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2037697

EFFECTS OF GROUND FISSURES ON DEVELOPMENTIN THE LAS VEGAS VALLEY, NEVADA ? |

1««?3

James L. Werle, R.G. and A.N. Stilley, P.E. §•»4 ^f

Converse Consultants Southwest, Inc., Las Vegas, Nevada f |** §Psr

ABSTRACT: Ground fissures - eroded ground tension cracks - commonly o-s?occur in the Las Vegas Valley of Southern Nevada and pose problems for new ||development. The fissures are generally believed to be associated with land »subsidence caused by groundwater withdrawal. Other possible localizedcauses include desiccation of highly expansive clays or saturation ofhydrocollapsible soils. The majority of earth fissures in the Valley are concen-trated near compaction faults and around high capacity wells. Nevertheless,the actual locations of fissure zones are unpredictable.

Based on field observations, the initial tensional breaks in the soil are less than1/2 inch wide. Subsequent subsurface erosion by infiltration of storm runoffmay enlarge cracks into cavities measuring up to 8 feet across. Cracks andvoids may later be filled by deposition or collapse.

Recent growth in the Las Vegas Valley has resulted in development on or nearzones of fissuring. Damage to existing structures has occurred due to groundfissures and therefore, design of new developments requires identification ofthe fissure zones. The following paper outlines methods which have been usedto locate areas of fissuring, to determine the degree and extent of fissuringpresent, and design considerations for structures located in fissure areas.

INTRODUCTION

Land subsidence due to groundwater withdrawal from alluvial basins is com-prised of vertical and horizontal components. The "vertical component can beestimated accurately because it is directly related to the amount of drawdownof the artesian head and by the compressive nature of the unconsolidatedaquifer sediments. The horizontal component is difficult if not impossible topredict because these tensional cracks often originate at depth and may not beexpressed at the surface. Piping through the subsurface fractures is an ero-sional feature and is dependent on local stratigraphy and drainage rather thanwater pumpage in the area.

Ground fissures have formed during the past 50 years in the Las Vegas Valleyof Southern Nevada {Bell, 1981}. Until recently, the fissuring - and theirassociated hazards to man-made structures - have been generally confined tooutlying undeveloped areas of the Valley. Rapid growth in Las Vegas Valley inthe last 5 years has resulted in increasing development of sites containingfissures or those with potential for fissuring. At the same time, extensive andpossible fissure-related damage at an established residential community hasfocused the attention of local and federal agencies to the problem. A compre-hensive study by the Nevada Bureau of Mines and Geology (NBMG) is currentlyin progress to evaluate land subsidence and related hazards in the Las VegasValley since 1981.

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"•" * -••:' *••:' "-" + '-*.-••• ', • • ' . ' •:-—- - -. » . V • . • • * "-^ ••<- • -• • ... "' '•>;'---. •.. . • .••• . - • - - - .-—^ .*r. • *,

Figure 1. Typical ground fissure in the Las Vegas Valley.

GEOLOGIC SETTING

Geologic and Hydrologic Conditions

The Las Vegas Valley covers an area of approximately 350 square miles in thesouthern portion of Nevada.- The Valley is a structural basin of late Mesozoicand Tertiary block faulting origin and is physiographically characteristic of theBasin and Range Province of the Western United States. The Valley is filledwith Tertiary and Quaternary Age unconsolidated sediments derived from thesurrounding mountains, the primary source of deposition being the SpringMountains to the west. The alluvial and lacustrine sediments are as thick as4,000 feet in the Las Vegas Valley and consist of clay and silt intercalated withfine sand to coarse gravel and calcareous cemented deposits (caliche). Ingeneral, the sediments grade increasingly finer with distance from the.sourceareas with decreasing elevation. The Valley drains into Lake Mead by LasVegas Wash and its system of tributary washes.

Two separate aquifers exist in the Las Vegas Valley: a shallow relativelyunconfined aquifer and a series of deep confining water-bearing zones(Plume,1984). Each zone typically contains granular sediments that are contained bylow permeability silts and clays. The fine-grained sediments occur as lenses orlayers which act as semi-confining barriers or aquitards that impede verticalflow. The majority of the groundwater withdrawn in the Valley is from thedeeper aquifer zone at depths greater than 200 feet.

The aquifers are faulted as a result of differential consolidation of theinterbedded fine-grained sediments. The features termed compaction faults(see next section) significantly affect the movement of groundwater, both

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laterally and between aquifers. Prior to artesian head decline as a result ofheavy pumping, flowing springs were reported along compaction fault scarps.

Compaction Faults

The system of prominent escarpments which trend in a general north-southdirection across the Las Vegas Valley were termed compaction faults by Maxeyand Jameson (1948). These compaction faults are believed to be non-tectonicfeatures of Quaternary Age in which the east side of the escarpment is downrelative to the west side.

