Non-liquefiable Soil Research Articles

Soil liquefaction analysis based on soil and earthquake parameters

Two basic parameters are needed to evaluate liquefaction analyses. While one of these parameters is the level of cyclic loads formed in the soil by the earthquake, the other is the liquefaction resistance of the soil. The objectives of the present study are: 1) to determine the levels of cyclic loads using shear wave velocity, soil and earthquake parameters; 2) to determine the shear resistance ratio which depends on the shear wave velocity and the magnitude of the earthquake event; 3) to present a liquefaction analysis method derived from the shear resistance ratio, the cyclic stress ratio and the thickness of the soil. The proposed method was examined by using in situ measurements of soils ranging from fine sands to sandy gravel and also soil containing layers of silt clay. This study is based on 315 case history data gathered from 22 earthquakes. In addition, these data were examined using another liquefaction analysis method and the results were compared with the proposed method. The proposed method was applied to 315 data collected from liquefied or non-liquefied regions. This method determined the liquefaction potential of 118 data collected from liquefied regions with 100% reliability. Additionally, the method correctly predicted the non-liquefaction conditions of 197 data from non-liquefied regions with 66% reliability. In order to increase the reliability of this method, additional region work for either liquefied or non-liquefied soil types is required, using denser soils (Vsc > 250 m/s) subject to more powerful ground movement (amax > 0.48 g), especially in deeper deposits (z > 15 m).

Influence of Motion Energy and Soil Characteristics on Seismic Ground Response of Layered Soil

The present study focuses on assessing local site effects (especially large-scale soil heterogeneity) and motion characteristics on seismic ground response using nonlinear one-dimensional numerical analysis. All nonlinear and curve-fitting parameters used for soil models were verified using the Class C1 prediction of centrifuge test results available in the literature. The comparison demonstrates that the available MKZ (pressure dependent Modified Kondner Zelesko) formulation with non-Masing hysteresis loading and unloading rule can reliably compute the 1-D ground response of cohesionless soil. Horizontal soil layers with different relative densities were considered next in various hypothetical models to assess the effect of subsurface properties on responses. One novel aspect of this study is that 51 different ground motions with a wide range of variation in their spectral accelerations, frequency contents, and duration characteristics were used to evaluate the effect of ground motion characteristics on the soil response. The results reveal that layering conditions play a significant role in modifying the seismic ground response of heterogeneous soil, especially when the loose liquefiable sand layer is sandwiched between two non-liquefiable soil layers. Relations were obtained to quantify the effect of different seismic inputs and varying site conditions on seismic ground response. The best correlation was obtained between the maximum excess pore water pressure (EPWP) development and the damage potential (Arias intensity) of an input ground motion. These relations can be used for estimating seismic ground response of an identical soil profile to that used in the present study for known design motion characteristics.

Post-liquefaction re-compaction effect on the cyclic behavior of natural marine silty soil in the Yellow River delta

Re-compaction process after previous liquefaction exerts significant effects on subsequent cyclic behaviour of soil. Due to the complexities in environmental loading conditions, the relevant studies on this topic are limited. In this paper, we collected natural submarine silts from non-liquefied and post-liquefied seabed in the Yellow River subaqueous delta via offshore hydraulic sampling method, respectively. Monotonic and cyclic triaxial tests were conducted between them to investigate the re-compaction effect after previous wave-induced liquefaction on the undrained static and cyclic shear behaviours of seabed soil. Our experiments indicated that due to re-compaction process, the static and cyclic strengths of seabed silt in post-liquefied area are both enhanced in comparison to non-liquefied soil. A transformation in static shear failure mode from plastic to brittle after wave-induced liquefaction could be identified in seabed silt. Furthermore, the developments of pore water pressure and cyclic axial strain with number of cycles for seabed silts could be characterized by the hyperbolic and power models, respectively, and an empirical relationship between pore water pressure and cyclic axial strain is proposed for natural seabed silt. Finally, compared with non-liquefied seabed silt, the threshold cyclic stress ratio (CSR) for post-liquefied seabed silt is also elevated due to re-compaction effect.

