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10 Instrumentation INTRODUCTION The safety of an existing dam can be improved and its life lengthened by a carefully planned and implemented surveillance program. A key part of such a program is a visual examination of the structure, the reservoir, and the appurtenant works (as discussed in Chapter 2~. However, surveillance must be more than visual observations. Settlements may go undetected without proper measurements of the dam. Comparison of seepage quanti- ties from one inspection to another and over the years is difficult by visual observation and estimation. There are also conditions within a dam that cannot be seen but that can be measured by instrumentation. Thus, even for a simple structure, some type of instrumentation may be needed to im- prove and supplement the visual examination. The purpose of instrumentation in an existing dam is to furnish data to determine if the completed structure is functioning as intended and "to provide a continuing surveillance of the structure to warn of any develop- ments which endanger its safety" (ICOLD 1969~. The means and methods available to monitor geotechnical phenomena that can lead to a dam failure extend over a wide spectrum of instrumenta- tion devices, consisting of very simple to very complex ones. The program for dam safety instrumentation requires detailed design that is consistent with all other project components; it must be based on prevailing geotech- nical conditions of the dam and impoundment site and on the hydrologic 278

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Instrumentation 279 and hydraulic factors prevalent both before and after the project was in operation. A proposal for instrumentation for monitoring potential deficiencies at existing dams must take into account the threat to life or property that the project presents. The extent and nature of the instrumentation depends on the complexity of the dam, the size of the impoundment, and the potential for loss of life and property damage downstream. The program should in- corporate instrumentation and evaluation methods that are as simple and straightforward as the location for installation and monitoring will allow. The ownerts) must make a definite commitment to a continuing monitor- ing program because any installation of instrumentation devices is wasted without a continuing program. A primary factor of any system is the involvement of qualified personnel at all times, especially during the installation of instrumentation devices. The preparation of comprehensive installation reports is a necessary ad- junct to future data evaluation. While instrumentation can be tied to automatic warning systems, the experience of the committee indicates that no computer or automatic warning system can replace engineering judgment. Instrumentation data must be carefully reviewed periodically by an engineer experienced in the field. This chapter discusses general deficiencies that may be noted or sus- pected during an examination of the dam and describes the instrumenta- tion that will monitor a deficiency. Increased knowledge of the potential deficiency through such monitoring may provide sufficient data to deter- mine the cause and prescribe the necessary treatment. Table 10-1 summa- rizes the deficiencies and the instrumentation to monitor that deficiency. Various types of instrumentation and their manufacturers or suppliers are listed, and methods of installation are discussed. After the instrumentation is in place, the data collection and analysis provide the owner and engineer with the information to define the problem more clearly. This will be dis- cussed in the section Data Collection and Analysis. MONITORING OF CONCRETE AND MASONRY DAMS Concrete and masonry dams must be inspected and monitored on a contin- uous basis, following a carefully planned monitoring program. To aid in these inspections and in the analysis of the condition of the dam, a number of monitoring methods and devices are used. Where these devices are in- stalled, they should be maintained in good condition, and the data ob- tained should be regularly recorded and evaluated. Changes in the magni- tude of the measurements recorded are the significant factors to be

