Land Subsidence in the Santa, Clara Valley, California, as of 1982
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-F
Land Subsidence in the Santa Clara Valley, California, as of 1982
By J.F. POLAND and R.L. IRELAND
MECHANICS 0 F AQUIFER SYSTEMS
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-F
A history of land subsidence in the Santa Clara Valley from 1916 to 1982 caused by water-level decline
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary
U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director
Library of Congress Cataloging-in-Publication Data
Poland, J.F. (Joseph Fairfield), 1908- Land subsidence in the Santa Clara Valley, California, as of 1982.
(Mechanics of aquifer systems) (U.S. Geological Survey Professional Paper 497-F) Bibliography Supt. of Docs. No.: I 19.16:497-F 1. Subsidences (earth movements)-California-Santa Clara Valley (San Benito County and Santa Clara County). I. Ireland, R.L. II. Series. III. Series: U.S. Geological Survey Professional Paper 497-F. QE75.P9 No. 497-F 557.72 s 87-600268 [QE600.3. U6] [551.3'53]
For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS
Page Page Abstract Fl Hydrology-Continued Introduction ------1 Recovery of artesian head, 1967-80------F18 Location and general features------1 Land subsidence ------21 Scope of study and purpose of report ------1 History of leveling control ------21 Well-numbering system ------3 Extent and magnitude of subsidence ------23 Acknowledgments------3 Economic and social impacts ------30 Annotated bibliography of reports by U.S. Geological Survey Monitoring of compaction and change in head ------33 authors ------3 Measurement of compaction ------33 Hydrogeology ------5 Analysis of stresses causing subsidence ------39 Hydrology ------14 Example of stress-strain graph ------39 Rainfall ------14 Computer simulation of aquifer-system compaction ------40 Ground-water pumpage ------14 Compaction-subsidence ratios ------42 Surface-water imports ------ 14 Computer plots of field records ------43 Decline of artesian head, 1916-66------14 Selected references ------45
ILLUSTRATIONS
' Page FIGURE 1. Generalized geologic map of north Santa Clara County ------F2 2. Geologic section A-A' from Alviso southeast to Edenvale Hills ------7 3. Geologic section B-B' from Los Altos east to Milpitas------8 4. Geologic section C-C' from near Los Gatos northeast to Alum Rock ------9 5. Composite log of core hole 6S/2W-24C7 in Sunnyvale ------10 6. Composite log of core hole 7S/1E-16C6 in San Jose ------11 7-12. Graphs showing: 7. Consolidation-test curves for samples from core hole 6S/2W-24C7 ------12 8. Consolidation-test curves for samples from core hole 7S/1E-16C6 ------13 9. Cumulative departure of rainfall at San Jose from the seasonal mean for the 106-year period 1874-75 to 1979-80 ------15 10. Ground-water pumpage in north Santa Clara County, 1915-80 ------15 11. Surface-water imports to north Santa Clara County, 1955-80 ------18 12. Artesian-head change in San Jose in response to rainfall, pumpage, and water imports ------20 13-17. Maps showing: 13. Generalized water-level contours for spring and summer, 1915, north Santa Clara County------21 14. Change in water level, 1915-67, north Santa Clara County------22 15. Network of level lines in the San Jose subsidence area; initial numbering of level lines ------23 16. Network of level lines in the San Jose subsidence area; numbering of level lines revised to three digits in the late 1950's ------24 17. Times and extent of leveling in the San Jose subsidence area ------25 18. Graph showing artesian-head change and land subsidence, San Jose------29 19-21. Maps showing: 19. Land subsidence from 1934 to 1960, north Santa Clara County ------30 20. Land subsidence from 1960 to 1967, north Santa Clara County ------31 21. Land subsidence from 1934 to 1967, north Santa Clara County ------32 22. Profiles of land subsidence, D-D ', Redwood City to Coyote, 1912-69 ------33 23. Profiles of land subsidence, E-E ', Mountain View to Milpitas, 1933-67 ------34 24. Profiles of land subsidence, F-F', Los Gatos to Alum Rock Park, 1934-69 ------35 25. Diagram of recording extensometer installations ------36 26-30. Graphs showing: 26. Elastic response of multiple extensometers at the Sunnyvale site 6S/2W-24C, 1981-82------37 27. Measured annual compaction in wells 1,000 ft deep in Sunnyvale and San Jose ------40 28. Stress change and compaction, San Jose site------41 29. Simulation of compaction based on water-level data for well 7S/1E-7Rl (1915-74) and on subsidence measured at bench mark P7 ------::------43 30. Compaction/subsidence ratios versus depth at extensometer wells in north Santa Clara County ------45
III IV CONTENTS
Page FIGURES 31-48. Computer plots showing: 31. Hydrograph, change in applied stress, compaction, subsidence, and stress-strain relationship, 6S/1 W -23E 1- - F 48 32. Hydrograph, change in applied stress, compaction, and stress-compaction relationship, 6S/2W-24C4 ----- 49 33. Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C4------50 34. Hydrograph, change in applied stress, compaction, and stress-compaction relationship, 6S/2W-24C3 ----- 51 35. Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C3------52 36. Hydrograph and change in applied stress, 6S/2W-24C4; compaction and stress-compaction relationship, 6S/2W-24C3 and 6S/2W-24C4------53 37. Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C3 and 6S/2W-24C4------54 38. Hydrograph and change in applied stress, 6S/2W-24C7; compaction, subsidence, and stress-compaction relationship, 6S/2W-24C3 and 6S/2W-24C7 ------55 39. Hydrograph, change in applied stress, compaction, subsidence, and stress-compaction relationship, 7S/1E-9D2------56 40. Hydrograph, change in applied stress, compaction, subsidence, and stress-strain relationship, 7S/1E-16C5 -- 57 41. Hydrograph and change in applied stress, 7S/1E-16C5; compaction, subsidence, and stress-strain relation- ship, 7S/1E-16C6-11------58 42. Hydrograph, compaction, and subsidence, 7S/1E-16C6-11------59 43. Hydrograph and compaction, 6S/2W-24C3 ------59 44. Hydrograph and compaction, 6S/2W-24C4 ------59 45. Hydrograph and compaction, 6S/2W-24C7 ------60 46. Hydrograph and compaction, 7S/1E-16C5------60 47. Hydrograph 7S/1E-16C5; compaction 7S/1E-16Cll ------,------60 48. Hydrograph and compaction, 7S/1E-16Cll ------60
TABLES
Page TABLE 1. Ground-water pumpage, north Santa Clara County, 1915-80 ------F16 2. Surface-water imports to north Santa Clara County, 1955-80 ------19 3. Annual compaction at compaction-measuring sites in north Santa Clara County ------38 4. Values of parameters used for simulating observed compaction at selected sites in north Santa Clara County ------42 5. Compaction versus subsidence for periods of leveling in north Santa Clara County ------44 6. Wells for which records are included in figures 31-42 ------61
CONVERSION FACTORS
The inch-pound system of units is used in this report. For readers who prefer the International System of Units (SI), the conversion factors for the terms used are listed below:
Multiply By To obtain
acre 4047 m2 (square meter) acre-ft (acre-foot) 1233 m3 (cubic meter) ft (foot) 0.3048 m (meter) gal/min (gallons per minute) .003785 m3/min (cubic meters per minute) (gallmin)/ft (gallons per minute per foot) .207 (Lis)lm (liters per second per meter) in. (inch) 2.54 em (centimeter) mi (mile) 1.609 km (kilometer) mi2 (square mile) 2.590 km2 (square kilometer)
National Geodetic Vertical Datum of 1929 (NGVD of 1929): A geodetic datum derived from a general ad justment of the first-order level nets of both the United States and Canada, formerly called mean sea level. NGVD of 1929 is referred to as sea level in this report. MECHANICS OF AQUIFER SYSTEMS
LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982
By J.F. PoLAND and R.L. IRELAND
ABSTRACT about 30 mi from Redwood City and Niles to Coyote, at From 1916 to 1966 in the San Jose area of the Santa Clara Valley, the bedrock narrows about 10 mi southeast of downtown California, generally deficient rainfall and runoff was accompanied by San Jose. a fourfold increase in withdrawals of ground water. In response, the The subsidence area is almost wholly in Santa Clara artesian head declined 180 to 220 feet. As a direct result of the artesian County. Southeast of Coyote Hills, the subsidence area head decline, the land surface subsided as much as 12.7 feet in San Jose, extends into Alameda County, and north of Palo Alto it due to the compaction of the fine-grained compressible confining beds and interbeds as their pore pressures decreased. The subsidence resulted extends into San Mateo County (fig. 1). in flooding of lands bordering the southern part of San Francisco Bay, Santa Clara County extends 30 mi southeast of Coyote. and the compaction of the sediments caused compressional failure of Ground water is pumped from wells in this area. There well casings in several hundred water wells. The gross costs of subsidence fore, to avoid confusion, the project area is defined as to date are estimated to be $30 to $40 million. north Santa Clara County when clarification is needed. The recovery of artesian head since 1967 has been substantial. In downtown San Jose, the artesian head recovered 70 to 100 feet in the For example, tables of ground-water pumpage and 8 years to 1975. Recovery of water levels was due to a fivefold increase surface-water imports listed in this report are for north in surface-water imports from 1965 to 1975, favorable local water supply, Santa Clara County, as indicated by their titles. decreased withdrawal, and increased recharge. Minor subsidence also has occurred in the Hollister area In 1960, the U.S. Geological Survey installed extensometers in core (not shown). It is reported that as of the late 1960's, sub holes 1,000 feet deep in San Jose and Sunnyvale. Measurements obtained from these extensometers demonstrate the marked decrease in annual sidence of 1 to 2 ft had occurred at bench marks in compaction of the confined aquifer system in response to the major head Hollister (T .H. Rogers, Woodward-Clyde Consultants, recovery since 1967. In San Jose, for example, the annual compaction oral commun., November 1983.) However, Hollister is in decreased from about 1 foot in 1961 to 0.24 foot in 1967 and to 0.01 San Benito County, 10 mi south of the south boundary foot in 1973. Net expansion (land-surface rebound) of 0.02 foot occurred of Santa Clara County, and thus this subsidence is not in 1974. discussed further in this report.
INTRODUCTION SCOPE OF STUDY AND PURPOSE OF REPORT LOCATION AND GENERAL FEATURES Since the late 1950's the U.S. Geological Survey has The Santa Clara Valley is a long, narrow structural carried on a modest study of land subsidence in the Santa trough that extends about 90 mi southeastward from San Clara Valley, financed by Federal funds for study of the Francisco. San Francisco Bay occupies much of its north mechanics of aquifer systems. The study has been similar em one-third. The valley is bounded on the southwest by to concurrent studies of land subsidence in the San the Santa Cruz Mountains and on the northeast by the Joaquin Valley (Poland and others, 1975; Ireland and Diablo Range (fig. 1). The San Andreas fault system is others, 1984), but on a much smaller scale. The principal a few miles southwest of the valley, and the Hayward fault field activities of the program have been twofold. First, borders the valley on the northeast. Both fault systems in 1960 the Geological Survey drilled core holes at the are active. centers of subsidence in San Jose and Sunnyvale. They Land subsidence occurs in the central one-third of the were drilled to a depth of 1,000 ft-the maximum depth valley in an area of intensive ground-water development. tapped by most water wells as of 1960. Cores were tested This report discusses only this densely populated central in the laboratory for physical and hydrologic properties one-third of the valley, which extends southeastward and for clay-mineral analysis. Results have been released
F1 F2 MECHANICS OF AQUIFER SYSTEMS
in several publications (Johnson and others, 1968; Meade, artesian head was monitored by the Geological Survey, 1967). and subsidence was monitored by the National Geodetic Second, in 1960 extensometers (compaction recorders) Survey. These records from 1960 to 1980 are included in were installed in the 1,000-ft core holes and in nearby full in this report as computer-plotted stress-strain or satellite holes in order to measure the compaction or ex stress-compaction relationships (see figs. 31-42). The pansion of the sediments in that depth range and com Geological Survey terminated measurements of water pare it with change in artesian head and with subsidence level and of compaction or expansion at the end of 1982. in the same time interval. Aquifer-system compaction and Measurements of these data for the 3 years 1980-82, expansion and water-level change (change in applied inclusive, have also been included in this report (table 3; stress) have been measured for 22 years. The change in figs. 43-48).
122"00' R 1 W R 1 E EXPLANATION D Alluvium and bay deposits T ~ Santa Clara Formation and assoc 4 ~ fated deposits s ~ Consolidated rocks
----Fault-Dashed where approx- imate, dotted where concealed ~'Trace of geologic section-See figures 2-4 Types of observation wells and iden tification numbers 37° 1 30' ~M Water level T :JE 1 Extensometer (listed in tables) 5 s 24C7 ® Well and (or) extensometer xP7 Bench mark and number
T 6 s I I I -----t---I
1 I
T 7 s
I - ---t--~--'-"M~A'lt'....,..-c
report I 37° I 15' I I I I T 8 I I s I I 0 5 KILOMETERS 0 5 MILES I
FIGURE !.-Generalized geologic map of north Santa Clara County. LAND SUBSIDENCE IN THE SANTA _9LARA VALLEY, CALIFORNIA, AS OF 1982 F3