Compaction faults in the Las Vegas Valley are generally considered to be theresult of differential consolidation of deep alluvial and lacustrine sedimentshaving dissimilar grain size and compressibility characteristics. Sedimentswithin the Las Vegas Valley grade increasingly finer from west to east. Associ-ated with decreasing grain size would be an increase in compressibility. Insuch a situation, differential consolidation would tend to develop across an areain which compressibility variations are greatest; hence, the characteristicescarpments. The escarpments probably represent a rather broad zone ofadjustment. It has been suggested that the apparent displacements may havebeen at least partly induced by a seismic event.

These compaction fault features have probably been present throughout mostof the geologically Recent Age. The Eglington Scarp, located in the northwest-ern part of the Las Vegas Valley transects both fine and coarse alluvial sedi-ments and was formed about 14,000 years ago (Haynes, 1967). This scarp isconsidered by most investigators to be the north extension of the western-mostcompaction fault in the Valley.

It has been postulated that, the compaction fault scarps tend to be natural hingelines expressing land subsidence; however, until recently, available data onsubsidence in the Las Vegas Valley did not indicate any marked change in therate of subsidence, or well defined differential ground subsidence across thescarps. New land level survey data now suggests this may be occurring atsome locations (NBMG progress report, 1990).

Land Surface Subsidence

Significant land subsidence within the Las Vegas Valley has been well estab-lished. It is assumed to be almost entirely the result of excessive groundwaterwithdrawal from the unconsolidated deep alluvial aquifers; however, a risingshallow water table in some parts of the Valley may produce local subsidence.Documented land subsidence is generally believed to be associated withproximity to large scale pumping areas.

In theory, the drop in hydraulic head due to pumping creates an increasedapplied stress in saturated soils which causes water to be squeezed out of thefiner grained soils. The expulsion of water continues until the pore pressuresare reduced to zero and the applied stress is carried by the soil structure. Thisis the consolidation process and the volume reduction should be equivalent tothe amount of water dispelled. The rate of consolidation will be a function ofdrainage path distance and permeability.

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Groundwater levels have dropped as much as 250 feet in the Las Vegas Valleysince 1955, while the magnitude of land subsidence has been steadily increas-ing since it was first documented in 1940 (Mafmberg, 1964). The relativedegree of subsidence has changed in time in response to the distribution ofpumping centers and their associated water level declines. Presently, thecenter of the subsidence bowl in the Valley is near downtown Las Vegas.Maximum subsidence on the order of about five feet has been measured withdecreasing subsidence toward the edge of the bowl (Mindling, 1971, Bell,1981, and NBMG progress report, 1990).

GROUND FISSURES

Tensionaf Cracking

Two expressions of ground fissures, formed in response to tensional stressesfrom regional land subsidence, have been observed in the Las Vegas Valley.While fissures do not necessarily form in areas of differential consolidation,they are characterized by close proximity to existing compaction fault scarps,and by location within areas of large scale groundwater withdrawal. The firstforms of expression are usually concentrated near compaction faults and areeither expressed surficially as a single, continuous, linear crack or groups ofdiscontinuous, dendritic cracks. The second are found in the vicinity of highcapacity wefls, and the fissures are represented in the form of radial andconcentric cracks around the well.

Highly expansive clays are common in the vicinity of compaction faults in theLas Vegas Valley. In localized areas, earth cracking may be caused by desicca-

Figure 2. Tensional cracks preserved in a surficial caliche cap.

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A/I tensional

M

N

siren /

/

I. Lateral stresses indue* tension cracking! II. Surface runoff and infiltration inljnj« crackthrough subsurface piping.

III. As piping continues; fissure begins to appearat surface as series of polhol*s and inwll cradu.

IV. Ax infiltration and iroiion continue, finuri•nlarjM and complottly op«n> to urfac* Jitunn«i roof collapses.

V. The intira finur* is opined to the surfi.cearxl enlargement continue* as fitfure wells arewidened; extenov* flumping and lide-nreemgullying occur.

Vt. Fiisure becornn filled with ilump and run-off debrn and is marked by vegetation linea-ment and slight surface depression; it maybecome reactivated upon renewal of tensilestress.

Figure 3. Generalized stages of fissure development (Bell, 1981).

tion and shrinkage of these clays, a phenomena unrelated to the horizontalcomponent of land subsidence. However, the drying out of these clays maybe associated with the elimination of springs along the compaction faults due tothe reduction of artesian head in the basin as a result of overpumping.

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Another possible cause for ground cracking in the Valley is the saturation oflocalized areas of soils susceptible to hydrocollapse. Found to be prevalent inthe Las Vegas Valley, hydrocollapsible soils settle rapidly upon introduction ofwater and local ground subsidence from this phenomenon may be a source ofcracks and fissures. Development in the Valley has resulted in a rising shallowwater table due to landscape irrigation. This along with other developmentrelated introductions of water to the subsurface may be increasing the water-induced collapse of these soils, and thus the potential for subsidence andfissuring.

