Excess Pore Water Pressure Ratio Research Articles

Analysis of Offshore Wind Turbine by Considering Soil-Pile-Structure Interaction: Effect of Sea-Wave Load Duration

ABSTRACT Offshore Wind Turbines (OWTs) confront different types of environmental loads during their lifetime. One of the most significant loads is the cyclic sea-wave load which affects the OWT system during the approximate 25 years of design life. This lateral cyclic load can influence the response of the OWT system because of accumulated permanent displacement and excess pore water pressure generation in soil. This procedure can change pile-soil stiffness. However, the behavior of OWT and its foundations is mostly studied under short-term cyclic loading, and the effects of duration of the cyclic loads on pile-soil interaction and performance of the foundation are not well understood and documented. Therefore, there is a lack of guidance in codes for the duration effects of cyclic loads on the structural and geotechnical response of OWTs. In this regard, the current study considers the effects of duration of the cyclic loads on serviceability and performance of the OWT system by considering soil-foundation-structure interaction using a 2-D finite element method. The behavior of the OWT system is evaluated based on the internal forces and deformation of the monopile foundation, shear strain, and excess pore water pressure ratio in the surrounding soil. Besides, liquefaction susceptibility in the sandy soil layer at the vicinity of the monopile and its effect on the performance of the foundation is investigated. Finally, the results can provide guidance on estimation of the dynamic performance of the OWT system during long-term cyclic loads. They can be used to specify the need for consideration of the duration of cyclic lateral loads for the design of OWT structures.

The friction-weakening cause by localized wear-induced damage and its effect on liquefaction resistance of sandy soil using DEM

Localized wear-induced damage caused by stress concentration at the contact point can lead to the decrease of the frictional coefficient (μ) of sandy soil, further affecting the liquefaction resistance. In this study, a bi-linear the friction-weakening model is proposed to describe the decreasing trend of μ with the increase of normal contact force (fn). Through a series of undrained cyclic triaxial tests, the influence of localized wear-induced damage on the characteristics of cyclic liquefaction is investigated from both macroscopic and microscopic perspectives. The macroscopic results indicate that samples have weaker liquefaction resistance due to the friction-weakening. Under low confining pressure (P0), the influence of the friction-weakening on the liquefaction resistance is relatively limited. With the increase of P0, the decrease of liquefaction resistance gradually becomes significant. However, the effect of friction-weakening on the liquefaction resistance does not remain constant, but shows a decreasing trend with the accumulation of excess pore water pressure ratio (EPWPR). From the microscopic perspective, the reduction of liquefaction resistance can be explained by the coordination number (Z) and the contact sliding ratio. Although higher Z caused by the friction-weakening may lead to greater liquefaction resistance, higher sliding contact ratio have been found to weaken the liquefaction resistance, and its influence exceeds Z.

The Correlation of Liquefaction Potential and Probability on Excess Pore Water Pressure in Kretek 2 Bridge Area

Liquefaction is soil condition associated with the drastic increase in pore water pressure of uniform sandy soil due to an enormous earthquake. Therefore, this study aimed to investigate the correlation of liquefaction potential with excess pore water pressure ratio in nine boreholes located at Kretek 2 Bridge area using empirical and numerical methods. Liquefaction potential was estimated based on a semi-empirical method simplified by Idriss and Boulanger (2008), and safety factor (SF) value of <1.0 was used to represent the existence of its potential. The result showed that liquefaction potential was dominant at depths of 1.5 to 6.0 m, with the exception of BH-9 with 16.5 m and BH-4. Furthermore, the excess pore water pressure ratio was estimated using empirical method developed by Yegian and Vitelli (1981) as well as Serafini and Perlea (2010). Numerical analysis was also conducted for comparison purposes and the process focused on using Deepsoil v7.0 to generate excess pore water pressure by considering soil conditions and dominant seismic sources in Kretek 2 Bridge area. The result showed that the ratio of excess pore water pressure was greater or equaled 0.8. Both empirical and numerical methods produced similar values for BH-1, BH-2, BH-8, and BH-9 at a depth of 1.5–3.0 m, 3–4.5 m, 3.0 m, and 16.5 m, respectively. This showed a correlation between excess pore water pressure ratio and liquefaction potential values at the same depth. However, numerical method tended to overestimate the ru value, necessitating the use of empirical method to obtain a more reliable result.

