Liquefiable Site Research Articles

Seismic Response of Utility Tunnels with Different Burial Depths at the Non-Homogeneous Liquefiable Site

Damage to underground structures induced by soil liquefaction under cyclic loads such as earthquakes has long been an important issue in underground engineering practice. In this paper, five models are developed using Flac3D software to analyze the effect of burial depth on the force, deformation characteristics, and uplift behavior of utility tunnels in the non-homogeneous site containing a liquefied layer. The cyclic shear property of saturated sand and the increase in pore water pressure during the earthquake are modeled using a cyclic load-volume strain increment model, using Shell-Type structural elements to model underground utility tunnels, and by using plastic hinges to represent the bending moment capacity of member’s joints. Numerical results show that for shallow-buried utility tunnels, increasing the burial depth increases the bending moment, shear force, and deformation of the structure while significantly reducing its uplift. Therefore, for high-strength shallow-buried utility tunnels, appropriate increase in burial depth can improve its seismic safety.

Site response and liquefaction hazard analysis of Hawassa town, Main Ethiopian Rift

The study area is located in one of the most earthquake prone regions in southern Ethiopia, which is characterized by small-to-intermediate earthquake occurrences causing damage to buildings. Predicting liquefaction hazard potential and local site effects are imperative to manage earthquake hazard and reduce the damage to buildings and loss of lives. The objectives of this work were to perform the equivalent linear response analysis (ELA) and shear wave velocity (Vs.)-based liquefaction hazard analysis and classify the site into different seismic site classes based on the European and American codes. The SPT-N and Vs.30 values showed the site falls in the C and D classes based on the NEHRP (2015) code but falls in the B and C classes based on the EC8 (2003) code. The susceptibility of liquefaction was evaluated using grain size analysis curves. Moreover, peak ground acceleration (PGA), spectral acceleration (SA), and maximum strain (%), which are very critical to understanding the local site effects, were estimated by the DeepsoilV.7 program. The cyclic stress ratio and cyclic resistance ratio were used to calculate the factor of safety (FS). A liquefaction potential index (LPI), probability of liquefaction (PL), and probability of liquefaction induced ground failure (PG) were used to assess the probability of liquefaction. The peak ground acceleration (g) values ranged from 0.166 to 0.281 g, whereas spectral acceleration (g) was found to be high at 0.1–1s. The liquefaction susceptibility screening criteria revealed that the study area is highly susceptible to liquefaction. FS is < 1 for a liquefied site, but FS is > 1 for non-liquefied sites. In comparison to non-liquefied sites, the liquefaction forecast site has a liquefaction potential index value of 0–54.16, very likely high PL, and high PG. The findings will be helpful in the design of structures and in solving practical challenges in earthquake engineering.

Fragility Analysis of Helical Piles Supporting Bridge in Different Ground Conditions

This paper examines the seismic performance of a bridge–helical pile foundation based on the seismic fragility analysis, considering the element fragility of a coupled bridge–soil–foundation system. Nonlinear time history analyses were conducted using a finite-element modeling scheme that was validated using the results of large-scale shaking table tests of soil–piles–structure systems involving both liquefiable and nonliquefiable soils. Incremental dynamic analysis was conducted to generate the fragility curves for a two-span bridge supported on helical piles in nonliquefiable and liquefiable sites considering a suite of ground motions. The damage limit states were defined to describe the capacity of the bridge components. In total, 440 nonlinear time-history analyses were performed to evaluate the seismic demand of the helical piles and the bridge-reinforced concrete pier components, and the results were used to establish their fragility curves. The results revealed that the helical piles were the most fragile component in the nonliquefiable and liquefiable tests. The liquefiable soil could decrease the seismic demand on the column lateral deformation and increase the demand dispersion. On the other hand, the reinforced concrete pier exhibited a large drift response in the nonliquefiable soil, causing it to be more vulnerable to seismic hazards than in the case of liquefiable soil.