In addition to the principal field activities, development level data utilized in this report were chiefly from field of a one-dimensional mathematical model that calculates measurements by the Geological Survey, but the Santa idealized aquifer-system compaction and expansion was Clara Valley Water District was most helpful in supply a highly significant accomplishment. The model has been ing records of ground-water levels from its well-measur applied to observed water-level fluctuations and to the ing programs. The California Division of Highways and resulting observed transient compaction and expansion the San Jose Water Works granted permission to drill behavior of a total thickness interval at several sites in core holes and maintain compaction-measuring equipment the Santa Clara Valley (Helm, 1977). on their properties. The principal purposes of this report are (1) to present Core descriptions in the field were by W.B. Bull, J.H. a summary of measured land subsidence through 1982, Green, R.L. Klausing, R.H. Meade, G.A. Miller, and F.S. including the overall extent and magnitude; (2) to report Riley (all of the Geological Survey). J.H. Green prepared the measured annual compaction and water-level change figures 2, 3, and 4 (geologic sections), figures 5 and 6 (com from 1960 through 1982 at extensometer sites; (3) to show posite logs of core holes), and figure 19 Oand subsidence the relationship between water-level change (stress from 1934-60). change), measured compaction, and subsidence, chiefly by use of computer plots of Geological Survey field data; (4) to record the yearly pumpage of ground water in north ANNOTATED BIBLIOGRAPHY OF REPORTS BY Santa Clara County from 1915 to 1980; and (5) to report U.S. GEOLOGICAL SURVEY AUTHORS the yearly surface-water imports from 1955 to 1980 via the Hetch Hetchy and South Bay Aqueducts. 1962, Poland, J.F., and Green, J.H., Subsidence in the Reports and papers published to date on various aspects Santa Clara Valley, California-A progress report: U.S. of the Geological Survey study in the Santa Clara Valley Geological Survey Water-Supply Paper 1619-C, 16 p. have been published in several different sources. There By maps and profiles, this report illustrates the sub fore, to assist the interested reader, this report includes sidence that had occurred by 1954; bench mark P7 in a brief annotated bibliography of published reports pre San Jose had subsided 7.75 feet. The first complete pared by Geological Survey authors as a result of these survey of the bench-mark network was in 1934. Sub studies. sidence maps are included for 1934-54, 1940-54, and 1948-54. WELL-NUMBERING SYSTEM 1962, Green, J.H., Compaction of the aquifer system and land subsidence in the Santa Clara Valley, California: The well-numbering system used in California by the U.S. Geological Survey Professional Paper 450-D, p. U.S. Geological Survey and the State of California shows 175-178. Results of laboratory tests on the compres the locations of wells according to the rectangular system sibility of fine-grained cores from two 1,000-ft core holes for the subdivision of public land. That part of the number and records of the fluctuation of artesian head since preceding the slash (as in 7S/1E-16C6) indicates the 1915 were used to compute the compaction of the con township (T. 7 S.); the number following the slash is the fined aquifer system up to the 1960 releveling of the range (R. 1 E.); the digit following the hyphen is the sec bench-mark network. Computed compaction was in tion (sec. 16); and the letter following the section number general agreement with actual subsidence, but the pro indicates the 40-acre subdivision of the section. Within cedure is highly subjective when applied to such hetero each 40-acre tract the wells are numbered serially, as geneous deposits. indicated by the final digit. 1964, Green, J.H., The effect of artesian-pressure decline In this report, wells are referenced to the Mount Diablo on confined aquifer systems and its relation to land sub base and meridian; all wells are south of the base but are sidence: U.S. Geological Survey Water-Supply Paper both east and west of the meridian. 1779-T, 11 p. This report summarizes the use of one dimensional consolidation tests on cores and known declines of artesian head to compute compaction of fine ACKNOWLEDGMENTS grained clayey beds. It also describes the use of void ratio values derived from one-dimensional consolidation We acknowledge the cooperation of Federal, State, and tests to compute change in porosity in response to local agencies, private companies, and individuals. All change in effective stress. leveling data used in the preparation of the subsidence 1967, Meade, R.H., Petrology of sediments underlying maps and graphs and in calculating magnitude and rates areas of land subsidence in central California: U.S. of subsidence were by the National Geodetic Survey Geological Survey Professional Paper 497 -C, 83 p. This (formerly the U.S. Coast and Geodetic Survey). Water- paper describes petrologic characteristics that influence F4 MECHANICS OF AQ"Q_IFER SYSTEMS
compaction of water-bearing sediments in the San 1969, Poland, J.F., Land subsidence and aquifer-system Joaquin and Santa Clara Valleys, including particle size, compaction, Santa Clara Valley, California, USA: in clay minerals, and associated ions. The analytical pro Tison, L.J., ed., Land subsidence, v. 1, International cedures used in the clay-mineral studies are discussed. Association of Scientific Hydrology Publication 88, (is In addition, particle-size data for cored sediments in the now International Association of Hydrological Sci Santa Clara Valley are tabulated, as are clay minerals ences), p. 285-292. Intensive withdrawal of ground and associated ions from fine-grained cored sediments water from the confined aquifer system, 800 ft thick, and from fine-grained surface sediments (seep. 38-45 in the San Jose area of the Santa Clara Valley has and 63-78). Montmorillonite is the dominant clay drawn down the artesian head as much as 220 ft since mineral, comprising between 5 and 25 percent of the 1915. Resulting land subsidence, which began about sediments, depending on the general fineness of their 1916, was 12.7 ft in 1967. Report includes graphs of particle size. Calcium is the dominant exchangeable measured compaction, water-level change, and sub cation. sidence to 1969 in Sunnyvale and San Jose, log-log plots 1968, Johnson, A.I., Moston, R.P., and Morris, D.A., of compressibility of fine-grained sampJes from the Physical and hydrologic properties of water-bearing Sunnyvale core hole, and a map of land subsidence from deposits in subsiding areas in central California: U.S. 1934 to 1967. Geological Survey Professional Paper 497-A, 71 p. To 1972, Poland, J.F., Lofgren, B.E., and Riley, F.S., provide information on the water-bearing deposits in Glossary of selected terms useful in studies of the the areas of maximum subsidence (in San Jose and in mechanics of aquifer systems and land subsidence due Sunnyvale), two core holes were drilled in the Santa to fluid withdrawal: U.S. Geological Survey Water Clara Valley. In all, 87 samples from these core holes Supply Paper 2025, 9 p. The glossary defines 25 terms were tested by the Geological Survey for physical and as they are used in Geological Survey research reports hydrologic properties, and 21 samples were tested by concerned with the mechanics of stressed aquifer the U.S. Bureau of Reclamation for engineering prop systems and of land subsidence. Most are terms that erties, including one-dimensional consolidation tests. have appeared in engineering or hydrologic literature, The report presents core-hole logs and laboratory data but several have been· introduced as a result of the in tabular and graphic form, describes methods of Survey's studies. laboratory analysis, and shows interrelationships of 1973, Webster, D.A., Map showing areas bordering the some of the physical and hydrologic properties. For a southern part of San Francisco Bay where a high water summary of the results of laboratory analyses, see table may adversely affect land use: U.S. Geological p. 43-45. Survey Miscellaneous Field Studies Map MF-530, 1 1968, Meade, R.H., Compaction of sediments underlying sheet, scale 1:125,000. Map presents general informa areas of land subsidence in central California: U.S. tion about the depth to the top of the saturated zone Geological Survey Professional Paper 497-D, 39 p. A (water table), shown in four depth zones 0-5, 5-10, study, partly statistical, of the factors that influence the 10-20, and >20ft below land surface. Depth zones are pore volume and fabric of water-bearing sediments com based on data obtained from more than 1,000 soil pacted by effective overburden loads ranging from 40 borings made for foundation engineering studies. The to 1,000 lb/in2• Paper includes summary of pertinent problems caused by a shallow water table are also previous studies that identify these factors and that in discussed. dicate the nature and degree of their influence on com 1977, Helm, D.C., Estimating parameters of compacting paction processes (p. 3-14). fine-grained interbeds within a confined aquifer system 1969, Poland, J.F., and Davis, G.H., Land subsidence due by a one-dimensional simulation of field observations: to the withdrawal of fluids: Geological Society of International Association of Hydrological Sciences, America, Reviews in Engineering Geology, v. 2, International Symposium on Land Subsidence, 2d, p. 187-269. A review of the known (1963) examples of Anaheim, California, December 1976, Publication 121, appreciable land subsidence due to fluid withdrawal p. 145-156. A one-dimensional mathematical model that throughout the world, with a brief examination of the calculates idealized aquifer-system compaction and ex principles involved in the compaction of sediments and pansion has been applied to observed water-level fluc of aquifer systems as a result of increased effective tuations and to the resulting observed transient stress. Special emphasis is given to subsidence studies compaction-and-expansion behavior of a total thickness in the San Joaquin (p. 238-252) and Santa Clara interval at several sites in the Santa Clara Valley. For (p. 252-262) Valleys and to the Wilmington oil field established values of total cumulative thickness of fine (p. 200-214) in the Los Angeles and Long Beach Harbor grained interbeds (aquitards) within the confined aquifer area. system, the weighted average thickness of these inter- LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F5
beds, and the initial distribution of preconsolidation HYDROGEOLOGY pressure, it is demonstrated that carefully evaluated values of the vertical components of hydraulic conduc A generalized geologic map of north Santa Clara County tivity and nonrecoverable compressibility can be used (the central part of the Santa Clara Valley) is shown in to predict aquifer-system behavior with reasonable figure 1. The map was compiled chiefly from geologic accuracy over periods of several decades. The paper maps prepared by Jennings and Burnett (San Francisco includes plots of simulated compaction based on meas sheet, 1961), Rogers (San Jose sheet, 1966), and Dibblee ured water level and compaction data at eight sites in (Palo Alto 15-minute quadrangle, 1966). The consolidated the Santa Clara Valley for periods of record ranging rocks, shown as a single unit on the map, range in age from 12 to 59 years. from Jurassic to Pliocene; they consist principally of con 1977, Poland, J.F., Land subsidence stopped by artesian solidated sedimentary rocks but also include substantial head recovery, Santa Clara Valley, California: Inter areas of metamorphic rocks-the greenstones of the Fran national Association of Hydrological Sciences, Inter ciscan Formation, which are probably metamorphosed national Symposium on Land Subsidence, 2d, Anaheim, submarine basalt flows (Dibblee, 1966). These rocks are California, December 1976, Publication 121, p. 124-132. highly compressed but yield small quantities of water to From 1916 to 1966, generally deficient rainfall and domestic wells from fractures induced by tensional and runoff was accompanied by a fourfold increase in compressional forces. withdrawals of ground water. In response, the artesian Except for the two major regional faults-the San An head declined 180 to 220 ft, causing the land surface dreas and Hayward faults-no attempt has been made to to subside as much as 12.7 ft in San Jose, due to com include faults on this generalized map because detailed paction of the fine-grained compressible aquitards as geologic mapping was beyond the scope of the study. For the pore pressures decreased. From 1967 to 1975 the more geologic detail, in addition to the sources cited, the artesian head recovered 70 to 100ft. This water-level reader is referred to a geologic map published by the recovery was due to a fivefold increase in surface-water California Department of Water Resources (1967, pl. 3). imports from 1965 to 1975, favorable local water supply, That map shows subsea contours on the buried surface decreased withdrawal, and increased recharge. Exten of rocks of the Franciscan Formation for much of the someters installed in 1960 to measure change in thick valley. From the Dumbarton Bridge southeast to Sunny ness of the confined system demonstrated the marked vale, the buried surface plunges from 300 to 3,000 ft below decrease in annual compaction in response to the major sea level. head recovery. In San Jose, the annual compaction The Santa Clara Formation of Pliocene and Pleistocene decreased from about 1 ft in 1961 to 0.01 ft in 1973. age unconformably overlies the consolidated rocks and is 1978, Helm, D.C., Field verification of a one-dimensional exposed intermittently along both flanks of the valley. mathematical model for transient compaction and ex Dibblee (1966) designated a type area as the exposures pansion of a confined aquifer system: in Verification of between Saratoga and Stevens Creeks, with a type sec mathematical and physical models in hydraulic en tion as exposed in Stevens Creek. About 2,200 ft of the gineering: American Society of Civil Engineers, Pro Santa Clara Formation is exposed in the type area. Where ceedings of Specialty Conference, College Park, exposed, the formation consists of semiconsolidated Maryland, p. 189-195. Land subsidence due to ground conglomerate, sandstone, siltstone, and claystone. The water withdrawal from a confined aquifer system is an conglomerate and sandstone are poorly sorted and have expression at land surface of net compaction (vertical a fine-grained matrix; thus the formation has a low consolidation) at depth of compressible layers within the permeability and, where exposed or at shallow depth, system. Water-level fluctuations within coarse-grained yields only small to moderate quantities of water to wells, aquifers produce stress changes on the upper and lower rarely enough for irrigation purposes. boundaries of slow-draining aquitards. Within a con Along the western border of the valley, the Santa Clara fined system, any lag in compactive response to such Formation was warped and folded during the last uplift a load increase is usually ascribed to slow vertical of the Santa Cruz Mountains. Dips of 25 o to 35 ° are com drainage from the aquitards. The emphasis of this paper mon. Beneath the valley the formation may be undis is threefold: First, the author describes the trial-and turbed and in part conformable with overlying beds. error method used to find vertical hydraulic conduc In his map of the Palo Alto 15-minute quadrangle, tivity, K', and nonrecoverable specific storage, S~kv; Dibblee (1966) discriminated a geologic unit overlying the second, the values of these parameters so estimated are Santa Clara Formation that he calls "older alluvium." The listed in table 1; and finally, based on these values, only extensive outcrop covers about 4 mi2 in the Saratoga predictions of critical stress and ultimate compaction area. He described the unit as stream-laid gravel com values are presented. posed of cobbles and pebbles in a matrix of sand and silt, F6 MECHANICS OF AQUIFER SYSTEMS derived from adjacent hills, generally undeformed, dis to Palo Alto and Milpitas and beneath the bay. In much sected where elevated, as much as 50 ft exposed, and age of the area of confinement, wells more than 200 ft deep presumably late Pleistocene. flowed in the early years of urban development (see Because this unit has been mapped only on the west side fig. 13). The confined aquifer system is as much as 800ft of the valley and is of little importance in water supply, thick. Around the valley margins, grotind water is its outcrop area has been combined with that of the Santa generally unconfined, and most of the natural recharge Clara Formation in the stipple pattern of figure 1. to the ground-water reservoir percolates from stream Unconsolidated alluvium and bay deposits of clay, silt, channels crossing alluvial-fan deposits. sand, and gravel of late Pleistocene and Holocene age The confining member overlying the confined aquifer overlie the Santa Clara Formation and associated system has a thickness of 150 to 200ft (see fig. 3, central deposits; their upper surface forms the valley floor. Well part; also see fig. 5). Although predominantly composed logs indicate that permeable alluvial deposits attain a of clay and silt, it also contains some channel fillings and thickness of 1,000 ft or more in the valley trough. Wells lenses of permeable sand and gravel. This confining range in depth from 200 to 1,200 ft (California Depart member supports a shallow water table distinguished by ment of Water Resources, 1967, pl. 4). The deeper wells an irregular surface. As of 1965-70, the shallow water probably tap the upper part of the Santa Clara Forma table overlying much of the confined system was less than tion, although the contact with the overlying alluvium and 30ft below the land surface (Webster, 1973). At least near bay deposits has not been distinguished in well logs the