Harry G. Poulos

Simplified pile-slope stability analysis

Abstract This paper presents a simplified approach to the study of a row of piles used for slope stabilization. The approach is based on an uncoupled formulation in which the pile response and slope stability are considered separately. The pile response when subjected to external lateral soil movements from slope instability is analysed by a modified boundary element method. A conventional simplified Bishop slip circle approach is employed to analyse the slope stability. Applications of the approach are presented and discussed with emphasis on identifying and optimizing some of the important factors that control the performance of piles used for slope stabilization. Several conclusions are drawn regarding the pile-slope stability problem, that of prime importance being the location of the piles for most effective slope stabilization.

Analysis of pile-soil interaction under lateral loading using infinite and finite elements

Abstract In order to gain a better understanding of pile-soil interaction under lateral loading, this paper presents a numerical analysis which combines the infinite and finite element method. Interest is focused on the group effect on ultimate lateral soil resistance. Firstly, a single isolated pile is analysed and reasonably good agreement is found between existing analytical solutions and results obtained by the present method. A limited parametric study is also presented and some parameters influencing the ultimate lateral soil resistance are identified. The analysis of pile groups is then considered and it is shown that the group effect tends to reduce pile capacity when the spacings between piles are within the practical ranges. The extent of the reduction depends on the arrangement of piles within the group.

3-D elastic analysis of vertical piles subjected to "passive" loadings

Abstract There are many cases where piles are subjected to “passive” loadings by soil movement past the piles. This paper employs a 3-D coupled boundary element approach to analyze the response of vertical piles subjected to passive loadings. A number of theoretical expressions for soil movements are developed and presented. These expressions have been incorporated into the pile-soil governing equation previously developed by the authors. The analysis is employed to examine pile responses when subjected to some typical passive loadings, such as soil shrink/swelling, soil surface surcharge, tunnelling, soil movements arising from driving piles and cavity formation in soil. Reasonable agreement is found between some existing published solutions and those developed herein.

General elastic analysis of piles and pile groups

This paper describes the development of a boundary element analysis for the behaviour of single piles and pile groups subjected to general three-dimensional loading and to vertical and lateral ground movements. Each pile is discretized into a series of cylindrical elements, each of which is divided into several sub-elements. Compatibility of vertical, lateral and rotational movements is imposed in order to obtain the necessary equations for the pile response. Via hierarchical structures, 12 non-zero sub-matrices in a global matrix are derived for the basic influence factors. Solutions are presented for a series of cases involving single piles and pile groups. In each case, the solutions are compared with those from more simplified existing pile analyses such as those developed by Randolph and by Poulos. It is shown that for direct loading effects (e.g. the settlement of piles due to vertical loading), the simplified analyses work well. However, for ‘off-line’ response (such as the lateral movement due to vertical loading) the differences are greater, and it is believed that the present analysis gives more reliable estimates. Copyright

A numerical model for dynamic soil liquefaction analysis

This paper presents an effective stress-based numerical model, which can be used to obtain pore pressure build up and consequent loss of soil strength due to earthquake-induced shaking. The main advantage of the new method is that it needs few model parameters compared to many existing effective stress-based ground response analysis methods. The pore pressure generation is calculated using the equivalent cycle pore pressure model developed by Seed et al. [J Geotech Engng Div, ASCE 102 (1976) 323] but the equations are used in a different manner. Pore pressure generation calculated by the new method and the equivalent cycle method for different load patterns shows that the new method can predict pore pressures which are in better agreement with experimental data, irrespective of the loading pattern. The equivalent cycle method predicts results in agreement with experimental data only when the loading pattern is highly irregular, and tends to under-predict pore pressure ratios for other loading patterns. To demonstrate the ability of the new method in simulating earthquake-induced site response and liquefaction-related ground deformations, the Kobe, 1995 earthquake has been analysed. The results obtained from the new analysis agree reasonably well with recorded accelerations and lateral ground displacements at Port Island, Kobe.

NUMERICAL ANALYSIS OF AXIALLY LOADED VERTICAL PILES AND PILE GROUPS

Abstract A numerical method, based on a simplified elastic continuum boundary element method, is presented for the settlement analysis of axially loaded vertical piles and pile groups. The soil flexibility coefficients are evaluated using the analytical solutions for a layered elastic half space. Results are presented and compared with existing published solutions for the following cases: (i) piles in homogeneous soil, (ii) piles in finite soil layer, (iii) piles end-bearing on stiffer layer, (iv) piles socketted into stiffer bearing layer, and (v) piles in Gibson soil. Reasonably good agreement is obtained between the present solutions and existing published solutions.

