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INSTRUMENTATION 278 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 quantities 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 improve 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 developments 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 instrumentation 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 geotechnical conditions of the dam and impoundment site and on the hydrologic
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 incorporate instrumentation and evaluation methods that are as simple and straightforward as the location for installation and monitoring will allow. The owner(s) must make a definite commitment to a continuing monitoring 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 adjunct 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 suspected during an examination of the dam and describes the instrumentation that will monitor a deficiency. Increased knowledge of the potential deficiency through such monitoring may provide sufficient data to determine the cause and prescribe the necessary treatment. Table 10-1 summarizes 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 discussed in the section Data Collection and Analysis. MONITORING OF CONCRETE AND MASONRY DAMS Concrete and masonry dams must be inspected and monitored on a continuous 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 installed, they should be maintained in good condition, and the data obtained should be regularly recorded and evaluated. Changes in the magnitude of the measurements recorded are the significant factors to be
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INSTRUMENTATION 282 observed and evaluated by a trained observer. However, the devices must be properly maintained to ensure that the readings and measurements obtained are appropriate. Drainage Systems In many concrete and masonry dams a foundation drainage system is installed to reduce uplift pressures on the dam. These systems are usually installed 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 important 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 chemical and suspended sediment content to aid in evaluation of solution or erosion 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 important to review such flows for changes in magnitude and material, both dissolved 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 observed 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 routinely as indicators of stability or instability. Changes in pressure should be looked for; increases may result in instability. Uplift pressures are mea
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 expected during and after construction. These movements will occur as the reservoir is filled and may periodically cycle as it is emptied and filled during succeeding seasons. Small movements are of little concern, but increases 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 located at various points on or adjacent to the dam. The benchmark or starting 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 surveyed 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 frequent if any tendency toward weakness or unsatisfactory performance is indicated. The data collected should be carefully recorded and should in-elude observations on the relative water levels in the reservoir and downstream. The records accumulated should be plotted to provide graphic displays of the locations of the monuments and displacements between monuments. Computers can be used to create and display three-dimensional time-lapse sequences of the structure. This allows the normal seasonal movement cycles to be differentiated from changes that may be indicators of potential problems.
INSTRUMENTATION 284 Deterioration 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 Condition 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 deterioration 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 ultrasonic 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, depends on the size and location of the dam. Such installations are desirable on larger structures, dams of unique design, and dams with large downstream 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 structure. Surveillance can be aided by devices that measure seepage and leakage 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
