Non-displacement Piles In Sand Research Articles

Discussion on “A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand”

In a recent paper (Lashkari, 2017), the author proposed a simple constitutive model for predicting the behaviour of a sand-structure interface under the framework of critical state soil mechanics. However, there are some issues, such as the parameters for the proposed interface model and the algorithm used for the pile segment analysis, that should be discussed in detail.

Response to the discussion on “A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand” by R.-F. Feng, Q.-Q. Zhang, S.-W. Liu, X.-M. Li

Response to the discussion on “A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand” by R.-F. Feng, Q.-Q. Zhang, S.-W. Liu, X.-M. Li

A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand

A simple semi-hyperbolic state-dependent constitutive model for sand-structure interfaces is proposed. The model formulation is consistent with critical state soil mechanics since void ratio evolves continuously with shear strain from initial state towards asymptotic critical state at extremely large shear strains. The model takes into account influence of normal stiffness on volume change and stress path. The proposed interface model is implemented in a pile segment analysis scheme for simulation of shaft resistance mobilization in non-displacement piles. Results reveal that the proposed pile segment analysis can well predict shaft resistance of model piles embedded in different sands.

Shaft and base resistance of non-displacement piles in sand

This paper presents the results of three-dimensional, finite element analyses performed with an advanced, two-surface-plasticity, constitutive sand model to investigate the response of non-displacement piles to axial loading. The analysis domain is carefully meshed such that the formation and evolution of shear bands next to the pile shaft and near the pile base can be properly captured. Analyses considering various soil profiles and pile geometries show that the mobilized lateral earth pressure coefficient K along the pile shaft increases with increasing relative density and decreasing initial confining stress. The ultimate unit base resistance is independent of pile diameter, increasing with increasing relative density and increasing initial confining stress at the pile base. Based on the analysis results, design equations are proposed to estimate the limit shaft resistance and ultimate base resistance of non-displacement piles in sandy soil. In proposing these relationships, the pile slenderness ratio is considered. The effect of layer proximity to the base of the pile or pile base embedment in a layer is also considered.

The base resistance of non-displacement piles in sand. Part I: field tests

This paper presents the results of full-scale load tests performed to investigate the end bearing pressure mobilised by continuous flight auger piles installed in sand. In particular, the tests considered the effects of the footing width and, by varying the load test procedure (from maintained load test to constant rate of penetration), allowed quantification of creep effects. By comparing the load test results with in situ test results from cone penetration tests, correlations between the end bearing pressure mobilised at normalised settlement levels of 10% of the footing width and the cone penetration test qc value were studied. For the maintained load tests, these correlations were found to be similar to those used in routine design practice. When creep effects were reduced using constant rate of penetration load testing, the end bearing pressure mobilised was significantly higher than that assumed in normal practice, and it was in keeping with the results of finite-element analyses performed using a soil model that ignored creep. In the final section, the field test results are compared to database pile load tests performed on non-displacement piles in sand.

Prediction of the shaft resistance of nondisplacement piles in sand

SUMMARYUsing pile segment analysis, the mobilized shaft resistance of axially loaded nondisplacement piles in sand is investigated here. It is accepted that the shaft capacity of piles constructed in granular soils is highly influenced by the mechanical behavior of soil–structure interfaces forming adjacent the piles skin. Adopting the thin interface layer as a load transfer mechanism, a simple but accurate critical state compatible interface constitutive model is introduced. After evaluation, the interface model in conjunction with the pile segment analysis is applied for the prediction of the shaft resistance mobilized in nondisplacement piles. The proposed approach takes into account the influences of pile diameter and surface roughness together with the effects of the surrounding soil density and stiffness on the mobilized shaft resistance. The performance of the proposed method is verified by comparing its predictions with the experimental data of various model piles covering wide ranges of length, diameter, roughness, and surrounding soil properties. Copyright © 2012 John Wiley & Sons, Ltd.

Analysis of the shaft resistance of non-displacement piles in sand

The paper examines, using numerical modelling, the problem of the limit shaft resistance of non-displacement piles installed in sands. The modelling makes use of an advanced, two-surface-plasticity constitutive model. The constitutive model predicts the soil response in both the small- and the large-strain range, while taking into account the effects of the intermediate principal effective stress and of the inherent anisotropy of the sand. Finite element analyses of shearing along the pile shaft are performed in order to examine the development of limit unit shaft resistance and the changes in stress state around the shaft upon axial loading of the pile. Special focus is placed on the operative value of the lateral earth pressure coefficient when limit shaft resistance is reached. The analyses offer useful insights regarding the factors controlling the value of unit shaft resistance in sands. The simulations predict a significant build-up of horizontal effective stress for dense sands. Based on these simulations, we propose a relationship between the lateral earth pressure coefficient for use in the calculation of the limit shaft resistance of the pile and the initial density and stress state of the sand.

On the Shaft Friction Modelling of Non-Displacement Piles in Sand

ABSTRACT To evaluate the capability of a centrifuge physical model to catch the main aspects of the shaft friction mobilisation of non-displacement piles in sand, a series of centrifuge tests was carried out by axially loading an instrumented model pile embedded without displacement, in homogeneous dry sand. Three model piles with different roughness were combined with two silica sands (Toyoura and FF) with different grain size. After a description of the experimental set-up, test results were analysed with reference to the current practice (β method). Moreover, the effects of the normalised pile skin roughness, Rn, on the shear stress mobilised were investigated. Finally, the shaft friction mechanisms observed from centrifuge tests, were interpreted by direct shear tests on sand/rigid plate interface under constant normal stiffness.

Analysis of calibration chamber plate load tests

The estimation of base resistance is a key step in the design of piles embedded in moderately dense to dense sand. Calibration chamber plate load tests are sometimes used to investigate the base load - settlement relationship of nondisplacement piles in sand. In such tests, the sand specimens are carefully prepared to simulate the installation of nondisplacement piles. In this paper, calibration chamber tests are analyzed using the finite element method; experimental and numerical results are compared. The finite element models are axisymmetric and use a nonlinear, elastic-plastic constitutive model. Plate resistance values predicted using the finite element analysis are shown to be in good agreement with measured values, which validates the proposed numerical model. Questions regarding the existence of calibration chamber size effects have not, to this date, been adequately addressed. Finite element analyses of both pile base resistance and plate resistance for sands with various relative densities and stress states show that size effects are usually small for settlement levels of interest in practice. This suggests that the use of calibration chambers in pile base capacity studies is justified.Key words: calibration chambers, plate load tests, constitutive modeling, size effects, piles, sands.

Analysis of Behavior of Sand Surrounding Pile Tips

A finite-element approach is presented to predict the behavior of dense sand surrounding the pile tip in a fairly large stress range, from low stress levels to very high ones, where particle breakage occurs. The approach is based on a constitutive model for sand, which is characterized by critical-state–type yield surfaces and by a nonlinear Mohr-Coulomb failure criterion, the latter being defined on the basis of constant-volume shear strength and of dilatancy. In addition, the concept of constant relative breakage surfaces, which represent families of curves characterized by constant amounts of particle crushing, is introduced. It is shown that the model, applied to the analysis of the behavior of the tip of nondisplacement piles in sand, can be used to visualize and to predict the distribution of strength, dilatancy, and grain breakage in the soil surrounding the pile base.