Soil-pile-superstructure interaction effects in seismically isolated bridges under combined vertical and horizontal strong ground motions

This paper presents a deterministic analysis carried out to study the influence of coupled vertical and horizontal ground motions on the response of an isolated bridge (or soil-pile-bridge system) considering soil-pile-superstructure effects. Towards this end, an advanced 3D finite element model is developed to evaluate the effects of combined Vertical Component (VC) and Horizontal Component (HC) of ground motion on the structural elements of the bridge including Soil Structure Interaction (SSI). Non-linear time-history analysis is carried out under strong ground motions using an advanced soil plasticity model for non-liquefiable soil. Three different bridge configurations consisting of (i) simply-supported girders (SSERB), (ii) continuous girder with fixed bearing (CFB) and (iii) continuous girder with elastomeric bearing (CERB) are considered. Ground motion selection procedure was applied among 21,000 records to select suitable ground motions for nonlinear time-history analysis. Critical damage parameters in the structural elements under the combined HC and VC of the ground motion are compared with those under only the HC of the ground excitation. Results indicate that the vertical components produce significant amplification in the pier's axial load with different lower and upper bounds for different seismic isolation systems. The Influence of higher modes on results of Incremental Dynamic Analysis (IDA) are presented for the different structural elements in a deterministic framework. The variations of shear capacity and demand are presented by IDA and the combined spectral acceleration is implemented as an Intensity Measure (IM) due to the inclusion of higher order modes. The simply-supported and continuous girder with elastomeric bearings have a trend for a higher drift ratio than the continuous girder with the fixed-bearings system under vertical excitation. Furthermore, the variation of axial loads and pile-cap displacements are highlighted with and without VC for piles at piers and abutments. In addition, pile-cap displacements, influence of bridge higher modes and VC effects on Soil-Pile-Superstructure Interaction (SPSI) relations are investigated with respect to frequency content of the ground motion. The kinematic and inertial SPSI effects on seismic isolation systems including VC are also explored comparatively. The influence of elastomeric bearing on SSI reduction was investigated by comparing the relative response of the superstructure, Free-Field and Foundation Input Motion (FIM) at the pile head in the frequency domain. The new information presented in this paper will be useful for realistic design of bridges in seismic areas considering coupled HC and VC of the seismic excitation.

Shake‐table investigation of scoured RC pile‐group‐supported bridges in liquefiable and nonliquefiable soils

SummaryThis paper presents results of one‐g shake‐table tests on scoured pile‐group‐supported bridge models in saturated (liquefiable) and dry (nonliquefiable) sands. The primary objective is to reveal the influence of liquefaction on seismic demands and failure mechanism of scoured bridges. To this end, two identical models, each consisting of a 2 × 2 reinforced concrete pile‐group with a center‐to‐center spacing of 3 times pile diameter, a cap and a single pier with a lumped iron block, were constructed and embedded into saturated and dry sands, respectively, with the same scour depth of 4 times pile diameter. Typical test results, including excess pore pressure, acceleration and displacement demands are interpreted first, followed by the focus on curvature demands and associated seismic failure mechanism identification. Finally, inertial and kinematic effects on pile curvature demands are estimated using cross‐correlation analyses. Results show that near‐pile liquefied soils exhibit more remarkable dilation tendency as compared to far field. For bridges under the given scour depth, soil liquefaction tends to significantly affect the failure modes via transferring damage positions from pier bottom to pile head and meanwhile from underground pile to pile head. In addition, pile group effects appear to be significant in nonliquefiable soils while to be relatively inessential in liquefied soils. Moreover, the inertial effect is more prominent in nonliquefiable soils, while the kinematic effect itself generally appears to be more significant in liquefiable soils. The test results can be used to validate numerical models for future studies.

