Pore Pressure Ratio Research Articles

The variation characteristics of dynamic shear stress reduction coefficient rd in loess tunnel stratum

In the simplified procedure of Seed for liquefying evaluation of free fields, the coefficient rd is the essential parameter, but presently the liquefaction assessment of loess tunnels are not available using this method. In this study, the variation characteristics of rd, which is suitable for the liquefying assessment of loess tunnels, are studied by 3D numerical simulation and analysis. The results reveal: the soils nearby underground structure have higher liquefaction potential than soils far it; the rd increases rapidly as nearing the tunnel excavation contour for distances smaller than the tunnel excavation span B; the formula of rd reveals the tunnels with shallower buried depth are more prone to liquefying than tunnels with larger burial depth h, and it can be used to estimate the liquefaction of loess tunnel; the pore pressure ratio Ru has the similar rule with the coefficient rd.

Seismically Evaluate Liquefaction-Induced Steel Tunnel–Sand–Pile Interaction (TSPI) Response

This paper presents the steel tunnel–sand–pile interaction (TSPI) response under scaled (0.05[Formula: see text]g and 0.1[Formula: see text]g) seismic excitations (Kobe and Loma Prieta) and sinusoidal wave (frequencies of 1, 5, and 7[Formula: see text]Hz) along with the sand relative densities of 48% and 80%. For this reason, a scaled shake table test was performed in the laboratory, and the 3D numerical analysis was performed by the Plaxis for the simulation. No joint was considered along the length of the tunnel to avoid complexity. In most cases, the excess pore pressure ratio exceeds the unit value at bottom and top locations of the TSPI model to inform the occurrence of liquefaction. Maximum lateral and translational displacements of the tunnel were found to be 143[Formula: see text]mm and 72[Formula: see text]mm, respectively, at the crown. In addition, the maximum vertical translation of sand and root mean square (RMS) value of the pile strain were obtained to be 61[Formula: see text]mm and 20[Formula: see text][Formula: see text]m/m, respectively. However, close variations of numerical and experimental studies may convince the accuracy of the simulation. By the way, it has a scope to enhance this study in the future by varying the geometry of the TSPI model.

PREDICTION OF SLOPES SAFETY FACTOR USING ANN AND TREE-BASED MODELS

Slope stability assessment is a non-linear engineering problem. The complex relationship between factors that affect slope instability is challenging to investigate theoretically and mathematically. This paper proposes the prediction of slopes safety factor, subjected to circular failure mode using artificial neural network (ANN) and two tree-based models using M5P and random forest (RF) algorithm. A dataset including 46 slope cases was divided into a 70/30 ratio to train and validate the model. The input parameters for slope stability evaluation include slope angle, slope height, cohesion, internal friction angle, unit weight and pore pressure ratio. The corresponding output parameter is a factor of safety (FS). The finest ANN structure was obtained as 6-7-1 using the trial-and-error method. The proposed model chooses elliot as the best activation function at a learning rate and momentum of 0.7 and 0.5 for the accurate estimation of FS. In validation phase of ANN, M5P and RF models, the coefficient of determination (R2) results as 0.94, 0.86 and 0.81, respectively. The result suggests that the developed ANN model provides higher accuracy than tree-based models which can be an effective tool for slope stability assessment. Furthermore, the significance of input variables for FS prediction has been investigated through sensitivity analysis, which identifies the complex relationship between the input and output variables. The accuracy achieved from the prediction model is crucial for preventing the risk of slope failure and increasing the slope safety during preliminary design stage.

Research on dynamic p-y curves in liquefied sand under cyclic loading conditions

Under dynamic cyclic loads, the pore water pressure in saturated sand rises continuously, which leads to the reduction of horizontal bearing capacity or the damage of the pile foundation. Under dynamic cyclic loads, the existing calculations methods of horizontal bearing capacity of pile foundations were obtained by proposing a reduction factor for the static p-y curve model mostly. Compared with the test results, the soil resistance around the pile obtained by this method is often overvalued and the pile-soil relative displacement has a relative underestimation. As a result, this method is not suitable for pile foundation design. Therefore, the variations of soil resistance around the pile and the pile-soil relative displacement are obtained by analyzing the effect of pore pressure ratio on soil strength around the pile in saturated sand in this paper. The theoretical formulas related to the cyclic vibration times and pore pressure ratio are proposed. The proposed formulas are introduced into the dynamic p-y curve model to obtain the dynamic p-y backbone curve in the process of sand liquefaction. The research results show that the variations of the soil resistance around the pile and the pile-soil relative displacement are related to the pore pressure ratio and the cyclic vibration times during the sand liquefaction. As the cyclic vibration times increases, the pore pressure increases, the soil resistance around the pile decreases and the pile-soil relative displacement increases.

