Sandpile Research Articles

Centrifuge study on the effect of the SCP improvement geometry on the mitigation of liquefaction-induced embankment settlement

Liquefaction of foundation soil causes significant damage to infrastructures. The sand compaction pile (SCP) method is a countermeasure against liquefaction to densify the ground by installing dense sand piles. In recent years, an innovative method has been developed to install sand piles into the ground at any angle. In this study, a series of dynamic centrifuge tests are carried out to investigate the effects of the angle of the SCP improvement zone on mitigating the liquefaction-induced settlement of embankment crests. The lateral displacement beneath the embankment toes is mitigated by the presence of the improvement zone installed in the foundation ground. In particular, the case with a 50° improvement zone is the most effective in mitigating lateral displacement and contributes to the lower settlement of the embankment.

Sandpile Models in the Large

This contribution is a review of the deep and powerful connection between the large-scale properties of critical systems and their description in terms of a field theory. Although largely applicable to many other models, the details of this connection are illustrated in the class of two-dimensional Abelian sandpile models. Bulk and boundary height variables, spanning tree–related observables, boundary conditions, and dissipation are all discussed in this context and found to have a proper match in the field theoretic description.

Nonlinear Sorption of Organic Contaminant during Two-Dimensional Transport in Saturated Sand

Multi-dimensional transport studies are necessary in order to better explain the fate of contaminants in groundwater. In this study, a two-dimensional transport experiment with organic contaminants in saturated sand was conducted to investigate the migration of the organic contaminant plume in multi-dimensional flow conditions. The transport test was conducted using toluene as a model organic contaminant in a saturated sand box under steady flow conditions. The initial plume was generated via injection at a point source. After 24 h, the plume distribution was delineated by interpolating toluene concentrations in the porewater samples. The mass centers of the toluene and the conservative tracer were almost coincident, but the size of the toluene plume was significantly reduced in longitudinal as well as transversal directions. The appropriateness of several types of sorption models were compared to describe the toluene sorption in two-dimensional transport system using numerical modeling. Among the sorption models, the Langmuir model was found to be the most appropriate to describe the sorption of toluene during two-dimensional transport. The results showed that two-dimensional experiments are better than one-dimensional column experiments in identifying the adsorption characteristics that occur during transport in saturated aquifers.

Studies on Bearing Characteristic of Open-Ended and Close-Ended Pipe Piles under High-Speed Railway Loadings

The pile foundation under a high-speed railway (HSR) not only bears the self-weight of the superstructure, but also bears a long-term vertical periodic load during its service. In this study, a large-scale vertical dynamic load test and particle flow numerical simulation method were adopted to investigate the axial cyclic performances of open piles in sand. The results from the dynamic test show that under a dynamic load with cyclic load ratios of 0.1 and 0.2, the surface displacement of the pile decays rapidly as the pile distance increases. Moreover, the higher the vibration amplitude and frequency, the larger the surface displacement. The influence range of the soil around the pile is approximately six times of the pile diameter. It is also found that the external frictional resistance of the upper part of the pile decreases, whereas that of the middle part increases. In addition, the internal friction resistance of the lower part of the pile decreases and gradually stabilizes after approximately 200 cycles. The simulation results show that the influence range of the pile on the surrounding soil is approximately 5–8 times the diameter of the pile under the action of dynamic loading; the vertical displacement of the pile mainly occurs at the beginning of the cycles. The total settlement of the pile foundation increases with an increase in the cyclic load ratio. With an increase in the cycle period, the friction resistance values on the outside of the pile and middle part significantly decrease and increase, respectively. After 10 cycles, the resistance gradually stabilizes, and no evident changes in the internal friction resistance of the pile are observed.

Self-consistent model of the plasma staircase and nonlinear Schrödinger equation with subquadratic power nonlinearity.