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282 SAFETY- OF EXISTING DAMS observed and evaluated by a trained observer. However, the devices must be properly maintained to ensure that the readings and measurements ob- tained are appropriate. Drainage Systems In many concrete and masonry dams a foundation drainage system is in- stalled to reduce uplift pressures on the dam. These systems are usually in- stalled during construction but can be installed or supplemented at any time. They consist of holes drilled through the base of the dam into the foundation and may contain pipes where the foundation formation will not remain open. Also, monolith joint drains and face drains are commonly installed to intercept seepage along monolith and lift joints. It is very im- portant to maintain the drain in usable condition; drains should be cleaned and periodically checked to maintain free flow conditions. Water levels in or flow from the individual drains should be routinely measured. The flows from all drains or groups of drains should be collected and measured at weir installations. The water should be checked for chem- ical and suspended sediment content to aid in evaluation of solution or ero- sion that may be taking place. The elevation of the reservoir and tailwater elevations should be recorded at the time of drainage measurements so that relationships between these parameters can be developed. Seepage and Leakage Seepage and leakage from the abutments, foundation, and joints or cracks in a dam should be collected and measured on a routine basis. It is impor- tant to review such flows for changes in magnitude and material, both dis- solved and suspended, transported by these flows. Increases in these items are early warning indicators of potential problems. Weirs and venturi flumes with upstream stilling basins are frequently used to measure seepage and leakage. Flow measurements in the downstream discharge channels can add information on the amount of seepage and leakage that is not ob- served at surface leaks or seeps. On critical and or remote structures it is sometimes desirable to telemeter the flow information to another location. Uplift Pressures Uplift pressures in the foundation and in the dam should be measured rou- tinely as indicators of stability or instability. Changes in pressure should be looked for; increases may result in instability. Uplift pressures are mea-

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Instrumentation 283 sured by piezometers inserted in holes drilled into the foundation of the dam. In some cases foundation drainage holes can serve as piezometers if packers are inserted temporarily in the top of the drain hole. Packers can also be used within the holes with connected gages to isolate the interval in which the measurement of water pressure is desired. Observations of the reservoir and tailwater elevations should be recorded when uplift pressures are measured. Movements Movement of concrete and masonry dams and their abutments can be ex- pected during and after construction. These movements will occur as the reservoir is filled and may periodically cycle as it is emptied and filled dur- ing succeeding seasons. Small movements are of little concern, but in- creases in the magnitude of the movement or direction of movement should be immediately evaluated as to their potentially adverse impact on the structure. Movements are measured by surveying the location of monuments lo- cated at various points on or adjacent to the dam. The benchmark or start- ing location for surveys should be located outside of the influence of the dam or reservoir if possible. Special measurement techniques may be used where very precise measurements are desired. Measurements of the locations of the monuments should be such that changes in vertical, horizontal (both longitudinal and transverse to the dam axis), and angular locations are measured. The number of monuments sur- veyed depends on the size and type of the structure. The locations are tailored to the structure and might include locations to measure movement between blocks, displacement at joints and cracks, deflections of various parts of the structure, settlement of the foundation, and movement of the abutments. The locations of the monuments should be recorded at relatively short intervals in the initial years of the life of the structure and less frequently as the satisfactory history of the dam lengthens. They should be more fre- quent if any tendency toward weakness or unsatisfactory performance is indicated. The data collected should be carefully recorded and should in- clude observations on the relative water levels in the reservoir and down- stream. The records accumulated should be plotted to provide graphic dis- plays of the locations of the monuments and displacements between monuments. Computers can be used to create and display three-dimen- sional time-lapse sequences of the structure. This allows the normal sea- sonal movement cycles to be differentiated from changes that may be indi- cators of potential problems.

