Table 2

Applications for instrumentation and monitoring of infrastructure slopes

ApplicationObjectives of monitoring schemeWhat to monitor? And number of instrumentsFrequency of readings, and duration of monitoringAnalysis and interpretation of dataExample case histories
(1) Monitoring the condition of problem slopes (including earthworks that are subject to significant changes in loading or profile, and ensuring function of remedial measures)To understand the depth and extent of an existing failure, and the conditions (such as porewater pressure) that may have caused it.
To ensure that any continuing displacements of materials that have already slipped remain small.
To demonstrate if remedial works are (or are not) required.
To check the performance of remedial measures
Displacements with depth using inclinometers; porewater pressure using piezometers; weather/climate.
For large areas of instability, ground surface displacements may be monitored using large-area approaches such as satellite-based LiDAR
If the hazard posed by the slope is low, and initial displacements are small, readings could be relatively infrequent, and data may not need to be logged continuously.
Duration may depend on hazard posed; may be months or years if monitoring is needed to limit risk to infrastructure
Readings may be plotted and analysed on a periodic basis (e.g. once a week or month)Monitoring of earthwork porewater pressures and displacements (Smethurst et al. 2015; Hughes et al. 2016)
(2) Obtaining parameters for use in design of remedial schemesTo understand the depth and extent of an existing failure, and groundwater conditionsDisplacements with depth using inclinometers; porewater pressure using piezometers; weather/climate parameters (precipitation, temperature)If the hazard posed by the slope is low, readings could be relatively infrequent (e.g. monthly).
Duration needs to be sufficient to make a reasonable assessment of extent of failure and likely worst porewater pressure conditions
Readings can be plotted and analysed on a periodic basis (e.g. once a week or month)For example, in stabilization of earthworks using piles; see Smethurst & Powrie (2007) and O'Kelly et al. (2008)
(3) Early warning systems to provide alarm of actual failure, or indication of incipient failureTo warn of actual or incipient failure that may pose a direct risk to safety of transport systemsDisplacement is the obvious indicator of incipient failure in many non-brittle materials; commonly assessed using inclinometers or tilt meters.
Climate, porewater pressures/suctions and soil moisture content may be secondary indicators
Frequency of readings may be high, to attempt to assess risk in ‘real time’ if failure may occur rapidly.
This would lean towards in-ground instrumentation, or tilt meters fixed to points on the slope surface, that are continuously datalogged
Data may need to be interpreted rapidly, and in part by machine (computer or datalogger), assessing monitoring data against pre-determined thresholdsA review of early warning systems has been given by Stähli et al. (2014). Details of a system using acoustic emission monitoring in a cutting slope have been given by Dixon et al. (2015)
(4) Monitoring slopes to manage risk at the infrastructure corridor scaleTo investigate changes in key parameters along significant lengths of asset.
To warn of incipient failure that may pose a direct risk to safety
Large numbers of instruments may be used along significant lengths of transport corridor.
The need to contain cost leads to measurements of ground surface displacement, or near-surface changes in porewater pressure/suction, or parameters such as soil moisture content. Ground surface displacements may be monitored using large-area approaches such as satellite-based LiDAR
Frequency of readings may be high (every few minutes), if there is a need to assess risk in real time because failure may occur rapidly.
Condition monitoring may take place over many years
Large volumes of data may need to be interpreted rapidly, and thus probably in part by machine (computer or datalogger), assessing monitoring data against pre-determined thresholdsUtili et al. (2015) described the use of monitoring information to consider the stability of longer lengths of asset. An overview for consideration of slopes at the corridor scale has been given by Dijkstra et al. (2014)
(5) Research: monitoring slopes to understand mechanisms of degradation and failureTo investigate particular modes of deterioration or failure.
To investigate processes (such as changes in porewater pressure) that lead to failure
A wide range of instrumentation may be used, including more unusual types to determine less commonly measured parameters (e.g. permeability).
Instrumentation may be extensive to obtain a detailed profile of variation with, for example, depth
Frequency of readings from instruments is likely to be high (hourly or sub-hourly), to obtain high-quality temporal datasets.
Duration of monitoring may be long, to assess, for example, long-term changes in porewater pressures over several years of climate
Readings may be collected and analysed infrequently, depending on the needs of the research programmeExamples include: Long-term variations of porewater pressure (Smethurst et al. 2012; Glendinning et al. 2014)
Investigations of extreme wet winter porewater pressures (Briggs et al. 2013)
Investigation of suctions supporting silt/silty sandy slopes (Casini et al. 2013; Westerberg et al. 2014, 2017)
Controlled failure of a full-scale test embankment (Lehtonen et al. 2015)
Understanding rainfall infiltration driven failure (Akca et al. 2011; Askarinejad et al. 2012)
(6) Development and testing of new types of instrumentationTo understand the performance of new instrumentation systems.
Calibration and validation of instruments
A mix of conventional and new instrumentsFrequency of readings is likely to be high (hourly or sub-hourly), to obtain high-quality temporal datasets.
Duration of monitoring may be longer, if new instrumentation needs to be proved in full range of conditions
Readings may be collected and analysed infrequently, depending on the needs of the research programmeResearch sites such as Hollin Hill, North Yorkshire, UK (Chambers et al. 2011) and Nafferton embankment, Northumberland, UK (Glendinning et al. 2014) are being used to assess the performance of new monitoring instruments and techniques.
Examples of new instrumentation include moisture and displacement monitoring using ERT (Wilkinson et al. 2010; Lehmann et al. 2013; Chambers et al. 2014; Gunn et al. 2015), and movement monitoring using AE (Smith et al. 2014b)