A new basis has been found for the theory of self-organization of transport avalanches and jet zonal flows in L-mode tokamak plasma, the so-called "plasma staircase" [Dif-Pradalier et al., Phys. Rev. E 82, 025401(R) (2010)PLEEE81539-375510.1103/PhysRevE.82.025401]. The jet zonal flows are considered as a wave packet of coupled nonlinear oscillators characterized by a complex time- and wave-number-dependent wave function; in a mean-field approximation this function is argued to obey a discrete nonlinear Schrödinger equation with subquadratic power nonlinearity. It is shown that the subquadratic power leads directly to a white Lévy noise, and to a Lévy fractional Fokker-Planck equation for radial transport of test particles (via wave-particle interactions). In a self-consistent description the avalanches, which are driven by the white Lévy noise, interact with the jet zonal flows, which form a system of semipermeable barriers to radial transport. We argue that the plasma staircase saturates at a state of marginal stability, in whose vicinity the avalanches undergo an ever-pursuing localization-delocalization transition. At the transition point, the event-size distribution of the avalanches is found to be a power law w_{τ}(Δn)∼Δn^{-τ}, with the drop-off exponent τ=(sqrt[17]+1)/2≃2.56. This value is an exact result of the self-consistent model. The edge behavior bears signatures enabling to associate it with the dynamics of a self-organized critical (SOC) state. At the same time the critical exponents, pertaining to this state, are found to be inconsistent with classic models of avalanche transport based on sand piles and their generalizations, suggesting that the coupled avalanche-jet zonal flow system operates on different organizing principles. The results obtained have been validated in a numerical simulation of the plasma staircase using flux-driven gyrokinetic code for L-mode Tore-Supra plasma.

The Limit Shape of the Leaky Abelian Sandpile Model

Abstract The leaky Abelian sandpile model (Leaky-ASM) is a growth model in which $n$ grains of sand start at the origin in $\mathbb{Z}^2$ and diffuse along the vertices according to a toppling rule. A site can topple if its amount of sand is above a threshold. In each topple, a site sends some sand to each neighbor and leaks a portion $1-1/d$ of its sand. We compute the limit shape as a function of $d$ in the symmetric case where each topple sends an equal amount of sand to each neighbor. The limit shape converges to a circle as $d\to 1$ and a diamond as $d\to \infty $. We compute the limit shape by comparing the odometer function at a site to the probability that a killed random walk dies at that site. When $d\to 1$, the Leaky-ASM converges to the ASM with a modified initial configuration. We also prove that the limit shape is a circle when simultaneously with $n\to \infty $ we have that $d=d_n$ converges to $1$ slower than any power of $n$. To gain information about the ASM, faster convergence is necessary.

Determination of depth-of-fixity point for laterally loaded vertical offshore piles: A new approach

Offshore piles are used as foundations for various marine structures such as quay walls, dolphins, and offshore platforms. Offshore piles are primarily subjected to lateral loads from wind, waves and currents, which is particularly challenging for piles with substantial free length above the seabed. This paper presents a new approach to determine the depth-to-fixity and the depth of the apparent plastic hinge of offshore piles considering the pile head condition, type of soil below, and the soil nonlinearity employing the p-y curve method. A comprehensive numerical investigation was conducted covering a wide range of key parameters governing the lateral response of offshore piles. The results of the parametric analysis were used to establish numerical equations to determine the depth-to-fixity and the depth of the apparent plastic hinge for free and fixed head piles embedded either in sand or in clay. In addition, a closed form solution has been developed utilizing the equivalent cantilever fixed-base model to estimate the lateral capacity of an offshore pile at specified lateral deflection and the corresponding maximum bending moment. The new approach was verified by field load test results conducted on partially embedded piles in sand and clay.

Ultimate lateral resistance of piles in sand based on the modified strain wedge model

This paper proposes a novel method for calculating the ultimate lateral resistance exerted by sandy soil against laterally loaded piles. The ultimate lateral resistance distribution of the pile on the front side of the pile above the rotation point is determined by a modified strain wedge model with the three-dimensional cone shape, while the ultimate lateral resistance on the back side of the pile below the rotation point is calculated according to the theory of the passive earth pressure. Using the proposed distribution form of the ultimate lateral resistance along the pile length, an explicit expression to calculate the ultimate bearing capacity is obtained based on the limit equilibrium principle. The rationality of the proposed method is verified by several laboratory and field tests from published literature, and the comparative analysis show that the prediction results coincide with the experimental results. The variations in the normalized modified strain wedge shape depth, radius and ultimate bearing capacity of the pile with the change in the relative stiffness are deeply explored by using the proposed method, and some fitted expressions to quickly determine the modified strain wedge shape and estimate the ultimate bearing capacity of different piles are obtained.

Time-Dependent Properties of Sandpiles

Bak, Tang, and Wiesenfeld (BTW) proposed the theory of self-organized criticality (SOC), and sandpile models, to connect “1/f” noise, observed in systems in a diverse natural setting, to the fractal spatial structure. We review some of the existing works on the problem of characterizing time-dependent properties of sandpiles and try to explore if the BTW's original ambition has really been fulfilled. We discuss the exact hydrodynamic structure in a class of conserved stochastic sandpiles, undergoing a non-equilibrium absorbing phase transition. We illustrate how the hydrodynamic framework can be used to capture long-ranged spatio-temporal correlations in terms of large-scale transport and relaxation properties of the systems. We particularly emphasize certain interesting aspects of sandpiles—the transport instabilities, which emerge through the threshold-activated nature of the dynamics in the systems. We also point out some open issues at the end.

