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<br /> <br /> <br /> <br />19 <br /> <br />Photo 3: Standpipe Piezometer <br />Settlement Monitoring <br />Typically, movement in dams is monitored to detect <br />settlement or deformations in the dam. These <br />conditions may be due to consolidation, creep, or <br />subsidence or other factor. Settlement within an <br />embankment dam may lead to loss of freeboard or <br />cracking due to differential settlement. Depressions, <br />sinkholes, scarps, sloughs or bulges, which may be <br />indicative of slope instability or internal erosion, are <br />often localized features that are more effectively <br />detected through frequent visual surveillance rather <br />than relying on periodic reading of instrumentation. <br />However, once identified, the known condition can be <br />more quantitatively monitored with instrumentation. <br />Instruments commonly used to monitor settlement <br />include survey monuments, settlement plates/sensors, <br />extensometers, piezometers, and inclinometers. <br />More Sophisticated Instruments <br />A large selection of instrumentation is available to <br />measure ground water and pore pressures; seepage, <br />flow, and turbidity; stress and strain; load; <br />temperature; precipitation and wind; reservoir and <br />tailwater levels; water quality; seismic measurements; <br />and deformation. Details of these types of instruments <br />are included in the ASCE Task Committee Guidelines <br />for Instrumentation and Measurements for Monitoring <br />Dam Performance (2000). <br />Planning an Effective Monitoring <br />Program <br />Identifying the Need for Instrumentation <br />Planning a monitoring program should begin by <br />identifying the need for instrumentation. The need <br />may be based on an observed condition or a known <br />vulnerability. Site conditions such as underground <br />mine workings, deep groundwater pumping, soft <br />foundation, or abrupt changes in the subsurface profile <br />could be known vulnerabilities that may lead to <br />subsidence or differential settlement. Observed <br />seepage, sediment deposition, or depressions, may all <br />be observed conditions that would warrant seepage <br />monitoring through collection and measurement, or <br />piezometers to evaluate seepage gradients. Signs of <br />slope instability (cracking, scarps, bulges) may be <br />justification to install piezometers to estimate internal <br />pore pressures for stability analyses. The need for <br />instrumentation is based on a comprehensive <br />understanding of the design and performance of the <br />dam. If an observed condition or known vulnerability is <br />identified, then one must assess whether it can be <br />reliably and efficiently monitored with instruments. If <br />so, which instrument is best suited for the specific <br />objective? <br />Selecting the Right Instrument <br />Once the need and specific monitoring objective is <br />identified, selecting the proper instrument is the next <br />step. Instrumentation should be selected based on the <br />answers to several pertinent questions: <br /> What is the observed condition, known <br />vulnerability, or PFM? <br /> What parameter would best monitor the <br />condition (flow, pore pressure, gradient, <br />deformation, etc)? <br /> Where would the instrument need to be <br />located to monitor the condition? <br /> How would the instrument need to be <br />installed?