Sandpile Research Articles

An advanced t-z curves method using cyclic shaft friction degradation for the design of piles in sand

A calculation model including an original cyclic t-z method is proposed for the analysis and the design of piles subjected to cyclic axial loads both in terms of displacements and resistance. This model allows the assessment of pile displacement and axial force distribution with depth for each load increment. The cyclic effects and especially the decrease of the shaft friction are accounted for by considering the current state of stress at the pile-soil interface. The approach is based on the ‘ABC’ method developed by the Imperial College. A procedure is developed to combine this method with t-z cyclic curves. Comparisons are made between the numerical results obtained from this cyclic t-z model and the experimental data of tests which were carried out during the project GOPAL (Grouted Offshore Piles For Alternating Loadings) and the French National Project SOLCYP (SOLlicitations CYlcliques sur Pieux): four cyclic loading sequences are considered for both driven and bored piles. The reliability of the proposed model is evaluated according to the loading level and the number of cycles. The use of this model also leads to the construction of cyclic stability diagrams.

Improvement of subgrade soil with sand columns for construction railway embankment: A case study

AbstractA railway on 4 m height embankment is being built in the south of Iraq. The railway alignment is extending on a compressible soil experienced impermissible settlement according to the results of soil investigation of the soil at the project site. A trial for reducing the settlement was done adopting sand piles. Nonlinear analysis was conducted to evaluate the settlement of soil before and after using the proposed technique.The results indicated that without any improvement, the expected total settlement of the railway embankment on the compressible clay layer is 170.2 mm. By inserting granular piles of diameter 0.3 m, it was obtained settlement decreases to 88.7 mm with reduction being in range of ∼48%.

3D FE analysis of bored pile- pile cap interaction in sandy soils under axial compression- parametric study

In this research study, a 3D elasto-plastic finite element was used to analyze the influence and the interaction between the bored concrete pile, pile cap, and surrounding ground under uniaxial compression load, using ABAQUS finite element software. Mohr-Coulomb plasticity model was used to simulate sand elements while the pile was modeled as elastic material with a modulus, Ep. The contact between pile and adjacent sand was simulated using contact pair with surface-to-surface type of interaction. Different sand properties and pile length / diameter (L/D) ratios were considered in the analysis. Pile cap bearing pressure (qcap), side shear stress (fside), tip bearing pressure (qtip), deformation, and stresses and strains around the pile were investigated. The results show that for the same soil, as L/D ratio increases qcap decreases. Moreover, the results show that for the same L/D ratio, qcap decreases as the soil modulus (Es) increases and qtip decreases slightly with the increase in Es whereas for the same Es, qtip increases with the increase in L/D ratio. The major contribution in the capacity of capped bored pile under compression loading is devoted to qtip. As L/D increases, the contribution of qtip increases. Furthermore, qtip increases as the relative stiffness, Es/Ep increases.

Research on dynamic p-y curves in liquefied sand under cyclic loading conditions

Under dynamic cyclic loads, the pore water pressure in saturated sand rises continuously, which leads to the reduction of horizontal bearing capacity or the damage of the pile foundation. Under dynamic cyclic loads, the existing calculations methods of horizontal bearing capacity of pile foundations were obtained by proposing a reduction factor for the static p-y curve model mostly. Compared with the test results, the soil resistance around the pile obtained by this method is often overvalued and the pile-soil relative displacement has a relative underestimation. As a result, this method is not suitable for pile foundation design. Therefore, the variations of soil resistance around the pile and the pile-soil relative displacement are obtained by analyzing the effect of pore pressure ratio on soil strength around the pile in saturated sand in this paper. The theoretical formulas related to the cyclic vibration times and pore pressure ratio are proposed. The proposed formulas are introduced into the dynamic p-y curve model to obtain the dynamic p-y backbone curve in the process of sand liquefaction. The research results show that the variations of the soil resistance around the pile and the pile-soil relative displacement are related to the pore pressure ratio and the cyclic vibration times during the sand liquefaction. As the cyclic vibration times increases, the pore pressure increases, the soil resistance around the pile decreases and the pile-soil relative displacement increases.

Research on the Influence of Spudcan Penetration and Extraction on Adjacent Platform Piles in Sand

Research on the Influence of Spudcan Penetration and Extraction on Adjacent Platform Piles in Sand

Sandpiles subjected to sinusoidal drive.

