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Numerical Analysis of Piles Under Cyclic Lateral Load
Abstract
The paper presents the results to study the response of piles in homogeneous and layered soils subjected to quasi-static and cyclic lateral loading using numerical analysis. The analyses are carried out using three-dimensional finite element software, ABAQUS v6.10. Hypoelastic constitutive model is used to simulate the small strain nonlinear behavior of soil observed during the application of cyclic lateral load on piles. The bending moment and deflection responses computed from ABAQUS analyses are compared with the results obtained from the reported case histories of field pile lateral load tests and centrifuge tests. The favorable comparisons established that the three-dimensional numerical analysis can be effective to model complex soil–pile systems. Numerical analysis was also able to simulate a cement injected layer as a reinforcement body to the soft foundation soil, which can affect the pile response. The study was further extended to understand, the effect of flow of surrounding soil around the laterally-loaded piles on the response of piles.
... Figures 5 and 6 show the scale model and scale section view of pile. The material property values of piles and raft models are provided in Tables 1 and 2 ( Banerjee and Shirole, 2014). ...
... However, the bending moments could not be directly obtained from the ABAQUS output as the pile was modelled as a solid element. This restriction may be overcome by adding a very flexible beam element along the pile axis ( Banerjee and Shirole, 2014). ...
... (2012) used static pushover analysis and provided a strategy by integrating the calculated plastic curvature at all integration points along the pile shaft. As mentioned before a very flexible beam element was added along the pile axis ( Banerjee and Shirole, 2014) to perform bending moments ( Fig.13), the maximum values of shear force, and displacement, max U for the pile. Eq. (7) was used to calculating the plastic hinge length values. ...
Liquefaction is one of the leading seismic actions that cause extensive damage to buildings and infrastructure during earthquakes. In many historic cases, plastic hinge formations in piles were observed at inexplicable locations. This project investigated the behaviour of piled foundations within soils susceptible to liquefaction by using numerical analysis carried out in Abaqus software in terms of plastic hinge development. Three different soil profiles were considered in this project that varied in the thickness of both the liquefiable and non-liquefiable layers, pile length and free- and fixed-head pile conditions. Modelling a single pile as a beam-column element carrying both axial and El Centro record earthquake loading produced results of the seismic behaviour of piles that could be assessed by force-based seismic design approaches. The displacements and deformations induced by dynamic loads were analysed for piles affected by liquefaction, and the results were used to demonstrate the pile capacity and discuss the damage patterns and location of plastic hinges. Parametric studies generally demonstrate that plastic hinge formation occurs at the boundaries of the liquefiable and non-liquefiable layers; however, the location can be affected by a variety of factors such as material properties, pile length and thickness of the liquefied soil layer.
... However, the pile bending moments could not be directly obtained from the Abaqus output as the pile was modelled as a solid element. This restriction was overcome by adding a very flexible beam element along the pile (Banerjee and Shirole, 2014). ...
... As a result of the pore water pressure build-up, the compressibility of the layer cannot change drastically (McGann et al., 2012) so the soil bulk modulus, Ƙ, is assumed to remain constant throughout the soil mass and the Poisson's ratio of liquefiable soils is assumed as u = 0·485 (McGann et al., 2012). Additionally, the Mohr-Coulomb failure criterion is used to simulate the soils behaviour (Helwany, 2007), while the hypoelastic model in Abaqus was used to simulate non-linearity below the yield envelope (Banerjee and Shirole, 2014). ...
... However, the pile bending moments could not be directly obtained from the Abaqus output as the pile was modelled as a solid element. This restriction was overcome by adding a very flexible beam element along the pile (Banerjee and Shirole, 2014). The dynamic load model requires boundary conditions that offer support to the elements whilst restricting unnecessary motions (Abaqus, 2012). ...
... As a result of the pore water pressure build up, the compressibility of the layer cannot change drastically (McGann, et al., 2012) so the soil bulk modulus, Ƙ, is assumed to remain constant throughout the soil mass, and the Poisson's ratio of liquefiable soils is assumed as υ = 0.485 (McGann, et al., 2012). Additionally, the Mohr-Coulomb failure criterion is used to simulate the soils behaviour (Helwany, 2007), while the hypoelastic model in Abaqus was used to simulate nonlinearity below the yield envelope (Banerjee and Shirole, 2014). ...
