Nonlinear Soil-pile Interaction Research Articles

On the mechanisms governing the response of pile groups under combined VHM loading

In this paper, an experimental, numerical and analytical study is conducted on a 2 × 1 bored pile group on saturated sand under combined loading. Initially, a single-degree-of-freedom system founded on the pile group is tested at the ETH Zurich drum centrifuge under vertical, lateral and pushover loading, gaining insights and deriving benchmark results for validation of finite-element (FE) models. The latter account for non-linear soil–pile interaction, using hypoplasticity for sand and appropriate modelling of interfaces and pile response. Combining centrifuge and FE modelling, the governing resistance mechanisms are identified and quantified. The transition from model to prototype scale is achieved after careful consideration of scale effects. The concrete damaged plasticity model is employed to model the non-linear response of the reinforced concrete (RC) piles, accounting for axial load dependency of bending moment capacity. The prototype problem is studied parametrically, deriving failure envelopes for different levels of vertical loading. Distinguishable failure modes are identified, and the contribution of different resistance mechanisms is quantified. Finally, analytical failure envelopes are derived based on limit equilibrium, expanding Broms’ theory to pile groups under combined loading. Accounting for the axial load dependency of RC bending moment capacity, the proposed closed-form solutions provide a useful design tool for engineering practice.

Dynamic Response of a Four-Pile Group Foundation in Liquefiable Soil Considering Nonlinear Soil-Pile Interaction

Piles, which are always exposed to dynamic loads, are widely used in offshore structures. The dynamic response of the pile-soil-superstructure system in liquefiable soils is complicated, and the interaction between the pile and soil and the pile volume effect are the key influencing factors. In this study, a water-soil fully coupled dynamic finite element-finite difference (FE-FD) method was used to numerically simulate the centrifuge shaking table (CST) test of a four-pile group in saturated sand soil. An interface contact model was proposed to simulate the pile-soil interaction, and a solid element was used to consider the volume effect of the pile. The acceleration responses of the soil and pile, settlement deformation, excess pore water pressure, and bending moment were examined. The results show that the bending moment response of the two piles parallel to the shaking direction show minor differences, while the two piles perpendicular to the shaking direction show almost the same distribution. The values of excess pore water pressure at the same depth but different azimuth angles around the pile are also different. The numerical simulation can accurately reproduce soil deformation and pile internal force during and after dynamic loading.

Examination of the reduction in natural frequency of laterally loaded piles due to strain-dependence of soil shear modulus

Society's transition to low-carbon energy sources has resulted in significant expansion in offshore wind technology. Most offshore wind turbines (OWTs) are founded on large-diameter steel tubular piles driven into the seabed, termed monopiles. OWTs are subjected to large numbers of dynamic and cyclic environmental loads during their lifetime. To avoid fatigue or resonance issues, accurate characterisation of soil-structure interaction under operating and extreme conditions is paramount. There is a need to further improve understanding of the dynamic lateral response of piles after large lateral load cycling (storm events). This paper presents the results of lateral load field tests on piles with two different slenderness ratios, where the natural frequency was measured after a series of applied lateral load-unload events. 1D numerical models of the piles were developed and several operating parameters were varied to identify the parameters of importance in the nonlinear soil-pile interaction. Approaches to estimate the operating soil shear modulus both in-situ and post load-unload were trialled, and five subgrade reaction models were used to couple soil and pile properties, to ascertain the performance at predicting the measured frequencies from the experiments. Results suggest that predicted frequencies are highly sensitive to choice of model and degradation method used.

Effects of Corrosion and Scouring on Barge Impact Fragility of Bridge Structures Considering Nonlinear Soil–Pile Interaction

Abstract This paper investigates the effects of corrosion and scouring on the barge impact fragility of bridge structures. A typical four-span continuous reinforced concrete (RC) bridge is selected...