This paper considers a sandpile model subjected to a sinusoidal external drive with the period T. We develop a theoretical model for the Green's function in a large T limit, which predicts that the avalanches are anisotropic and elongated in the oscillation direction. We track the problem numerically and show that the system additionally shows a regime where the avalanches are elongated in the perpendicular direction with respect to the oscillations. We find a crossover between these two regimes. The power spectrum of avalanche size and the grains wasted from the parallel and perpendicular directions are studied. These functions show power-law behavior in terms of the frequency with exponents, which run with T.

Scaling limits of fluctuations of extended-source internal DLA

In a previous work, we showed that the 2D, extended-source internal DLA (IDLA) of Levine and Peres is δ3/5-close to its scaling limit, if δ is the lattice size. In this paper, we investigate the scaling limits of the fluctuations themselves. Namely, we show that two naturally defined error functions, which measure the “lateness” of lattice points at one time and at all times, respectively, converge to geometry-dependent Gaussian random fields. We use these results to calculate point-correlation functions associated with the fluctuations of the flow. Along the way, we demonstrate similar δ3/5 bounds on the fluctuations of the related divisible sandpile model of Levine and Peres, and we generalize the results of our previous work to a larger class of extended sources.

A generalized elastoplastic load-transfer model for axially loaded piles in clay: Incorporation of modulus degradation and skin friction softening

The softening behavior of skin friction is commonly observed from load-displacement responses of axially loaded piles in stiff clay, sensitive clay, and dense sands at a large loading level. This paper presents a generalized and novel elastoplastic load-transfer model to predict the load-displacement response of axially loaded piles in clay considering the degradation of skin friction. In this model, the soil around piles is divided into the shear band immediately adjacent to piles and the soil outside shear bands. An elastoplastic model in terms of the critical state theory-based modified Cam-clay model is developed to describe the mechanical behavior of shear bands. The elastoplastic model can explicitly consider both hardening and softening behaviors of skin friction. The elastic behavior of the soil outside shear bands is represented by a modulus degradation model, which captures the shear modulus degradation with the shear strain. Subsequently, an elastoplastic load-transfer model possessing generalized ability to describe both hardening and softening behaviors is developed by combining these two theoretical models. Predictions of the load-settlement curves are compared with the measured values from three well-documented cases to verify the proposed model. Good agreements demonstrate that the proposed load-transfer model provides reasonable and effective prediction for bearing performance of piles.

Experimental Studies and Finite Element Modeling of Piles and Pile Groups in Dry Sand under Harmonic Excitation

Foundations supporting reciprocating engines, radar towers, turbines, large electric motors, and generators, etc. are subject to vibrations caused by unbalanced machine forces as well as the static weight of the machine. If these vibrations are excessive, they may damage the machine or cause it not to function properly. In the case of block foundation, if changes in size and mass of the foundation do not lead to a satisfactory design, a pile foundation may be used. In this study, the dynamic response of piles and pile Groups in dry sand is investigated experimentally. The analysis involves the displacement response under harmonic excitation. In addition, a numerical modeling by using finite element method with a three-dimensional formulation is adopted to simulate the experimental model. The results of the numerical model showed that a good agreement is achieved between the predicted dynamic response and that measured from the experimental model.

Some statistics about Tropical Sandpile Model

Tropical sandpile model (or linearized sandpile model) is the only known continuous geometric model exhibiting self-organised criticality. This model represents the scaling limit behavior of a small perturbation of the maximal stable sandpile state on a big subset of $\mathbb Z^2$. Given a set $P$ of points in a compact convex domain $\Omega\subset \mathbb R^2$ this linearized model produces a tropical polynomial $G_P{\bf 0}_\Omega$. Here we present some quantitative statistical characteristics of this model and some speculative explanations. Namely, we study the dependence between the number $n$ of randomly dropped points $P=\{p_1,\dots,p_n\}\subset[0,1]^2=\Omega$ and the degree of the tropical polynomial $G_{P}{\bf 0}_\Omega$. We also study the distributions of the coefficients of $G_{P}{\bf 0}_\Omega$ and the correlation between them. This paper's main (experimental) result is that the tropical curve $C(G_{P}{\bf 0}_\Omega)$ defined by $G_{P}{\bf 0}_\Omega$ is a small perturbation of the standard square grid lines. This explains a previously known fact that most of the edges of the tropical curve $C(G_{P}{\bf 0}_\Omega)$ are of directions $(1,0),(0,1),(1,1),(-1,1)$. The main theoretical result is that $C(G_{P}{\bf 0}_\Omega)\setminus (P\cap \partial\Omega)$, i.e. the tropical curve in $\Omega^\circ$ with marked points $P$ removed, is a tree.

