Nonlinear Soil Research Articles

An extended BNWF model for root piles incorporating soil–root interactions under lateral loading

Root piles constitute a new type of pile in which a series of prefabricated reinforced concrete roots are inserted into the surrounding soil to improve the bearing capacity of the pile foundation. To investigate the nonlinear behaviour of root piles under lateral loading, an extended beam on nonlinear Winkler foundation (BNWF) model incorporating the soil–root interactions is proposed in this paper. The proposed model is aimed at simultaneously describing the additional bearing capacity provided by roots and the nonlinear soil–root interactions for laterally loaded root piles. Lateral loading tests are conducted on two full–scale root piles to verify the applicability and reliability of the proposed model. It is demonstrated that the proposed model can describe the lateral behaviour of root piles with a reasonable calculation accuracy. Further parametric analyses are conducted to investigate the lateral bearing behaviour of the root piles. It is found that the root pile increases the lateral bearing capacity of the piles owing to the roots. Increasing the number or length of roots and locating them in shallow soil layers can improve the lateral bearing capacity of the root piles. The obtained results are useful for the design and application of root piles.

Scour protection effects on the dynamic response of jacket structures under extreme load events

A well-validated Finite Element (FE) model of a jacket structure (OC4 jacket) with pile foundation together with a Computational Fluid Dynamic (CFD) model (OpenFOAM-based) set-up for extreme waves on the structure are applied to perform systematic comparative analyses of the dynamic response of the structure to extreme wave loads with and without scour effects. The pile foundation model is considered by implementing a non-linear soil model to the foundation using load-deflection (p-y) approach to capture the non-linear behaviour of the soil for large pile foundation displacements. The results show that, the scour protection improves the foundation bearing capacity and consequently, decreases the failure probability in terms of both serviceability and ultimate limit states. Moreover, it enhances the dynamic response of the structure under extreme load events caused by breaking waves. Accordingly, an innovative solution for scour protection of offshore-fixed bottom structures has been introduced by “Biodegradable Soft Rock” (sand filled containers produced from bio-degradable geotextile product) which is the newest development in this section.

Parametric Analysis on the Effect of Dynamic Interaction between Nonlinear Soil and Reinforced Concrete Frame

The effect of dynamic soil–structure interaction on the seismic demand of a reinforced concrete frame is of great significance to seismic design, retrofit, and damage evaluation. To investigate the degree of influence of the consideration of the soil–structure interaction on the structural seismic response, an efficient numerical model considering the nonlinearities of both a reinforced concrete frame and soil was developed and validated against a shaking table test. Subsequently, detailed parametric analyses on the dynamic soil–structure interaction effect were conducted, where the influences of the length and diameter of the pile, span number and frequency of the structure, soil property, and natural uncertainty of the seismic record were investigated. The research results indicate that the base shear of the pile-supported reinforced concrete frame generally increases with a larger pile length and pile diameter. The influence of the span number and pile diameter on the soil–structure interaction effect is up to 40% in some cases while that of the pile length is within 10% in general. Consideration of the soil–structure interaction can also considerably increase the structural base shear in certain cases and the growth can be greater than 30%. The dynamic soil–structure interaction effect predominantly depends on the structure frequency, spectral characteristic and peak acceleration of the seismic record, and soil shear wave velocity while the influence of the pile diameter and number of spans cannot be neglected in some cases.

Study on the Intercropping Mechanism and Seeding Improvement of the Cavity Planter with Vertical Insertion Using DEM-MBD Coupling Method

In the dry areas of Northwest China, cavity planters with vertical insertions are used for seeding on film. Due to the uncertain mechanism between cavity planters and maize seeds and soil, research on the cavity planter has been slow. Several theoretical and experimental methods have been developed to investigate the interaction between the cavity planter and maize seeds in soil. These methods enable exploration of the mechanism to reduce soil disturbance and improve seeding performance. However, these methods are unable to predict the dynamic force of tools and soil behavior because of non-linear soil properties. A simulation experiment was conducted using the DEM-MBD coupling method to explore soil disturbance caused by cavity seeders and the resistance to entry. Additionally, the effect of the maize shape and the cavity planter motion on the seed number qualification and the empty cavity rate was investigated. It was proposed that the inverted hook be used to prevent the movement of maize seeds up and down in cavity seeders, thereby improving seed filling performance. Simulations and experiments were conducted, and the results showed that the average empty cavity rate and the seed number qualification were 2.0% and 91.3%, respectively, which met the requirements of the maize sowing standards.