Relationship between p-multiplier and force ratio at pile head considering non-linear soil–pile interaction

The p-multiplier concept has been widely used to analyse the response of laterally loaded pile groups. Conventionally, the p-multiplier fm is evaluated iteratively by trial and error comparisons of p–y curves and field measurements. However, these approaches could face numerical difficulties and often require subjective adjustment and therefore could result in non-unique solutions. This paper derives a power relationship between fm and the force ratio at the pile head ηhead, defined by the ratio of the lateral force acting at the head of a group pile to that of a single pile, where ηhead equals fm to the power of n. In the case of long piles, it was found that (a) n = 3/4 when the initial modulus of subgrade reaction kini and the ultimate soil resistance pult are constant with depth; and (b) n = 3/5 when both kini and pult increase linearly with depth. This analytical relationship can then be utilised to determine fm from the test results of group piles and single piles, and quickly estimate the lateral bearing capacity of pile groups.

Non-linear soil–pile interaction induced by ground settlements: pile displacements and internal forces

In urban areas, the construction of tunnels and deep excavations beneath and near pile foundations can be detrimental for the superstructure and the foundation. A two-stage continuum-based non-line...

Fragility reduction of offshore wind turbines using tuned liquid column dampers

High flexibility of offshore wind turbines (OWTs) makes them vulnerable to excessive vibrations. This paper studies vibration control of offshore wind turbines induced by multi-hazard excitations. A model consisting of entire offshore wind turbine foundation and tower controlled by tuned liquid column dampers (TLCD) considering nonlinear soil pile interaction is established. The model is subjected to wave, wind, and seismic loading. The effect of severity of earthquake on the performance of the structural control device is investigated. A fragility analysis based on acceleration capacity thresholds is performed to estimate reliability improvement using the structural control devices. The fitted fragility functions based on multiple stripes analysis are constructed and compared with the empirical cumulative distribution curves. The results suggest that the use of an optimal TLCD with a mass ratio of 2.5% reduces the fragility of the system by as much as 6% and 12% for operational and parked conditions, respectively.

Semi-Active Structural Control of Offshore Wind Turbines Considering Damage Development

High flexibility of new offshore wind turbines (OWT) makes them vulnerable since they are subjected to large environmental loadings, wind turbine excitations and seismic loadings. A control system capable of mitigating undesired vibrations with the potential of modifying its structural properties depending on time-variant loadings and damage development can effectively enhance serviceability and fatigue lifetime of turbine systems. In the present paper, a model for offshore wind turbine systems equipped with a semi-active time-variant tuned mass damper is developed considering nonlinear soil–pile interaction phenomenon and time-variant damage conditions. The adaptive concept of this tuned mass damper assumes slow change in its structural properties. Stochastic wind and wave loadings in conjunction with ground motions are applied to the system. Damages to soil and tower caused by earthquake strokes are considered and the semi-active control device is retuned to the instantaneous frequency of the system using short-time Fourier transformation (STFT). The performance of semi-active time-variant vibration control is compared with its passive counterpart in operational and parked conditions. The dynamic responses for a single seismic record and a set of seismic records are presented. The results show that a semi-active mass damper with a mass ratio of 1% performs significantly better than a passive tuned mass damper with a mass ratio of 4%.

Parametric investigation on the responses of laterally loaded piles in overconsolidated clay using nondimensional solutions addressing nonlinear soil-pile interaction

This paper presents nondimensional solutions and parametric studies of the responses of laterally loaded piles in overconsolidated clay. Nondimensional expressions of a nonlinear p-y model, termed as the H-model, and the governing equation of laterally loaded piles were derived and solved. Parametric studies by virtue of the nondimensional solutions were conducted, which revealed that the pile head response is highly sensitive to the change in the soil undrained shear strength (su), the maximum lateral bearing factor (np(max)), and the ratio of the elastic modulus of subgrade reaction to the soil undrained shear strength (nk), but is insensitive to other parameters in the model.

