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

Visual defects in a dam can indicate various issues affecting its safety and performance. These observations are crucial for identifying distress conditions and initiating appropriate emergency actions or repairs.

Key visual defects in dams include:

• Cracks:

• An incomplete separation into one or more parts, with or without space between.
• They are classified by direction (longitudinal, transverse, vertical, diagonal, random) and width.
• Width classifications are: Fine (less than 0.5 mm), Medium (between 0.5 mm and 2 mm), and Wide (over 2 mm).
• Cracks can be a sign of structural damage and may result from factors like temperature changes, shrinkage, settlement, or movement.
• Significant cracking often indicates displacement or misalignment of the structure.
• In concrete structures, cracks can lead to undermining by water seepage or piping, causing further structural damage and settlement.
• Metal cracking signifies a separation into parts, with deep or extensive cracking indicating potential failure.

• Seepage:

• The internal movement of water through the dam body, its foundation, or abutments, often emerging at the ground surface downstream.
• Unusual seepage or changes in the seepage regime are considered dam incidents.
• High turbidity (cloudiness) in seepage water is a dangerous sign, indicating the erosion of fine particles from within the dam body.
• The formation of springs downstream can also result from seepage through the foundation.
• Seepage issues can be aggravated over time and are closely monitored for discharge rates, color, and signs of pressure.

• Boils:

• A disruption of the soil surface caused by water discharging from below the surface.
• Eroded soil may form a ring (like a miniature volcano) around the disruption.
• Small boils with clear water and no increase in flow may indicate an "Internal Alert Condition BLUE," requiring constant monitoring.
• Boiling or artesian conditions observed below an earth dam are signs of a dam incident.

• Erosion and Scour:

  • Erosion is the deterioration caused by the abrasive action of fluids or solids in motion.
  • Abrasion erosion specifically refers to damage caused by the grinding action of waterborne silt, sand, gravel, rocks, and ice on concrete surfaces, resulting in a smooth, worn appearance.
  • Internal erosion is a broad term for processes where water moves within or adjacent to soil zones of embankment dams and foundations, leading to material loss. This includes processes like piping (progression of erosion through a conduit).
  • Scour is the loss of material at an erosional surface where concentrated flow is present, such as in a crack or at the dam/foundation contact. It can affect spillways and outlet works.
  • Overtopping failure of an embankment dam begins with erosion of the dam crest, which is very difficult to stop once it starts.
  • Evidence of erosion, such as rain cuts on embankment slopes or disturbed rip-rap, should be noted during inspections.

• Deterioration:

  • A general term for the decline in condition of dam materials or structures.
  • It can manifest as:
    • Spalling: Breaking or flaking of small fragments from concrete, masonry, or stone surfaces due to weather or abrasion.
    • Pitting: Formation of small holes or depressions on a surface, often due to cavitation.
    • Scaling: Local flaking or peeling of the near-surface concrete or mortar.
    • Dusting: Development of a powdered material on the surface of hardened concrete.
    • Corrosion: Disintegration or deterioration of concrete or reinforcement (e.g., steel) due to chemical attack or electrolysis.
    • Leaching: Removal of soluble material from concrete by water.
    • Alkali-Aggregate Reaction (AAR): A chemical reaction causing expansion and cracking in concrete.
    • Cavitation: Damage caused by the rapid formation and collapse of vapor bubbles in high-velocity water flows, resulting in pitting and holes in hydraulic surfaces like spillways or outlet works.
    • Disintegration: The breaking down of material into smaller components.
    • Delamination: Separation of a material into layers.
    • Joint Sealant Failure: Breakdown of materials used to seal joints between concrete sections.

• Movement and Deformation:

  • Observable changes in the shape or position of dam components.
  • These can include settlement, distortion, and vertical or horizontal displacement.
  • While large movements may be visually apparent, instruments like extensometers, inclinometers, and tilt meters are used to detect smaller, more precise movements.

• Other Physical Anomalies:

  • Discolouration: A change in the normal or desired color of a surface.
  • Sand Pocket: A localized area within concrete that contains sand but lacks cement, indicating poor mixing.
  • Segregation: The uneven distribution of components within mixed concrete, leading to non-uniform proportions in the mass.
  • Wet Spots: Localized damp areas on dam surfaces, often indicative of seepage.
  • Sinkholes: Depressions on the surface that suggest subsurface settlement or movement of particles.
  • Exposed Reinforcing: Visible steel reinforcement bars due to the loss or deterioration of surrounding concrete.
  • Vegetation: Unwanted plant growth on the dam embankment, which can hinder inspections and create pathways for seepage along root systems.
  • Obstructions: Accumulations of floating debris (such as wood logs, vegetation), ice, or sediments that block spillways, outlet works, or intake channels, reducing their discharge capacity.
  • Undermining: The erosion of material beneath a structure, leading to a loss of support.