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 continuous 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 transported 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 attributed 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 surveyor's markers imbedded in concrete placed in excavations on the embankment and abutments. Differences in elevation and location of the monuments 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
INSTRUMENTATION 286 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 monuments 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 history 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 embankments, in abutments, and in the reservoir rim. 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 furrows 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 excavations, 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 pressures in the embankments are measured by piezometers. There are basically three types of piezometers in common usage: (1) a hydraulic piezometer in which the water pressure is obtained directly by measuring the
INSTRUMENTATION 287 elevation of water standing in a pipe or vertical tube, (2) an electronic piezometer 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 piezometers 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 augered 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 piezometer 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 piezometer units designed for special applications and to provide various levels 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 piezometer 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 material 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
INSTRUMENTATION 288 terials 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 Instrumentation , 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. Preconstruction and postconstruction hydrogeologic studies and instrumentation are recommended. The instrumentation can primarily be done by installing and monitoring observation wells, weirs, and piezometers. Komie suggests that postconstruction monitoring should be continued for several cycles of reservoir operation to document cyclic changes. Periodic observation 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 uplift water pressure in bedrock is a potential danger. Tockstein (USCOLD 1979/1981) states that whenever the in situ conditions of an area are disturbed by the impoundment of a reservoir a potential for ground movements is created along the reservoir slopes and the slopes adjacent to, but outside of, the reservoir rim. The damage potential can be reduced by identifying 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 understand the purpose of the instrumentation. The reduced data must be reviewed periodically by a professional engineer or engineering geologist with expertise in slope stability. Another excellent reference on instrumentation to monitor ground movements relative to slope instability is by
INSTRUMENTATION 289 Wilson (1970). Figure 10-5 is an example of slope instrumentation and data plots from Wilson's paper. Figure 10-1 Typical observation well installations. Source: ICOLD (1969, 1981). INDUCED SEISMICITY As discussed in Chapter 9, controversy exists over whether there is a significant increase in seismic activity associated with impoundment of some large reservoirs. Increased instrumentation for new and old projects can assist in solving this controversy. Bolt and Hudson (1975) discuss the need for seismic instrumentation and recommend the minimum instrumentation for recording basic earthquake data. They recommend that where there may be potential for determining if earthquakes are induced from reservoir loading a network of seismographs should be in operation prior to impounding the reservoir. They recommend simple and reliable instruments. The seismographs should be
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INSTRUMENTATION Figure 10-2 Water-level measurements by (A) steel tape, (B) air-line probe, (C) electric probe, (D) sonic sounder, (E) pressure transducer, and (F) float. Source: ICOLD (1969, 1981). 291
INSTRUMENTATION Figure: 10-3 Typical 90° V-notch weir. Source: ICOLD (1969, 1981). 292
INSTRUMENTATION 293 Figure 10-4 Typical parshall flume. Source: ICOLD (1969, 1981). Figure 10-5 Point Loma landslide, California. Source: Wilson (1970).