A simplified approach for predicting the non-linear critical speed of railway tracks

When trains approach the railway track-ground critical speed ground deformations are amplified, potentially exceeding the linear-elastic strain range in the supporting earthworks. This leads to non-linear soil behavior which can be difficult to model due to high computational cost and the need for detailed characterization of soil properties. As a solution, for the first time this research presents a simplified methodology to calculate the non-linear critical speed of track-ground structures. The methodology uses a novel semi-analytical approach that combines dispersion curves with a simplified linear equivalent model. Using the approach, a track-ground static strain field is first calculated, and then converted into a dynamic one using a non-dimensional dynamic amplification function, independent of the elastic properties of the soil. The result allows the non-linear critical speed to be calculated using static simulation results which are relatively straightforward to generate. The methodology is validated using a 3D non-linear Hardening Soil-small (HSSmall) numerical model and then a parametric study is used to compare results for both concrete slab and ballasted tracks. The results show the proposed approach is a reliable tool for predicting the non-linear critical speed.

Resonance of a structure with soil elastic waves released in non-linear hysteretic soil upon unloading

Abstract High-frequency motion is often observed in small-scale experimental works carried out in flexible containers under simplified seismic loading conditions when single harmonic sine input motions are introduced at the base of a soil specimen. The source of the high-frequency motion has often been sought in experimental inaccuracies. On the other hand, the most recent numerical studies suggested that high-frequency motion in the steady-state dynamic response of soil subjected to harmonic excitation can also be generated as a result of soil elastic waves released in non-linear hysteretic soil upon unloading. This work presents an example of a finite element numerical study on seismic soil–structure interaction representative of an experimental setup from the past. The results show how high-frequency motion generated in soil in the steady-state response, apparently representative of soil elastic waves, affects the steady-state response of a structure, that is, it is presented how the structure in the analysed case resonates with the soil elastic waves. The numerical findings are verified against the benchmark experimental example to indicate similar patterns in the dynamic response of the structure.

Combined loading of RC pile groups in clay accounting for N–M interaction

Motivated by the need for retrofit of existing bridges, this paper explores the nonlinear response of pile groups in clay under combined loading. The problem is analysed numerically, employing a kinematic hardening model for soil, and the Concrete Damaged Plasticity (CDP) model for the reinforced concrete (RC) piles. Using a reference 2 × 1 pile group, it is shown that three resistance mechanisms are mobilized: axial pile loading (Max), pile bending (Mb), and pilecap resistance (Mcap). While current design practice typically considers only the first mechanism, it is shown that allowing for strongly nonlinear soil response and full mobilization of all three mechanisms may lead to a significant increase of pile group moment capacity. The analysis also reveals the need to account for axial force–bending moment (N−M) interaction of the RC piles, which is feasible through the CDP model. Especially for existing pile groups, more realistic estimation of ultimate moment capacity may facilitate the development of rational retrofit strategies. A parametric study is subsequently conducted, exploring the effect of the safety factor against vertical loading (FSv), the moment to shear (M/H) ratio, interface modelling, and pilecap contribution. The results are generalized by analysing more complex 3 × 1 and 4 × 1 pile group typologies and by deriving 3D failure envelopes in the V – H – M space. Thanks to the contribution of the pilecap, the decrease of FSv leads to an expansion of the failure envelope, which can be of relevance for existing pile group assessment after bridge widening. Finally, the interaction diagrams are normalized, leading to a unique non-dimensional 3D failure envelope for all examined pile group typologies.

