PhD Defence Ms Fekerte Arega Yitagesu

Department of Earth Systems Analysis

Title of defence

Remote sensing and geotechnical investigation of expansive soils: integrated approach for understanding soil geotechnical and spectral characteristics

Summary

Expansive soils are weak and unstable when subjected to changes in moisture content either due to significant seasonal, climatic variations (alternate dry and wet periods) or artificial causes. Their presence in construction sites is challenging, as it is one of the critical factors that can substantially affect the overall cost, especially of lightweight engineering infrastructure development. Increase in volume (swelling) of expansive soils often exceeds the downward pressure exerted from lightweight structures, causing deformations and formation of cracks. Considerable decrease in volume (shrinkage) associated with drying, on the other hand, is responsible for differential settlement. According to literature, soil swelling and shrinkage cause damage to infrastructure that cost billions of dollars each year in various parts of the world. Thus, it is rated as one of the most costly natural geo-hazards. The areas that are predominantly prone to soil swelling and shrinkage problems are those located in arid and semiarid regions of tropical and temperate climate zones, where the annual evapotranspiration exceeds precipitation. Expansive soils also occur in Ethiopia and are notorious for posing a wide range of problems in the construction sector.

As investment in infrastructure development forms a significant portion of the global economy, expansive soils are a prime focus of research in geotechnical engineering and soil science. Two foremost topics in expansive soil research are (1) characterization and (2) treatment or stabilization. While the first deals with identification and quantitative analysis of expansive soils, the second strives to improve their geotechnical characteristics (such as reducing their swelling and shrinkage potential). Site characterization is a prerequisite at the onset of any construction, also to promote a better land-use planning. A comprehensive geotechnical investigation ensures an optimal planning, design, construction and satisfactory functionality of structures. Confronted by prohibitively expensive customary testing procedures, use of advanced techniques such as remote sensing is highly sought for characterizing expansive soils.

In recent years, significant development has been made to improve geotechnical characterization of expansive soils and better account their spatial variability, by taking advantage of remote sensing techniques. Wavelength coverage and spectral resolution of spectrometers are continually enhanced. Prices of spectrometers have decreased. Portable spectrometers are available. Spatial and spectral resolutions of sensors are also continuously improved apart from the availability of large archives of remotely sensed data sets. In addition, advances in spectral and image data processing techniques have created the opportunity to extract soil geotechnical information from remote sensing data.

Shallow landslide in slopes of collvium with expansive soil matrix

In this research, geotechnical and spectral characteristics of expansive soils are described. The main aim was to investigate interrelationships between these two aspects of expansive soils across the visible-near infrared (VNIR), short wave infrared (SWIR), and mid infrared (MIR) wavelength regions (0.35-14 µm). Expansive soil geotechnical and spectral characteristics are determined by the mineralogical assemblage, principally the presence and abundance of active clay minerals, which provides a prospect to link the two. Emphasis was put on establishing new techniques for characterization including identification, estimation of commonly used geotechnical parameters, and mapping spatial variability in geotechnical characteristics of expansive soils.

On a laboratory scale, expansive soils can be identified and characterized from their spectra. The technique requires a small amount of material per sample, minimal sample preparation and time for analyses. Thus, opposed to the arduousness to accomplish using conventional techniques, a dense sampling can be achieved. In addition to the VNIR and SWIR wavelength regions, spectral characteristics of active clay minerals and expansive soils were also established in the MIR wavelength region. This has given new insights as there was a little understanding of the spectral responses of active clay minerals and expansive soils in the MIR wavelength region. Several spectral and statistical analyses techniques were used to derive compositional information, and relate soil spectral characteristics to routinely determined geotechnical parameters such as consistency limits. Statistical techniques such as a multivariate calibration, partial least square (PLS) regression analysis, were effective for deriving quantitative estimation of soil geotechnical characteristics from spectral data. Spectra of clay minerals in the MIR wavelength region are found to provide additional remarkable information for identification and subsequent characterization of clay minerals. Statistically significant correlations are observed between geotechnical and spectral characteristics of expansive soils.

Cracks, deformation and potholes on road surface due to expansive subgrade

The potential of multispectral images in combination with a multivariate statistical analysis (PLS) for mapping variation in soil geotechnical characteristics was also investigated. To date, no direct remote sensing maps of soil geotechnical parameters have been published. The approach presented in chapter seven highlighted the potential of multispectral remote sensing in mapping geotechnical characteristics of expansive soils. It is possible to acquire large-scale (covering a wide area) estimate of soil geotechnical parameters such as those indicating subgrade characteristics. Availability of such maps during project planning can give a reasonable indication of soil geotechnical characteristics. Identification of problematic sites for narrowing down the need for detailed assessment is of significance.  Furthermore, important contribution is anticipated as a constraining factor in spatial infrastructure planning such as choice of optimal road alignment.

Remote sensing allows for mineralogical composition analyses and quantitative estimation of various soil geotechnical characteristics, which was previously possible through a combination of at least two methods. In practice, this unique capability can provide information necessary for planning and design of infrastructure. Overall, remote sensing can substantially contribute to the understanding of active clay minerals and play a significant role in geotechnical investigation of expansive soils. Therefore, integrating these techniques with routine geotechnical analyses of soils is highly beneficial, as it can serve as basic means for recognizing the presence of expansive soils. Finally, the issue of standardizing remote sensing techniques requires further research and collaboration among the domain of experts in the discipline to universalize the approach.

Biography

Fekerte Arega Yitagesu has several years of experience in construction sector: geotechnical site investigation and engineering geological mapping; earthworks; slope stability and settlement analyses; sampling, in-situ and laboratory testing of various construction materials; environmental impact assessment (EIA), strategic environmental assessment (SEA) etc. Her experience includes assessing geotechnical and geo-environmental inputs for preparation and review of road projects, tender and construction document, quality control and management (supervision), preparation of geotechnical investigation and material reports, preparation of environmental impact assessment reports and environmental management and monitoring plans for road projects etc. She also gained an ample experience in remote sensing and geographic information systems (GIS) in relation to infrastructure development and various aspects of environmental applications including developing geo-database, modeling spatial earth processes relevant to the planning, design, monitoring, and construction of road infrastructure, as well as related technical and environmental issues. She did her PhD research on one of the main challenging soil (expansive soil) in engineering practices. Her study is focused on identification, characterization and mapping expansive soils using remote sensing techniques. Other than expansive soils, she has a very good knowledge and keen interest in ground improvement research and the application of remote sensing and geo-information technologies for analyzing and mapping other problem soil, geomorphic processes and geo-hazards such as mass movement.

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