Liquefaction is an important seismic hazard that can cause extensive damage and high economic impact during earthquakes. Despite the extensive research, methodologies and approaches for managing liquefaction for pilesupported structures, failures of structures due to liquefaction have continued to occur to this day. The main aim of
this paper is to develop a simplified methodology for reducing potential structural damage of structures founded in soils susceptible to liquefaction. In order to implement a successful remediation technique, the current methods for pile failure in liquefiable soils and remediation schemes of earthquake-induced liquefaction are critically reviewed and discussed. Cementation and lattice structure techniques for reducing liquefaction hazard are proposed, while numerical analysis for unimproved and stabilised soil profiles using the finite-element method is carried out to simulate the analysis of both stabilisation techniques. The results showed that both techniques are effective and economically viable for reduction or avoidance of potential structural damage caused by liquefied soil and can be used in isolation or in combination, depending on the ground profile and pile type.
... Numerous contributions on the analysis of piles under lateral cyclic load using 2 D finite element modelling (FEM) are available (Jin et al. 2010;Banerjee and Shirole 2014;Abbasa, Chik, and Taha 2015;Raddatatz and Taiba 2016), but those with the 3 D FEM have been rather limited (Sinha and Hanna 2017;Azizkandi, Baziar, and Yeznabad 2018;Mali and Singh 2018). However, a more rigorous analysis and development of design methodology on laterally loaded pile group demands a detailed analysis using 3 D FEM, despite the possible enormous computational effort (Basack and Nimbalkar 2018). ...
Major structures like offshore platforms, wind turbines, transport infrastructure, tall buildings, etc., resting on soft compressible clays, are often supported by pile foundations. Apart from usual vertical loading (dead load, live load, etc.), these piles are subjected to significant cyclic loads arising from actions of waves, ship impacts, winds or moving vehicles. Under such circumstances, the lateral mode of cyclic loading is predominant and affects the overall foundation stability. Such repetitive loading leads to stress reversal in adjacent soft clay initiating progressive degradation in soil strength and stiffness, deteriorating the pile capacity with unacceptable displacements. Although several past studies investigated the response of single pile under lateral cyclic loading, a detailed investigation on pile group in clay subjected to cyclic lateral loading, which is of immense practical interest to field engineers, is yet to be carried out. In this paper, in-depth study has been carried out by developing a three-dimensional dynamic finite element model. Comparison of the computed results with available test data validates the numerical model. Extensive parametric studies with field data indicate that both the axial and lateral pile capacities and displacements have been significantly influenced by the cyclic loading parameters. Relevant design curves are also constructed.
... A research computed the bending moment and deflection of pile under lateral cyclic loading by ABAQUS and the results were compared with previous reported field cases. The comparison results illustrated that three dimensional numerical analysis imposes good effect on modeling of complex soil-pile system [19]. Soil condition affects the behavior of pile. ...
Rapid urbanization creates a demand to expand the cities where using pile foundation became a recurrent practice. To ensure sustainability of projects pile load tests are important but may not be always feasible in terms of costing, on-site constrains etc. In this circumstance numerical analysis is a good alternate to estimate precise pile load capacity rather than conventional conservative approaches. This research illustrates the pile group efficiency fluctuation due to pile diameter, spacing, pile number and orientation in prescribed sandy soils. Using the conventional method the individual pile capacities are calculated for a constant depth with variable diameters and soil profiles. For simulating the piles, geometric models of sandy soils with sufficient boundaries are generated in PLAXIS 3D FOUNDATION software where the parameters of pile and soil components are considered as per predetermined values from reliable references. The analysis results have thoroughly been scrutinized by plotting several graphs at different aspects. The outcomes indicate that the conventional pile spacing i.e. 2.5D to 3.5D has an insignificant effect on pile group efficiency, irrespective to pile diameter and soil type. It also exhibits that the increment of pile number significantly decreases pile group efficiency for diameters of 600mm, 800mm, 1000mm and 1200mm in sandy soils. With a few exceptions as the diameter of the pile increases, the group efficiency decreases. The arrangement of piles in group has minor impact on pile group efficiency which enhances onsite flexibility. It is expected that these outcomes will facilitate the practicing engineers for efficient solutions.
... The soil, raft, and liner were meshed using C3D8R elements. A flexible beam element was embedded inside the solid pile elements to analyse the bending moment developed in the pile (Varghese et al. 2019;Banerjee and Shirole 2014). A monitoring section was selected at the centerline of the pile group (i.e., y = 0) for reference. ...