Intelligent warehouse monitoring based on distributed system and edge computing

This paper mainly focuses on the volume calculation of materials in the warehouse where sand and gravel materials are stored and monitored whether materials are lacking in real-time. Specifically, we proposed the sandpile model and the point cloud projection obtained from the LiDAR sensors to calculate the material volume. We use distributed edge computing modules to build a centralized system and transmit data remotely through a high-power wireless network, which solves sensor placement and data transmission in a complex warehouse environment. Our centralized system can also reduce worker participation in a harsh factorial environment. Furthermore, the point cloud data of the warehouse is colored to visualize the actual factorial environment. Our centralized system has been deployed in the real factorial environment and got a good performance.

Physical model on effects of tunnelling towards single piles under zoned of influence

Tunnel excavation usually will causes soil redistribution and settlement. Hence, for urban areas where large building used pile for foundation, tunnelling might affect the pile integrity. Engineers pose great role in order to correctly estimating the stress changes onto pile structures. It is particularly vital to estimate the tunnelling effects when new tunnels are to be built near an existing pile. Therefore, this paper presents a physical modelling to investigate the tunnel-pile-soil interaction. Sand box test with pile and tunnel model was developed to imitate the field condition. The laboratory testing conducted with constant parameter tunnel cover to tunnel diameter ratio of 2.5 with pile to tunnel distance of 1.5 of tunnel diameter. Two rate of tunnel excavation were carried out. Laboratory tests determined the longitudinal and transverse ground surface settlement induced by tunnelling, as well as the movement and induced bending moment on a single pile foundation. For validation, initial results were compared with previous findings and shows an agreement. Further test then conducted with higher tunnelling speed. With higher speed, it was found that surface settlements, pile movements and bending moment are more affected.

Skin Friction Directionality in Monotonically- and Cyclically-Loaded Bio-inspired Piles in Sand

Piles can be subjected to axial loading in opposite directions during their installation and service life. For instance, piles for offshore jacket structures and load testing reaction systems are subjected to compressive loading during installation and tensile or cyclic loading during service life. This creates a design dilemma: while a large skin friction can lead to refusal at shallower depths than required during driving, it also promotes a large pile axial capacity. This paper describes the load-transfer behavior of piles with surfaces inspired by the belly scales of snakes that mobilize a direction-dependent skin friction. The investigation presented herein consists of a series of twelve centrifuge pile load tests on bio-inspired and smooth reference piles in dense and loose deposits of Ottawa F65 sand. Test results indicate that greater skin friction forces are mobilized when the bio-inspired piles are displaced in the cranial direction (i.e. soil moving against asperities) relative to the caudal direction (i.e. soil moving along asperities). This is observed during pushing and driving installation, where greater skin friction forces were mobilized during installation by pushing in the cranial direction and driving in the cranial direction required more blows per meter. Similarly, the skin friction mobilized during pullout tests was between 82% and 198% greater in the cranial direction than in the caudal direction, and the skin friction mobilized during pullout by the bio-inspired pile in the cranial direction was between 560% to 845% greater than that mobilized by the reference untextured pile. During cyclic loading, degradation of the skin friction magnitude and pile secant stiffness was observed in both cranial and caudal directions; however, the mobilized magnitudes were generally greater in the cranial direction. Discussion is provided on the potential benefits that the bio-inspired surface texture could realize on the overall performance of axially-loaded piles.

Influence of helix bending deflection on the load transfer mechanism of screw piles in sand: Experimental and numerical investigations

This study investigates the load transfer mechanism that includes the effect of helix bending deflection on end-bearing capacity, distribution of ground pressure under the helix and soil deformation around the screw pile. The helix to shaft diameter ratio of 2.5 and 2.8 with a strong helix and a weak helix were used. The model ground was prepared with fine sand at 80% of relative density. To investigate the load transfer mechanism, the experimental tests were modelled in a 3D finite element code. A good agreement between the experimental and numerical approaches was found. The numerical analyses showed that large influence zone exists under screw pile with strong helix, which resulted in higher mobilized soil shear strength that contributed to higher end-bearing capacity. In the case of strong helix, uniform pattern of pressure distribution was observed under the central shaft and the helix. Similar pattern of pressure distribution under the central shaft was observed in weak helix case but the pattern of pressure distribution under the helix changed from uniform to triangular to trapezoidal at various stages during the load test. The normalized end-bearing capacity decreased linearly with the increase in normalized helix bending deflection in both approaches, i.e. experimental and numerical.

