Behavior Of Pile Groups Research Articles

Effect of air injection on pile and pile group behavior in liquefiable soil

Soil liquefaction during large earthquakes causes serious damage to structures built on foundations. In recent years, artificially injecting small amounts of air into liquefaction-susceptible soils has been proposed as a newly developed and cost-effective liquefaction mitigation method. Nevertheless, the liquefaction response of piles through this method has not been investigated so far. A series of 1g shaking table tests were conducted in this research to evaluate the liquefaction response of pile and pile group in air-injected partially saturated soils. The results have shown that air injection effectively reduces the liquefaction-induced settlements of pile and pile group. Also, this method has relatively reduced the excess pore pressure ratio. According to visual results and bending moments in piles, the depth of a liquefied soil layer decreases in air-injected partially saturated soils. Also, the deformation of soil structure is observed to decrease by injecting air into the samples. Despite these effects, larger accelerations are transmitted to the surface of the soil and consequently to the structure, and larger bending moments are applied to the piles in air-injected partially saturated samples. It was observed that at the low confining pressure, the effectiveness of this method is not a function of air injection duration.

Scour effects on lateral behavior of pile groups in sands

Scour that removes soils around foundations supporting bridges and marine structures is the major cause for structural failures. Recently, many studies have been conducted to investigate scour effects on lateral behavior of single piles using numerical analyses, laboratory model tests, or centrifuge tests. However, very limited research is focused on scour effects on lateral behavior of pile groups. No simple method is currently available for practicing engineers to analyze pile groups under scour conditions. In this study, a simple and practical method was proposed based on the existing p-y curve theory to facilitate the design of laterally loaded pile groups under scour conditions. Feasibility of the proposed method was examined against 3D finite element (FE) analyses through a series of parametric studies by varying scour-hole dimensions, soil properties, and pile group configurations. The results showed that the proposed method generally produced agreeable results with the 3D FE analyses. Scour-induced lateral capacity loss of pile groups was 10–13% more in dense sands than in loose sands. Pile groups were more susceptible to scour than single piles under the equivalent scour conditions. A pile group with smaller pile spacing and larger pile numbers showed a less loss of lateral capacity due to scour.

Model Studies on Load Sharing for Shaft and Tip of Pile Groups in Saturated and Unsaturated Soils

The behavior of pile foundation is dictated by its structural properties and stiffness and strength of the soil supporting the pile. This complexity is further increased when the soil is in partially saturated condition. In this work, an attempt has been done to investigate the influence of matric suction on the behavior of single pile and pile group, and the amount of increase in the bearing capacity due to the influence of this suction. The load transfer for shaft and tip of pile groups embedded in soil with different properties (saturated and unsaturated) is investigated. First, the stresses below the center of piles are measured by a soil pressure transducer located at a depth of 0.25 L, (L = length of pile). These stresses are transferred directly to pile tip according to approximate 2 V:1 H method and then converted to load at the tip and shaft of pile. The general trend is that in a single pile, at the start of load applying, the percentage of distributed load is 70% for shaft and 30% for tip, then as the applied load increases, the pile settles and a differential movement between the soil and pile occurs, creating more adhesion between the soil resulting in increase in shaft capacity. For pile groups, the same trend is obtained, but the percentage of load transferred to the shaft increases as the number of piles increases, in contrast to the load transferred to the tip. The results showed that the sharing of load between shaft and end resistance for single pile in saturated soil is halved when the pile reaches its ultimate resistance, while this response is different for unsaturated soil where the load is transferred as a whole to the pile tip at approximately the intermediate stages of loading according to the analysis criteria adopted.