Across the globe, rapid urbanization demands the construction of tunnels within the vicinity of high-rise buildings supported on piled raft foundations. As a consequence, ground movements caused by such tunneling works could interfere with the serviceability of the foundations. Hence, the prediction of tunnel-induced ground deformation is of utmost importance. In the current study, a three-dimensional numerical analysis was carried out to study the response of an existing piled raft foundation with a 2 × 2 pile arrangement subjected to ground movements induced by a 6 m-diameter tunnel. The results obtained from the numerical simulations were further compared with a field study. The field measured lateral pile deflections were found to be in fair agreement with those computed using the present method. The numerical analysis was then extended to understand the influence of various factors, such as tunnel diameter, tunnel longitudinal axis, pile diameter, pile position from tunnel, and soil stiffness on the response of pile raft in the vicinity of an adjacent tunnel. The results obtained from the para-metric study was finally summarized in simple semi-empirical equations to estimate the maximum lateral deflection and bending moment for both short and long piles.
This study is concerned with evaluating the response of single pile subjected to cyclic horizontal uniaxial loading. Combination of lateral cyclic loading and constant vertical or lateral load in two-orthogonal directions is also studied. Moreover, the influence of one- and two-way loading, number of cycles and the intensity of constant load in conjunction with the cyclic lateral load is presented. The study is further continued by comparing the performance of long and short single pile embedded in clayey soil. A series of three-dimensional finite element models using ANSYS code are conducted to model three-dimensional transient analysis as well as the complicated soil pile interaction. In order to verify the validity numerical model, the numerical response is compared with reported results obtained by experimental cyclic load test performed on single pile penetrated in soft clay. Results show strong coupling between the two-orthogonal direction of lateral loading that decreases the lateral pile capacity. The interaction relation of biaxial loading with the factor b = 1.2 and 1.15 for one- and two-way loading respectively may well approximate the analysis results. The results of study are presented in terms of lateral cyclic capacities, pile head deflection, displacement profile and bending moment over the pile length.
A series of dynamic triaxle test on soft soil in Tangshan Binhai area was carried out to understand the horizontal dynamic characteristics of the soil around the offshore wind power pile foundation under wave load. The influence of confining pressure, dynamic stress amplitude and vibration frequency on horizontal dynamic characteristics of soft soil were studied. The results show that the horizontal dynamic strength of soft soil increases with the increase of confining pressure increases, and decreases with the increase of vibration. When the dynamic stress amplitude increases, the vibration times decreases. The horizontal dynamic strain εd increases with the increase of vibration frequency, and the larger the dynamic stress amplitude is, the more significant the growth is. The change agrees well with that of the Monismith model. The change of the horizontal dynamic modulus of soft soil is significantly influenced by the dynamic stress amplitude. When confining pressure decreases, dynamic modulus decreases. The Caofeidian soft soil has distinct structural horizontal dynamic characteristics. Under different confining pressures, the damping ratio shows an increasing trend when dynamic stress amplitude increases. © 2016, China National Publications Import and Export (Group) Corporation. All right reserved.
Simple expressions are developed for the equivalent horizontal spring and damping coefficients at the top of an end-bearing pile embedded in a uniform linear soil. Beam on elastic foundation and dynamic finite element analyses are used. It is demonstrated that, for long end-bearing piles, the top displacement response at high frequencies is independent of the pile length, and the pile behaves identically to a long floating pile in a half-space. A numerical criterion is presented to decide when a pile is long or short.
A series of centrifuge model tests of the lateral response of a fixed-head single pile in soft clay is reported. Both monotonic and cyclic episodes of loading are described, with varying amplitude and with intervening periods of reconsolidation. The soil conditions are characterized by cyclic T-bar penetrometer tests. The ultimate capacity under monotonic load for virgin and for postcyclic conditions was found to be comparable with calculations based on existing design methods, including theoretical plasticity solutions and empirical methods. The lateral stiffness was observed to degrade with cycles, with the rate of degradation being greater for larger cycles. The degradation pattern has been tentatively linked to the cyclic T-bar response, by considering the 'damage' associated with the cumulative displacement and remolding, in each case. This approach provides a consistent interpretation of the tests. Although episodes of pile movement and soil remolding led to a reduction in lateral resistance, intervening periods of reconsolidation led to a similar magnitude of recovery and a reduction in the level of softening in subsequent cyclic episodes. During an initial episode of cyclic lateral movement, the stiffness degraded by a factor of 2.3, which is comparable with the strength sensitivity derived from a cyclic T-bar test. In contrast, after five episodes of reconsolidation, the stiffness had recovered back to within 25% of the stiffness observed in the first cycle of the first episode, and it showed negligible degradation during subsequent cycling. This observation implies that, over a long period of cyclic loading, the lateral stiffness of a pile may tend towards a value that is independent of cycle number, and that represents a balance between the damaging effects of remolding and pore pressure generation and the healing effects of time and reconsolidation.
