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6 Literature Review 2.1 Literature Review This discussion summarizes a literature review of slope instrumentation and monitoring technology that became available within approximately the last decade in the United States and internationally. In general, the unstable slope instrumentation and monitoring techniques that have advanced during this period address detection of movement from a distance, increased quantity of data at more frequent intervals, change detection, and updates to existing tech- nologies. These technologies are being used by some DOTs and, in fact, some of the literature has been supported or authored by DOTs. The literature review discussion is categorized by advancement and applications as follows: ⢠Remote-sensing methods ⢠Ground-based instrumentation and monitoring ⢠Instrumentation Software and Data Management 2.2 Remote Sensing Advancements Remote sensing of ground deformation can be completed using sensor systems that are ground based, airborne, or from a satellite. In general, these methods include using global navigation satellites, optical sensors (active and passive), and microwave sensors. While some of these technologies were available more than two decades ago, advancement has occurred in many aspects including interpretation of the data to improve resolution and precision, cost- effectiveness of technology deployment, and use in different terrain and ground cover conditions. While the technology is advancing, there is not a single remote-sensing method or technology that can cover the entire range of unstable slope monitoring needs. A summary of remote- sensing techniques is introduced in Table 2.1 with an expanded discussion for the methods in the following subsections. For the monitoring of unstable slopes, technological gains in the field of remote sensing and interpretation of 3-dimensional (3D) data interpretation have advanced the capability of geo- technical engineers to understand natural and anthropogenic slope processes at unprecedented resolution and accuracy (Lato et al., 2014). Published examples of remote sensing for unstable slopes include mapping topographical change (Young et al., 2010, Derron et al., 2013), assessment of rockslide susceptibility (Gigli et al., 2013), and characterization and monitoring of rockslides (Oppikofer et al., 2009). While there are rapid advancements occurring in remote-sensing technology, it is important for users of the different remote-sensing technologies and platforms to understand the advantages and limitations with each method when applied to mapping of rock slopes (Francioni et al., 2018). C H A P T E R 2
Literature Review 7 A survey of European practice reported that most agencies (83% that answered survey) use remote sensing for both landslide detection/mapping and monitoring, while a few (17%) use it only for detection and mapping (Tofani et al., 2013). Global Navigation Satellite Systems Global navigation satellite systems (GNSS) consist of satellite networks that enable the measurement position (longitude, latitude, and elevation) of a given point. In the United States, the global positioning system (GPS) is a form of a GNSS. Early applications of this technology used digital photography combined with static GNSS measurements to measure surface dis- placements and estimate the volume of an unstable slope; however, in the early 2000s many studies started using the Fast-Static and Real-Time Kinematic (RTK) techniques that not only improve the accuracy of the measurements, but also reduce the level of analysis effort (Gili et al., 2000; Mora et al., 2003). These techniques use a high-density grid, where the coordi- nates of the points are obtained by continuously moving one receiver with respect to a fixed station. If several receivers are used simultaneously, and the control points are reliable, the accuracy of the measurements can be on the order of millimeters to centimeters. Advantages of using GNSS/GPS include reliability, coverage over large areas, having similar accuracy to elec- tronic distance measurements, measurement in all weather conditions, having coherent results between consecutive surveys, and ease of operation (Coe et al., 2003). Within the last decade, GNSS/GPS equipment has become progressively less expensive, lighter, and easier to use. New operating modes, methods, and software have been developed for the data recording and postprocessing. In the past years, several case studies that monitor slow landslide movements using these techniques have been published (Malet et al., 2002; Tarchi et al., 2003; Coe et al., 2003; Komac et al., 2015; Bouali et al., 2019). Passive Optical Sensors (Photogrammetry) Passive optical sensors record radiation that is reflected from the ground surface or other features within a field of view. Photography is a form of passive optical sensor technology that relies on reflection of visible light to create static images. In general, remote sensing of ground deformation can be quantified with this technology by processing multiple images from the same georeferenced location and applying corrections for sensor type and image precision and through orthorectification, which removes distortions and creates a constant scale across the image. Numerous approaches for these corrections have been developed in the last 10 to 15 years (i.e., Sertel et al., 2007; Zitova and Flusser, 2003). Method/ Technology Sensor Platform Measurement Parameters Techniques GPS/GNSS Satellite Longitude, latitude, elevation of each point in grid Static, Fast-Static, Real-Time Kinematic (RTK) Passive optical sensors (photogrammetry) ground-based, aerial, satellite Displacement, features orientation surface elevation, change detection SLR camera, UAV equipped with GPS and optical sensors Active optical sensors ground-based, aerial Displacement, 3D coordinates/ models, change detection Electronic Distance Measurement, LiDAR: terrestrial (TLS) or airborne (ALS) Active microwave sensors ground-based, satellite Displacement, soil moisture, 3D models, surface roughness/ cover, change detection SAR, InSAR, Differential InSAR, Advanced InSAR, Ground-Based InSAR, PSP-DIFSAR Table 2.1. Summary of remote-sensing technologies.
