Table 3

New monitoring technologies

Instrument/techniqueDescriptionAccuracyResolutionOther notes
TemporalSpatial
Surface deformation monitoring
Global positioning system (GPS)GPS system receives time signals from orbiting satellites and positioning is based on signal travel times.
Minimum of 4 satellites required for position calculation (i.e. x, y and z) accurate to c. 15 m.
Accuracy improvements can be achieved by using: (1) differential GPS: correction of atmospheric disturbances from comparison of GPS position with known fixed position of a base station; accuracy c. 0.1 m; (2) real time kinematic (RTK) GPS: for positioning the carrier phase of the signal is used rather than the actual time signal; accuracy <0.01 m.
Accuracy of RTK-GPS is required for monitoring of mass movements (Millis et al. 2008).
m to mmLow: manual repeated surveys
High: continuous monitoring on spatially fixed receivers
High: depending on number of monitoring locationsDependent on satellite coverage; reception limited in strong topographic depressions (e.g. alpine valleys).
Affected by signal scattering: limited accuracy in forested areas.
Considerable time requirement for full site surveys.
Instrumentation and processing software are of high cost.
Permanent installations have been used to monitor movements of large natural landslides causing damage to transport infrastructure (e.g. Massey et al. 2013).
Most likely to be used for applications (1), (3), (4) and (5) in Table 2
Photogrammetry3D reconstruction of surface topography from overlapping photographs taken from different positions (at least 2).
Accuracy mainly dependent on photograph resolution and number of overlapping photographs (i.e. number of shot positions per covered area, Bemis et al. 2014).
Both aerial (e.g. using manned or unmanned aircraft/aerial vehicles) and terrestrial photogrammetry can be used
m to mmLow: restricted by time requirements for photograph acquisition and data processing
High: continuous monitoring on permanently installed cameras
High: high accuracy point cloud/DEM, deformation monitoring for entire study siteApplication limited by high cost and time requirements.
Post-processing of data relatively complex (e.g. see Akca et al. 2011).
Widely used for digital terrain mapping and monitoring surface change for natural rock slopes and landslides; a small number of examples of application to infrastructure slopes (e.g. Jang et al. 2008).
Most likely to be used for applications (1), (4) and (5) in Table 2
Remote sensingTerrestrial-, aerial-, or satellite-based recording of reflected electromagnetic energy from the Earth's surface.
Typical examples used in investigations of surface deformation (Scaioni et al. 2014; Petley et al. 2005): (1) LiDAR (light detection and ranging): distance measurement employing backscattered energy of laser beam, used to create digital elevation models (DEMs); (2) InSAR (interferometric synthetic aperture radar): mapping of phase differences between reflected radar waves of different acquisition times, representative of surface deformation
m to mmMedium to low: restricted by time required for survey (i.e. in case of terrestrial and aerial surveys) and processingHigh: high accuracy point cloud/DEM, deformation monitoring for entire study siteApplication limited by high cost and time requirements (i.e. terrestrial and aerial surveys).
Post-processing of data relatively complex.
Temporal resolution dependent on satellite orbit (i.e. time between repeated data acquisition over same location).
Accuracy dependent on signal wavelength and atmospheric condition.
Positioned reflectors may be required to overcome seasonal changes in vegetation.
Aerial surveys (e.g. Miller et al. 2012) have been used to characterize and look at longer duration changes within infrastructure earthworks.
Most likely to be used for applications (1), (4) and (5) in Table 2
Fibre optics (e.g. Brillouin optical time domain reflectometry; BOTDR)Determination of locally applied strain to a single optical fibre cable by time-domain analysis of frequency spectra of backscattered light pulses (Thévenaz 2010).
Frequency shifts caused by changes in fibre density.
Time-domain analysis allows for determination of strain/deformation location.
Other optical fibre strain measurement approaches can be used, e.g. Bragg gratings (Glisic & Inaudi 2007)
Strain measurement: 0.2% (e.g. 2 mm for 1 m spatial resolution)High: continuous monitoring of permanent installationsHigh: cable layout can be adapted to site conditions to optimize coverage and resolutionNo absolute measure for displacements.
Relatively high cost.
Need for correction of temperature effects.
Complex processing required.
Can also be used subsurface, such as in a borehole.
Widely used to measure strain in structural elements, but no known applications to unreinforced transport infrastructure slopes.
Most likely to be used for applications (1), (3) and (4) in Table 2
Accelerometer, geophoneRecording of ground surface velocity or acceleration in response to: (1) earthquakes (i.e. as trigger for slope destabilization); (2) rapid (i.e. brittle) landslide movements.
