POTENTIAL FOR SUBSIDENCE FISSTJRING IN THE PHOENIX ARIZONA USA AREA Michael K. Larson Kansas Department of Health and Environment Bureau of Oil Field and Environmental Geology 10U Cody Hays, Kansas 67601 USA Former address Department of Geology Arizona State University Tempe, Arizona 85287 Abstract Earth fissures associated with as much as 2 m of subsidence caused by water level declines of more than 100 m have the potential for damage to urban infrastructure in the Phoenix metropolitan area. The long, narrow eroded cracks open in alluvium by differential subsidence localized by buried bedrock hills and scarps, the edge of a stable ("hinge-line") or advancing subsidence area, sedimentary facies changes, manmade changes in vertical loading, and groundwater recharge mounds. In Paradise Valley, gravity and subsurface data indicated a fissure in Phoenix occurred over a buried hill. Nearby similar features and a buried fault may localize fissuring in the basin, including parts of Scottsdale. Fissuring near Mesa has been associated with two shallow bedrock areas, and a similar nearby area may soon also undergo fissuring. No fissures have yet to occur in Deer Valley, however two large buried bedrock ridges may influence fissure formation as subsidence continues. Introduction Earth fissures, long narrow eroded tension cracks, have been an increasing problem in south-central Arizona basins over the last several decades as a result of land subsidence caused by a tremendous overdraft of groundwater supplies. Until recently, the fissure-subsidence problem was largely confined to agricultural areas where pumping was most concentrated, but now the problem is also becoming a major concern in rapidly growing urban areas. Some of the most important parts of the municipal infrastructure are the most vulnerable to damage. The operation of canal, storm drainage, and sewer systems dependent on gravity flow may be impeded because of altered gradients; wells can be damaged by casing protrusion or collapse; and buildings, roads, bridges, and utility lines may be directly damaged by fissuring. Description and Origin of Fissures Fissures occur in alluvial basins of southern Arizona where groundwater levels have declined from 45 m to more than 90 m accompanied by generally more than 0.3 m of subsidence. The openings are typically several hundred meters long, 1 m or more wide, and 1 to 20 m deep. Little or no vertical offset is observed; however, there are some notable exceptions. A fissure in the Picacho basin, located about 100 km SE of Phoenix, measures 15-8 km in length where as much as 3-8 m of land subsidence has occurred (Laney, Raymond, and Winikka, 1978). 291
The fissures begin below the surface as small tensional cracks, later eroded to the surface by heavy rains or application of irrigation water. Runoff enlarges the fissure in length and frequently opens other cracks subparallel to the initial one. Ground fissures may originate from a variety of processes including: 1) tectonic movements, 2) desiccation of expansive clay soils, 3) hydrocompaction (caused by the artificial wetting of low density sediments or artificial fill), 4.) horizontal seepage stress related to groundwater flow toward pumped areas, and 5) localized differential subsidence. The majority of fissures in Arizona basins are caused by the last process (differential subsidence) which is the basis for fissure prediction as outlined in this report. The other processes are of either little importance or are a shallow soil phenomena caused by application of surface water rather than groundwater withdrawal (expansive soil and hydrocompaction). Horizontal seepage stress may in some cases act together with differential subsidence to cause fissures. In central Arizona basins, subsidence generally begins after water levels decline more than 30 m (Holzer, 1981). With continued subsidence, fissures form at points of maximum horizontal tensile stress near maximum convex-upward curvature of the subsidence profile. The temporal and spatial distribution of fissures is determined by the historical development of water resources, hydrogeologic characteristics, and subsidence history of a particular basin. The term "bedrock" as defined in this study is defined as consolidated rock that is essentially incompressible under the magnitude of effective stress changes resulting from water level decline.