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284 Deterioration SAFETY OF EXISTING DAMS Routine visual inspection of concrete and masonry dams can be of great value in determining the integrity of the structure. Descriptions of concrete conditions should conform with the appendix to "Guide for Making a Con- dition Survey of Concrete in Service" (American Concrete Institute 1968~. Comparative photographs can aid the trained observer in distinguishing changes that might otherwise be more difficult to identify. Where deterio- ration of concrete, mortar, or masonry appears to be taking place, cores and samples can be taken and tested in the laboratory to provide absolute strength values. A routine schedule of nondestructive testing, such as ultra- sonic velocity measurements, can be useful in determining trends of changes in strength. These types of tests should be carried out whenever deterioration appears or is suspected to be taking place. Seismic Instrument Program A seismic instrument program is an essential part of evaluating existing dams in areas of high potential for seismic activity. Devices to measure ground motions and dam response can facilitate rational design decisions for repair and strengthening of a structure if damage has occurred as a result of an earthquake. These records are also desirable to compare the performance of the structure with design expectations and to estimate the structure's performance during other, larger shocks. However, the type of seismic instrument installation, or whether there even should be one, de- pends on the size and location of the dam. Such installations are desirable on larger structures, dams of unique design, and dams with large down- stream hazard potential. MONITORING OF EMBANKMENT DAMS A visual examination by a trained professional of an embankment dam is a reliable way to detect potential malfunctions or deteriorations of a struc- ture. Surveillance can be aided by devices that measure seepage and leak- age through and around the embankment, movements of the embankment and foundation, and water levels and pressures within the embankment and the foundation. Adequate records of such measurement devices, along with the visual observations, should be maintained. To be effective, these records should be continuous and periodically reviewed by a professional engineer versed in the design and vulnerability of embankment structures. These reviewers should be able to distinguish the important indicators from the unimportant. A tendency toward change in behavior of the dam should

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Instrumentation 285 signal a need for further review and analyses. The record review must focus on the anomalies as opposed to the norm. Obviously this requires a continu- ous data base reflecting measurements made and records kept over a period of time. Seepage and Leakage Seepage and leakage through the embankment, abutments, or foundation can be measured by various types of weirs. It is important to be able to check changes in amounts of seepage or leakage and in the material trans- ported by these flows. Water from seeps can be collected by various drain structures into a weir box and measured by flow over the weir. The weir box would serve as a settling basin for some materials that may be carried from the embankment or abutment. Therefore, the weir box should be watched for deposition of materials. When measuring the flow at the weir plate the water should be observed for turbidity and changes in color, and samples should be taken for analyses of dissolved minerals if the abutment or foundation contains soluble solids. Springs and stream flow downstream of the embankment should be periodically monitored since changes in flow could indicate that piping or solution may be taking place. Movements Considerable movement of embankment dams can be anticipated during and immediately after construction. Much of the movement may be attrib- uted to foundation settlement under the loading of the embankment. The embankment will also move as the reservoir is filled for the first time and may periodically cycle movements as the reservoir is emptied and filled in succeeding seasons. Movements are determined by periodic measurements of monuments placed in or on the structure and abutments during construction or located on the structure and the abutments after construction. For existing dams monumentation to measure movements is usually limited to the crest and downstream slopes. The monuments usually consist of steel rods or sur- veyor's markers imbedded in concrete placed in excavations on the embank- ment and abutments. Differences in elevation and location of the monu- ments are measured by transit and level surveys of the monuments. Measurements of the locations of the monuments on the surface of the embankment should be such that changes in both vertical and horizontal locations are measured. The measurements should be reduced to graphical displays of changes in vertical location, changes in longitudinal location along the axis of the embankment, and changes in horizontal location transverse to the axis of the embankment (upstream and downstream). Re-