Effect of layered liquefiable deposits on the seismic response of soil-foundations-structure systems

The soil liquefaction is a major cause of damage in structures during earthquakes. This damage varies from small settlements to complete failure due to the loss of bearing capacity. To deal with these problems, piled foundations have been utilized in the presence of liquefiable soils in seismic zones. More recently, rigid inclusion foundations have been also considered. A fundamental approach to study the soil-foundation-structure interaction requires the determination of the influence of the kinematic and inertial effects in the system. In order to investigate the effects of this interaction, numerical models with a 3-storey reinforced concrete building founded on pile systems (soil-pile-structure) and rigid inclusion systems (soil-inclusion-platform-structure) were analyzed. Finite difference numerical models were developed using Flac 3D. The SANISAND constitutive model was utilized to represent the behavior of the liquefiable soil layer. This model predicts with accuracy the soil response for various soil densities, stress levels and loading conditions. The linear elastic perfectly plastic constitutive model with a Mohr-Coulomb failure criterion was used to represent the behavior of the non-liquefiable soil layers. Different relative density values of the sand layer were considered. Two earthquake signals were used to study the influence of the frequency of the systems excitation. For each case, the spectrum response, shear forces and rocking of foundations were obtained. Maximum shear strains and excess pore pressures were presented at different depths. The efforts and displacements in the rigid elements (piles or rigid inclusions) were also compared for the different systems. The results show that the relative density, the pile length and the frequency of the input motion greatly influence the response of the reinforced systems.

Seismic response of shallow foundations over liquefiable soils improved by deep soil mixing columns

There are limited information on how an artificially non-liquefiable soil layer created by ground improvement beneath shallow foundations can help the reduction of liquefaction induced foundation settlements and the deterrence of bearing capacity failure effectively. This is addressed herein, numerically through a three-dimensional finite difference model using FLAC3D. The validity of the developed model was evaluated by comparing the results obtained from the model with the results of numerical studies and experimental centrifuge tests available in the literature. Using the validated model, the seismic response of a shallow foundation including bearing capacity and settlement was evaluated parametrically over a single DSM column in terms of its diameter and depth and also DSM group columns in terms of the diameter of columns and their center-to-center distance. Afterward, the influence of shallow foundation characteristic parameters was evaluated including thickness, width and embedded depth on the seismic response of shallow foundations over liquefiable soils improved by DSM columns. The obtained results can be used in practical engineering applications and provide new insight into the seismic performance of shallow foundations with DSM columns located over liquefiable soils.

Numerical investigation of the influence of non-uniform factors on the monotonic/cyclic behaviour of coarse-grained soil

Coarse-grained soil (CGS) has been extensively used in engineering practices because of their advantages of relatively high strength, easy compaction and small deformation. In engineering design, CGS is usually regarded as a non-liquefiable soil. However, many post-earthquake surveys have shown that CGS, such as sandy gravel, can be seriously liquefied, meaning that under certain seismic loads, CGS with loose to moderate densities might have the potential to be liquefied. In this study, soil-water coupling finite difference-finite element (FD-FE) deformation analyses, based on the cyclic mobility (CM) model, were conducted to investigate the monotonic and cyclic characteristics of CGS. The model can accurately describe the complicated mechanical behaviour of CGS, considering the influences of the stress-induced anisotropy, the density and the naturally deposited structure of soils in a unified way. The material parameters of CGS were calibrated by large-scale triaxial tests. Particularly, the influences of non-uniform factors on the element behaviour of CGS subjected to monotonic/cyclic loading were systematically studied, such as the gravitational stress field (GSF) of the specimens, the friction between the loading plate and specimen (FLPS), the confining pressure, the cyclic shear stress ratio (CSR), and the loading frequency, etc. It is found that the CM model can reproduce the stress-strain-dilatancy relation of CGS in the CU‾ and CD tests well and that the GSF and FLPS have a certain impact on the element behaviour of CGS. However the average value results from the numerical simulations can quantitatively and qualitatively describe the monotonic/cyclic mechanical behaviour. In addition, under lower confining pressure or higher CSR, the non-uniform distribution of volumetric and axial strain inside the specimen will be prominent, and the specimen enters the cyclic-mobility stage more easily. High loading frequency and CSR will cause a large distortion of the specimen, at which point the element test, strictly speaking, is no longer an element test.