- Thus, bedrock includes all crystalline rock, indurated conglomerate, and evaporites (anhydrite, salt) that are common in basins of south-central Arizona. Differential subsidence may be localized by several different geological environments: 1) buried bedrock hills, 2) buried bedrock scarps, 3) the "hinge" or "zero-line" of subsidence, 4-) sedimentary facies changes, 5) the edge of an advancing subsidence front, 6) manmade changes in vertical loading, and 7) near recharge mounds. Geophysical and land survey data indicate most fissures are associated with the first three environments, particularly with bedrock hills (Anderson, 1973; Jennings, 1977; Jachens and Holzer, 1979; Larson, 1982a) (Fig. 1). The other environments are of lesser importance, but may explain the occurrence of transient fissures or fissures at the center of a basin. Subsidence varies as a function of the thickness of alluvium; therefore differential compaction over buried hills (Fig. 1A) can produce a convex-upward subsidence profile with fissuring either directly over the buried ridge crest or offset some distance away from the crest. Buried bedrock scarps (Fig. 1B) of tectonic or erosional origin can also cause a rapid change in the amount of compaction and subsidence within a short distance. Such a subsurface feature may cause vertical offsets on fissures, as apparently has occurred along the Picacho fissure previously mentioned. Another common environment of fissuring is near the "hinge-line" of subsidence (Fig. 1C) (Larson, 1982a). The location of the hinge-line is delineated by the area where the original depth to groundwater (prior to pumping or application of irrigation water) approximates the depth to bedrock. Little or no compaction and hence no land subsidence should occur where depth to bedrock is less than the original depth to groundwater. At greater depth there is an increasing amount of subsidence. Fissures can form along or parallel to the hinge-line in the subsiding area. 292
A B ' S, f S, T 7////7T/7/ y / / / / / ) \ ^ ^ ~ Î L _ 1A. Buried bedrock hill 1B. Buried bedrock scarp c D ( S, "^V~~~~" _S2_ / / ^ \ ^ ^ ^ ^ ^ ^ ^ a _ <//K: / / / / >~. i =._^._^._ = ^_SB. = i. _ K szs^rzjriszjrzz-^rjt-rir-- 1C. Hinge-line 1D. Sedimentary facies changes FIG. 1A-D Geologic Environments of Fissuring. Finer-grained sediments such as silt and clay tend to compact more than coarser-grained sands and gravel? thus differential subsidence and fissuring may be localized by abrupt facies changes in basins (Fig. 1D). The growth of a subsidence "bowl" or area may be rapid enough to cause fissuring along the edge of the subsidence front similar to ground failures which occur over mined areas, particularly for coal (Larry Powell, U.S. Bureau of Mines, personal communication) (Fig. 1E). These fissures are transient, and may close or heal because of the short length of time such areas undergo tensile strain. Fissures may result from artificial changes in vertical loading or recharge mounds in subsiding areas (Figs. 1F and 1G). Because compaction of sediments from declining water levels is caused by increases in effective stress[difference between total stress (exerted downward) and 293
1E. Edge of advancing subsidence front 1F. Manmade changes in vertical loading G LEGEND Fissure Si ' f\ "i~\ «si _S^^-^\ J" i y -2î- _.21- """^/ \ T 21 _ V2 U:JM Alluvium V//A Bedrock [ 2 Subsided Surface Subsidence Surface P^lj Water Land Surface table (original) r-^-l Water table 1G. Groundwater recharge mound FIG. 1E-G Geologic Environments of Fissuring. hydraulic stress (exerted upward )J of the deposits, cultural activities that affect either total or hydraulic stress (in addition to groundwater removal) may be important in influencing fissure location. For example, construction activity or fluctuating levels of impounded water behind dams could change total stress, therefore also affecting effective stress and creating loci of differential subsidence. Potential Fissure Areas in the Phoenix Metropolitan Complex Earth fissures have been noted in northeast Phoenix, Queen Creek, Luke Air Force Base, and east Mesa-Apache Junction areas where continued fissuring is likely (Fig. 2). In addition, there are at least five other specific areas that can be considered "high risk" for future fissuring where fissures have not occurred (Larson, 1982b). Two of the areas are in Paradise Valley; two in Deer Valley; and one locality is in northeast Mesa. 29^
