Radiotelemetry wireline extensometer used for remote slope deformation monitoring

Radiotelemetry wireline extensometer for slope monitoring. Source: Wikimedia Commons (CC BY-SA 3.0)

Purpose of monitoring on active slopes

A monitoring programme on a coastal slope serves two distinct functions. The first is observational: it records whether a slope is stable, slowly creeping, or accelerating. The second is operational: it provides the data that triggers alarms or enables decisions about evacuation, road closure, or the activation of emergency stabilisation measures.

Monitoring alone does not stabilise a slope. Its value lies in reducing uncertainty — replacing speculation about slope behaviour with measured evidence. On Italy's more populated coastal stretches, where roads and buildings sit close to hillside edges, the ability to detect accelerating movement before collapse has direct safety implications.

ISPRA's risk classification system for landslides (PAI — Piano di Assetto Idrogeologico) identifies high-risk areas on Italian coasts requiring active monitoring. Several regional authorities in Liguria, Campania, and Calabria operate dedicated slope monitoring networks connected to civil protection alert systems.

Inclinometers

An inclinometer measures the tilt of a borehole at depth, detecting the lateral displacement profile of a slope. A guide casing is grouted into a borehole drilled through the suspected failure zone into stable ground below. A torpedo-shaped probe fitted with accelerometers is lowered through the casing at regular intervals and measures the inclination at each depth.

By comparing successive readings over time, the location and magnitude of lateral movement can be identified. When the casing begins to shear — at the failure surface — the probe can no longer pass through, marking the point at which the instrument's useful life on that installation has ended. On coastal slopes in southern Italy, inclinometer arrays of three to five casings are common for slopes of moderate size.

Automated inclinometers

In-place inclinometers (IPI) carry a fixed array of sensors inside the casing, transmitting readings automatically at programmed intervals. This eliminates the need for manual surveys and allows near-real-time monitoring. Data is typically transmitted via cellular modem to a remote server. The disadvantage is higher installation cost and the need for power at the instrument head.

Extensometers and crack gauges

A wireline extensometer connects an anchor grouted in stable ground to a reference head at the surface via a wire under constant tension. As the slope deforms, the wire length changes, and this change is recorded by a displacement transducer at the surface. The instrument shown above is a radiotelemetry type that transmits readings without requiring a site visit.

Crack gauges — simpler versions consisting of a steel gauge bridge mounted across a visible tension crack — are widely used as low-cost supplementary sensors. They do not require boreholes and can be installed quickly after a slope shows visible signs of distress.

Active coastal erosion on a sandy shoreline illustrating the scale of slope loss over time

Active coastal erosion. Source: Wikimedia Commons (CC BY-SA 2.0)

GPS and GNSS benchmarks

Precise GNSS surveying of fixed benchmarks on a slope provides three-dimensional displacement vectors over time. The benchmarks — steel pins or bolts grouted into stable outcrops — are occupied periodically with survey-grade receivers, or permanently instrumented with continuously operating GNSS stations.

A network of benchmarks distributed across a landslide body can distinguish between uniform movement (a single sliding block) and differential movement (multiple blocks or a rotational slide). On coastal slopes where the failure mechanism is not yet understood, the spatial pattern of displacement from a GNSS network can guide decisions about which zones to prioritise for more detailed investigation.

Satellite InSAR

Interferometric Synthetic Aperture Radar (InSAR) uses the phase difference between radar signals acquired on two or more satellite passes over the same area to measure surface displacement with millimetre-scale precision. The technique does not require any instrumentation on the ground: displacement maps are generated by processing publicly available satellite data from missions such as Sentinel-1 (ESA).

InSAR is particularly useful for retrospective analysis — detecting slow movement that was not captured by ground instruments — and for screening large areas to identify unstable zones before any infrastructure has been installed. The Copernicus Ground Motion Service, operated under the European Union's Copernicus programme, provides a publicly available dataset of ground motion over Europe including the Italian coastline, derived from Sentinel-1 data.

Alert thresholds and civil protection

The output of monitoring systems is only as useful as the decision framework that interprets it. For slopes near roads or settlements, alert thresholds are set in displacement velocity (e.g. mm/day or mm/week) with three levels: routine observation, enhanced monitoring, and emergency. These thresholds are agreed between the geotechnical engineer, the infrastructure manager, and the relevant civil protection authority.

In Italy, Decreto Legislativo 1/2018 establishes the legal framework for civil protection risk management including landslide events. Regional civil protection agencies (Protezione Civile) are responsible for activating and communicating alerts on monitored slopes in their jurisdiction.

Further reading