The Lugeon Test: Methodology, Interpretation, and Applications in Geotechnical Engineering

 Dam instrumentation plays a pivotal role in understanding the foundation and structural behavior of dams during both their construction and operation. An effective monitoring program provides the necessary information to evaluate a dam's performance, reassure dam owners and stakeholders, and detect critical changes in performance, as the dam owner is directly responsible for any consequences of a dam failure. The importance of a well-planned monitoring installation for maintenance and operation, especially in aging structures, is widely accepted and often legally mandated.

India currently ranks third globally with 5,264 large dams in operation and another 437 under construction, with approximately 80% of existing dams being over 25 years old. Their health and safety are of paramount importance for the sustainable use of these assets and for protecting downstream populations and property.

Purpose and Use of Instrumentation in Dams

The primary purpose of instrumentation is to supply data that aids in evaluating the safety of a structure by collecting quantitative data on its performance and detecting problems at an early, preventable stage. This information can be crucial for ensuring the safe functioning of a dam, which is vital for economic benefit and public safety.

The uses of dam instrumentation include:

• Verifying Expected Behavior: Instruments confirm that a dam is behaving as designed, providing reassurance to dam owners and the public.
• Warning of Problems: Instrumentation can detect issues such as uplift pressures exceeding design values in gravity dams, excessive settlements in earth dams, or high tensile stresses, providing early warnings of potential problems.
• Defining and Analyzing Problems: When an issue arises, instrumentation data helps analyze and define the extent of the problem, such as determining if movement in a gravity dam is normal or increasing, and whether it originates in the dam or its foundation. This information is vital for planning and designing corrective measures.
• Evaluating Remedial Actions: For dams undergoing modifications to correct deficiencies, instrument readings taken before and after the work help analyze and evaluate the effectiveness of these remedial actions.
• Long-Term Health Assessment: Instrumentation facilitates the monitoring, evaluation, and long-term health assessment of dams, bringing uniformity and standardization to health monitoring practices.
• Understanding Dam Behavior: It provides a better understanding of the dam's foundation and structural behavior during both construction and operation.
• Risk Management: An effective dam safety monitoring program, utilizing instrumentation, is essential for dam owners to manage the risks associated with dam operation and maintenance.

Types of Instruments and Their Uses

Instruments are chosen and placed to answer specific questions about dam safety. The basic physical parameters crucial for assessing dam safety and performance include water pressure, seepage and leakage, movement, reservoir and tail water levels, and weather conditions. Other important data include local seismic activity, stress, strain, and temperature.

Here are some types of instruments and what they measure:

• Water Pressure:

  • Piezometers: These devices measure water pressure (pore pressure) at specific locations within dam bodies, foundations, and abutments. They can be open standpipe, closed standpipe (also called pore pressure cells), hydraulic, pneumatic, or vibrating wire types. Uplift pressure, the upward component of pore pressure, reduces the effective downward weight of the dam and affects stability, hence its measurement is critical.
  • Total Pressure Cells: Used to measure total stress within soil mass or at contact with structures or rock.

• Seepage and Leakage:

  • V-notch Weirs and Flumes: These hydraulic structures are installed across open channels to measure seepage flow rates by relating water depth above the crest to discharge. Any sudden change in collected seepage quantity without clear cause (like reservoir level or rainfall changes), or changes in turbidity, discoloration, sediment content, or chemical content, can signal a severe problem.
  • Seepage/Leakage Monitoring: Direct observation of seepage quantity and turbidity is simple, with internal drain systems often directed to calibrated weirs.

• Movement (Deformation and Displacement):

  • Plumb lines (Direct and Inverted): Measure the deformation of the dam and its foundation.
  • Inclinometers and Tiltmeters: Measure the deformation of the dam. Inclinometers measure a casing's inclination to the vertical at different points to detect settlement.
  • Joint Meters/Crack Meters: Measure movement along and across joints and cracks.
  • Extensometers (Borehole and Fill): Measure relative movement between the dam and abutment, dam foundation, or within weakness zones.
  • Survey Markers, Total Station, GPS: Used for measuring relative movement between abutment and dam, different blocks of the dam, and the absolute movement of the dam.
  • Settlement Gauges and Systems: Measure vertical movement and settlement within the embankment.