Hazard-dependent soil factors for site-specific elastic acceleration response spectra of Italian and European seismic building codes

To define the seismic input in non-liquefiable soils, current seismic standards give the possibility to treat local site effects using a simplified approach. This method is generally based on the introduction of an appropriate number of soil categories with associated soil factors that allow modifying the shape of the elastic acceleration response spectrum computed at rocky (i.e. stiff) sites. Although this approach is highly debated among researchers, it is extensively used in practice due to its easiness. As a matter of fact, for standard projects, this method represents the driving approach for the definition of the seismic input. Nevertheless, recent empirical and numerical studies have risen doubts about the reliability and safety of the simplified approach in view of the tendency of the current soil factors of Italian and European building codes to underestimate the acceleration at the free surface of the soil deposit. On the other hand, for certain soil classes, the current soil factors seem to overestimate ground amplification. Furthermore, the occurrence of soil nonlinearity, whose magnitude is linked to both soil type and level of seismic intensity, highlights the fallacy of using constant soil factors for sites with a different seismic hazard. The objective of this article is to propose a methodology for the definition of hazard-dependent soil factors and simultaneously quantify the reliability of the coefficients specified in the current versions of Eurocode 8 (CEN 2005) and Italian Building Code (NTC8 2008 and revision NTC18 2018). One of the most important outcome of this study is the quantification of the relevance of soil nonlinearity through the definition of empirical relationships between soil factors and peak ground acceleration at outcropping rock sites with flat topological surface (reference condition).

Using peak ground velocity to characterize the response of soil-pile system in liquefying ground

Performance of a soil-pile system can be significantly influenced by many characteristics of an earthquake ground motion, and it is vitally important to identify the ground motion parameters that have the most significant effects on the response when predicting the level of movement or damage in the pile. In this paper, three-dimensional finite element (FE) analysis was conducted to simulate a centrifuge experiment on the nonlinear behavior of a pile founded in liquefiable soil subjected to strong earthquake motions. The result of the FE analysis was found to be in reasonable agreement with the experimental data. As such, the calibrated FE model was used to investigate the influence of ground motion parameters on the pile-soil response in both liquefied and non-liquefied soils. It was found that peak ground velocity (PGV) is an appropriate ground motion parameter to characterize the response of the soil-pile system in liquefying ground. The maximum pile bending moment, pile lateral displacement, and soil lateral displacement increased with increasing PGV. Moreover, near-fault ground motions could result in more severe damage to the pile compared to far-fault grounds motions. This study provided a new insight on the influence of ground motion parameters, in particular PGV, on the dynamic performance of a pile foundation.

Application of MLR Procedure for Prediction of Liquefaction-Induced Lateral Spread Displacement

I presented the Kenneth L. Lee lecture at the 2016 Queen Mary Seminar, ASCE Los Angeles Geo-Institute Chapter, and publish it herein. The topic was lateral spread problems I have encountered as a consultant. The first issue was how deep to bury a pipeline at stream crossings to mitigate the lateral spread hazard. My answer is twice the bank height (2H) beneath approaches or 1H beneath the channel. For shallow liquefiable layers, a shear zone would form well above and not harm the pipe. For deep liquefiable layers, nonliquefiable soil above the pipe should buttress the channel against lateral spread. The second issue was finding the thinnest liquefiable layer that is susceptible to lateral spread. In the database of Youd et al. database, the thinnest layer is 1.0 m. For case histories used by Zhang et al. to verify their procedure, the minimum thickness is 0.6 m. Extrapolation to thinner layers adds uncertainty and a tendency for overprediction. The third issue was how to apply multiple linear regression (MLR) at a site with insufficient SPT data. CPT data were used to create ψ profiles; we then applied the criterion that ψ<–0.08 indicates material too dilative for lateral spread to develop. Summing layer thicknesses with ψ>–0.08 yielded estimates of T15 for use with MLR.

Comparison of the sand liquefaction estimated based on codes and practical earthquake damage phenomena

Conducting sand liquefaction estimated based on codes is the important content of the geotechnical design. However, the result, sometimes, fails to conform to the practical earthquake damages. Based on the damage of Tangshan earthquake and engineering geological conditions, three typical sites are chosen. Moreover, the sand liquefaction probability was evaluated on the three sites by using the method in the Code for Seismic Design of Buildings and the results were compared with the sand liquefaction phenomenon in the earthquake. The result shows that the difference between sand liquefaction estimated based on codes and the practical earthquake damage is mainly attributed to the following two aspects: The primary reasons include disparity between seismic fortification intensity and practical seismic oscillation, changes of groundwater level, thickness of overlying non-liquefied soil layer, local site effect and personal error. Meanwhile, although the judgment methods in the codes exhibit certain universality, they are another reason causing the above difference due to the limitation of basic data and the qualitative anomaly of the judgment formulas.

