Characterization of Unsaturated Soils Using Elastic and Electromagnetic Waves

Recent advances in laboratory instruments and techniques enabled researchers to explore new aspects of the behavior of geomaterials and perform measurements that would be otherwise impossible to acquire using traditional geotechnical laboratory techniques. This dissertation is focused on utilizing elastic and electromagnetic wave measurements and SMCT imaging to non-destructively characterize different aspects of the behavior of unsaturated soils. A model that relates P-wave velocity in soils to the volumetric water content was used to develop a new methodology to determine the in situ density and moisture content of soils. It was numerically and experimentally verified to assess it validity and range of applicability. On the other hand, a triaxial apparatus that enables the measurement of P- and S- wave velocities in unsaturated soil specimens under controlled net stress and matric suction was also developed. Several verification experiments were performed using the apparatus and the results were compared to theoretical models as well as previous experimental results. Moreover, a drying cell was used to examine the effect of the presence of fine clay and silt particles on the elastic waves` velocity and the small strain stiffness of unsaturated soils. The results were confirmed by analyzing SMCT images of similar samples at different drying stages. The proposed methodology yielded good predictions of the density and the moisture content of soils. However, different experimental and numerical error sources caused the predicted density and moisture content values to slightly differ from the measured values. For the majority of the tested specimens, the density was estimated within ±10% of the measured values while the water content was estimated within ±20%. On the other hand, the experimental results from the new triaxial apparatus showed a significant effect of matric suction on the recorded wave velocities. It was also documented that wave velocity values increase with increasing percentage of fine silt particles in the specimens. The results of the drying cell experiments as well as the SMCT image analysis showed the profound effect of the presence of fine silt and clay particles on the small strain stiffness of unsaturated soils. The density of the pore fluid increased during drying due to the higher fine concentration. The concentration of fine particles was found to be significantly higher at areas close to the interparticle contact than in pore bodies away from the contacts causing an increase in the interparticle contact stiffness

Spatial and Temporal Evolution of Particle Migration in Gap-Graded Granular Soils: Insights from Experimental Observations

This study presents physical observations and insights into particle migration characteristics throughout the suffusion process. Using a purpose-built coaxial permeameter cell, suffusion experiments were conducted on idealised internally unstable gap-graded granular soils at varying fines content and hydraulic loading conditions. The specimens were prepared with a mixture layer comprising finer and coarser fractions underlying a coarse layer composed of the coarser fraction alone. This enabled the finer fraction within the mixture layer to migrate through the coarse layer with upward seepage flow. The local porosity profile along the specimen was determined using spatial time domain reflectometry and an inversion algorithm, which enabled the development of a novel field map of the difference in porosity from the initial condition. This field map provided a visual guide of the spatial and temporal variation in porosity and enabled particle migration internally within the specimen to be quantitatively characterised from onset to progression to washout. The limiting onset condition identified from the field map was shown to be comparable to that obtained using conventional approaches, thereby providing strong validation for the application of porosity-based field maps. As suffusion progressed, the height of infiltrating finer particles into the coarse layer increased linearly with time, while the overall rate of particle migration from the mixture layer to the coarse layer evolved in a non-linear manner with the rate of migration increasing as the specimen reached a complete mixture condition, where the finer fraction infiltrated the entire coarse layer. The attainment of a complete mixture condition was dependent on the fabric of the gap-graded soil ... (see PDF for full abstract).Comment: 28 pages, 20 figures. Article published in Journal of Geotechnical and Geoenvironmental Engineering 2023 (see https://ascelibrary.org/doi/10.1061/JGGEFK.GTENG-11094

Time-Domain Reflectometry (TDR) monitoring at a lab scale of aerobic biological processes in a soil contaminated by diesel oil

