Excess Pore Pressure Ratio Research Articles

Effect of soil permeability on soil–structure and structure–soil–structure interaction of low-rise structures

Earthquake-induced soil liquefaction can generate excessive damage to building structures due to significant reduction in soil effective stress. With increasing urbanisation and population growth, the performance of closely spaced buildings, such as those in towns and cities, is of greater concern where structure–soil–structure interaction (SSSI) may occur in conjunction with full or partial liquefaction. This study investigates the seismic performance of isolated and adjacent structures built on shallow foundations on soils of different permeability (i.e. with different amounts of drainage and therefore generating different amounts of excess pore water pressure) using a combination of dynamic centrifuge modelling and finite-element modelling. The results demonstrate that the reduction in free-field ground surface intensity measure (specifically, Housner intensity) due to increasing liquefaction can be correlated to an index which is the normalised integral of excess pore pressure ratio (ru) with depth. This index can express the amount of liquefaction uniquely, even when full liquefaction occurs to only partial depth, or where excess pore water pressures are increased but below the level of full liquefaction at all depths. This underlying correlation means that structural demand reduction (e.g. reduction in inter-storey drift ratio) with liquefaction (SSI effect) can be linked to either: (a) the ru–depth index; (b) the depth of the liquefaction front, which can be estimated from a liquefaction triggering analysis; or (c) the ground surface intensity amplification/attenuation in the free field from the results of one-dimensional free-field soil column analyses. Where there are adjacent structures, a strongly beneficial SSSI effect on co-seismic settlement and a detrimental effect on drift in non-liquefied soil, as observed in this and previous studies, reduced towards a null effect in both cases with increased liquefaction, with liquefaction appearing to isolate the structures from each other (at least for the configuration considered herein). This has also been linked to the amount of amplification/attenuation of surface ground motion in the free field (or amount of liquefaction) as a simple indicator of the likely importance of SSSI effects in liquefiable soil.

Liquefaction potential along with pore water pressure generation of coastal sand of Digha in West Bengal, India

Stress controlled cyclic triaxial tests have been done on coastal sand of Digha, West Bengal, India, at different frequencies, confining pressures, and relative densities and cyclic stress ratios. Number of cycles for initial liquefaction (NL) has been determined for that number of cycle when excess pore pressure ratio has become equal to 1. Significant influences of density of sand, confining pressure and number of cycles for initial liquefaction on coastal Digha sand have been found. The test results have shown that increasing density of sand increases liquefaction potential, whereas cyclic strength of sand decreases with increase of confining pressure. An empirical correlation has been developed on cyclic strength of sand based on these parameters and this correlation fits quite well with the observed experimental results. Bender Element tests have been performed to determine maximum shear modulus (Gmax) of Digha sand at different densities and confining pressures. A high correlation coefficient between cyclic strength and Gmax of Digha sand at any NL has been found. A new pore water pressure generation model has been introduced for this sand along with upper bound and lower bound curves to predict excess pore water pressure build up due to seismic loads.

Methodologies Available for the Determination of Seismic Active Thrust Acting on Retaining Walls: A Critical Review

Design of retaining walls (RWs) in earthquake (EQ)-prone areas requires the knowledge of the distribution of lateral earth pressure behind it. The lateral earth pressure acting on RW in case of dynamic/ seismic loading comprises of two components: (a) earth pressure due to static loading and (b) dynamic incremental pressure due to seismic forces. Pseudo-static and pseudo-dynamic methods are mostly preferred methods used to estimate the seismic earth pressure acting on the RW. Analysis based on pseudo-static approach (PSA) assumes seismic forces as equivalent constant inertial force acting on the wall, whereas pseudo-dynamic approach (PDA) includes the effect of phase change and dynamic amplification of seismic waves. This state-of-the-art paper presents a systematic review on the methodologies available for the determination of seismic active thrust, which are based on PSA and PDA. In addition, several other methods, e.g. methods involving numerical techniques, methods based on arching effect, etc., are also reviewed in this paper. While doing so, various limitations of above-stated approaches are pointed out. Further, it is found that there is scarcity of any rational method for the determination of seismic coefficients used in the analysis based on PSA. In addition, the effect of damping characteristics and excess pore pressure ratio, on seismic active thrust, is attempted in very limited studies.