Influence of seismic motions on behavior of piles in liquefied soils

SummaryIn the present study an analytical procedure based on finite element technique is proposed to investigate the influence of vertical load on deflection and bending moment of a laterally loaded pile embedded in liquefiable soil, subjected to permanent ground displacement. The degradation of subgrade modulus due to soil liquefaction and effect of nonlinearity are also considered. A free headed vertical concrete elastic nonyielding pile with a floating tip subjected to vertical compressive loading, lateral load, and permanent ground displacement due to earthquake motions, in liquefiable soil underlain by nonliquefiable stratum, is considered. The input seismic motions, having varying range of ground motion parameters, considered here include 1989 Loma Gilroy, 1995 Kobe, 2001 Bhuj, and 2011 Sikkim motions. It is calculated that maximum bending moment occurred at the interface of liquefiable and nonliquefiable soil layers and when thickness of liquefiable soil layer is around 60% of total pile length. Maximum bending moment of 1210 kNm and pile head deflection of 110 cm is observed because of 1995 Kobe motion, while 2001 Bhuj and 2011 Sikkim motions amplify the pile head deflection by 14.2 and 14.4 times and bending moment approximately by 4 times, when compared to nonliquefiable soil. Further, the presence of inertial load at the pile head increases bending moment and deflection by approximately 52% when subjected to 1995 Kobe motion. Thus, it is necessary to have a proper assessment of both kinematic and inertial interactions due to free field seismic motions and vertical loads for evaluating pile response in liquefiable soil.

Seismic liquefaction performance of strip foundations: Effect of ground improvement dimensions

Evidence from field case studies, as well as from experimental and theoretical research, suggest that the detrimental effects of seismic liquefaction on the performance of surface foundations on level ground may be mitigated in presence of a non-liquefiable soil crust of adequate dimensions and shear strength. This paper refers to the case where the non-liquefiable crust is not natural, but it has been artificially created by ground improvement, and focuses upon the effect of the size (thickness and width) of the improved area on the seismic settlement and the degraded post-seismic bearing capacity of the foundation. Size effects are evaluated numerically, starting from the reference case of an infinitely extending improved soil crust and progressing to more realistic cases of ground improvement of gradually decreasing lateral extend of improved soil. Guidelines are provided for a cost-effective design of ground improvement, based on the rate of seismic settlement reduction with increasing dimensions of the improved ground.

Impact of Ground Response Analysis on Seismic Behavior and Design of Piles in Kolkata City

Kolkata city is a major metro city of eastern India and spread in a north–south direction along the east bank of the Hoogly river with the soil being predominantly soft, thick and alluvial in origin. According to seismic code of India, Kolkata city is located along the boundary of seismic zone III and IV which indicates moderate to high seismic hazard. In this paper, using derived empirical correlations between shear wave velocity V s and SPT N values, soil site classification is carried out and it indicated CLASS E category, highlighting the seismic hazard of Kolkata city. The alluvial nature of soil located in Kolkata city is prone to soil amplification when subjected to different earthquake motions at the bedrock level and amplification factor for bedrock level acceleration varying between 1.7 and 2.5 is obtained in the present study. The spectral acceleration at 5% damping ratio is observed to be 1.94 g, when subjected to 1995 Kobe earthquake motion. The results obtained from ground response analysis is used for seismic design of pile foundations passing through liquefiable and non-liquefiable soil strata by considering both kinematic and inertial loading. The maximum bending moment is observed to occur at the interface of liquefiable and non-liquefiable soil layers. Both bending moment and pile head deflection showed a substantial increase in magnitude when inertial loads are considered in the analysis in addition to kinematic loading. Hence the present study illustrates the significance of deflection and bending response of pile foundations in liquefiable soil as important parameters for seismic design of pile foundations in Kolkata city.