Fictitious pile method for fixed-head pile groups subjected to horizontal loading

A novel fictitious pile method based on the elastic theory is presented for the analysis of horizontally loaded pile groups. By considering the strengthening effect of intervening piles, the interaction between the piles and the soil is decomposed into the extended soil and fictitious piles. The proposed approach is based on the superposition of the horizontal displacement of individual piles. It enables the avoidance of a full analysis of pile groups, and thus, improves the calculation efficiency. The pile-to-pile interaction factors, horizontal load distributions, and group efficiency factors produced by the proposed method are very close to those calculated by the finite element method. Several examples are then presented to discuss the influence of nondimensionalized piles and soil parameters on the behavior of pile groups.

Investigation of Seismic Behavior of Clay‐Pile Using Nonlinear Kinematic Hardening Model

In geotechnical mediums where the bearing capacity of upper layers of soil is not suitable for use of the shallow foundations, piles are usually used as deep foundations to transfer loads to the stronger lower layers. Here, the seismic behavior of single pile and pile group constructed in saturated soft kaolin clay under three different earthquakes using Abacus 3D software is investigated. The aluminum material considering the linear elastic model has been used for the piles, and the nonlinear kinematic hardening model with Von Mises failure criterion has been considered for clay. This model can consider the soil stiffness degradation by increasing the number of cyclic loading. Three different methods have been used to calibrate the model parameters, two of them are new methods. In all calibration methods, the cyclic shear and undrained cyclic triaxial tests are used. The results obtained from the numerical analysis of the soil‐pile model are in relatively good agreement with the centrifuge model results. According to the results, the variation of earthquake frequency and intensity affects the bending moment created along the pile and also the distance between piles in a pile group affects the amount of the interaction between them.

Response of passively loaded pile groups – an experimental study

Preventing or reducing the damage impact of lateral soil movements on piled foundations is highly dependent on understanding the behavior of passive piles. For this reason, a detailed experimental study is carried out, aimed to examine the influence of soil density, the depth of moving layer and pile spacing on the behavior of a 2x2 free-standing pile group subjected to a uniform profile of lateral soil movement. Results from 8 model tests comprise bending moment, shear force, soil reaction and deformations measured along the pile shaft using strain gauges and others probing tools were performed. It is found that soil density and the depth of moving layer have an opposite impact regarding the ultimate response of piles. A pile group embedded in dense sand requires less soil displacement to reach the ultimate soil reaction compared to those embedded in medium and loose sands. On the other hand, the larger the moving depth, the larger amount of lateral soil movement needs to develop the pile group its ultimate deformations. Furthermore, the group factor and the effect of pile spacing were highly related to the soil-structure interaction resulted from the transferring process of forces between pile rows with the existing of the rigid pile cap.

Experimental Investigation of Single Model Pile and Pile Group Behavior in Saturated and Unsaturated Sand

AbstractThe research focus of this paper is directed to investigate the behavior of a single model pile and pile groups in sand under both saturated and unsaturated conditions. Forty different mode...

Experimental Investigation on Efficiency Factor of Pile Groups Regarding Distance of Piles

There are a lot of the parameters which affect pile group behavior in soil. One of these factors is the distance of piles from each other. The impact of distance on pile groups in sand has been investigated through some researches, whereas most of them have not represented an exact estimation according to the continuous change of the distance in sand. Moreover, most of previous investigations have considered two piles as a perfect group. Since two-pile group has the least interaction effect among piles, it cannot suitably demonstrate the influence of spacing. In this lecture, several 4-pile groups modeled with different spacing were subjected to axial loading in laboratory. The pile groups were free-head with length to diameter ratio of 13.5. The piles are designed in a way which the shaft resistance of piles can be completely mobilized through the test. Then, the bearing capacities of pile groups are measured and compared with the single pile's resistance in order to calculate the efficiency coefficient of the groups. It is revealed that the distance is noticeably effective in efficiency factor and this effectiveness, non-linearly decreases by increase of spacing. The results show that the efficiency coefficient is changing between almost 1 and 1.4.

