Pore Water Pressure Generation Research Articles

A semi‐analytical model for coupled THM consolidation of saturated clays improved by PVTD considering thermal contraction

AbstractPrevious studies have demonstrated that saturated normally consolidated and lightly over‐consolidated clays undergo contraction when heated due to a reduction in preconsolidation pressure. A linear constitutive model is proposed to describe the thermal contraction, with this model, governing equations are developed for the coupled thermo‐hydro‐mechanical (THM) consolidation induced by a prefabricated vertical thermo‐drain (PVTD). Corresponding semi‐analytical solutions are derived employing the Laplace transform and validated via comparison with experimental results, existing numerical model, and custom finite element method (FEM) model. Subsequently, comprehensive parametric analyses are carried out to investigate the THM coupling consolidation behaviors of the clays. Outcomes show that aside from surcharge load, generation of excess pore water pressure in soils can also be induced by thermal contraction, difference in thermal expansibility between pore water and soil grains, and thermo‐osmosis, where the influence of thermal contraction on the excess pore water pressure is the most prominent among the three factors.

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.

Insights on seasonal solifluction processes in warm permafrost Arctic landscape using a dense monitoring approach across adjacent hillslopes

Solifluction processes in the Arctic are highly complex, introducing uncertainties in estimating current and future soil carbon storage and fluxes, and assessment of hillslope and infrastructure stability. This study aims to enhance our understanding of triggers and drivers of soil movement of permafrost-affected hillslopes in the Arctic. To achieve this, we established an extensive soil deformation and temperature sensor network, covering 48 locations across multiple hillslopes within a 1 km2 watershed on the Seward Peninsula, AK. We report depth-resolved measurements down to 1.8 m depth for May to September 2022, a period conducive to soil movement due to deepening thaw layers and frequent rain events. Over this period, surface movements of up to 334 mm were recorded. In general, these movements occur close to the thawing front, and are initiated as thawing reaches depths of 0.4–0.75 m. The largest movements were observed at the top of the south-east facing slope, where soil temperatures are cold (mean annual soil temperatures averaging −1.13 °C) and slopes are steeper than 15°. Our analysis highlights three primary factors influencing movements: slope angle, soil thermal conditions, and thaw depth. The latter two significantly impact the generation of pore water pressures at the thaw–freeze interface. Specifically, soil thermal conditions govern the liquid water content, while thaw depth influences both the height of the water column and, consequently, the pressure at the thawing front. These factors affect soil properties, such as cohesion and internal friction angle, which are crucial determinants of slope stability. This underscores the significance of a precise understanding of subsurface thermal conditions, including spatial and temporal variability in soil temperature and thaw depth, when assessing and predicting slope instabilities. Based on our observations, we developed a factor of safety proxy that consistently falls below the triggering threshold for all probes exhibiting displacements exceeding 50 mm. This study offers novel insights into patterns and triggers of hillslope movements in the Arctic and provides a venue to evaluate their impact on soil redistribution.

Effect of thermal cycling on the isotropic consolidation and triaxial shear behavior of lateritic clay

It is crucial to comprehend soil thermomechanical behavior while designing underground energy structures to ensure safety. Studies on the soil response to thermal cycles in terms of the generation of thermal-induced volume change and pore water pressure are rare, and relevant research on how these responses might affect soil consolidation parameters and shear strength is very limited. To experimentally investigate the effect of thermal cycling under drained and undrained conditions on the isotropic consolidation parameters and triaxial shear strength of lateritic clay, this paper employs a temperature-controlled triaxial apparatus to conduct a series of isotropic mechanical consolidation and thermal consolidation tests, as well as undrained triaxial shear tests. The thermal response in volume change and pore water pressure are discussed, and the changes in the consolidation parameters, the preconsolidation pressure, and the shear strength are identified. It is concluded that increments of irreversible contraction of lateritic clay are observed during thermal cycling under drained conditions and further lead to a slight increase in the preconsolidation pressure. Nevertheless, thermal cycling hardly affects the swelling and compression index. The shear strength increases after being subjected to thermal cycling under drained conditions, which can be attributed to the increase in cohesion. When drainage is not allowed during thermal cycling, the generation of pore water pressure occurs during temperature variations and completely dissipates after the thermal cycling phase, and its reversibility is unaffected by the stress level and number of cycles. Furthermore, thermal cycling has little effect on the consolidation parameters, preconsolidation pressure, and shear strength. This study provides new insights into the mechanisms controlling the response of clay to thermal cycling.