Enhancing displacement coefficient method for multi degree of freedom buildings (MDOF) considering nonlinear soil structure interaction

Enhancing displacement coefficient method for multi degree of freedom buildings (MDOF) considering nonlinear soil structure interaction

Impact of Design Parameters on the Dynamic Response and Fatigue of Offshore Jacket Foundations

The lifetime of offshore foundations is governed by a combination of harsh environmental conditions and complex service loads. The fatigue limit state (FLS) analysis needs to be performed in the time domain to capture the complex phenomenon. This study aims to investigate different parameters and design modifications that can impact the design life of an offshore jacket foundation. An OC4 jacket foundation is designed in industrial software from DNV and reduced to a super-element model. The super-element model is connected to an NREL 5-MW wind turbine designed in Bladed. The time-series loads are used to compute the fatigue damages faced by the foundation during the service life. The impact of soil non-linearity, marine growth, scour size, the mass of the transition piece, and the grouted connection’s design on the dynamic response and fatigue damages are compared. A 30% increase in life was observed by replacing the concrete transition piece with a lightweight steel configuration. The fatigue damages were considerably greater for the inclined pile in the leg grouted connection than for the leg in the pile concept. The study provides a different perspective by analysing the effect of design parameters and design changes in the complex and computationally expensive time-series domain.

Pipeline element for upheaval buckling analysis of submarine pipelines with geometric-imperfections under high temperature high pressure

Submarine pipeline is used for transporting oil from wells to resource storage facilities. The oil inside the pipeline is heated and pressured for long-distance transportation, making the pipeline under high temperature and high pressure (HTHP). The pipeline is usually placed inside a trench where surrounding soil restraints its longitudinal expansion. The pipeline is not perfectly straight, but with geometric imperfections, so the upheaval buckling may be triggered when the accumulated compression force has reached its buckling strength. It is not easy to conduct a geometrically nonlinear analysis for the pipeline because of the nonlinear soil–pipeline interactions (SPIs). This paper proposes a new line element aiming for the geometrically nonlinear large deformation analysis of the pipeline under HTHP. The axial expansion force due to high working temperature and pressure is included in the element formulation. Furthermore, a Gauss–Legendre integral method is employed to consider continuously distributed nonlinear SPIs in both lateral and longitudinal directions. Since the pipeline might exhibit large deflections, the analysis adopts the Updated-Lagrangian approach to establish the equilibrium conditions. A Newton–Raphson procedure is developed to execute the analysis. Detailed element formulations are given. Six groups of examples are provided to examine the robustness of the numerical method.

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.

Effect of Soil Damping on the Soil–Pile–Structure Interaction Analyses in Cohesionless Soils

Pile foundations in earthquake-prone regions must be analyzed and designed considering the dynamic loads. In the fully nonlinear dynamic analyses, the soil nonlinearity can be considered using the modulus degradation curves in the total-stress approach, and the soil damping is controlled by the unloading/reloading rule. Several researchers have investigated the effect of soil damping on the free-field soil response analyses, but the effect on the soil–pile–structure system response has not been studied thoroughly. In this study, the nonlinear elastic method (hyperbolic model) and the elastoplastic Mohr–Coulomb (MC) models were implemented to investigate the effect of soil damping on the pile and structure response. Dynamic soil–pile–structure interaction analyses were performed by simulating two different centrifuge tests published in the literature, and the analysis results were compared with the centrifuge test results. The analyses with the low-intensity input motions showed that the superstructure accelerations and the bending moments in the single pile were estimated with reasonable accuracy. However, the superstructure accelerations might be underestimated under high-intensity motions, especially in the MC model. Additional analyses were performed under six earthquake records. The constitutive models (MC and hyperbolic) may significantly vary the maximum structural acceleration and pile maximum moments (up to 50%). As a result, the responses of the superstructure and the pile in soil–pile–structure interaction problems are highly dependent on the soil constitutive model that must take the damping into account accurately; in turn, due account must be given to the selection of the constitutive model.