Usually measured employing spring-mounted magnetic masses moving within wire coils generating electric signals. Microchip micro-electrical mechanical system (MEMS) accelerometers are widely used
Acceleration: 0.1 m s−2High: continuous monitoring of permanently installed sensorsLow to high: dependent on number and distribution of accelerometers or geophonesRecording of movement changes only; limited detection capability of low-velocity ductile movements (e.g. creep).
Extraction of movement periods from background noise may be difficult.
Requires complex post-processing.
No known applications for transport infrastructure slopes.
Most likely to be used for applications (1) and (3) in Table 2
Electrode tracking using electrical resistivity monitoringResistivity measurements are sensitive to the subsurface resistivity distribution and electrode separations.
Monitoring installations usually consist of either a line or grid of electrodes, with electrode spacing ranging from 0.5 to 5.0 m.
Measured resistivities can be inverted to track electrode, and thus landslide movement (Wilkinson et al. 2010, 2015), along a line or surface grid
5–10% of electrode spacing (e.g. 0.025–0.5 m, dependent on electrode layout)Medium to high: dependent on measurement layout; 2D lines can be measured hourly, 3D grids usually dailyMedium to high: dependent on measurement layoutAccuracy dependent on resistivity data quality.
Other data streams required to calibrate/confirm measurements.
Requires complex installation and post-processing.
High-cost measurement system.
The approach has been demonstrated using an installation installed within a natural landslide (Wilkinson et al. 2010).
Most likely to be used for applications (1) and (5) in Table 2
Subsurface deformation monitoring
Time domain reflectometry (TDR)Deployment of coaxial cables (or optical; see BOTDR) in vertical boreholes.
Measurement of reflections along a conductor.
Localized deformation of coaxial cable leads to local impedance contrast at which a pulse is reflected.
Time-domain analysis allows for determination of deformation location.
Rate of impedance change is indirectly proportional to ground movement rate (Kane et al. 2001; Millis et al. 2008)
cm to mm (dependent on cable length)Low: manual surveys using portable pulse generators.
High: continuous monitoring of permanently installed systems
Low to medium: depending on whether used in single borehole or borehole networkNo direct measurements of deformation or deformation rate.
Costs range from low (infrequent, manual surveys) to high (continuous, permanent monitoring or borehole network).
Sold as a commercial system, and has been installed into numerous natural and engineered slopes (Kane et al. 2001).
Most likely to be used for applications (1), (2), (3) and (5) in Table 2
Shape acceleration array (SAA)Comprises a string of MEMS sensors, installed inside boreholes.
Sensors are placed at regular intervals.
Each section of the array measures 3D displacements (Abdoun et al. 2013)
±1.5 mm per 30 m array lengthHigh: continuous monitoringLow to medium: depending on whether used in single borehole or borehole networkInstrumentation and processing software are of high cost.
SAA string can be retrieved from the borehole.
Can provide early warning of slope instability.
Care should be taken with processing software (Buchli et al. 2016).
Sold as a commercial system and has been used fairly widely in stable and unstable infrastructure slopes (e.g. Dixon et al. 2015).
Most likely to be used for applications (1), (2), (3) and (5) in Table 2
Active waveguide and slope ALARMS sensor (i.e. acoustic emission monitoring)Comprises a steel waveguide (i.e. as conductor for acoustic emission signals) and angular granular backfill.
Host slope deformation causes deformation of granular backfill, creating high-energy acoustic emission (AE) signals travelling along the waveguide (Dixon et al. 2003).
AE rates are proportional to slope movement rates, highlighting accelerations and decelerations of movements (Smith et al. 2014b; Dixon et al. 2015; Smith & Dixon 2015)
Differentiation of movement rates that differ by an order of magnitude (e.g. 0.01 and 0.1 mm h−1)High: continuous monitoringLow to medium: depending on whether used in single borehole or borehole networkSensitive to slow rates and small displacements.
Most applicable to slopes failing along a defined shear surface.
Relatively low-cost instrumentation.
Can provide early warning of slope instability.
Emerging technology; has been trialled in a clay cutting slope (Dixon et al. 2015) and at the BIONICS facility (Glendinning et al. 2014), with a number of other installations in natural landslides.
Most likely to be used for applications (1) and (3) in Table 2
Electrical resistivity tomography (ERT)ERT measurements consist of electrodes placed at the surface and/or in boreholes.
Resistivity is sensitive to the subsurface lithology, e.g. clay content; inverted resistivity models represent a volumetric image of the local lithology.
Temporal changes in the resistivity distribution can inform about mass movements.
Changes can be quantified using emerging boundary extraction algorithms (e.g. Chambers et al. 2015; Uhlemann et al. 2016)
m to cm, dependent on data quality and depth of changesMedium to high: varies between daily and hourly, depending on measurement layoutMedium to high: depending on measurement layout (i.e. 2D or 3D acquisition)Measurement sensitivity reduced with increasing distance to electrodes.