This study aims to monitor the biological processes ongoing in a hydrocarbon polluted soil. The experiments were carried out at a laboratory scale by measuring the evolution of its geophysical electromagnetic parameters. Time-domain reflectometry (TDR) probes were used to measure dielectric permittivity and electrical conductivity in columns of sandy soil artificially contaminated with diesel oil (Voil/Vtot = 0.19). To provide aerobic conditions suitable for the growth of microorganisms, they were hydrated with Mineral Salt Medium for Bacteria. One mesocosm was aerated by injecting air from the bottom of the column, while the other had only natural aeration due to diffusion of air through the soil itself. The monitoring lasted 105 days. Geophysical measurements were supported by microbiological, gas chromatographic analyses, and scanning electron microscope (SEM) images. Air injection heavily influenced the TDR monitoring, probably due to the generation of air bubbles around the probe that interfered with the probe–soil coupling. Therefore, the measurement accuracy of geophysical properties was dramatically reduced in the aerated system, although biological analyses showed that aeration strongly supports microbial activity. In the non-aerated system, a slight (2%) linear decrease of dielectric permittivity was observed over time. Meanwhile, the electrical conductivity initially decreased, then increased from day 20 to day 45, then decreased again by about 30%. We compared these results with other researches in recent literature to explain the complex biological phenomena that can induce variations in electrical parameters in a contaminated soil matrix, from salt depletion to pore clogging

REAL TIME MONITORING OF VOLUMETRIC WATER CONTENT IN RECLAIMED MINE WASTE USING CONE PENETRATION - TIME DOMAIN REFLECTOMETRY

Reclamation soil covers over mine waste are designed to store water as a means of controlling water balance in order to support re-vegetation and minimize the net percolation of water into mine waste. Measurement of stored volumes of water in reclamation material is a critical element in monitoring the performance of the soil cover. The purpose of this project is to undertake laboratory and field trials of a sensor to measure volumetric water content profiles through reclaimed mine waste using a new time domain reflectometry (TDR) component added to conventional geotechnical cone penetration testing (CPT). Time domain reflectometry measures the volumetric water content of a soil by characterizing the dielectric constant of the soil – a property that is strongly dependant on volumetric water content. The sensor design is constrained by the ability to be adapted to standard CPT equipment as well as being operational in soils with elevated electrically conductivity (EC). The engineering problem addressed in this project is the measurement of detailed profiles of stored water volume over large areas of reclaimed mine waste (i.e. soil covers placed over tailings or overburden) in real time. If successful, this will be a new method to obtain real time, spatial distributions of stored water volume to depths of up to 10 m within reclaimed mine closure landscapes. Other methods exist to obtain stored water volume profiles however these methods are rather complicated. The laboratory component of this project was intended to measure the accuracy and operational range of the device using prepared samples under a controlled setting. Two field based calibration studies were used to assess the robustness of the device under extreme forces and to evaluate its performance under actual field conditions. It is expected that the developed CPT-TDR probe can be utilized by industry to evaluate the performance of reclamation covers in a wide variety of closure designs

Characterisation of sedimentary structure and hydraulic behaviour within the unsaturated zone of the Triassic Sherwood Sandstone aquifer in North East England