Bearing capacity of shallow foundations accounting for seismic excess pore pressures

The method of characteristics has been used to develop a rigorous solution for the evaluation of seismic bearing capacity of shallow foundations resting on homogenous fully submerged soil accounting for the effect of earthquake-induced pore water pressures. The solution, derived with reference to both the Hill and Prandtl plastic mechanisms, is essentially referred to the bearing capacity factor Nγ, is of general validity and has been checked against finite element analysis results. An original empirical expression providing the corrective coefficient to be introduced in the usual trinomial bearing capacity formula has been proposed, accounting for the reduction in bearing capacity due to combined effect of soil inertia and earthquake-induced pore water pressures. It has also been demonstrated that, even considering earthquake-induced excess pore pressures, the soil and superstructure inertial effects are still decoupled. This allowed introducing an overall corrective coefficient accounting for both the inertial (soil and superstructure) and excess pore pressure effects, computed as the product of the corresponding corrective coefficients. Finally, original simple equations were derived giving a threshold value of the excess pore pressure ratio required to trigger a bearing capacity failure in the static post-seismic conditions because of the soil shear strength reduction.

Study on the dynamic pore water pressure model of centerline tailings sand based on dynamic triaxial tests

The increase of pore water pressure under a dynamic load is an important reason for the vibration liquefaction of saturated sand. Therefore, studying the dynamic pore water pressure model of tailings sand plays an important role in the seismic liquefaction analysis of tailings ponds. In this paper, the dynamic pore water pressure development characteristics of tailings fine sand and tailings silt in a tailings pond with the centerline method are studied by dynamic triaxial tests. The research results show that the liquefaction dynamic pore pressure ratio of tailings fine sand shows a trend of "rising–smooth–rising" with increasing vibration frequency ratio, and the liquefaction dynamic pore-pressure ratio of tailings silt shows an approximate linear and stable increasing trend. According to the development law of dynamic pore pressure, a new tailings sand pore pressure stress model is proposed in this paper, and the fitting effect is compared with four different pore pressure models, which verifies the applicability of the new model to predict the growth of dynamic pore pressure in tailings and silt.

Numerical Simulation of Piles in a Liquefied Slope Using a Modified Soil–Pile Interface Model

The liquefaction of soil surrounding a pile significantly affects the dynamic interaction between the soil and the pile. In particular, liquefaction of the sloping ground can induce permanent deformation and a bending moment on the pile due to the lateral displacement of the liquefied soil in the downslope direction. However, numerical analysis studies on piles installed in a liquefiable slope have been very limited and have not properly simulated the behavior of the pile. Therefore, a modified soil–pile interface model was proposed, which linearly decreases the interface friction angle with the increase in the excess pore pressure ratio. The proposed model was validated by comparing it with the centrifuge test results of Yoo et al. (2023). Simulation results on the slope crest settlement and the pile-bending moment showed good agreement with the centrifuge test results. A parametric study was conducted by applying the validated model to analyze the effect of slope inclinations and the amplitude of input motions on the slope displacement and the pile moment. The simulation results showed that the slope inclinations affected the area of the sliding mass, causing a larger pile-bending moment with a larger inclination. When the amplitude of the input motion was sufficiently large to trigger the failure of the liquefied slope, the slope displacement and the pile-bending moment did not increase any further.

