Displacement Piles Research Articles

Nonlinear lateral response of offshore large-diameter monopile in sand

This paper presents a nonlinear analysis method for laterally loaded large-diameter monopiles. The method accounts for the effects of pile diameter and soil deformation on the modulus of horizontal subgrade reaction by considering a modified modulus of horizontal subgrade reaction. The solution considers the force and moment equilibrium of soil-pile system to calculate the response of the monopile to lateral loads. The influence of the soil side friction reaction on the ultimate horizontal resistance is evaluated considering the degradation of modulus of horizontal subgrade reaction due to large pile displacement. The predictions of the developed method compared favorably with the results obtained from full-scale field tests, centrifuge model tests and laboratory model tests as well as existing analytical methods. The developed analytical method is then applied to study the rigid rotation behaviour of offshore monopiles. The results demonstrated that under lateral loading at the pile head, the depth for rigid rotation point lies within 0.62L–0.73L (L is pile length), and the depth of the rotation point increases as the pile diameter increases. It was also found that the load eccentricity has a significant effect on the depth of the rotation point.

Kinematic response of pipe piles subjected to vertically propagating seismic P-waves

This paper presents an analytical solution for determining the time-harmonic response of single open-ended pipe piles subjected to vertically propagating seismic P-waves. Following the presentation of the formulation, we employ the solution to derive closed-form expressions of seismic pipe pile displacements, as well as robust expressions to determine the depth- and frequency-dependent parameters of complex Winkler springs, for use with beam-on-dynamic-Winkler-foundation models. Finally, the importance of considering the contribution of the inner soil in the seismic analysis of pipe piles is quantified via a parametric sensitivity analysis.

Impact of the installation on the long-term cyclic behaviour of piles in sand: A numerical study

The installation process changes the state of the soil in the vicinity of a pile and therefore influences its load-displacement behaviour during the operational phase following the installation. The installation process itself and its effect on the pile behaviour under a subsequent high-cyclic loading are numerically investigated in this work. Two different installation techniques, namely impact driving and jacking, are investigated using a coupled Eulerian-Lagrangian approach in combination with a hypoplastic material model. Either ideally drained or partially drained conditions are assumed. Afterwards the pile response to a high-cyclic loading with up to five million load cycles is studied using a high-cycle accumulation (HCA) model. Compared to a wished-in-place (WIP) pile, the simulations accounting for the installation process showed considerable less permanent pile displacements in case of initially medium dense sand. However, the opposite was observed in case of initially dense sand. Current practice assuming a WIP state as the starting point in numerical simulations seems thus to be conservative for initially medium dense sand but leads to an underestimation of permanent deformations for initially dense sand.

Sand-steel interface behaviour under large-displacement and cyclic shear

Large-displacement shear appears between pile shaft and its surrounding soil during the penetration process, and cyclic shear is the typical feature in subsequent service load condition. In this paper, a series of interface shear tests have been conducted to investigate the sand-steel interface behaviors under large-displacement and cyclic shear. Three types of particles (quartz sand, carbonate sand and steel bead) and two normal boundary conditions (constant normal load, CNL; constant normal stiffness, CNS) were used in these tests. To simulate the large-displacement shear process for monotonic installed pile, the large-displacement shear is performed using monotonic shear method. It is found that under the large-displacement shear with CNL condition, the quartz sand and carbonate sand show different breakage degrees and modes. Obvious particle breakage changes both the particle gradation and sample densification, which are the main reasons for the increase of interface friction angle. While for the cyclic shear with CNS condition, the interface strength shows a quick degradation feature, which is mainly affected by the initial shape of particles. It is also found that the previous interface shear plays a key role in this kind of degradation. When the shear displacement is small, the degradation is promoted with only a small amount of sand breakage. But if the sand breakage is significant under the large-displacement shear, abundant broken fine particles fill into the void and make the sand sample much denser. So, the degradation trend of interface strength will be suppressed. It is suggested that the previous large-displacement shear process for displacement pile should be adequately considered in the design.