Prediction of nonlinear vertical settlement of a pile group consisting of new and existing displacement piles in clay strata

Reusing existing displacement piles is economical, time-saving, and environment-friendly. This paper presents an analytical approach for predicting the load carrying behavior of pile groups consisting of new and existing displacement piles. The evolutions of the undrained shear strength and shear modulus of clay adjacent to the piles from installation through consolidation to long-term ageing are investigated to determine the load carrying behavior of displacement piles. The nonlinear load-settlement behavior of an individual pile is modelled by load-transfer method, where the exponential function-based load-transfer models integrating the two developed soil parameters are employed to represent the nonlinear behavior at the pile-soil interface. The pile-pile interaction in the pile group is explored based on the shear displacement method. Combining the load-transfer method and the shear displacement method, an analytical framework is proposed for predicting the load-settlement behavior of pile groups consisting of new and existing piles. The proposed framework is validated by predicting the vertical settlement of a high-rise apartment built on a pile group consisting of 74 new displacement piles and 22 existing displacement piles during construction. Good agreement is achieved between the predicted and measured results. A parametric study is performed to explore the stiffness efficiency and the load-settlement behavior of pile groups with different layouts of new and existing piles. The results indicate that the pile group with larger ratio of the number of existing piles to the number of total piles shows both stronger stiffness and a higher load carrying capacity.

Experimental and Numerical Investigation of Pile Group With and Without Building Frame Subjected to Axial Load

The interactions between soil, piles and building frames are very complex in the analysis of structures. The existing design produces the conservative estimation of pile group capacity which is subjected to axial loads. The realistic behavior of pile group may change due to the soil structure interaction. For the proper design of pile foundations, one has to consider the interaction of building frame, soil and the pile group. In this study, experimental studies are carried out on a single pile and pile group (2 × 3) with and without building frame entrenched in cohesionless soil to understand the influence of building frame on pile group behavior when it subjected to axial loads. Also, numerical analysis has been carried out by using the finite element method based on ANSYS software. The effect of building frame on the behavior of pile group was investigated in terms of pile settlements, pile cap rotation, pile head deflection and bending moment in piles by varying the parameters (aspect ratio of the pile, the spacing between the piles and the relative density of sand bed). The numerical analysis results are well in agreement with the experimental results with a percentage of deviation in the range 5–20% for all the cases considered in the study.

Three-dimensional centrifuge modelling of the effects of tunnelling on vertical behaviour of pile group

In densely built urban cities, tunnels are likely to be constructed adjacent to existing deep foundations. Tunnelling activity inevitably induces soil stress changes and soil movements in the ground, which may affect nearby existing pile foundation. Although a number of studies have been carried out by various researchers to investigate tunnelling effects on piles, the three-dimensional effects of tunnelling on pile group have not been well-investigated and understood. In this paper, a three-dimensional centrifuge test was carried out to investigate the effects of tunnel construction on a pile group in dry sand. The diameter of each pile was designed as 0.8m (in prototype) and the diameter (D) of the simulated tunnel is 6.08m. Tunnel advancement was modelled in-flight by controlling a volume loss of 1.3%. The tunnelling process was divided into four stages to model the three-dimensional tunnelling effects. The closest distance between the tunnel and the pile group was 1.5m at the pile toe. Measured settlement of pile group, as well as axial force distribution of piles in the pile group are reported and discussed.

Comparison of lateral response of pile group in series and parallel arrangement near sloping ground

ABSTRACT Several studies have been presented till now to predict the behaviour of piles subjected to lateral load located in horizontal ground. However, the effect of nearby slope on pile response is not thouroughly investigated yet. Assessment of pile response encountering sloping ground is still a challenging task. In the present study, experimental investigations were planned to examine the behaviour of pile group near sloping ground. The paper presents the results of series of 26 tests in medium sandy soils considering three ground slopes and four edge distances from the slope crest. Similarly, pile group response for horizontal ground condition was obtained for quantification of effect of sloping ground. Displacements and bending moments had considerable increment for pile group near slope. As the edge distance was increased, pile response was approaching to level ground case. From the measured bending moments, the p–y curves and p-multipliers were established for pile group near slope.