Quasi-site-specific prediction of shear wave velocity from CPTu

A rapid and reasonable estimation of the local site shear wave velocity (Vs) profile is indispensable for site assessment. For soil deposits, researchers have developed numerous empirical correlations between Vs and piezocone penetration test (CPTu) based on a global/regional (generic) database. However, these transformations are usually biased for local sites because the generic database contains all available data without regard to site-specific differences, and the transformation uncertainty is usually non-negligible due to the complicated geology factors and measurement errors. Moreover, it is challenging to construct a site-specific correlation due to the sparse in-situ measurements. To this end, this study develops a quasi-site-specific CPTu-Vs transformation model that algorithmically combines both the generic database and site-specific measurements to provide reliable Vs prediction with uncertainty description. A generic database containing 3116 cases of 6 parameters is collected from different countries, including corrected cone tip resistance, sleeve frictional resistance, depth, soil behavior index, pore pressure ratio, and Vs. As the significant impact of soil type on CPTu-Vs correlation, independent Johnson transformation (JS) for different soil types is first adopted to normalize the generic database as a whole, ensuring the constructed model is applicable to any soil type site. Three state-of-art quasi-site-specific models are investigated for CPTu-Vs transformation: probabilistic multiple regression (PMR), the hybridization method (HYD), and the hierarchical Bayesian model (HBM). Empirical correlations and the recently-developed data-driven model (XGBoost) are also considered to illustrate the novelty of the quasi-site-specific model. The applications on three sites with different soil types show that considering soil type in JS improves model accuracy and reduces the transformation uncertainty. The HBM with multiple basic CPTu parameters performs best regarding accuracy, uncertainty, and robustness.

Shaking table test on the dynamic responses of shield tunnel in liquefiable stratum

Seismic liquefaction affects badly the safety of the underground structures. For the dynamic responses of the tunnel in liquefied watered sandy stratum in one of the rail transit projects in Shanghai, this paper proposes a tunnel-foundation shaking table model test and analyses the dynamic responses of electrolytic desaturation technology on surrounding soil, which provides some theoretical guidance for seismic designs of Shanghai rail transit projects. The test result shows that the excess pore pressure ratio of soil decreases gradually with the increase of buried depth; sand is easier to liquefy than silt so the tunnel liquefaction and uplift during the sandy section should be paid more attention to; electrolytic desaturation technology works significantly on improving the liquefaction resistance abilities of sand and silt which can effectively reduce the damage caused by tunnel uplift due to seismic liquefaction.

Control of pressurized microbubble generation by multi-channel membranes: Experiments and modeling

Microbubbles have been attracting a lot of attention in the fields of chemical and petrochemical engineering, environmental protection, medicine, and aquaculture etc. Herein, a controlled pressurized microbubble generation system is developed. Numerous microbubbles are generated under certain of pressure by multi-channel ceramic membranes in a surfactant-free system consisting of dispersed gaseous and continuous aqueous phases. The average diameter and size distribution of the microbubbles are significantly affected by the membrane pore size, membrane area, liquid viscosity, pressure and gas-liquid flow ratio. A mathematical model about the average diameter of microbubbles is established for the first time by directly introducing the membrane parameters including the membrane area and membrane pore size, and the error of the as-established model is less than 10%. More importantly, the model can be used to predict the optimal membrane pore size and membrane area under different operating conditions under the premise of achieving the expected bubble size. The work will aid the understanding and development of microbubble generation with high performance.

Energy-based approach to study liquefaction triggering in homogeneous and stratified soils under consolidated undrained cyclic loading

Pore water pressure development through seismic effect is one of the key reasons for liquefaction failures in the foundations, dams, bridges, slopes and other vulnerable geotechnical engineering structures. The energy dissipation theories have provided a remarkable understanding of the pore water pressure generation inside a soil skeleton. In the present study, a series of consolidated undrained cyclic triaxial tests have been performed to evaluate the normalized dissipated energy (Wn) required for triggering liquefaction in both homogeneous and stratified soil specimens (prepared using silt interlayer) to replicate the site conditions nearby marine or alluvial deposits. The generated hysteresis loops using cyclic loading facilitate the calculation of dissipated energy. Also, the test results indicated a significant variation in the dissipated energy of homogeneous specimens at different magnitudes of cyclic stress ratios (CSR) and relative densities (Dr). With an increase in the cyclic stress ratio from 0.14 to 0.20, the trend of dissipated energy was observed to be different when relative densitie of sand varied from 30% to 65%. In the stratified specimens, thickness (t) and location of the silt layer (d) in the given soil specimen were the major influencing factors for estimating the liquefaction potential in loose conditions. Additionally, the highest value of Wn,max was obtained as 0.51 for the stratified specimen with topt = 28 mm at a cyclic stress ratio of 0.14. Based on the trendline variations, empirical models have been developed to estimate the normalized dissipated energy (Wn) for both homogeneous and stratified conditions. The developed models for homogeneous specimens are functions of CSR and Dr, whereas, in the case of stratified specimens, t, d and σd/σ'c (σd = deviator stress, σ'c = effective confining stress) have been utilized to predict the Wn with an satisfactory R2 value above 0.90. From the analysis of liquefaction resistance curves, the energy-based approach used in this study has been observed to provide more realistic results than the pore pressure ratio and strain-based failure criteria.