INSTRUMENTATION 294 spread in azimuth around the reservoir with the interstation distance not greater than 30 kilometers or less than 5 kilometers. The instruments are best located on bedrock outcrop and should be remote from construction activities, quarries, and streams. For seismographic characteristics, Bolt and Hudson recommend two alternative schemes that will meet the minimum requirement and that have been field tested. One system uses portable seismometers and visual recording units; the network stations are not connected. The second system telemeters the signals from individual seismometers to a central recording room, often using commercial telephone lines. This system is more expensive but has the advantage of recording at one central location. The operation of either system does not require an instrumentation specialist or seismologist. Another excellent paper on reservoir-induced seismicity is by Sharma and Raphael (USCOLD 1979/1981). They stress that instrumentation to study local reservoir-induced seismicity must be installed prior to reservoir filling to establish the extent to which local seismicity is a consequence of the reservoir or part of a more general seismic pattern. These authors discuss objectives of seismic instrumentation, type of measurements, instruments, schedule for making measurements and data analysis. TYPES OF INSTRUMENTS The need and purpose for instrumentation was described earlier in this chapter. This section describes various types of instruments which may be used to monitor dams. Table 10-1 compares causes of deficient behavior with the types of measurements or observations used to monitor that behavior. A brief inventory of the instruments and the factors measured and monitored is listed in Table 10-2. Table 10-3 lists the various commercially available instrument types with the name of the manufacturer. The information on addresses of the U.S. or Canadian suppliers was current at the time of preparation of the report by Dunnicliff (1981). Installation of instruments presupposes the observation (measurements), reduction, and evaluation of the data. In a later section of this chapter, Monitoring of Concrete Dams, a proposed observation schedule is provided, including tables on the frequency of readings (see Tables 10-4 and 10-5). These data are described fully in U.S. Bureau of Reclamation (1974). The procurement and timely evaluation of instrumentation data are primary prerequisites for determining the conditions of dams. The physical conditions existing at the dam should be observed and noted when making observations. Such information supplements periodic inspections. In some instances, such as where dams are not readily accessible, data acquisition
INSTRUMENTATION 295 by automated means may be appropriate. A method to achieve these prerequisites, including automated data evaluation and plotting, is found in a paper by Lytle (ICOLD 1972). TABLE 10-2 Inventory of Geotechnical Instruments Phenomena Measure Instrument Suppliers' Numbers Refer to Table 10-3 Pore water and ground Piezometer, closed system 3, 6, 8, 14, 16. 18, 22, measurement and open system, 25, 29, 30, 31, 32, 36, observation wells 37, 41, 42, 44, 45, 49, 50, 53 Earth pressure Earth pressure cells, 6, 16, 18, 19, 27, 36, measurements wholly embedded in soil; 42, 44, 49, 50 at contact plane between soil and structure Deformation Survey equipment transits 1, 6, 9, 16, 18, 19, 22, measurements; horizontal theodoloites, electronic 23, 24, 26, 27, 28, 29, and vertical distance measurement 33, 35, 36, 41, 42, 43, equipments (EDME), and 44, 45, 46, 47, 48, 49, levels 50, 53, and numerous others Internal deformation, Extensometers, 4, 8, 9, 10, 15, 16, 19, rotational and tilting inclinometers, tiltmeters 20, 22, 26, 27, 32, 41, 42, 43, 44, 47, 48, 53 Load and strain Strain meters Load cells 2, 5, 7, 8, 12, 13, 14, measurement: Surface Concrete stress cells 15, 16, 18, 19, 22, 26, installation or embedded 27, 32, 33, 38, 39, 40, 44, 45, 47, 48, 49, 51, 52 Temperature measurement Temperature sensors 8, 16, 27, 32, 42, 44, 48, 49, and numerous others Seepage Weirs, flow meters, and Local flumes SOURCE: Adapted from Dunnicliff (1981). METHODS OF INSTALLATION Proposed methods and procedures suggested for installation of the various instrumentation types are presented in the U.S. Army Corps of Engineers (1971, 1976, 1980) and U.S. Bureau of Reclamation (1974). A timely reference is Geotechnical Instrumentation for Monitoring Field Performance (National Research Council 1982). Chapter 5 of that report presents numerous instrumentation types and installation procedures. A caveat is expressed, however, concerning the application of the recommendations
