Assessment of soil–pile–structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations

The role of the seismic soil–pile–structure interaction (SSPSI) is usually considered beneficial to the structural system under seismic loading since it lengthens the lateral fundamental period and leads to higher damping of the system in comparison with the fixed-base assumption. Lessons learned from recent earthquakes show that fixed-base assumption could be misleading, and neglecting the influence of SSPSI could lead to unsafe design particularly for structures founded on soft soils. In this study, in order to better understand the SSPSI phenomena, a series of shaking table tests have been conducted for three different cases, namely: (i) fixed-base structure representing the situation excluding the soil–structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by floating (frictional) pile foundation in soft soil. A laminar soil container has been designed and constructed to simulate the free field soil response by minimising boundary effects during shaking table tests. In addition, a fully nonlinear three dimensional numerical model employing FLAC3D has been adopted to perform time-history analysis on the mentioned three cases. The numerical model adopts hysteretic damping algorithm representing the variation of the shear modulus and damping ratio of the soil with the cyclic shear strain capturing the energy absorbing characteristics of the soil. Results are presented in terms of the structural response parameters most significant for the damage such as foundation rocking, base shear, floor deformation, and inter-storey drifts. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil–structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by floating pile foundations in comparison to the fixed base structures. However, the floating pile foundations contribute to the reduction in the lateral displacements in comparison to the shallow foundation case, due to the reduced rocking components.     

A p–y curve model for laterally loaded XCC pile in soft clay

Acta Geotechnica - This paper presents a p–y curve model for laterally loaded X-sectional cast-in-place concrete (XCC) piles, which are a type of non-circular cross-sectional pile, in soft...     

Effect of the soil–pile–structure interaction in seismic analysis: case of liquefiable soils

The design of earthquake-resistant structures depends greatly on the soil–foundation–structure interaction. This interaction is more complex in the presence of liquefiable soils. Pile and rigid inclusion systems represent a useful practice to support structures in the presence of liquefiable soils in seismic zones. Both systems increase the bearing capacity of soil and allow reducing the settlements in the structure. Numerical models with a 3-storey reinforced concrete frame founded on inclusions systems (soil–inclusion–platform–structure) and pile systems (soil–pile–structure) were analyzed. Finite difference numerical models were developed using Flac 3D. Two different soil profiles were considered. A simple constitutive model for liquefaction analysis that relates the volumetric strain increment to the cyclic shear strain amplitude was utilized to represent the behavior of the sand, and the linear elastic perfectly plastic constitutive model with a Mohr–Coulomb failure criterion was used to represent the behavior of the earth platform. Two earthquakes were used to study the influence of the different frequency of excitation in the systems. The results were presented in terms of maximum shear forces distribution in the superstructure and spectrum response of each system. The efforts and displacements in the rigid elements (piles or rigid inclusions) were compared for the different systems. The bending and buckling failure modes of the pile were examined. The results show that the pile system, the soil profile and the frequency of excitation have a great influence on the magnitude and location of efforts and displacements in the rigid elements.

     

The effect of cemented soil strength on the frictional capacity of precast concrete pile–cemented soil interface

Acta Geotechnica - The pre-bored grouted planted (PGP) pile is a composite pile consisting of a precast concrete pile and the cemented soil around the pile. Thus, the PGP pile shaft capacity is...     

Safety factor calculation of soil slope reinforced with piles based on Hill’s model theory

How to evaluate reasonably the stability of a soil slope reinforced with piles (SSRP) still is an urgent problem. At present, the three-dimensional (3D) finite element strength reduction method has been used for the soil slope stability analysis. However, to accurately determine the global instability of soil slopes is the key to implementing the strength reduction finite element method. In this paper, the 3D finite element strength reduction algorithm (FESRA), based on Hill’s model theory, is proposed to assess the stability of SSRP and study on the relationship between the safety coefficients of SSRP and the displacements of slope mass. The results show that: (1) the relationship between the safety coefficients of SSRP and the displacements of slope mass agrees with the Hill’s model; (2) the proposed method (3D FESRA based on Hill’s model theory) in this study may take into account simultaneously the pile response and slope stability, and makes the results of SSRP stability analysis reasonable and reliable, which could be used as a reference for the evaluation of stability of the same type of slope; and (3) further study should be done to confirm whether the proposed method in this study is suitable for other types of slopes.     