Impact of ground motion duration and soil non-linearity on the seismic performance of single piles

Pile foundations strongly influence the performance of supported structures and bridges during an earthquake. In case of strong earthquake ground motion, soft soils may be subjected to large deformation manifesting aspects typical of the non-linear behaviour such as material yielding, gapping and cyclic degradation. Therefore, nonlinear soil-pile interaction models should be able to capture these effects and improve the prediction of the actual seismic loading transferred from the foundation to the superstructure. In this paper, a beam on nonlinear Winkler foundation (BNWF) model is used, which can simulate cyclic soil degradation/hardening, soil and structural yielding, slack zone development and radiation damping. Incremental Dynamic Analyses (IDAs) are performed to evaluate the effects of Ground Motion Duration (GMD) and soil non-linearity on the performance of single fixed-head floating piles. Various homogeneous and bilayer soil profiles are considered, including saturated clay and sand in either fully dry or saturated state and with different levels of compaction. In order to evaluate the effect of nonlinearity on the response, the results of the nonlinear analyses are compared with those obtained from linear soil-pile analysis in terms of bending moment envelope. Results show the relevance of considering the GMD on the performance of the single pile especially when founded on saturated soils. For low intensities and dry sandy soils, the linear soil-pile interaction model can be used for obtaining reliable results.

杭位置の違いによる群杭の非線形水平地盤抵抗のモデル化

1. Introduction Pile foundation response during earthquakes is strongly affected by nonlinear soil-pile foundation interaction. The damages to pile foundations during the 1995 Hyogo-ken Nanbu Earthquake and the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake were obviously attributed to nonlinear interaction of soil-pile foundation-superstructure. The dynamic nonlinear behavior of soil around each pile affects lateral load distribution and displacement of each pile. Therefore, shaking table tests were conducted to clarify the dynamic nonlinear behavior of soil around piles, further earthquake response analyses were conducted to simulate the effect of the nonlinear soil-pile interaction system on performance of the superstructure supported by pile group. 2. Outline of shaking table tests 25-pile group foundation model was set up in Toyoura sand deposits within a laminar container. The sand deposits were prepared by air pluviation method in the laminar container. The model piles were acryl cylinder with 12mm in diameter and 400mm long. 25 piles were arranged in squares and pile spacing was 2.0 times pile diameter. The superstructure was modeled as a rigid body. The input waves were seismic waves in notification with random phase of Japan and the Hyogo-ken Nambu earthquake. 3 input acceleration levels were used to investigate the influence of input motion level. Tests with and without the mass of superstructure were conducted to investigate lateral load distribution of each pile. 3. Outline of shaking table tests simulation Two analysis methods were conducted for simulation of tests, one is nonlinear 3D-FEM, and the other is lumped mass model. Two case of Lumped mass model are conducted, one is with soil springs around pile group which were different according to pile arrangements and direction of loading, and is with soil springs without pile group effect. Soil springs were calculated by using nonlinear 3-D FEM, which were modeled appropriately according to the location of pile. 4. Conclusions The concluding remarks of these shaking tests and analytical studies are as follows. (1) In shaking tests, lateral subgrade reaction around piles depends on the location of each pile in pile group and the direction of loading. That of the corner pile is remarkably larger than that of the middle pile. (2) In shaking tests, hysteresis curve of soil spring at the corner pile is found to be asymmetric loop, while that at the middle pile be symmetric loop. (3) In shaking tests, bending moment at each pile head depends on the location of each pile in pile group. That of corner pile head is remarkably larger than that of middle pile head. As the input level of acceleration increase, bending moment at each pile head is equalized. (4) The proposed lumped mass model of the soil-pile foundation system, which consists of beam elements and nonlinear interaction springs considering the location of pile, is verified validation. It is confirmed that this proposed model well represents response of the superstructure supported by pile group.

Seismic analysis of highway skew bridges with nonlinear soil–pile interaction

Performance of highway bridges during an intense seismic event is an issue of utmost importance. Of particular interest is the response of skewed highway bridges, where the skew angle and other related factors make the problem more complex. Although a number of studies had been carried out to address these issues in the past, most of them have neglected or over-simplified the soil–structure interaction effects, primarily relying upon the assumption that soil–structure interaction generally leads to a conservative estimation of seismic demands. The present study focuses on investigating the effect of skew angle on seismic response of a bridge-foundation system including nonlinear soil–pile interaction subjected to bi-directional ground motions. It has been observed that the rotational demand of the bridge deck is greatly affected by the skewness, indicating an increased vulnerability of skewed bridges due to rotational movement of the deck leading to deck unseating. It is also observed that the shear and moment demands of the piers increase significantly with increasing skew angle, as much as 54% and 37%, respectively. The maximum bending moment of the pile shaft is also found to increase upto 55%, indicating higher design requirements for the foundation components of the skew bridges compared to a similar normal bridge.