INSTRUMENTATION 296 TABLE 10-3 Names and Addresses of Manufacturers and North American Suppliers 1. A & S Co., 9 Ferguson Street, Milford, MA 01757 2. Ailtech, 19535 E. Walnut Drive, City of Industry, CA 91748 3. Apparatus Specialties Co., Box 122, Saddle River. NJ 07458 4. Bison Instruments, Inc., 5708 West 36th Street, Minneapolis, MN 55429 5. BLH Electronics, 42 Fourth Avenue, Waltham, MA 02254 6. *Borros Co., Ltd., Box 3063, S-17103 Solna 3, SWEDEN (NA supplier: Roctest, Ltd.) 7. Brewer Engineering Lab., P.O. Box 288, Marion, MA 02738 8. Carlson Instruments, 1190-C Dell Avenue, Campbell, CA 95008 9. *Coyne et Bellier, 5 Rue d'Heliopolis, 75017 Paris, FRANCE (NA supplier: Roctest, Ltd.) 10. Eastman Whipstock, Inc., P.O. Box 14609, Houston, TX 77021 11. Engineering Laboratory Equipment, Inc. (ELE), 10606 Hempstead, Suite 112, Houston, TX 77092 12. Evergreen Weight, Inc., 15125 Highway 99, Lynnwood, WA 98036 13. *Gage Technique, Ltd., P.O. Box 30, Trowbridge, Wilts, England (NA supplier: Terra-metrics, Inc.) 14. Gentran, Inc., 1290 Hammerwood Avenue, Sunnyvale, CA 94086 15. Geokon, Inc., 7 Central Avenue, West Lebanon, NH 03784 16. *Geonor, Grini Molle, P.O. Box 99, Roa, Oslo 7, Norway (NA suppliers: ELE, Roctest Ltd., Slope Indicator Co., and Terrametrics) 17. Geotechniques International, Inc., P.O. Box E, Middleton, MA 01949 18. *Franz Gloetzl, D-7501 Forchheim, Baumesstechnik, West Germany (NA suppliers: Terrametrics, Inc., and Roctest Ltd.) 19. Hall, Inc., 1050 Northgage Drive, San Rafael, CA 94903 20. Hamlin, Inc., Lake and Grove Streets, Lake Mills, WI 53551 21. Hitec Corporation, Nardone Industrial Park, Westford, MA 01886 22. *Huggenberger AG Zurich, Hohlstrasse 176, CH-8040, Zurich, Switzerland (NA supplier: Slope Indicator Co.) 23. Hewlett-Packard: contact local office 24. *Dr. Ing. Heinz Idel, Potthoffs Borde 15, 43 Essen, West Germany (NA supplier: Terra-metrics, Inc.) 25. *Ingenjorsfirman Geotech AB, Varslevagen 39, S-43600, Askin, Sweden (NA supplier: Roctest, Ltd.) 26. *Interfels, Zweigniederlassung, Bentheim, West Germany (NA supplier: Roctest, Ltd.) 27. Irad Gage, Etna Road, Lebanon, NH 03766 28. Kern Instruments, Inc.: contact local office 29. Keuffel & Esser Co.: contact local office 30. Landtest Ltd., 43 Baywood Road, Rexdale, Ontario, Canada M9V 3Y8 31. *Linden-Alimak AB, S-93103 Skelleftea, Sweden (NA supplier: Burcan Industries, 1255 Laird Boulevard, Montreal, Quebec, Canada H3P 2T1) 32. *H. Maihak, 2000 Hamburg, 39 Semper Street, Hamburg. West Germany (NA suppliers: Ampower Corporation, 1 Marine Plaza, North Bergen, NY 07047 and Roctest, Ltd.) 33. W. H. Mayes & Sons, Ltd., Vansittart Estate, Arthur Road, Windsor, Berkshire, England 34. Micro-Measurements, Box 306, 38905 Chase Road, Romulus, MI 48174 35. Walter Nold Company, 24 Birch Road, Natick, MA 01760 36. Petur Instrument Company, Inc., 11300 25th Avenue NE, Seattle, WA 98125 37. Piezometer R & D, Inc., 33 Magee Avenue, Stamford, CT 06902 38. Prewitt Associates, Dawson Building, 1634 N. Broadway, Lexington, KY 40505
INSTRUMENTATION 297 without proper guidance by a competent engineer for an evaluation of the materials in which the instruments will be installed and the purpose and period for which monitoring is required. 39. Proceq, SA, Riesbachstrasse 57, CH-8034, Zurich 8, Switzerland 40. Remote Systems, Inc., P.O. Box 12914, Pittsburgh, PA 15241 41. Roctest Ltd., 665 Pine, St. Lambert (Montreal), Quebec, Canada J4P 2P4; also Roctest, Inc., 7 Pond Street, Plattsburgh, NY 12901 42. Slope Indicator Co., 3668 Albion Place North, Seattle, WA 98103 43. *Peter Smith Instrumentation Ltd., Gosforth Industrial Estate, Newcastle Upon Tyne NE3 1XF, Gosforth, England (NA supplier: Roctest, Ltd.) 44. *Soil Instruments, Ltd., Bell Lane, Uckfield, East Sussex, TN22, 10L, England (NA supplier: Solinst Canada, Ltd., 5-2440 Industrial Street, Burlington, Ontario, Canada L7P 1A5) 45. Soiltest, Inc., 2205 Lee Street, Evanston, IL 60202 46. Spectra-Physics, Inc., 1250 W. Middlefield Rd., Mountain View, CA 94042 47. Structural Behavior Engineering Laboratories (SBEL), P.O. Box 23167, Phoenix, AZ 85063 48. *Telemac, 17 Rue Alfred Roll, 75 Paris 17eme, France (NA Supplier: Roctest, Ltd.) 49. Terrametrics, Inc., 16027 West 5th