Theoretical analysis of three-dimensional effect in pile integrity test

The one-dimensional (1D) rod model, which is used in several pile dynamics applications, has two main simplifying assumptions: only vertical reaction is considered and excitation on the pile top is treated as uniform. These two simplifications negatively affect its application in pile integrity tests, especially for large-diameter piles. To overcome these two limitations, a new non-uniform axisymmetric loading (NAL) theoretical model that considers bidirectional (vertical and radial) reactions of the pile body and the non-uniformity of the excitation on the pile top is proposed. Based on the developed model, displacement and velocity fields of an end-bearing pile embedded in homogeneous soil under non-uniform impact are solved theoretically. Analytical solutions are then verified by existing solutions and finite element results. Compared with 1D rod pile model, the NAL model can predict accurately the three-dimensional (3D) effect in the pile integrity test. Extensive parametric analysis is conducted using the present solutions to evaluate the 3D effect in the pile integrity test and provide practical suggestions to diminish the 3D effect in the actual test.

Analysis of nonlinear load-displacement behaviour of pile groups in clay considering installation effects

This paper presents an analytical approach for predicting the nonlinear load-displacement behaviour of pile groups consisting of displacement piles in normally consolidated and lightly overconsolidated clayey soils. The effects of squeezing and remolding on the soil around the piles caused by pile installation are reflected by the soil strength and shear modulus. The nonlinear load-displacement behaviour at the pile-soil interface is modeled by the load-transfer method with exponential load-transfer functions and the pile-pile interaction effects are reflected by the shear displacement method. A hybrid analytical method, incorporating the developed soil parameters and combining the load transfer method and the shear displacement method, is proposed for the problem considered. Four sets of centrifuge model tests, involving pile groups with different pile diameters, pile lengths and pile spacings, were conducted to verify the proposed analytical framework. The validity of the proposed method, the significance of the installation effects as well as the nonlinear behaviour of the pile groups are demonstrated by comparing the load-displacement curves from the centrifuge model tests with those from the proposed method and other relevant methods. Parametric studies are conducted to investigate the pile diameter, pile length, pile spacing and pile number on the stiffness efficiency of pile groups. The results indicate that increasing the pile diameter and length can effectively improve the stiffness efficiency and the load carrying behaviour of displacement pile groups.

Lateral response of rigid monopiles subjected to cyclic loading: centrifuge modelling

In this study, a total number of 20 centrifuge tests were carried out to investigate monopile behaviour under lateral cyclic loading. The instrumented model pile simulates an offshore wind turbine foundation with an embedment ratio of 5 installed in sand layers with two relative densities of 80% and 50%. The influence of the directional characteristic and amplitude of cyclic load on pile lateral behaviour was studied. The data analysis focused on the influence of cyclic load on the accumulation of lateral displacement and evolution of secant stiffness of the foundation system. The most damaging cyclic load type (which can cause the most accumulated pile displacement) is identified as two-way loading, and it was observed that cyclic load always increases the pile secant stiffness. A new model for the prediction of evolution of accumulated displacement and change in secant stiffness has been formulated. An example of the procedure developed is presented for a typical field monopile subjected to cyclic loading. Lastly, the performance of the new model is demonstrated and predicted results are compared with field test data.

Assessment of the Posttunneling Safety Factor of Piles under Drained Soil Conditions

The need to tunnel closely beneath piles is increasing due to the development of urban areas. This poses a risk to the stability and serviceability of overlying structures (e.g., buildings, piers, and piled embankments). The impact of tunneling on piles is usually assessed using a displacement threshold, yet this provides no information about the posttunneling pile safety factor. Knowledge of a pile’s safety factor under serviceability or extreme loading conditions is important, especially if future repurposing of the associated superstructure is a possibility. Tunneling can reduce the safety factor of a pile up to the point of geotechnical failure (i.e., when the pile capacity reduces to that of the applied load), yet little guidance is available to enable a straightforward means of assessing the posttunneling safety factor of a pile. This paper aims to address this shortcoming by providing design charts based on an analytical tunnel-single pile interaction approach that provides a means of determining a posttunneling pile safety factor. The methodology and design charts are applicable to drained soil conditions and include for the effects of the initial pile safety factor, the pile installation method [displacement (driven and jacked), nondisplacement (bored) with only the shaft capacity, and nondisplacement with base and shaft capacity] and varying water table depths. In the paper, as a validation exercise, analytical predictions are compared against data from geotechnical centrifuge tests designed to model both displacement and nondisplacement piles in sands, including a variety of tunnel–pile relative locations and initial pile safety factors. For a specified design value of a posttunneling pile safety factor, the design charts enable a quick assessment of the safe location of a pile or a tolerable tunnel volume loss considering ground parameters, water table position, pile installation method, and initial safety factor.