Regardless of the source of the cracking, no vertical offset has been observedwhere the tensional cracks are encountered in the field (Figure 2). Fieldobservations have shown that the initial tensional breaks in the soil are usuallyless than 1/2 inch wide.

Secondary Erosional Features

Fissures generally originate as subsurface tensional cracks or fracture zonesdue to the mechanisms described above and subsequent erosion enlarges themand exposes them at the surface. Surficia! expressions of these tension crackssuch as those shown in Figure 2 allows storm water runnoff to enter theground and begin subsurface erosion of susceptible soil horizons (as depicted inFigure 3). Often this erosion is in the form of piping or tunnelling through thesubsurface soils. Eventually these features collapse or are filled-in by depositseroded from elsewhere in the fissure system. Additional runoff promotesfurther piping and tunnelling which would result in more slumping. The roofs ofsome subsurface cavities collapse to the surface leaving shallow depressions,sinkholes or open voids. Eventually, the fissure may be temporarily filled andobscured, but may be marked by lineations of vegetation that thrives on moistsoil in the fissure.

In general, the path of subsurface erosion is found to be controlled by the initialtensional ground crack. In some instances however, the piping and tunnellingdeviates from the initial cracking and has followed subsoils that are highlysusceptible to erosion. The most susceptible soils include silts, sandy silts,clayey silts and fine sands with large amounts of soluble gypsum (Figure 4).The formation of these secondary erosional features are very unpredictablebecause their paths are dependent only on the nature and dip of the subsurfacedeposits. This stratigraphic dependence makes prediction of areas with exten-sive piping nearly impossible because evidence for this phenomena may not befound at the surface.

GEOLOGIC SITE INVESTIGATIONS

Extensive site investigations are performed in areas where excessive groundwa-ter pumping has occurred and where compaction faults are in the site vicinity.These investigations begin with a review of all published maps and geologicliterature to determine if the site is located within a known fissure zone, dis-tance to the nearest compaction fault, groundwater pumping history of thearea, and recorded land subsidence in the vicinity of the site.

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Figure 4. A subsurface erosional channel shown between twobackhoe trench explorations.

Aerial photographs are interpreted to identify any visible compaction faults orfissure zones on the site. These zones are most often identified by the linearorientation of vegetation that preferentially grows in the fissures where wateraccumulates in the low areas. Aerial photographs are valuable because the sitecan be examined prior the extensive development of the area.

A site reconnaissance is performed and this may give the first evidence forexistence of fissures. Vegetation lineation may be more evident in the field.Compaction faults and large capacity wells in the site vicinity are identified andinvestigated for fissure development. This reconnaissance involves covering asmuch of the site as possible and recording all observations. If any evidence forfissuring is found, the site area is extensively mapped. The location and extentof fissures and other minor subsidence features are noted.

Trenching is often performed to determine existence and depth of tensionalcracks and subsurface piping features. Past experience has indicated thatcracking usually extends to depths of 5 to 10 feet below the surface. Erosionalchannels can extend deeper. However, this technique is expensive and withoutsurficial evidence of fissuring in the area, trenching may not be feasible as thelocation of erosional tunnelling is unpredictable.

in one instance, a geophysical technique was used to locate and determine theextent of subsurface fissures. Evidence for fissures had been found duringgrading of a proposed roadway. The area was mapped and design recommen-dations were made. Ground penetrating radar was later used, before construc-tion had continued and bridged voids and fissures were successfully locatedwith the technique (Figure 5). Future use of ground penetrating radar mayprove to be valuable in subsurface fissure analysis.

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Occasionally, the presence of fissures on the site is not discovered until sitegrading commences. The use of heavy equipment on-site sometimes results inthe collapse of subsurface tunnels.

Two field techniques have been used to locate filled or obscured fissures duringgrading. The first is used in clayey soils. The blade from the excavation equip-ment (dozer) is used to scrape the surface. Filled fissures can often be ob-served on the abraded surface by the variation in color or materials in the filledfissure as compared with the adjacent ground.

It has been observed that fissures filled or obscured during the grading opera-tions often become visible following an rainstorm. In this manner, surfacerunnoff infiltrates a fissure on a cut surface producing erosion of the feature.Hence, the second field technique which has proven successful in locatingfissures involves using a water truck to artificially "reactivate" the fissures.

rof of suaonAoe StWFACS

1.7 f BET

SUBSURFACE PROFILE AT SITE PROFILE INTERPRETATION

Figure 5. Ground penetrating radar located fissuring in street alignment.