Excess pore water pressure generation in poorly graded sands with varying particle shapes and relative densities

In this study, a series of cyclic triaxial tests were conducted on two poorly graded sands with different particle shapes. The experimental results were integrated with test results reported in the literature to construct a comprehensive database for investigating the effects of particle shape and relative density on excess pore water pressure (EPWP) generation. The sand types in the database were divided into three groups based on the particle shape: subrounded, subangular, and angular. The test results showed that the particle shape and relative density influenced the axial strain development and EPWP generation during undrained cyclic loading. As the relative density increased, the effect of the particle angularity on the shape of the EPWP generation curves decreased. The normalized EPWP generation curves of the subangular particles (C306 sand) exhibited a strong dependence on the relative density. However, the angular and subrounded particles exhibited a weaker influence than the subangular particles on EPWP change for different relative densities. A model was established to predict the EPWP generation with variations in the particle shape and relative density.

Generation and origin of persistent high excess pore water pressure in debris flows

Due to building up excess water pressure, debris flows differ markedly from other water-sediment mixtures in mountain basins. Although a number of mechanisms are known to create excess water pressure, persistent high excess water pressure develops in static-quasistatic debris-flow slurries when possible triggers such as saturation, contraction, and Reynolds stress are absent. This suggests that there exist more fundamental mechanisms for the creation and persistence of high excess water pressure. Using 12 clastic materials with four maximum diameters up to 40 mm from three typical debris flows in Southwest China, we make comparative measurements of excess water pressure, gravity drainage, and decanting of water on 12 pairs of debris-water mixtures. It is found that saturation is not a mechanism for excess water pressure generation, which can be ascribed to the heterogeneity of saturated sediments. Mixing undoubtedly creates persistent high excess water pressure in the mixed saturated sediments. Thorough mixing causes the homogenization of water-saturated sediments through random grain rearrangements. The particles in the homogeneous debris-water mixtures stay suspended in water, resulting in excess water pressure derived from particle weight. More significant is the integration of clasts and water over the timescales for debris-flow durations promoted by mixing, which leads to the persistence of high excess water pressure. The development of persistent high excess water pressure in sediment-water mixtures is controlled by external factor—mixing, and internal factors, including sorting of sediments and water content of mixtures.

Monotonic and cyclic triaxial testing of untreated and polyurethane-treated soil and soil–rubber mixtures

The present research focuses on developing alternate sustainable base materials for a high-speed slab track. In this study, a series of monotonic triaxial, cyclic triaxial and permeability tests were conducted on four types of materials, viz. mix-A (gravel soil), mix-B (soil mixed with rubber), mix-C (polyurethane foam adhesive (PFA)-treated soil), and mix-D (PFA-treated soil–rubber mixture). The influence of cyclic loading frequency, effective confining pressure, drainage condition and relative density on the deformation, excess pore water pressure, resilient modulus and damping ratio of these different mixes is evaluated. The monotonic triaxial test results indicate that the PFA treatment of mix-A and mix-B increased their shear strength and critical state strength. In contrast, incorporating rubber into mix-A and mix-C helped enhance their ductility. The cyclic triaxial test results show that the PFA treatment of mix-A and mix-B significantly reduced the magnitude of deformation and generation of excess pore water pressure, which caused these untreated mixes to fail prematurely under lower confinement to which a typical base layer is subjected. The influence of cyclic loading frequency and effective confining pressure on the material's response differed for untreated and treated soil. The permeability test results indicate good drainage for mix-D comparable to mix-A.