Pull-Out Response of a Laboratory-Scale Energy Pile Subjected to Cooling Cycles

During winter, energy piles are subjected to cooling cycles due to the cyclic operation of the ground source heat pump (GSHP). The cyclic operation of the heat pump causes the energy pile to contract and expand axially and radially, affecting the soil–pile interaction. The effects of cooling cycles on the thermomechanical behavior of energy piles have not been fully investigated. A fully instrumented model concrete energy pile was installed in dry sand and subjected to cooling cycles. Cooling was achieved by circulating a heat exchange fluid with a target inlet temperature of 20°C lower than the initial temperature of the soil (ΔT=−20°C). Following the application of thermal loading, pull-out axial load tests were performed to evaluate the effects of cooling cycles on the load–displacement response of the energy pile. Baseline (room temperature), 5-cooling-cycle (5CC), and 100-cooling-cycle (100CC) tests were performed and summarized in this paper. During the thermal and mechanical load applications, temperature in the soil and the pile, axial strains, radial strains, and radial pressures at the soil–pile interface were measured. For the 1g scaled energy pile and test conditions (pile in dry sand subjected to cooling cycles with ΔT=−20°C), the peak pull-out loads decreased by 32% and 38% for the 5CC and 100CC tests, respectively.

Seismic limit states for reinforced concrete bridge pile in sand

Lateral responses of a pile are governed by its mutual interaction with the supporting soil, known as the Pile-Soil Interaction (PSI), which in turn influences its structural limit states. The present study evaluates the seismic limit states for bridge pile in sand, while incorporating the parametric influences of pile and soil assessed through PSI. Such influences have not been dealt with in the past, except for the p (soil-pile reaction) – y (pile deflection) responses (though with only few pile parameters). Pile cross sectional size, grades of concrete and reinforcement, longitudinal and transverse reinforcement ratios, axial load ratio and the sand effective friction angle are observed to affect the PSI and consequently the limit states. Thus, limit states are proposed as parameterised expressions, generated through regression on the limit state data obtained by analysing the damage model (developed herein) for the pile-soil system samples generated over wide ranges of the influential parameters. These expressions, validated through the compliance with the expected trends and a fair agreement with a test result, can be readily used to obtain the expected values of the limit states over a broad range of parametric combinations and without requiring analysis repetition. These, in turn, can contribute to reliable estimation of fragility of a bridge system with the given properties for its pile foundation.

Effect of pile inclination on the lateral deformation behaviour of pipe piles in calcareous sand

The lateral deformation behaviour of inclined piles in calcareous sand is investigated with model testing and numerical analysis. A model pile is instrumented with quasi-distributed strain sensors and installed in calcareous sand with various inclination angles. The lateral deformation, bending moment and shear force of the pile are analysed by investigating the effect of the inclination angle and embedded depth. The experimental results of piles with inclination angles of −30°, 0° and 30° are analysed. A random finite element method (RFEM) is established to investigate the effect of the pile inclination. A single exponential function is used to characterize the spatial autocorrelation of soil parameters. A random field is obtained by the Cholesky method. The RFEM is verified with experimental results and used to further analyse the lateral deformation behaviour. The results indicate that both of the orthogonal maximum influence distances from the pile centre have a linear relationship with the pile inclination. A linear relationship between the maximum lateral deformation on the pile top and the pile inclination is proposed. The outcome of this study may be helpful for understanding the lateral deformation behaviour of piles in calcareous sand and designing inclined piles.

Evaluation of Incipient Motion of Sand Particles by Different Indirect Methods in Erosion Function Apparatus

An experiment was carried out in an acrylic glass-sided re-circulating closed conduit with a rectangular cross section, which is similar in construction to an erosion function apparatus. An adjustable sand box, made of acrylic glass, was attached to the bottom of the conduit as the sand zone or the test section. The hydraulics of the flow in the erosion function apparatus is complicated due to the limited part of the non-smooth and erodible soil surface attached to the closed conduit. As the bed shear stress changes with the bed roughness, even though the flow velocity does not change, establishing a method to estimate the incipient motion is an important challenge for an erosion function apparatus. The present study was conducted to explore the incipient motion of sands from bed shear stress estimated by four different indirect methods on both the sand bed and the smooth bed installed in the erosion function apparatus. In the experiment, particle image velocimetry (PIV) was used to investigate flow dynamics and incipient motion in terms of dimensionless critical bed shear stress. The experimental results show that the bed shear stress estimated from the log-law profiles in the sand zone and the smooth zones are relatively higher than those of the other indirect methods. The dimensionless critical bed shear stress of threshold condition evaluated by all indirect methods was found in good agreement with those of previous results in both zones. The Manning roughness and Darcy–Weisbach friction coefficients were evaluated based on the critical shear velocity at the incipient motion. Although these coefficients were found slightly greater in the smooth zone than in the sand zone, in both zones, they showed good agreement with previous studies.

