Landslide monitoring

Why monitor a landslide?

Landslides are one of the most complex geotechnical phenomena to analyse and anticipate. Each year, they cause considerable damage to road and rail infrastructure, networks, buildings, and retaining structures.

Most landslides do not occur instantaneously; they generally result from a gradual evolution, characterised by phases of deformation that are more or less prolonged before the appearance of a rupture.

Landslide monitoring relies on the use of various techniques and instruments that allow for continuous observation of slope behaviour and detection of the first signs of instability.

Slope stability monitoring is based on geotechnical instrumentation consisting notably ofinclinometers, piezometers, and soil moisture sensors, designed to continuously track displacements, ground deformations, as well as the hydrogeological conditions influencing slope stability.

Understanding slope stability

The stability of a slope depends on the balance between the factors that promote landslides and those that allow the soil to remain in place.

Among the main factors likely to destabilise a slope :

• the increase in interstitial pressures ;

• prolonged rainy episodes ;

• the variations in groundwater level ;

• erosion at the foot of the slope ;

• earthquakes ;

• freeze-thaw cycles ;

• earthworks ;

• overloads at the top of the slope.

In many cases, the increase in water pressure in the soil pores reduces effective stresses according to Terzaghi's principle. As the soil loses strength, the risk of landslide gradually increases.

Displacements : an essential but insufficient indicator

The measurement of displacements generally constitutes the first parameter monitored on a landslide.

However, the absolute value of the displacement is often not the most relevant information.

Geotechnical engineers are mainly interested in :

• the rate of displacement ;

• accelerations ;

• differential displacements ;

• the location of active zones.

The analysis of speeds allows in particular to identify the different phases of evolution of a landslide :

Schéma explicatif simplifié

The early identification of these changes in behaviour is one of the main objectives of an instrumented monitoring system.

Pore water pressure: one of the main causes of many landslides

In fine soils and weathered materials, variations in interstitial pressure play a decisive role in triggering instabilities.

An increase in water pressure directly reduces effective stresses and therefore the mechanical resistance of the ground.

This is why piezometers play a central role in the geotechnical instrumentation of unstable slopes.

The joint analysis of piezometric data and displacement measurements often allows for the identification of very strong correlations between episodes of hydraulic recharge and the accelerations observed on the ground.

In certain contexts, piezometric variations can even serve as a precursor indicator several days before the occurrence of significant movements.

Rainfall measurement 

Ground deformation and crack opening

The appearance of cracks at the crest of a slope or in structures located near a landslide often constitutes one of the first visible signs of destabilisation. Their evolution can reveal a progressive movement of the ground and allow for the identification of phases of acceleration of the phenomenon. Continuous monitoring of crack openings using crackmeters allows for the quantification of displacements and the anticipation of a potential worsening of instability.

Structural inclination and rotation

Ground movements frequently generate progressive rotations of structures located on the slope.

Retaining walls, buildings, or engineering works can exhibit very slight angular variations but are indicative of an evolution of the phenomenon.

Inclinometers record angular variations, (and/or informative in mm) and provide a geometric dimension to linear displacement measurements.

The value of a multi-parameter approach

One of the most common mistakes is to monitor a landslide using only one type of sensor.

Experience shows that the understanding of a geotechnical phenomenon relies primarily on the correlation of several parameters :

• displacements ;

• crack openings ;

• inclinations ;

• pore pressures ;

• water table levels ;

• soil moisture ;

• precipitation ;

• temperature.

This multi-parameter approach not only allows for the detection of anomalies but also helps to better target the mechanisms responsible for the observed movements.

From Monitoring to Alerting

The objective of a monitoring system is to provide reliable information that allows for understanding the evolution of a phenomenon and aids in decision-making.

The analysis of trends, accelerations, and correlations between the different parameters allows for the definition of alert thresholds tailored to the specific behaviour of the monitored site.

These early warning systems today play a major role in the protection of infrastructure, property, and people exposed to natural risks.

FEELBAT Monitoring Solutions for Landslides

FEELBAT develops a range of connected sensors dedicated to geotechnical monitoring and the surveillance of ground movements. The aim is to provide infrastructure managers, engineering firms, and experts with reliable data to track the evolution of unstable slopes and better understand the mechanisms behind landslides.

Depending on the stakes of the site and the phenomena to be monitored, several technologies can be deployed :

DELTA L+: monitoring of crack evolution and relative displacement;

DELTA R: monitoring of tilts and rotational movements;

DELTA X-P: groundwater level monitoring;

DELTA X-Hs: soil moisture monitoring;

DELTA X-Ra: rainfall monitoring.

All sensors also include temperature measurement, enabling correlations between observed movements and environmental conditions.

Data is automatically transmitted to a centralized platform providing real-time visualization, trend analysis, automated reporting, and customizable alert management.

Thanks to their wireless design, FEELBAT sensors can be deployed quickly in the field with minimal maintenance requirements. Depending on the selected configuration, their battery life can reach several years, ensuring continuous long-term monitoring.

Multiple communication protocols are available to adapt to site-specific network coverage constraints: Sigfox, LoRaWAN, LTE-M and 4G.

Sensors can be installed individually or in a mesh to cover extensive areas and cross-reference different monitoring indicators. This multi-sensor approach provides a more comprehensive view of slope behaviour and better identifies the factors that may influence its stability.

Connected monitoring is now an essential tool for detecting significant changes, tracking the activity of a landslide, and anticipating risks.

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