Free-field soil response to bidirectional horizontal non-uniform ground motion from multi-point shaking table tests

This paper assesses the dynamic nonlinear free-field soil response under bidirectional horizontal non-uniform excitation through a series of multi-point 1 g shaking table tests. The soil bed was enclosed in an innovative soil box supported by an array of three shake tables and was subjected to uniform and non-uniform ground motions in one and two horizontal directions. The soil response to bidirectional and unidirectional excitations, both uniform and non-uniform, were compared and discussed in terms of acceleration time history and Fourier spectra, acceleration amplification factor and soil settlement, as well as shear stress-shear strain behavior. The comparison of soil responses to bidirectional and unidirectional shakings demonstrated that the trends of peak acceleration amplification factors are similar along soil depth, but the response difference between bidirectional and unidirectional shaking is increasingly higher along the soil profile towards the ground surface, especially for uniform excitation. The variations of soil settlement, shear stress-strain and damping curves under bidirectional excitation were similar to that under longitudinal excitation; however, they were much different than those observed under transverse excitation. The soil damping in Y-direction under non-uniform excitation was larger than that under uniform excitation. These different site response characteristics under bidirectional non-uniform excitation should be properly considered in the seismic design of extended infrastructure.

Application of nonlinear soil resistance-pile lateral displacement curve based on Pasternak foundation model in foundation pit retaining piles

Application of nonlinear soil resistance-pile lateral displacement curve based on Pasternak foundation model in foundation pit retaining piles

Variability of physics-based simulated ground motions in Thessaloniki urban area and its implications for seismic risk assessment

An accurate characterization of earthquake ground motion and its variability is crucial for seismic hazard and risk analysis of spatially distributed portfolios in urban areas. In this work, a 3D physics-based numerical approach, based on the high-performance spectral element code SPEED (http://speed.mox.polimi.it/), is adopted to generate ground shaking scenarios for strong earthquakes (moment magnitude MW=6.5–7) in the Thessaloniki area (Northern Greece). These simulations account for kinematic finite-fault rupture scenarios and a 3D seismic velocity including the two main geological structures present in the area (Thessaloniki and Mygdonia basins). The numerical model is successfully validated by comparing simulated motions, on the one hand, with the recordings of a real small-magnitude (MW4.4) earthquake and, on the other, with empirical Ground Motion Models for the historical MW6.5 1978 earthquake. The sensitivity of results to the velocity model, anelastic attenuation, and non-linear soil effects is evaluated. The variability of the ground motion intensity measures in Thessaloniki as a function of the finite-fault rupture realizations (causative fault, magnitude, hypocenter location) is explored to gain insight into its potential impact on seismic risk assessment in urban areas.

Nonlinear fictitious-soil pile model for pile high-strain dynamic analysis

Existing numerical models for simulating the pile behaviours in high-strain dynamic load tests (DLTs) cannot account for the wave propagation in base soil. This paper proposes a nonlinear fictitious-soil pile (FSP) model to better simulate the base soil. The proposed model regards the base soil as a FSP with a cone angle extending from pile toe to bedrock, and simulates shaft resistance by improved Randolph model. The pile-soil system is discretized into a series of mass-nonlinear springs, and was solved based on the Newmark’s β method. The proposed model was validated by comparing its predictions with those obtained from Randolph model and Smith model, as well as field measurements of driven and bored piles in different sites. Parametric study demonstrated that the calculated results were significantly affected by the discretization degree, FSP dimensions and soil nonlinearity. The displacement attenuation in base soil is nonlinear, and the impact energy is consumed rapidly near pile toe. Besides, large-diameter piles tend to have a larger affected zone, in which the wave phenomenon in base soil should be considered. Empirical formulas for soil affected zone and required pile displacements in DLTs were proposed to facilitate the practical application of FSP model.