Mitigating liquefaction-induced displacements of shallow foundation using helical piles

During previous earthquakes, displacements of shallow foundations on liquefiable sites caused significant damage to overlying super-structures leading to casualties and catastrophic economic loss. Various countermeasures have been developed to lessen liquefaction-related damages while minimising cost and environmental impacts. This study aims to evaluate the use of helical piles as a possible technique to mitigate the settlement, tilting, and sliding of shallow foundations during seismic liquefaction. For this purpose, 12 1g shaking table tests were conducted on loose saturated sand under harmonic base input for (a) free-field condition, (b) a model foundation on the soil surface and (c) a model foundation underpinned by helical piles. The effects of input amplitudes and the number of helical piles were investigated in terms of acceleration response, excess pore-water pressure and foundation displacements. The results confirmed the satisfactory performance of helical piles in reducing shallow foundation displacements. In particular, the mean permanent settlements were reduced by 45 and 75% when using four and eight helical piles, respectively. Similar trends were observed for the shallow foundation permanent tilting and sliding: permanent tilting was reduced by 30 and 59% when using four and eight helical piles, respectively, while these results were 45 and 68% for the sliding.

Identification of optimal ground-motion intensity measures for assessing liquefaction triggering and lateral displacement of liquefiable sloping grounds

This article aims at identifying the optimal scalar- and vector-valued intensity measures (IMs) for predicting liquefaction triggering and the consequence of liquefaction (i.e. permanent deformation of sloping grounds), respectively. A total of 169 ground motions were selected from the NGA-West2 database. Using the selected ground motions as input, one-dimensional dynamic analyses were conducted for generic sloping ground soil profiles consisting of a loose-sand liquefiable layer implemented in OpenSees. The excess pore water pressure ratio and the liquefaction-induced lateral displacement were regarded as the engineering demand parameters (EDPs) of interest. A total of 20 scalar-IMs and 21 vector-IMs representative of various characteristics of seismic loading were evaluated as candidate IMs. The criteria for identifying optimal IMs include consideration for the efficiency, sufficiency, and predictability of the given IMs. An error propagation approach was employed to evalute the overall uncertainty of EDPs under scenario earthquakes using different IMs as predictor variables. Based on the results of extensive comparative analyses, acceleration spectrum intensity (ASI), and the vector of peak ground acceleration and spectral acceleration at 0.3 s (peak ground acceleration (PGA) and Sa(0.3 s), respectively) are identified as the optimal scalar- and vector-IM for liquefaction triggering evaluation, respectively. However, Arias intensity ( Ia) and vectors of [ Ia, cumulative absolute velocity] and [PGA, Ia] are identified as the optimal scalar- and vector-IM for predicting the liquefaction-induced lateral displacements. The optimal IMs identified could be used in performance-based liquefaction hazard evaluations, to assess the liquefaction triggering and consequence of liquefiable sites.

Nonlinear Solid–Fluid Coupled Seismic Response Analysis of Layered Liquefiable Deposit

A seismic response analysis of layered, liquefiable sites plays an important role in the seismic design of both aboveground and underground structures. This study presents a detailed dynamic site response analysis procedure with advanced nonlinear soil constitutive models for non-liquefiable and liquefiable soils in the OpenSees computational platform. The stress ratio controlled, bounding surface plasticity constitutive model, PM4Sand, is used to simulate the nonlinear response of the liquefiable soil layers subjected to two seismic ground motions with different characteristics. The nonlinear hysteretic behavior of the non-liquefiable soil under earthquake excitations is captured by the Pressure Independent Multi Yield kinematic plasticity model with a von Mises multi-yield surface. The soil elements are modelled utilizing the solid–fluid fully coupled plane-strain u-p elements. The seismic response of the layered liquefiable site in terms of the development of excess pore water pressure, acceleration, ground surface settlement, and stress–strain and effective stress path time histories under two representative earthquake excitations are investigated in this study. The numerical results indicate that both the characteristics of ground motions and the site profile have a significant influence on the dynamic response of the layered liquefiable site. Under the same intensity of ground motion, the loose sand layer with a 35% relative density is more prone to liquefaction and contractive deformation, which causes irreversible residual deformation and vertical settlement. The saturated soil layer can effectively filter the high-frequency components and amplify the low-frequency components of ground motions. Moreover, the liquified site produces a 40% post-earthquake consolidation settlement after the excess pore pressure dissipation.