bay, the shallow water table did not fluctuate ap because of the lithologic similarity. preciably during the period of prolonged artesian-head Coarse-grained deposits predominate in the alluvial-fan decline terminating in 1966, indicating little change of and stream-channel deposits near the valley margins, stress in the shallow confining member. j where the stream gradients are steeper. The proportion In 1960, the Geological Survey drilled core holes 1,000 of clay and silt layers increases hayward. For example, ft deep at the two centers of subsidence: in Sunnyvale a well-log section extending 12 mi northward from Camp (well6S/2W-24C7) and in San Jose (well7S/1E-16C6). An bell to Alviso (Tolman and Poland, 1940, fig. 3) shows that electric log was obtained for each core hole after coring to a depth of 500 ft the cumulative thickness of clay layers was completed. Graphic logs and generalized lithologic in the deposits increases from 25 percent near Campbell descriptions were prepared from the geologists' logs made to 80 percent near Alviso. at the drill site, supplemented by interpretation of the Well yields in the valley range from 300 to 2,500 gallons electric log in zones not cored or of poor recovery. These per minute (gal/min), and the specific capacity [(gal/min)/ft three elements were combined to give a composite log for drawdown] ranges from 10 near the valley margin to 70 each core hole (figs. 5, 6). The depths of the samples tested in mid-valley, exceeding 300 in both San Jose and Santa also were plotted on the composite logs. Clara (California Department of Water Resources, 1967, In all, 87 samples from these core holes were tested by pl. 6). the Geological Survey in the laboratory for physical and Geologic sections A-A' (fig. 2), B-B' (fig. 3), and C-C' hydrologic properties, including particle-size distribution, (fig. 4) illustrate the general nature of the alluvial water specific gravity of solids, dry unit weight, porosity and bearing deposits that underlie the valley floor. Fine void ratio, hydraulic conductivity (normal and parallel to grained materials such as clay, silt, and sandy clay, which bedding}, and Atterberg limits (Johnson and others, 1968, retard the vertical movement of confined ground water, p. A1 and tables 5 and 6). constitute the major part of the valley fill near the bay Compressibility characteristics of fine-grained com (figs. 2, 3). Sand and gravel occur in lesser amounts near pressible layers (aquitards) can be obtained by making the bay, but they are more abundant near the valley one-dimensional consolidation tests of "undisturbed" fine margins, where locally they are predominant. None of the grained cores in the laboratory. As one phase of the distinctive layers or lenses can be traced laterally for mor€ research on compaction of the aquifer system, laboratory than a mile or two. Inspection of the three figures in consolidation tests were made by the U.S. Bureau of dicates that the well-log section B-B' (fig. 3), which Reclamation on 21 selected fine-grained cores from the traverses mostly lowland areas near the bay, contains a two core holes (see sample depth and number in composite preponderance of clay. On the other hand, sections A -A' log). Properties tested included the compression index, and C-C' (figs. 2, 4) traverse chiefly upland areas, and Cc, a measure of the nonlinear compressibility of the sam many of the well logs indicate a preponderance of coarse ple, and the coefficient of consolidation, Cv, a measure grained material. of the time rate of consolidation. Complete results of these In the central two-thirds of the valley below a depth of laboratory tests have been published (Johnson and others, 150 to 200 ft, ground water is confined. The confinement 1968, tables 8 and 9, and figs. 12 and 21c). The 21 samples extends northward from the southern part of San Jose tested spanned a depth range from 141 to 958 ft below LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F7 land surface. The range of the compression index, Cc, was consolidation-test curves for the 11 samples from core hole small compared with the range in the San Joaquin Valley: 6S/2W-24C7 and the 10 samples from core hole the maximum value was 0.33, the minimum was 0.13, and 7S/1E-16C6 are shown in figures 7 and 8. the mean was 0.24. Of the 21 samples, 15 had Cc values The plot of void ratio against the logarithm of load falling between 0.20 and 0.30. This suggests that the (effective stress) is known as thee-log p plot. The com nonlinear compressibility characteristics of the aquitards pression index, Cc, representing the compressibility of the in the confined aquifer system do not vary widely. The sample, can be obtained graphically from the e-log p
NW SE ALVISO EDENVALE
FEET A A' 300
200 N ...., N a...... , 0 LO l en ..... I I I _I 100 ~ .....~ ~ (/) co lh
100
200
300
400 Sand
500 ~ Silt ~ "Ci""D o· 600 Sand and gravel ::f Gravel 700 Clay
800 ~ Clay, sand, See figure 1 for location and gravel of section 900
0 2 3 4 5 KILOMETERS
0 2 3 4 5 MILES
FIGURE 2.-Geologic section A-A' from Alviso southeast to Edenvale Hills. r -n 0 (0 00 ~ m (Jl ~ w N men Nm 0 0 0 0 0 0 0 0 0 0
sw NE
c C' N ~ ALUM a J: N ("') ROCK "=f" "=f" U I FEET 7 ("') ~ !: ....1 <( 300 !: ~ I .- ~ co co 1 N LO .... >!:en I a: ~ a: t-,..... ~ N en ___ I "='" CO L0 !: N N ("') ...... cn en; .... I N N I I ...... en ~ !: ("') "=f" N ~ Q.. ~ J: w w ,!,..~r .....,. cn !: ("') ("') .... 5i .... 0 "='" ("') 200 ,, f " "" 0') I I I 0~4? ~ ~ I W I en -- w w w ...... w...... en .... en ...... en ·~ .....,. I ·.~_6, I 100 v ·~ ~ -.- SEA LEVEL
i:D -o.= -:_-_- =~- ~ =.:=.:.= .:..--:-:::.:-: •:-;:;; -_-__ ~-:- ----_ 100 ~:~ ~--::: ~l,~ - ;-=-:-:::- _:.. :::-:..: -:::-::- ~
200 -~"- illim~ ~~~ ~~~ ~ :-;:=: _._._ ~~ ·=-·-,_ ~-=.:.-=-==- 300 ~
400
500
600
700
800
See figure 1 for location of section 900
See figure 2 for explanation
1000
0 2 3 4 5 KILOMETERS
0 2 3 4 5 MILES
FIGURE 4.-Geologie section C-C' from near Los Gatos northeast to Alum Rock. FlO MECHANICS OF AQUIFER SYSTEMS
SPONTANEOUS GENERALIZED LITHOLOGIC SAMPLE NUMBER POTENTIAL ~~~:~~~:y GR~;:IC lfrom co~E~!!'~~~. drilling USBR 1 USGS 5 millivolts AM -16 in. FEET performance, and lllctric logsl ~------~~------~--~-~~~+~--~~------~~50~---o-.~.------~~~~~~~~~------1500
60 CAL10 0-150 Clay, silty, sandy, and gravely. Olive to blue. Calcareous nodules
23 L255
60 CAL 20 150-185 Clay. Greenish gray to olive. Calcareous nodules
23 L256 -t-----=:::::::=::::====t- 200-:~.;,-;.: 185-215 Gravel, clayey, silty, and sandy. Greenish gray ~~~~~~~~~~~~~~~-! 23 L258
;-=-~~ 256-318 Clay, silty, sandy, and gravely. Greenish gray 1------=+--+--~::;;..,,....------~ 300 --~-_:::.:~ to olive brown. Calcareous nodules 23 L257 ~ 318-328~G~ra~v~el~,s=a~nda------1 60 CAL 30 23 L259 328-415 Clay and dt, sandy. Olive gray .to yellowish brown. Calcareous nodules
415-434 Sand, gravelly and . silty. Olive brown to vellow brown 23 L 261
434-506 Clay, silty, and sandy . Olive gray to yelow brown. Calcareous nodules
500 r----~5:--t----.~======r- ~ 506-516 Gravel 23 L262 60 CAL 40 !':0~:0. 516-536 Clay, silty, sandy, and graveHy. Ohe gray to yellowish brown. Calcareous nodules ::~-:_- ~-547 Sand, silty and clayey .. Yelow brown to gray
23 L263 54 7-6 78 Clay and silt , sandy. Oarll greenish gray to olive ---- brown
678-742 Clay, silty. Greenish gray 23L265
60 CAL 50 ~~~~~-~-~-~~------~-4 -:~=~:-~:~: 742'-752 c~r.:.iltlilc~~o::nn~:.liv• brown to greenish - 752-802 Clay, silty. Oarll greenish gray to olive gray. 1----""""""1'..:;__+-.....:::.-....:::------+- BOO~ Calcareous nodules. Gatropod shels at 755-760 It ~ 802-814 se::,dJ:ta~:W'J::dv· clayey silt. Olive gray ~ :~ 814-839 Sil'.iod~'f:Y and sandy. Olive gray. Calcareous ~- 839-845 - ...... ·-.... 60 CAL 60 23 L 267
1-----...... L..--+-~-...... ,------+-- 900 --=::.:~ 845-1000 Clay, silty and sandy. Greenish gray to olive brown. ~ ~ Calcareous nodules
23 L269 60 CAL 69
~------L------~------~-L~------~1000~~------~ ~ Prepared by J. H. Green 1u.s. Bureau of Reclamation FIGURE 5.-Composite log of core hole 6S/2W·24C7 in Sunnyvale. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 Fll
SAMPLE NUMBER SPONTANEOUS RESISTIVITY GRAPHIC GENERALIZED LITHOLOGIC POTENTIAL oluns m21m LOG DESCRIPTION USBR 1 USGS 5 millivolts AM -16 in. (from core descriptions. drillinQ ~------+------+-~-~~~·~----~o~o------s~:~~+--F~T~-~-~-~------~P~•rl~o-~~·~~~·~.end~~·~c~•~~-~~~~~·'~------4
0-70 Clly and dt
== r------~ -==~..-_;. ~_=.·· 70-75 Clly. grevely and sendy. Gr11nish gray to yelowish brown. Wood It 73 It r-----+------+ 100-:~~ 75-148 Gravel. sandy and cllyey ::. ~ ;:.0: =~-·------~ "---= 148-214 Clly end slit. sendy. Greenish grey to olive { brown. Grevel 11 172- 17 4 h 1-- 60 CAL 70 r- t-~o:::!"---+---c-""-L ...... ,-----+ zoo-::-.::-~ ---::. ~ ~214-246 Cley, silty IIIII sillily. Greenish grey to Ytlowish --=-=- brown. Pltnt r1111ains at 230-231 It, grevll 23L271-I- at 240-241 ft 246-290 Grevtl. sendy end clayey. Rock fragments as ···- largt IS 3 in. . ~ c ~ 1290-303 Clay, silty. sandy, end gravely. Light olive 23 L 272 -I- r---~-;--~--~===------+300-~ grey "5 ------=== >--· 303-326 liancl. clayey 8RCI grevlly. Modlrete lll'own I I- to dirk vtllowish brown ~ -~:~::.~_=-~-=-:..=·-~ 326-390 Clay. silty. sandy, and grevely. Greenish grey 23 l 273 -~- _ _ _ _ to olive gray. Calcareous ---- ~---.::~=-+-----.,;.;~~~§==1400-E • .~ ~ 390-426 Send 1nd grevel Sand is pale olive to yelowish 1 .•. ••. brown. Calcareous :._·~:...-426-438 tlly end silt, sandy. Yelowish brown. Cal· 23 L275 -1- 60 CAL 80 :..-=-..:. careaus 1- - ? ~--~-~-~ _4_3_8_-4_7_3_G_re_vtl_._•_•ndy---end--silt--y._Modl___ r_••_•_brow __ n____ -l 473-512 Silt end clay, sendy. Greyish red to llllldlrete < 500 - ~-==: olive brown. Cerbone-•. caltereous 23 L 277 - <------~ ~------~ ~ 512-528 Greve!. Rock freg111111ts u largt as 2.5 in. ~ =---- :=: --:_: 528-583 S~t IIIII clay, sandy. Bluish grey to yelowish 23L 279- ~- --- · brown. Celcereous. Cerboneceous et L_ ---- 530-531 h. gravel It 546-550 ft ~ -.~ r--~~----+-7------+600-:~~I ~ t ~---=-=------=-= j~ 58HI70---.. _, 7--- 670-706 Silt. clayey. sendy, end gravely. Olive brown ~ ~ -::: to yelowish brown. Caltereous 700 ~ -~ 706-722 Clay end grevel Yelowish brown ~ L __721-748 ... , ...... -· ...... -60 CAL 90 ~ } J 748-785 Grevtl, sendy, end dty. Silt 11 756-759 It ~ _s------~------1 23 L 282- _____ 785-817 Silt end clay, sendy end grevely. Olrk bluish r--__:;~r--+---~~;::;:.. + 800 -~-; 0 {______. ~~ -8-1-7--8-:34--:-Gr-::-:-.-':-ye-.~-n-:-is:-.-~- :-~-cla--ye_y_____ -1 23 L 283- ~- ~ 834-855 Silt end sand. clayey end grevely. Olive ~- -~= grey to olive brown - ~ 855-865 Grevel ~ :::: 865-897 Silt end clay J----:;~~+-::-====;§§§§:=~=:f- 900 -'?.~ 897-929 GriYII end sind. Green to yelow~h b~wn. ~ • ~-.~ Rock fregments II llrge IS 2 II., silt II ~ 903-906 h 284 23 -- 60 CAL96 _- _-: 929-973 Silt end clay. slndy. Yelowish brown to ~ : : olive. Send or grevelly slllll at 964-967 It ~------~ ---"'~ 973-981 Sand
'------IL------1~-==-=---1~---=5:10..-----...&. 1000 =-= :-= 981-1000 Silt end clay 1u.s. Bureau of Reclamation Prepered by J. H. Gr11n FIGURE 6.-Composite log of core hole 7S/1E-16C6 in San Jose. F12 MECHANICS OF AQUIFER SYSTEMS
SAMPLE DEPTH 141 FEET SAMPLE DEPTH 437 FEET 0.90 0.70 0.80 0. 60 0. 70 0.50
0.60 0.40 0. 50 0.30 SAMPLE DEPTH 522 FEET 0.40 0.60 0.30 0.50 SAMPLE DEPTH 191 FEET 0.70 0.40
0.60 0. 30 0. 50 0.20 SAMPLE DEPTH 605 FEET 0.40 0.80 0.30 0. 70 DEPTH 253 FEET O. BO SAMPLE 0.60
0.70 0. 50 Q) . o. 60 0.40 0 1- ....: 0.30 a: 0.50 SAMPLE DEPTH 715 FEET c 0.40 0.80 0 ;:::oo. SAMPLE DEPTH 308 FEET 0.70 0. 70 0.60 0.60 0. 50 0.50 0.40 0. 40 0.30 0.30 SAMPLE DEPTH 344 SAMPLE DEPTH 865 FEET 0.70 0. 70 0.60 0.60 0. 50 0.50
0.40 0.40 0.30 0.30 0 10 100 1000 SAMPLE DEPTH 958 FEET 0. 70 0.60
0.50 0.40 0 1 10 100 1000 10,000 LOAD, IN POUNDS PER SQUARE INCH FIGURE 7.-Consolidation-test curves for samples from core hole 6S/2W-24C7. Sample depths are in feet below land surface. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F13 curve: It is the slope of the straight-line portion of the the types of deposits encountered to the 1,000-ft depth loading curve or its dashed extension. Note that several as nonmarine and mostly alluvial, with possibly a few points were obtained on the rebound as well as the loading lacustrine deposits at depths below 700 ft. Although both branch of the curve in order to compare void-ratio in core holes are near San Francisco Bay (the Sunnyvale hole creases due to unloading. is only 3 mi from the bay), Meade reported that neither Based on a study of the cores recovered from the two core contained sediments that could be recognized as core holes, Meade (1967, table 1 and p. 39-43) described estuarine. He listed several pieces of evidence indicating
SAMPLE DEPTH 554
0.60 0. 50 0.40 SAMPLE DEPTH 301 FEET 0.70------~----~----~----- 0.30 REBOUND
0.60 0. 2 0 L..,_____ ....._ ____ ...... _ ____ .....J... ______.. ______J
0. 50 0.40 0. 50
0.30------~--~~--~~ 0.40
0. SAMPLE DEPTH 354 FEET 0. 30 L...-____.....______.______---L,. ______.. ______, 60 SAMPLE DEPTH 790 FEET Q) 0. 50 0.6 0
~ C) 0. 40 0. 50 I- cc a:: 0. 3 0 .______....._ ____~ ____...... 0.40 0.30
0.60 0.20L------~----__.__ ___---L,. ___ __.. ___ __J O. SAMPLE DEPTH 832 FEET 0.50 60 0.40 0. 50 0.40 SAMPLE 0.70~----~----~----~----~----~ 0:30._____ ....._ ____~------L..-~~~---.J SAMPLE DEPTH 937 FEET 0.60 0.70~----~--~~--~---~------0.50 0.60 0.40 0.50
0.30 0.40.__---~---~---_.---~~--__j 0 10 1000 10,000 0.20 .______....._ ____~----~----~----~ 0 10 100 1000 10,000 LOAD, IN POUNDS PER SQUARE INCH FIGURE B.-Consolidation-test curves for samples from core hole 7S/1E-16C6. Sample depths are in feet below land surface. F14 MECHANICS OF AQUIFER SYSTEMS the absence of estuarine sediments, including a complete GROUND-WATER PUMPAGE lack of marine or estuarine fossils. It is of interest to note that at the Dumbarton Bridge The intensive development of irrigated agriculture in crossing of San Francisco Bay, 9 mi northwest of the the valley began about 1900. Tibbetts and Kieffer (1921, Sunnyvale core hole, Atwater and others (1977, pl. 1, pl. 7) reported that the number of irrigation wells in north cross section C-C') found two intervals of estuarine Santa Clara County increased from 115 in 1890 to 1,590 deposits above a depth of 180 ft, respectively 0 to 60 ft in 1920. and 120 to 160 ft below sea level and each about 6 mi wide. Their study of late Quaternary depositional history is Year 1890 1900 1910 1920 based on detailed examination of sediments from 18 Wells 115 235 440 1,590 boreholes at three San Francisco Bay toll crossings (Bay Bridge, San Mateo Bridge, and Dumbarton Bridge). Three Agricultural pumpage increased from about 40,000 acre geologic cross sections show detailed descriptions of ft/yr in 1915-20 to a maximum of 103,000 acre-ft/yr in samples to a maximum depth of 200 ft below present sea 1945-50 (fig. 10, table 1). After 1945, population pres level. Emphasis is on the estuarine deposits. sures caused a great transition of land use from agricul X-ray diffraction studies of 20 samples from the two tural to urban and industrial development. By 1970-75 core holes indicate that montmorillonite composes about most of the orchards had been replaced by houses, and 70 percent of the clay-mineral assemblage in these agricultural pumpage had decreased to 20,000 acre-ft/yr. deposits. Other constituents are chlorite (20 percent) and Municipal and industrial pumpage, on the other hand, in illite (5 to 10 percent) (Meade, 1967, p. 44, fig. 33, and creased from 22,000 acre-ft/yr in 1940-45 to 131,000 acre tables 14 and 15). Montmorillonite is the most compres ft/yr in 1970-75. Total pumpage (fig. 10, table 1) increased sible of the clay minerals. nearly fourfold from 1915-20 to 1960-65-from 49,000 to 185,000 acre-ft/yr-but then decreased 19 percent to 150,000 acre-ft/yr by 1970-75 in response to a rapid in HYDROLOGY crease in surface-water imports.