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286 SAFETY OF EXISTING DAMS lationships to the water surface elevation in the reservoir at the time of measurement of the monuments are important and should be recorded along with the monument location data. Whenever possible the monu- ments should be tied to a benchmark that is outside the influence of the dam and reservoir. Monuments should be located such that they are not damaged by normal traffic or operations. The number of monuments used depends on the size of the structure. The interval between measurements would depend on the history of the embankment. The interval should be relatively short during the initial years of the embankment life and may be extended as the satisfactory his- tory of the embankment lengthens. If the structure shows any tendency toward weakness or unsatisfactory performance, the time interval between measurements should be shortened appropriately to provide analytic data that can warn of impending problems. Inclinometers can be used to measure internal movements within em- bankments, in abutments, and in the reservoir rind.. An inclinometer is a vertical tube placed in the embankment after construction. An electronic device is lowered within the inclinometer tube to detect any change in the location of the tube since the last measurement. The tube has vertical fur- rows or ridges that control the location of the pendulum device used to detect movement. The use of an inclinometer can give a continuous record of movement from the surface to the bottom of the inclinometer allowing differentials to be calculated at any elevation. This information has been useful in plotting the movements of slide masses and can provide valuable information on movements within embankment dams. They are not cheap to install and are relatively expensive to monitor. Therefore their use in existing embankments may be limited to the more important structures and to those with obvious deficiencies. Another device to measure movement is the extensometer. It is generally used to measure strain in rock masses but can also be used in soil. The best use of an extensometer is to study relaxation or movement in rock excava- tions, such as tunnels or mines. The extensometer assembly can be sensed either mechanically or electrically. Piezometric Pressures A primary indicator of the performance of an embankment is the water pressure distribution within the structure and its foundation. Water pres- sures in the embankments are measured by piezometers. There are basi- cally three types of piezometers in common usage: (1) a hydraulic piezome- ter in which the water pressure is obtained directly by measuring the

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Instrumentation 287 elevation of water standing in a pipe or vertical tube, (2) an electronic pi- ezometer in which the water pressure deflects a calibrated membrane and the deflection is measured electronically to give the water pressure, and (3) a gas pressure unit in which the water pressure is measured by balancing it with a pressurized gas in a calibrated unit. The electronic and gas piezome- ters are usually installed during construction of the embankment, whereas the standpipe type can be installed at any time and is commonly used on existing structures. One of the simplest piezometers is a performance type installed vertically in the embankment. These may be driven or angered into place or installed in holes drilled specifically for the purpose. Care should be taken during drilling to prevent hydraulic fracturing within the embankment. If there is a chance that embankment material might move through the perforations in the piezometer tube, a graded filter should be placed around the piezom- eter pipe in the drilled hole, and the annular space above the piezometer tip location should be backfilled with material of low permeability. The surface area around the piezometer should be sealed to prevent the entry of surface water along the casing. There are many variations of hydraulic pi- ezometer units designed for special applications and to provide various lev- els of accuracy and ease of measurement. Piezometers should be installed in an embankment structure so that the location of the free water surface or phreatic line can be determined. The line of piezometers would be perpendicular to the longitudinal axis of the embankment. In large structures there may be several lines of piezometers, while in smaller structures and existing dams perhaps one line would be adequate. The time interval between measurements of the water levels or pressures in piezometers depends on the age and condition of the structure. More frequent measurements are appropriate for relatively new structures and those with apparent or suspected defects. The elevation of the water surface in the reservoir and other conditions should be noted at the time of measurement of water levels and pressures in piezometers. If the piezome- ter is of the vertical standpipe type it should be kept capped at all times when measurements are not being made in order to prevent entry of mate- rial that would render future measurements impossible. RESERVOIR RIM The construction of a dam and the subsequent impoundment cause more interference with natural conditions than do almost any other works of the civil engineer (Legget 1967~. The groundwater level along the reservoir valley will be directly affected by the rise in water level, generally for a considerable distance away from the actual shore line of the reservoir. Ma-