An Explicit Finite Element Method for Saturated Soil Dynamic Problems and Its Application to Seismic Liquefaction Analysis

An explicit finite element method is proposed to solve the u-p-formed dynamic equation of saturated porous media. In this method, a special discretization is implemented to discretize the global computational domain into local node systems of the individual nodes, and the dynamic equation of each local node system corresponding to each node is discretized by the completely explicit integration method in the time domain. By cycling through all the nodes, the dynamic responses of the global computational domain are obtained. In addition, a viscoelastic artificial boundary is added in the method. In numerical examples, the proposed FEM is verified by the good agreements between the results obtained from the proposed method and the analytical and numerical solutions of existing methods, respectively. After being embedded in OpenSees software, the proposed method is implemented into analysis of the seismic responses of liquefiable site.

Nonlinear site response at liquefiable sites: Insights from downhole seismic observations

The nonlinear behavior of shallow soil plays an important role in modifying surface ground motions. We analyze the nonlinear site response at liquefiable sites using the compiled long-term ground motion data (ranging from 1992 to 2020 with over 2000 earthquake events) at four high-quality downhole arrays that have liquefied during at least one earthquake. Two nonlinear event parameters, the percentage of nonlinearity (PNL) and the frequency shift parameter (fsp), are used to quantify the modification of site response of strong events to that of weak events. Site-specific regression analyses are performed to correlate nonlinear event parameters to incident motion parameters. Results indicate that site response at liquefiable sites varies from linear to nonlinear to liquefied states as the incident motion intensity changes, and the occurrence of liquefaction could enhance the amplification of low-frequency and very-high-frequency ground motions. As strong-motion observations for liquefiable sites are very limited, the findings of this work are of significance for further study of strong-motion seismology at liquefiable sites.

Impact of Ground Densification on the Response of Urban Liquefiable Sites and Structures

Ground densification is a common countermeasure against soil liquefaction. The state-of-practice for designing ground densification is typically based on empirical estimates of liquefaction triggering and ground settlement in the free field, ignoring seismic soil-structure interaction (SSI) near building structures. The existing guidelines for the geometry of ground densification also do not take into account constraints introduced by the presence of a neighboring foundation, or the impact of densification on seismic structure-soil-structure interaction (SSSI) and performance of adjacent buildings in urban settings. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite-element analyses, validated with centrifuge experimental results, are used to evaluate the influence of ground densification on the seismic performance of isolated and adjacent, similar and dissimilar, inelastic structures on liquefiable soils. The response is evaluated for treatment of varying dimensions (depth [dDS] and width [WDS]), location (e.g., below one or both neighboring buildings), and symmetry configurations. Ground densification is shown to effectively reduce the permanent settlement of isolated structures when covering the full depth of the critical layer, while potentially amplifying column strains and structural deflections. Depending on the properties of the structure and symmetry of treatment, densification could also adversely impact a foundation’s permanent tilt. The influence of densification on SSSI is shown to depend strongly on the geometry of densification, dynamic properties of the neighboring structures, and building spacing. For the spacings (S) considered, ground densification effectively reduced the permanent settlement of two adjacent mitigated structure(s). However, the combination of SSSI and ground densification typically notably amplified asymmetrical deformations below the foundations (hence, permanent tilt) as well as column strains, particularly for an unmitigated neighbor. This effect was strongest when closely spaced and when WDS=S. The results indicate that ground densification location and geometry must be designed with extreme care in urban settings, particularly when near a taller and weaker neighboring structure.