Numerical simulation of air–soil two-phase flow based on turbulence modeling

Soil flow and induced air blasts are of great harm to humanity, and historically they have caused a lot of damage to infrastructure. However, these phenomena cannot be described by traditional analog modeling methods that limit their use in disaster prevention efforts. Computational fluid dynamics (CFD) is an applied technique commonly used in a range of fields including the chemical industry, and aircraft and automobile manufacturing, but little is reported on the use of this method to simulate flowing soil in geotechnical engineering applications. The CFD method can effectively make up for the deficiency of normal calculation methods in the analysis of soil flow and air blasts. This paper uses the FLUENT (version 6.3) CFD calculation software to simulate the processes of soil flow and induced air blast changes during soil flow with an Eulerian air–soil two-phase model included in a standard k-ε turbulence model. Velocity vectors of air blasts at different times during soil flow are obtained, and the characteristics of turbulent flow can be found based on the velocity vectors. The numerical simulation techniques adopted in this paper captured precise configurations of soil flow. The results show that the CFD method is especially suitable for simulating the process of soil flow; hazard assessments can be implemented, and the performance of structures involved with disaster prevention can be improved based on the numerical simulation of changing air blasts.     

Rock–support interaction analysis based on numerical modelling

The use of yield in supports to control the final loading that develops upon a support system has been one of the most important deformation control techniques used by tunnelling engineers, both historically and currently. Successful use of this approach requires a thorough understanding of the process of rock–support interaction as it is an approach that can fail dramatically if incorrectly applied. There is a fine line between the yield support technique improving the conditions, and the approach resulting in the development of a large area of failed rock, which could ultimately be detrimental. The relationship between the support action and the rock has historically been studied using analytical approaches with the application of significant simplifying assumptions.This paper presents a new approach, where a state-of-the-art numerical model is run repeatedly to develop rock–support interaction curves. This has the advantage of allowing more realistic tunnel geometry, stress states and ground conditions to be simulated. It does, however, use the familiar output form of the relatively simple rock–support interaction curve as opposed to complex and voluminous graphics. Its disadvantage lies in the considerable number of computer runs required to develop the full solutions. Computer software has, however, been written to automate much of this process using a programming language within the modelling package.The analysis approach has been further improved by plotting not one rock–support interaction curve but a whole family of curves representing variations in the rock mass quality of the assumed ground, since this is the most variable of the input parameters for most tunnelling situations. This form of output allows engineers to study the practical range of yield they may require for their rock conditions and also to define at what rock mass quality they can expect the yielding approach to cease to be an effective strategy. This new approach has been presented on a test case history with idealized rock mass properties to illustrate the approach. However, it is an approach that can be specially tailored to any set of rock conditions, tunnel geometry or stress.     

Influence of spatial variability of soil friction angle on sheet pile walls’ structural behaviour

The uncertainty in terms of soil characterisation is studied to assess its effect on the structural behaviour of extended structures as sheet pile walls. A finite element model is used. This integrates a numerical model of the soil–structure interaction together with a stochastic model that allows characterising the soil variability. The model serves in propagating the variability and the system parameter uncertainties. Discussion is mainly focused on two points: (1) testing the sensitivity of the structural behaviour of a sheet pile wall to different geotechnical parameters and (2) assessing the influence of spatial variability of soil properties on the structural behaviour by identifying the most sensitive geotechnical parameter and the most significant correlation length values. The findings showed that in assessing the sheet pile wall’s structural behaviour, there are spatial variability parameters that cannot be considered negligible. In this study, soil friction angle is found to be an important parameter.     