Finite-difference solution for dissipation of excess pore water pressure within liquefied soil stabilized by stone columns with consideration of coupled radial-vertical seepage

The implementation of stone columns is a widely accepted method for improving the stability of liquefiable soil. A comprehensive understanding of the behavior of the composite ground is crucial for accurate design and calculation in practical applications. Several existing mathematical models were established to assess characteristics of the stone column-improved ground by typically ignoring the vertical seepage within liquefiable soil. This negligence will inevitably lead to significant calculation errors, particularly when the vertical permeability of liquefiable sites is high or the installation spacing of stone columns is large. In this context, a new mathematical model which accounts for coupled radial-vertical seepage within liquefiable soil is proposed to determine the reinforcement performance of stone columns. The equal strain assumption and new boundary conditions are incorporated to obtain numerical solutions with the finite difference method. Then the present solution is degenerated to the conventional calculation model to verify the reasonability of the proposed model. Finally, a parametric analysis is conducted to investigate the impacts of crucial parameters on the performance of stone columns for excess pore water pressure variation during soil liquefaction. The results reveal that the peak value of the maximum excess pore water pressure ratio increases with the increment of both the column spacing and cyclic stress ratio. Moreover, the increasing radial and vertical consolidation parameters Tb and Th will accelerate the dissipation rate of the excess pore water pressure of liquefiable sites. Furthermore, the conventional model neglecting the vertical seepage will underestimate the variation rate of the excess pore water pressure, and the calculation error will become larger with the increase of Th.

Experimental investigation of liquefaction characteristics of saturated coral sand subjected to complex cyclic stress paths: Effects of loading frequency

How cyclic loading frequency affects the liquefaction characteristics of saturated coral sand was investigated by performing undrained cyclic shear tests subjected to a 90° jump of principal stress, with consideration given to cyclic stress ratios (CSR) of 0.25, 0.30, 0.35, and 0.40, orientations of major principal stress of 0°, 22.5°, 45°, 67.5°, and 90°, and cyclic loading frequencies ( f ) of 0.05, 0.1, 0.5, and 1 Hz. The test results show that increasing f decreases the generation rate of excess pore water pressure and deformation. Under the same cyclic stress path, the amplitude of deviatoric strain and the excess pore water pressure ratio (ru) have a unique relationship. The unit cyclic stress ratio (USR) was introduced to represent the liquefaction resistance of saturated coral sand under different cyclic loading modes, and increased f leads to increased liquefaction resistance. Under isotropic consolidation, the relationship between the cumulative dissipated energy (Wc) and ru is almost independent of f, CSR, and cyclic stress path. A hyperbolic function model of Wc with the development of ru is proposed, and its applicability is verified against data from the literature.

Method for Estimating Fully Coupled Response of Deep Excavations in Soft Clays

The influence of excavation rate, geometric layout, and hydraulic conductivity of basal and retained soils on the fully coupled solid–fluid behavior of braced excavations in cohesive soils is presented. Fully coupled excavation analyses were performed using the hypoplasticity clay model to reproduce the constitutive soil behavior combined with Biot’s consolidation theory. A method aimed at predicting excavation-induced ground movements using excess pore-water pressure ratios as a function of site-specific groundwater considerations is presented for the determination of the recommended type of excavation analysis (i.e., drained, partially drained, or undrained). Results obtained from the numerical simulations indicated that excavation rate to hydraulic conductivity ratios, ER/k, smaller than 0.1 and larger than 10,000 can be analyzed under drained and undrained conditions, respectively. The proposed method was validated with published case histories, and its ability to estimate excavation performance in terms of excess pore-water pressures and excavation rates is presented in this paper.