Integrated Model for Wave-Induced Oscillatory and Residual Soil Response in a Poro-Elastic Seabed: Partially Dynamic Model

The evaluation of wave-induced residual pore pressure in a porous seabed and associated seabed liquefaction is essential for designing marine infrastructure foundations. The strength and stiffness of the seabed could be weakened due to the build-up of pore pressures under cyclic wave action, further leading to residual liquefaction. Existing models for residual liquefaction are limited to the quasi-static uncoupled approaches, which do not account for the effect of oscillatory pore pressure on the accumulative pore pressure acceleration of solid particles, despite the mutual influence of these two mechanisms. To overcome these limitations, this paper proposes a new model for residual soil response with u−p approximation (partial dynamic model) that couples oscillatory and residual mechanisms. The proposed model is validated through wave flume tests and centrifuge tests. Based on the coupling model, a new criterion of liquefaction integrating both oscillatory and residual mechanisms is also proposed. Numerical examples demonstrate that the coupling effect significantly affects the wave-induced seabed liquefaction potential. Furthermore, a new parameter (Ω) representing the ratio of oscillatory and residual pore pressure is introduced to clarify which mechanism dominates the pore pressure development.

Laboratory investigation on liquefaction of sands and cemented sand mixes

ABSTRACT Liquefaction is considered one of the most common phenomena resulting in serious damage during earthquakes. Appropriate estimation of liquefaction potential, damping characteristics and shear modulus of soil under dynamic loading is crucial for precise dynamic response analysis and soil modelling problems. The present paper emphasises the influence of relative density, confining pressure, and shear strain on the cyclic response of sand specimens. A parametric study comprising varying relative densities (35–65%), confining pressures (100–150 kPa), and shear strain (0.015–1.3%) under 1 Hz loading frequency was conducted in the laboratory. The sand samples were subjected to a series of consolidated undrained cyclic triaxial tests. It is observed that with an increase in confining pressure and relative density, the pore pressure ratio reduced; whereas an increase in pore pressure ratio was witnessed with an increase in shear strain. The influence of cement inclusion on the cyclic response of the specimens is evaluated for varying cement contents (1–4%). The addition of 4% cement considerably enhanced the strength of sand and arrested the liquefaction instigation.

Numerically liquefaction analysis of tunnel‐sand pile interaction (TSPI) under seismic excitation

AbstractLiquefaction is a vital issue for the tunnel‐sand pile interaction (TSPI) system under seismic excitation. Therefore the present study proposed a TSPI model for the liquefaction analysis numerically using the numerical code of Plaxis 3D under local seismic excitation. Validation results of the numerical code inform a convincing level of accuracy to ensure the analysis of the TSPI model using this code. In this research, the geometry of the TSPI model is considered smaller compared to the tunnel diameter to influence the probability of the liquefaction occurrence, and the only variable geometric dimension is the tunnel pile clear distance of the interaction zone while other geometric and material properties are considered to be the standard value. UBC3D‐PLM (two yield surfaces consisting of kinematic hardening rules) constitutive model is used to ensure the liquefaction effect in the TSPI model; however, the excess pore pressure ratio exceeds the limiting value of 1 to inform the existence of liquefaction in the TSPI model. In addition, the uplift of sand is exceeded from the settlement due to liquefaction. However, further investigations such as experimental and analytical studies may have to be performed in the future to enhance the present study.