Analytical model for lateral response of piles considering inhomogeneous disturbance characteristics of homogeneous sand

This paper proposes an analytical model for calculating the lateral response of piles in homogeneous sand by considering inhomogeneous disturbance characteristics distribution. The disturbed soil in front of the pile is separated into strain wedge (SW) model elements, with the characteristics of each element calculated through coordinated pile-soil deformation. The limit of the stress level of the SW model element is also considered, and two geometric characteristic distributions are provided. A hyperbolic model is used to calculate the shear stress on the pile side. The proposed model is verified through four pile tests, and the accuracy of the new model is demonstrated by comparison. Additionally, the distributions of soil strain, stress level, fan angle and three critical depths are discussed. The results demonstrate that the model can realistically reveal the inhomogeneous stress–strain status distribution and geometric characteristics of the disturbed soil.

On the Fluctuations of Internal DLA on the Sierpinski Gasket Graph

Internal diffusion limited aggregation (IDLA) is a random aggregation model on a graph G, whose clusters are formed by random walks started in the origin (some fixed vertex) and stopped upon visiting a previously unvisited site. On the Sierpinski gasket graph, the asymptotic shape is known to be a ball in the graph metric. In this paper, we improve the sublinear bounds for the fluctuations known from its known asymptotic shape result by establishing bounds for the odometer function for a divisible sandpile model.

Simulation study of oxytetracycline contamination remediation in groundwater circulation wells enhanced by nano-calcium peroxide and ozone

The widespread use of antibiotics in recent years has led to increasing antibiotic contamination of shallow groundwater. As the most widely used tetracycline antibiotic, oxytetracycline has received a lot of attention from researchers due to its stable molecular structure and difficulty in degradation. Aiming at remediation of oxytetracycline pollution in shallow groundwater, nano-calcium peroxide (nCaO2) and ozone (O3) are used to enhance the degradation of oxytetracycline in groundwater circulation well (GCW). A three-dimensional sand box test device for circulation wells is designed to explore the repair efficiency of circulation wells strengthened by different oxidants. The results show that after nCaO2 and O3 enhancing circulation wells operate for 10 h, the average removal rate of OTC reaches 83%, and the highest removal rate is 88.13%, which is 79.23% and 13.96% respectively higher than that of nCaO2 and O3 enhanced circulation wells alone, and there is no rebound phenomenon after aeration stops. The in-situ treatment of enhanced GCW by nCaO2 and O3 has potential applications for the removal of OTC in groundwater environments.

Code provisions evaluation for the punching shear capacity of R.C footings without reinforcement for punching shear