8 Advances in Unstable Slope Instrumentation and Monitoring The measurement parameters that can be obtained with this technology include displace- ment, orientation of surface features, and elevation. Measurement accuracy can be on the order of centimeters for ground-based (terrestrial) systems, and in the past decade accuracy has improved to about 25 cm from aerial or satellite-based platforms (Delacourt et al., 2007; Stumpf, 2013). Advances in image processing and high-resolution digital cameras within the last 5 years have enabled several applications of photogrammetry in the field of geotechnical and geological engineering for unstable slopes. One method, âstructure from motionâ (SfM) uses the informa- tion from many detailed and overlapping digital images to generate dense three-dimensional (3D) models of surfaces through remote sensing (James and Robson, 2012; Westoby et al., 2012). Image processing and the photogrammetric SfM analysis for constructing 3D terrain models can be accomplished using proprietary software, while other proprietary and open- source software is useful in conducting temporal change analysis between 3D models constructed over time. Ground-Based Photogrammetry Examples from literature indicate that equipment for ground-based photogrammetry can range from off-the-shelf SLR camera technology to more specialized, professional cameras (Cardenal et al., 2008; Honkavaara et al., 2009). Recent advances in the technology, such as higher-resolution cameras, direct georeferencing, and telecommunication, are allowing new advancements such as implementation of mobile mapping for rock cuts (MacPhail et al., 2018) and near real-time, automated, remote photogrammetric rock slope monitoring systems (Kromer et al., 2019). Proprietary software platforms are common tools for geological engineering analysis and mapping of rock slope data captured through photogrammetry. While ground-based photogrammetric 3D model construction and SfM technology are rapidly advancing, reliability in the analysis is dependent on georeferencing the model to known points through GNSS or conventional survey methods. To address this constraint, new analysis methods have advanced that enable the creation of 3D photogrammetric models using commer- cial smartphone cameras and without precise georeferencing from GNSS or survey (Francioni, et al., 2019). Aerial Photogrammetry Aerial images can be collected from conventional aircraft platforms and within the last decade the use of unmanned aerial vehicles (UAVs) has enabled considerable advancement in this form of remote sensing. UAVs also are providing considerable reductions in the cost of data collection from airborne and satellite platforms (Watts and Keaton, 2017). In the last decade, low-cost UAVs with improved battery technology and improved optical sensors have enabled obtaining spatial data in real time, with flexible collection schedules and at high image resolutions that are applicable for investigations of unstable slopes (Smith, 2015; Farina et al., 2017). In addition, this technology allows data collection in hazardous and inaccessible areas (Rossi et al., 2016; Casagli et al., 2017). Analysis of aerial photographs for unstable slopes often involves the measurement of change across a series of multitemporal digital terrain models (DTMs) or digital elevation models (DEMs). Such change detection between detailed 3D slope models acquired through aerial photography at different times can obtain movement detection limits in the range of 0.5 to 1 m3 depending on data collection and processing processes (Gauthier et al., 2015). When combined with models developed through ground-based photogrammetric methods, the 3D model coverage can be improved through the capture of areas not visible from the ground (Meeks, et al., 2017).