A practical method for construction of p-y curves for liquefiable soils

In practice, the analysis of laterally loaded piles is often carried out using a “Beam on Non-linear Winkler Foundation method” whereby the lateral pile-soil interaction is modelled as a set of non-linear springs (also known as p-y curves). During seismic liquefaction, the saturated sandy soil changes its state from a solid to a viscous fluid like material, which in turn alters the shape of the p-y curve. Typically, p-y curves for non-liquefied soil looks like a convex curve with an initial stiff slope that reduces with increasing pile-soil relative displacement (y) i.e., elasto-plastic softening response. However, recent research conclusively showed that p-y curve for liquefied soil has a different shape, i.e., upward concave with practically-zero initial stiffness (due to the loss of particle to particle contact) up to a certain displacement, beyond which the stiffness increases due to reengaging of the sand particles. This paper presents a practical method for construction of the newly proposed p-y curves along with an example.

Construction of simplified design p–y curves for liquefied soils

In practice, laterally loaded piles are most often analysed using a ‘beam-on-non-linear-Winkler-foundation’ approach, whereby the soil–structure interaction is modelled by means of p–y curves. Although well-calibrated p–y curves exist for non-liquefied soils (e.g. soft clay and sand), the profession still lacks reliable p–y curves for liquefied soils. In fact, the latter should be consistent with the observed strain-stiffening behaviour exhibited by liquefied samples in both element and physical model tests. It is recognised that this behaviour is induced by the tendency of the liquefied soil to dilate upon undrained shearing, which ultimately results in a gradual decrease in excess pore pressure, and consequent increase in stiffness and strength. The aim of this paper is twofold. First, it proposes an easy-to-use empirical model for constructing stress–strain relationships for liquefied soils. This only requires three soil parameters which can conveniently be determined by means of laboratory tests. Second, it introduces a method for the construction of p–y curves for liquefiable soils from the proposed stress–strain model, based on the scaling of stress and strain into compatible soil reaction p and pile deflection y, respectively. The scaling factors for stress and strain are computed following an energy-based approach that is analogous to the upper-bound method used in classical plasticity theory. To validate the proposed p–y curves, results from a series of centrifuge tests are employed to back-calculate p–y curves for liquefied soils. The latter are compared with those obtained from the proposed method and the conventional p-multiplier approach.

Ground response at liquefied sites: seismic isolation or amplification?

The seismic response of liquefied ground is parametrically investigated via analytical visco-elastic wave propagation theory, as well as nonlinear, effective stress, numerical analyses. It is found that a minimum liquefied layer thickness is required in order to ensure seismic isolation effects, i.e. significant attenuation of the seismic motion at the ground surface relative to that at the base, while for thinner layers attenuation of the seismic motion becomes marginal and may even turn into amplification. For harmonic excitations, the limiting thickness for seismic isolation and for seismic amplification are expressed as fractions of the corresponding wave length in the liquefied layer, and are also correlated to the thickness of the non-liquefiable soil crust reduced relative to that of the underlying liquefied layer. For a given soil profile, the above criteria may be inversely utilized in order to identify the harmonic excitation components that will be eventually filtered out and those that will be amplified. Application examples verify the validity of the proposed criteria for site and excitation conditions of engineering interest.

Calibration of a CRR model based on an expanded SPT-based database for assessing soil liquefaction potential

A new calibrated empirical liquefaction triggering correlation equation with five parameters determined by trial and error is derived from the expanded SPT-based database of the Idriss and Boulanger (2010) and Xie (1984) case histories. The new calibrated correlation equation is reasonably conservative for the expanded SPT-based database and is insensitive to reasonable variations in the components of the liquefaction analysis framework adopted in this study. In addition, a probabilistic version of the new calibrated correlation equation expressed in the form of a mapping function that relates the liquefaction probability to a nominal safety factor obtained using the weighted maximum likelihood technique is verified using a weighted empirical probability approach and Monte Carlo simulations. Liquefaction, transition and non-liquefaction zones for clean sands are defined based on different probability contours, i.e., empirical correlations with different liquefaction probabilities. Liquefaction triggering correlations for liquefaction probabilities of 4%, 15% and 25% represent the very low, low, and moderate probability events, respectively, of a liquefiable soil being mistaken for a non-liquefiable soil in the liquefaction potential evaluation.

Coupled behavior of pile foundations in liquefied and non-liquefied soils during earthquakes including case study

Coupled behavior of pile foundations in liquefied and non-liquefied soils during earthquakes including case study