A study of the sedimentological framework and permeability characteristics of the Sherwood Sandstone has been undertaken together with a detailed investigation of moisture migration in the vadose zone at a single field site. Sedimentary structure and likely permeability variations were studied by use of laboratory grainsize analysis, logging of nearby outcrops, borehole geophysics and Ground Penetrating Radar (GPR). GPR is ideal for vadose zone hydrogeological applications as the majority of the features imaged are visible to radar as a result of variations in capillary, held moisture, and the amount of capillary retention is controlled by the size of the pore throats in the sediments, which directly influences their permeability properties. Combined use of GPR, reconstruction of sedimentary facies and quantification of permeability characteristics has provided detailed 3D models of the sedimentary subsurface. Time Domain Reflectometry (TDR) was used to monitor water movement within the Sherwood Sandstone at a site near Selby in Yorkshire, creating a vertical and lateral profile of groundwater movement within the unsaturated zone. Moisture content has also been monitored using a neutron probe, and a commercially-available portable packer system, which have provided verification of the accuracy of the custom-made TDR system. The TDR installations consisted of automated arrays of TDR probes, permanently installed upon borehole packers at varying depths, and these have provided moisture content data of a high temporal resolution. The TDR system has allowed monitoring of seasonal moisture variation under natural rainfall loading, and the results have been interpreted in order to gain a better understanding of groundwater migration at a different scale to data previously available. The bulk of the rock in the Sherwood Sandstone aquifer study area consists of relatively permeable medium-grained sandstones. However, results suggest that vertical flow in the unsaturated zone may be impeded by the presence of relatively impermeable fine sandstone units, which correspond to bar top and slack water environments, and occasional mudstone layers representing overbank deposits. This restriction to vertical flow may cause localized perched aquifer conditions, which provide sufficient hydrostatic head to initiate horizontal migration in the overlying rock. Modelling of real rainfall events suggests that 25% of the water present in the perched aquifer layers undergoes lateral or bypass flow (i. e. it drains laterally rather than through the fine-grained layers). In the saturated zone, the horizontal flow of groundwater in the Sherwood Sandstone aquifer is likely to be dominantly via the relatively coarse, trough-stratifed sandstone layers, so that a low proportion of the total aquifer porosity may provide a route for rapid contaminant transport

Studies of the use of time domain reflectometry to determine moisture content in finite regions

This thesis addresses several questions related to the sensitivity of TDR to the determination of the moisture in laboratory experiments where the measurement is done noninvasively; Some tests were performed on moistened sand in a plastic box, treating the moisture content as the independent variable. Another set of tests was performed on moistened sand in a plastic tube to find the relationship between the outputs from the TDR with moisture content in systems both with and without water outflow. In addition, a set of tests was performed on a layered arrangement of large household ceramic tiles where the gaps between the tiles were or were not saturated with water. In all these tests, it is shown that TDR is effective to measure the moisture content noninvasively; A numerical analysis of a geometry related to those studied experimentally has been performed. Results show the effect of gap location on the TDR indication

In situ water content measurement of compacted highway subgrades using time domain reflectometry

The water content is a principle factor of subgrade soil stability in shallow foundation pavement systems. Currently data do not exist for seasonal subgrade water content changes in Tennessee pavement systems. Four test sites across the state were instrumented with equipment to monitor subgrade and flexible asphalt pavement water content and temperature, water infiltrated through the pavement layers, and climatic conditions. These data will later be used to develop a rational method of flexible asphalt pavement design that accounts for environmental factors and seasonal water content changes. Five-segment TDR probes, manufactured by Environmental Sensors, were installed in the soil subgrade, the stone base, and the asphalt stabilized base. Single segment TDR probes, constructed in the laboratory, were installed in the asphaltic concrete. The multiple-segment TDR probes allow changes in water content to be tracked two-dimensionally, between the probes and along probe segments. Temperature sensors were installed at depths corresponding to the soil subgrade and the stone base TDR probes. Three temperature sensors were also installed in the top 20 cm of the pavement. A weather station was installed at each test site to monitor air temperature, relative humidity, solar radiation, wind speed, and rainfall. The Moisture Point TDR system, manufactured by Environmental Sensors, was designed for use in agricultural soils. The materials in which the TDR probes were installed at each test site are quite dissimilar from agricultural soils. For this reason, a calibration study was performed with the Moisture Point TDR equipment using test site subgrade soils and a single crusher run gravel sample. Ten previously published TDR calibration equations were evaluated to determine which equation most accurately predicts water content for the subgrade soils and for crusher run gravel. The relationship between inverse signal velocity and water content proposed by Herkelrath et al. (1991) most accurately predicted water content for all subgrade soils; however, this relationship requires the derivation of a soil specific slope and intercept. The three- and four-phase dielectric mixing models, proposed by Dobson et al (1985) and rewritten to calculate volumetric water content by Weitz et al. (1997), provided results within the accuracy of the Moisture Point equipment for subgrade soils. The equation proposed by Baran (1994) most accurately predicted water contents for the crusher run gravel