Local site characteristics and earthquake ground motion parameters around the Nile River, south of Cairo, Egypt

Parameters related to local site conditions represent an essential information for earthquake hazard evaluation and seismic risk mitigation at any region. In this research, the authors determine the local site characteristics and seismic ground motions around the Nile River, south of Cairo City, Egypt. This area suffered from a moderate earthquake (Mw 5.8) on 12 October 1992 and influenced by a soil liquefaction. Furthermore, the area is covered by soft sediments with large thickness, which may amplify seismic waves and lead to more damage in the event of earthquakes occurrence.Microtremor survey was carried out at six liquefiable sites in the study area by array of seismic stations. The recorded data was analyzed by the Centerless Circular Array (CCA) method to infer the shear wave velocity (Vs). The dispersion curves yielded from this analysis cover frequency range from 2.3 to 13.5 Hz. An inversion process was applied to get the Vs profile. The obtained velocities were averaged at 30 m depth (Vs30) which in turn have been used for a soil classification and for calculating the bearing capacity of soil. The H/V spectral ratio method was used for an experimental estimation of the site predominant frequency and its corresponding amplitude of seismic waves. Moreover, seismic amplification was calculated using the shear wave velocities. The Peak Ground Acceleration (PGA) and seismic intensity (MMI) were calculated for the October 12, 1992, earthquake.The results reveal Vs30 values from 170 to 240 m/s. Soil Class D is predominant in the study area. The amplification factor ranges from 2.5 to 4.5, corresponding to predominant frequency from 0.5 to 1.4 Hz. The allowable bearing capacity ranges from 0.7 to 1.0 Kg/cm2. The PGA at the surface ranges from 0.16 to 0.4 g. The calculated seismic intensity (MMI) ranges from VII to VIII which are the same with those observed during the 12 October 1992 earthquake. The parameters estimated in this research may effectively contribute to the earthquake risk reduction in the study area.

Experimental Study on Seismic Instability of Pile-Supported Structure Considering Different Ground Conditions

The stability of pile-supported structures under earthquake loads involves complex dynamic soil–structure interaction (SSI). A series of large-scale shaking table experiments was performed to investigate the seismic performance and SSI of pile-supported structures considering different ground conditions. Lateral soil resistance and dynamic displacements of superstructures located in nonliquefiable and liquefiable sites as well as a rigid foundation were measured during shaking tests. The measured responses were analyzed to investigate the P-Δ effect on structural stability. It was observed that the soil lateral resistance to the pile cap at the nonliquefiable site was large and represented the main component of the lateral bearing capacity resisting structure overturning. On the other hand, the lateral resistance of liquefied soil on piles was insignificant and the lateral bearing capacity was derived primarily from soil resistance to pile cap. The analysis of the measured dynamic displacements demonstrated that the seismic instability of the structure was mainly due to the P-Δ effect caused by the rotation of the foundation, especially in liquefied sites. In addition, comparison with the results of base shears and bending moments with the rigid base assumption allows for an in-depth discussion of the effect of SSI on structural seismic design.

Seismic Soil Liquefaction Mitigation Using Stone Columns for Pile-supported Wharves

ABSTRACT Stone columns (SCs) can reduce the seismic response of pile-supported wharves in liquefiable sites. A series of parametric studies were conducted to investigate the influence of SC diameter, SC space ratio, SC permeability, and typical earthquake characteristics on the seismic response of pile-support wharves and to assess the SC retrofit effect. The ratio of the SC space and diameter is found to play an important role in reducing the deck displacement. Based on the experimental results, the damage state of a pile support can be restricted to a potentially tolerable level using SCs in different liquefiable sites. Preliminary design suggestions are proposed for improving the seismic response of pile-supported wharves in earthquake-prone regions.