Seismic fragility analysis of 5 MW offshore wind turbine

Considering nonlinear soil–pile interaction, seismic fragility analysis of offshore wind turbine was performed. Interface between ground soils and piles were modeled as nonlinear spring elements. Ground excitation time histories were applied to spring boundaries. Two methods of applying ground motion were compared. Different time histories from free field analysis were applied to each boundary in the first loading plan (A). They were compared with the second loading plan (B) in which the same ground motion is applied to all boundaries. Critical displacement for wind turbine was proposed by using push-over analysis. Both the stress based and the displacement based fragility curves were obtained using dynamic responses for different peak ground accelerations (PGAs). In numerical example, it was shown that seismic responses from loading plan A are bigger than from plan B. It seems that the bigger ground motion at surface can cause less response at wind turbine due to phase difference between ground motions at various soil layers. Finally, it can be concluded that layer by layer ground motions from free field analysis should be used in seismic design of offshore wind turbine.

Nonlinear Interaction of Soil–Pile in Horizontal Vibration

This paper presents a new model for analyzing a nonlinear soil–pile interaction subject to horizontal shaking of a vertical circular pile embedded in a soil layer of finite thickness. The pile rests on bedrock with either a pinned or a clamped support. The soil mass is assumed composing of a “semi-nonlinear” inner soil zone around the pile and a linear viscoelastic soil zone outside the inner zone. When the inner soil behaves linearly, the present solutions are identical to those obtained by Nogami and Novak in 1977. Numerical results show that soil resistance of less slender piles developed against the vibration is larger than that of more slender piles. Soil resistance depends more strongly on the size of the nonlinear inner zone when the pile is vibrating at a frequency higher than the natural frequency of the soil. Soil nonlinearity, in general, results in a smaller damping and stiffness of the soil–pile system, except at high frequency. At higher vibration frequency, the situation can be very complicated. The exact value of the dynamic stiffness of the soil–pile system depends on elastic shear wave speed, soil nonlinearity, vibration frequency, slenderness ratio of the pile, magnitude of vibration, and tip conditions of the pile. Generally speaking, the dynamic stiffness is smaller than the static stiffness. The normalized dynamic stiffness for pile with a pinned tip is, in general, larger than that with a clamped tip, while the reverse is true for the damping.

Phenomenological Model Applied to Inelastic Response of Soil-Pile Interaction Systems

ABSTRACT A dynamic nonlinear Winkler spring model, is developed for the seismic response of deep foundations. The model utilizes the phenomenological “BWGG” model (outlined in a companion paper), and it can treat the inelastic response of both the soil and the pile. The nonlinear reaction of the soil is modeled realistically by the BWGG interaction springs and dashpots, with due consideration to effects such as : separation (gapping) of the pile or caisson from the soil, radiation damping, and loss of strength due to pore-water pressure development. The modeling of pile inelasticity is also versatile, and can treat from well-reinforced to poorly-reinforced concrete sections. The necessity for the proposed model arises from the difficulty to predict the large-amplitude dynamic response of piles up to failure. The BWGG-Winkler model is validated through the results of in-situ monotonic and dynamic pile load tests. It is further utilized to study the nonlinear soil-pile interaction under lateral monotonic loading. The results of the model are compared with the venerable Broms (1964) theory for pile lateral capacity

Seismic response of tall building considering soil-pile-structure interaction

The seismic behavior of tall buildings can be greatly affected by non-linear soil-pile interaction during strong earthquakes. In this study a 20-storey building is examined as a typical structure supported on a pile foundation for different conditions: (1) rigid base, i.e. no deformation in the foundation: (2) linear soil-pile system; and (3) nonlinear soil-pile system. The effects of pile foundation displacements on the behavior of tall building are investigated, and compared with the behavior of buildings supported on shallow foundation. With a model of non-reflective boundary between the near field and far field, Novak’s method of soil-pile interaction is improved. The computation method for vibration of pile foundations and DYNAN computer program are introduced comprehensively. A series of dynamic experiments have been done on full-scale piles, including single pile and group, linear vibration and nonlinear vibration, to verify the validity of boundary zone model.