Chapter 8 — Deformation Monitoring: Survey and Geotechnical Sensors

8.1 Movement is the ultimate signal

Deformation — movement of the dam or foundation — is often the most direct sign of distress. It is monitored both at the surface (survey) and internally (geotechnical sensors).

8.2 Surface deformation — survey

A survey control network of monuments on the dam crest, slopes, and abutments is the backbone of deformation monitoring:

• Total station / GNSS measures horizontal and vertical position to millimeters.
• Frequency ranges from monthly in steady state to daily during construction or after an event.
• Datums must be on stable ground outside the zone of influence.

Survey reveals settlement, crest shortening, and slope bulging.

8.3 Internal deformation — inclinometers

An inclinometer measures lateral displacement of a casing installed in a borehole. It detects shear surfaces (the depth where movement concentrates) — the signature of slope instability or internal erosion.

• Install casing through the zone of concern, anchored below it.
• Read from the bottom up; the bottom is the reference.
• Casing integrity is critical: a cracked or offset casing gives false readings.

8.4 Internal deformation — extensometers and settlement systems

• Extensometers (rod, magnetic, wireline) measure movement between an anchor deep in stable ground and a point nearer the surface — detecting settlement or heave at specific depths.
• Settlement systems (liquid, magnetic, settlement gauges) track vertical movement across an embankment.

8.5 Tiltmeters and crackmeters

• Tiltmeters sense rotation of a surface; useful on concrete structures and steep slopes.
• Crackmeters / joint meters measure opening of cracks or joints in concrete dams, abutments, and foundations.

8.6 Combining measurements

Deformation is most powerful when correlated: a surface survey shows where the dam moved; an inclinometer shows at what depth; piezometers show why (pore-pressure change). Together they localize the mechanism.

Build a deformation baseline early

Embankments settle as they consolidate; the first years define the normal settlement rate. Later acceleration stands out clearly against that baseline.

Figure. Deformation monitoring: crest survey monuments (settlement, downstream movement) plus an inclinometer for internal shear (ASCE MOP-135 scope).

8.7 Key takeaways

• Deformation is the most direct distress signal; monitor it inside and out.
• Survey networks give surface movement; inclinometers reveal shear depth.
• Extensometers and settlement systems give depth-resolved vertical movement.
• Correlate deformation with piezometers to identify mechanism and location.