Seismic behaviour of piles in non-liquefiable and liquefiable soil

This paper investigates the nonlinear soil–pile–structure interaction employing three-dimensional nonlinear finite element models verified with the results of large-scale shaking table tests of model pile groups-superstructure systems. The responses of piles in both liquefiable and non-liquefiable soil sites to ground motion with varying intensities were evaluated considering both kinematic and inertial interaction. The calculated piles and soil responses agreed well with the responses measured during the shaking events. The numerical models correctly predicted the different pile deformation modes that were exhibited in the experiments. The finite element analysis was then employed to perform a parametric study to evaluate the kinematic and inertial effects on the piles' response, considering different ground motion Intensity and piles characteristics. It was found that the bending moment of piles in the liquefiable site increases significantly, compared to the non-liquefiable site, due to the loss of lateral support of the liquified soil, and the maximum bending moment occurs at the interface between the loose and dense sand layers. The inertial interaction contributes the most to the bending moments at the pile top and the interface between the top clay and liquefied loose sand layers. For piles with a larger diameter, the bending moment due to kinematic interaction increases significantly, and the bending moment distribution corresponds to short (rigid) pile behaviour. In addition, the piles at the saturated site displace laterally as a rigid body during strong ground motions because the pile base loses the lateral support due to the soil liquefaction. Finally, the kinematic interaction effect becomes more significant for piles with higher elastic modulus.

Influence of structure on the aseismic stability and dynamic responses of liquefiable soil

In previous major earthquakes, the damage and collapse of structures located in liquefied field which caused by site failure a common occurrence, and the problem of assessment of liquefaction possibility and discussion on the seismic response and stability of liquefied site is still issue of common concern in geotechnical earthquake engineering. To study the influence of the existence of structure on the seismic responses and stability of liquefiable sites, two groups of shaking table tests on liquefiable non-free field (with structure) and a free field (without structure) are performed. It can be summarized from the experimental results as following. The natural frequency of non-free field is larger and the damping ratio is smaller than that of a free field. For the weak seismic loading condition, the dynamic response of sites shows similar rules and trend. For the strong ground motion condition, soils in both experiments all liquefied obviously and the depth of liquefaction soil in the free field is significantly greater than that in the non-free field, besides, porewater pressure in the non-free field accumulated relately slow and the dissipated quickly from analysis of porewater pressure ratios in both experiments. The amplitudes of lateral displacements and acceleration of soil in the non-free field are obviously smaller than that in the free field caused by the effect of presence of the structure. In a word, the presence of structures will lead to the increase of site stiffness, site more difficult to liquefy, and the seismic stability of the non-free site is higher than that of the free site due to soil–structure interaction.

Time–Frequency Filter for Computation of Surface Acceleration for Liquefiable Sites: Equivalent Linear Stockwell Analysis Method

This paper presents a novel method for performing equivalent linear analysis that allows for a variation of the stiffness and equivalent viscous damping properties throughout the duration of shaking. The ability to change the dynamic properties throughout the analysis comes from the conversion of the input ground motion into the time-frequency domain using the Stockwell transform whereby different transfer functions can be applied at different times before converting back to the time domain. The equivalent linear Stockwell analysis (ELSA) method provides a fully decoupled approach to modeling the dynamic site response of liquefiable soil deposits that can account for changes in properties due to strain effects and the build-up of excess pore pressure. The simplicity of the method means that only a limited number of dynamic soil properties are required, the same as those used in an equivalent linear analysis, as well as an estimation of the build-up of excess pore pressure and the relative density of the soil. While there are drawbacks of decoupling the estimation of the build-up of excess pore pressure from the dynamic response, this approach means that the effects of liquefaction on ground shaking can be independently assessed using different models for estimating pore pressure. Furthermore, the influence of liquefaction mitigation interventions, or the presence of a building, can present significant modeling challenges for fully-coupled or loosely coupled approaches, whereas they can easily be assessed using simple decoupled tools. Changing the dynamic properties throughout time using the ELSA method provides a rational way to correct equivalent linear analyses for the known drawback of using the same properties throughout the whole duration. Validation studies are presented of the ELSA method against field downhole recordings from the Wildlife Array and fully-coupled nonlinear effective stress analyses.

Use of ambient noise records in seismic engineering: An approach to identify potentially liquefiable sites

Nakamura and Takizawa (1990) proposed a shear strain model from experimental studies using the horizontal-to-vertical spectral ratio (HVSR) in potentially liquefiable sites. This model allows estimating the maximum surface layer shear strain during an earthquake. In this paper, the model's experimental verification is performed by analyzing more than 130 earthquakes in two monitoring centers with potentially liquefiable soils. It is concluded that the model may be used to estimate the maximum soil shear strain for earthquakes with peak ground acceleration greater than 5 cm/s2 and when the soil deposit has a soft surface layer over a rigid layer. A methodology to identify potentially liquefiable sites based on this model was proposed and tested in an alluvial soil deposit in Santiago de Cali (Colombia). The results are compared with those obtained from the liquefaction assessment using CPT and Vs, and a good match between the methodologies is observed.