Variations in the dynamic response of structures founded on piles induced by obliquely incident SV waves

SummaryAlthough the seismic actions generally consist of a combination of waves, which propagates with an angle of incidence not necessarily vertical, the common practice when analyzing the dynamic behavior of pile groups is based on the assumption of vertically incident wave fields. The aim of this paper is to analyze how the angle of incidence of SV waves affects the dynamic response of pile foundations and piled structures. A three‐dimensional boundary element‐finite element coupling formulation is used to compute impedances and kinematic interaction factors corresponding to several configurations of vertical pile groups embedded in an isotropic homogeneous linear viscoelastic half‐space. These results, which are provided in ready‐to‐use dimensionless graphs, are used to determine the effective dynamic properties of an equivalent single‐degree‐of‐freedom oscillator that reproduces, within the range where the peak response occurs, the response of slender and nonslender superstructures through a procedure based on a substructuring model. Results are expressed in terms of effective flexible‐base period and damping as well as maximum shear force at the base of the structure. The relevance and main trends observed in the influence of the wavefront angle of incidence on the dynamic behavior of the superstructure are inferred from the presented results. It is found that effective damping is significantly affected by the variations of the wave angle of incidence. Furthermore, it comes out that the vertical incidence is not always the worst‐case scenario.

Experimental investigation of batter pile groups behaviour subjected to lateral soil movement in sand

ABSTRACT A series of laboratory model tests on batter pile groups embedded in the sand was carried out in a specially designed testing box. The lateral responses were investigated for 1 × 2 capped batter pile groups when subjected to lateral soil movements (passive loading) with different configurations; Vertical-Vertical (VVL), Batter-Vertical (BVL), Vertical-Batter (VBL), and Both- Batter (BBL). The effect of pile group arrangement and batter angle on the bending moment, shear force, soil reaction, pile rotation and deflection behaviour of the passive batter pile groups were studied. It is observed that the behaviour of the individual piles in a group was significantly affected by the batter angle and the pile group arrangement. It is also shown that under passive loading, batter pile groups with (BBL) configuration of (−10°, +10°) offered more resistance to the lateral soil movement compared to other pile group arrangements, while (VVL) configuration offered the least resistance.

Axial load testing of helical pile groups in glaciolacustrine clay

The behaviour of helical pile groups has not previously been experimentally investigated through field testing. In the present study, field compressive load tests of 2 × 2 helical pile groups and single piles were conducted in a glaciolacustrine clay in Edmonton, Canada. The group pile spacing, interhelix spacing, and soil setup time were varied. Piezometers were used to measure the excess pore pressure (ue) response to pile installation. Selected groups contained a strain gauge instrumented pile that was used to estimate the pile load transfer and failure mechanism. Group performance was evaluated by estimating the group efficiencies and settlement ratios. Results show that the group interaction of small-diameter helical piles was lower than that of conventional pile groups. The short-term performance of groups was significantly reduced by ue, and groups with smaller group pile spacing had a greater reduction in short-term performance. Instrumented grouped piles and one single pile, both having an interhelix spacing ratio of 5, exhibited individual bearing failure. The measured group capacities and load distributions indicated that individual pile failure occurred, as opposed to block failure.

Numerical simulation of concrete pile groups' response bored in cemented sand deposit under axial static load testing

For a safe foundation to perform as desired, the ultimate strength of each pile must fulfil both structural and geotechnical requirements. Pile load testing is considered as a direct method of determining the ultimate bearing capacity of a pile. Pile groups are commonly used in foundation engineering and due to the difficulties and cost of full-scale load tests, most pile group tests are scaled down regardless of whether performed in the field or laboratory. In this paper, it is aimed to simulate the behaviour of concrete bored pile groups under axial static load testing using PLAXIS 3D software and to compare the obtained results with measured curves in an experimental study introduced in the literature. In numerical simulation, to account for the stiffness variation existing inside the pile group and to achieve a reasonable correlation between measured and predicted load-settlement curves three different analyses, including linear elastic, completely non-linear, and a combination of non-linear and linear analyses were performed. The results indicate that the combined non-linear and linear analysis seems a suitable analysis for pile group behaviour prediction.