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288 SAFETY OF EXISTING DAMS serials in the sphere of influence of the reservoir water may fail to retain their former stability and landslides may result. Leakage from the reservoir is another potential source of trouble. In the publication General Considerations on Reservoir Instrumenta- tion, by the Committee on Measurements (ICOLD 1969), Komie discusses several consequences of altering natural groundwater regime relative to the reservoir rim. Seepage into adjoining basins is a potential problem, seepage from solid waste disposal reservoirs can cause deterioration of the regional groundwater, and seepage may contribute to local subsidence. Precon- struction and postconstruction hydrogeologic studies and instrumentation are recommended. The instrumentation can primarily be done by install- ing and monitoring observation wells, weirs, and piezometers. Komie sug- gests that postconstruction monitoring should be continued for several cycles of reservoir operation to document cyclic changes. Periodic obser- vation of natural springs and comparison of pre- and postreservoir water temperature are also often helpful. Figures 10-1 through 10-4 are typical instrumentation installations from Komie's paper. Instability of reservoir slopes caused by saturation of overburden or up- lift water pressure in bedrock is a potential danger. Tockstein (USCOLD 1979/1981) states that whenever the in situ conditions of an area are dis- turbed by the impoundment of a reservoir a potential for ground move- ments is created along the reservoir slopes and the slopes adjacent to, but outside of, the reservoir rim. The damage potential can be reduced by iden- tifying existing and potential landslide areas, monitoring these areas, and implementing a preplanned course of action in the event that excessive movement of an unstable area is imminent. In planning a monitoring program it is necessary to remember that a landslide will occur when the driving forces exceed the resisting forces. Thus, the parameters that affect or measure these forces on a given slope should be identified. The parameters most commonly used are pore water pressure and displacement, both at the surface and at various depths within the slope, which indicate if the resisting forces are being exceeded and at what location. By correlating changes in these parameters with each other and with external influences on the slope, the cause of the movement can be identified and appropriate remedial action initiated. A systematic approach to planning a monitoring program is presented by Dunnicliff (1981~. It is important that both the person installing the instruments and the person making the readings and maintaining the instruments under- stand the purpose of the instrumentation. The reduced data must be re- viewed periodically by a professional engineer or engineering geologist with expertise in slope stability. Another excellent reference on instrumen- tation to monitor ground movements relative to slope instability is by

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298 SAFETY OF EXISTING DAMS National Research Council, Geotechnical Instrumentation for Moni- toring Field Performance, April 1982, Chapter 5, Details of Instrumenta- tion. Adding instrumentation to existing dams will require specialized equip- ment and drilling techniques for boreholes in which the instruments are to be installed or in fastening to existing structures; therefore, it is recommended that firms having subsurface exploration experience and expertise should be obtained for making the installations. During the drilling of boreholes sam- ples of material and logs of the borings should be obtained. These data will be of significant value for subsequent data evaluation and prognostication concerning the ongoing safe operating conditions. The drilling methods and operation procedures must be specified and carefully monitored to prevent damage from hydraulic fracturing of earth embankments. Portable instruments also have been useful for investigation of deficien- cies at dams. For example, borehole cameras and periscopes have been used successfully in examining concrete structures and foundations. Televi- sion cameras have important applications in underwater work, such as in surveys of conduits and the submerged faces of dams. Sonic devices have been used effectively to locate leaks in dams. They have been particularly useful in measuring leakage through the concrete facings of rockfill em- bankments where disruption of the slab joints or cracks in the panels were primary sources of leakage. A basic instrument used for this purpose is a hydrophore, which essentially is a waterproof microphone that can be lowered on a cable. The test involves the comparison of background sound intensity with intensity measured in the vicinity of leaks. The microphone may be lowered from a boat located over the points of suspected leakage. Although areas of high sound intensity can be found by using such equip- ment, the size of the leak does not necessarily correlate with the indicated intensity. A small leak can produce a high sound level if there is a sharp disturbance at the entrance. On the other hand, a large leak with a smooth entrance could produce a much lower reading. Two effective methods of visual inspection of a dam face involve the em- ployment of divers and closed-circuit television. Divers can be effective in water depths up to about 150 feet without very expensive equipment. At greater depths the long decompression time required for each dive may be- come prohibitive. An effective way to operate at relatively shallow depths is to have the diver carry a television camera with him so that leaks can be viewed by an engineer at the surface or recorded on videotape. When in- specting with closed-circuit television, a hydrophore and a "pigtail" mounted on the camera facilitate location of the leak. The sonic level will increase and the pigtail will be drawn in the direction of flow as the camera is moved into the region of higher velocity (Jensen 1968~.