One-Dimensional Consolidation Test with Pore Pressure Measurements — An Accelerated Procedure

Abstract Consolidation properties of cohesive soils are often determined using the controlled-strain loading (CSL) consolidation test. It was well recognized in the literature that the consolidation properties, such as compression index (Cc), coefficient of consolidation (cv), and preconsolidation pressure (σc′), depend on the axial strain rate adopted for conducting the CSL test. Fixing the appropriate axial strain rate is often a challenge in CSL tests. The conventional incremental load (IL) consolidation test suffers from the limitation that it takes long time to complete the test. In the present study, an accelerated IL consolidation testing procedure with pore water pressure measurements is developed in an attempt to overcome the limitations of the CSL and conventional IL consolidation tests. The duration of the IL is controlled such that the subsequent IL is applied once the excess pore pressure dissipates to 15 % of the total applied stress (pore pressure ratio, Ru, is 0.15). The testing procedure is validated by performing a series of experiments on six reconstituted and four intact soil samples. The consolidation parameters obtained from the proposed method compares very well with conventional IL test as well as with the CSL consolidation test, suggesting its validity. The proposed method is about 3 times faster when compared with the standard CSL test. Therefore, the proposed method is a viable alternative to the CSL consolidation test.

Multidirectional Vibroseis Shaking and Controlled Blasting to Determine the Dynamic In Situ Response of a Low-Plasticity Silt Deposit

In this paper, efforts to characterize and compare the full-scale in situ three-dimensional (3D) dynamic response of a low-plasticity silt deposit to multidirectional loading from two different sources, a vibroseis shaker named T-Rex and controlled blasting, are presented. Horizontal vibroseis shaking at a frequency, f, of 10 Hz, produced dynamic responses in the silt that ranged from linear-elastic to nonlinear-inelastic, inducing maximum equivalent direct simple shear (DSS) shear strains, γDSS,max, up to 0.15% and residual excess pore pressure ratios, ru,r, of 14.1%. Blast-induced shear waves with predominant frequencies ranging from 9.6 to 14.6 Hz excited nonlinear-elastic and nonlinear-inelastic responses in the silt deposit, with γDSS,max of 1.14% and maximum ru,r of 61%. Importantly, these responses were observed to be minimally influenced by high frequency compression waves. Multidirectional loading, and excess pore pressure, ue, migration and impedance were identified as the predominant factors for achieving the large ru,r in the silt deposit from these two in situ testing techniques. The cyclic threshold shear strain, γtp, to trigger ru,r observed from the T-Rex shaking equaled 0.007% to 0.011% and varied with the initial soil stiffness. The two testing techniques demonstrated that the in-situ shear modulus, G, reduced to 90% of the maximum shear modulus, Gmax, at γDSS,max≈γtp, whereas by γDSS,max≈1%, G further reduced to 10 to 30% of Gmax corresponding to ru,r of ∼60%. Changes in soil fabric were quantified using small-strain shear-wave velocity measurements performed before and/or upon initiation and after each stage of dynamic testing, and were linked to the observed increase and decrease in γtp for the shallower and deeper 3D elements, respectively, following T-Rex shaking. The side-by-side comparison of the dynamic responses and soil properties derived from these two distinctly different field-testing techniques validate the use of controlled blasting for quantifying in situ dynamic soil properties and responses.

Sedimentary facies characterization through CPTU profiles: An effective tool for subsurface investigation of modern alluvial and coastal plains

ABSTRACTCone penetration tests, a method that is typically used to determine the engineering properties of soils, can be used as an effective tool for refined subsurface stratigraphic investigations of alluvial and coastal plains, aside from the geographic location. High‐resolution calibration of piezocone penetration tests (CPTU) with 20 sediment cores enabled the detailed characterization of alluvial, deltaic and coastal depositional systems of the Po Plain. Twelve cored facies associations, typical of alluvial and coastal plain environments, were characterized based on four distinct CPTU profiles: basic cone resistance (Qc), sleeve friction (Fs), water pore pressure (U) and friction ratio (FR). Sandy facies associations (fluvial/distributary channel, bay‐head delta, transgressive barrier, delta‐front/beach‐ridge) typically have high (>4 MPa) Qc, low‐to‐negative U and low (<2%) FR. Muddy deposits (well‐drained/poorly‐drained floodplain, swamp, lagoon and prodelta) exhibit opposite trends. Heterolithic facies associations (crevasse‐levée, offshore/delta‐front transition) display characteristic seesaw profiles. Plotting of late Quaternary deposits onto the latest version of the cone penetration test typical soil behaviour chart (Robertson, 2010) enables the identification of distinctive facies associations reflecting distinctive grain size. CPTU interpretation leads to sedimentary facies recognition well beyond the simple lithological differentiation and, in particular, allows the refined characterization of clay‐rich and silt‐rich depositional units (swamp clays and peats, central‐inner and outer lagoon, proximal/distal prodelta deposits) that exhibit only subtle differences in lithology. CPTU data can also serve for the accurate detection of key stratigraphic surfaces with potential engineering applications, such as the Pleistocene–Holocene boundary. This latter, a common feature of several alluvial and coastal plain successions, is commonly marked by an abrupt upward decrease of basic cone resistance and sleeve friction from Late Pleistocene, pedogenized, stiff strata to overlying Holocene, organic‐rich, soft deposits. This study offers an updated CPTU‐facies characterization method that could be suitable for subsurface investigations of modern alluvial and coastal plains worldwide.