Behavioural analysis of vertical and batter pile groups under vertical and lateral loading in sand

Laboratory model tests were carried out to study the behaviour of vertical piles and batter pile groups under vertical and lateral load. The model pile groups were made up of mild steel rod of 8-mm diameter. Parameters such as degree of batter and different length to diameter ratios of 7.50, 15.00 and 22.50 were considered in this study. The size of the model tank was 1000 mm × 1000mm × 800 mm. Experiments were performed on 3 × 3 model pile groups with a row of batter piles both positive and negative in addition to vertical pile groups with batter angle 0° in sandy soil subject to vertical and lateral loads. It was observed that the behaviour of vertical pile groups and group of piles with batter piles were similar but it showed substantial variation in the capacity of pile groups. Results indicated that the load–settlement relationships were non-linear for all model pile groups both under vertical and lateral loading. Numerical FEM analysis using ABAQUS/CAE 6.11 was also used to compare and validate the load carrying capacity of pile groups obtained from the experimental model tests.

An Approach for Capacity Calculation of Shaped Pile Group Under Uplift Load

Model tests were conducted to investigate the behavior of a special-shaped pile group (2×1, and 2×2) and a single pile under uplift loading. A theoretical method was developed to estimate the ultimate uplift capacity of the special-shaped pile group based on a unified composite failure surface. The method was validated by the current model test results and those published in the previous literatures. The predicted uplift capacities generally agree well with the measured test results, showing that the method is suitable for the estimation of uplift capacity of both special-shaped pile groups and equal diameter pile groups.

Dragload of pile groups in consolidating non-Darcian clays

In the present study, the behaviour of pile groups with flexible cap in consolidating non-Darcian soil is investigated in terms of dragloads and downdrag. The effective stresses and the settlements are computed at different depths of the soil layers using a non-linear, one-dimensional consolidation equation for variable permeability and the non-Darcian condition. The relationship between displacements and shear stresses at the pile–soil interface and at the pile tip are modelled using a hyperbolic function. The proposed model is validated with the reported experimental results. The effects of pile spacing, slenderness ratio, soil modulus, pile modulus and the effects of non-Darcian parameters (α and n) on the developed dragload and downdrag have been studied for line pile groups (3 × 1), square pile groups (2 × 2, 3 × 3 and 5 × 5) and rectangular pile groups (3 × 2). It is observed that the central pile of a 5 × 5 pile group mobilises only about 30–45% of dragload as compared to the single piles during 1 year to 10 years of consolidation. In a 2 × 2 pile group, only 20–30% of ultimate pile settlement is attained after 5 years of consolidation owing to the non-Darcian condition, in comparison to 90% of ultimate settlement for the Darcian flow condition.

Thermo-mechanical behaviour of floating energy pile groups in sand

This paper presents the experimental results of small-scale model tests of an instrumented floating energy pile group in which the piles were embedded in dry medium-dense sand and subjected to the seasonal temperature pattern of the city of Nanjing in China. The study also included a model test to assess the effect of including nonthermal piles on the thermo-mechanical behaviour of the floating energy pile group. For comparison, a model test of a single floating energy pile embedded in the same soil and subjected to a similar temperature pattern was also conducted. The results show that the thermo-mechanical behaviour of an energy pile group is different from that of a single energy pile in terms of the thermally induced change in axial pile stress and the displacement of the pile top and tip. This difference in behaviour could be explained by the higher lateral confining pressure expected on a single pile than on a pile in a group due to pile interaction effects, which could lead to different end restraint boundary conditions. We conclude that the thermo-mechanical behaviour of an energy pile is controlled mainly by the end restraint boundary conditions.