There is no agreement on how to predict punching shear for footings among different design codes, which is inconsistent with experimentally observed capacity. Such inconsistency is due to the following issues: (1) the different definitions used by design codes for the critical section that resists the punching shear; (2) the different handling of the main parameters' influences, such as the shear slenderness, longitudinal reinforcement ratio, yielding strength of reinforcement, and compressive strength of concrete; (3) the different modelling of the subsoil underneath the footings is affecting the prediction of punching; and (4) the different modelling of the subsoil beneath the footings is affecting the prediction of punching. The current study quantified the level of safety for the various international design codes using the 195 RC column footings that had been punching shear tested and obtained from earlier investigations. Six design codes, standards, or guidelines were selected, including ACI 318–19, PrEC2, JSCE (1997), CSA and BS 8110–1 (1997), and fib Model Code (2010). The effect of the main parameters on the safety of the punching shear capacity calculated using selected methods was examined. These main parameters are concrete and reinforcement material characteristics, effective depth, column dimension, shear slenderness, and reinforcement ratio. In addition, the effect of modelling the subsoil underneath the footing is considered a parameter. It is found that all selected methods underestimate calculated punching shear capacity compared to those measured experimentally. In addition, the BS8110 is the most conservative of all selected design codes, while the second generation of the Euro code (PrEC2) is the closest to measurements and the most reliable. Moreover, the safety (Vexp/Vcalc.) of the predicted capacity using all selected methods is inversely proportional to the effective depth, the concrete compressive strength, and the column dimensions. i.e., the predicted capacity is increased with increasing these parameters, while being directly proportional to the yielding strength of reinforcement and the shear slenderness ratio, i.e., the predicted capacity is decreased with increasing these parameters. All design codes do not consider the effect of reinforcement material characteristics, even though this parameter significantly affects the results. For the flexure reinforcement ratio, the safety of the predicted punching shear capacity using ACI, CSA, and fib Model Code is directly proportional. While it is inversely proportional to the safety calculated using the PrEC2, and JSCE design codes. This is due to the fact that the PrEC2, and JSCE design codes considered the flexure reinforcement ratio while the ACI, CSA, and fib model codes neglected its effect. All selected design codes predict the punching shear capacity well if the concrete compressive strength ranges from 20 to 30 MPa, the reinforcement yielding strength is less than 500 MPa, the reinforcement ratio is less than 0.4, the effective depth ranges from 200 to 300 mm, the shear slenderness ratios are less than 2 or more than 3, and the column periphery is related to the effective depth (bo/d) between 2.0 and 6.0. Additionally, the spring simulation of the subsoil provides capacity values close to those obtained experimentally, while the sand box model, which assumes uniform soil pressure, provides the divergence values of punching capacity. The PrEC2 and fib MC2010 LOAIII design codes provided less safety than the predicted punching shear capacity for different types of sub-soil simulations.

Mechanical behavior analysis and bearing capacity calculation of CFG pile composite foundation on coral sand site

Coral sand foundation formed by hydraulic fill often faces the problem of poor bearing capacity. This paper proposed for the first time to apply CFG pile composite foundation to coral sand sites to verify the feasibility of this scheme and understand its mechanical characteristics. Firstly, taking on-site coral sand as the research object, a pile sand interface shear test was conducted to clarify the mechanism of pile side friction. At the same time, the ultimate bearing capacity of CFG pile and its composite foundation was measured through in-situ static load tests. Then, based on the strength parameters of the pile sand interface revealed by indoor tests, numerical simulations were conducted to analyze the bearing characteristics of CFG piles and their composite foundations. Finally, a method for calculating the vertical bearing capacity of rigid piles in composite foundation considering interface parameters was proposed. The results showed that the bearing capacity characteristic values of single pile and composite foundation meet the design requirements; The interface friction angle and cohesion together increased the ultimate side friction by 64.41%; The load is mainly borne by the pile tip resistance, and the increase of the interface friction angle will make the proportion of the side friction load first increase and then decrease more obviously; The pile soil stress ratio first increased and then tended to stabilize as the interface strength increased. Compared with the field static load test results, the rationality of the calculation method for composite foundation rigid piles was verified. This study may have reference significance for the design and construction of coral sand foundation treatment in offshore island and reef projects.

Geotechnical behavior of a single pile in sand with varied cross-section geometries and construction techniques

Geotechnical behavior of a single pile in sand with varied cross-section geometries and construction techniques

CPT-based design method for helical piles in sand

Helical piles are used extensively for low and medium rise developments and are a popular foundation solution in cases where resistance to significant uplift loads is required. The ability to re-use helical piles and recent interest in their use as foundations in offshore wind and solar energy applications has renewed interest in improving existing design methods. This paper presents the results from field investigations in medium dense and dense sand that examine effects on the axial tension and compression capacity of varying the helix pitch, shaft diameter, advancement ratio and shaft tip geometry. The results from the 20 pile tests conducted (many of which included instrumentation) indicate low sensitivity to a range of the investigated parameters and a strong correlation of the capacity to the cone penetration test resistance. A design method based on these findings is proposed for typical helical pile geometries and is shown to provide predictions that are generally within 10 to 15% of the axial capacities reported for 30 other well documented pile tests reported in the literature. The method, referred to as UWASP-22, incorporates a simple means of estimating the load-displacement response of a helical pile as well as a formulation enabling prediction of the installation torque which is a common quality control measure for helical piles.