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GEOLOGIC PROBLEMS

There are several problems associated with designing for structures in fissurezones. Actual sites of fissures are unpredictable. Although fissures are oftenassociated with compaction faults and high capacity wells, erosional piping maybe stratigraphically controlled and can be found at some distance from thesource of the tensional crack. In essence, fissures and cavities can be found inunexpected areas upon further erosion and collapse of the tunnel. Problemsarise in deciding whether to design for tensional cracking or the erosionalfeatures.

Another problem .arises in time prediction. It is unknown if and when fissuringwill occur in an area, or in the case of existing fissures, whether fissuring willcontinue. In addition, there is a delayed time factor from the time pumpingbegins to the point where clays are dewatered. It is difficult to predict howlong it takes dewatering to occur and thus, effects of subsidence may notappear for years after pumping has commenced.

Highly expansive clays found near compaction faults may form fissures causedby desiccation and shrinkage of these clays due to cessation of springs alongthese faults. Hydrocollapsible soils may become saturated from man's activitiescausing localized subsidence and the formation of shallow ground cracks thatmay lead to fissuring. These processes further complicates design considera-tions.

CONSIDERATIONS FOR FOUNDATION DESIGN

After the presence of fissuring has been identified in an area, and the extent,nature, and mechanism, has been investigated, it is necessary to determine ifdevelopment in an area is feasible, and to design structure foundations. Beforefoundation design can be detailed, it is necessary to evaluate aspects of thefissure zone and the structures planned for development.

Probably one of the most important factors to consider in deciding to develop ina fissure zone, is whether the actual cause of the fissuring has been identified,and whether it is continuing to occur. If the process which originally causedthe fissures has stopped, then foundation design can be limited to dealing withthe existing fissures, without concern for formation of new features. If themechanism which causes the fissures is continuing to occur, development ofcertain types of structures may be limited, and foundation design often involvessupport of structures with a rigid system.

One of the most common causes of fissures in the Las Vegas Valley is pumpingfrom high volume water wells. Prior to the early 1970's, most of the water forthe Las Vegas Valley was obtained from deep wells. In 1971, the first phase ofa water supply line system from Lake Mead was completed, and pumping atmany wells was discontinued. Currently, the Las Vegas Valley obtains amajority of its water from Lake Mead and the remainder from deep wells. Inmany areas it is possible to trace the most probable cause of the fissuring to awell or well field which is not currently being operated. If it can be determinedthat future pumping of wells in these areas will not occur and land subsidence

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from past water extraction has ceased, foundation design can usually be limitedto dealing with existing fissures.

Another important consideration in foundation design is the depth of thefissuring below ground surface. This is usually determined by trenching orgeophysical methods during the field investigations. Past experience hasindicated that fissures or in-filled fissures usually extend to depths of about 5 to10 feet below the surface. Where significant erosion has occurred, erosionchannels can extend deeper than this. This will normally establish the depthrequired for foundation soil modification and determine costs of development.

Another consideration is whether there are actually voids present, or whetherthe voids have been in-filled by erosion. If a void is present, then it is neces-sary to design the structure to bridge the void or to excavate the void and fill itin. If a void has been in-filled with soil, it may create a "soft spot" in thesubgrade or an area with different support characteristics. If large voids arepresent, they are normally the result of erosion of smaller fissures.

The type of development and site grading required on a specific site, has animportant influence on foundation design. Foundation design to accommodatethe ground fissures may be minimal for such facilities as parks, golf courses, orlandscaped areas. Other facilities, such as buildings or buried utilities mayrequire extensive design precautions. If cut and fill are required to level the sitefor use, it may be possible to locate settlement sensitive facilities in areaswhere the zone of fissuring has been overexcavated.

METHODS OF FOUNDATION DESIGN FOR GROUND FISSURING

When the nature and mechanism of the fissures and the details of the type ofdevelopment have been determined, requirements for foundation modificationand design can be developed. If investigation indicates the ground fissures arecontinuing to occur, it may be necessary to recommend no-build zones, orlocation of non-sensitive types of development in the high risk fissure zones.Most often, when an active zone, where fissuring is continuing to occur isidentified, location of buildings within the active zone is not recommended.Where plans for the development will allow, location of non-sensitive features,such as greenbelts, parks, or landscaped areas, has been recommended. Sincethe fissure zones are often linear and extend over large areas, this can severelyrestrict development of a site.

In some instances, location of certain facilities, such as streets and utilities,outside of an active fissure zone, can not be avoided. If it is necessary tolocate.streets in an area where fissures may form, use of a flexible pavement(asphaltic concrete) is usually recommended and periodic inspection, mainte-nance and repair of the street may be required. Location of movement sensi-tive utilities, such as gas or water transmission lines, in active fissure zones isdifficult. In some instances, protection of utilities, by locating them in anoversize tunnel or conduit has been recommended to provide protection fromdamage due to fissure formation.