Pile foundations in a soft ground will usually be subjected to lateral cyclic loading during earthquakes. In a major earthquake, it is reasonable to think that the mechanical behavior of the pile foundation and the surrounding ground is nonlinear. In recent years, the limit state design method has become predominant in the design of foundations for railway bridges and other structures. Investigations of the mechanical behavior of pile foundations subjected to lateral cyclic loading up to the ultimate state, therefore, are very important in providing evidence for the design method. In this paper, field tests on a pile foundation, composed of cast-in-place reinforced concrete piles and subjected to one-side cyclic lateral loading up to the ultimate state, are simulated with the three-dimensional elasto-plastic finite element analysis (DGPILE-3D). In the numerical analysis, particular attention is paid to the stress-strain relation of the soil which, to the author's point of view, plays a dominant role in the mechanical behavior of the pile foundation. Such constitutive models as the Drucker-Prager model, Cam-clay model, and t ij model are adopted for the analysis in order to find the differences in the results due to the application of different constitutive models. Based on the analysis, the authors try to provide an applicable numerical way of evaluating the mechanical behavior of a pile foundation subjected to cyclic lateral loading at the ultimate state.
Equations and graphs for the determination of shear modulus and damping of soils were presented. The equations and graphs were based on test results on both remolded and undisturbed saturated cohesive soils and on clean sands. For various saturated cohesive soils tested, the frequencies were greater than about 200 cps and they were for about 100,000 cycles of loading.
A dynamic beam on a nonlinear Winkler foundation (or "dynamic p-y") analysis method for analyzing seismic soil-pile-structure interaction was evaluated against the results of a series of dynamic centrifuge model tests. The centrifuge tests included two different single-pile-supported structures subjected to nine different earthquake events with peak accelerations ranging from 0.02 to 0.7g. The soil profile consisted of soft clay overlying dense sand. Site response and dynamic p-y analyses are described. Input parameters were selected based on existing engineering practices. Reasonably good agreement was obtained between calculated and recorded responses for both structural models in all earthquake events. Sensitivity of the results to dynamic p-y model parameters and site response calculations are evaluated. These results provide experimental support for the use of dynamic p-y analysis methods in seismic soil-pile-structure interaction problems.
This paper presents test results from cast-in situ reinforced concrete single and group piles subjected to strong horizontal excitation. The tests were conducted for different eccentric moments simulating different excitation levels to obtain the frequency-amplitude response of the pile. Moderate nonlinear behavior is observed in both horizontal and rocking components of vibration. The experimental results were compared with dynamic interaction factor approach using nonlinear solutions. The accuracy of the nonlinear analysis in predicting the dynamic response depends on the choice of parameters that best characterize the response of boundary zone around the pile and the realistic length of pile separation. It is shown in this study that by allowing for boundary zone and separation between pile and soil, close agreement between theoretical predictions and measured response curves can be achieved.
The necessity of earthquake resistant reinforcement for structural foundations in large-scale earthquakes is rising. Especially, earthquake resistance reinforcement methods for pile foundations for bridges and buildings and pipeline networks for water, sewage, gas etc. are limited by various unfavorable construction conditions. Urano, Adachi, Mihara and Kawamura proposed a new method for earthquake resistance reinforcement applicable to either new or existing pile foundations, which was confirmed by model shaking table test under 1g gravitational field and the numerical simulation analysis. The proposal method aims to increase the stiffness of the pile foundation by the effect of a two layered Rahmen structure by the reinforcing body made in the ground. This paper shows a full scale field test of the proposed method and its numerical simulations that were conducted to examine deformation characteristics of the reinforced foundation and the effect of the reinforcement. Based on the test results, applicability of the proposed reinforcement method for a pile foundation has been confirmed.