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An appropriate procedure for evaluating known or suspected subsidence areas is to conduct a geophysical survey tied to well drilling and other subsurface data to determine buried bedrock configuration (Larson, 1984). The most critical depths for bedrock features are from about 30 to 500 m because the greatest amount of compaction of sediments occurs at this interval as the water level declines. Gravity has been the most effective geophysical method in terms of correlation of anomalies with fissure occurrences, particularly where buried topographic highs exist. Convexupward gravity anomalies can be generally correlated with a buried convexupward bedrock surface. At sites where geophysical or other data indicate a high risk of fissuring, land survey measurements to monitor horizontal and vertical ground movements can be used to predict possible fissure locations and subsidence patterns. In January 1980 the first fissure occurred in the Paradise Valley basin in an area where water levels have declined more than 90 m (A in Fig. 2), and land subsidence of at least 1.05 m has occurred. The 120 m long crack opened in a residential construction site in north Phoenix. Gravity and land survey data indicate the ground failure resulted from differential subsidence over a bedrock hill buried at a depth of approximately 45 m. The hill is part of a series of hills on a pediment that trends NE away from the Phoenix Mountains (Larson, 1982a; Larson and Pewé, 1983). A large cone of depression of water level decline is directly east of the Phoenix Mountains in Scottsdale that may also be an area of future fissuring (Cordy, Holway, and Pewe /, 1980) (B in Fig. 2). Lausten (1973) indicated fissures may form in areas of steep gravity gradients over inferred buried bedrock scarps in Paradise Valley. Land survey data indicated measured subsidence of less than 0.3 m for the area (Arizona Department of Transportation, 1981), however, continued monitoring by the U.S. National Geodetic Survey and the city of Scottsdale will probably reveal even more subsidence has occurred in the area. Another potential fissure area in Paradise Valley is south of the McDowell Mountains (C in Fig. 2). A prominent convex-upward gravity anomaly in this area of suspected rapid subsidence may represent an area of high potential for fissuring (Larson, 1982b). Because of the proximity of the Central Arizona Project canal (designed to transport water more than 300 km to central Arizona) and rapid urbanization of this area, more detailed investigations should be conducted. Groundwater level declines greater than 90 m have occurred directly west of the Phoenix Mountains in northwest Phoenix and north Glendale (Deer Valley) north of the Arizona Canal (D in Fig. 2, and Fig. 3) (Larson, 1982b). Fissuring has not yet occurred in Deer Valley. Reconnaissance gravity and well data indicate an area covering more than 60 knr where two large NW-SE bedrock ridges are buried at depths less than 300 m that may result in fissuring. Fissuring is also likely south of the Hedgepeth Hills where water levels have dropped over 90 m over a broad area (E in Fig. 2) with as much as 0.14 m of subsidence (Arizona Department of Transportation, 1981). Since 1963, fissuring has occurred in the east Mesa-Apache Junction areas where water levels have declined more than 120 m accompanied by as much as 2 m of subsidence (Laney, Raymond, and Winikka, 1978; Arizona Department of Transportation, 1981) (E in Fig. 2). Fissures are localized near two gravity highs associated with small isolated bedrock outcrops at Double Knolls and Hawk Rock (Fig. 4). Fissuring has occurred over buried bedrock hills and scarps at both of these locations as determined by 296