A method which has been used to reduce risk of damage to structures infissure zones, is to design a rigid or heavily reinfprced support system. This

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may consist of a relatively rigid, reinforced concrete slab and footing system, ora post-tensioned concrete slab support system. The idea with these systems isto provide a foundation which will span a void or fissure, and resist lateralmovements, if they occur.

A system which has been used extensively in the Las Vegas Valley, particularlyfor residential structures, is the post-tensioned cable foundation system. Withthis method, a concrete slab and beam system beneath the structure is con-structed. Embedded cables within the beams running across the structure arethen tensioned and tied off. This induces a cornpressive stress in the slab andbeam system, and allows the concrete to resist more tension or bending,without distress. Post-tensioned foundation systems have been used exten-sively for structures in expansive soils in this area in the past, and thus contrac-tors and builders are experienced with their use.

Another method which is often used, particularly in non-active fissure areas, isoverexcavation and recompaction. With this method, the depth of fissuring isestablished by trenching or excavation of test pits in the field. When themaximum fissure depth has been established, this depth is overexcavated andthe on-site material is recompacted. This procedure has several benefits. Itactually fills in any voids or fissures which may be present and may preventerosion of fissures by surface runoff which could affect future structures. Itmay also remove filled in voids or fissures and provide uniform support beneaththe structure. If an area is suspected of containing fissures, overexcavationduring construction can be used as a method of exploration. Overexcavationand recompaction obviously does not have the benefits if the site is located inan active zone, and ground fissures are likely to occur in the future on the site.

Geotextiles and geogrids have recently been used to improve foundationsupport in areas of ground fissuring. Geotextiles are normally used where thereis a concern with loss of ground into a fissure or a fissure caused void. Thegeotextile is placed on the subgrade and acts as a filter to prevent movement offines into the fissure or void. Traditional methods, based on the soil gradationand the properties of the geotextile, are used in design to select and specify thegeotextile.

Geogrids have been used where there is concern with support of a structureacross a fissure zone. Usually, a geogrid, or series of geogrids are embedded inthe subgrade beneath a street or structure, with the idea that if a fissure ispresent or forms, the geogrid will provide support to help bridge the void.Geotextiles and geogrids are often used in combination to provide supportacross a void and to prevent movement of soil into the void.

EXAMPLES OF FOUNDATION DESIGN

Storm Drainage Channel

A system of flood control facilities designed to handle runoff from large precipi-tation events is under construction in the Las Vegas Valley. During 1986 to1989, a section of open channel, approximately one mile in length, was de-signed and constructed in North Las Vegas, in the northern part of the LasVegas Valley. During construction of the structure in 1989, a series of fissures

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and fissure related features were discovered along a portion of the proposedchannel alignment. This required a detailed investigation and design to accountfor the fissures.

The storm drainage channel is trapezoidal in shape and extends below theground surface to depths of about 8 to 12 feet. The channel is concrete linedand the route extended roughly along a shallow wash through relatively unde-veloped area. During initial geotechnical investigations, test borings indicatedthe subsurface soils consisted of sands, silts, and clays. Field reconnaissanceindicated the presence of several ground fissures and numerous erosionalfeatures associated with fissures. A detailed geologic reconnaissance andfissure study was recommended.

The fissure study consisted of a review of available aerial photos of the routeand geologic references of the area. The ground surface along the alignment ofthe channel was then traversed and fissures were mapped and located on aplan.

The zone of fissuring was located in an area which contained mapped andreferenced zones of fissuring, although it was not within a known active fissurezone. The route was also located within 1000 feet of the Nellis Air Force Basewell field, and near a well field used to water a local golf course. Both of thesewell fields are pumped extensively.

Field mapping indicated fissure features were concentrated in an area along thealignment about 2400 feet in length. Features observed varied from narrowsurface cracks to large linear depressions. The linear features often consisted ofa series of potholes interconnected by shallow linear depressions. The potholesgenerally ranged from 1 to 3 feet wide and deep. Several much deeper sink-holes were observed outside the alignment of the channel.

Relocation of the channel outside the fissure area was not feasible, and designhad to provide that fissures would not adversely affect the structure. Severalalternates were considered for foundation design. Since the base of thestructure was normally 8 to 10 feet below ground, it was felt that most of thesurface features would be excavated and removed during construction.Although it was not known how deep the fissures extended, it was felt neces-sary to provide a zone of known uniform support beneath the channel bottomand side slopes.

Several alternates were considered and cost estimates developed for construc-tion. Alternates which were considered included overexcavation beneath thechannel by depths of two to four feet and providing a geotextile at the base ofthe fill. The alternate which was selected for construction involvedoverexcavating beneath the channel base by a depth of two feet and placingengineered fill to the base of the channel. The channel base and sides werethen reinforced structurally with concrete beams and designed to span voids ofup to two feet in diameter. A geogrid was also placed beneath the base of thechannel. The sloped portion of the channel was also overexcavated andreinforced with geogrids.