Development of impedance spectroscopy based in-situ, self-calibrating, on-board wireless sensor with inbuilt metamaterial inspired small antenna for constituent detection in multi-phase mixtures like soil

Real time and accurate measurement of sub-surface soil moisture and nutrients is critical for agricultural and environmental studies. This work presents a novel on-board solution for a robust, accurate and self-calibrating soil moisture and nutrient sensor with inbuilt wireless transmission and reception capability that makes it ideally suited to act as a node in a network spread over a large area. The sensor works on the principle of soil impedance measurement by comparing the amplitude and phase of signals incident on and reflected from the soil in proximity of the sensor. The permittivity of the soil dielectric mixture which is calculated from these impedance measurements is used as input parameter to the dielectric mixing models which are used to estimate the ionic concentration in soil. The inbuilt wireless transceiver system is connected to a specially designed metamaterial inspired small antenna in order to reduce the sensor size while keeping the path losses to a minimum by using a low frequency. This composite right-left handed (CRLH) antenna for wireless transmission at 433 MHz doubles up as an underground, sensing element (external capacitor) and integrates with the on-board sensor for soil moisture and nutrient determination. The input impedance of the CRLH sensor, surrounded by the soil containing moisture and nutrient and other ions, is measured at multiple frequencies. It is shown that the change in moisture and ioinic-concentration can be successfully detected using the sensor. The inbuilt self-calibrating mechanism makes the sensor reliable at different environmental conditions and also useful for remote, underground and hand-held applications. A multi-power mode transceiver system has been designed to support the implementation of an energy efficient medium-access-control

pH, geoelectrical and membrane flux parameters for the monitoring of water-saturated silicate and carbonate porous media contaminated by CO2

Characteristics of potable water aquifer contaminated by CO2 are investigated using well-defined laboratory experiments. The porous media domain was prepared with silica sand and limestone in separate experiments. The investigations used combinations of techniques to measure various parameters in the water-saturated porous media domain on which pressure of CO2 was imposed, under various conditions, which correspond to different geological depths. Measured parameters included the pH, geoelectrical parameters, and the diffusion of the CO2 gas through the water-saturated porous media domain using non-porous silicone rubber sheet. Experimental results revealed the existence of three stages in the profile of pH change with time as CO2 dissolved and diffused in the water-saturated porous media domain, which was composed of silica sand. The first stage was characterised by rapid decline in the pH. This is associated with quick dissolution of CO2 and the formation of carbonic acid together with bicarbonate. The second stage showed short rise in pH value, which was attributed to the reverse reaction, i.e., the formation of aqueous and gaseous CO2 and water from the carbonic acid. The third stage was that of the equilibrium in the forward and the reverse reactions, marked by steady state in pH value, which remained unchanged till the end of the experiment. The bulk electrical conductivity (σb) of the water-saturated porous domain increased in the presence of CO2. This is attributed to the formation of ionic species, especially bicarbonate, as CO2 dissolved in the domain. The rise in σb coincided with the first stage of the change in the pH of the system. In addition, the σb was higher in limestone than silica sand, and it increased with pressure of the domain. But, the bulk dielectric constant (εb) showed no change with the dissolution of the CO2 under different conditions. Furthermore, permeation of CO2 through the silicone rubber indicated the diffusion of the CO2 gas through the water-saturated domain. CO2 flux through the membrane was shown to increase with depth or pressure of the domain. A mathematical expression derived in this work shows the dependence of σb on the pH and the initial value of σb. Predictions of the changes in the σb for different porous domains show the reliability of the mathematical expression developed in this work