FIG. 3 Depth to bedrock and potential fissure areas in Deer Valley, north Glendale and northwest Phoenix (areas D and E in Fig. 2). Contour interval 60 m (200 ft). (Source: Larson, 1982b) geophysical and drilling data (Richard Raymond, U.S. Bureau of Reclamation, personal communication). A third gravity high in the same area of water level decline correlates with a mountain buried at a depth of 200 m near Falcon Field in northeast Mesa (Larson, 1982b). As subsidence continues in this area the probability of fissures will greatly be increased. Conclusions Officials and the general public should be made aware of the value of geological and geophysical studies with the subsidence-fissure problem. 297
FIG. 4 Bouguer gravity, fissures, and associated bedrock outcrops in east Mesa basin. Gravity contour interval 2 milligals. (Source: Peterson, 1968) The objectives of these investigations to assess and alleviate problems should always be within the context of a much broader problem the problem of conserving a limited water supply in an arid environment. Acknowledgments Information provided by the staffs of the U.S. Geological Survey, U.S. Bureau of Reclamation, Arizona Department of Water Resources, Arizona Department of Transportation, City of Phoenix Water Production Office, and the City of Scottsdale Engineering Department was most helpful. Appreciation is due to Allen Moody, Department of Geology, Fort Hays State University for his drafting work and thoughtful review of this paper, and Lynn Vogler, also of Fort Hays for insights on subsidence problems. Gene Rohr and Bryce Bickford, Hays Daily News provided technical and duplicating assistance. Gratitude is also extended to Jeanie Michaelis and Mrs. Harold Chambers of Hays for typing of preliminary and final copies of this paper. 298
References Anderson, S. L., 1973, Investigation of the Mesa earth crack, Arizona, attributed to differential subsidence due to groundwater withdrawal: Arizona State University, Unpublished Master's thesis, 111 p. Arizona Department of Transportation, 1981, 1981 NGS Level Line, Apache Junction to 1-10 at Miller Road, ADOT preliminary adjustment: Unpublished report, 192 p. Cordy, G. E., Holway, J. V., and Péwe, T. L., 1977, Environmental geology, Paradise Valley quadrangle, Maricopa County, Arizona: Unpublished report, City of Scottsdale, 14- maps, scale 1:24,000. Holzer, T. L., 1981, Preconsolidation stress of aquifer systems in areas of induced land subsidence: Water Resources Research, v. 17, no. 3, p. 693-704- Jachens, R. C, and Holzer, T. L., 1979» Geophysical investigations of ground failure related to ground water withdrawal, Picacho basin, Arizona: Ground Water, v. 17, no. 6, p. 574-585. Jennings, M. D., 1977, Geophysical investigations near subsidence fissures in northern Pinal and southern Maricopa counties, Arizona: Arizona State University, Unpublished Master's thesis, 102 p. Laney, R. L., Raymond, R. H., and Winikka, C. C, 1978, Maps showing water level declines, land subsidence, and earth fissures in southcentral Arizona : U.S. Geological Survey Water Resources Investigations Open File Report 78-83, scale 1:250,000, 2 sheets. Larson, M. K., 1982a, Origin of land subsidence and earth fissures, northeast Phoenix, Arizona: Arizona State University, Unpublished Master's thesis, 151 p. Larson, M. K., 1982b, Prediction of earth fissures related to groundwater withdrawal in the greater Phoenix area, Maricopa County, Arizona: Unpublished report, 16 p. Larson, M. K., 1984, Fissure Prediction: Geologic Models and Applications, in Second Arizona Symposium on Subsidence (Land Subsidence: Designs and Solutions, Arizona Consulting Engineers Association: Arizona Bureau of Geology and Mineral Technology Special Paper (in press). / / Larson, M. K., and Pewe, T. L., 1983, Earth fissure and land subsidence hazards in northeast Phoenix: Fieldnotes, Arizona Bureau of Geology and Mineral Technology, v. 13, no. 2, p. 8-11. Lausten, C. D., 1973, Gravity methods applied to the geology and hydrology of Paradise Valley, Maricopa County, Arizona: Arizona State University, Unpublished Master's thesis, 137 p. Peterson, D. L., 1968, Bouguer anomaly map of parts of Maricopa, Pima, Pinal, and ïuma Counties, Arizona: U.S. Geological Survey Geophysical Investigations Map GP-615, scale 1:250,000. 299