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Figure 6. Placement of geogrid below a drainage channel base duringconstruction.

During construction, the exposed subgrade was inspected in detail for evidenceof fissures. None were detected at that time. Figure 6 shows a portion of thechannel base under construction.

Construction of the channel was completed in 1989 and the facility has func-tioned satisfactorily to the present time.

Street Section

During investigations for an urban collector street leading into a large residentialdevelopment in North Las Vegas, a zone of ground fissures was detected.Construction of the street was stopped and additional studies were conductedto determine the extent of the fissures and to revise design of the streetsection.

The roadway under consideration is approximately 0.8 mile in length. It wasdesigned as a four lane roadway, with four inches of asphaltic concrete surfac-ing, four inches of crushed aggregate base and nine inches of uncrushedaggregate base overlying compacted subgrade. During construction, fissureswere observed along about 2200 lineal feet of the route.

Field investigations included a detailed field reconnaissance and mapping of thefissure features. Backhoe test pits were then excavated at several of thefissure locations to investigate fissure widths and depths.

Subgrade soils in the area of the fissures consisted of clayey and sandy silts.Fissure features varied from narrow linear cracks to large linear depressions and

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sinkholes. Cracks in the surface of the ground ranged from less than one inchup to six inches in width. Some of the cracks could be probed to depths of sixfeet. Several near-surface cavities were observed with in-place soil partiallybridged over a void. The cavities were observed near the ground surface andappeared to be the result of erosion from surface runoff. Backhoe test pitsexposed several areas where voids were obviously filled in with eroded materi-al. In some areas, considerable organic material was present in the filled-invoids.

The street section was reevaluated and it was decided that design measureswere required to prevent the possibility of loss of material from beneath thestreet section into the fissures. Several alternates were considered.Overexcavation and recompaction to the maximum depth of the fissures (6feet) was not feasible from a cost standpoint. The alternate that was finallyselected consisted of overexcavating below the pavement section by 18 inches,scarifying, compacting and proofrolling the subgrade, laying a geotextile overthe subgrade, and placing structural fill to the base of the pavement section.The revised street section is shown in Figure 7.

OVEREXCAVATEAND RECOMPACT

4' TYPE II AGGR. BASE9" TYPE I AGGR. BASE

WOVEN GEOTEXTILE8- SCARIFY AND RECOW>ACT

IN PLACE

Figure 7. Revised street section through area of ground fissures.

Construction of the street section was completed in 1989 and the roadway hasbeen in use since then without distress.

CONCLUSIONS•

Ground fissuring and fissure related features present a hazard to developmentwithin certain areas of the Las Vegas Valley in Southern Nevada. Fissuresappear to be related to groundwater withdrawal and the subsequent ground

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subsidence. They are most prevalent near high volume well fields and adjacentto compaction faults. Several mapped zones of ground fissures are present butthe actual locations are unpredictable. Specialized foundation design measuresare required for structures or development within fissure zones.

REFERENCES

Bell, J.W., 1981, "Subsidence in Las Vegas Valley," Nevada Bureau of Minesand Geology, University of Nevada, Reno, Nevada, Bulletin 95, 84p.

Haynes, C.V., 1987, "Quaternary Geology of the Tule Springs Area, ClarkCounty, Nevada," Nevada State Museum Anthropological Papers, No. 13,Part 1, 90p.

Malmberg, G.T., 1964, "Land Subsidence in Las Vegas Valley, Nevada,1935-63" Nevada Department of Conservation and Natural ResourcesInformation Report 5, 10p.

Maxey, G.B., and Jameson, C.H., 1948, "Geology and Water Resources of LasVegas, Pahrump, and Indian Springs, Clark and Nye Counties, Nevada,"Nevada State Engineer, Water Resources Bulletin 5, 121 p.

Mindling, Anthony, 1971, "A Summary of Data Relating to Land Subsidence inLas Vegas Valley," University of Nevada, Desert Research InstituteReport, 55p.

Nevada Bureau Of Mines and Geology (NBMG), 1990, "Subsidence in LasVegas Valley, Progress Report," University of Nevada, Reno, Nevada27p.

Plume, Russell W., 1984, "Ground-water Conditions in the Las Vegas Valley,Clark County, Nevada: Part 1.- Hydrogeologic Framework," U.S. Geologi-cal Survey, Open-File Report 84-130, 25p.

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Proceedings of the 1991 Annual Symposium on

Engineering Geology &

Geotechnical Engineering

(No. 27)

Compiled and Edited by

James P. McCa/pinDepartment of GeologyUtah State University

Published April,1991

Copies of this and previous Proceedings volumes may be obtained from:Lee Robinson

Idaho State UniversityP.O. Box 8371

Pocatello,ID 83209

Printed byPublication Design & Production

Utah State University

1991 SYMPOSIUM ON ENGINEERING GEOLOGYAND GEOTECHNICAL ENGINEERING

TABLE OF CONTENTS

SESSION 1; FOUNDATIONS

1. Computation of stress and settlement at an arbitrary point due to a uniformly-loadedrectangular area; Charles M. Schlinger.