RAINFALL SURFACE-WATER IMPORTS Precipitation that falls within the drainage basin is the The import of surface water to Santa Clara County ultimate source of all local ground water. The rainfall at began about 1940 when San Francisco began selling sur San Jose has developed definite trends in the past 100 face water imported from Hetch Hetchy Reservoir in the years (fig. 9). A plot of the cumulative departure of rain Sierra Nevada to several municipalities. This import in fall at San Jose from the seasonal mean for the period creased from 6,000 acre-ft in 1955 to 12,000 acre-ft in 1 of record clearly defines these trends. The rainfall record 1960 and to about 50,000 acre-ft in the 1970's (fig. 11, for San Jose began in 1874-75. For the 106-year period table 2). Imports before 1955 are not known. to 1979-80, the seasonal mean rainfall (July 1 to June 30) Surface water imported from the Central Valley was 14.13 in. From about 1890 to about 1915 the generally through the State's South Bay Aqueduct first became rising curve defined by the yearly points indicates that available in 1965; during the 1970's, deliveries through the cumulative surplus exceeded the cumulative deficiency the aqueduct exceeded 100,000 acre-ft four different years by about 55 in. Thus, the average yearly surplus in this (table 2, fig. 11). wet 25-year period exceeded the seasonal mean by about Unused imported water was recharged to the ground 2 in. water reservoir through stream channels and percolation On the other hand, for the 50 years from about 1916 ponds. The yearly quantity diverted to channels (stippled to about 1966 the declining curve defined by the yearly segment of yearly bars, fig. 11) in the 15 years has aver points indicates that the cumulative deficiency exceeded aged about 40,000 acre-ft and represents about 48 per the cumulative surplus by about 60 in. Thus, the average cent of the total import from the South Bay Aqueduct. yearly deficiency in the 50-year dry period was below the seasonal mean by about 1.2 in. DECLINE OF ARTESIAN HEAD, 1916-66 In the spring of 1916, the artesian head in index well 1The cumulative departure curve is detennined by calculating the seasonal mean 7S/1E-7R1 in San Jose stood 12ft above the land surface for a period of record, calculating each season's departure (plus or minus) from (fig. 12). At that time there were only about 1,000 irriga that mean, and then summing those departures. Thus, the first season had a defi ciency of 6 in. and the second season had a surplus of 5 in., resulting in a cumulative tion wells in north Santa Clara County. The cumulative deficiency of 1 in. for the first two seasons. departure graph of rainfall at San Jose (fig. 9) shows that LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F15
UJ j: en 40
UJ-' > ~en-~ ~ i::5 20 :::Een ::::J (AVERAGE RAINFALL FOR 106 SEASONS IS 14.13 INCHES,SEASONAL YEAR JULY 1 TO JUNE 30) (.) 40~----~----~------L-----J-----~----~~----J-----~----~~----~-----L----~ 1860 1880 1900 1920 1940 1960 1980 YEAR FIGURE 9.-Cumulative departure of rainfall at San Jose from the seasonal mean for the 106-year period 1874-75 to 1979-80. (Data from U.S. Weather Bureau and National Weather Service.) MUNICIPAL AND INDUSTRIAL 1920 1930 1940 1950 1960 YEAR FIGURE 10.-Ground-water pumpage in north Santa Clara County, 1915-80. F16 MECHANICS OF AQUIFER SYSTEMS TABLE 1.-Ground-water pumpage, north Santa Clara County, 1915-80, in acrejeet [From 1915 to 1966 agricultural pumpage was estimated by the Santa Clara ValldY Water District from ~cultural power sales of Pacific Gas and Electric Company and is reported~ fiscal~ear. To 1966 municipal an industrial pump~ is listed by calendar year; municipal pumpage is metered, industrial is estimated. ince 1 6 ~umpage is metered and is reporteil here ~ fiscal year July 1-June 30. Indust:fial fuumpage was estimated as follows: 1915-34, 5,000 acre- yr; 1935-43, 10,000 acre-ftlyr; 1944-50, 1 ,000 acre-ftlyr; 1951-60, 20,000 acre- yr; 1961-66, 22,000 acre-ftlyr] Municipal Agricultural Total Year and Yearly 5-year average Yearly 5-year average industrial 1915-16 22,800 8,600 31,400 1916 31 '500 9,200 40,700 1917 41,300 9,800 51,100 1918 45,500 10,700 56,200 1919-20 56,000 39,420 10,200 66,200 49,120 1920-21 63,000 10,800 73,800 1921 54,600 10,500 65' 100 1922 56,700 10,700 67,400 1923 97,500 10,800 108,300 1924-25 82,500 70,860 14,700 97,200 82,360 1925-26 76,000 12,600 88,600 1926 74,000 12,500 86,500 1927 90,000 12,000 102,000 1928 118,000 12,700 130,700 1929-30 102,000 92,000 15,400 117,400 105,040 1930-31 122,000 14,600 136,600 1931 84,000 17,000 101,000 1932 104,000 14,400 118,400 1933 108,000 14,900 122,900 1934-35 56,000 94,800 15,100 71, 100 110,000 1935-36 68' 100 18,900 87,000 1936 63,700 19,800 83,500 1937 61,400 20,100 81,500 1938 77' 600 20,200 97,800 1939-40 73,000 68,760 22,100 95' 100 88,980 1940-41 72,600 20,700 93,300 1941 84,000 20,000 104,000 1942 97,600 18,700 116,300 1943 117,900 19,600 137,500 1944-45 96,100 93,640 33,500 129,600 116, 140 1945-46 100,800 34,200 135,000 1946 104,900 36,400 141,300 1947 114, 100 38,200 152,300 1948 98,200 41,600 139,800 1949 95,700 102,740 44,200 139,900 141,600 1950-51 84,000 49,500 133,500 1951 81,100 46,600 127,700 1952 95,700 50,700 146,400 1953 92,500 52,700 145,200 1954 121,800 95,020 55,600 177,400 146,040 1955-56 90,200 64,900 155,100 1956 94,300 66,100 160,400 1957 78,600 77,100 155,700 1958 102,600 79,800 182,400 1959 90,000 91,140 93,200 183,200 167,360 LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F17 TABLE 1.-Ground-water pumpage, north Santa Clara County, 1915-80, in acre-feet-Continued Municipal Agricultural Total Year and Yearly 5-year average Yearly 5-year average industrial 1960-61 84,800 106,400 191,200 1961 63,400 121,300 184,700 1962 54,400 119,400 173,800 1963 76,200 109,700 185,900 1964-65 57,200 67,200 131,400 188,600 184,840 1965-66 40,200 122,300 162,500 1966-67 29,800 140, 100 155,390 1967-68 35,400 139,200 174,630 1968-69 28,600 136,670 165,270 1969-70 29,320 32,660 126,040 155,360 162,630 1970-71 23,710 124,500 148,210 1971 24,090 142,390 166,480 1972 17,450 137,820 155,270 1973 15,940 129,125 145,060 1974-75 16,903 19,620 120,526 137,430 150,490 1975-76 20,036 148,764 168,800 1976-77 16,714 136,329 153,043 1977-78 11,702 115,581 127,283 1978-79 12,027 137,678 149,705 1979-80 9,793 14,050 139,764 149,557 149,680 1980-81 9,568 151,558 161,126 1916 was the end of a wet period beginning about 1890. The bottom graph of figure 12 shows the estimated total Hunt (1940) reported that in 1912, 29 percent of the valley ground-water pumpage for the 65-year period 1915-80; lands were irrigated, but that by 1920, 67 percent were the plot is similar to figure 10 except that the increase irrigated, including 90 percent of the orchards; nearly all in pumpage is plotted downward to reflect its influence of these lands were irrigated with ground water from on the artesian head in the index wells. Estimates from wells. By the autumn of 1966, the head in well 7S/1E-6M1 1915 to 1965 are based chiefly on electric-power consump in San Jose was about 210ft below land surface (fig. 12). 2 tion for agricultural use and are graphed as 5-year The principal factors in this major 50-year decline of averages; since then, virtually all pumpage has been about 220 ft are shown in figure 12. The upper line is a metered. replot (fig. 9) of the cumulative departure, in inches, from The 50-year decline in artesian head plainly was caused 1910 to 1980 of the seasonal rainfall at San Jose from the by generally deficient rainfall and constantly increasing seasonal mean for the 106-year period since 1874-75. Ex pumping draft. The curve of artesian-head decline con cept for the 6-year wet period 1936-42, the departure in forms reasonably well with the cumulative departure the 50 years from 1916 to 1966 was substantially negative, curve of the seasonal rainfall at San Jose. and the cumulative departure represents a cumulative Water levels in a relatively full ground-water basin at deficiency of 60 in. the end of the prolonged wet period beginning in 1890 are shown in figure 13. The water-level contours are generalized from plate XIV of Clark (1924), which was drawn from measurements of depth to water in wells in the spring and summer of 1915. Both figures 13 and 14 2The record of depth to water in index well 7S/1E-7R1 extends from 1915 to are maps prepared by P.R. Wood and K.S. Muir (U.S. date. However, during the last two decades the artesian head in this well has been Geological Survey, written commun., June 1972). Clark's higher than the head in nearby representative deep wells, such as well 7S/1E-6M1 (803ft deep). Accordingly, the composite hydrograph of figure 12 represents the area of artesian flow as of 1915 (from pl. XIV) and his artesian head in well 7R1 to 1959 and the head in well 6M1 from 1959 to 1980. "boundary of former area of artesian flow" (from pl. XV) F18 MECHANICS OF AQUIFER SYSTEMS are both shown in figure 13. Obviously the confined generally northwesterly movement of ground water aquifer system must extend beyond Clark's ''former area toward San Francisco Bay in 1915. In the San Jose area, of artesian flow." The boundary between confined and the water levels in wells ranged from 60 to 100ft above free ground-water conditions is not easily determined or sea level, and artesian wells were still flowing in down recognized in parts of the Santa Clara Valley. In this town San Jose. report, however, the 1-ft subsidence line of 1934-67 By 1967, after a half century of exploitation, water (fig. 21) is considered to be a general approximation of levels in wells had been drawn down below sea level in the boundary of the confined area, and accordingly it is much of the ground-water basin. The hydraulic gradient plotted in figure 13. toward San Francisco Bay in wells tapping the confined It is of interest to note that the California Department system had been reversed, and movement was now of Water Resources (1967, p. 85 and pl. 11) defined the toward large cones of pumping depression as much as boundary of the confined aquifer system as the boundary 200ft below 1915 levels, surrounding heavily pumped of their San Jose subarea. That boundary agrees in municipal well fields. In central San Jose, the artesian general with the 1-ft subsidence line of 1934-67 (fig. 21), head of 1967 was 160 to 180ft below 1915levels (fig. 14). except near Oak Hill and Penitencia Creek. In part of the confined aquifer system in 1967, water In general, the water-level contours of figure 13 are levels in wells had been drawn down below the base of assumed to represent a free water table near the valley the confining member, which near the bay is 150 to 200 margins and the potentiometric surface of the confined ft below the land surface. Therefore, it is not practical aquifer system in the central area included within the 1-ft to draw a boundary between water table and confined con subsidence line. The water-level contours by 1915 had not ditions as of 1967 on the basis of figure 14. yet been distorted appreciably from the conditions that prevailed before man began the steady increase in ground RECOVERY OF ARTESIAN HEAD, 1967-80 water withdrawal. The water-level contours indicate a The recovery of water level since the middle 1960's has been substantial. By 1975, the spring high-water level at 200.------~~------~------~ index well 7S/1E-6M1 (fig. 12) was 60ft higher than in 1966 and about equal to the 1935 level. Subsequent recovery was affected by the drought of 1976 and 1977, but the high level of 1980 was still 60 feet higher than 1- w the 1963-66 lows. w LL. From 1958 through 1982, the Geological Survey ~ 150r------measured depth to water in well 7S/1E-16C5 at the 12th o: u SOUTH BAY and Martha Street well field of the San Jose Water <( AQUEDUCT LL. Works. This well is 908ft deep and is about 1.5 mi east 0 of well 7S/1E-7R1 (fig. 1). The record of depth to water en 0 z in well 7S/1E-16C5 is shown from 1958 through 1979 in <( en figure 41A and from 1980 through 1982 in figure 47A. ~ 100r------~ 0 The recovery of artesian head in this index well from the :I: SOUTH BAY 1966 low to the 1978 low was about 80ft. 1- AQUEDUCT z Diverted to channels This major recovery of artesian head was due to several 1-- factors, including increased imports of surface water, 0: 0 favorable local water supply, decreased pumpage, and in 0.. creased recharge (fig. 12). The most important factor was the increase in imports. ; 50 ~------illll\~~~fili'~::t:::~~ 1- The import of surface water to Santa Clara County 0 1- began in about 1940. As shown by table 2, by 1966-67 yearly import through the Hetch Hetchy and South Bay Aqueducts was 36,000 and 32,000 acre-ft, respectively. By 1979-80, total import was about 158,000 acre-ft, con o~--~~~~~~~~~~~~~~~~ siderably more than double the import of 1966-67. 1950 1960 1970 1980 The average seasonal rainfall at San Jose was above YEAR normal in the 14-year period 1966-80. The cumulative FIGURE 11.-Surface-water imports to north Santa Clara County, departure graph (fig. 9) indicates a positive increase of 1955-80. about 15 in. in the 14 years. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F19 TABLE 2.-Surface-water imports to north Santa Clara County, 1955-80, in acrefeet per fiscal year [Data from Santa Clara Valley Water District; total imports rounded to 5 acre-ft] South Bay Aqueduct Year Hetch Hetchy Total Diverted to (July 1-June 30) Aqueduct Total imports channels 1955-56 5,965 1956 6,300 1957 7,445 1958 9,690 1959-60 12,300 1960-61 12,765 1961 15,578 1962 22,970 1963 24,072 1964-65 29,536 495. 495 30,030 1965-66 33,572 29,680 29,680 63,250 1966 35,784 31,785 31,480 67,570 1967 39,902 63,032 55,213 102,935 1968 41,675 57,663 46,466 99,340 1969-70 47,203 77,234 38,072 124,435 1970-71 45,288 88,486 44,024 133,775 1971 49,744 92,432 43,596 142,175 1972 48,890 92,582 47,586 141,470 1973 49,467 86,565 41,076 136,030 1974-75 53,273 104,052 36,196 157,325 1975-76 58,229 118,692 52,607 176,920 1976 47,657 89,693 29,318 137,350 1977 38,052 79,776 20,126 117,830 1978 49,434 105,107 41,308 154,540 1979-80 51' 806 105,868 35,628 157,675 The average yearly pumpage of ground water, which involved (1) salvage of flood waters from local streams reached a peak of about 185,000 acre-ft in 1960-65, that would otherwise waste to the bay, and (2) importa decreased to about 150,000 acre-ft in 1970-75 and tion of water from outside the valley. In 1935-36, five 1975-80 (table 1). A principal reason for this 19-percent storage dams were built on local streams to provide deten decrease is the tax on ground-water pumpage applied tion reservoirs with combined storage capacity of about since 1964 for water extracted from the ground-water 50,000 acre-ft, to retain flood waters, and to permit con basin. In 1970-71, for example, the ground-water tax was trolled releases to increase streambed percolation (Hunt, levied at $8 per acre-ft for ground water extracted for 1940). The storage capacity of detention reservoirs was agricultural purposes and at $29 per acre-ft for ground increased to 144,000 acre-ft by the early 1950's (Califor water extracted for other uses. For water delivered on nia State Water Resources Board, 1955, p. 51). the surface in lieu of extraction, the cost was $10.50 per Recharge from stream channels and percolation ponds acre-ft for water used for agriculture and $31.50 per acre to the ground-water reservoir has been augmented since ft for water used for other purposes. The economic ad 1965 by water from the South Bay Aqueduct that could vantage of using surface water where available is obvious not be delivered directly to the user. The quantity diverted when one considers the cost of ground-water extraction to recharge areas in the 15 years has averaged about in addition to the tax. 40,000 acre-ft per year and represents 48 percent of the Local agencies have been working since the 1930's to total import from the South Bay Aqueduct. (See stippled obtain water supplies adequate to stop the ground-water segment of yearly bars, fig. 11, and upper right graph, overdraft and raise the artesian head. Their program has fig. 12). I 2oo "'%j ~ I I I 1 0 Cl) 0 z Cl) HETCH HETCHY < wo AQUEDUCT Cf.IJ::::.w :::J:ooum ow z ..... :::J:U. I-I ;o 50 w -1- za:: - u w'Lt') 40 ._:< a:,..... a::U...... ;::::) co 30 50 oo a:: a: Q.. FIGURE 12.-Artesian-head change in San Jose in response to rainfall, pumpage, and water imports. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F21 LAND SUBSIDENCE In 1919, a line of first-order levels was run southward from San Jose. This leveling revealed subsidence of 0.4 HISTORY OF LEVELING CONTROL ft at San Jose (fig. 22, bench mark P7). The first relevel The first precise leveling of bench marks within the sub ing early in 1932 showed subsidence since 1912 ranging siding area in Santa Clara County was carried out in 1912 from 0.35 ft in Palo Alto (bench mark I7) to 3.66 ft in San on a line from Bingham, Utah, to San Francisco through Jose. More extensive leveling early in 1933 confirmed the Niles, San Jose, and Redwood City. This initial line and subsidence and provided additional control on its extent. all subsequent leveling described in this report were ac As a result of the great scientific interest in this regional complished by the National Geodetic Survey (formerly the subsidence, and through the joint efforts of A.L. Day, C.F. U.S. Coast and Geodetic Survey). Tolman, and the National Geodetic Survey (Tolman and R 1 E EXPLANA D Alluvium and bay deposits ~ Santa Clara Formation and l..!.!.i..!l associated deposits T 4 ~ Consolidated rocks s ----- Fault-Dashedwhereapprox- lmate, dotted where con cealed ----Boundary of area of artesian low, 1915. (Data from Clark, 1924,Piate XIV) ---- Boundary of former area of artesian Row (pre-1915). (Data from Clark, 1924, plate XV). ----Approximate boundary of pres sure area (Line of 1 foot sub sidence, 1934-67)-Dashed where uncertain --10--Water-level contour-General- ized contour' on the water table In the recharge area and on the potentiometric surface of the confined aquifer system in the con T 6 lined area. Dashed where s approximately located. Con tour interval 10 and 20 feet. Datum is sea level. Contours Clark, i T I 7 I s I I I I I I ----~------~-- 1 I 37° 15' T 8. s 0 1 2 3 4 5 KILOMETERS : 0 2 3 4 5 MILES FIGURE 13.-Generalized water-level contours for spring and summer, 1915, north Santa Clara County. F22 MECHANICS OF AQUIFER SYSTEMS Poland, 1940, p. 29), an extensive network was laid out westward to San Jose and then northward along the east in 1933 to determine by periodic releveling the extent, side of San Francisco Bay past Niles. Line 2 extends along magnitude, and rate of subsidence. the west side of the bay from San Jose northwestward The extent of the network is shown in figures 15 and past Redwood City. Three transverse lines (3, 18, and 9) 16. Initially numbered with single or double digits (fig. 15), extend southwestward across the San Andreas fault, and the lines were renumbered by the National Geodetic three (1, 4, and 6) extend eastward across the Hayward Survey in the late 1950's using three-digit numbers fault. (fig. 16). The network was completely leveled for the first The times and extent of leveling of the net are shown time in the spring of 1934. The total length of survey lines in figure 17. Heavy lines drawn on the screened back composing this bench-mark net is about 250 mi. As shown ground define the leveling accomplished in a designated in figure 15, level line 1 extends from Morgan Hill north- year, which may range from a single line to complete R 1 E EXPLANATION Alluvium and bay deposits T D 4 Santa Clara Formation and s associated deposits ~ Consolidated rocks -----Fault-Dashed where approximate, dotted where concealed --2o--Line of equal water-level change, 1915-67-Dashed where approx 37° imate. Interval 20 feet 30' T 5 s T 6 s T 7 s 37° 15' T 8 s 0 5 KILOMETERS 0 5 MILES FIGURE 14.-Change in water level, 1915-67, north Santa Clara County. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F23 coverage of the net, as in 1954 and 1960. In 1956, the 371222. When requesting bench-mark elevations in sub National Geodetic Survey published a chronologie com siding areas from the National Geodetic Survey, one pilation of adjusted elevations, in feet, through 1954 for should specify quadrangle, line number, and year or years all bench marks in the network (U.S. Coast and Geodetic of leveling. Survey, 1956). Since the late 1950's, the primary control for identify EXTENT AND MAGNITUDE OF SUBSIDENCE ing bench-mark elevations is the National Geodetic Survey's 30-minute quadrangle (for example, see parts The subsidence record for bench mark P7 in San Jose of National Geodetic Survey quadrangles 371221, 371222, is plotted in figure 18, together with the fluctuation of 371213, and 371214 in figure 16). Almost all the network artesian head in nearby index well 7S/1E-7Rl-6Ml taken is in National Geodetic Survey quadrangles 371213 and from figure 12. The black dots on the subsidence curve EXPLANATION ~ Bedrock ----•• Fault- Dashed where approxi· mate, short dash where infer· red, dotted where concealed 15 ------Number of level line 0 City 0 Airport 0 5 10 KILOMETERS 0 5 10 MILES ' ' FIGURE 15.-Network of level lines in the San Jose subsidence area; initial numbering of level lines. F24 MECHANICS OF AQUIFER SYSTEMS indicate times of bench-mark surveys. The fluctuation of Maps showing lines of equal subsidence for the periods artesian head represents the change in stress on the 1934-54, 1948-54, and 1940-54 have been published aquifer system, and the subsidence is the resulting strain. earlier (Poland and Green, 1962, figs. 4, 5, and 6). Three Subsidence at bench mark P7 began about 1916 and was additional subsidence maps are included in this report. 5.4 ft by 1938. From 1938 to 1947, subsidence stopped They show subsidence from 1934 to 1960 (fig. 19), sub during a period of artesian-head recovery but resumed sidence from 1960 to 1967 (fig. 20), and overall subsidence in 194 7 coincident with a rapidly declining head. It at from 1934 to 1967 (fig. 21). To date (1984), 1967 was the tained its fastest average rate in 1960-63 (0.7 ft/yr). By last year of releveling of the bench-mark network. 1967 the bench mark had subsided 12.67 ft. Releveling From 1934 to 1960, maximum subsidence exceeded 5 ft of bench mark P7 in 1969 showed a further small increase at centers in San Jose and Sunnyvale (fig. 19). In that in subsidence to 12.88 ft. period subsidence beneath the bay tidelands ranged from 122"00' EXPLANATION ~ Bedrock ----··Fault-Dashed where approxi mate, short dash where infer red, dotted where concealed 371221 Quadrangle numbers 110 (National Geodetic Survey) ---- Number of level line 0 City 371221 371214 371222 371213 0 5 10 KILOMETERS 0 5 10 MILES FIGURE 16.-Network of level lines in the San Jose subsidence area; numbering of level lines revised to three digits in the late 1950's. -.:._"', ~ ::<('\ "'\ '\ ~-- Nile~..,_, N~ (j ,~· \'<.. ,..,. \,, ' ' ' . '- ., ;: ~~' >\- ~~''. ·. ·.A---~ 'Y A .. -(' '\(() \.~---~- /. "' \\ . X , \ R ' 'v' ... •. "" \\. ' ?, / v,· (>. X '· - ,-- . ") \-.:.: . . edwoodC \ -v -. . r : >~ "t ·~ .' "ty . '>I ~ . · ' ' I · y '-·1 \~\.< ~~~- "*"_1.--·~ ·,.-: f; ' (~":'~~~:_"':w\>·-\ ,:"\(() \ ~ \~I z ~\. 1 •· '?\ ~- ; L · \Sa ~ 1~--- t::l .. \ -...j >;--. ..,:.J I y r:n ~)\ ~;.\\\s.>-,.., ~~ ~t·' ~~~<.,:o. "/ i / c::: ~: " .> ' '··<. / t:O \ / "-,, \i '· \ /, '1- '/~'-----"' / ·, :X ,·> ~\" ~ ..., ·~·; \, t~;I .. ·-; ~; \~'1 : / ·-~ r:n 1!i ' / .··. ~ "·"-) /( \!'l /' // .!,'l>l-~·· • ·<: ... < "'~-J. y / i '\ 'i-..J A .-•'{ /', ~- /' 8 \ ;., :/'··,._/"-..::::, ;\.c/Y~ i/ '"1".:;_/'l·J '· ;> ,, "y'\.. .:;:.....- \ .. 'v . \__ 1<' ·,, t_%j ;r:~~:·~~-~- v, z "'-··~ I · •.~\~)-.0~<.:: ''-·~," / !">~\~ "" .'/-, 7 0 'Y/ ,?·. ./ ~)?';., f""/ }· ,' /'r-/·; '"~'· ~ t-t t-t t_%j ~t-< &? t-t ~ z~ ~ >r:n 0 ~ ...... (.0 00 ~ Morgan Hill 1933 1934 1934 JANUARY~EBRUARY MAY~EPTEMBER NOV EMBE R"DECEMBE R FIGURE 17.-Times and extent of leveling in the San Jose subsidence area. ~ ~en ~ ~ 0") ...:) ' ' a:: t_:Ej \ Morgan Hill\ Morgan Hill 1935 1936 1937 ~ MAY·JUNE MARCH-JUNE MARCH-DECEMBER 0 00 0 to:j ~ 9 -''l ;1 ;~ a:: 00 ~ Morgan Hill' Morgan Hill Mo,gan ~ 1937-38 1938·39 1939 NOVEMBER 1937 • MARCH 1938 OCTOBER 1938 • JANUARY 1939 FEBRUARY-MARCH FIGURE 17.-Continued. § r:n 53r:n ~ ~ zt:r;j 1-3 Morgan gj r:n 1939-40 1940 1948 NOVEMBER 1939 • MARCH 1940 MARCH-MAY JANU ARY·JULY ~ C) :::-:'! ~ '\.- ~t"' t:r;j ~to< ~ t: d ~ ~ v' r:n> 0 lozj c.o~ ~ Morgan Morgan Hill 1954 1956 1960 APRIL-JULY AUGUST OCTOBE~NOVEMBER FIGURE 17.-Continued. l'%j ~ -::J ~ t..:> 00 y \"'<' )' .. />-\... ~~';'; - \ '-!' ;-.~-' '-\( Redwood· jv.~.;'( · " \t .. CitY )j ,, 'c.J '\·\V,_, \ ! 't '<~/ r• . ' ~ ~.,, _ _\~, '-.., ·~... aiY\.X ~ "{\,\. f:1(1 · -,· , ,~-~-A~.( i ... , · o t '-1 ;, t / / ~·- & '\ A, ':>("-..;-'\~ J- \ ~--~..,.')'-,...,_ / Y' ~''l•_,...., /'-f-'> \<';' l \ 1969 MARCH-MAY FIGURE 17 .-Continued. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F29 1 to 3 ft. As of 1960, the area of subsidence lay wholly figure 22 (for location oflineD-D~, see fig. 21). The spring within the area of alluvium and bay deposits. From 1960 1934leveling was used as a reference base because it was to 1967, maximum subsidence exceeded 3.5 ft in San Jose the first complete leveling of the net. Subsidence before and Santa Clara (fig. 20) and at one bench mark in San 1934 is plotted above the reference base, and subsidence Jose was 3.9 ft. The average rate of subsidence was 0.5 after 1934 is shown below it. The marked increase in rate ft/yr, the most rapid rate of subsidence in any 7-year of subsidence after 1948 from Palo Alto to San Jose plain period. ly is due to the increased decline of artesian head (fig. 18) From 1934 to 1967, the National Geodetic Survey re in response to increased ground-water withdrawal (fig. surveyed the network from "stable" bedrock ties a dozen 10). Note that from 1934 to 1967 maximum subsidence times to determine changes in altitude of the bench marks. of 8.6 ft was near bench mark W111 3 mi northwest of During the 33-year period 1934-67, subsidence ranged bench mark P7; in addition, note that from 1934 to 1960 from 2 to 5 ft under the bay and its tidelands to 8 ft in the greatest subsidence along line D-D I was 5. 7 ft at San Jose and Santa Clara (fig. 21). The volume of sub bench mark J111 reset in Sunnyvale. Changes in the rate sidence (pore-space reduction) planimetered from the and magnitude of withdrawal and of artesian-head decline 1934-67 subsidence map is about 500,000 acre-ft. If thE~ doubtless have caused much geographic variation in sub ratio of the pre-1934 subsidence volume to the 1934-67 sidence rate and magnitude with time. subsidence volume is assumed to be equal to the ratio o:f Profiles of land subsidence along transverse line E-E I the pre-1934 subsidence of bench mark P7 to the 1934-67 from Mountain View to Milpitas from 1933 to 1967 subsidence of that bench mark, then the total subsidencE~ (fig. 23) have a pattern similar to that of the profiles in volume from 1912 to 1967 is about 800,000 acre-ft. figure 22. The rate of subsidence prior to 1948 is low com When a line of bench marks in a subsidence area has pared with the great increase in the rate thereafter. been releveled a number of times, one of the most infor Benchmark J111 reset is common to both linesD-DI and mative means of presenting subsidence information is to E-E~; from 1934 to 1967 it subsided 7.8 ft. draw a series of subsidence profiles (figs. 22-24). Land-subsidence profiles along line F -F~ from Los Land-subsidence profiles along line D-D I from Redwood Gatos through San Jose to Alum Rock Park from 1934 City to Coyote from 1912 through 1969 are shown in to 1969 are displayed in figure 24. Bench mark 119 in San 40 ARTESIAN HEAD 0 WELL 7S/1E-7R1 WELL 7S/1E-6M1 w (.) cc 1.1-a: ::I 0 40 Cl) c z cc_. ~ 2 80 c_. 1- w w w r:::a 1.1- a: !: c 4 120 w u.i SUBSIDENCE BENCH MARK P7 > (.) c z r:::a w cc c 1- w enr:::a 6 160 w ::I 1.1- Cl) !: a:w 8 200 1-cc 3:: c 1- 10 240 ~ Cl.. w Note: c Dotted segments show 12 author's interpretation 280 1910 1920 1930 1940 1950 1960 1970 1980 YEAR FIGURE 18.-Artesian-head change and land subsidence, San Jose. F30 MECHANICS OF AQUIFER SYSTEMS Jose is common to both line D-D' and F -F'; it subsided 1916, requiring construction and repeated raising of 7. 7 ft from 1934 to 1969 and was exceeded only by bench levees to restrain landward movement of the saline bay mark E 176, which sank 8. 7 ft in the same period. water as well as construction of flood-controllevees near Although line F -F' crosses the concealed Hayward fault the hayward ends of the valley streams. The subsidence at Alum Rock, about 2.5 mi from the east end of the line, has affected stream channels in two ways: Channel grades the relevelings from 1934-69 do not indicate any differen crossing the subsidence bowl have been down warped, and tial displacement across the fault. saline bay water has moved upstream. These changes tend to cause channel deposition near the bay and reduce chan ECONOMIC AND SOCIAL IMPACTS nel capacity, creating the need for higher levees. Intru sion may occur where wells tapping shallow aquifers are Subsidence has caused several major problems. Lands pumped, inducing downward movement, especially adjacent to San Francisco Bay have sunk 3 to 9 ft since through rusted well casings. EXPLANATION D.. Alluvium and bay deposits ,·,, ~ Santa Clara Formation and .. .. associated deposits ~ Consolidated rocks -----Fault-Dashed. where approx imate, dottetl where concealed -4-- Une of equal subsidence-Dashed where poorly controlled. Com pded from leveling of National Geodetic Survey in May September 1934 and October- November 1960. Interval, in feet, is variable 37° 15' 0 1 2 3 4 5 KILOMETERS 0 2 3 4 5 MILES Compiled by J. H. Green FIGURE 19.-Land subsidence from 1934 to 1960, north Santa Clara County. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F31 About 30 mi2 of evaporation ponds from Palo Alto subsidence that occurred from 1969 through 1982. The around the south end of the bay are used for salt produc measured compaction at 6S/2W-24C7 from 1969 through tion. Behind the landward chain of dikes bordering these 1982 was 0.24 ft. If the ratio of compaction to subsidence ponds, at least 17 mi2 of land lie below the highest tide for the 13-year period remained at 105 percent, the sub level of 1967. Currently these lands are protected by the sidence would be about 0.25 ft. The measured compac dikes and stream-channel levees, but they as well as the tion at 7S/1E-16C11 from 1969 through 1982 (table 3) was bay-front levees are considered inadequate. Extensive 0.44 ft. If the ratio of compaction to subsidence for the flooding of the Alviso area in March to April1983 attests 13-year period remained at 99 percent, the subsidence to the inadequacy of the stream-channel levees. would also be 0.44 ft. These figures suggest that the sub By making use of the table of annual compaction (table sidence for the 13-year period was on the order of half 3) and the compaction-subsidence ratios for the 1,000-ft a foot at San Jose (16C11) and a quarter of a foot in core holes (table 5), we can derive a rough estimate of the Sunnyvale (24C7). EXPLANATION D Alluvium and bay deposits • •e I Santa Clara Formation and ::.:• associated deposits Consolidated rocks -----Fault-Dashed where approx Imate. dotted where concealed -O.t""':""Une of equal subeidence-DMhed where poorly controllecl. Com piled lrom leveling of Natioul Geodetic Survey In October- November 1960 and February March 1967. Interval 0.5 and 0.1 foot 37° 15' 0 1 2 3 4 5 KILOMETERS 0 2 3 4 5 MILES FIGURE 20.-Land subsidence from 1960 to 1967, north Santa Clara County. F32 MECHANICS OF AQUIFER SYSTEMS About $9 million of public funds had been spent to 1974 funds spent on maintaining the salt-pond levees, estab on flood-control levees to correct for subsidence effects, lishing and resurveying the bench-mark net, repairing according to Lloyd Fowler, former Chief Engineer of the railroads, roads, and bridges, replacing or increasing the Santa Clara Valley Water District. In addition, the Leslie size of storm and sanitary sewers, installing drainage Salt Co. has spent an unknown but substantial amount pumping plants, and making private engineering surveys, maintaining levees on the 30 mi2 of salt ponds to counter the direct costs of subsidence must have been at least $30 as much as 9 ft of subsidence in the Alviso area. Several to $40 million to date. Fowler (1981) has estimated that hundred water-well casings have failed in vertical com the direct costs of subsidence, including the estimated cost pression as a result of compaction of the sediments. The of a proposed new levee system, all figured in 1979 dollars, protrusion of well casings as much as 2 to 3 ft above land would exceed $100 million. surface also has been observed (Tolman, 1937, p. 344). The A major earthquake could cause failure of the bay cost of repair or replacement of such damaged wells has margin levees, which would result in the flooding of areas been estimated as at least $4 million (Roll, 1967). Including presently below sea level. The bay-margin levees were R 3W R 2 W 1 22000' R 1 W R 1 E EXPLANA I D Alluvium and bay deposits ~ Santa Clara Formation and ~ • • • assocIated deposits I T I ~ Consolidated rocks 4 s I -----Fault-Daahed where approx Imate, dotted where concealed --2--Line of equal subaldence-Daah ed where approximately located. Interval, In feet, Is D D' variable 1---.JAIIgnment of subsidence profiles- 24C7 See figures 22-24 e Core hole and number T OC5 Observation well and number 5 JG2 s )( Bench mark and number T ii s I I -;-----t--- 1 I I T I 7 I s I I I ----r------I I 1 I 37° I I 15' I I T 8 I : s I 0 1 2 3. 4 5 KILOMETERS I 0 2 3 4 5 MILES FIGURE 2C-Land subsidence from 1934 to 1967, north Santa Clara County. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F33 constructed of locally derived weak materials and were interval in which compaction is occurring and to measure designed only to retain salt-pond water under static con the magnitude and time distribution of the compaction ditions (Rogers and Williams, 197 4). The potential for such where possible. Such information, together with periodic an earthquake poses a continuing threat to flooding of the measurement of land subsidence as determined by spirit estimated 17 mi2 of land standing below the high-tide level surveys to surface bench marks, is essential for level of 1967. Such a threat has probably reduced the value determining the cause of subsidence and for monitoring of this land substantially compared with its value if it all the magnitude and the change in rate of subsidence. When still stood above sea level as it did in 1912. This decrease coupled with measurement of water-level or head change in land values should be included in the gross costs of in the stressed aquifer systems, these data supply the subsidence. numbers required for stress-compaction or stress-strain analysis. MONITORING OF COMPACTION AND The first extensometer (compaction recorder) in the CHANGE IN HEAD Santa Clara Valley was installed in 1958 at well 6S/1W- 23E 1 in Agnew. In 1960, extenso meters were installed MEASUREMENT OF COMPACTION in the cased core holes 1,000 ft deep at the centers of sub sidence in San Jose (7S/1E-16C6) and in Sunnyvale Two principal objectives of the Federal program on (6S/2W-24C7), in two satellite holes in Sunnyvale (24C3 mechanics of aquifer systems are to determine the depth and 24C4), and in two unused water wells. They have D San Jose D' "'<» (l. .... 4 Menlo Redwood Park Sunnyvale \ City Palo Alto \ 2 Ill (.!) ,..... ,.....~ w 1940 ~ 1- w 0 w u. ~ w u z ~ 2 u; Ill => en Dashed lilies indicate no control between 4 bench marks 6 Note: All leveling to bench marks by the National 8 Geodetic Survey FIGURE 22.-Profiles of land subsidence, D-D ', Redwood City to Coyote, 1912-69. F34 MECHANICS OF AQUIFER SYSTEMS N .... ~ E Q) ~ a: a: Q) ~ Milpitas Q) .... a: a: ~ co co ~ t9 ~ Q) 'o:t Q) a: 'o:t a: ~- ~- :::c l!) 00 .....(0 'o:t (0 00 t9 ~ ::; c; ..... 'o:t (0 L.O 0 w :::c 0 I X u l!) 'o:t 0 ..... 00 0 0 0 w· ..... co N coco co co ~191_3 ..... > co N 1- a: ::::> > ~ X >- --- 0 E' I 1934 BASE ------/ --- / -- / - / / / / / 2 / 1- / w w / u. / 3 ~ / / w· (.) / z w / 0 / CJ) m :::> 4 CJ) / / / / / / 5 / / / 6 / / Note: Dashed lines / indicate no leveling / / / / / / Note: All leveling by the 0 4 KILOMETERS National Geodetic Survey 0 4 MILES SL------~ FIGURE 23.-Profiles of land subsidence, E-E', .Mountain View to Milpitas, 1933-67. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F35 QJ ~ ~ ~ <0 QJ ~ ~ QJ ~ QJ QJ QJ "C a: a: a: a: a: a: "3 c: Los Gatos ,....co co co co San Jose co ,....~ ~ <0 i5 Alum Rock ,.... ,.... ,.... QJ u. co a:l Park en ~ a: 1:i (.!) 5 i= ;n cr u. > co lii (I) ,.... 8 ,.... ,.... ,.... co cO cO co" tO co co ~- COco~ ...... ,.... ,.... ,.... :::l ,.... ,....cO ,.... ,.... ,....tO ,.... ,.... ,....tO ,.... cO,.... N ,.... COr-.,_ ill ~m > co o~co u ;;; z s: M 5 i= (I) cr a: z :::E:= w :2 ~ a: > OJ:r F I I I I I( 0 2 3 f UJ UJ u.. 4 w· (.) z UJ Cl ~ 5 U)=> 6 8 0 4 KILOMETERS 0 4 MILES Note: All leveling by the National Geode.tic Survey 9~------~ FIGURE 24.-Profiles of land subsidence, F-F', Los Gatos to Alum Rock Park, 1934-69. F36 MECHANICS OF AQUIFER SYSTEMS recorded compaction or expansion of the confined aquifer Extensometers of two types are in use (fig. 25). In the system for 22 years. cable type (fig. 25A), an anchor weight is attached to the Core hole 7S/1E-16C6 was destroyed in 1969 during the extensometer cable, lowered into the well, and set below construction of a freeway. It was replaced in 1969 by the bottom of the well casing. This anchor acts as a depth another 1,000-ft extensometer, well 7S/1E-16C11. bench mark. The extensometer cable is maintained Sheaves Recorder mounted in Compaction tape - Steel tab I e ____. Clamp /Bench mark _...J::::::L-.....C::::::;).,._ Con c r et e .--~___.___.____, I I ~"'":":'T":"r."':"":"'''~~sl ab~+.-:-:-~~~~ 1 1 ··=;<) Anchor weight, 200 to 300 puunds, in open hole A Extensometer pipe cemented in open hole 8 FIGURE 25.-Recording extensometer installations. A, Anchored cable assembly. B, Pipe assembly. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F37 (ideally) under constant tension by a counterweight at the land surface. As the aquifer system compacts, an equivalent length of cable appears to emerge from the well; this movement is measured by a mechanical recorder attached to the compaction cable at land surface. In the free-standing pipe extensometer (fig. 25B), the pipe is lowered into the well and set below the bottom of the well casing. A mechanical recorder attached to the exten someter pipe measures the movement of the land surface relative to the nonmoving pipe and thus measures the compaction or expansion of the aquifer system. The accuracy of the compaction record is dependent ~ 00 largely on the ability of the extensometer cable or pipe ...... I 00 (j) to maintain a constant length between the bottom-hole ...... reference point and the above-ground reference point. 6 Friction tends to develop between the extensometer cable """C'\1 ~ or pipe and the deforming well casing, which shortens and C\J ,... 00 moves downward in the grip of the compacting sediments. .,...~ ~ »- Q.l Because of its much greater cross section, the pipe is more ~ ~;g. ·;.; competent than the cable to resist frictionally induced Q.l ~ length changes. Therefore it generally produces a better ~ ~ record in typical wells of moderate depth, but other fac § tors may be involved in selecting the design for a specific rn Q.l ...c: site. ~ ~ When initally installed, all the extensometers were of 1:1! 00 the anchored-cable type. To reduce the friction problem :r:: Q.l 1- ~""' 2 Q.l and increase the accuracy of measurement, the three 0 :2 s0 paired Sunnyvale extensometers (6S/2W-24C3, C4, and 00 ~ Q.l C7) and 7S/1E-16Cll in San Jose were modified in the ~ > Compaction and expansion in feet [minus (-) indicates expansion] C4 C3 C7 Date (250ft) (550ft) (1,000 ft) 4/1/81 0 0 0 10/3/81 0.015 0.061 0.076 3/26/82 -.015 -.061 -.076 Net change from 4/1/81 to 3/26/82 0 0 0 These compaction plots are of interest for two reasons. N ' extensometers to very small changes in applied stress. wells 7S/1E-16C5 and C11 in San Jose; for location, see Secondly, we can see from the plots in figure 26 that the fig. 1). The five wells will continue to be monitored by the measured compaction from April1, 1981, to March 26, Santa Clara Valley Water District for compaction of the 1982, at the Sunnyvale site all occurred within the elastic aquifer system and water-level change. range of stress; that is, no permanent deformation The net annual compaction or expansion (negative com occurred in the aquifer systems. paction) at each site for the entire period of record Continuous or periodic water-level measurements are through 1982 is summarized in table 3, as are records of· also necessary in a subsidence-monitoring program in compaction or expansion in two additional depth inter order to define the cause-and-effect relationship of the vals defined by multiple-depth installations. For example, subsidence process. Moreover, in order to produce reliable at 6S/2W-24C, wells C3, C4, and C7 are respectively 550, stress-strain or stress-compaction plots that can be utilized 250, and 1,000 ft deep. The extensometer in well C7 to derive storage and compressibility characteristics of records total compaction from land surface to the 1,000-ft the aquifer system, water-level measurements must repre depth, and the extensometer in C3 measures the com sent the average change in stress in the compacting inter paction from land surface to the 550-ft depth. By sub val being measured. tracting the compaction in C3 from that in C7, the At the Sunnyvale site, for example, wells 24C4 (250-ft compaction of the 550- to 1,000-ft depth interval is depth) and 24C3 (550-ft depth) are open-bottom blank calculated. casings drilled primarily as extensometers. Periodic The marked decrease in the annual compaction in water-level measurements have been made and are response to the substantial head recovery since 1966 is recorded in the computer plots of field records (figs. 32, demonstrated graphically by the compaction records from 34, 36) and supplementary records of 1980-82 (figs. 43, the two deep extensometers in Sunnyvale and San Jose 44). The hydrograph for the 250-ft well (24C4) shows an (fig. 27). The annual compaction in well 7S/1E-16C6-11 annual fluctuation of about 50 ft and may be a fair in San Jose decreased from about 1 ft in 1961 and 1962 representation of stress change for the confined depth to 0.24 ft in 1967 and to 0.01 ft in 1973. Net expansion interval from 200 to 250ft (fig. 32A). The hydrograph for Oand-surface rebound) of 0.02 ft occurred in 197 4. In 1976 the 550-ft well (24C3), on the other hand, was very slug the drought caused a sharp decline of artesian head (fig. gish from 1964-74 (fig. 34A) and is not considered reliable 40), which in turn increased compaction to 0.11 ft at the as a measure of stress change in the depth interval 250 San Jose site. In Sunnyvale, compaction of the aquifer to 550ft. system in well24C7 decreased from about 0.45 ft in 1961 The Geological Survey terminated field monitoring of to 0.04 ft in 1973; net expansion of 0.016 ft and 0.04 ft compaction and water-level change on this study in occurred in 1974 and 1975, respectively. January 1983. At that time, extensometers were in use The subsidence of surface bench marks from 1960 to in five wells at two compaction and water-level monitor 1967 (or 1969), plotted on the long-term compaction plots ing sites (wells 6S/2W -24C3, C4 and C7 in Sunnyvale and for wells 6S/2W-24C7 (fig. 38) and 7S/1E-16C6-11 (fig. TABLE 3.-Annual compaction at compaction lin order to arrive at consistent sums, the amount of annual compaction is shown to 0.001 ft; however, many Anchor Depth Start ~Tell depth when . . 1nterva1 of 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 number Installed (f ) (feet) eet record 6S/1W-23E1--- 425 0-425 9/10/58 0.093 0.214 0.141 0.270 0.253 0.119 0.153 0.142 0.145 0.027 6S/2W-24C4--- 250 0-250 1/29/60 .037 .058 .040 .030 .053 .018 .027 .017 -24C3--- 550 250-550 1/29/60 .200 .151 .152 .123 .144 .104 .125 .070 -24C3--- 550 0-550 1/29/60 .237 .209 .192 .153 .197 .122 .152 .087 -24C7--- 1,000 550-1,000 6/30/60 .244 .172 .177 .182 .174 .144 .126 -24C7--- 1,000 0-1,000 6/30/60 . 211 .453 .364 .330 .379 .296 .296 .213 7S/1E-9D2---- 605 0-605 5/5/60 .094 .333 .408 1.047 -9D2---- 605 0-605 1/14/65 .210 .200 .087 7S/1E-16C5--- 908 0-908 8/16/60 .080 .712 .918 .429 .394 .397 .418 .226 -16C6--- 1,000 0-1,000 6/30/60 .276 .966 1.021 .399 .506 .410 .477 .236 -16Cll-- 1,000 0-1,000 6/2/69 1Record ends April 22, 1963 (equipment failure) 2Site abandoned May 1978 3c6 record ends June 2, 1969, and Cl1 star.ts on June 2, 1969 ' LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F39 41), demonstrates that the measured compaction to the seepage stress across the bed is altered in direction or 1,000-ft depth is approximately equal to the measured sub magnitude. sidence as determined by bench-mark surveys. The close The change in applied stress within a confined aquifer agreement of the compaction and subsidence values at the system, due to changes in both the water table and the Sunnyvale and San Jose sites is shown in table 5. At the artesian head, may be summarized concisely (Poland and San Jose site (16C6), measured subsidence from October others, 1972, p. 6) as 1960 to May 1969 was 4.21 ft, and measured compaction for the same time period was 4.17 ft; thus the ratio of com paction to subsidence was 99 percent, which means that all of the subsidence is due to the measured compaction where Pa is the applied stress expressed in feet of water, to the 1,000-ft depth. he is the head (assumed uniform) in the confined aquifer system, hu is the head in the overlying unconfined ANALYSIS OF STRESSES CAUSING SUBSIDENCE aquifer, and Ys is the average specific yield (expressed as a decimal fraction) in the interval of water-table Increase in effective stress (grain-to-grain load) is the fluctuation. cause of compaction of sediments. The withdrawal of water from wells reduces the head in the aquifers that EXAMPLE OF STRESS-STRAIN GRAPH are tapped and increases the effective stress borne by the aquifer matrix. The reader interested in a quantitative Field measurements of compaction and correlative analysis of stresses causing subsidence is referred to change in water level serve as continuous monitors of sub Lofgren (1968) and to Poland and others (1975, p. 39-44). sidence and indicators of the response of the system to In summary, water-level fluctuations change effective change in applied stress. They also can be utilized to con stresses in the following two ways: (1) A rise of the water struct stress-compaction or stress-strain curves from table provides buoyant support for the grains in the zone which, under favorable conditions, one can derive storage of the change, and a decline removes the buoyant support; and compressibility characteristics of the measured part these changes in gravitational stress are transmitted of the aquifer system, as demonstrated by Riley (1969). downward to all underlying deposits. (2) A change in posi Nineteen years (1960-79) of measured water-level tion of either the water table or the artesian head, or both, change (well 7S/1E-16C5) and compaction (well 7S/1E- may induce net vertical hydraulic gradients across con 16C6-11) in San Jose are shown in figure 41, together with fining or semiconfining beds and thereby produce a net a computer plot of stress change versus strain. Well16C5 seepage stress. The vertical normal component of this is 908ft deep, and compaction in well16C6-11 is measured stress is algebraically additive to the gravitational stress to 1,000 ft, spanning the full 800-ft thickness of the aquifer that is transmitted downward to all underlying deposits. system from the 200- to the 1,000-ft depth. As the water A change in effective stress results if the preexisting levels at this San Jose site rose rapidly after 1967 (fig. measuring sites in north Santa Clara County measured yearly values are not accurate ~o less than a few hundredths of a foot. Minus (-)indicates expansion] Total measured 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 compaction (fE>et) 0.028 -0.022 -0.031 0 0.024 -0.021 -0.024 -0.012 0.043 0.018 -0.027 0.003 1. 536 .009 0 .009 .044 -.027 -.006 -.002 -.005 .004 .003 -.003 -.005 -0.001 0.003 -0.005 .298 .064 .021 -.008 -.032 .028 -.023 -.020 -.016 .022 -.002 .001 .004 -.002 .002 -.005 1.103 .073 .021 .001 .012 .001 -.029 -.022 -.021 .026 .001 -.002 -.001 -.003 .005 -.010 1.401 .092 .081 .020 .018 .024 .069 .006 -.016 .025 .008 .023 .001 -.004 .008 -.001 1. 573 .165 .102 .021 .030 .025 .040 -.016 -.037 .051 .009 .021 0 -.007 .013 -.011 2.948 .882 .041 .038 .010 .007 .057 -.003 -.019 -.014 .043 2 .036 .693 .156 .054 .044 .106 .123 .030 .021 .026 .095 .049 .027 .029 -.027 .040 -.028 4.319 .129 3 .023 4.443 .071 .075 .064 .050 .007 -.019 .011 .114 .043 .006 .042 -.018 .028 -.035 .439 F40 MECHANICS OF AQUIFER SYSTEMS 41A), the stress-strain curves obtained from the paired line is the component of the storage coefficient attribut measurements of compaction and artesian head began to able to elastic or recoverable deformation of the aquifer show seasonal expansion during the winter months when system skeleton, Ske, and equals 1.5 x 10-3. The the water level was highest and the effective stress on component of average specific storage due to elastic the confined system was lowest (fig. 41D). These stress deformation, Sske, equals Ske/800 ft = 1.87 x 10-6/ft. If strain loops can be used to obtain the compressibility of stresses are expressed in feet of water, and if Yw (the unit the confined system in the recoverable or elastic range weight of water) equals unity, the average elastic com of stresses (less than preconsolidation stress). The stress pressibility of the aquifer system skeleton, ake, is-equal strain curve for 1967-74 has been enlarged in figure 28. numerically to Sske. Depth to water is plotted increasing upward. Change in In these computations we have assumed that in the depth to water represents an average change in stress range of stresses less than preconsolidation stress, the in all aquifers of the confined aquifer system tapped by compressibility of the aquitards and of the aquifers is the well16C5. The lower parts of the descending segments same. Therefore, the full thickness of the confined aquifer of the annual loops for the winters of 1967-68, 1969-70, system, 800 ft, was used to derive the average specific and 1970-71 are approximately parallel, as shown by the storage component, Sske, in the elastic range of stress. dotted lines, indicating that the response is essentially elastic in both aquifers and aquitards when the depth to COMPUTER SIMULATION OF water is less than about 180 ft. The heavy dashed line AQUIFER-SYSTEM COMPACTION drawn parallel to the dotted lines represents the average slope of the segments in the range of stresses less than As stated by Helm (1978), land subsidence due to preconsolidation stress. The reciprocal of the slope of this ground-water withdrawal from a confined aquifer system SUNNYVALE WELL 6S/2W-24C7 ...... w w u.. 2 z 0 1.2~~--~---r--li---~--r---r-~--~--~---,---,---,---,--,---,---.