Size distribution law of earthquake-triggered landslides in different seismic intensity zones

Abstract. The Ms 8.0 Wenchuan earthquake in 2008 and Ms 7.0 Lushan earthquake in 2013 produced thousands of landslides in the southern region of the Longmen Mountains in China. We conducted field investigations and analyzed remote sensing data to determine the distribution law of earthquake-triggered landslides. The results show a strong negative power law relationship between the size and frequency of landslides in VII, VIII, and IX seismic intensity zones, a weak power law in the X seismic intensity zone, and a lognormal distribution in the XI seismic intensity zone. Landslide density increases with increasing seismic intensity. A sandpile cellular automata model was built under the conceptual framework of self-organized criticality theory to simulate earthquake-induced landslides. Data from the simulations demonstrate that, with increasing disturbance intensity, the dynamical mechanism of the sandpile model changes from a strong power law to a weak power law and then to a lognormal distribution. Results from shaking table experiments of a one-sided slope sandpile show that, for peak ground acceleration (PGA) in the range of 0.075–0.125 g, the relation between the amount and frequency of sand follows a negative power law. For PGA between 0.15 and 0.25 g, the relation obeys a lognormal distribution. This verifies that the abovementioned distribution of earthquake-induced landslides should be a universal law from a physical viewpoint and may apply to other areas. This new perspective may be used to guide the development of an inventory of earthquake-triggered landslides and provide a scientific basis for their prediction.

Numerical Study of Laterally Loaded Piles in Soft Clay Overlying Dense Sand

This paper presents the results of three dimensional finite element analysis of laterally loaded pile groups of configuration 1×1, 2×1 and 3×1, embedded in two-layered soil consisting of soft clay at liquid limit overlying dense sand using Plaxis 3D. Effects of variation in pile length (L) and clay layer thickness (h) on lateral capacity and bending moment profile of pile foundations were evaluated by employing different values of pile length to diameter ratio (L/D) and ratio of clay layer thickness to pile length (h/L) in the analysis. Obtained results indicated that the lateral capacity reduces non-linearly with increase in clay layer thickness. Larger decrease was observed in group piles. A non-dimensional parameter Fx ratio was defined to compare lateral capacity in layered soil to that in dense sand, for which a generalized expression was derived in terms of h/L ratio and number of piles in a group. Group effect on lateral resistance and maximum bending moment was observed to become insignificant for clay layer thickness exceeding 40% of pile length. For a fixed value of clay layer thickness, lateral capacity and bending moment in a single pile increased significantly with increase in pile length only up to an optimum embedment depth in sand layer which was found to be equal to three times pile diameter and 0.21 times pile length for pile with L/D 15. Scale effect on lateral capacity has also been studied and discussed. Doi: 10.28991/cej-2021-03091686 Full Text: PDF

Centrifuge testing on monotonic and cyclic lateral behavior of large-diameter slender piles in sand

The existing studies have been primarily focused on the lateral behavior of large-diameter stubby pile or small-diameter slender pile in sand, with little attention paid to large-diameter slender pile. This study presents a unique series of centrifuge tests on monotonic and cyclic lateral behavior of heavily instrumented large-diameter slender piles in medium dense sand. Two typical length to diameter ratios (L/D) are considered with the same length (L = 60 m) but different diameters (D = 4 and 6 m). It is found that the lateral behaviors of large-diameter slender pile, including its monotonic p-y response, cyclic accumulation of lateral displacement and cyclic stiffness evolution, are marginally different from those of small-diameter slender piles, but significantly deviate from the large-diameter stubby piles. This may suggest the longstanding argument of ‘diameter effect’ is relatively minor, while the lateral behavior of monopile in sand is more significantly governed by the relative pile-soil stiffness. The API (2011) non-conservatively predicts both stiffness and capacity of the large-diameter slender piles, leading to development of a new p-y formulation. These centrifuge testing results form a unique database to support development of new design methods for large-diameter slender piles, and to verify advanced numerical analyses involving cyclic models.