Dynamic Assessment of OWT under Coupled Seismic and Sea-wave Motions

The effect of soil-monopile-structure interaction is of great importance in the design of offshore wind turbines (OWTs). Although sea waves play the most effective role in the performance of OWTs, the coupled effect of sea-wave loads and seismic motion on the performance of the OWT system in seismic-prone areas is a factor that is less investigated and should not be ignored. In this regard, a 2-D porous model based on Biot’s poro-elastic theory is considered to capture the pore water pressure generation in the soil domain surrounding the OWT foundation. The coupled effect of sea waves and seismic motion through a comparative study is considered for the reference OWT system based on the monopile foundation by using the FE program, OpenSees. The results of the analyses are presented in specific locations. Upon the obtained results, the dynamic behavior of the OWT system and the possibility of liquefaction in the soil surrounding the OWT during applied loads are investigated and compared. This comparison is a good representative of the effect of the seismic motion on the performance of the OWT system and the soil medium by considering soil-monopile-structure interaction in seismic-prone areas.

An Experimental Study on Excess Pore-Water Pressure Generation of a Poorly Graded Sand Using Combined RCTS Device

Abstract The combined dynamic torsional resonant column and cyclic torsional shear (RCTS) device is employed to study the generation of excess pore-water pressure, especially the threshold shear strain of excess pore-water pressure, γtPP, in an uncemented poorly graded granular soil. Reconstituted specimens of uncemented sand were constructed under water and then tested in the RCTS device using a newly designed base pedestal, with which pore-water pressure is measured at four points systematically located at 0.75 times the radius of the base pedestal. Specimens were tested under different mean effective confining pressures, and all specimens had B-values above 0.95 at the start of testing. During testing, the excess pore-water pressure was measured continuously during 30 cycles of loading at multiple increasing loading levels. The three major factors influencing γtPP that were investigated are as follows: (1) mean effective confining pressure, (2) relative density, and (3) loading frequency. The results are presented in terms of the following: (1) excess pore-water pressure ratio versus shear strain, and (2) threshold shear strain versus number of loading cycles. Other related behaviors such as softening during cycling and post-cyclic moduli changes are also discussed.

Experimental investigation for dynamic characteristics of paraffin-graphite based CPCM (composite phase change material) amended expansive soil under dry-wet cycles

Expansive soils are known to exhibit complex behavior under varying moisture and stress conditions, leading to soil cracking and instability. In this study, the effects of composite phase change material (CPCM) on the dynamic characteristics of expansive soils before and after dry and wet cycles were investigated. Further, the soil with and without CPCM amendment was also subjected to dry-wet cycles so as to monitor cracking phenomenon. Dynamic triaxial tests were conducted to evaluate the damping ratio and elastic modulus of expansive soil amended with CPCM. For interpretation of the cracking and dynamic properties of CPCM amended soil, laser thermal conductivity and electron microscope scanning tests were conducted. The results showed that the incorporation of CPCM material effectively inhibited soil cracking by reducing the temperature gradient difference within the soil and actively regulating the temperature distribution. Moreover, the CPCM amended expansive soil demonstrated better stability under the combined effect of alternating load and wet and dry cycles. The dynamic modulus of elasticity of the 10% CPCM amended soil was found to be about three times that of the bare soil. Additionally, the CPCM material improved the soil structure and reduced the damping ratio and the generation of pore water pressure in the expansive soil. Overall, this study demonstrates the effectiveness of using CPCM materials to improve the dynamic characteristics and stability of expansive soils, making it more suitable as roadbed filler.

Liquefaction Behavior of Typical River Channel Deposit in Kolkata City

Liquefaction susceptibility of river channel deposit in Kolkata city is studied using laboratory cyclic triaxial tests. River channel deposit, which supports a large part of rapidly growing urbanization of Kolkata city, mainly consists of sandy soil with little amount of silt. Cyclic triaxial tests have been performed on this soil sample with varying relative density, confining pressure, and cyclic strain amplitude. Results are presented to show how these parameters influence the pore water pressure generation within soil. Relative density, confining pressure and cyclic strain amplitude are found to affect the pore pressure generation characteristics in the soil, and the number of cycles required to reach liquefaction phenomena vary significantly. Finally, pore pressure generation characteristics are modeled using a hyperbolic model and a pore pressure generation equation is proposed for the typical river channel deposit soil. The model exhibits a certain threshold value of cyclic strain amplitude, up to which the initial rate of pore pressure generation decreases and then increases.