2. Analysis of foundation resting on swelling soils; Ashraf El-Ashal, F. El-Nahhos, F. El-Kadi.

3. ** *paper withdrawn* **

4. Estimation of ultimate capacity of piles in the Utah region; Reza Rahman, Bret G.Dixon, Loren R. Anderson, Ed Keene.

5. Predicting and monitoring settlement of highway embankments; Djan Chandra, HantoroWalujono.

SESSION 2: MATERIAL PROPERTIES

6. Relationships among engineering characteristics of peaty soils; Hilary I. Inyang.

7. Geotechnical studies on coal overburden rock from underground coal gasificationprojects in Wyoming; Lary K. Burns, Thomas R. Marks, Barbara A. Marcouiller-Hippe.

8. Preliminary classification of transitional materials; Paul M. Santi, Raymond H. Rice.

SESSION 3: DAMS AND TUNNELS

9. Geotechnical investigations for the reconstruction of Quail Creek Dike, Utah; C. CharlesPayton.

10. Trial Lake Dam; Sheldon Talbot, David Pitcher, James Kuenzli, Lee DeHeer.

11. Foundation evaluation and preliminary design for proposed reservoir in Farmington Bayof the Great Salt Lake; Bill Leeflang, Ben Everitt, Dan Aubrey, Jim Palmer.

12. Reliability assessment of a rockfill dam under stochastic earthquake loading; Casan L.Sampaco, Martin Wieland.

ii

13. Remedial chemical grouting of deteriorated concrete, Soda Dam, Idaho; Michael P.Bruen, C.M. Koncarski.

14. US 189 Provo Canyon tunnels, Utah; L, W. Abramson, E.G. Keene.

SESSION 4: GEOLOGIC HAZARDS 1. Rivers

15. Impact of raft removal on the sedimentary processes and sedimentary budget of the RedRiver, Arkansas-Louisiana; Christopher C. Mathewson, Thomas Weirich.

16. A combined GIS-HEC computational procedure for defining 100-year floodplainelevations; S,G. McLin, G.W. Fuller, G.W. Tauxe.

SESSION 5: SLOPE STABILITY

17. Summary of landslides in Utah, 1987-1990; Kimm M. Harty.

18. A probabalistic investigation of slope stability in the Wasatch Range, Davis County,Utah; James S. Eblen.

19. Geologic controls on landsliding, Siskiyou Mountains, Oregon; William R. Henkle,William T. Gentry, Stephen P. Bingham.

20. Use of multiple working hypotheses and multiple geologic/geophysical technologies toanalyze a complex landslide; Derrick D. Crowther, Michael B. Phipps, Thomas L. Slosson,James E. Slosson.

21. Influence of stress path on soil strength parameters and analysis of rainfall-inducedslope failures; Scott A. Anderson, Nicholas Sitar.

22. A review of Spencer's method for slope analysis; Sunil Sharma, Abdul Moudud.

23. The design of debris fan dam repair techniques employing numerical methods; TrevorD. Smith, Bob Slyh, Kelly Uhacz, Stanley Soliday, Clifton Deal.

24. Systematic liquefaction-induced ground failure hazard analysis; Jeffrey R. Keaton,Loren A. Jalbert.

SESSION 6: SEISMIC HAZARDS

25. Geologic aspects of shear-wave velocity and relative ground response in Salt LakeValley, Utah; John C. Tinsley, Kenneth W. King, David A. Trurnm, David L. Carter, RobertWilliams.

26. Fault rupture hazard analysis using trenching and borings, Warm Springs fault, SaltLake Valley, Utah; Robert M. Robison, Ted N. Burr.

iii

27. Multiple late Quaternary surface faulting events along the southern Lemhi Fault,southeastern Idaho; Mark A. Hemphill-Haley, Thomas L. Sawyer, Peter L.K. Knuepfer,Steven L. Forman, Richard P. Smith, Ivan G. Wong.

28. A slip rate based on trenching studies, San Jacinto fault zone near Anza, California;P.M. Merifield, T.K. Rockwell, C.C. Loughman.

29. Opinion surveys as an integral part of earthquake policy implementation; Gary R.Madsen, Loren R. Anderson.

SESSION 7: GEOLOGIC HAZARDS 2. Problem Soils

30. Soil and rock causing engineering geologic problems in Utah; William E. Mulvey.

31. Collapsible soil hazard mapping for Cedar City, Utah; K.M. Rollins, Tonya Williams.

32. Effects of ground fissures on development in the Las Vegas Valley, Nevada; James L.Werle, A.N. Stilley.

SESSION 8: HYDROGEQLOGY AND GEOPHYSICAL TECHNIQUES

33. A preliminary hydrogeological assessment of the Fort Hall landfill, Bannock County,Idaho; Gary L. Kindel, H. Thomas Ore, John Welhan.