Three dimensional Finite Element modeling of seismic soil–structure interaction in soft soil

Earthquakes in regions underlain by soft clay have amply demonstrated the detrimental effects of soil–structure interaction (SSI) in such settings. This paper describes a new three dimensional Finite Element model utilizing linear elastic single degree of freedom (SDOF) structure and a nonlinear elasto-plastic constitutive model for soil behavior in order to capture the nonlinear foundation–soil coupled response under seismic loadings. Results from an experimental SSI centrifuge test were used to verify the reliability of the numerical model followed by parametric studies to evaluate performance of linear elastic structures underlain by soft saturated clay. The results of parametric study demonstrate that rigid slender (tall) structures are highly susceptible to the SSI effects including alteration of natural frequency, foundation rocking and excessive base shear demand. Structure–foundation stiffness and aspect ratios were found to be crucial parameters controlling coupled foundation–structure performance in flexible-base structures. Furthermore, frequency content of input motion, site response and structure must be taken into account to avoid occurrence of resonance problem.     

Study of hidden factors affecting the mechanical behavior of soil–rock mixtures based on abstraction idea

Because of spatial variability and complex compositions, the mechanical test results of natural soil–rock mixtures (SRMs) are often discrete and lack reproducibility, which has greatly restricted the practical application of the experimental findings. The objective of this study was to examine the general mechanical behavior of SRMs under the influences of some hidden factors (e.g., structural parameters, parent rock type and weathering degree). To that end, the abstraction idea was adopted to prepare purified SRM samples. Large-scale triaxial tests were performed on these purified materials. On this basis, the influences of three structural parameters on the mechanical behavior of SRMs were studied. Moreover, the relationship between the shear strength and parent rock type and that between the shear strength and the spatial distribution of rock blocks were quantified. Some additional intrinsic behavior was distinguished from individual experimental phenomena through the comparative analysis of the test data in this study and those reported in the literature. The results show that the hidden factors had significant influences on the mechanical behavior of SRMs. A greater saturated uniaxial compressive strength of rock blocks generally led to a larger shear strength of SRMs. According to the significance of their influences on the shear strength parameters of SRMs, the structural parameters are ordered as: the gradation of rock blocks, the initial dry density of sample and the spatial distribution of rock blocks. The deformation and failure feature of SRMs were considerably affected by the spatial distribution of rock blocks and shear rate. And the shear strength parameters of SRMs were mainly influenced by the content of grains between 40 and 60 mm. The findings of this study would provide useful guidance for engineering practice.

     

Numerical study of the soil–structure interaction within mining subsidence areas

Structures affected by mining subsidence are exposed to heavy damage potential in relation to the induced tensile or compressive horizontal ground strains. This study intends to specify and compare the mining subsidence effect in terms of building transmitted movements or induced stresses, given the soil–structure interaction phenomena produced at the interface between a “stiff” elastic structure and a “flexible” elastoplastic soil.     

3D bearing capacity probabilistic analyses of footings on spatially variable c–φ soil

Acta Geotechnica - The paper examines three-dimensional (3D) analyses of the load bearing capacity of square and strip footings on a spatially variable cohesive–frictional (c–φ)...     

An embedded bond-slip model for finite element modelling of soil–nail interaction

Soil nailing has been widely used as a reinforcing technique to retain excavations and stabilise slopes. Proper assessment of the interaction between the nails and the surrounding soil is central to safe and economical design of the composite reinforced soil structure. In this note, a new interface model, denoted as “embedded bond-slip model”, is proposed to model the soil–nail interaction numerically in a simplified manner. Combining the key features of the embedded element technique and the conventional interface element method, the proposed plane–strain interface model has the advantages that no special considerations have to be given to the arrangement of the finite element mesh for the soil nails, and that possible tangential slippage along the interface can be modelled. The formulation also allows pore water flow across the soil nails to be incorporated into the analysis. The proposed model has been implemented into a finite element code and verified by simple element tests under different uni-direction loading conditions. Using the proposed interface model, back analyses of a field test involving a soil-nailed cut slope subjected to a rise in groundwater table have been conducted. This note presents the details of the embedded bond-slip model and the numerical results which demonstrate that the proposed model is capable of simulating soil–nail interaction conveniently and realistically.     