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302 DATA COLLECTION AND ANALYSIS SAFETY OF EXISTING DAMS The reason for installing instruments in dams is to monitor them during construction and operation. One of the specific applications of measure- ments is to furnish data to determine if the completed structure will con- tinue to function as intended. The processing of large masses of raw data can be efficiently handled by computer methods. The interpretation of the data requires careful examination of measurements as well as other influ- encing effects, such as reservoir operation, air temperature, precipitation, drain flow and leakage around the structure, contraction joint grouting, concrete placement schedule, seasonal shutdown during construction, con- crete testing data, and periodic instrument evaluations. The display of data should be both tabular and graphical and should be simple and readily un- derstood. The data should be reviewed periodically by a professional engi- neer versed in the design, construction, and operation of embankment and/ or concrete dams. Because of the various types of instrumentation used in the different kinds of dams, data collection and analysis will be discussed for embank- ment dams and for concrete gravity and concrete arch dams separately. Monitoring of Embankment Dams The U.S. Corps of Engineers' Engineer Manual (1976) discusses collecting, recording, and analyzing data and makes recommendations on frequency of reading, recording, and reporting measurements and analysis. Profiles of piezometric levels in the foundation and contours of pore water pressures in the embankment and foundation should be made periodically to evalu- ate the data. Examples of plotting the data are shown in Figures 10-6 and 10-7. After construction and during reservoir filling, embankment and foundation piezometers should be read weekly or once for every S-foot rise in the water level. Readings after reservoir filling should be continued on a quarterly or semiannual basis depending on the dissipation of pore water pressures. It is also recommended in the Corps' Manual that additional ob- servations be made at times of varying heads. The movement measuring devices should be read immediately after in- stallation, since all subsequent readings will be referred to the initial read- ing. The field data should be reduced to a reportable form promptly and prepared in graphical form to evaluate relations and trends. Figure 10-8 shows a typical presentation of vertical and horizontal movement data. Wilson (1973) states that it is customary to install surface monuments on the top of a dam after completion and to observe postconstruction settle- ment for several years. Internal movement instruments should be installed inside a dam before the embankment is topped out to provide a history of

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Instrumentation 305 interaction between the core and the rockfill shells and the wetting of the dry rockfill and that the dam had an acceptable margin of safety. For earth pressure measuring devices, after a structure is completed, the Pressure cells should be read at least annually to evaluate chances in stress . ~ . ~ . ~ . 1 1 1 1 1 1 1_ _ 1 _ _ 1 _ __ ~ ~ __ 1 _ ~ _ with time. Data from earth pressure cells ShOUlCt ne rectuceu ano time blots maintained for each cell during and after construction. Analysis of data should inclucle a comparison of observed earth pressure with earth pressure assumed for the design of the structure. The U.S. Bureau of Reclamation (1974) has developed a frequency of readings for earth dam instrumenta- tion. A copy of this illustration is shown in Table 10-4. Monitoring of Concrete Dams The U.S. Bureau of Reclamation (1976, 1977) states that to determine the manner in which a concrete dam and its foundation behave during con- ,,~ 370 ,,, 160 - - ``, 1 5 0 o in Dec 14-64 Sep'-27- 6 Oct 15 644: ~ . ~ l ~t 1 ,iJu i17-64 1 1 s~lt2937^ 1 .1' Oct 27-65 Oct 15-66 / Jon 6-67 / . ,L J' fete- 65 a Mor 14-67~\ Ju1~20-66 1 1} ~ ~~ I U June4 -676 Jo r~ 6 ~ _ _ 1 1 T , \ oval ~ a. >5-68~- on 9-68 . ~ on 28-69 Croci 6-70 ~ 1 ','~Jon 12-71 g IseP1 9 - 7 ~ _ `tJuly7-71 ~Jur~7-70 July 7- 69d I Do141O~ ) ~ - - , ~e23-6 1 ~ 60E~: ~~: -16 -8 0 ~ 16 o o ~ 100 20 80 1 1 1 1 1 - 1 lOm _M 10 24 32 40 Horizontol displacements, centimeters Upstream ~ Downstream FIGURE 10-9 Horizontal (river direction) displacements versus pool levels, Monument 10. SOURCE: Marsal and de Arellano (1972~.