Literature Review 9 Satellite Imagery In approximately the last decade, passive optical imaging in the remote sensing of unstable slopes from satellite platforms has improved in (1) ground sampling distance (due to transfer delay and integration sensors like IKONOS, Quickbird) with resolutions as low as 0.5 meter, (2) acquisition of stereo imagery with minimal time delay (ALOS Prism), (3) relatively low costs for high-resolution images, and (4) access to very high resolution (VHR) satellite imagery up to one day after major disasters (e.g., Google, OpenStreetMap). Accuracy is generally in the order of meters, with days of temporal time lapse between acquisitions of images. The application of this technology has been used to monitor 169 slow-moving landslides in the mountains of southern Europe, enabling measurement of landslide size, velocity and seasonal acceleration for large landslides (Stumpf et al., 2017). Active Optical Sensors Active optical sensors consist of devices that produce and emit laser beams that can be reflected back to the sensor for the purpose of measurement. These devices include electronic distance meters (EDM) and terrestrial and aerial-based laser scanners (LiDAR). The precision of EDM can approach millimeter scale at a range of up to 5,000 meters, whereas the LiDAR can be used to cover larger areas with accuracy that approaches centimeter scale. EDM technology advance- ments include robotic total stations that allow for near real-time monitoring by transmitting measurements via satellite to a control station (Stiros et al., 2004; Gumilar et al., 2017). LiDAR has been used increasingly in landslide characterization and monitoring (Van Den Eeckhaut et al., 2007; Sturzenegger and Stead, 2009; Gorsevski et al., 2016; Cannon et al., 2017; Abdulwahid and Pradhan, 2017). LiDAR monitoring for slopes from ground-based stations is classified as terrestrial laser scanning (TLS), while LiDAR from aircraft or UAVs is identified as airborne laser scanning (ALS). Data from TLS and ALS can be used to perform a kinematic analysis of the discontinuities in a slope, quantify displacement vectors in an unstable slope, support rockfall analysis (SafeLand, 2012, Lato et al., 2014), and evaluate frequency and magnitude relationships for rockfall from slopes (van Veen et al., 2017). ALS also can be used for assessment of unstable slopes at a regional scale, such as monitoring of deep-seated landslides (Pedrazzini et al., 2010; Jaboyedoff et al., 2012). In the past decade, LiDAR measurements have been combined with newer data processing approaches such as artificial neural network, object-based image analysis, and machine learning algorithms (Gorsevski et al., 2016; Li et al., 2015). The 3D models generated from LiDAR also are useful in the process of change detection, which involves quantitatively comparing several models and mapping locations of change. Through change-detection methods, reported detection accuracy includes 1 m3 for rockfall from slopes (Abellan et al., 2013), landslide mapping (Lato et al., 2016), and near millimeter accuracy for in situ movements of unstable blocks in rock slopes (Kromer et al., 2015). For a railway corridor exposed to unstable slopes, collaborations between researchers and practitioners have resulted in the successful implementation of analytical methods using LiDAR and photogrammetry data to aid in the understanding of landslide dynamics, regions and rates of change, direction of motion, and the ability to forecast rock fall source zones and volumes. Integrating 3D analysis alongside traditional subsurface borehole, field mapping, and instru- mentation data is enabling engineers and geoscientists to understand geomorphic processes at a previously unattained level of granularity (Lato et al., 2017). In one study, change detection from TLS is used to evaluate the mechanisms that generate debris flows and the variation flow volumes above a railroad corridor (Bonneau and Hutchinson, 2017).