Avenue, Golden, CO 80401 50. Terra Technology Corporation, 3860 148th Avenue, N.E., Redmond, WA 98052 51. Texas Measurements, Inc., P.O. Box 2618, College Station, TX 77840 52. Transducers, Inc., 14030 Bolsa Lane, Cerritos, CA 90701 53. Westbay Instruments, Ltd., Suite lB, 265-25th Street, West Vancouver, British Columbia, Canada V7V 4H9 54. Wild-Heerbrugg Ltd: contact local office * Contact North American (NA) supplier, not manufacturer. SOURCE: Dunnicliff (1981). The following listed chapters from the referenced material cover details for installation of the instrumentation most basic for monitoring dams: ⢠U.S. Corps of Engineers Manual, EM 1110-2 1908, Part 1 of 2, 31 Aug 71: Instrumentation of Earth and Rockfill Dams (Groundwater and Pore Pressure Observations); Chapter 5, Installation, Maintenance of Piezometers, and Observations. ⢠U.S. Corps of Engineers Manual, EM 1110-2-1908, Part 2 of 2, 19 Nov. 76: Instrumentation of Earth and Rockfill Dams (Earth Movement and Pressure Measuring Devices); Chapter 2, Movement Devices for Embankments and Foundations. ⢠U.S. Corps of Engineers Manual, EM 1110-2-4300, 15 Sept. 1980: Instrumentation for Concrete Structures, Chapter 3, Uplift and Leakage, and Chapter 4, Plumbing Instruments and Tilt Measuring Devices.
INSTRUMENTATION 298 ⢠National Research Council, Geotechnical Instrumentation for Monitoring Field Performance, April 1982; Chapter 5, Details of Instrumentation. Adding instrumentation to existing dams will require specialized equipment 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 samples 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 deficiencies at dams. For example, borehole cameras and periscopes have been used successfully in examining concrete structures and foundations. Television 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 embankments 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 hydrophone, 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 equipment, 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 employment 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 become 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 inspecting with closed- circuit television, a hydrophone 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 (Jansen 1968).
INSTRUMENTATION Figure 10-6 Piezometer heads and contours in embankment at end of construction, Table Rock Dam. Source: U.S. Army Corps of Engineers (1971/1976). 299
INSTRUMENTATION 300
INSTRUMENTATION Figure 10-7 Piezometric heads and contours in the foundation, Waco Dam. Source: U.S. Army Corps of Engineers (1971/1976). 301
INSTRUMENTATION 302 DATA COLLECTION AND ANALYSIS The reason for installing instruments in dams is to monitor them during construction and operation. One of the specific applications of measurements is to furnish data to determine if the completed structure will continue 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 influencing 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, concrete testing data, and periodic instrument evaluations. The display of data should be both tabular and graphical and should be simple and readily understood. The data should be reviewed periodically by a professional engineer 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 embankment 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 evaluate 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 5-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 observations be made at times of varying heads. The movement measuring devices should be read immediately after installation, since all subsequent readings will be referred to the initial reading. 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 settlement for several years. Internal movement instruments should be installed inside a dam before the embankment is topped out to provide a history of
INSTRUMENTATION 303 deformations. The reading of these instruments should continue for years to establish performance during operation. Figure 10-9 shows a plot of horizontal displacements versus pool levels at E1 Infiernillo Dam (Marsal and de Arellano 1972). The observations of the well-instrumented E1 Infiernillo Dam led the authors to conclude that the unexpected behavior that developed two years after the first filling of the reservoir was the result of the Figure 10-8 Displacements and settlements of central points at crest. Source: U.S. Army Corps of Engineers (1971/1976).