Rate, Water Pressure, and Temperature Effects in Landslide Shear Zones

AbstractThe behavior of slow‐moving landslides is controlled by the residual strength of the shear zone. Despite their mobile state, such slides can often withstand extreme events such as heavy rainfalls and earthquakes. A rate‐hardening in the shear zone is suspected to be one of the main stabilizing factors and has been well investigated, in particular for clayey soils. Soils from steep alpine landslides, on the other hand, are often dominated by silts and sands with various clay contents, and while understanding of their rate dependency is less advanced, it remains critical for the reliable risk assessment. In this article, an improved ring shear apparatus is presented and applied to investigate the rate, pore water pressure, and temperature effects in landslide shear zones. The testing program on samples from two alpine landslides in Switzerland revealed a moderately positive rate effect explaining their mobile state. An achieved insight into the thermo‐hydro‐mechanical processes of fast shearing provides physical evidence to the widely debated hypothesis on the generation of excess pore water pressure due to frictional heating. However, for the tested materials, this effect only occurs after extended shearing and does not lead to a complete loss of shear strength. Based on that and the observed positive rate effects, catastrophic failure seems unlikely for these landslides. At the same time, the results suggest that for less permeable soils, frictional heating may indeed be a source of negative rate effect during very rapid shearing.

Influences of particle size distribution and fines content on the excess pore water pressure generation of sand-silt mixtures under cyclic loading

The mechanical behaviors of binary mixtures are different from that of uniform soils. However, experimental studies on the dynamic responses of sand-silt mixtures considering the combined effects of fines content (FC) and host sand gradation are limited in the literature. In this paper, an experimental program was designed concerning five binary mixtures with three grain size ratios (Rd) and three sand uniformity coefficients (Cus) for investigating the effects of fines content and host sand gradation on both the liquefaction resistance and the excess pore water pressure (EPWP) generation of sand-silt mixtures. Results show that the effect of FC on the liquefaction resistance of sand-silt mixtures depends on relative density (Dr), grain size ratio, and sand uniformity coefficient. The liquefaction resistance of sand-silt mixture can be uniquely characterized by the equivalent relative density Dr* regardless of FC, Dr, and host sand gradation. Relationship between the EPWP ratio Ru and the cycle ratio N/NL for the sand-silt mixtures can be well expressed by the Seed model, which is significantly affected by cyclic stress ratio (CSR), FC, and Dr but insensitive to the change of host sand gradation. A trained artificial neural networks (ANN) model is conducted to predict the fitting parameter β of the Seed model with a maximum difference of 25% between the predicted and measured values. Moreover, there is an arctangent relationship between Ru and shear strain amplitude γa of each specimen, and the fitting parameter A for the relationship is sensitive to the particle size distribution and stress amplitude. A unique relationship between a density-corrected parameter A/Dr* and CSR is proposed regardless of FC and host sand gradation.

Impact liquefaction mechanism of sandy silt with a change in impact energy

The impact liquefaction of sediments in the path of a landslide would reduce the friction of the sliding surface, leading to an increase in the speed and distance of the landslide. In this study, a self-designed impact-liquefaction test was used to explore the pressure development of soil under the effect of different impact energies. The microscopic mechanisms were unravelled by using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) methods. The results show that soil at the middle depth shows a higher porewater pressure than at the top and bottom of the layer. Furthermore, the variation in the porewater pressure in these parts is relatively stable, and the peak pressure linearly increases with impact energy. The collapse of large pores in sandy silt is the primary reason for the generation of high porewater pressures in sandy silt. But when the soils are high-energy impacted, the mesopores and small pores are compressed and blocked, which also stimulates the porewater pressure. The results also demonstrate weak drainage behaviour of sandy silt under conditions of rapid impact. If the impact energy is sufficient, the porewater pressure in sandy silt suppresses dissipation, causing quick liquefaction in the interior of the sandy silt. Supplementary material: Language embellishment proof and SEM image binarization method are available at https://doi.org/10.6084/m9.figshare.c.6655482

