W. M. Yan

Effect of Particle Grading on the Response of an Idealized Granular Assemblage

The effects of particle-size distribution on a granular assemblage’s mechanical response were studied through a series of numerical triaxial tests using the three-dimensional (3D) discrete-element method. An assemblage was formed by spherical particles of various sizes. A simple linear contact model was adopted with the crucial consideration of varying contact stiffness with particle diameter. Numerical triaxial tests were mimicked by imposing axial compression under constant lateral pressure and constant volume condition, respectively. It was found that an assemblage with a wider particle grading gives more contractive response and behaves toward strain hardening upon shearing. Its critical state locates at a lower position in a void ratio versus mean normal stress plot. Nevertheless, no obvious difference in the critical stress ratio was shown. Model constants in a simple but efficient phenomenologically based granular material model within the framework of critical-state soil mechanics were calibrated ...

On the Proper Estimation of the Confidence Interval for the Design Formula of Blast-Induced Vibrations with Site Records

Abstract Blast-induced ground vibration has received much engineering and public attention. The vibration is often represented by the peak particle velocity (PPV) and the empirical approach is employed to describe the relationship between the PPV and the scaled distance. Different statistical methods are often used to obtain the confidence level of the prediction. With a known scaled distance, the amount of explosives in a planned blast can then be determined by a blast engineer when the PPV limit and the confidence level of the vibration magnitude are specified. This paper shows that these current approaches do not incorporate the posterior uncertainty of the fitting coefficients. In order to resolve this problem, a Bayesian method is proposed to derive the site-specific fitting coefficients based on a small amount of data collected at an early stage of a blasting project. More importantly, uncertainty of both the fitting coefficients and the design formula can be quantified. Data collected from a site formation project in Hong Kong is used to illustrate the performance of the proposed method. It is shown that the proposed method resolves the underestimation problem in one of the conventional approaches. The proposed approach can be easily conducted using spreadsheet calculation without the need for any additional tools, so it will be particularly welcome by practicing engineers.

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.

Coupled-Consolidation Modeling of a Pile in Consolidating Ground

When a pile is embedded in a consolidating ground (e.g., newly reclaimed land), soil may settle more than the pile, thus generating negative skin friction along the pile shaft. This negative friction induces additional axial load to the pile (dragload) and pulls the pile further downward (downdrag). In this paper, the problem is investigated numerically with the finite-element package ABAQUS. It was found that the package defaults an interface model that models the mobilized interface strength in a way that the effect of water pressure was overlooked. Therefore, a modified numerical algorithm is proposed in this study. It amends the model by correctly bringing pore water pressure into the calculation steps. which properly simulates the effective stress-dependent nature of the shear strength at the soil-pile interface. The algorithm is then verified by a self-contained simple to understand simulation. A case history of two piles (one of them coated with bitumen) embedded in a consolidating soft ground is then back analyzed with the proposed algorithm. Fully coupled consolidation and geometric nonlinearity are also considered in the analyses. The transient response of the problem is investigated, including the development of dragload, downdrag, and neutral plane with time. The simulation generally fits well with the field measurements. Parametric studies of the effects of pile head loading reveal that the position of the neutral plane depends not only on the magnitude of the applied pile head loading, but is also affected when the load is applied with respect to the consolidation process. DOI: 10.1061/(ASCE)GT.1943-5606.0000651.

Prediction of pile set-up in clays and sands

Increase in pile capacity after initial driving has been well observed in clays and sands over decades. The phenomenon is referred to as pile set-up by geotechnical engineers. More economical pile design may benefit from this time-dependent increase subject to a reliable prediction. Simple empirical relations of the current capacity with the initial capacity and elapse time after driving are available in the literature with different model parameters being suggested for clays and sands, respectively. Nevertheless, appropriateness of the relations and confidence interval of the model parameters are rarely investigated and this hinders the application of these formulae. In this study, a revised single-parameter empirical relation is proposed based on the existing formulae. A comprehensive database from pile field tests data in clayey and sandy ground in the literature is compiled and Bayesian analysis is conducted on both these empirical formulae independently for clays and sands. Bayesian inference allows not only the estimation of the uncertain parameter but also the quantification of the associated uncertainty in the form of probability distribution. This study sheds lights on the confidence interval of the model parameter and it provides designers more reliable prediction of the additional capacity due to pile set-up.

Fully Coupled Consolidation Analysis of Shear Strength Mobilization and Dragload of a Pile Subject to Negative Skin Friction

Mobilization of negative skin friction (NSF) along a pile shaft increases the axial load (dragload) and pile head settlement (downdrag) of the pile, which may exceed the structural capacity of the pile and violate its serviceability performance, respectively. Therefore, the dragload estimation is essential for a pile embedded into a settling ground, where NSF is very likely to occur. This paper presents the results of a series of fully coupled consolidation numerical parametric studies of a pile embedded into a consolidating ground. The investigation focuses on the influences of pile geometry, ground compressibility and loading condition on pile responses. The soil compressibility and time to apply the pile head load are found to have little influence on the degree of skin friction mobilization in the long term. The long-term neutral plane (NP) lies within the 60–70% range of the pile embedded length when the pile head load is absent. The presence of a head load shifts NP upwards, and the amount of upshift increases with decreasing pile diameter and length. A simple design chart is proposed to estimate the skin friction distribution in the long term in cases where the pile head load is absent. Finally, an illustrative example is provided to demonstrate how the chart is used and performs. The study offers practicing engineers a simple and quick approach to estimating the dragload of a pile subject to NSF.