10 Advances in Unstable Slope Instrumentation and Monitoring Interferometric Synthetic Aperture Radar Interferometric Synthetic Aperture Radar (InSAR) is a remote-sensing technology that can obtain measurements of ground surface deformation, often at regular intervals and enabling change detection of unstable slopes. InSAR devices operate with sensors that use radiowaves or microwaves from ground, aerial, or satellite-based platforms. The sensors are less dependent on atmospheric conditions when compared to photogrammetry methods, and data can be acquired during storms, fog, day, or night. In general, sensors with longer wavelengths have lower resolution but better penetration capacity and stability with respect to environmental conditions. Remote sensing with InSAR methods can produce accuracy that is near millimeter scale and has broad spatial coverage. Processing steps within InSAR can include Synthetic Aperture Radar (SAR), Differential InSAR (DInSAR), and Advanced DInSAR (A-DInSAR) (Ferretti et al., 2007; Colesanti & Wasowski, 2006; Notti et al., 2015). Information obtained from these methods include geo- graphic coordinates and elevation, deformation rates, and time series of displacements. The methods have been applied in the detection of landslides in both regional scale studies and location-specific deformation modeling (Tizzani et al., 2007; Rossi et al., 2008; Herrera et al., 2009; Bordoni et al., 2018; Bonì et al., 2018). The analysis of historical SAR data can enable the mapping of ground deformations over time, with durations reported in the literature of up to 27 years (SafeLand, 2012). If the method is ground based (GB InSAR), the monitoring equipment can be installed in a few hours, producing near millimeter accuracy and near real time mapping of deformation in unstable slopes (Dario et al., 2002; Luzi, 2010; Barla and Antolini, 2016). Data from SAR remote sensing also can provide other empirical estimations including soil moisture and the thickness of the active layers above permafrost, which can be important inputs toward the evaluation of unstable slopes (Dubois et al., 1995; Luo et al., 2016; Widhalm et al., 2017). Selection of Remote-Sensing Methods for Unstable Slopes Within the United States, guidance for the selection of remote-sensing applications for unstable slopes and other applications has been developed for use by DOTs (Anderson, 2013). This guidance has been developed through projects that were initiated more than 10 years ago, such as an effort by the Federal Lands Highway Program of the FHWA to evaluate the use of InSAR for unstable slope movements that can impact roadways. The report published at the completion of this project, InSAR Applications for Highway Transportation Projects, recom- mends guidelines for the incorporation of InSAR into other data collection activities for unstable slopes, such as photogrammetry, field surveys, boreholes, and slope inclinometer installations (USDOT, 2006). Other early applications for LiDAR and photogrammetry for rock slopes also were in use more than a decade ago (USDOT, 2008). More recent work includes a cooperative research project sponsored by the U.S. Department of Transportation and completed by Michigan Technologi- cal University. This project produced several deliverables related to remote sensing to support geotechnical asset management, including unstable slopes. The deliverables include evaluation and validation of remote-sensing methods, performance measurement, decision support systems, and cost-benefit guidance for remote-sensing applications (Wolf et al., 2015). Outside of the United States, SafeLand was a project for a comprehensive analysis of landslide risk in Europe with the purpose of protecting people and property from a changing pattern of landslide hazard and risk caused by climate changes and demographic changes. The project
Literature Review 11 began in May 2009 and concluded in April 2012 and involved 27 agency and university partners from 12 European countries. The project also included collaboration with advisors from the United States, China, India, Japan, and Hong Kong. In the U.K., Highways England has devel- oped guidance for remote survey techniques for embankments and slopes based on benefit and cost considerations (Pritchard, 2018). The SafeLand project website is maintained by the Norwegian Geotechnical Institute and includes several reports and publications that are useful for selection and planning of remote sensing on unstable slopes. These reports are included in the SafeLand Work Area 4: Development of monitoring technology, especially early warning systems and remote-sensing techniques, and applications (NGI, 2019). Figure 2.1 provides guidance developed from the SafeLand project on selecting remote-sensing methods for different unstable slope conditions. 2.3 Ground-Based Instrumentation and Monitoring Advancements Installed Geotechnical Instrumentation Conventional geotechnical instrumentation that is installed on or in the ground has not changed significantly in the past decade. The advancements that have occurred involve integration of technology from other uses into existing systems, improvements that increase the frequency Figure 2.1. Guidance for selection of remote-sensing methods (from NGI, 2019).