A method is presented for calculating the seismic response of a system of horizontal soil layers. The essential element of the method is a rheological model suggested by Iwan which takes account of the nonlinear hysteretic behavior of soils and has considerable flexibility for incorporating laboratory results on the dynamic behavior of soils. Finite rigidity is allowed in the underlying elastic medium, permitting energy to be radiated back into the underlying medium. Three alternate ways of integrating the equations of motion are compared, an im-plicit technique, an explicit technique, and integration along characteristics. An example is set up for comparing the different methods of integration and for comparing the nonlinear solution with a solution based on the widely used equivalent linear assumption. The example consists of a 200-m section of firm alluvium excited at its base by the N21E component of the Taft accelerogram multiplied by four to produce a peak acceleration of 0.7 g and a peak velocity of 67 cm/sec. The three techniques of integration give very similar results, but integration along characteristics has the advantage of avoiding spurious high-frequency oscillations in the acceleration time history at the surface. For the chosen example, which has a thick soil column and a strong input motion, the equivalent linear solution underestimates the intensity of surface motion for periods between 0.1 and 0.6 sec by factors exceeding two. The discrepancies, however, wonld probably be less for input motion of lower intensity. At longer periods the equivalent linear solution is in essential agreement with the nonlinear solution. For the same example both solutions show that, compared to a site with rock at the surface, motion at the surface of the soil is amplified for periods longer than 1.5 sec by as much as a factor of two. At shorter periods the amplitude is reduced.
The behavior of pile foundations under earthquake loading is an important factor affecting the performance of structures. Observations from past earthquakes have shown that piles in firm soils generally perform well, while the performance of piles in soft or liquefied ground can raise some questions. Centrifuge model tests were carried out at the National University of Singapore to investigate the response of pile-soil system under three different earthquake excitations. Some initial tests were done on kaolin clay beds to understand the pure clay behavior under repetitive earthquake shaking. Pile foundations comprising of solid steel, hollow steel and hollow steel pile filled with cement in-fill were then embedded in the kaolin clay beds to study the response of clay-pile system. Superstructural inertial loading on the foundation was modeled by fastening steel weight on top of the model raft. The model test results show that strain softening and stiffness degradation feature strongly in the behaviour of the clay. In uniform clay beds without piles, this is manifested as an increase in resonance periods of the surface response with level of shaking and with successive earthquakes. For the pile systems tested, the effect of the surrounding soft clay was primarily to impose an inertial loading onto the piles, thereby increasing the natural period of the piles over and above that of the pile foundation alone. There is also some evidence that the relative motion between piles and soil leads to aggravated softening of the soil around the pile, thereby lengthening its resonance period of the soil further. The centrifuge model tests were back-analyzed using the finite element code ABAQUS. The analysis shows that the simple non-linear hypoelastic soil model gave reasonably good agreement with the experimental observations. The engineering implication arising from this study so far is that, for the case of relatively short piles in soft clays, the ground surface motions may not be representative of the raft motion. Other than the very small earthquakes, the raft motion has a shorter resonance period than the surrounding soil.
Mathematical models of soil nonlinearity in common use and recently developed nonlinear codes are compared to investigate the range of their predictions. We consider equivalent linear formulations with and without frequency-dependent moduli and damping ratios and nonlinear formulations for total and effective stress. Average velocity profiles to 150 m depth with midrange National Earthquake Hazards Reduction Program site classifications (B, BC, C, D, and E) in the top 30 m are used to compare the response of a wide range of site conditions from rock to soft soil. Nonlinear soil models are compared using the amplification spectrum, calculated as the ratio of surface ground motion to the input motion at the base of the velocity profile. Peak input motions from 0.1g to 0.9g are considered. For site class B, no significant differences exist between the models considered in this article. For site classes BC and C, differences are small at low input motions (0.1g to 0.2g), but become significant at higher input levels. For site classes D and E the overdamping of frequencies above about 4 Hz by the equivalent linear solution with frequency-independent parameters is apparent for the entire range of input motions considered. The equivalent linear formulation with frequency-dependent moduli and damping ratios under damps relative to the nonlinear models considered for site class C with larger input motions and most input levels for site classes D and E. At larger input motions the underdamping for site classes D and E is not as severe as the overdamping with the frequency-independent formulation, but there are still significant differences in the time domain. A nonlinear formulation is recommended for site classes D and E and for site classes BC and C with input motions greater than a few tenths of the acceleration of gravity. The type of nonlinear formulation to use is driven by considerations of the importance of water content and the availability of laboratory soils data. Our average amplification curves from a nonlinear effective stress formulation compare favorably with observed spectral amplification at class D and E sites in the Seattle area for the 2001 Nisqually earthquake.