Characteristics of solid–liquid phase transformation of saturated coral sand subjected to various patterns of cyclic loading

This study conducts a series of undrained cyclic shear tests on saturated coral sand with different initial relative densities ( Dr) subjected to a 90° jump rotation of the principal stress with various initial orientations. Considering liquefiable coral sand as a fluid, an important finding is that the variation in the apparent viscosity with the number of cycles is significantly affected by Dr, the effective mean principal stress ( [Formula: see text] ), cyclic stress ratio, cyclic loading path, and cyclic loading frequency. The average flow coefficient (κ) is introduced to describe the fluidity of saturated coral sand under cyclic loading by considering the variation in the shape of the stress–strain rate curve. A positive exponential correlation is observed between κ and the excess pore water pressure (EPWP) ratio ( ru) under the given cyclic loading modes. Another significant finding is that the solid–liquid phase change time is the turning point of the apparent viscosity and the average flow coefficient gradients with ru. The inversion point’s EPWP ratio is approximately 0.9, defined as the EPWP ratio during the solid–liquid phase transformation ( ruth). The ruth value is not affected by cyclic loading mode, [Formula: see text] , and Dr.

Liquefaction behavior evaluation of a multi-layered site with fines-dominated soils

Characterization of liquefaction response in terms of triggering and manifestation at sites with soils containing significant amounts of fines is critically important. Saturated fines-dominated soils, that are known as liquefaction-susceptible materials, would represent a range of extreme responses to earthquake excitations from cyclic mobility to cyclic liquefaction causing excessive displacements. In a multi-layered soil profile, the interaction of adjacent layers in porewater pressure redistribution along the site profile plays an important role in liquefaction response. In this study, an advanced Nonlinear Dynamic Analysis (NDA) procedure in predicting the liquefaction mechanism and manifestation at a site with multiple fines-dominated soil layers is presented. The procedure specifically aims at simulating the interactions in a multi-layered system containing both sand-like and clay-like materials. The liquefaction data recorded by Wildlife Liquefaction Array (WLA) in 1987 Superstition Hills Earthquake (M = 6.6) was deployed to validate the NDA procedure. WLA site consists of sandy silt, silty sand, clayey silt, silty clay, and silt deposits in its upper 20 m and has provided invaluable records of porewater pressure and ground motions. Two critical-state-based constitutive models, PM4Silt and PM4Sand, in a coupled dynamic-fluid finite difference platform were employed. Calibration of the constitutive models was conducted using Cyclic Direct Simple Shear (DSS) test single-element modeling. The simulated response of the site from the proposed approach was compared with those of other NDA procedures conducted on this case history in terms of liquefaction parameters such as excess porewater pressure ratio and maximum shear strain. These comparisons revealed that incorporating multi-layer effects in an advanced NDA procedure can enhance the liquefaction behavior predictions at sites with fines-dominated materials.

Liquefaction Potential of Saturated Sand Reinforced by Cement-Grouted Micropiles: An Evolutionary Approach Based on Shaking Table Tests

Cement-grouted injections are increasingly employed as a countermeasure material against liquefaction in active seismic areas; however, there is no methodology to thoroughly and directly evaluate the liquefaction potential of saturated sand materials reinforced by the cement grout-injected micropiles. To this end, first, a series of 1 g shaking table model tests are conducted. Time histories of pore water pressures, excess pore water pressure ratios (ru), and the number of required cycles (Npeak) to liquefy the soil are obtained and modified lower and upper boundaries are suggested for the potential of liquefaction of both pure and grout-reinforced sand. Next, adopting genetic programming and the least square method in the framework of the evolutionary polynomial regression technique, high-accuracy predictive equations are developed for the estimation of rumax. Based on the results of a three-dimensional, graphical, multiple-variable parametric (MVP) analysis, and introducing the concept of the critical, boundary inclination angle, the inclination of micropiles is shown to be more effective in view of liquefaction resistivity for loose sands. Due to a lower critical boundary inclination angle, the applicability range for inclining micropiles is narrower for the medium-dense sands. MVP analyses show that the effects of a decreasing spacing ratio on decreasing rumax are amplified while micropiles are inclined.