A Vs-Based Logistic Regression Method for Liquefaction Evaluation

The current liquefaction evaluation methods mainly focus on the success rate for liquefied sites so that the evaluation result tends to be conservative at different seismic intensities. Therefore, a new formula about liquefaction evaluation by introducing logistic regression theory is proposed to solve the deficiencies of the current evaluation method, which is based on 225 sets of shear wave velocity data reported by Andrus. The reliability of the new formula is verified based on 336 sets of Vs data collected from the Kayen database. The performance of the new formula on liquefaction evaluation is compared with existing liquefaction evaluation methods including the Andrus method and the Chinese code method. Compared with the Andrus method and Chinese code method, the success rates of liquefaction evaluation given by the new formula under different seismic intensities are more balanced between liquefied site and nonliquefied site. The new formula at 50% probability of liquefaction is more adaptable for a wide range of seismic intensities, ground water table, and sand buried depth. In addition, the new formula at different probabilistic levels of liquefaction can be adopted based on the importance of the engineering site in risk analysis.

Estimation of the critical buckling load of pile foundations during soil liquefaction

Buckling instability of slender piles may occur based on the effect of excessive axial loads acting on a pile with diminishing effective stress and shear strength owing to surrounding soil liquefaction. However, several guidelines for pile design often provide insufficient consideration of buckling failure, and it is significant to study the buckling failure of piles in liquefiable sites for the design and inspection of piles. In this paper, an efficient method of determining the critical buckling load of piles considering the material and geometric nonlinearities of piles based on the excess pore pressure distribution is proposed based on the beam-on nonlinear Winkler foundation (BNWF) model. Then, the method is verified and validated using results from centrifuge tests. Next, plots of the main effects and analysis of variance (ANOVA) were used for the sensitivity analysis of pile failure modes under axial loading. The initial imperfections and flexural rigidity of the pile have significant effects on the pile failure modes, and the pile is susceptible to buckling failure with low initial imperfections and flexural rigidity. Furthermore, the effects of the flexural rigidity of the pile, unsupported length ratio, initial imperfections, and degree of soil layer liquefaction on the critical buckling load of the pile in liquefiable soil are examined. The axial load-carrying capacity of the pile decreases with increasing initial imperfection amplitude, decreasing flexural rigidity of the pile, decreasing unsupported length ratio and severer degree of soil layer liquefaction. Finally, an estimating formula for predicting the critical buckling load of piles in liquefiable soil and an application example are presented.

Seismic Response Characteristics of Liquefiable Sites with and without Sediment Ejecta Manifestation

Dynamic nonlinear effective stress analysis of 45 well-documented liquefaction field case histories provides insights on the seismic response of sites where simplified liquefaction triggering procedures indicated liquefaction effects would be severe but either surface manifestations were not observed or inconsistent amounts of sediment ejecta were observed. The occurrence of sediment ejecta and its severity depend significantly on hydraulic processes after strong ground shaking ceases, which the simplified procedures do not consider. Postshaking advection processes govern the amount of upward seepage–induced artesian flow that determines ejecta severity. The ejecta potential index (EPI) captures these key aspects of the hydraulic processes of liquefaction manifestation. EPI can be sensitive to variables such as hydraulic conductivity, groundwater level, and input ground motions. However, it proves to be a useful index that correlates well to the severity of ejecta manifestation observed in the field case histories. Sites with severe ejecta have high EPI values, and sites without ejecta have low EPI values.

Centrifuge Modeling for Seismic Performance of Floating Piled Raft with and without Drainage Wells in Liquefiable Site

Abstract Drainage techniques can be employed to reduce damage of pile rafts in liquefiable ground by decreasing the build-up of excess pore water pressure and maintaining soil stiffness during eart...