Yu-Hsing Wang

A particulate-scale investigation of cemented sand behavior

In this paper, triaxial tests and numerical simulations using the discrete element method (DEM) are combined to explore the underlying mechanisms of the unique behavior of artificially cemented sands. The experimental results show that strength enhancement, volumetric dilation, and the shear banding associated failure mode are observed in Portland cement sand; these features become more pronounced with increasing cement content. Different responses are found in gypsum-cemented sand even though both types of cemented sand specimens were prepared under very similar void ratios before shearing. The DEM simulations on the Portland cement sand were carried out under two particular arrangements (i.e., the use of small cementing particles and flexible membrane boundaries). The simulation results reveal that particles in the bonding network jointly share the loading and many micro force-chains associated with cementation are created. Compared with uncemented sand, a more stable and stronger force–chain complex su...

Mechanisms of Aging-Induced Modulus Changes in Sand under Isotropic and Anisotropic Loading

AbstractIn this paper, experimental studies were conducted using a true triaxial apparatus with a bender element system to examine the mechanisms of aging-induced, small-strain shear modulus changes in sand samples under isotropic and anisotropic loading. Numerical simulations based on the discrete element method (DEM) were also carried out in parallel. In the isotropic loading cases, the three measured shear moduli, Gxy, Gyz, and Gzx, and associated aging rates, in terms of the modulus changes, are similar in every loading stage. DEM simulations reproduced the experimental findings and suggested a general trend. A sample with a lower shear modulus before aging, because of a greater percentage of weak forces, allows more forces to be redistributed from the strong force network to the weak force network through the process of contact force homogenization during aging and therefore can have a higher aging rate. In the anisotropic loading cases where σz>σx=σy, the measured modulus increase (i.e., the aging r...

Calibration of Tactile Pressure Sensors for Measuring Stress in Soils

This paper provides a method of properly calibrating tactile pressure sensors for the measurement of stress in soils subjected to short-term and long-term static loading. Tailor-made cells were used for the sensor calibration and examination of the calibration accuracy. It was found that if the calibration results for each sensing element on the sensor (i.e., each sensel) are used, the precision of the measured stress in soil under short-term static loading can be greatly enhanced. The sensor creep has to be quantified and removed from the measurement when the soil is subjected to long-term static loading. Because the creep response of each sensel depends on the applied pressure, the associated calibration also has to be carried out under different levels of pressure. This is time consuming and not feasible in reality. A quick method that can significantly shorten the calibration time is therefore proposed in this work. The examination results prove the accuracy of the quick method for calibrating sensor creep.

Mechanisms of Aging-Induced Modulus Changes in Sand with Inherent Fabric Anisotropy

AbstractIn this paper, experiments and discrete element method (DEM) simulations were conducted in parallel to examine the underlying mechanisms of aging effects on the stiffness changes of sand with inherent fabric anisotropy. The true triaxial apparatus, equipped with a bender-element system, was used to monitor the evolution of the small strain shear moduli of sand samples, i.e., Gxy (or Ghh), Gyz (or Ghv), and Gzx (or Gvh), during the aging process. DEM simulations where clumped particles were used to mimic fabric anisotropy replicated the experimental observations closely and provided insights from the micromechanical perspective. The inherent fabric anisotropy gave rise not only to the stiffness anisotropy, i.e., Gxy>Gyz≈Gzx, but also to a higher aging rate for Gxy than for Gyz (or Gzx), i.e., Gxy increased more than Gyz (or Gzx) during aging, thereby increasing the stiffness anisotropy. As the particle aspect ratio or fabric anisotropy increased, the distribution of contact forces became more inhom...

Characterizing Bond Breakages in Cemented Sands Using a MEMS Accelerometer

In this study, a microelectromechanical systems (MEMS) accelerometer, MEMSA (1–3 kHz), and a commercially available piezoelectric acoustic emission (AE) sensor, PZT (125–750 kHz), were used to capture AE responses in uncemented and cemented sands during triaxial compression tests. The AE rates measured in the cemented sands by both sensors demonstrated a similar trend and showed a strong resemblance to the stress-strain response. The bond breakages and the associated AE activities were mild at small strains and increased afterwards to initiate yielding. After the peak stress, shear banding gradually formed and the AE rate distinctly dropped. These observations suggest that a MEMS accelerometer can function as an effective AE sensor to detect the bond-breakage process in cemented sands. In addition, a PZT is more sensitive to the AE detection for cemented sands but a MEMSA starts earlier to capture AE and also continue to capture AE from the shear band at large strains while the PZT only measures a few or no AE activities.