Excess pore water pressure generation in saturated sandy soils subjected to various cyclic stress paths

Excess pore water pressure (EPWP) plays an essential role in determining the cyclic undrained behavior of saturated sandy soils. The influence of cyclic stress paths on the EPWP generation in saturated sandy soils has emerged as an interesting issue in recent years. By using a hollow cylinder torsional shear apparatus, this paper presents the results of a systematic experimental study on the isotropically consolidated undrained responses of saturated coral sand under the 90° jump rotation of cyclic principal stresses with various orientations. Laboratory cyclic tests showed that the patterns of EPWP generation in saturated coral sand exhibits three modes that are related to the patterns of cyclic loading and levels of cyclic stress. Moreover, the unit cyclic stress ratio (USR) defined by Chen et al. (2020) can be used as an index for distinguishing among the three modes of EPWP generation under varying cyclic loadings. Furthermore, a unique form of the relationships exists among the EPWP ratio (ru), USR, and cycle ratio (N/NL) for all cyclic loading paths considered. Following this, we propose a model for the generation of ru as a function of the USR and N/NL for application. The wide applicability of this model is validated by using independent experimental data on four siliceous sandy soils in this study and from the literature.

Liquefaction Evaluation of Microbial Induced Calcium Carbonate Precipitation (MICP) Treated Sands; A Strain Energy Approach

ABSTRACT The present research investigates liquefaction behavior of microbial induced calcium carbonate precipitation (MICP)-treated Nevada sand with different silt content, for the first time in terms of dissipated strain energy. The obtained results indicated that normalized liquefaction capacity energy for MICP-treated soil was dependent on the CSR and NL. Moreover, acceptable relationships between the excess pore water pressure ratio and dissipated energy were found for both MICP-treated and untreated samples. Furthermore, Bender element tests results proved that shear wave velocities had a stronger relationship with rather than CSR, while, MICP treatment changed the relationship between and liquefaction resistance.

Performance of a Subgrade-Embankment-Seawall System Reinforced by Drainage PCC Piles and Ordinary Piles Subjected to Lateral Spreading

Coral sand is widely distributed in coastal areas and used as a soil foundation in an increasing number of projects. Historical records show that the liquefaction and lateral spreading of coral sand foundations occurred many times during earthquakes, resulting in serious geological and engineering disasters. Based on a rigid drainage pile, a PCC pile (Large Diameter Pipe Pile by using Cast-in-place Concrete) and drainage body are innovatively combined into a new drainage PCC pile, to reduce the harm caused by liquefaction and lateral spreading of the coral sand foundation. In this study, the seismic response of the subgrade-embankment-seawall system on the coral sand liquefaction site was simulated using a shaking table. The excess pore water pressure ratio, acceleration, average foundation settlement, seawall displacement, embankment deformation, and pile body bending moment of the ordinary pile foundation and new drainage PCC pile foundation under seismic loads of PGA   peak   ground   acceleration = 0.05   g , PGA = 0.1   g , and PGA = 0.2   g on the coral sand site were analyzed and compared. Compared with ordinary pile foundations, the excess pore water pressure ratio, pile bending moment, seawall displacement, foundation settlement, and embankment deformation of the new drainage PCC pile foundation were markedly reduced, while acceleration increased, which showed that the new drainage PCC pile had a positive anti-liquefaction effect on the coral sand foundation.