--.,--.---~--~~ ...... u <( c.. ~ o.a~~~~~~~~~~~~~~~~~~~~ WELLS 7~~1~!~~~ and C11 o.: l!lillilllllillillll ililillilllillilill! 11111111111111111111 :::~:::::::::::=:::: ::;:;::;: :::;:;:;:: ·:·:·:·:· .·.·.·.·.· 1962 1964 1966 1968 1970 1972 1982 YEAR FIGURE 27.-Measured annual compaction in wells 1,000 ft deep in Sunnyvale and San Jose. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F41 is an expression at land surface of net compaction (ver adjacent aquifer. This model has been applied by Helm tical consolidation) at depth of compressible layers within (1977) to observed water-level fluctuations and to the the system. Water-level fluctuations within coarse-grained resulting observed transient compaction-and-expansion aquifers produce stress changes on the upper and lower behavior of the confined aquifer system at seven sites in boundaries of slow-draining aquitards (compressible fine the Santa Clara Valley. grained interbeds). Indirectly, irregular and cyclic pump According to Helm, values for total cumulative thick ing drawdowns generate complex loading and unloading ness b' sum of fine-grained interbeds within the confined stress patterns in the aquifer system, causing compaction aquifer system, the weighted-average thickness b' equiv of and expansion of the aquitards. Within a confined system, these interbeds, recoverable vertical compressibility S~ke, any lag in compactive response to such a load increase and the initial distribution of preconsolidation pressure is usually ascribed to slow vertical drainage from the p' max (z, 0) must be approximated from field data in aquitards. After water levels in coarse-grained material dependently of any subsequent simulation process. Only rise during a period of water-level recovery, it has been values for vertical components of hydraulic conductivity observed during the next cycle of declining water levels K' and nonrecoverable compressibility S ~kv are adjusted that nonrecoverable compaction is reinitiated before by trial-and-error to fit calculated-to-observed compaction water levels reach their prior lowest level. history. The estimated values forK' at the sites studied Helm (1977) developed a one-dimensional mathematical range from 0.56 x 10-6 to 12.5 x 10-6 ft/d, those for model of vertical deformation based on a modification of s~kv range from 1.4 X 10-4 to 13.1 X 10-4fft, and those Karl Terzaghi' s consolidation theory; the model computes for s~ke range from 2.2 X 10-6 to 15.8 X 10-6/ft transient aquitard compaction and expansion due to arbi (table 4). trarily specified changes in effective stress within an For established values of b' sum, b' equiv, and p' max (z, 0), 340 300 • I • • • • • • • • ..... • + • • 260 • 4) • ...... ~ • • • I- w • • • w LL. • • • 1972 • • ~ "' • 220 7 • / r£ • w • I- • <{ ~ 1967 /1973 0 I- 180 :I: I-a.. w 0 • 140 •• •• • 100 •• •• • COMPACTION, IN FEET FIGURE 28.-Stress change and compaction, San Jose site. Depth to water in well 7S/1E-16C5, and compaction in well 7S/1E-16C6-11. F42 MECHANICS OF AQUIFER SYSTEMS TABLE 4.-Values of parameters used for simulating observed compaction at selected sites in north Santa Clara County [Modified from Helm (1977, table 1)] Period of record Estimated thickness Estimated values of parameters (years) (feet) Site of water-level S' S' Weighted K' data Water-level Compaction Total skv ske average _6 1 4 1 6 data data b' feetx 10 - -x10 - - -x10 - sum b' day foot foot equiv 6S/1W-23E1 1958-72 1958-72 189 15.1 1.18 1.4 7.5 6S/2W-24C3 1960-72 1960-72 225 18 .56 13.1 3.96 6S/2W-24C7 1960-72 1960-72 422 53.1 12.5 3.35 2.60 6S/2W-25C1 1960-72 1960-72 255 18 .75 3.26 15.8 6S/2W-25C1 1921-74 1932-74 255 18 .72 3.29 7S/1E-7R1 1915-74 1916-74 477 18 1.87 2.25 7S/1E-9D2 1958-72 1960-63, 258 18 .82 6.1 7.92 1965-72 7S/1E-16C5 1958-72 1960-72 477 18 3.28 2.3 2.2 Helm (1977, figs. 9, 10) demonstrated that carefully February 1967. The 5-percent excess is attributed to evaluated values of K' and S~kv can be used to predict mechanical problems in recording the compaction. aquifer-system behavior with reasonable accuracy over At extensometer site 7S/1E-16C5 (908 ft deep) the periods of several decades. A good example of the simula aquifer system is about 800-ft thick, from 200 to 1,000 tion result for six decades of record using values listed ft below land surface. If the compaction potential was in table 4 is shown in figure 29 (from Helm, 1977, fig. 10). uniform with depth, the part measured by extensometer 16C5 would be 708/800, or 88.percent of the subsidence. COMPACTION-SUBSIDENCE RATIOS Actually, for the 8.5 years from October 1960 to May 1969 the compaction measured at 16C5 equaled 87 percent of The amounts of measured compaction and measured the subsidence. subsidence at extensometer sites where leveling data are At extensometer site 7S/1E-16C6, the extensometer available are shown in table 5. The ratio of measured com cable anchor was landed at the 1, 000-ft depth, believed paction to measured subsidence, expressed as a percent to be the bottom of the aquifer system. For the full age, was computed for the period of record available. 8.5-year period to May 1969, the measured compaction Percentages are sometimes erratic for short periods of was equal to 99 percent of the subsidence, indicating that control at some sites, as shown by table 5; some periods the anchor truly was landed at the base of the compact of measured compaction and leveling are combined to ing system. make the compaction/subsidence ratios more consistent, In the 13.5 years from May 1969 through 1982 the cu and some are not shown as a result of incomplete data mulative compaction at the two 1,000-ft extensometers, or mechanical problems at the extensometer site. 6S/2W-24C7 in Sunnyvale and 7S/1E-16C6-11 in San At extensometer site 6S/2W-24C, the compaction/sub Jose, was 0.24 ft and 0.44 ft, respectively (table 3). Dur sidence ratio for the full period of the leveling record ing the six years 1977 to 1982, inclusive, the cumulative (November 1960 to February 1967) increased from 11 per compaction was very small, about 0.03 ft at 6S/2W-24C7 cent for C4 (the 250-ft extensometer) to 50 percent for and 0.07 ft at 7S/1E-16C6-11. C3 (the 550-ft extensometer) and to 105 percent for C7 A.K. Williamson (U.S. Geological Survey, oral commun., (the 1,000-ft extensometer). For the shorter periods 1981) noted an interesting relationship in the San Joaquin between levelings, the ratio for 24C3 was surprisingly Valley between extensometer depth and the compaction consistent, whereas the ratio for 24C4 was erratic. For subsidence ratio (Ireland and others, 1984, p. 153 and fig. extenso meter 24C7, the measured compaction during 67). We have plotted a similar figure for the few exten each of the shorter periods exceeded the measured sub someters in north Santa Clara County (fig. 30). Compac sidence and averaged 105 percent for the six years to tion/subsidence ratios based on the full period of available LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F43 records were used in preparing figure 30. Although there COMPUTER PLOTS OF FIELD RECORDS were only seven ratios to plot, they surveyed a wide range of extensometer depths: one was at shallow depth, three The records of compaction and of depth to water in the were at mid-depth (400 to 600ft deep), and three were extensometer wells or in nearby observation wells have at full depth of the aquifer system (about 1,000 ft). All been computerized on a daily basis, and computer plots were centrally located within the confined system and at of these records through 1979 are included here as figures either of the two centers of maximum subsidence. 31-42. Graphs of subsidence of a surface bench mark Together, they defined a relatively "tight" curve, which located at the measuring site, determined by periodic indicates that all the compaction due to ground-water instrumental surveys to a stable bench mark, are also in withdrawal occurs at depths between about 200 and 1,000 cluded for the first part of the period of compaction ft. Between these depths the compaction/subsidence ratio measurements. (No bench-mark surveys have been made is shown to approximate a linear function of the logarithm since 1969.) Site locations are shown in figure 1. of the depth. The least-squares regression equation of the The Geological Survey continued collecting field records function has an R-squared value of 0.95, which indicates of compaction and depth to water at these sites in the that 95 percent of the variation of the ratio is explained three years 1980-82 (figs. 43-48). Early in 1983 the opera by the equation, whereas 5 percent of its variation is ran tion and maintenance of the principal sites was taken over dom (or attributable to other factors not considered). by the Santa Clara Valley Water District. On the basis of the relation shown in figure 30, a 300-ft The computer program and extensometer and water extensometer in a subsidence area in the Santa Clara level data collected on this long-term (mechanics of Valley would record approximately 18 percent of the total aquifers) study on land subsidence are stored on computer subsidence measured at the well site, and a 700-ft exten tape Number 220691, type 6250BPI, at the U.S. Geo someter would record approximately 72 percent of the logical Survey Information Services Division, Reston, total subsidence at the site. Virginia. ~ w 500 zwa:: -LL.w w z ~ 600 (!)-<( zen~ 1oo 2 3 ~w w 4 LL. z 5 z 6 0 i= u 7 <( ~ 8 ~ 0 u 9 10 11 12 13 1920 1930 1940 1950 1960 1970 FIGURE 29.-Simulation of compaction based on water-level data for well 7S/1E-7R1 (1915-74) and on subsidence measured at bench mark P7 (from Helm, 1977, fig. 10). F44 MECHANICS OF AQUIFER SYSTEMS TABLE 5.-Compaction versus subsidence for periods of leveling in north Santa Clara County [Ratio of compaction/subsidence: value shown on total line is ratio for period of record] Ratio of Bench Extensometer Depth Dates of Subsidence Compaction compaction mark number (feet) leve;Ling (feet) (feet) to subsidence number (percent) 6S/1W-23El 425 Al76 11/60-2/67 2.392 1.033 43 6S/2W-24C4 250 Jlll 11/60-2/63 .807 .091 11 2/63-4/65 .683 .085 12 4/65-2/67 .544 .045 8 Total ...... 2.034 .221 11 6S/2W-24C3 550 Jlll 11/60-2/63 .807 .414 51 2/63-4/65 .683 .341 so 4/65-2/67 .544 .267 49 Total ...... 2.034 1.022 so 6S/2W-24C7 1,000 Jlll 11/60-2/63 .807 .845 105 2/63-4/65 .683 .738 108 4/65-2/67 .544 .547 101 Total ...... 2.034 2.130 105 7S/1E-9D2 605 JG4 10/60-2/63 1. 552 .762 48 2/63-2/67 1. 551 (1) 2/67-5/69 .230 .118 51 7S/1E-16CS 908 JG2 10/60-2/63 l. 919 1. 677 87 2/63-3/67 1. 923 1.604 83 3/67-5/69 .366 .365 100 Total ..•...... •...... 4.208 3.646 87 7S/1E-16C6 1 '000 JG2 10/60-3/67 3.842 3.812 99 3/67-5/69 .366 .363 99 Total ...... •...... 4.208 4.175 99 1No extensometer data from 4/22/63 to 6/18/65. The primary purpose of including these records is to stress-strain relationships are plotted for figures 31-41. show graphically the measured compaction and sub In these figures, compaction equals the change in thick sidence at specific sites, and (so far as possible) the change ness of the compacting interval, and strain refers to the in effective stress in the pertinent aquifers at these sites, compaction divided by the thickness of the compacting as indicated by the hydrographs. Because of the confin interval. ing beds and multiple-aquifer/aquitard system, it is dif As indicated in table 6, stress-strain relationships ficult to obtain water-level measurements that specifically are shown for figure 31 (extensometer 68/1 W-23E1) represent the average stress change for the interval in and for figures 40 and 41 at the San Jose core-hole which compaction is being measured. site (extensometers 7S/1E-16C5 and 16C6-11, Change in applied stress and stress-compaction or respectively). LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F45 120.------r------~------~------~------~------~ • 100 RATIO (%) = 149{LOG 10 OEPTH)-352 • ..._ z I.U (.) a: 80 I.U 0... ~ w (.) z I.U c Ci) co :::::> (/) 0..._ 60 z 0 i= (.) cs:: 0... ~ • 0 (..') ~ 0 0 40 i= cs:: a: EXPLANATION 20 • EXTENSOMETER WELL OL------L-L------L------L------L------~------~ 0 200 400 600 800 1000 1200 DEPTH OF EXTENSOMETER, IN FEET FIGURE 30.-Compaction/subsidence ratios versus depth at extensometer wells in north Santa Clara County. SELECTED REFERENCES California Department of Water Resources, 1967, Evaluation of ground Atwater, B.F., Redel, C.W., and Helley, E.J., 1977, Late Quaternary water resources, South San Francisco Bay, appendix A, Geology: depositional history, Holocene sea-level changes, and vertical crustal Bulletin 118-1, 153 p. movement, southern San Francisco Bay, California: U.S. Geological __ 1975, Evaluation of ground water resources, South San Fran Survey Professional Paper 1014, p. 1-15. cisco Bay, v. 3, Northern Santa Clara County area: Bulletin 118-1, California State Water Resources Board, 1955, Santa Clara Valley 133 p. (contains 11 geologic sections, fault map, and 10 sheets of investigation: Bulletin 7, 154 p. subsurface deposits by depth intervals). F46 MECHANICS OF AQUIFER SYSTEMS Clark, W.O., 1924, Ground water in Santa Clara Valley, California: U.S. Scientific and Cultural Organization, 7 Place de Fontnoy, 75700 Geological Survey Water-Supply Paper 519, 209 p. Paris, France, p. 279-290. Dibblee, T.W., 1966, Geologic map and sections of the Palo Alto Poland, J.F., and Davis, G.H., 1969, Land subsidence due to the 15-minute quadrangle, California: California Division of Mines and withdrawal of fluids: Geological Society of America, Reviews in Geology, map sheet 8. Engineering Geology, v. 2, p. 187-269. Fowler, L.C., 1981, Economic consequences of land surface sub Poland, J.F., and Green, J.H., 1962, Subsidence in the Santa Clara sidence: Journal of Irrigation and Drainage Division, Proceedings Valley, California-A progress report: U.S. Geological Survey of the American Society of Civil Engineers, v. 107, no. IR2, Water-Supply Paper 1619-C, p. C1-C16. p. 151-158. Poland, J.F., Lofgren, B.E., Ireland, R.L., and Pugh, R.G., 1975, Land Green, J.H., 1962, Compaction of the aquifer system and land subsidence subsidence in the San Joaquin Valley, California, as of 1972: U.S. in the Santa Clara Valley, California: U.S. Geological Survey Pro Geological Survey Professional Paper 437-H, 78 p. fessional Paper 450-D, p. 175-178. Poland, J.F., Lofgren, B.E., and Riley, F.S., 1972, Glossary of selected __ 1964, The effect of artesian-pressure decline on confined aquifer terms useful in studies of the mechanics of aquifer systems and land systems and its relation to land subsidence: U.S. Geological Survey subsidence due to fluid withdrawal: U.S. Geological Survey Water Water-Supply Paper 1779-T, 11 p. Supply Paper 2025, 9 p. Helm, D.C., 1977, Estimating parameters of compacting fine-grained Riley, F.S., 1969, Analysis of borehole extensometer data from central interbeds within a confined aquifer system by a one-dimensional California, in Tison, L. J., ed., Land subsidence, v. 2: International simulation of field observations: International Association of Association of Scientific Hydrology (is now International Associa Hydrological Sciences, International Symposium on Land Sub tion of Hydrological Sciences) Publication 89, p. 423-431. sidence, 2d, Anaheim, California, December 1976, Publication 121, Rogers, T.H., 1966, Geologic map of California, Olaf P. Jenkins, ed.: p. 145-156. California Division of Mines and Geology, San Jose sheet, scale __ 1978, Field verification of a one-dimensional mathematical model 1:250,000. for transient compaction and expansion of a confined aquifer system, Rogers, T.H., and Williams, J.W., 1974, Potential seismic hazards in in Verification of mathematical and physical models in hydraulic Santa Clara County, California: California Division of Mines and engineering: American Society of Civil Engineers, Proceedings of Geology, Special report 107, 39 p., 6 pl. Specialty Conference, College Park, Maryland, p. 189-195. Roll, J.R., 1967, Effect of subsidence on well fields: American Water Hunt, G.W., 1940, Description and results of operation of the Santa Clara Works Association Journal, v. 59, no. 1, p. 80-88. Valley Water Conservation District's project: American Geophysical Santa Clara Valley Water Conservation District, 1964, Hydrologic data Union Transactions, part 1, p. 13-32. for north Santa Clara Valley (California), seasons of 1935-36 to Ireland, R.L., Poland, J.F., and Riley, F.S., 1984, Land subsidence in 1960-61, v. 3, surface-water summary and ground-water fluctua the San Joaquin Valley, California, as of 1980: U.S. Geological tion: Santa Clara Valley Water Conservation District, mimeo report, Survey Professional Paper 437-1, 93 p. p. 828-1230. Jennings, C.W., and Burnett, J.L., 1961, Geologic map of California, __ 1967, Hydrologic data for north Santa Clara Valley (California), Olaf P. Jenkins, ed.: California Division of Mines and Geology, San seasons of 1961-62 to 1965-66, v. 4, precipitation, evaporation, Francisco sheet, scale 1:250,000. reservoir