Partially drained uplift behaviour of plant roots in dilative soils

A large volume of research reporting the pull-out behaviour of root systems is available, but no study has considered the effects of soil drainage. This work implemented a modified three-dimensional embedded beam element model in a finite element platform that solved model equations by using a fully hydromechanically coupled algorithm. The model was validated against published centrifuge pull-out tests on root analogues, and the validated model was then applied to study parametrically the influence of the ratio of uplift rate to soil hydraulic conductivity on pull-out behaviour. The results demonstrated that the model can well capture the prepeak behaviour of the root systems up to the peak pull-out resistance. The generation of negative pore–water pressure ( p e x) owing to soil dilation upon root–soil interfacial shearing was the major reason for increased pull-out resistances under partially drained conditions. Compared with other root systems, root systems with smaller branch angles and deeper branch depths mobilised considerably more significant plastic deviatoric strains in the soil in their vicinity, generating more negative p e x. Hyperbolic dimensionless backbone curves were derived to explain the transitional pull-out behaviours of root systems of different geometries under drainage conditions that ranged from fully drained to undrained.

Effects of the Properties of Fines on the Pore Water Pressure Generation Characteristics of Sand–Silt–Clay Mixtures during Cyclic Loading

To investigate the effects of the properties of plastic fines on the pore water pressure generation characteristics of fine-grained soils during cyclic loadings, we used 29 sets of test data from the literature and prepared another 21 reconstituted specimens with different types of fines and fine contents (FCs) for cyclic triaxial testing. Two types of undisturbed soil specimens and three types of reconstituted soil specimens were also included for testing. The results indicated that under cyclic loading, the pore water pressure (PWP) ratios of clean sands increased slowly, stagnated, then finally accelerated until initial liquefaction, whereas those of the plastic soils containing fines with a plastic index (PI) value of >5 increased sharply in the initial stage. In addition, the cyclic stress ratio of specimens containing mudstone (PI = 12.4) and kaolinite (PI = 32.0) fines increased by 1.5–3.0 times more than non-plastic fines if the cyclic number chosen was 100. The range of the upper and lower limits of the PWP curves of the specimens with an FC of 30% were smaller that of the limits of the PWP curves of the specimens with an FC of 15%. The above results were further analyzed using a mathematical model. This paper systematically uses both the literature and laboratory test data to demonstrate that plastic fines and non-plastic fines have significantly different effects on water pressure generation under cyclic loading conditions, and a mathematical model also demonstrated the same trends. These findings are able to clarify previous unclear arguments. Thus, the model results developed in this study could also provide the field of engineering with a complete advanced calculation, requiring analysis only via software.

Validation of strain/energy-based pore pressure model in one-dimensional response analyses using centrifuge tests

The results of cyclic strain-controlled tests performed on reconstituted specimens of Ottawa sand F-65 using the combined triaxial simple shear (TxSS) apparatus were used to establish a strain/energy-based pore pressure model. The model was utilized in conjunction with the sigmoid function (SIG4) to simulate the cyclic behavior of Ottawa F-65 sand under stress and strain loading conditions using FLAC at the element-level and in 1-D effective stress analysis. A counterpart set of cyclic stress-controlled direct simple shear (DSS) tests was performed to assess the predictive capability of the numerical model to determine the liquefaction potential curves. Two dynamic centrifuge tests were simulated using the proposed model and Finn model, and a satisfactory comparison of the observed and computed responses in terms of pore water pressure generation at different depths was obtained. Furthermore, model validation was carried out by applying real earthquakes from the Western United States (WUS) to a hypothetical soil deposit and then comparing the liquefaction triggering according to published liquefaction charts. A good agreement between the numerical results and the published charts confirms the applicability of the proposed strain/energy-based model in 1-D response analysis and liquefaction triggering assessment.