INSTRUMENTATION 304 TABLE 10-4 Frequency of Readings for Earth Dam Instrumentation Progress Report During Periodic Report Operation Construction Frequency of Readings Frequency of Readings Construction Shutdown First Year Regular Piezometer Twice Monthly Monthly Monthly readings monthly (separate gages) Piezometer Monthly Alternate Approximately Annually readings months 6 months after on same (master gage) completion of date as a dam set of separate gage reading Porous tube Twice Monthly Monthly Monthly piezometer monthly readings Internal Complete set Monthly Complete set Every 2 vertical and of readings approximately 6 years horizontal each time a months after movement unit is dam is readings installed completed (crossarm or HMD) Foundation Complete set Monthly Approximately Every 2 settlement of readings 6 months after years readings each time an dam is (baseplates) extension is completed added Measurement Monthly, if Monthly, if Approximately Every 2 pointsâ required, or required 6 months after years cumulative when dam is dam is settlement and completed completed deflection readings Measurement Monthly as Monthly Approximately Every 2 pointsâ portions of 6 months after years cumulative structures are structure is settlement and completed completed deflection readings spillway and outlet works Measurement Monthly as Monthly, if Approximately Every 2 pointsâ slabs on required 6 months after years cumulative structure are structure is settlement completed completed readings- spillway floor slabs SOURCE: U.S. Bureau of Reclamation (1974).
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 changes in stress with time. Data from earth pressure cells should be reduced and time plots maintained for each cell during and after construction. Analysis of data should include 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 instrumentation. 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 Figure 10-9 Horizontal (river direction) displacements versus pool levels, Monument 10. Source: Marsal and de Arellano (1972).
INSTRUMENTATION 306 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 instruments 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. TABLE 10-5 Frequency of Readings for Concrete Dam Instrumentation Type Reading Frequency Embedded instrument Seven to 10 days during construction; semimonthly or monthly afterward. More frequently during periods of reservoir filling or rapid drawdown. Deflection measuring devices Weekly; closer intervals during periods of special interest. Uplift pressure measurement Monthly, except for initial filling, which should be a 7- to 10-day interval. Target deflection and pier net Semiannually during period of triangulation measurements minimum and maximum air temperature. Additional measurements during early stages of reservoir filling. Leveling across top of dam and vicinity 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 (1977). 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 remaining measurements the interval between successive readings may be lengthened. The U.S. Corps of Engineers' Engineering Manual (1980) discusses instrumentation to measure the structural behavior of concrete gravity dams under five major headings: (1) Carlson-type instruments, (2) uplift 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).
INSTRUMENTATION 307 REFERENCES 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. CT11 (November), pp. 1095-1104. Dunnicliff, J. (1981) Measurements Committee Report, U.S. Committee on Large Dams Section VI, Inventory of Geotechnical Instruments, Manufacturers or Suppliers. International Commission on Large Dams (ICOLD) (1969) General Considerations Applicable 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 Instantaneous Safety Control of Dams and Reservoirs, Bulletin No. 41, January. Jansen, R. B. (1968) A Prescription for Dam SafetyâInstrumentation and Surveillance, Conference of College of Engineering, University of California, Berkeley. Legget, R. F. (1967) "Reservoirs and Catchment Areas," Chapter 14 in Geology and Engineering, 2d ed., McGraw-Hill International Series, New York. Lytle, J. D. (1982) Dam Safety Instrumentation; Automation of Data Observations, Processing 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 I and 2, EM 1110-2-1908, August 1971 and November 1976. U.S. Army Corps of Engineers (1980) Instrumentation for Measurement of Structural Behavior of Concrete Structures, EM 1110-2-4300, September. U.S. Bureau of Reclamation (1974) Earth Manual, 2d ed., Government Printing Office, Washington, DC. U.S. Bureau of Reclamation (1976) "Design of Gravity Dams," Chapter XIII in Design Manual 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 Related 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 Engineering, Casagrande Volume, John Wiley & Sons, New York. Recommended Reading ASCE (1981) Conference Proceedings on Recent Developments in Geotechnical Engineering for Hydro Projects, Fred Kulhawy, ed.
INSTRUMENTATION 308 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 Institute of Technology, Zurich. O'Rourke, J. 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, J. M., and Carlson, R. W. (1965) Measurement of Structural Action in Darns, 3d ed., James J. Gillick and Co., Berkeley, Calif. Sarkaria, G. S. (1973) Proceedings of the Engineering Foundation Conference on Inspection, 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, General 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.