Performance of ground penetrating radar in root detection and its application in root diameter estimation under controlled conditions

A plant is stabilized by its root system. In congested urban cities such as Hong Kong, ground trenching is frequently seen due to the installation of utility lines along the roadside. Soil nailing, which involves soil coring in slopes, is a common solution to improve the slope stability. However, both activities inevitably pose a risk to the integrity of any root systems present, and thus reduce the root anchorage. To prevent or minimize such damage, a careful design of the excavation/drilling location is of prime importance. Ground penetrating radar (GPR) provides a non-destructive method for locating roots by examining the contrast between the dielectric properties of the roots and the surrounding soil. To examine the performance of GPR and promote its use in Hong Kong, a test bed was prepared using local materials to create a controlled environment in which to conduct a series of systematic tests evaluating the performance of a 900 MHz GPR. The reflected radargrams were subject to the influence of the following factors: size and depth of roots, horizontal distance between roots, and contrast between the root and soil water content. Correlations between root size and a number of waveform parameters were also explored. Limiting values for root size, root embedded depth, horizontal separation distance between roots, and water content contrast between root and soil were obtained. A significant correlation was found between the root diameter and time travel parameter T2 (p<0.001, r=0.795). Because GPR root detection is highly site-specific, this study provides a local reference for GPR performance in the Hong Kong environment. The findings demonstrate that the 900 MHz GPR is applicable in Hong Kong for the detection of main roots.

Particle Elongation and Deposition Effect to Macroscopic and Microscopic Responses of Numerical Direct Shear Tests

In this study, a series of numerical direct shear tests is carried out by the three-dimensional discrete element method. The box is filled by either spherical or elongated particles of mono-size. Particles of three different aspect ratios (defined as length/width of a particle), 1 (i.e., spherical), 1.5 and 2, are modeled. Elongated particles are created by joining primary spherical balls together, and no particle breakage is allowed. The granular specimen is prepared by either depositional method or by random generation of particles inside the box. By controlling the interparticle friction coefficient, number of particles and deposited direction, particle assemblies with very close initial density but different packing or microstructure can be obtained. Various measurement spheres are defined at different locations of the box to reveal the local stresses by considering interparticle interaction forces. The results show a significant spatial variations of the stresses, which deviate noticeably from the global measurements recorded at the box boundaries. Furthermore, global measurements appreciably conclude higher ultimate strength of the assemblage as compared to the local ones from the measurement spheres, regardless the particles’ aspect ratio and packing. The ultimate shear strength increases with particles’ aspect ratio. Initial fabric affects the ultimate shear strength such that the assemblage having more particles aligning parallel to the shear direction (Dep⊥S) yields the lowest strength. On the other hand, randomly packed assemblage exhibits the highest strength. Furthermore, Dep⊥S specimen shows the least amount of dilation. Particle orientation is described by a tensorial parameter, and its evolution during shear is discussed. Analysis shows that only particles close to the shear plane exhibit significant rotation and thus a noticeable change in the fabric. It is found that the evolution of fabric tensor is closely linked to the macroscopic response of an assemblage. Fabric analysis helps to explain the macroscopic responses from a microscopic particle rearrangement perspective.

Cyclic Degradation of a Multidirectionally Laterally Loaded Rigid Single Pile Model in Compacted Clay

AbstractIn practice, the lateral loads applied to piles are often multidirectional. To examine the effect of the loading path on the behavior of piles embedded in clay under lateral cyclic loading, a series of model tests under various displacement paths was conducted using a computer-controlled biaxial motion platform. The displacement paths at the pile head were the unidirectional regular path, the cross path, the eight-shape path, the unidirectional irregular path, and the multidirectional irregular path. The experimental results revealed that under the same displacement amplitude, degradation of the lateral resistance with the number of cycles was more significant under the eight-shape path than under the cross path, and the degradation under the cross path was more significant than under the unidirectional path. The degradation factor was as high as 38% under the eight-shape path with a relatively large displacement amplitude, which was approximately 65% larger than the corresponding value under the ...

Development of Neutral Plane on a Pile in a Consolidating Ground

When a pile is installed in a layer of soft and compressible soil, the pile would be subjected to skin friction as the consequence of the relative settlements between the soil and the pile. Negative skin friction (NSF) is mobilized when soil settles more than the pile while positive skin friction (PSF) occurs vice versa. The location where NSF changes to PSF is called the neutral plane which is also the position where the relative movement is zero. This paper presents the results of a series of axis‐symmetric coupled‐consolidation finite element modeling of a wish‐in‐place pile embedded in a consolidating ground. Geometric non‐linearity technique is adopted and the pile‐soil interface is assumed to follow a bi‐linear Coulomb type frictional behavior. Attempts have been made to study the effects of the soil’s compressibility, pile head loading and pile end‐bearing type on the development of skin friction and neutral plane with time. It is found that NSF is fully mobilized near the ground surface. In the ca...