tend toweaken the strength of the materialalong such geologic features as joints andfaults. The size of the boulders releasedis therefore dependent upon the spatialrelationship of the planes of weakness,and ranges from small granular particlesto large individual blocks. Dependingupon the strength of the material and thedistance of falling, the rock will remaineither relatively intact or disintegrate intoa myriad of smaller pieces upon impact.Accumulation of this debris will form atalus, or loose rock slope, at the base ofthe cliff. Active retreat of the cliff faceonly occurs when the slide rock accumula­tion can be removed and bare rock slopesat the base are again established (Koons,1955).

The precipitous cliff faces on thewestern side of the Santa CatalinaMountains laclt appreciable talus slopesat their bases. It is therefore presumedthat active rock falls can be anticipated inthe-future. Due east of the town of VistaCatalina, two varieties of rock falls areseen. The first is exfoliation of thegranitic outcrops which resemble the

UPPER CONVEXITY;

Soil creep, d!bris fkrw ..

Slope Stability continued

between the disintegration of the under­lying material and the rate of removal ofthis debris from the sloping surface. Theerosional processes of water, wind, andmass movement further control the shapeof a slope. These properties may work incombination or independently on anysegment of the slope. Thus, a streamcutting the base of a hillside may exercisethe dominant control on the actual formassumed by the surface. By using aqualitative and sometimes quantitativeexamination of slope forms and processes,conclusions can be reached on the evolu­tion and stability of hillside elements.

Investigators of slope form are ofteninterested in the profile of the landsurface. In this case, measurements arerecorded along selected lines of traverse,and the corresponding slope profile is thendrawn up for inspection and inter­pretation. An idealized slope profilecontaining four components was used byWood (1942) for an attempt to arrive at ageneral theory of slope formation in aridand semi-arid climates (Figure 1). Thoughmuch controversy has resulted from hisclassification, it still provides the userwith a general scheme in which to studyslope profiles. In the following section,each component of his classification will bebriefly discussed and analyzed for theindirect evidence it provides in a slopestability study. These components are anupper convexity (waxing slope), cliff face(free face), straight segment (constantslope), and basal concavity (waning slope).

Upper convexity. Arid topography ischaracteristically stepped, with a succes­sion of low-gradient surfaces separatedby steep slope segments above andbelow. Two types of upper convexity, orwaxing slopes, can be observed .(thoughmany intermediate steps may be pres­ent). These are: (1) a convexity boundedbetween two straight segments, and (2) a

Vol. 6, No.2 FIELDNOTES Page II

• ,t I.,.. ~..... :.' .. ,.~•••• ~ ••O, ••• f ••••••'l~

.~' ..". ~~.","';: f:. :::~~'. '.~".':.'.' ':

-........:.:.~,

Slope Failures Caused ByArtificial Modifications

smaller depth of slope is necessary toproduce a mass movement. But withinthe Mt. Lemmon quadrangle area, wefind many vertical slope (scarps) whichhave been cut by incising streams orartificial excavations. Thus, the value ofslope angle (i) is 90° and tension cracksmay develop. These cracks will interceptthe failure plane before the critical heightis reached, and will reduce the height by50 percent (see Figure 2b).

Substituting physical testing para­meters in this method can sometimesaccurately predict the maximum height ofa cliff or embankment. Lohner and Handy(1968), for instance, working in the loessarea of Iowa, found close agreementbetween actual and theoretical heightbased on the above method.

The application of the above case islimited to certain models. Slopes that failin circular arcs, or in rock, would betreated differently. As we will see later,the effects of the water pore pressure isan important parameter determining thestability of slopes. Regardless of thepresent difficulty in duplicating actualfield conditions in the laboratory, soilmechanics offers the most realistic quan­titative basis for analyzing slopes inengineeri!1g terms.

I. FClU'IDATIONS ONBEDROCK AT SURFACE

Man's modifications of natural slopesfor construction purposes is the chiefcause of many mass movements in rockand soil. The use of design methods

2. FOUNDATIONS CARRIEDTHROUGH SOIL TO BEDROCK

.~..I"',,,, ...~,t, .•• ·.,.· ••.o •.••"h~.•:.· ...: •. : ...;:trot ......