Examining Setup Mechanisms of Driven Piles in Sand Using Laboratory Model Pile Tests

AbstractIn this study, model pile tests were carried out in a custom-made pressurized chamber to explore the mechanisms of a driven pile setup in dry sand. A bender element system and tactile pressure sensors were used in parallel to monitor the stiffness and stress changes in the soil surrounding the pile during the test. The experimental results demonstrate that pile setup is not caused by the increase in at-rest radial stress σrp′. Instead, it is mainly attributed to the increase in radial stress during pile loading Δσrp′, as a result of soil aging (or creep). Pile installation pushes the surrounding soil to the side, thereby imposing additional loading on the soil inside the influence zone. This loading action initiates an associated aging (or creep) process during the setup period; the aging effects ultimately give rise to an increase in both Δσrp′ and pile shaft resistance. The measurements also reveal that the increase in aging-induced soil stiffness is due to contact normal forces among soil parti...

A Broadband Dielectric Measurement Technique: Theory, Experimental Verification, and Application

A wide-frequency dielectric spectrum can provide meaningful information that is intimately related to the properties of soils. A small-sized sensor allows the assessment of local dielectric characteristics of soils and is conducive for long-term monitoring as it can be buried in the soil sample without causing significant perturbations. Considering these advantages, the objective of this study is to explore the broadband measurement of complex permittivity ranging from kHz up to GHz using a slim-form open-ended coaxial probe with an outside diameter of 2.2mm. The measurements are conducted in three different frequency ranges, i.e., high frequency (HF, 500MHz∼20GHz), medium frequency (MF, 10MHz∼1GHz), and low frequency (LF, 1kHz∼15MHz), in view of pertinent aperture admittance models, feasible measurement principles, and required calibrations for system biases. These considerations are described and discussed in turn from the high-frequency measurements to the low-frequency tests. Verification of the assoc...

Making a Biaxial Testing System With the Aid of 3D Printing Technique to Examine the Kinetic Behavior of Particulate Media

This study presented the details of a specially designed biaxial testing system with a flexible boundary to examine the features of particle motion and associated contact movement during shearing. The whole testing system was modified from a CKC triaxial testing system so that the production cost was low, and this approach was applicable to other types of triaxial testing systems. The 3D printing technique was applied to ease manufacture of the testing device and to improve the resolution of the subsequent image analyses. The middle part of the biaxial cell was printed using the 3D printer because it is often difficult to make by the traditional means due to a complicated geometry and the requirement for a one-piece material to ensure the cell is seamless and leak-proof. A packing of elliptical rods, also produced by the 3D printer, is used as the test sample. In principle, any shape of particle with designed properties can be printed. The goal of tracing the particle motion and movement at contacts during shearing is achieved by means of particle image velocimetry (PIV) and close-range photogrammetry, based on the reference dots marked on the observation windows of the biaxial cell and two object dots printed on each rod. The high-resolution 3D printer ensured the accuracy of marking the object dots on the rod and therefore improved the resolution of subsequent analyses. The experimental results demonstrated the validity of the biaxial testing system and the ability and effectiveness of the system to capture the kinematic features of particles and associated contact movements in response to biaxial shearing.

An Instrumented Flume to Characterize the Initiation Features of Flow Landslides

Rainfall-induced landslides are on the rise due to global warming but the associated initiation mechanisms remain unclear. To observe the local initiation features of flow landslides under different rates of water supply, a well-instrumented flume capable of tightly controlling the inflow from the groundwater supply pipes and upstream discharge was built. Basal porewater pressure transducers (PPT) were installed to monitor the water pressure evolution, and a simple method to retrofit pressure transducers into PPTs was introduced. Internal movement behaviors of the slope body prior to the onset of and during a full failure were monitored with low-cost and small-size MEMS (Micro-Electro-Mechanical-Systems) accelerometers. A MEMS sensing package, termed the Smart Soil Particle (SSP, first generation), was also developed in conjunction with the laboratory testing program for field implementation in the future. In a set of experiments designed to investigate the initiation mechanism of a loose soil sample under a small and slow constant groundwater inflow, the MEMS accelerometers and basal PPTs successfully captured the intricate internal movement features and porewater pressure patterns. This warrants more systematic studies of the flow landslide initiation mechanism using the newly developed flume instrumentation.

Towards a Better Understanding of the Electro-Magnetic Properties of Soils

Given the sensitivity of soils to disturbance, the use of low perturbation (i.e., non-destructive) electromagnetic waves provides a viable option for studying soil properties. The measured parameters during the excitation of a material by an electromagnetic wave are electrical conductivity, dielectric permittivity, and magnetic permeability. Electrical conductivity is a measure of charge mobility in response to an electrical field. Dielectric permittivity represents the polarizability of a material. Magnetic permeability indicates the degree of magnetic dipole alignment within a material under the excitation of the magnetic field. Factors that affect electromagnetic wave parameters include water content/degree of saturation, porosity, spatial distribution of the phases (e.g., anisotropy), particle properties (e.g., specific surface), pore fluid characteristics, and temperature. This paper presents a review of electromagnetic parameters, provides experimental data showing the impact of the various factors on the electromagnetic response, and gives a physical explanation of why these factors affect electromagnetic properties. Finally, various applications of electromagnetic wave-based techniques are presented, including monitoring the diffusion of salt through a soft kaolinite sediment and the hydration process in cemented paste backfill.