12 Advances in Unstable Slope Instrumentation and Monitoring and quantity of data collection, greater data storage and transmission capabilities, and viewing and use of data. Examples of advancements to installed geotechnical instrumentation are summarized below. Extensometers Extensometers are a form of geotechnical instrumentation that is commonly used to measure displacement at the surface of rock discontinuities and other unstable slopes types. Advancements in extensometer technology have involved improvements to durability, corrosion resistance, and calculations of landslide displacements from extensometer data (Corominas et al., 2000). Another improvement involves connecting electronic transducers, dataloggers, and telemetry systems, which allows extensometer data to be collected remotely and at regular and more frequent intervals (Shulz and Ellis, 2007). Inclinometers Inclinometers have been used for over 40 years to measure the magnitude and location of deformation across discrete intervals in the subsurface, such as on the sliding plane or dis- continuity in an unstable slope. Innovations within approximately the last decade for traditional inclinometers include integration of automatic inclinometers with other modules so that major physical and mechanical parameters as well as groundwater level are also measured (Machan and Bennett, 2008). The use of microelectromechanical systems (MEMS) accelerometers is a new advance- ment in about the last 10 years for inclinometers in unstable slope applications. The in-place inclinometer consists of a string of MEM sensors spaced at 1-foot (0.3-meter) increments. Early uses of in-place inclinometers include installations by New York DOT for two demonstration projects on unstable slopes (Barendse, 2008; NYDOT, 2012) and by Minnesota DOT for two unstable slopes (Dasenbrock, 2010). The MEMS inclinometer technology also has been evaluated by Alaska DOT for use in cold regions and applications that include unstable slopes, frozen ground creep, and settlement from thawing ground (Darrow and Jensen, 2012). Piezometers Observation wells and piezometers are a means for measuring and monitoring groundwater levels and fluctuations, including applications on unstable slopes. Vibrating wire piezometers are a form of geotechnical instrumentation that is installed below the ground surface and enables monitoring for groundwater change with time and pore water pressures. The recent advance- ments for piezometer applications involve the expanded use of VW piezometer systems in different installation methods, low permeability applications, and with systems that include temperature measurements and remotely monitor data continuously in near real time (Contreras et al., 2008; Simonsen and Sorensen, 2012). Pressure Cells Contact earth pressure cells (diaphragm or hydraulic) are a geotechnical instrument that is placed within the ground to measure total stress distribution, magnitude, and direction. Pres- sure cells are typically installed during construction of new earthworks, such as embankments and dams, and also have been used to monitor the performance of an unstable slope mitigation project (Srinivansan and Schroeder, 2008). Fiber Optics Fiber optic instruments are a form of geotechnical and structural instrumentation of the measurement of displacements. Similar to pressure cell applications, fiber optic instruments are
Literature Review 13 often incorporated into new construction for bridges, walls, and dams. There are some examples of fiber optic sensor use to measure differential displacements in unstable slope applications (Liu et al., 2010; Zhu et al., 2011); however, the application in monitoring and warning for geohazard applications is considered to be in a state of infancy (Zhu et al., 2017) Geophysical Methods In general, geophysical methods in the literature are related to exploration methods; however, there are some instances of applications for monitoring of unstable slopes. Geophysical methods measure seismic, gravitational, magnetic, electrical, and electromagnetic properties below the ground surface. Geophysical methods generally are minimally invasive and can cover large areas and depths. When used in combination with subsurface explorations (boreholes), geophysics can be used to construct 3D models of the subsurface and these models can be updated with new data through time for temporal analysis of unstable slopes. In the past decade, advancement has occurred for equipment that can be used in landslide applications, in difficult terrain, and with new methods such as geophysical inversion (Jongmans and Garambois, 2007; Maurer et al., 2010; Whiteley et al., 2019). When using geophysical methods, the collected data are related to geological or geotechnical properties through empirical equa- tions. There is not a single method that is applicable