• Reservoir Water Level and Tail Water Elevations:

  • Staff Gauges: Simple elevation gauges for manual reading.
  • Automatic Water Level Recorders (AWLR) / Water Level Sensors: Complex devices that sense water level and allow for automated, frequent monitoring.
  • Radar Type Water Level Recorders: Modern instruments for precise water level measurement.

• Weather Conditions (Hydro-meteorological):

  • Rain Gauges (Ordinary, Natural Syphon, Weighing Type, Tipping Bucket Recording): Measure precipitation.
  • Automatic Weather Stations (AWS): Provide data on air temperature, atmospheric pressure, humidity, water temperature, ice thickness, and other environmental factors.
  • Evaporation Pans: Measure evaporation rates.

• Seismic Activity:

  • Strong Motion Accelerometers and Broadband Seismometers: Detect local seismic activity and measure ground motion and dynamic response of the dam to seismic forces. They are part of associated data acquisition and analysis systems. Seismological stations around the reservoir record seismic activity in the region.

• Stress and Strain:

  • Stress Meters and Strain Meters: Used to measure stress and strain at critical points within the dam body. Examples include Carlson stress meters.
  • Reinforcement Meters, Penstock Meters, Load Cells: Also used to measure stress and strain in specific components.

• Temperature:

  • Thermometers and Temperature Sensing Systems: Measure concrete temperature, air temperature, reservoir water temperature, and temperature gradients in concrete dams for thermal studies. Temperature sensors can provide valuable data on seepage flow time and source.

Instrumentation Planning and Implementation

Instrumentation system planning should be a logical, systematic process, starting with defining objectives and ending with predetermined actions based on the data obtained. Key steps involve determining appropriate measurements, deciding on the number and locations of measuring points, and selecting suitable hardware. For larger dams, it is advisable to consult with design specialists and manufacturers.

For existing dams, installing instrumentation can be more challenging and costly, especially for concrete/masonry dams where many instruments (like stress/strain meters, inverted pendulums) cannot be retroactively installed. However, piezometers can often be installed in existing dams. The program for instrumenting an old dam needs to be site-specific, considering the age, condition, purpose, strategic/locational/functional importance, and hazard potential of the structure.

Data Collection, Analysis, and Automation

Monitoring data analysis is crucial for evaluating dam safety. Dam design engineers should establish ranges or limiting values for instrumentation data to indicate acceptable performance. Exceeding these values should trigger investigations and corrective actions.

Data collection can be done manually or automatically.

• Manual Data Collection involves recording data in field books, paper forms, or handheld devices, noting complementary information like date, time, water levels, temperature, and precipitation.
• Automated Systems (ADAS) utilize electronic sensors, remote data loggers, and communication links (cell phone, landline, radio, satellite) to read and store data, reducing labor and elapsed time for interpretation. They can provide automatic warnings if limiting values are exceeded and allow for more frequent readings. While automated systems are sophisticated, they should never replace actual onsite monitoring and visual inspection.

Data maintenance is vital; for dams managed by government agencies, instrumentation data is required to be maintained for the life of the dam. The data should be presented in both tabular and graphical formats and periodically reviewed by a professional engineer.

Instrumentation Levels based on Hazard Potential

The level and frequency of instrumentation and monitoring vary based on several factors, including the dam's hazard potential, size, complexity, known problems, and foundation conditions.

• Hazard Classification: Dams are categorized into Hazard Classes (e.g., Class I, II, III, IV) based on the potential consequences of their failure.
• Monitoring Frequency:
  • For Dams in Hazard Class I (lowest hazard), automatic instruments are generally not required, and manual readings may be taken quarterly.
  • For Dams in Hazard Class IV (highest hazard), automatic instruments may be read daily, and manual readings twice a month or weekly.
  • During initial filling of a reservoir or when water levels are high, and after significant storms or earthquakes, more frequent (even daily) readings are suggested, regardless of the hazard class.
  • Medium and high dams generally require priority and denser instrumentation.
  • A weaker dam in distress needs closer observations and thus more instruments to monitor its behavior.

While the sources don't provide a list of how many instruments each specific dam has, they emphasize that the type and nature of the dam determine the number and locations of instruments to be embedded. The goal is a modest level of instrumentation for new dams to provide adequate warning of serious conditions, and for existing dams, to install what is necessary to evaluate safety and performance given their unique conditions and history.