Effect of nonlinear soil−structure interaction and lateral stiffness on seismic performance of mid−rise RC building

In this study, the nonlinear soil-structure interaction (SSI) is considered to analyse the mid-rise structure's seismic performance with proportional lateral stiffness of columns in the orthogonal direction. The model's geometry is created using a finite element-based software package (ABAQUS). Cap plasticity and hysteretic models are used to mimic the nonlinear behaviour of soil and structural components, respectively. Truncated soil mass is incorporated using the spring and dashpot at the boundaries node with a MATLAB code for the dynamic analysis. The methodology validation is carried out on the shallow foundation under the cyclic loading and found in good agreement. Static pushover analysis of 4-storey, 8-storey mid-rise structures and 12-storey dual RC frame system is carried out to estimate the yielding displacement of the building structure at the roof level, which is further used in the incremental dynamic analysis (IDA) of the SSI and fixed base system. IDA is carried out for earthquake intensities ranging from 0.1g to 0.6g. IDA results are represented in terms of displacement ductility demand, normalised peak settlement, base shear, normalised peak foundation sliding and drift ratio. From the IDA analysis, the fragility assessment of the building is carried out for both the cases (fixed and SSI base). The present study concluded that lateral stiffness and SSI significantly affect the seismic performance of mid-rise buildings.

Numerical simulation of seismic response of Slope–Foundation–Structure interaction for mid–rise RC buildings at various locations

This study is intended to evaluate the seismic response of the Slope–Foundation–Structure interaction of the mid–rise building at various locations with varying slope angles for linear and nonlinear homogenous soil strata using finite element modelling. The dynamic implicit method is used for the time domain analysis, and the Kelvin element boundary is applied at the truncated soil domain to absorb the reflecting waves. The nonlinearity of soil is incorporated using the hyperbolic stress-strain model by developing the user-defined material (UMAT). The model is validated with published literature, and found the result in good agreement. Further, slope stability is evaluated using FLAC3D and found the factor of safety >1. The results are shown in terms of vertical displacement, horizontal displacement, drift ratio, peak horizontal floor acceleration (PHFA), the rocking of the foundation and base shear. The effects are calculated for the different locations of the building, nonlinearity and slope angle. Nonlinear soil behaviour shows the permanent vertical deformation of the foundation. The lateral displacement of the buildings constructed at the top of the slope and the bottom of the slope are more affected compared to the building constructed on the slope for all slope angles. The nonlinearity of soil is more likely to affect the lateral seismic response of the buildings located at the top and on the slope as compared to the building located at the bottom of the slope. The PHFA is reduced by incorporating the nonlinearity of soil for all slope angles. All buildings are experiencing a high interstorey drift ratio for slope angle 20. The base shear seems to increase with the decrease in slope angle for all buildings in case of linear and nonlinear soil behaviour. The building located on the slope is more vulnerable to rocking failure compared to buildings located at the top and bottom of the slope. With the increase in slope angle, the rocking of the foundation is also increasing.

Seismic Performance Assessment of Semi Active Tuned Mass Damper in an MRF Steel Building Including Nonlinear Soil–Pile–Structure Interaction

Seismic Performance Assessment of Semi Active Tuned Mass Damper in an MRF Steel Building Including Nonlinear Soil–Pile–Structure Interaction

Analysis of nonlinear soil-structure interaction using partitioned method

For the current state-of-practice in soil-structure interaction (SSI) of nuclear facilities, the strain-dependent characteristics of soil are considered indirectly via equivalent linear methods in the frequency domain, typically represented by the SASSI program. SSI analysis in the time-domain, although directly involving material nonlinearity of the soil, is inefficient in practice up to now. Based on Partitioned Analysis of SSI (PASSI), a method for nonlinear soil-structure interaction analysis is developed in this paper, by applying implicit time-step integration and explicit one with different time steps to nuclear island and to soil, respectively. The incremental equilibrium equation and explicit decoupling method are used to analyze the soil nonlinearity described by the Davidenkov model with simplified loading-reloading rules, which avoids solving algebraic equations and iteration processes. An asynchronous parallel algorithm for nonlinear SSI analysis is given. A simple example is given to verify the partitioned approach with explicit-implicit co-computation against the full-explicit approach. Seismic response characteristics of nuclear power plants are investigated by comparing responses of nonlinear analysis with linear analysis. Different results address the importance of nonlinear SSI analysis for the seismic performance of nuclear structures.