The Ardabil plain, with an approximate area of 1097.2 km2 in northwestern Iran, has experienced land subsidence due to intensive groundwater withdrawal and long seasons of drought in recent years. Different techniques have been used to investigate and evaluate subsidence in this region including: Global Positioning Systems (GPS), Levelling, and Geotechnical methods. These methods are typically expensive, time-consuming, and identify only a small fraction of the areas prone to subsidence. This study employs an Interferometric Synthetic Aperture Radar (InSAR) technique to measure the long-term subsidence of the plain. An open-source SAR interferometry time series analysis package, LiCSBAS, that integrates with the automated Sentinel-1 InSAR processor (COMET-LiCSAR) is used to analyze Sentinel-1 satellite images from October 2014 to January 2021. Processing of Sentinel-1 images shows that the Ardabil plain has been facing rapid subsidence due to groundwater pumping and reduced rainfall, especially between May 2018 to January 2019. The maximum subsidence rate was 45 mm/yr, measured at the southeastern part of the plain. While providing significant advantages (less processing time and disk space) over other InSAR processing packages, implementation of the LiCSBAS processing package and its accuracy for land subsidence measurements at different scales needs further evaluation. This study provides a procedure for evaluating its efficiency and accuracy for land subsidence measurements by comparing its measurements with the results of the GMTSAR and geotechnical numerical modeling. The results of geotechnical numerical modeling showed land subsidence with an average annual rate of 38 mm between 2006 and 2020, which was close to measurements using the InSAR technique. Comparison of the subsidence measurements of the Ardabil plain using the LiCSBAS package with results obtained from other techniques shows that LiCSBAS is able to accurately detect land deformation at large scales (~ km). However, they may not be optimized for more local deformations such as infrastructure monitoring.
Comprehensive characterization of the small strain shear modulus Gmax of an unsaturated soil specimen during hydraulic hysteresis requires precise control of the stress state, matric suction, degree of saturation, as well as knowledge of the void ratio. This paper describes the details and typical results from a test method which incorporates the axis translation technique for suction control, a flow pump for degree of saturation control, and a vertically oriented proximeter to infer changes in void ratio into a fixed–free resonant column setup. A unique aspect of this test method is the operation of the flow pump to reach equilibrium points on the hysteretic soil–water retention curve (SWRC) for measurement of Gmax. The flow pump, which controls water flow to or from the soil specimen through a high air-entry ceramic disk, is guided using a feedback loop involving the matric suction measured at the boundary of the specimen. Values of Gmax were measured at different equilibrium points on the primary drainage path of the SWRC up to the point of water occlusion, then along a scanning imbibition path of the SWRC. The results indicate a greater shear modulus during imbibition, consistent with trends and magnitudes noted in the technical literature.
This paper presents an experimental methodology for using multistage, drained triaxial tests on compacted soils under unsaturated conditions to estimate soil-specific relationships between mean effective stress and matric suction. Tests were performed by applying a matric suction to a soil specimen in a triaxial cell using the axis translation technique with back-pressure, then shearing the specimen under drained conditions until reaching stress-path tangency. The specimen was then unloaded, a new suction was applied, and the shearing process was repeated. The points of maximum principal stress difference for the unsaturated specimen were plotted versus mean effective stress, defined using the degree of saturation as the effective stress parameter, and they were found to correspond well with the critical state line defined from triaxial tests on saturated specimens. The suction stress for the compacted soil tested in this study was observed to increase nonlinearly with suction, tending toward a constant value with increasing suction. Although there are potential changes in soil structure in the specimen during loading, unloading, and reloading, the results indicate that the multistage testing method may be useful for estimating soil-specific effective stress parameters for compacted soils in unsaturated conditions. Furthermore, the fact that differences in the soil-water retention curve of soil specimens subjected to different net confining pressures were observed for the soil tested in this study emphasizes the importance of using soil-specific tests to validate predictive relationships between suction stress and matric suction.
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