The influence of the soil constitutive models on the seismic analysis of pile-supported wharf structures with batter piles in cut-slope rock dike

Abstract In coastal regions, earthquakes caused severe damage to marine structures. Many researchers have conducted numerical investigations in order to understand the dynamic behavior of these structures. The most frequently used model in numerical calculations of soil is the linear-elastic perfectly plastic model with a Mohr-Coulomb failure criterion (MC model). It is recommended to use this model to represent a first-order approximation of soil behavior. Therefore, it is necessary to accommodate soil constitutive models for the specific geotechnical problems. In this paper, three soil constitutive models with different accuracy were applied by using the two-dimensional finite element software PLAXIS to study the behavior of pile-supported wharf embedded in rock dike, under the 1989 Loma Prieta earthquake. These models are: a linear-elastic perfectly plastic model (MC model), an elastoplastic model with isotropic hardening (HS model), and the Hardening Soil model with an extension to the small-strain stiffness (HSS model). A typical pile-supported wharf structure with batter piles from the western United States ports was selected to perform the study. The wharf included cut-slope (sliver) rock dike configuration, which is constituted by a thin layer of rockfill overlaid by a slope of loose sand. The foundation soil and the backfill soil behind the wharf were all dense sand. The soil parameters used in the study were calibrated in numerical soil element tests (Oedometer and Triaxial tests). The wharf displacement and pore pressure results obtained using models with different accuracy were compared to the numerical results of Heidary-Torkamani et al.[28] It was found that the Hardening Soil model with small-strain stiffness (HSS model) gives clearly better results than the MC and HS models. Afterwards, the pile displacements in sloping rockfill were analyzed. The displacement time histories of the rock dike at the top and at the toe were also exposed. It can be noted that during the earthquake there was a significant lateral ground displacement at the upper part of the embankment due to the liquefaction of loose sand. This movement caused displacement at the dike top greater than its displacement at the toe. Consequently, the behavior of the wharf was affected and the pile displacements were important, specially the piles closest to the dike top.

Numerical investigations of the effects of different factors on the displacement of energy pile under the thermo-mechanical loads

The displacement characteristics of energy pile are very important for its safe and sustainable operation. In order to obtain the influences of various factors on the displacement of energy pile, a thermo-mechanical coupled numerical model of energy pile with groundwater advection was established based on COMSOL Multiphysics software. The effects of thermal load, thermo-mechanical coupling load, groundwater advection, soil type, soil stratification and cooling and heating cycle operation on the displacement variation of energy pile operated in summer and winter modes were studied. The results indicate that for thermal load alone, the pile displacement in summer mode can almost be restored to its initial state, and an irrecoverable pile head displacement will occur in winter mode. The lower the inlet temperature, the larger the displacement variation degree. For the thermo-mechanical coupling load, the change ratio of pile head displacement increases with the decrease of mechanical load and increases with the increase of inlet temperature in summer. For winter mode, it increases with the decrease of mechanical load and inlet temperature. In addition, the larger the advection velocity of groundwater, the more heat (cold) can be taken away from the pile, and thus the smaller the displacement changes. For the soil types and stratification, the pile head displacement in soft clay is the largest, followed by silty clay and hard clay, and the rock soil is the smallest. Moreover, the soil stratification and cooling and heating cycle operation has a significant effect on the displacement changes of energy piles and should be considered in the design of energy piles.

Observed behaviors of a long and deep excavation and collinear underlying tunnels in Shenzhen granite residual soil

The performances of a long and deep excavation and underlying collinear existing tunnels in Shenzhen granite residual strata are presented and evaluated. The collinear section for the excavation and tunnel had a length of 3.1 km and the maximum excavation depth was 18.0 m. The excavation was supported with retaining piles, one level of reinforced concrete strut, and 1–3 levels of steel pipe strut. Emphasis is put on the retaining pile displacement, ground surface settlement, and tunnel deformation, correlated with ground conditions, construction activities, excavation geometries, and time-dependent behaviors. An appreciable increase in retaining pile displacement occurred after the end of each soil layer excavation. The ground surface settlements were close to the observations due to metro station excavation with high length to width ratio. In addition, the deformation of the existing tunnel at a certain cross section was governed by the exposure time for the excavation base, base excavated length ahead, and unloading ratio urd (urd = He/C, He was the excavation depth and C was the tunnel cover depth). The upper and lower bounds of the graph of urd plotted against δt/C (δt was the tunnel heave) can be expressed by exponential equations. The observations of this project can provide a reference for cases in similar conditions.