A DEM Investigation on the Influence of Cyclic and Static Stress Ratios and State Variables on the Pore Water Pressure Generation in Granular Materials

A granular soil may undergo liquefaction failure due to excess pore water pressure (Δu) generation when subjected to cyclic loading such as an earthquake. Several past studies indicated Δu generation may be independent of cyclic stress ratio (CSR), whereas others contradicted this observation. A soil element in its natural state often remains in K0 consolidation state, i.e., subjected to nonzero static shear stress ratio (SSR). Using the discrete element method (DEM), this study performs a large number of cyclic triaxial tests and evaluates various factors influencing the magnitude and rate of Δu generation under symmetric and nonsymmetric cyclic loading conditions. The study also evaluates the capabilities of several existing Δu generation models to predict the development of Δu under different failure scenarios. It was observed that the generation of Δu can be influenced by CSR for specimens subjected to symmetrical cyclic loading and by both CSR and SSR for nonsymmetric cyclic loading conditions. A new prediction model for Δu generation was developed along with an objective framework to estimate its input parameters. The model showed good agreement with the DEM simulation results.

Earthquake-induced large deformations and failure mechanisms of silty sands in sloped ground conditions

In practical engineering, sand deposits or fills usually contain fines and are subjected to cyclic shear stresses induced by earthquakes, traffic, or waves which are superimposed on the initial static shear stress in natural or artificial slopes or beneath existing structures. To explore the combined and complex effect of an initial static shear stress and fines content of non-plastic silty sands on their deformation characteristics, pore pressure generation, and liquefaction susceptibility, the results of a comprehensive experimental program of cyclic simple shear (SS) tests are presented. Test conditions cover different fines contents fc (0-40%), initial void ratios e0, and initial static shear stress ratios (factor α=0-0.30). The observed types of failure are divided into four cyclic patterns: flow liquefaction, limited flow liquefaction, cyclic mobility, and plastic strain accumulation, depending on the initial state of the specimens and cyclic loading characteristics. Moreover, the threshold fines content (fthre), denoting the specific value of the fines content at which the behavioral properties of the mixture are reversed, is not affected by α level, and a practically unique value of around 24.5% is identified. The Kα values of sand-silt mixtures measured under cyclic simple shear loading would either increase or decrease with an increasing initial static shear level based on the initial global void ratio and fines content of mixtures; in particular, for a given initial global void ratio, the reduction of cyclic resistance due to the addition of non-plastic fines (fc < fthre) is much more pronounced as α increases. Finally, the larger the initial shear stress, the smaller the cyclic pore-water pressure (PWP) measured at failure. Therefore, a modified stress-based PWP generation model is proposed to predict the cyclic residual excess pore pressures developed under various initial static shear stress conditions in non-plastic silty sands in a satisfactory way.

Liquefaction characteristics of saturated marine coral sand with different non-plastic fines content subjected to complex cyclic stress paths

Coral reefs thrive in complex marine geological environments and are likely to suffer from dynamic marine disasters. The liquefaction characteristics of saturated marine coral sand subjected to dynamic loading, such as ocean waves, submarine earthquakes, storm tides, volcanic activities, and hurricanes, are key factors affecting the safety of coral reefs. A systematic study is conducted through a series of hollow-cylinder torsional shear experiments on the undrained response of saturated marine coral sand with different non-plastic fines contents (FCs) under cyclic linear stress paths with various cyclic loading direction angles (αd). An interesting discovery is that the cyclic linear stress paths with angles αd and 90°−αd have the same stress effect on liquefaction characteristics under isotropic consolidation condition. The test results show that if the near-zero effective stress state is defined as the criterion for initial liquefaction, then the liquefaction resistance of saturated coral sand decreases with increasing αd or FC. The generation of excess pore water pressure (EPWP) presents three modes: (1) “rapid–stable–fast,” (2) “rapid–stable,” and (3) “fast linear-like.” Moreover, an energy-based EPWP prediction method is established. A novel discovery is that the generalized shear strain amplitude (γga) is uniquely related to the EPWP ratio of coral sand for a given relative density and FC. In addition, by introducing unit cyclic stress ratio (USR) as a new index of liquefaction resistance, a common correlation between equivalent skeleton void ratio esk* and USR15 (required for initial liquefaction in 15 cycles) is established for all test cases considered. This is confirmed by the experimental data of five types of terrestrial siliceous sands reported in the literature.