to unstable slopes; rather, a combination of methods is used depending on the material contrast, depth, and required information. Example applications from the literature are discussed in the following subsections. Seismic Methods Seismic data can be used to measure elastic parameters related to the bulk modulus and shear modulus of the slope materials. The methods include (1) seismic tomography, which generally is applied in rock conditions; (2) seismic reflection for estimating the subsurface geometry of an unstable slope; (3) surface waves analysis to evaluate the soil shear modulus with depth; and (4) 1D ambient-noise measurement for detecting sliding surfaces of unstable slopes (Wang et al., 2016; Stucchi et al., 2017). Electrical Methods These methods measure the resistivity, conductivity, and self-potential of subsurface materials. Typically, measurements involve inserting two electrodes in the ground and maintaining a direct current flow. Resistivity is used for identifying layers with resistivity contrast, self-potential is deployed for identifying groundwater levels, and induced polarization to identify groundwater in unstable slopes (Morse et al., 2012; Lapenna et al., 2005; Solberg et al., 2008). Electrical resistivity tomography (ERT) is one such method that can provide a monitoring framework to manage moisture movement and identify failure trigger conditions within embankments (Gunn et al., 2015; Smethurst et al., 2017). Electromagnetic Methods This method involves the transmission of an electromagnetic field through the soil and comparison of the primary and secondary magnetic fields. Typical applications in unstable slopes include the determination of the limits of the moving mass; however, this method should be combined with other methods for proper interpretation (Méric et al., 2007; Yamazaki et al., 2017). Acoustic and Micro-seismic Monitoring Seismic sensors consist of devices such as seismometers, geophones, accelerometers, and acoustic emission transducers that can monitor acoustic waves produced by materials undergoing
14 Advances in Unstable Slope Instrumentation and Monitoring deformation, such as in unstable slope masses. Even though this is not a new technique and its use in landslide applications is limited, advancements in the past decade have overcome previous limitations such as the need for secure instrument housing and electricity. Advantages of this method include lower equipment cost, ability of capturing precursory signals of failure, and applications over small and large areas. Based on previous studies, an Assessment of Landslides using Acoustic Real-time Monitoring Systems (ALARMS) has been validated and suggested as a tool for early warning system for low-income communities (Dixon et al., 2015, 2018; Hu et al., 2018). In addition, this technology applies well to rain-triggered landslides, because they often involve loose granular material and large flow-like failures. 2.4 Instrumentation Data Management Advancements With expanded geotechnical instrumentation use and technology advancements, the frequency of data collection and volume of data collected from monitoring programs are increasing. When combined with multiple installations and long-term monitoring programs, data management processes and systems are beneficial for efficient use, analysis, and visualization data. Several proprietary and commercial data management and visualization software applications exist in addition to many custom contractor or agency-specific software systems. The non- proprietary literature on these systems appears to be limited, or where presented is presented as secondary information to the primary topic. As an example, such use is presented in a study on displacement monitoring to support rock slope designs on an I-90 project in Washington (Norrish et al., 2011). 2.5 Summary of Literature Review Most of the recent instrumentation and monitoring literature is related to remote-sensing technologies and increased data quantities and at more frequent intervals; however, there have also been advancements in older ground-based technologies. There also is guidance in the literature for selecting technologies best-suited for a given site. In remote sensing, advancement has occurred in many aspects in the last decade, including advanced data interpretation techniques that improve resolution and precision, cost-effectiveness of technology deployment, and use in different terrain and ground cover conditions. Conventional ground-based geotechnical instrumentation has not changed significantly in the past decade; however, advancements that have occurred involve integration of technology from other uses into existing systems, improvements that increase the frequency and quantity of data collection, greater data storage and transmission capabilities, and viewing and use of data.