.••~ •••••• o .o.t ••~. • , 't,.'· ". •.~~ .•••' e ..t •••

3. FOUNDATIONS ON SOILA. SIlaliow FOOIndotions(footillQs)B. Deep Foundations (pilesorcoi'lIOnS)

Soil

THE INFLUENCE OF TENSION FRACTUREON SLOPE STABILITY

Figure 3. THREE GENERAL GROUND CONDITIONS FOR ESTABLISHING SAFESTRUCTURAL FOUNDATIONS ON NATURAL SLOPES.CASE 1. Stable bedrock exposed at ground surface or close to it; foundations can beshallow.CASE 2: Stable bedrock lies below deposits of unconsolidated soil; foundations arecarried through the soil which might be remnants of a soil failure or an old stream;other alternatives are stablizing the soil and placing shallow foundations or removingthe soil in the process of building a multi-level house.CASE 3. Stable bedrock lies too deep to reach economically with foundations;foundations needed might be shallow as in House A. where high bearing strength hasbeen determined by soils engineer, or deep as in House B where poor soil conditionsexist (after Leighton, 1966).

IA

/ / \POTENTIAL SLIDINGSURFACE

/10.

H

A

Figure 2b. Tension features developingparallel to the edge of the slope willintercept the failure plane and reducethe critical height of the slope.

planar failure passing through the toe ofthe slope, the Culmann method (Ter­zaghi, 1967) can be used to determine thiscritical height (Figure 2a). Importantparameters used to determine this criticalheight is the cohesion (C), angle ofinternal friction (0), bulk weight of thematerial, and angle of the sloping surface(i).

The results of this method can be inter­preted in two ways. As the slope angle (i)becomes increasingly steeper, the criticalheight (Hc) will become smaller. Second­ly, the critical angle becomes smaller asthe cut becomes increasingly deeper.

Therefore. as the potential failureplane is inclined at a higher angle, a

H

f)e"" 6 /p /

"y//

/~//Lj ,/ / A POTENTIAL FAILURE

-,-_-1......-"",-,,,- 0. PLAN E8

We have seen in the previous sectionhow a geologist's understanding of theshape of slopes can lead to indirectconclusions regarding the stability of thematerial. But what will happen if thenatural shape, or angle of the slope, isaltered by man? Soil engineers have longbeen interested in the problems of massmovement of material in cases where itbecomes necessary to modify naturalslopes or create new ones, as in embank­ment cutting and other excavations. Bycreating new slopes which are higher orsteeper, landslides can be induced be­cause the shear stresses in the soil massare increased. An understanding of themechanics of mass movement and thephysical properties of various soil mater­ials has thus enabled the engineer toredesign slopes for maximum safety.

The most prominent and widespreadtype of slope failure within the study areais slab failure. The simplest example ofthis process is the slumping of bankmaterial along a stream channel or from acut bank excavated for a highway cut orhomesite (see photograph 4). The reasonthat river banks or cut slopes can remainvertical, and do not become unstable up~ tocertain heights is that they possesscohesion. There remains, however, amaximum height at which the soil willstand before it fails. Various methodshave been designed in the attempt topredict their maximum height for anembankment. Assuming that we have a

THE CeLMANN APPROACHTO SLOPE STABILITY

Figure 2a. A quantitative approach toslope stability assuming a potentialplane failure passing through the toe ofthe slope.

drainage systems than by any othererosional process. Mass movement onthese slopes is nil except where streamshave been deeply incised. In this situa­tion, planar failure of soil, whether instream channels or in cut slopes behindhouses, constitutes the main type of massmovement.

Engineering Approach toSlope Stability in Soil

Pagel2 ARIZONA BUREAU OF MINES June, 1976

Slope Stability Map

Figure 4. FOUR DIFFERENT CUT-SLOPE ANGLES (1 :1, 1.5:1, 2:1, and 3:1). A 2:1slope has a horizontal length twice the vertical height. A safe cut slope angle is themost vital requirement for stability. Some cut slopes in residential development arestable at 1:1, others at 1.5:1, others at 2:1, or 3:1 and still others have to be evenflatter (or retained). The steeper the cut slope angle, the more level lot pad space iscreated and the more material has to be excavated. Grading in most residentialhillside developments involves cutting the hilltops and placing this material in thecanyons and along the lower hillside slopes as fill. In most cases civil engineersbelieve that it is more economical to produce cut and fill lots by earth-work than toconstruct tracts of homes on the natural slopes (after Leighton, 1966).