Investigation of drying and wetting effects on response of highly densified tailings to cyclic loadings: Shaking table test results

In this technical paper, layered deposits of thickened tailings (TT) and paste tailings (PT) are first subjected to a period of drying, and then to manually simulated rainfall. Thereafter, the bottom layer of the deposits of TT and PT is subjected to shaking with the use of a one-dimensional shaking table. The pore water pressure and displacement are monitored at different depths of the layers. The obtained results show that excess pore water pressure at mid-depth of each layer of deposited TT and PT develops immediately until reaching the peak value during shaking. However, the excess pore water pressure ratio is smaller than 0.8 during shaking in the layered TT and PT deposits that are initially exposed to a drying and wetting cycle. In other words, these deposits did not liquefy during the cyclic loadings despite exposure to heavy rainfall. The excess pore water pressure ratio is smaller than 0.85 in the layered PT deposit (initially not exposed to a drying and wetting cycle), so this layer does not liquefy during shaking. However, the layered TT deposit (initially not exposed to a drying and wetting cycle) liquefies during the cyclic loadings. After the applied shaking loadings, the test results reveal that the layered TT and PT deposits initially exposed to a drying and wetting cycle resist post-shaking liquefaction while those not initially exposed to a drying and wetting cycle are susceptible to post-shaking liquefaction. Contraction and dilation are observed in all of the layered deposits during the shaking.

Response of Bridge Foundation with Drainage Structure in the Liquefied Inclined Site under Sinusoidal Waves

Many earthquake damage investigations have shown that lateral spreading is one of the main causes of damage to bridge foundations. However, the seismic research on bridge foundations with drainage systems is relatively lacking. Therefore, based on the shaking table test, the seismic response of a drained sheet pile-reinforced bridge foundation on a liquefied inclined site was studied under the action of sinusoidal waves. Compared with the conventional group, the peak excess pore water pressure ratio and the lateral displacement of the sheet-pile wall of the test group were smaller, but the acceleration amplification factor was larger, indicating that the anti-liquefaction performance of the site was effectively improved. Meanwhile, the acceleration amplification factor of the test group was larger, and the lateral displacement of the bridge superstructure was smaller. These results indicated that the drainage structure significantly improved the stability and safety of the bridge system.

Correlation of excess pore water pressure ratio on flow liquefaction phenomenon in Sibalaya – Central Sulawesi Province

On September 28, 2018, liquefaction in Sibalaya damaged irrigation canals and hit 51.2 hectares fields and roads. This phenomenon was triggered by an earthquake of magnitude 7.5 that shook Sigi Regency, Central Sulawesi Province. As an increase in soil pore water pressure is one of the causes of liquefaction, the pore water pressure ratio (ru) at the study site was analyzed. Then, this figure is used as a point of reference to determine the liquefaction potential. Standard Penetration Test (SPT) data and laboratory tests from four boreholes resulting from soil investigations in 2021 with a maximum depth of 20 meters were utilized in this study, along with microtremor data down to the depth of bedrock from test results in 2023. Based on the data, the liquefaction potential was assessed using the one-dimensional nonlinear site response approach, the GQ/H + PWP model, and the DEEPSOIL V.7 program. At multiple layers of boreholes, ru is more than or equal to 0.8, indicating that the Sibalaya liquefaction flow area is still susceptible to liquefaction.

Study on the liquefaction characteristics of saturated sands by millisecond delay blasting

The liquefaction in saturated sands under blast loading is an important research topic in geotechnical engineering. The effect of blast loading on the liquefaction characteristics of saturated sands was studied through a large-scale millisecond delay blasting (MDB) liquefaction field test. Based on the field test, a numerical model was established by the LS-DYNA finite element program to study the propagation of blast waves and the effects of explosive mass, scaled distance, and blast delay on the liquefaction characteristics of saturated sands. The results show that there was a significant sand boiling phenomenon in saturated sands under blast loading, and high values of the excess pore water pressure ratio were observed at monitoring points during the field test. The numerical simulation results show that the scaled distance and the blast delay significantly affected the excess pore water pressure. While the blast delay increased from 110 ms to 330 ms, the excess pore water pressure of the central area can increase by more than 18%. Based on the field and numerical tests, an empirical equation is established that can take into account both the scaled distance and blast delay, which provides a theoretical basis for future large-scale liquefaction sites by MDB.