Shielding Piles from Downdrag in Consolidating Ground

Negative skin friction (NSF) can induce an increased compressive force on piles, called dragload, and additional pile settlement caused by the downward pull of the soil, called downdrag. To investigate the efficiency of shielding effects on dragload and downdrag of piles, centrifuge tests have been carried out to study the shielding mechanisms created by installing sheet piles sleeves around an existing pile in con- solidating ground. The effects of various shielded lengths have also been investigated. Comparisons between centrifuge test results and finite- element(FE)analysesaremadeanddiscussed.Basedoncentrifugetests,itisclearthattheshieldingeffectonthedragloaddecreasesonlygently with a decrease in the shielded length, whereas the shielding effect on the downdrag decreases exponentially with a decrease in the shielded length. Numerical simulations of the centrifuge model tests on the sleeved center piles reveal that the observed shielding effects on the center pile are attributed to the stress transfer from the consolidating soft soil to the sheet pile sleeve. As consolidation proceeds, the relatively stiff sheetpilessleevehangsupthesoil,leadingtoasignificantreductionintheverticalandhorizontaleffectivestressesinthesoilandintheNSFon the center piles. The deeper the depth, the greater the hang-up effects. Thus, the shielding effect increases with the shielded length of the center pile. The reduction in the NSF on the center piles protected by the sheet pile sleeve is more significant than the reduction in the NSF from the sacrificial piles with the same shielded length. DOI: 10.1061/(ASCE)GT.1943-5606.0000764.

Ground Movements – A Hidden Source of Loading on Deep Foundations

Abstract Ground movements can arise from a large number of sources and can have a significant effect on nearby piles and deep foundations. The loading of the piles by ground movements is a different mechanism to that arising from direct applied loading to the pile head, and consequently it is not generally possible to adequately analyze the effects of ground movements simply by applying some type of equivalent loading to the pile head. The main effects of ground movements are the development of additional movements, axial forces and bending moments in the piles, and thus the key design aspects are related to movements and to the structural integrity of the pile. However, the ultimate geotechnical load carrying capacity is generally not affected by the ground movements themselves. This paper will describe an approach to the analysis of ground movement effects on piles, considering axial and lateral movements separately. Some of the main features of pile response will be discussed for three specific problems involving ground movements: 1. Piles near and within embankments;2. Piles near an excavation for a pile cap;3. Piles subjected to seismic ground motions.

Approximate computer analysis of pile groups subjected to loads and ground movements

This paper describes the development of an approximate approach for the analysis and design of piles subjected to axial and lateral loading and also to vertical and horizontal ground movements. The analysis involves a number of simplifications in order to make it feasible to implement. For example, it considers the behaviour of a ‘representative’ pile in a group to characterize the behaviour of all piles in the group, and adopts approximations to derive free-field interaction factors from the conventional interaction factors for direct loading. The analysis has been implemented via a computer program called EMbankment PIle Group (EMPIG) and has the ability to incorporate the following features: 1. single piles or pile groups, 2. applied vertical, lateral and moment loading on the pile cap, 3. the effects of axial and lateral soil movements caused by embankment construction, 4. a layered soil profile, 5. non-linear axial and lateral response of the piles. Comparisons between solutions from EMPIG and other independent programs suggest that it is capable of providing results of adequate accuracy for practical design purposes. The analysis has been used to investigate the effects of pile rake on a typical bridge abutment group. The presence of raked piles can have a detrimental effect on group behaviour, especially in the presence of ground movements. Large lateral deflections can be generated and axial forces and moments in the piles are increased. Comparisons are also made with the results of centrifuge model tests on abutment pile groups. Copyright

Numerical simulation of soil liquefaction due to earthquake loading

Abstract This paper presents a numerical model that can be used to analyse soil liquefaction when a saturated soil deposit is subjected to earthquake loading. Although many methods are available to study the gradual build-up of pore water pressures and the subsequent loss of soil strength based on effective stresses in the soil, they need many model parameters to analyse a problem. This paper presents a numerical model, which needs only a few model parameters when compared with many existing methods, which are based on the effective stress principle. Pore pressure generation due to earthquake loading is calculated via the pore pressure model developed by Seed et al. [J. Geotech. Engng Div., ASCE 102 (1976) 323–346]. Earlier, this method had been used with numerical codes based on the total stress method where the generated pore pressures are based on the total stress level in the soil. Here, a numerical procedure based on the effective stress method has been devised which takes into account the stiffness and strength degradation of the soil due to progressive build-up of pore water pressures. Results given by the new model are compared with shake table test data, centrifuge test data and existing numerical models. It is found that this numerical model predicts pore pressures which are in good agreement with other methods, despite its simplicity.