Monitoring of drying-induced soil moisture loss with PZT-based EMI technique

Air-drying process of soil is a crucial procedure in geological and geotechnical engineering. Drying-induced ground subsidence and damage to overlying buildings is a widespread and urgent problem. Monitoring of drying-induced water evaporation in soil is of great importance. In this paper, soil moisture loss monitoring based on lead zirconate titanate (PZT) transducer using electro-mechanical impedance (EMI) technique was investigated. A physical model test in our laboratory was conducted to study the feasibility and applicability. In the experimental research, three identical PZT transducers that were wrapped with waterproof insulation glue were pre-embedded inside a cohesive soil specimen. In addition, another PZT transducer was embedded in a sandy soil specimen to explore the application effect in soil with different composition. EMI signatures of these four PZT patches during the air-drying process were collected and analyzed. Experimental results indicated that the peak frequency in the conductance signatures presented a rightward shift as the water evaporates. Moreover, the corresponding peak magnitude keep decreasing with the continuous development of soil moisture loss. To better quantify the variations, two statistical metrics including root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD) were employed to study the changing characteristics of the EMI signatures. All these two metrics increase coincidentally in the process. Experimental results demonstrate that cohesive and sandy soil moisture loss monitoring by using the embedded PZT transducer is feasible and reliable. This work also serves as a proof-of-concept study to demonstrate the performance of the EMI technique in monitoring the soil moisture content.

Strength Characterization of Soils’ Properties at High Strain Rates Using the Hopkinson Technique—A Review of Experimental Testing

The paper presents a review of crucial experiments and the latest publications, presenting the previous and current trends in experimental research in 2018–2021 in the area of soil dynamic interaction based on the Hopkinson bar technique. A review of investigated experimental test stands was made, in particular, cohesive and non-cohesive soil specimens prepared with different dimensions and densities. From this study, it can be concluded that the dynamic response of the soil depends on many factors, e.g., density, cohesion, moisture and grain structure of the soil specimen. There is still a noticeable interest in SHPB experiments performed in both 1D and 3D versions under modified conditions (frozen/heated soil specimen, different degree of water saturation content of the soil sample) in a wide range of strain rates 102–104 s−1, which is a large field for further research. The need to learn about the characteristics of various types of soil (both cohesive and non-cohesive) for the selection of structural design solutions for the protection elements of critical infrastructure was emphasized.

Experimental Investigation and Numerical Modeling of Piezoelectric Bender Element Motion and Wave Propagation Analysis in Soils

The bender element (BE) test has been widely used for the characterization of soil specimens to determine the dynamic or low-strain shear modulus. However, the actual behavior of the BE inside the soil specimen still remains unknown. The current ASTM standard does not consider the interference of P waves in BE measurements, which can lead to significant errors in the evaluation of shear wave velocities. In this paper, the BE motion inside different media is numerically studied through a coupled piezoelectric and solid mechanics finite element (FE) model. The numerical results are calibrated and compared with the real motion of the BE monitored using a high-frequency laser vibrometer. The proposed model successfully captured the measured motion of the BE in the air as well as transparent soils.More importantly, the proposed model provided a method for understating the interactions of P waves and S waves in a soil specimen. Simulated signals for an Ottawa sand specimen showed a good agreement with independent results from resonant column tests. The proposed piezoelectric-solid mechanics FE model can be used to study the soil-bender element interaction so that sound recommendations can be given to improve the interpretation of BE tests for different soils.

Enhanced Electrokinetic Remediation for the Removal of Heavy Metals from Contaminated Soils

The electrokinetic remediation of an agricultural soil contaminated with heavy metals was studied using organic acids as facilitating agents. The unenhanced electrokinetic treatment using deionized water as processing fluid did not show any significant mobilization and removal of heavy metals due to the low solubilization of metals and precipitation at high pH conditions close to the cathode. EDTA and citric acid 0.1 M were used as facilitating agents to favor the dissolution and transportation of metals. The organic acids were added to the catholyte and penetrated into the soil specimen by electromigration. EDTA formed negatively charged complexes. Citric acid formed neutral metal complexes in the soil pH conditions (pH = 2–4). Citric acid was much more effective in the dissolution and transportation out of the soil specimen of complexed metals. In order to enhance the removal of metals, the concentration of citric acid was increased up to 0.5 M, resulting in the removal of 78.7% of Cd, 78.6% of Co, 72.5% of Cu, 73.3% of Zn, 11.8% of Cr and 9.8% of Pb.