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306 SAFETY OF EXISTING DAMS TABLE 10-5 Frequency of Readings for Concrete Dam Instrumentation Type Reading Frequency Embedded instrument Deflection measuring devices Uplift pressure measurement Target deflection and pier net triangulation measurements Leveling across top of dam and . . . vicinity Seven to 10 days during construction; semimonthly or monthly afterward. More frequently during periods of reservoir filling or rapid drawdown. Weekly; closer intervals during periods of special interest. Monthly, except for initial filling, which should be a 7- to 10-day interval. Semiannually during period of minimum and maximum air temperature. Additional measurements during early stages of reservoir filling. Periodically. More frequently in the early stages of operation and less frequently at later stages, depends on conditions encountered. SOURCE: U.S. Bureau of Reclamation (1977J. struction and operation, measurements should be made to obtain data on strain, temperature, stress, deflection, and deformation of the foundation. There are two general methods of measurement for obtaining the essential behavioral information. The first method involves several types of instru- ments that are embedded in the mass concrete of the structure and on the features of the dam. The second method involves several types of precise surveying measurements from targets at various locations on and in the dam. The suggested schedules for collecting data are shown in Table 10-5. The planned program for measurements should cover a time period that will include a full reservoir plus two cycles of reservoir operation, after which a major portion of the measurement may be suspended. For the re- maining measurements the interval between successive readings may be lengthened. The U.S. Corps of Engineers' Engineering Manual (1980) dis- cusses instrumentation to measure the structural behavior of concrete grav- ity dams under five major headings: (1) Carlson-type instruments, (2) up- lift and leakage, (3) deflection plum line, (4) precise alignment facilities, and (5) thermocouples. The frequency of collection and evaluation of data is also discussed in detail for each category. Some excellent illustrations concerning dam foundation uplift pressure histories, methods of showing uplift pressure gradients, typical deflection history, and typical precise alignment marker layout and details can be found in the U.S. Corps of Engineers' Engineering Manual (1980~.

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Instrumentation REFERENCES 307 American Concrete Institute (1968) "Guide for Making a Condition Survey of Concrete in Service," Journal of the American Concrete Institute, Proceedings, Vol. 65, No. 11, pp. 905-918. Bolt, B. A., and Hudson, D. E. (1975) "Seismic Instrumentation of Dams," Journal of the Geotechnical Engineering Division, ASCE, Vol. 101, No. GTll (November), pp. 1095- 1104. Dunnicliff, J. (1981) Measurements Committee Report, U.S. Committee on Large Dams Sec- tion VI, Inventory of Geotechnical Instruments, Manufacturers or Suppliers. International Commission on Large Dams (ICOLD) (1969) General Considerations Applica- ble to Instrumentation for Earth and Rockfill Dams. Committee on Observations of Dams and Models, Bulletin No. 21, November. International Commission on Large Dams (ICOLD) (1981) Automated Observation for In- stantaneous Safety Control of Dams and Reservoirs, Bulletin No. 41, January. Jansen, R. B. (1968) A Prescription for Dam SafetyInstrumentation and Surveillance, Con- ference of College of Engineering, University of California, Berkeley. Legget, R. F. (1967) "Reservoirs and Catchment Areas,'' Chapter 14 in Geology and Engi- neering, 2d ea., McGraw-Hill International Series, New York. Lytle, J. D. (1982) Dam Safety Instrumentation; Automation of Data Observations, Process- ing and Evaluation; Question 52, in International Commission on Large Dams (ICOLD), Response 14th Congress, Rio de Janeiro. Marsal, R. J., and de Arellano, L. R. (1972) "Eight Years of Observations at E1 Infiernillo Dam," Proceedings of the Specialty Conference on Performance of Earth and Earth- Supported Structures, ASCE. National Research Council (NRC) (1982) Geotechnical Instrumentation for Monitoring Field Performance, National Academy Press, Washington, D.C. Sharma and Raphael (1979/1981) General Considerations on Reservoir Instrumentation, Committee on Measurements, USCOLD. U.S. Army Corps of Engineers (1971 and 1976) Instrumentation on Earth and Rock-Fill Dams, Parts 1 and 2, EM 1110-2-1908, August 1971 and November 1976. U.S. Army Corps of Engineers (1980) Instrumentation for Measurement of Structural Behav- ior of Concrete Structures, EM 1110-2-4300, September. U.S. Bureau of Reclamation (1974) Earth Manual, 2d ea., Government Printing Office, Washington, DC. U.S. Bureau of Reclamation (1976) "Design of Gravity Dams," Chapter XIII in Design Man- ual for Concrete Gravity Dams. U.S. Bureau of Reclamation (1977) "Design of Arch Dams," Chapter XIII in Design Manual for Concrete Arch Dams. Wilson, S. D. (1970) "Observational Data on Ground Movements Belated to Slope Instability, The Sixth Terzaghi Lecture,'? Journal of the Soils Mechanic and Foundation Division, SM5, September. Wilson, S. D. (1973) "Deformation of Earth and Rockfill Dams," in Embankment-Dam Engi- neering, Casagrande Volume, John Wiley & Sons, New York. RECOMMENDED READING ASCE (1981) Conference Proceedings on Recent Developments in Geotechnical Engineer- ing for Hydro Projects, Fred Kulhawy, ed.