iNITIAL CUT

A relative slope stability map wasprepared for the northwest quarter of theMount Lemmon quadrangle (see Figure6). The ability of a slope to remain stableis dependent on the slope angle, type ofmaterial, geologic structure, and watertable condition. Slope stability was classi­fied into 5 major groups, from moststable to least stable. Interpretation wasbased on field observation of hillsidessubject to failure and their probableresponse to excavations. Slopes seeming­ly stable in natural cuts were many timesfound to be rendered unstable duringexcavation.

The relationship between stabilityunits and geologic material is outlined inthe table.

Generally, the stability of slopes in thestudy area is considered high whencompared to areas outside the aridsouthwest. Although unstable conditionsin some areas are widespread, landslidingphenomena as encountered in Californiaand on the East Coast have not beenobserved in the Mt. Lemmon area. Thismay be due to the following factors:

(1) An analysis of the natural slopeangles indicates that the alluvium is in aperiod of equilibrium. Coatings of desertvarnish, staining some surficial alluvium,indicate that they have remained in placefor at least a few hundred years.

(2) There is a general lack of clay in thesediments. Clay has a high tensilestrength when dry and almost nostrength when wet. This material, in thewet state, acts as a lubricant and is oftenthe triggering mechanism in landslides.

(3) Rainfall is relatively low. Water canserve as a cumulative driving force bycausing seepage, adding weight, andlubricating or hydrating clay minerals.

Conclusion

The development of hillside areas inSouthern Arizona will continue to in­crease as the availability of flat-lyingareas decreases. The construction prob­lems encountered when building on flat,low-lying land are only magnified whenapplied to sloping regions. The stabilityof the natural slopes, then, should be aprime consideration during the precon­struction phase in order to minimizecostly problems in the future.

An analysis of hillside elements in the. Mt. Lemmon quadrangle indicates that a

much larger area of potentially unstableslopes exists' than was previously real­ized. Further development of interpre­tive maps, as provided in this text, willbenefit both engineers and land-useplanners in future decision-making roles.Enactment of local slope ordinancesshould strive to meet both the safety andeconomic requirements of the public.

FILL

I

Cutting into a slope. Because the valleyfloors of the region are sometimes rathernarrow, excavation at the foot of a slopeto make more flat ground is verycommon. The prime requirement for thestability of cut-back slopes is that theyare graded at a safe angle (Leighton,1966). Proper design of the cut shouldminimize rilling and gullying while at thesame time yield the greatest possiblearea for the housing surface (Figure 4). Itis obvious from the figure that by steep­ening the cut, more flat area can bedeveloped for a given lot size. In the Mt.Lemmon study area, the Fort LowellFormation can stand at 1:1 while themaximum angle of cut for the surficialdeposits is 1:111z or 1:2. The Tinaja bedselsewhere in the Tucson basin will remainstable at 1:1 but in the study area only at1:3 due to its weathered condition andthe presence of clay. Slides that occurafter grading are often an indication thatthe problem was not detected duringexcavation. Four ways that a stable cutslope can be made unstable are illustratedin Figure 5.

Fills. The improper design of fills onwhich foundations are placed can result insevere settlement of the structure (seephotographs 5 and 6). This can be causedby

(1) Situating the fill on existing vegeta­tion or compressible soil;

(2) Inadequate drainage of the fill,causing seepage and saturation of thebuilding site;

(3) Improper compaction of the fill.In order to avoid excessive erosion of

the fill, vegetation should be planted as·quickly as possible. Placing fragments ofrock to minimize surface erosion on thesloping sides should be avoided, as thiswill tend to initiate small-scale failures onpoorly consolidated fill.

ORIGINAL GROUND PROFILE

1PAVED TERRACE DRAIN

IN CUT-SLOPE

50'

1

CUT.

I

presented previously can help an engin­eer to estimate areas of potential failure.The construction of houses or engineeringstructures on relatively stable ground canalter conditions to such an extent thatfailure is likewise inevitable. Three con­struction conditions that will have aneffect on the stability are loading, cut­ting, and filling of a slope.