Nonlinear analysis of laterally loaded rigid piles at the crest of clay slopes

This paper proposes a method for nonlinear analysis of a rigid pile under lateral load at the crest of clay slope. It is assumed that the ultimate soil resistance in front of the pile and behind the pile vary nonlinearly with depth. The modulus of horizontal subgrade reaction in front of the pile and behind the pile vary nonlinearly with the pile displacement at ground surface. The horizontal shear resistance at the pile base is considered and a bilinear relationship between the shear resistance and the displacement is used. Three distribution patterns of the soil resistance along a pile are also considered. Based on the force and moment equilibrium, the system equations are derived for a laterally loaded rigid pile in sloping ground with different soil resistance distribution patterns. The proposed method is validated by comparing its results with those from field tests, finite element analyses and a theoretical method. Finally, the method is used to analyze the effect of slope angle, pile-soil adhesion factor, height above ground surface of point of load application, pile diameter and embedded length on the lateral response of rigid piles.

Centrifuge study on the effect of pile bending stiffness on the slope stabilised by piles

The stability and failure of a slope stabilised by piles are influenced by the pile bending stiffness. In this paper, the effect of the bending stiffness is studied by centrifuge model tests. Two kinds of piles made of aluminium and acrylic tubes are chosen to represent relatively rigid and flexible piles, respectively. The piles are installed in a row at various pile-spacing ratios in a slope with an inclination of 45°. The model slope consists of a compacted inclined soil layer sliding on a stable wooden structure along a prescribed slip surface. Strain gauges are installed on an aluminium pile to obtain the bending moment. Images taken during the test from the side and the top are used to monitor the soil and pile displacements. The results show that the slope is well stabilised by aluminium piles with small spacing. In the case of relatively flexible piles, the top part of the pile becomes ineffective and the slope stability is reduced.

DISPLACEMENT ANALYSIS OF SHEET PILE WITH TIE BEAM ON BREAKWATER CONSTRUCTION BY USING PLAXIS

The breakwater construction usually founded in coastal area with many public or company facilities. It is very useful to break the ocean waves so it can not reach the shore line. Furthermore, the breakwater construction is also applied as a barier for a port for ship docked. One of the type of breakwaters is tube column filled with compacted sand reinforced by sheet file and tie beam. This research will discuss about the sheet pile displacement in static and dynamic condition. Plaxis is applied as the main tool to built the 2-D model based on given data from field observation. The input parameter for this model is applied in soil stratigraphy, compacted sand column, sheet pile, tie beam, pseudostatic wave force, sea level and earthquake acceleration. The running of this model divided in three condition. There are static condition, static condition with pseudostatic wave force, and dynamic condition. The results show different displacement between the conditions. The sheet pile displacement in static condition is 3.35 cm at harbor side and 1.91 cm at open sea. The sheet pile displacement in static condition with pseudostatic wave force is 12.14 cm at harbor side and 12.08 cm at open sea. The sheet pile displacement in dynamic condition has 5 cm peak both harbor side and open sea.

A new dynamic interaction factor for the analysis of pile groups subjected to vertical dynamic loads

Τhis paper presents an analytical method for calculating the steady-state impedance factors of pile groups of arbitrary configuration subjected to harmonic vertical loads. The derived solution allows considering the effect of the actual pile geometry on the contribution of pile-soil-pile interaction to the response of the group, via the introduction of a new dynamic interaction factor, defined on the basis of soil resistance instead of pile displacements. The solution is first validated against a published solution for single piles that accounts for the effect of pile geometry on the generated ground vibrations. Accordingly, we show that the derived soil attenuation factor agrees well with existing solutions for pile groups in the high frequency range, but considerable differences are observed in both the stiffness and damping components of the computed impedance when the relative spacing between piles decreases. Numerical results obtained for typical problem parameters suggest that ignoring pile geometry effects while estimating the contribution of pile-soil-pile interaction in the response may lead to inaccurate results, even for relative large pile group spacings.