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308 SAFETY OF EXISTING DAMS ASCE (1973) Inspection, Maintenance and Rehabilitation of Old Dams, New York. Bannister, J. R., and Pyke, R. (1976) "In-Situ Pore Pressure Measurements at Rio Blanco," Journal of the Geotechnical Engineering Division, ASCE, Vol. 102, No. TG10, October, pp. 1073-1091. De Alba, P., and Seed, H. B. (1976) "Sand Liquefaction in Large-Scale Simple Shear Test," Journal of the Geotechnical Engineering Division, ASCE, Vol. 102, No. GT9 (September), pp. 909-974. ICOLD (1973) Rock Mechanics and Dam Foundation Design. International Commission on Large Dams, Boston, Mass. ICOLD Bulletin No. 23 (1972) Reports of the Committee on Observation on Dams and Models, July. Jansen, R. B. (1980) Dams and Public Safety, U.S. Department of the Interior, Bureau of Reclamation (reprinted 1983~. Kovari, K. (1977) Field Instrumentation in Rock Mechanics, Vols. I and II, Federal Insti- tute of Technology, Zurich. O'Rourke, I. E. (1974) "Performance Instrumentation Installed in Oroville Dam," Journal of the Geotechnical Engineering Division, ASCE, Vol. 100, No. GTZ (February), pp. 157-174. Peck, R. B. (1969) "Advantages of Limitations of Observational Method in Applied Soil Mechanics," 9th Rankine Lecture, Geotechnique, Vol. 19, No. 2, pp. 171-187. Raphael, I. M., and Carlson, R. W. (1965) Measurement of Structural Action in Dams, 3d ea., James J. Gillick and Co., Berkeley, Calif. Sarkaria, G. S. (1973) Proceedings of the Engineering Foundation Conference on Inspec- tion, Maintenance and Rehabilitation of Old Dams, Safety Appraisal of Old Dams: An updated perspective, September 23-28, ASCE, New York, pp. 405-217. Sherard, J. L., et al. (1973) Earth and Earth Rock Dams, John Wiley & Sons, New York. Slobodulk, D., and Roof, E. F. (1980) "Monitoring of Dam Movements Using Laser Light," Transportation Engineering Journal, November, pp. 829-843. U.S. Bureau of Reclamation (1980) Safety Evaluation of Existing Dams (SEED Manual), Government Printing Office, Washington, D.C. Wahler, W. A. (1974) Evaluation of Mill Tailings Disposal Practices and Potential Dam Stability Problems in Southwestern United States, Prepared for Bureau of Mines, Gen- eral Report, Vol. I. Wilson, S. D., and Handcock, C. W., Jr. (1965) "Instrumentation for Movements Within Rockfill Dams," Instruments and Apparatus for Soil and Rock Mechanics, ASTM STP 392, American Society of Testing Materials, pp. 115-130.