Loading. The placing of a weightedmass on a slope, such as a fill to extendthe backyard of a house, is the mostcommon type of overloading. The in­creased loading can cause the formationof surfaces of rupture in underlying soiland rock, resulting in failure. Structuresalso are loading factors (see Figure 3). Inplaces where soil conditions are weak,foundations must be properly reinforcedto distribute the weight of the structureevenly to avoid concentrating stress andthus initiating a failure.

Figure 5. FOUR WAYS TO MAKE ASTABLE CUT SLOPE UNSTABLE. Thesmall slides shown have developedfrom the stable initial cut at the top byman's modification of the slope (afterLeighton, 1966).

Vol. 6, No.2 FIELDNOTES Page 13

Photo 1. Shear cliff face in Santa Catalina Mtns. with straightsegment below.

Photo 3. Slide in undifferentiated Tortolita granodiorite.

Photo 5. Series of retaining walls to stabilize fill slope. Homebeing constructed on fine-grained recent alluvium overlyingFt. Lowell Formation in map unit 4.

Photo 2. Slope in Tortolita Mtns. Debris slides predominatein this material.

Photo 4. Failure of a cut-back slope. During the summer rainsof 1973, many tons of material were released into thebackyard. Rilling and piping erosion is now causing furtherinstability.

Photo 6. Improper maintenance of fill sites. Erosion of theslope may cause future instability, while lowering theaesthetic value of the property.

Page 14 ARIZONA BUREAU OF MINES June, 1976

-References

Photo 7. Landslide in surficial alluvium. Failure of materialsimilar to model in figure 2a.

Photo 8. Close-up of photo 7. Looking into failure plane.More resistant flatlying coarse beds remain in place. Failureplane inclined at 35°

Photo 9. Outline of slope failure in Fort Lowell Formationadjacent to Pusch Ridge, Santa Catal ina Mtns.

Berkey, C.P., personal communication toC.F. Sharp (1937); in EngineeringGeology in Southern California; editedby R. Lung and R. Proctor (1966)

Carson, M.A., and Kirkby, M.J., Hillsideform and process; Cambridge Univer­sity Press (1972)

Davidson, E.S., Geohydrology and waterresources of the Tucson Basin, Ari­zona; U.S. Geological Survey water­supply paper 1539-E (1973)

Koons, D., Cliff retreat in the Southwest­ern United States; in American Jour­nal of Science, v. 253 (1955)

Leighton, F.B., Landslides and hillsidedevelopment; in Engineering geologyin Southern California; edited by R.Lung and R. Proctor (1966)

Lohnes, R.A., and Handy, R.L., Slopeangles in friable loess; in Journal ofGeology, v. 76 (1968)

Melton, M.A., Debris-covered hillslopesof the Southern Arizona desert - con­sideration of their stability and sedi­ment contribution; in Journal of Geol­ogy, v. 73 (1965)

Pashley, E.F., Structure and strati­graphy of the central, northern, andeastern parts of the Tucson Basin,Arizona; University of Arizona, Ph.D.dissertation (1966)

Smith, G.E.P., The physiography of Ari­zona valleys and the occurrence ofgroundwater; University of ArizonaAgricultural Experiment Station Tech­nical Bulletin 77 (1938)

Terzaghi, C., Soil mechanics in engineer­ing practice (1967)

Wood, A., The development of hillsideslopes; in Proceedings of the Geolo­gists' Association, v. 53 (1942)

AGS PublishesTectonic Di~est

The Arizona Geological Society has pub­lished the "Tectonic Digest," Volume X intheir continuing series of geologic bulle­tins.

The Tectonic Digest contains 430 pages,has a separate map supplement, andincludes 19 articles on tectonics inArizona.

Digests may be ordered by mail fromthe Arizona Geological Society, Box 4054,Tucson, AZ 85717, at a cost of $11.50 each,which includes postage. They also arebeing sold over the counter at the ArizonaBureau of Mines' offices at 845 NorthPark, Tucson for $10.50 each.

Articles in the Digest include "The Ageof Basin-Range Faulting in Arizona,""Free-Air Gravity Anomaly Map of Ari­zona," "Elements of Paleozoic Tectonics inArizona," and "Late Devonian Tectonicsin Southeastern Arizona."