Stability risk assessment of slopes using logistic model tree based on updated case histories.

A new logistic model tree (LMT) model is developed to predict slope stability status based on an updated database including 627 slope stability cases with input parameters of unit weight, cohesion, angle of internal friction, slope angle, slope height and pore pressure ratio. The performance of the LMT model was assessed using statistical metrics, including accuracy (Acc), Matthews correlation coefficient (Mcc), area under the receiver operating characteristic curve (AUC) and F-score. The analysis of the Acc together with Mcc, AUC and F-score values for the slope stability suggests that the proposed LMT achieved better prediction results (Acc = 85.6%, Mcc = 0.713, AUC = 0.907, F-score for stable state = 0.967 and F-score for failed state = 0.923) as compared to other methods previously employed in the literature. Two case studies with ten slope stability events were used to verify the proposed LMT. It was found that the prediction results are completely consistent with the actual situation at the site. Finally, risk analysis was carried out, and the result also agrees with the actual conditions. Such probability results can be incorporated into risk analysis with the corresponding failure cost assessment later.

Effect of coefficient of uniformity on cyclic liquefaction resistance of granular materials

Using three-dimensional discrete element method, we analyze the particle size distribution (PSD) effect on the cyclic liquefaction resistance of spherical particle assemblies. For the same mean particle size and log-linear type PSD, the coefficient of uniformity (Cu) is chosen as a descriptor of the PSD. Samples with five levels of Cu are isotropically compressed to the same pressure and two relative densities (Dr) informed by the maximum and minimum achieved void ratios determined for each Cu. The ten samples are subjected to constant volume cyclic simple shearing at different cyclic stress ratios until reaching initial liquefaction, in 56 simulations. The simulations suggest that at each Dr the evolution pattern of excess pore pressure ratio against the number of loading cycles normalized by the number of cycles to liquefaction is minimally affected by the Cu. For the samples with lower Dr, increasing the Cu in the range 1–3 first increases and then decreases the liquefaction resistance; this trend reverses at the higher Dr. Two critical state parameters based on the void ratio and the coordination number at the pre-shearing state of the samples correlate well with the cyclic liquefaction resistance for the ranges of Cu and Dr considered in this study.

Experimental and Empirical Studies to Evaluate Liquefaction Resistance of Partially Saturated Sands

Partially saturated sands with occluded gas bubbles may occur naturally in the field or can be created artificially as a way of mitigating liquefaction effects. This study aimed to investigate the main parameters that influence the liquefaction resistance of partially saturated sands containing occluded gas bubbles. It also adopted a simplified approach and proposed empirical relationships that linked the liquefaction-resistance ratio to the bulk modulus, maximum volumetric strain, and excess pore-pressure ratio. Considering these objectives, a total of 60 stress-controlled dynamic triaxial tests were conducted. Nearly/fully saturated and partially saturated sand specimens prepared using sodium percarbonate were subjected to undrained cyclic loading with a loading frequency of 0.1 Hz. The loosely or densely packed sand specimens at different degrees of saturation (79–100%) were examined under different initial effective confining stresses (25–100 kPa) and back pressures (100–400 kPa). The analysis of the test results indicated that the liquefaction resistance of the partially saturated sands constantly increased as the degree of saturation decreased, and this effect was more prominent under higher initial effective confining stresses and lower back pressures. The adopted method satisfactorily captured this trend and showed reasonable matches between the empirical predictions and experimentally observed results from this study and previous research.