34. The influence of rock discontinuity properties on ground water flow at the Bunker HillMine, Kellogg, Idaho; Thomas E. Lachmar.

35. Analysis of prefabricated wick drains by uncoupled finite strain consolidation theory;S. Bang, P. Zhao.

36. Recent use of portable x-ray fluorescence on heavy metal contamination sites; BlairMcDonald.

37. Achievable detection limits using a portable x-ray fluorescence spectrometer; AlanSeelos.

38. Ground penetrating radar, electrical resistivity, and soil and water quality studiesintegrated to determine the source(s) and geometry of hydrocarbon contamination at a sitein north-central Arizona; Nathan Brett Mustoe, Clyde Frederickson.

39. Applications of shear, compressional, and v,/vp data to evaluate subsurface geology andassociated conditions and hazards; Alvin K. Benson.

40. Identification of radon-hazard areas along the Wasatch Front, Utah, using geologictechniques; Barry J. Solomon, Bill D. Black, Dennis L. Nielson, Linpei Cui.

iv

POSTER SESSION

41. Surging flow in alluvial channels; Ben Everitt

AUTHOR INDEX

Paper:Abra.rn.son, L.W 14Anderson, L.R 4,29Anderson, S.A. 21Aubrey, D 21Bang, S 35Benson, AK. 39Bingham, S.P 19Black, B.D 40Bruen, M.P 13Burns, L.K. 7Burr, T.N 26Carter, D.L 25Chandra, D 5Crowther, D.D 20Cui, L 40Deal, C 23DeHeer, L 10Dixon, B.G 4Eblen, J.S 18El-Ashal, A. 2El-Kadi, F 2El-Nahhos, F 2Everitt, B 11,41Forman, S.L 27Frederickson, C 38Fuller, G.W 16Gentry, W.T. 19Harty, K.M 17HemphiU-Haley, M.A. 27Henkle, W.R 19Inyang, H.1 6Jalbert, L.A, 24Keaton, J.R 24Keene, E 4,14Kindel, G.L 33King, K.W 25Knuepfer, P.L.K 27Koncarski, C.M 13Kuenzli, J 10Lachmar, T.E 34

vi

Leenang, B 11Loughman, C.C 28Madsen, G.R 29Marcouiller-Hippe, B.A. 7Marks, T.R 7Mathewson, C.C 15McDonald, B 36McLin, S.G 16Merifield, P.M 28Moudud, A 22Mulvey, W.E 30Mustoe, N.B 38Nielson, D.L 40Ore, H.T 33Palmer, J 11Payton, C.C 9Phipps, M.B 20Pitcher, D 10Rahman, R 4Rice, R.H 8Robison, R.M 26Rockwell, T.K 28Rollins, K.M. 31Sampaco, C.L 12Santi, P.M 8Sawyer, T.L. 27Schlinger, C.M 1Seelos, A. 37Sharma, S 22Sitar, N 21Slosson, J.E 20Slosson, T.L 20Slyh, B 23Smith, R.P 27Smith, T.D 23Soliday, S 23Solomon, B.J 40Stilley, AN 32Tauxe, G.W 16Tinsley, J.C 25Tnimm, D.A 25Uhacz, K. 23Walujono, H 5Weirich, T 15Welhan, J 33Wieland, M 12

vii

Werle, J.L 32Williams, R 25Williams, T 31Wong, I.G 27Zhao, P 35

viii

"Gaddy, Alan"<GaddyA@ repsrv.com>

08/30/200412:05 PM

To: Steve Wall/R9/USEPA/US@ EPAcc: David Basinger/R9/USEPA/US@EPA, "Cliff Anderson (E-mail)"

<cliff@ahymo.com>, "Lundell, Clarke" <LundellC@repsrv.com>, "CraigH. Benson (E-mail)" <benson@engr.wisc.edu>, "'Don Patterson(E-mail)" <dpatterson@bdlaw.com>, "Douglas Hamilton (E-mail)"<dhamilton@exponent.com>, "Steve Barringer (E-mail)"<steve@mgninc.com>, "Gardner, Tom" <GardnerT@repsrv.com>,Laurie Williams/R9/USEPA/US@EPA

Subject: FW: References: Werle and Stilley

Hi Steve attached is one of the referenced articlesrequested ; .Alan

Alan J. GaddyRepublic Services of Southern Nevada770 E. Sahara AvenueLas Vegas, Nevada 89104(702) 644-4210 x 237Pax (702) 644-3028GaddyA® RepSrv.com

Werle and Stilley 1991.p