A study on the influence of the plasticity index (PI) on the cyclic stress-strain parameters of saturated soils needed for site-response evaluations and seismic microzonation is presented. Ready-to-use charts are included, showing the effect of PI on the location of the modulus reduction curve G/G(max) versus cyclic shear strain-gamma-c, and on the material damping ration gamma-versus-lambda-c curve. The charts are based on experimental data from 16 publications encompassing normally and overconsolidated clays (OCR = 1-15), as well as sands. It is shown that PI is the main factor controlling G/G(max) and lambda for a wide variety of soils; if for a given gamma-c PI increases, G/G(max) rises and lambda is reduced. Similar evidence is presented showing the influence of PI on the rate of modulus degradation with the number of cycles in normally consolidated clays. It is concluded that soils with higher plasticity tend to have a more linear cyclic stress-strain response at small strains and to degrade less at larger gamma-c than soils with a lower PI. Possible reasons for this behavior are discussed. A parametric study is presented showing the influence of the plasticity index on the seismic response of clay sites excited by the accelerations recorded on rock in Mexico City during the 1985 earthquake.
The stiffness of soil of very small strain G0 is a useful parameter for characterizing the non-linear stress-strain behaviour of soil for monotonic loading. Tests were carried out on fine-grained soils in a hydraulic triaxial cell fitted with bender elements and with local axial gauges. From the results of these tests simple expressions were obtained which describe the variation of G0 with current state in terms of the current stress and overconsolidation ratio. The parameters in these expressions were found to depend on plasticity index. The simple expressions for G0 were found to apply generally at larger strains, with the values for the parameters also depending on the current strain. -from Authors
This paper presents test results from cast-in situ reinforced concrete single and group piles subjected to strong horizontal excitation. The tests were conducted for different eccentric moments simulating different excitation levels to obtain the frequency-amplitude response of the pile. Moderate nonlinear behavior is observed in both horizontal and rocking components of vibration. The experimental results were compared with dynamic interaction factor approach using nonlinear solutions. The accuracy of the nonlinear analysis in predicting the dynamic response depends on the choice of parameters that best characterize the response of boundary zone around the pile and the realistic length of pile separation. It is shown in this study that by allowing for boundary zone and separation between pile and soil, close agreement between theoretical predictions and measured response curves can be achieved.
The load-transfer (or t-z) curve, which reflects the interaction between a pile and the surrounding soil, is important for evaluating the load-settlement response of a pile subjected to an axial load using the load-transfer method. Preferably, the nonlinear stress-strain behavior of the soil should be incorporated into the t-z curve. This paper presents a practical approach for the estimation of t-z curves along bored piles by considering the nonlinear elastic properties and modulus degradation characteristics of the soil. A method for evaluating the modulus degradation curve from the results of a pressuremeter test is proposed. The results of load tests on one instrumented bored pile in Piedmont residual soil in Atlanta and another in the residual soil of the Jurong Formation in Singapore provide verification of the validity of the proposed approach.
This article reports a variational solution and its spreadsheet calculation procedure for the analysis of laterally loaded piles in a soil with stiffness increasing with depth. The aim of the paper is to provide solutions that can be used simply with recourse only to spreadsheet calculation to solve the displacement and bending moment of laterally loaded piles, so that they can be easily applied in practice as an alternative approach to analyze the response of laterally loaded piles.
Dynamic response of footings and structures supported by piles can be predicted if dynamic stiffness and damping generated by soil–pile interaction can be defined. An approximate analytical approach based on linear elasticity is presented, which makes it possible to establish the dimensionless parameters of the problem and to obtain closed-form formulas for pile stiffness and damping. All components of the motion in a vertical plane are considered; that is, horizontal as well as vertical translations and rotation of the pile head. The stiffness and damping of piles are defined in such a way that the design analysis of footings and structures resting on piles can be conducted in the same way as is applied in the case of shallow foundations.