fluctuation, surface-water flow, surface-water summary, Johnson, A.l., Moston, R.P., and Morris, D.A., 1968, Physical and ground-water fluctuation, and ground-water quality: Santa Clara hydrologic properties of water-bearing deposits in subsiding areas Valley Water Conservation District, mimeo report, p. 1231-1705. in central California: U.S. Geological Survey Professional Paper __ 1977, Historical ground-water level data, 1924-1977: Santa Clara 497-A, 71 p. Valley Water District, mimeo report, 423 p. Lofgren, B.E., 1968, Analysis of stresses causing land subsidence: U.S. Tibbetts, F.H., and Kieffer, S.E., 1921, Report to Santa Clara Valley Geological Survey Professional Paper 600-B, p. B219-B225. Water Conservation Committee on Santa Clara Valley Water Con Meade, R.H., 1967, Petrology of sediments underlying areas of land sub servation project: Santa Clara Valley Water Conservation District, sidence in central California: U.S. Geological Survey Professional unpublished typwritten report, 243 p. Paper 497 -C, 83 p. Tolman, C.F., 1937, Ground water: New York, McGraw-Hill, first edition, __ 1968, Compaction of sediments underlying areas of land sub 593 p. sidence in central California: U.S. Geological Survey Professional Tolman, C.F., and Poland, J.F., 1940, Ground-water, salt-water infiltra Paper 497-D, 39 p. tion, and ground-surface recession in Santa Clara Valley, Santa Clara Poland, J.F., 1969, Land subsidence and aquifer-system compaction, County, California: American Geophysical Union Transactions, pt. 1, Santa Clara Valley, California, USA, in Tison, L.J., ed., Land sub p. 23-35. sidence, v. 1: International Association of Scientific Hydrology (is U.S. Bureau of Reclamation, 1961, Central Valley Project, San Felipe now International Association of Hydrological Sciences) Publication Division geology and ground-water resources appendix: U.S. Bureau 88, p. 285-292. of Reclamation, Geology Branch, Project Development Division, __ 1977, Land subsidence stopped by artesian-head recovery, Santa 118 p. Clara Valley, California: International Association of Hydro U.S. Coast and Geodetic Survey, 1956, San Jose area, California, logical Sciences, International Symposium on Land Subsidence, 1912-54leveling (190A California); 42 p. (A compilation of adjusted 2d, Anaheim, California, December 1976, Publication 121, elevations for all bench marks in the level network.) p. 124-132. Webster, D.A., 1973, Map showing areas bordering the southern part __ 1984, Case history No. 9.14, Santa Clara Valley, California, USA, of San Francisco Bay where a high water table may adversely affect in Poland, J.F., ed., Guidebook to studies of land subsidence due land use: U.S. Geological Survey Miscellaneous Field Studies Map to ground-water withdrawal: United Nations Educational, MF-530, scale 1:125,000. FIGURES 31-48; TABLE 6 F48 MECHANICS 1JF AQUIFER SYSTEMS e 200 !;~ 160 QC 120 to~~ :~ 80 Stress-strain ~z- 40 ~ 0 ~o----~------,~o----~,5~----~20------2~5------3~0------3~5----~40~----~45------5~0------5~5------s~o----~65~~--~----~75x10~ STRAIN D FIGURE 31.-Hydrograph, change in applied stress, compaction, subsidence, and stress-strain relationship, 6S/1W-23El. A, Hydrograph of well 6S/1 W -23E 1, perforated 170-177, 244-255, 311-315, and 343-350 ft. B, Change in applied stress, water table assumed constant. C, Compac tion to 425-ft depth at well6S/1W-23E1 and subsidence of bench mark A176, 0.6 mi west. D, Stress change versus strain (225-ft thickness). LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F49 Q !i! :li FRt·-.. -- -1---... t.~r·.t .. !-- ... :-RSi- --._,.- f· ~ 1:i I I I I I I I I I I I I I I +-~ 1-· I I I I I i 1977 1978 1979 1980 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 ..1975 ,. 1976 c ~ 160 ~~ Q c 120 .... ~ ~~ ~ Q 80 c.,_ : ~ 40 c,:,Zz- 0 ~ 0 0.1 0.2 0.3 0.4 COMPACTION, IN FEET D FIGURE 32.-Hydrograph, change in applied stress, compaction, and stress-compaction relationship, 6S/2W-24C4. A, Hydrograph ofwell6S/2W-24C4, depth 250ft. B, Change in applied stress, water table assumed constant. C, Compaction to 250-ft depth at well6S/2W-24C4. D, Stress change versus compaction of deposits above 250-ft depth. F50 MECHANICS OF AQUIFER SYSTEMS c ji: J!l~1n~--1----l-===:=--tf~-[-----J-~-J-1~----1--~--r- r~=:=:==+~~r·~=--;-~II u 8 ~ i l~ [? I I I I I I c~~K'"'~ o- 250 jMt I I I I I I I ~ 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 . 1970 1971 1972 1973 1974 1975 u c h·:: b·...... ' ~ .... ~ ~ 40 !!:t: ~ ~ 0 ~!! 0 0.1 0.2 0.3 0.4 ~ COMPACTION, IN FEET D FIGURE 33.-Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C4. A, Hydrograph of well 6S/2W-25Cl, depth 500ft. B, Change in applied stress, 6S/2W-25Cl, water table assumed constant. C, Compaction to 250-ft depth at well6S/2W-24C4. D, Stress change versus compaction of deposits above 250-ft depth. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F51 ~ i1it!! ft II I I I I I I t~·~t· .. ·rt~l I I I~ I~ u 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 c 160 r 120~ ~ ::f.~.o~~~~--~~0.~5~--~~~~~.-o~~--~~~1.5 COMPACTION, IN FEET D FIGURE 34.-Hydrograph, change in applied stress, compaction, and stress-compaction relationship, 6S/2W-24C3. A, Hydrograph ofwell6S/2W-24C3, depth 550ft. B, Change in applied stress, water table assumed constant. C, Compaction to 550-ft depth at well6S/2W-24C3. D, Stress change versus compaction of deposits above 550-ft depth. F52 MECHANICS OF AQUIFER SYSTEMS A B ~ i I!!tffll i I I ,r ..··t·"l ... I I II 1:r I I Ii 1: I I L) 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 c s;~ ~ 120~8o ~'4- a..C c ..... 40 Stress-compaction ~~ ~z o~~~~--~~~~~~~~~~--~~~ ~- 0 0.5 1.0 1.5 ~ COMPACTION, IN FEET D FIGURE 35.-Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C3. A, Hydrograph of well6S/2W-25Cl, depth 500ft. B, Change in applied stress, 6S/2W-25Cl, water table assumed constant. C, Compaction to 550-ft depth at well 6S/2W-24C3. D, Stress change versus compaction of deposits above 550-ft depth. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F53 c D 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 E .n 160 e:cz:~ Cl) ~ 120 QC ~~ ~~ 80 z""t- ...... -~ 40 z-t:)Z % 0 u"" 0 0.2 0.4 0.6 0.8 1.0 1.2 COMPACTION, IN FEET F FIGURE 36.-Hydrograph and change in applied stress, 6S/2W-24C4; C, Compaction to 250-ft depth at well 6S/2W-24C4. D, Compaction compaction and stress-compaction relationship, 6S/2W -24C3 and to 550-ft depth at well 6S/2W-24C3. E, Compactio11 in 250-550-ft depth 6S/2W-24C4. A, Hydrograph of well 6S/2W-24C4, depth 250ft. B, interval. F, Stress change versus compaction of deposits from Change in applied stress, 6S/2W -24C4, water table assumed constant. 250-550-ft depth. F54 MECHANICS OF AQUIFER SYSTEMS c D ~~~!11ft c~·r .. ~t~"ll 2.5 L..--..L...... -...L...... --L--...... L-~-L-...... l..-...... L--..I--'---.J...... -~..J....--...L--...... __-J...._....J....Im I I .I I: M _ __, 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 E u) ~ e:ffi 120 "'t-cc !!!~ 80 ~ .. ~c c ..... 40 zw Stress·compaction ....-~ z-tDZ 0 c 0 0.2 0.4 0.6 0.8 1.0 1.2 X (.) COMPACTION, IN FEET F FIGURE 37.-Hydrograph and change in applied stress, 6S/2W-25Cl; compaction and stress-compaction relationship, 6S/2W-24C3 and 6S/2W-24C4. A, Hydrograph of well 6S/2W-25Cl, depth 500 ft. B, Change in applied stress, 6S/2W-25Cl, water table assumed constant. C, Compaction to 250-ft depth at well 6S/2W-24C4. D, Compaction to 550-ft depth at well6S/2W-24C3. E, Compaction in 250-550-ft depth interval. F, Stress change versus compac tion of deposits from 250-550-ft depth. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F55 ll:~ !!1~ 1~-1------1~-g;ot:·j"'---·1· ·---1--i<,l·····----1-··--···1·---i·--···1------1---~~1--~-~·-~ ~c c-' A 1~ Jill Ift I I~ I I I c -0.5 I ~ 0.0 ~ ~ 0.5 .... ~r----... ~ 1.0 ~ ~ "'----r-.... Compaction, 0-1000 feet ~ z 1.5 '--- 0.0 ~ C) 2.0 ----... 0.5 ;:. ~ 2.5 r--- t----. ....___ : Bench- mark J 111 r--... 1.0 ~ ~ 3.0 -- 1.5 --- 2 I r-- 8 3.5 2.0 ~ 4.0 -- 2.5 ~ D 'tt·-.. iliEIII1960 1961 1962 1963 1964 1965 1966 1967 : I 1968 I1969 I1970 I1971 I1972 I1973 I1974 1975 1976 ·1···11977 1978 I1979 I1980 I E !f::f~~/}~~~ 40 ; m Stress-compaction ~~ o~~~~~------~~~----~~--~~------~~~._~ 5 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 COMPACTION, IN FEET F FIGURE 38.-Hydrograph and change in applied stress, 6S/2W-24C7; depth at well 6S/2W-24C3. D, Compaction to 1,000-ft depth at well compaction, subsidence, and stress-compaction relationship, 6S/2W-24C7, and subsidence at bench mark J111, 400ft southeast. 6S/2W-24C3 and 6S/2W-24C7. A, Hydrograph of well 6S/2W-24C7, E, Compaction in 550-1,000-ft depth interval. F, Stress change versus perforated 807-851 and 880-903 ft. B, Change in applied stress, compaction of deposits from 550-1,000-ft depth. 6S/2W-24C7, water table assumed constant. C, Compaction to 550-ft F56 MECHANICS OF AQUIFER SYSTEMS -0.5 ,_ 0.0 I I ...... ____ I 0.5 ...... __ --- t----- 0.0 t;:; I l iE 1.0 1'---. t-- 0.5 ~ ...... Compaction, 0-605 feet z 1.5 1.0 iE 0 2.0 ~ t; Bench mark JG4 1.5 ~ : 2.5 2.0 ffi 2 3.0 t-- 2.5 ~ 0 ---- (..) --- 1-- 3.5 3.0 ~ 4.0 3.5 U) 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 c en ~ 160 en ~~ ~ 160 oc 120 ~ffi !!:!ll: ~ !« 120 ~~ .... :J: c,_ 80 z .... ~ :5 80 -~ c,_ .... Stress-compaction ~zz- 40 c : ti! 40 z-~z 5 0 c 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 COMPACTION, IN FEET COMPACTION, IN FEET D E FIGURE 39.-Hydrograph, change in applied stress, compaction, sub and subsidence of bench mark JG4 at well7S/1E-9D2. D, Stress change sidence, and stress-compaction relationship, 7S/1E-9D2. A, versus compaction of deposits above 605-ft depth, May 1960 to April Hydrograph of well 7S/1E-9D2, depth 605 ft. B, Change in applied 1963. E, Stress change versus compaction of deposits above 605-ft stress, water table assumed constant. C, Compaction to 605-ft depth depth, January 1965 to January 1968. from May 1960 to April1963 and from January 1965 to January 1978, LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F57 B -0.5 0.0 0.5 '- 10 '--. 0.0 8 ...... 1.5 '\. 0.5 ..... :!!!: 2.0 ~ r----...... _.._ ~--.....__ 1.0 ~ ~ 2.5 1.5 :!!!: '-., Compaction. ·o-908 feet ~ 3.0 ...... _ "--- 2.0 ...· ~ 3.5 '--- 2.5 ~ Q 4.0 ~- 3.0 ~ (.) 4.5 Bench mark JG2 ...... 3.5 ~ 5.0 --. 4.0 "' 5.5 4.5 1958 1959 1960 1961 1962 1963 1964 1965 1966-- 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 c 200 180 ~ 160 Q 140 120 Q.. ..Q.. 100 :!!!: 80 60 sz 40 ~ 20 0 0 10 15 20 25 30 35 40 45 50 55 60 63 x1o-• STRAIN D FIGURE 40.-Hydrograph, change in applied stress, compaction, sub 7S/1E-16C5 and subsidence of bench mark JG2, located in top of north sidence, and stress-strain relationship, 7S/1E-16C5. A, Hydrograph east concrete curb of 12th Street, approximately 20 ft west of well of well 7S/1E-16C5, depth 908ft. B, Change in applied stress, water 7S/1E-16C5. D, Stress change versus strain (708-ft thickness). table assumed constant. C, Compaction to 908-ft depth at well F58 MECHANICS OF AQUIFER SYSTEMS .:! I ~... 160 I Q 200 ji,Qg'~iFfi\fvf~J qq Gf'1it9t9?=f'1YEtffH B -0.5 0.0 I I 0.5 '----+---.. l l ..... 1.0 \ I I 0.0 f--...... I I 1.5 Bench mark JG2 '--.... Compaction 0-1000 feet 0.5 _'-..,. 1.0 ~ 2.0 .._____,_ 2.5 '--.... ""'-.., ----...... _,_ 1.5 ~ 3.0 2.0 3.5 ---...... ----.... ~ Bench mark JG3 ...... 2.5 ri 4.0 3.0 8 4.5 I ----- ""------.... 3.5 5.0 ----- 4.0 5.5 ------1- 4.5 6.0 5.0 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 c 180 160 ....."" c 140 ~ 120 100 :5 Stress·strain ..... 80 60 ~ 40 20 0 4 0 10 15 20 25 30 35 40 45 50 55 60 63 x 10- STRAIN D FIGURE 41.-Hydrograph and change in applied stress, 7S/1E-16C5; mark JG3 from February 1963 to May 1969. (Bench mark JG2 is compaction, subsidence, and stress-strain relationship, 7S/1E-16C6-ll. located in top of northeast concrete curb of 12th Street, approximately A, Hydrograph ofwell7S/1E-16C5, depth 908ft. B, Change in applied 20ft west ofwell7S/1E-16C5; bench mark JG3 is located approximate stress, 7S/1E-16C5, water table assumed constant. C, Compaction to ly 300ft north of bench mark JG2.) D, Stress change versus strain 1,000-ft depth at well 7S/1E-16C6-ll (see fig. 42B) and subsidence (800-ft thickness). at bench mark JG2 from October 1960 to February 1963 and bench LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F59 -0.5 0.0 0.5 ~ t;:; 1.0 "\. 0.0 "'"h. h. ~ 1.5 0.5 ..... Bench mark JG2 ~ "\. ~ 2.0 1.0 ~ !--.. Compaction 0-1000 feet z 2.5 ~"' --r-----... 1.5 ; § 3.0 " ...... _ 2.0 • ~ 3.5 r---... 2.5 ~ ...... _ ...... :::E 4.0 3.0 ::!l Bench mark JG3 ,...... _ 8 4.5 "'--- 3.5 ~ I -...... 5.0 ---- - 4.0 ~ 5.5 - 1- 4.5 6.0 5.0 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 8 FIGURE 42.-Hydrograph, compaction, and subsidence, 7S/1E-16C6-ll. 1960 to February 1963 and bench mark JG3 from February 1963 to A, Hydrograph of well 7S/1E-16Cll, perforated 551-571, 598-618, May 1969. (Bench mark JG2 is located in top of northeast concrete and 640-660 ft. B, Compaction to 1,000-ft depth at well 7S1E-16C6-11 curb of 12th Street, approximately 20ft west of well 7S/1E-16C5; (7S/1E-16C6 from June 1960 to June 1969 and 7S/1E-16Cll from June bench mark JG3 is located approximately 300 ft north of bench mark 1969 to January 1980) and subsidence at bench mark JG2 from October JG2.) -, 0 . . . ~ 1- 0 .... !' T ..... ~~-- Hydrogr~ph, 24C3 Hydrograph, 24C4 ow g~ _,w /'...... --.. .A. ___...-'\ wLL. 40 ...... __. -----...: ...... __, wLL. 40 - - IDz \.__/ r--' mz a:- a:- w • 80 ~ u.i 80 v \ I 1-w 1.1 . . . 0.0 ....,.. "TO , I I I I I . ' ' ' . .... ' 1- 1.2 1- w w 0.1 w w LL. LL. 1.3 ~ ~ 0.2 Compaction, 0-550 feet Compaction, 0-250 feet z _.,. z 0 1.4 0 0.3 i= -~rv i= u ~ ~ u - <( 1.5 <( Q. Q. 0.4 ~ ~ 0 1.6 0 u u 0.5 1.7 . . 0.6 .. . 1980 1981 1982 1983 1980 1981 1982 1983 B B FIGURE 43.-Hydrograph and compaction, 6S/2W-24C3. A, Hydrograph FIGURE 44.-Hydrograph and compaction, 6S/2W-24C4. A, Hydrograph of well 6S/2W-24C3, depth 550ft. B, Compaction to 550-ft depth at of well6S/2W-24C4, depth 250ft. B, Compaction to 250-ft depth at well 6S/2W-24C3. well 6S/2W-24C4. F60 MECHANICS OF AQUIFER SYSTEMS ~~--- ow 0 40 wu.....JW ~~--- Hydrograph, 16C5 mz ow 80 a:- 40 ------r ------...JW w • _I wu.. 1-w <(u Hydrograph, 24C7 lllz a:- 120 80 w • ~~ 1-w 0~ <(u 1- (/) 120 ~~ 160 J:c 1-z 0~ a..<( 160 I- (f) 200 ~...J J:c A 1-z a..<( 240 ~...J 2.6 -r 280 1- A w 2.7 w u.. 4.6 -r ~ 2.8 . .. Compaction, 0-1 000 feet z 1- 0 2.9 w 4.7 i= w u u.. Compaction.! 0-1000 feet <( ~--- / a.. 3.0 ~/ ~ ~ 4.8 ::iii - 0 z _,...-...., ...... 0 4.9 I~ ....__., u 3.1 i= - u -- "-.../ <( 3.2 . . . . . a.. 5.0 1980 1981 1982 1983 ::iii 0 B u 5.1 FIGURE 45.-Hydrograph and compaction, 6S/2W-24C7. A, Hydrograph 5.2 of well6S/2W-24C7, perforated 807-851 and 880-903 ft. B, Compac 1980 1981 1982 1983 tion to 1,000-ft depth at well 6S/2W-24C7. B FIGURE 47.-Hydrograph 7S/1E-16C5, and compaction 7S/1E-16C11. A, Hydrograph of well 7S/1E-16C5, depth 908ft. B, Compaction to 1,000-ft depth at well 7S/1E-16C11. 0 40 ~~--- Hydrograph, 16C5 ow 40 ~~--- ...JW ow 80 wu.. Hydrograph, 16C 11 wu.....JW mz a:- 80 mz w • a:- 120 1-w w . <(u 1-w 120 <(u 160 ~~ ~~ 0~ 0~ I- (f) 160 I-(/) 200 J:c J:c 1-z 1-z a..<( 200 a..<( 240 ~...J ~...J 240 280 A A 4.6 ' -.- ~ 4.0 -T . 1- 1- w 4.7 I w 4.1 w w u.. u.. Compaction, 0-1 000 feet ~ 4.8 ~ 4.2 z· Compaction, 0-908 feet z ~ 0 4.9 I~ ~ ....__., 0 4.3 i= - i= v ~ u -- "-.../ u /~ <( <( ~ a.. 5.0 a.. 4.4 ::iii ::iii 0 0 u 5.1 u 4.5 __.__._ 5.2 . 4.6 1980 1981 1982 1983 1980 1981 1982 1983 B B FIGURE 46.-Hydrograph and compaction, 7S/1E-16C5. A, Hydrograph FIGURE 48.-Hydrograph and compaction, 7S/1E-16Cll. A, Hydrograph of well 7S/1E-16C5, depth 908ft. B, Compaction to 908-ft depth at of well 7S/1E-16Cll, perforated 551-571, 598-618, and 640-660 ft. well 7S/1E-16C5. B, Compaction to 1,000-ft depth at well 7S/1E-16Cll. LAND SUBSIDENCE IN THE SANTA CLARA VALLEY, CALIFORNIA, AS OF 1982 F61 TABLE 6.-Wellsfor which records are included in.figures 31-42 Water-level Depth Perforated Figure Well Type of recorder or of well interval Remarks number number extensometer observation (feet) (feet) well 31 6S/1W-23E1 Cable Recorder 425 170-177 Stress-strain plot 244-255 for 225-ft interval. 311-315 343-350 32 6S/2W-24C4 Cable Observation 250 Changed from cable to and pipe. pipe assembly in October 1973. 33 -24C4 ---do------do----- 250 -25C1 None ----do----- 500 unknown 34 -24C3 Cable ----do----- 550 Changed from cable to and pipe. pipe assembly in October 1973. 35 -24C3 --do-- ----do----- 550 -25C1 None ----do----- 500 unknown 36 -24C4 Cable ----do----- 250 and pipe. -24C3 ---do------do----- 550 37 -24C4 ---do------do----- 250 -24C3 ---do------do----- 550 -25C1 None ----do----- 500 unknown 38 -24C3 Cable ----do----- 550 and pipe. -24C7 ---do------do----- 1,000 807-851 Changed from cable to 880-903 pipe assembly in October 1973. 39 7S/1E-9D2 Cable Recorder 605 unknown 40 -16C5 Cable ---do--- 908 unknown Stress-strain plot for 708-ft interval. 41 7S/1E-16C6-11 Cable ---do--- 1,000 551-571 Stress-strain plot for and pipe. 598-618 800-ft interval. 640-660 -16C5 Cable ---do--- 908 42 -16C6-11 Cable ---do--- 1,000 Changed from cable to and pipe. pipe assembly in April 1972. GPO SBS-045/78029