An analytical model of soil–structure interaction with swelling soils during droughts

Lightly loaded structures constructed on expansive soils may develop structural damage as a result of changes in the soil’s moisture content. This study investigated an analytical model of soil–structure interaction to assess the settlement of dwellings built on swelling soils when droughts occur. The building behavior was investigated with the Euler–Bernoulli beam theory, and the ground behavior was investigated with a Winkler-derived model based on the state surface approach. The analytical model results were compared to those of a finite element analysis using the Barcelona Expansive Model (BExM) performed with Code_Bright.The analytical model was then used to assess the settlement transmission ratio for a typology of clayey soils and different parameters of building. The results indicated that the final deflection of the building increased with the building length and soil suction. The building deflection due to the suction variations was inversely proportional to the load, the rigidity of the building and the embedding depth of the foundation. Increasing these parameters made the building less vulnerable to shrinkage and swelling action.     

GPU-accelerated iterative solutions for finite element analysis of soil–structure interaction problems

Soil–structure interaction problems are commonly encountered in engineering practice, and the resulting linear systems of equations are difficult to solve due to the significant material stiffness contrast. In this study, a novel partitioned block preconditioner in conjunction with the Krylov subspace iterative method symmetric quasiminimal residual is proposed to solve such linear equations. The performance of these investigated preconditioners is evaluated and compared on both the CPU architecture and the hybrid CPU–graphics processing units (GPU) computing environment. On the hybrid CPU–GPU computing platform, the capability of GPU in parallel implementation and high-intensity floating point operations is exploited to accelerate the iterative solutions, and particular attention is paid to the matrix–vector multiplications involved in the iterative process. Based on a pile-group foundation example and a tunneling example, numerical results show that the partitioned block preconditioners investigated are very efficient for the soil–structure interaction problems. However, their comparative performances may apparently depend on the computer architecture. When the CPU computer architecture is used, the novel partitioned block symmetric successive over-relaxation preconditioner appears to be the most efficient, but when the hybrid CPU–GPU computer architecture is adopted, it is shown that the inexact block diagonal preconditioners embedded with simple diagonal approximation to the soil block outperform the others.     

A finite element–based approach for predictions of rigid pile group stiffness efficiency in clays

This paper presents a finite element parametric study of several variables that affect the stiffness efficiency of rigidly capped pile groups with a view to developing a solution for preliminary design purposes. Previous empirical solutions from linear elastic work had identified a significant dependence of stiffness efficiency on pile group size and group spacing, and in this study, the effect of the pile length-to-diameter ratio, the compressibility of a stiff bearing stratum beneath the pile group and the depth below ground level to the stiff bearing stratum are also considered. Pile groups in a soft clay/silt are modelled using PLAXIS 3D Foundation in conjunction with a soil model that captures the stress dependency of soil stiffness. The trends from the soft soil study have been formulated into a set of equations which can be used to predict the stiffness efficiency of pile groups. This new approach captures more variables than previous simpler empirical prediction methods and performs better when applied to a database of 29 published pile group case histories.     

Tectonic stress accumulation in Bohai–Zhangjiakou Seismotectonic Zone based on 3D visco-elastic modelling

Future earthquake potential in the Bohai–Zhangjiakou Seismotectonic Zone (BZSZ) in North China deserves close attention. Tectonic stress accumulation state is an important indicator for earthquakes; therefore, this study aims to analyse the stress accumulation state in the BZSZ via three-dimensional visco-elastic numerical modelling. The results reveal that the maximum shear stress in the BZSZ increases gradually as the depth increases, and the stress range is wider in the lower layer. In the upper layer, the maximum shear stress is high in the Zhangjiakou area, whereas in the lower layer, relatively high values occur in the Penglai–Yantai area, which may be affected by the depth of the Moho surface. Besides, weak fault zones will be easily fractured when the maximum shear stress is not sufficiently high due to their low strengths, resulting in earthquakes. Therefore, based on the modelling results, the upper layer of the Zhangjiakou area and the lower layer of the Penglai–Yantai area in the BZSZ in North China are more likely to experience earthquakes.