A case study is presented of the interaction between the bending due to laterally spreading forces and axial-load induced settlement on the piled foundations of the Kandla Port and Customs Tower located in Kandla Port, India, during the 2001 Bhuj earthquake. The 22m tall tower had an eccentric mass at the roof and was supported on a piled-raft foundation that considerably tilted away as was observed in the aftermath of the earthquake. The soil at the site consists of 10m of clay overlaid by a 12m deep sandy soil layer. Post-earthquake investigation revealed the following: (a) liquefaction of the deep sandy soil strata below the clay layer; (b) settlement of the ground in the vicinity of the building; (c) lateral spreading of the nearby ground towards the sea front. The foundation of the tower consists of 0.5m thick concrete mat and 32 piles. The piles are 18m long and therefore passes through 10m of clayey soil and rested on liquefiable soils. Conventional analysis of a single pile or a pile group, without considering the raft foundation would predict a severe tilting and/or settlement of the tower eventually leading to a complete collapse. It has been concluded that the foundation mat over the non-liquefied crust shared a considerable amount of load of the superstructure and resisted the complete collapse of the building.
The near-surface geological site conditions in the upper tens of meters
are one of the dominant factors in controlling the amplitude and
variation of strong ground motion during large earthquakes. The
understanding of these site effects comes primarily from surface
recordings. For instance, different methods to estimate site response
and their variability are studied using aftershock data for the 17
January 1994 M6.7 Northridge, California earthquake. A second approach
corresponds to borehole measurements. We use the Garner Valley Downhole
Array (GVDA), which consists of a set of seven downhole strong-motion
instruments ranging from 0 to 500 meters depth, to study site response
effects. The GVDA velocity structure is first studied, then the H/ V is
evaluated, and finally some considerations of 2D and 3D basin effects
are also shown. These previous studies considered small to moderate
earthquakes, where strain levels are small enough, so that linear wave
propagation is assumed. However, for strong motions produced during
large earthquakes, the soils behave nonlinearly. In this study we
present evidence that nonlinearity can be directly observed in
acceleration time histories such as Wildlife Refuge, 1987 Superstition
Hills, CA; Kushiro Port station, 1993 Kushiro-Oki, Japan; among others.
To understand the nature of nonlinear soil dynamics, we developed a
model that includes anelastic dissipation of energy due to hysteresis.
The hysteresis is described by the generalized Masing rules. This new
hysteresis formulation, based on the classical Masing rules, has a
functional representation, and depends only on one parameter that can be
related to damping ratio tests. The coupling with pore pressure
generation shows the degradation of the shear modulus and the yield
stress during the cyclic response of the material. The simulations show
amplitude reduction as well as the shift of the fundamental frequency to
lower frequencies as observed on vertical arrays. The synthetic
accelerograms show the development of intermittent behavior---high
frequency peaks riding on low frequency carrier---as observed in
acceleration records. Using the Kushiro Port and Port Island borehole
arrays, we have modeled the recorded ground motions at the surface and
different depths. The synthetic accelerations and response spectra show
good agreement with the data.
A series of static lateral load tests were conducted on a group of fifteen piles arranged in a 3x5 pattern. The piles were placed at a center-to-center spacing of 3.92 pile diameters. A single isolated pile was also tested for comparison to the group response. The subsurface profile consisted of cohesive layers of soft to medium consistency underlain by interbedded layers of sands and fine-grained soils. The piles were instrumented to measure pile-head deflection, rotation, and load, as well as strain versus pile depth. The average load resisted by each group pile was lower than the load resisted by the single pile at the same deflection. The lead row resisted loads similar to a single pile with the second row and third and subsequent rows resisting successively smaller loads. Maximum bending moments in the trailing row piles were larger and occurred at greater depths than the lead row piles. Group effects became more pronounced at larger deflection levels due to increased overlap of the shear zones that resisted the lateral motion of the piles thereby reducing the soil resistance. LPILE Plus version 4.0 (Reese et al., 2000) was used to model the single pile test. The initial input soil parameters were adjusted to obtain a good match between the measured and computed results. This refined soil profile was then used to model the pile group in GROUP version 4.0 (Reese et al., 2000). User-defined p-multipliers were adjusted to match the measured and calculated results. For deflections up to 38 mm, p-multipliers were 1.0, 0.87, 0.64, 0.81, and 0.70 for Rows 1 to 5, respectively. For larger deflections, the p-multipliers decreased to an average value of 1.0, 0.81, 0.59, 0.71, and 0.59. Thesis (M.S.)--Brigham Young University. Dept. of Civil and Environmental Engineering, 2004. Includes bibliographical references (p. 215-220).
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