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Chapter 8 — Deformation Monitoring: Survey and Geotechnical

8.1 Movement is the bottom line

Ultimately, a tailings failure is movement — of the crest, the beach, or the deposit. Deformation monitoring detects movement before it becomes a failure.

8.2 Surface deformation

  • Survey monuments on crest, beach, and toe measured by total station, GNSS, or leveling reveal settlement and lateral displacement.
  • Robotic total stations track prisms automatically and can alarm on threshold exceedance.
  • Drone photogrammetry builds repeatable surface models to map broad, low-rate movement and beach change.

8.3 Internal deformation

Some of the most dangerous movement is internal — a sliding surface within the deposit or foundation that no surface monument reveals.

  • Inclinometers (probe or in-place) detect lateral shear and the depth of a shear plane.
  • Extensometers (rod, magnetic, wire) measure settlement or heave at depth.
  • Multi-point borehole extensometers (MPBX) combine several depths in one borehole.
  • Tiltmeters sense rotation of crest or structure.

8.4 Choosing depth vs. surface

Surface survey catches gross movement; geotechnical instruments catch the internal shear and settlement that precedes it. For high-consequence facilities, use both, with geotechnical instruments placed along credible shear surfaces identified in the failure-mode analysis (Chapter 2).

8.5 Frequency and automation

During raises, storms, or seismic events, movement can accelerate within hours. Automated survey and in-place inclinometers provide the temporal resolution that manual surveys cannot. Pair them with trigger levels (Chapter 13).

Correlate movement with pore pressure

The most informative interpretation correlates deformation with pore pressure: movement under rising \(r_u\) (Chapter 6) is far more alarming than movement under stable pore pressure.

8.6 Common pitfalls

  • Monuments placed only on the crest miss beach and toe movement.
  • Inclinometer casing not grouted properly reads the wrong zone.
  • Manual surveys too infrequent to catch rapid change.

8.7 Key takeaways

  • Movement — surface and internal — is the ultimate failure signal.
  • Use both surface survey and internal geotechnical instruments.
  • Correlate deformation with pore pressure for context.
  • Automate where movement can accelerate quickly.