Displacement analysis of open pipe piles sinking based on strain path method

The displacement field in the process of opening pipe piles sink has always been a hot issue in the study, especially in the area of pile, the soil body has produced a large strain, regardless of the large deformation effect of the soil, there will be a large calculation error. The strain path method can obtain the flow velocity of the soil because it does not rely on the constitutive relationship of the soil, which is a favorable theory for solving large strain problems. In this paper, the modified strain path method is used as the theoretical basis. The open pipe pile is equivalent to a closed pipe pile through the soil plug effect, and the sunk pile displacement of the open pipe pile is calculated to further reveal the soil plug effect and the soil squeeze effect of the open pipe pile mutual restraint and mutual influence, thus providing a theoretical basis for the pile displacement of open pipe piles.

Soil parameters’ numerical back analysis for comparison of driven pile’s capacity assessment methods

The paper shows several methods of driven piles capacity assessment. The piling work consisted of driving around 10 000 prefabricated concrete piles for the indirect foundation of National Stadium’s located in Warsaw city center. A total number of 68 investigated piles was divided into three groups, depending on their length, because longer piles tend to be weaker, what is shown in the paper. Results of dynamic load test, CASE and CAPWAP limit capacity and piling reports provided by AARSLEFF company stood the basis for further analysis in this work. The given results of dynamic test were taken as the most reliable data to compare with other methods, as they gave surprisingly close values, while being nondependent. Large amount of displacement piles happened to impact existing soils’ mechanical properties. The difference in maximum resistance force was estimated, by carrying out a numerical back analysis with GEO5 software. The computation shows significant growth of internal friction angle, and compaction index which led to three times higher limit pile capacity. The extra compaction effect during pile driving should be considered, during the design process, both as positive for soil stiffness, and potential impediment for the contractor.

Effect of top soil liquefaction potential on the seismic response of the embedded piles

3D model was performed using the ANSYS program to investigate the behaviour of piles embedded in a liquefiable area during earthquakes. The model contained a CPT215 element that has an additional degree of freedom for water pressure. Two components of the movement were employed to the model for EL-CENTRO earthquake, 1940. The results showed good model accuracy compared to the previous tests. When the model was subjected to horizontal movement only, the lateral pile displacement decreased with the increase of top non-liquefiable layer thickness and vice versa. Whereas when both horizontal and vertical component of motions were applied to the model; the lateral pile displacement increased significantly in the case of the top non-liquefiable depth is 3 m only, and the vertical pile displacement increased with the reduction of top layer thickness. The presence of top liquefiable layer is very influential on the lateral pile displacement as it increased by 120%.

Linear analysis of a rectangular pile under vertical load in layered soils

In this paper, analytical and numerical solutions are developed for the pile with a rectangular cross-section under vertical load in layered soils. The rectangular cross-section is considered as a circular cross-section with a proposed formulation of equivalent radius. A number of bar elements models the pile and soil column below the pile tip while a series of independent springs distributed along the pile shaft with spring stiffness determined by properties of the corresponding soil layer models the surrounding soil. The method is based on energy principles and variational approach and the 1D finite element method is used in a pile displacement approximation. A new equation for modulus reduction appropriate for the rectangular pile is also developed to match the results of the proposed method to those of the three-dimensional (3D) finite element analyses. The proposed solution verified by comparing its results to the 3D finite element analyses and the comparisons are in excellent agreement.
 Keywords:
 rectangular piles; variational; energy principle; vertical load; finite element.

Experimental and Numerical Studies on Bearing Characteristics of Hexagonal-Section Composite Foundation Element

Hexagonal-section composite foundation is a new method of foundation reinforcement. The load–settlement characteristic and load-transfer mechanism play a critical role in the bearing capacity of pile composite foundation. The quantitative assessment of the hexagonal-section composite foundation element (Trilobal Pile for short) under vertical load was carried out through experimental and numerical studies. The variation rules and the influencing factors of stress and displacement of trilobal pile were compared and analyzed in ten model tests. And the numerical approach with trilobal pile (composite foundation) and equal diameter pile composite foundation were also conducted to simulate the loading process and the stress variation. The results show that the load–settlement curve of trilobal pile changed slowly and its ultimate bearing capacity was about 150 kN. Compared with the equal diameter pile, the ultimate bearing capacity of trilobal pile was increased by almost 275%, and the settlement was reduced by nearly 104.2%. The pile–soil stress ratio of trilobal pile was always greater than 1, and the maximum was about 7. And the axial force and lateral friction resistance of the trilobal pile changed abruptly at the variable cross section. The maximum axial force abrupt change rate of the trilobal pile was about 10%, and the abrupt change rate of the lateral frictional resistance of each test pile was about 65.2–82%. These results provide useful theoretical references for further development and optimization of the trilobal pile.