Continuum and discrete element modelling for describing coupled hydro-mechanical effects of earthworm burrow coatings on soil shrinkage

Structured soils with earthworm burrows are locally heterogeneous due to coatings along these biopore walls that are superimposed on the inter- and intra-aggregate pore network of the soil matrix. The partially compacted finer-textured and organic matter-rich coatings can limit the flow exchange between the macropores and the soil matrix during preferential flow. Still unknown are dynamic hydro-mechanical interrelations in coating and matrix domains that affect stress–strain behaviour at the macroscopic scale. Such hydro-mechanic interactions may be described with the discrete element method (DEM) coupled with a two-phase pore finite volume (2PFV) approach if relevant pore structures are represented in the model. The objective was to develop a coupled DEM-2PFV model together with a parameterization procedure. Major task was to create a parameterization procedure to calibrate micro parameters of the model by macro properties quantified from drainage experiments of soil samples with earthworm burrow wall coatings.The solid phase was modelled by particle aggregation creating inter and intra-aggregate pore network for the soil matrix in a cube of about 5 cm edge length and one side with the coating structure. This DEM model was coupled with a 2PFV model to simulate hydro-mechanic effects during drainage. Sand box drainage experiments were carried out on soil matrix and biopore samples with laser surface elevation measurements to obtain the mechanical stress–strain macro parameters necessary for model calibration. The poly-dispersed DEM-2PFV model was able to describe effects of two-phase air–water flow on stress–strain macro parameters. The micro parameters (i.e., particle stiffness and bond strength) of the pore scale model were obtained from macro parameters of the primary and secondary stress–strain stages by training a random forest meta-estimator. The model was able to reproduce the pore network of coating material and the inter- and intra-aggregate pore network of the matrix that are dynamically changing with the effective stress. The machine learning model revealed that the bond strength among particles within aggregates governed the shrinkage of soil matrix, while the particle stiffness of the coating material reduced the susceptibility of aggregate breakage producing a more stable inter-aggregated pore network during the drainage process. This study confirmed that coating material present in biopore surface increases the horizontal soil hydro structural stability. The microscale hydro-mechanic modelling can be useful for finding flow exchange parameters inputs for upscaled models and correlating pore-scale parameters to experimentally determined stress–strain macro parameters.

Metamodelling of the load-displacement response of offshore piles in sand

The paper illustrates the development of metamodels of the response of steel piles driven in sand and subjected to pull-out. The metamodels are created for the prediction of the pile tensile capacity and secant stiffness. They were developed using the results of finite element analyses, which made use of finite element models of robustness assessed employing a selection of available data from large-scale model pile tests. Four hundred finite element analyses allowed for the calibration of very accurate metamodels, which were also demonstrated to closely track the outputs of the experimental results. Once calibrated, the metamodels can be used independently from the finite element models they stemmed from. The outcomes of the study show that metamodels of piles response can yield very accurate results within a wide and realistic range of soil-pile configuration, avoiding the laborious implementation and computational cost which underpins the use of finite element models. As the use of metamodels in this context is new, the paper relies on particularly simplified problem, but the procedure could be extended to accommodate modelling features of higher complexity.

Experimental study on the behavior of single pile foundation under vertical cyclic load in coral sand

The bearing behaviors of piles in coral sand are critical for project safety due to the unique physical and mechanical properties of coral sand, such as high void ratio, high compressibility, high friction angle and easy to break. A series of model experiments were carried out in this study to evaluate the vertical behavior of single pile in coral sand foundation under cyclic dynamic load with the consideration of the influence of the amplitude of cyclic load, the static biased load and the number of cyclic. Load-settlement relationships, settlement-cyclic number relationships, pile tip and pile side resistances have been thoroughly investigated. Results showed that the load in coral sand foundation will transfer to the pile tip more obviously compared with the pile foundation in quartz sand. Therefore, the pile side resistance will be reduced while the corresponding pile tip resistance will be increased. Particularly, the weakening of side resistance mainly occurs in the first 500 cycles. The development of cyclic cumulative settlement is affected by the combination of the magnitude of static biased load and cyclic amplitude. The existence of static biased load is conducive to restrain the cyclic cumulative settlement when the amplitude of cyclic load is less than static load.