Dynamic Response of Rectangular Tunnels Embedded at Various Depths in Spatially Variable Soils

This study investigated the seismic response of rectangular tunnels with various embedment depths considering the spatial variability of soil shear modulus. The spectral representation method was adopted to simulate the anisotropic random field of soil. The excess pore water pressure, the liquefied zone, the ground displacement and the uplift displacement of the tunnel were obtained through the random finite difference method to analyze the seismic response. It was observed that the soil excess pore water pressure ratio under the tunnel gradually decreased and the liquefaction degree reduced with depth increase. The peak value of the liquefied zone range increased with the increase in embedment depth. The mean response of stochastic analysis was smaller than the deterministic calculation results when the tunnel embedment depth was less than 10 m. The maximum tunnel floating displacement obtained from random analyses had the probability of 67.3%, exceeding the value calculated by deterministic analyses when H = 12 m.

Seismic Response of a Liquefiable Site-Underground Structure System

To study the dynamic response of a saturated sand-underground structure system subjected to earthquakes, a series of shaking table tests with a geometric scale ratio of 1/30 were conducted. Based on the experimental acceleration records of testing soil deposits, the relationship between dynamic shear stress and horizontal soil displacement was analyzed by the 1D shear beam inverse calculation method. Meanwhile, the development law of the equivalent dynamic horizontal subgrade reaction coefficient and the dynamic strain of the sidewall in the underground structure has also been discussed. The testing results indicate that the dynamic shear stress of the soil deposit under the bottom plate of the underground structure is larger than that of the soils surrounding the sidewall and above the roof plate; in addition, the soil displacement tends to decrease with the buried depth. The dynamic shear stress–displacement hysteretic loop of the soil deposits tends to be plump as the input amplitude increases. The spectral characteristics of ground motions obviously influence both the dynamic shear stress–displacement hysteretic curve and the excess pore water pressure ratio of saturated sand soil, especially under medium and strong excitations. The equivalent dynamic horizontal subgrade reaction coefficient decreases with the increase of soil depth, and the difference between the coefficient above and underneath the underground structure model can reach 7.589 MN/m3. On the contrary, the equivalent dynamic horizontal subgrade reaction coefficient decreases with the increase of the input amplitude of ground motions, and the maximum reduction ratios of the coefficient are 74.4%, 66.7%, and 47.3%, corresponding to the El-Centro, Kobe, and Wolong ground motions, respectively. The soil liquefaction has a certain effect on the equivalent dynamic horizontal subgrade reaction coefficient. In general, the dynamic strain at the top of the sidewall in the underground structure is higher than that at the bottom of the sidewall, which illustrates that the deformation of underground structures is in good agreement with the seismic deformation mode of soil deposits.

Seismic responses of rectangular tunnels in liquefiable soil considering spatial variability of soil properties

The objective of this paper was to investigate the dynamic response of a tunnel-soil system in the soil with spatial variability of shear modulus with various coefficients of variation and scales of fluctuation. The soil shear modulus was highlighted and modeled as an isotropic random field using the spectral representation method, then mapped into the finite difference model. The dynamic stochastic responses of the excess pore water pressure ratio, the liquefied zone, the ground displacement, and the uplift displacement of the tunnel were discussed. It was observed that the effect of coefficient of variation was pronounced in which an increase of the coefficient of variation brought out a reduction in the corresponding dynamic stochastic responses. However, the scale of fluctuation was found to be insignificant in the tunnel soil system.

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.