Complex installation and processing required.
Used to measure ground movements for a range of applications, including natural landslides; no known applications to transport infrastructure slopes.
Most likely to be used for applications (1), (2) and (5) in Table 2
Subsurface condition monitoring
Conventional soil moisture probesBased on relative permittivity measurements, which are related to moisture content using Topp's equation (Topp et al. 1980).
Main techniques: (1) time-domain reflectometry (TDR): relative permittivity derived from the travel time of an electromagnetic pulse through a waveguide; (2) capacitance sensors: relative permittivity determined based on the charging time of a capacitor, employing the soil as dielectric
Relative permittivity: ±1; moisture content: ±3% of measurementHigh: continuous monitoring on permanently deployed sensorsLow to medium: sensor samples only surrounding medium, can be increased if used in sensor networksMoisture content derived through empirical relationships.
Usually requires calibration.
Robust and reliable sensor technology.
Latest developments include web-based real-time delivery of multi-location moisture data from sensor networks at field sites.
Several commercially available devices; fairly widely used to measure soil moisture content in the near-surface zone of infrastructure slopes (e.g. Smethurst et al. 2012; Glendinning et al. 2014).
Most likely to be used for applications (1), (3), (4) and (5) in Table 2
Electrical resistivity tomography (ERT) monitoring of soil moistureThe resistivity of a soil depends mainly on its mineralogy and degree of saturation.
Laboratory-derived relationships can be used to translate resistivity into moisture content.
Repeated ERT surveys on permanently installed electrodes can be used to image volumetric moisture movements (e.g. Chambers et al. 2014; Gunn et al. 2015).
ERT could also be used to monitor cavity development
Moisture content: < ±5%Medium to high: varies between daily and hourly, depending on measurement layoutMedium to high: depending on measurement layout (i.e. 2D or 3D acquisition)Measurement sensitivity reduced with increasing distance between electrodes.
Complex installation and processing required.
Measurement accuracy dependent on resistivity data quality.
Several installations have been used to image moisture changes in clay infrastructure slopes (Glendinning et al. 2014; Gunn et al. 2015). Many other examples of use in natural slopes.
Most likely to be used for applications (1), (4) and (5) in Table 2
High-capacity porewater suction probesProbes consist of (1) filter, acting as interface between soil and measurement device, (2) water reservoir and (3) pressure measuring device.
Recent improvements of measurement range and accuracy through reduction of water reservoir and higher air entry pressures of the ceramic filter (Toll et al. 2011, 2013).
Allows suction measurements in the range of 0 – 2000 kPa
Porewater pressure/suction: > ±5 kPaHigh: continuous monitoring of permanently installed sensorsLow to high: dependent on number and distribution of probesLimited accuracy if applied at low suctions.
Long-term measurement drift may occur.
Laboratory re-saturation necessary if water reservoir dries out.
Probes have been trialled in a clay embankment in the UK (Toll et al. 2011, 2013).
Most likely to be used for applications (1), (3), (4) and (5) in Table 2
Probes for indirect measurements of porewater suctionProbes consist of a soil moisture device encapsulated within a porous ceramic of known water retention properties. Soil moisture in ceramic measured, and related to suction in the soil.
Accuracy is dependent on correct calibration between suction and moisture content of ceramic (Smethurst et al. 2012)
Porewater suction: high readings ± 10%High: continuous monitoring on permanently deployed sensorsLow to medium: sensor samples only surrounding medium, can be increased if used in sensor networksRequires careful calibration.
Generally robust sensor technology.
Latest developments include web-based real-time delivery of multi-location suction data from sensor networks at field sites.
Several commercially available devices; fairly widely used to measure porewater suction in the near-surface zone of infrastructure slopes (e.g. Smethurst et al. 2012; Glendinning et al. 2014).
Most likely to be used for applications (1), (3), (4) and (5) in Table 2
Ground penetrating radar (GPR)Measurement based on the propagation of electromagnetic waves in the subsurface, i.e. wave speed dependent on dielectric properties.
Use of non-guided waves (in contrast to TDR where guided waves are used).
Properties of reflected, ground, and cross-borehole waves can be used (Huisman et al. 2003; Steelman et al. 2012).
GPR could also be used to characterize (Di Prinzio et al. 2010) and monitor cavity development
Moisture content: > ±0.02 m3 m−3Low to medium: manual surface or borehole surveysMedium to high: depending on measurement layout and employed frequencyHigh-cost measurement system.
Requires complex post-processing.
Limited applicability in highly conductive soils (i.e. clay) owing to attenuation of the GPR signal.
Commonly used to establish ballast depth in railway formations. Used by Donohue et al. (2011, 2013) to investigate an old clay railway embankment.
Most likely to be used for applications (1), (4) and (5) in Table 2