Shaking table tests on the seismic responses of underground structures in coral sand

To reveal the seismic response of underground structures in coral sand, a series of shaking table model tests on underground structures at four different groundwater levels were carried out. The seismic responses of the coral sand and the underground structure were obtained and analyzed. The results showed that the excess pore pressure ratio and the liquefaction degree of coral sand increased with increasing groundwater level, resulting in a more significant separation between the sidewall and the surrounding soil. However, the acceleration amplification factor of the coral sand site, the dynamic strain and bending moment of the underground structure could be reduced. Moreover, the increase in the groundwater level contributed to the settlement of the ground surface. However, due to the presence of an uplift force caused by the liquefaction of the coral sand, the settlement of the underground structure decreased as the groundwater level increased. Furthermore, the distribution of the dynamic soil pressure under the groundwater level was found to be quite different from that under dry soil condition.

The Effect of Lightweight Expanded Clay Aggregate on the Mitigation of Liquefaction in Shaking Table

Non-conventional materials are often applied to dissipate the energy of dynamic loads and mitigate liquefaction. In this study, the effect of Lightweight Expanded Clay Aggregate (LECA) on the mitigation of liquefaction was examined using shaking table tests. LECA, a common lightweight material, was added to saturated sand in a range of 0–20% to study the variation of excess pore water pressure (EPWP) generation, as well as the acceleration response of the soil. Based on the obtained results, the addition of 5% and 10% LECA to saturated sand resulted in a significant decrease in the generation of EPWP, while higher LECA content led to an increase in the generation of EPWP. The mechanisms behind such behaviors contradict the effects of increasing the damping of the soil mixture and reducing the effective initial stress as a function of overall mixture density. Adding 5 and 10% LECA reduces EPWP because aggregates accelerate the dissipation of EPWP during and after vibration. As the percentage of LECA increases by 15% and 20%, excess pore pressure ratio increases. The cause of this may be the reduction of effective initial stress leading to reduced soil density in higher-LECA contents. 10% LECA is presented as the threshold for changing the liquefaction behavior of sand-LECA mixtures. This study concludes that adding LECA to sand samples increases the rate of dissipation of EPWP, but also leads to a significant decrease in initial effective stress.

Cyclic Response of Loose Anisotropically Consolidated Calcareous Sand under Progressive Wave–Induced Elliptical Stress Paths

Observations of the performance of coastal and marine structures in weather events that produce standing and/or progressive waves have pointed to liquefaction of the seabed as a contributing cause of damage. The cyclic response of marine sediments under wave loading can serve to improve the understanding of seabed liquefaction triggering and its consequences. This paper presents the results of an experimental study of the cyclic behavior of loose isotropically and anisotropically consolidated calcareous sand under elliptical stress paths representing progressive waves. Various ratios, λ, of the axial stress difference and torsional shear stress amplitudes were investigated in a hollow-cylinder torsional shear apparatus. The results indicate that the failure mode of isotropically and anisotropically consolidated (IC and AC, respectively) specimens are cyclic mobility and residual deformation failure, respectively. The difference in the observed failure mode appears to be related to particle shape and is independent of λ. However, the cyclic resistance of the calcareous sand is strongly influenced by λ and increases with decreasing consolidation stress ratio, Kc. The cyclic resistance of IC specimens is relatively independent of the orientation of elliptical stress paths, whereas the AC specimens exhibited a marked dependence. The terminal peak excess pore pressure ratio, ru,t, and shear work required for cyclic failure, Wf, are independent of λ; however, ru,t and Wf increases with increasing and decreasing Kc, respectively. Moreover, the relationship between the normalized peak excess pore pressure ratio and normalized shear work is related to the type of soil, but is independent of λ and Kc.

Experimental study on the liquefaction process of iron concentrate on swing table

Liquefaction of the bulk particle cargoes is a serious hidden risk for the ore transportation ship. To further understand the liquefaction mechanism of the particle cargoes, the model experiments have been carried out, by adopting saturated iron concentrate samples and the six degrees of freedom swing table. The effect of external load frequency on pore water pressure and excess pore pressure ratio was investigated. Results show that the external load is the main factor causing liquefaction of the cargo, and the possibility of liquefaction of different parts of the whole cargo is different. The bottom part of the cargo changes the most under different external loads, while the top does not.

Caissons in Cohesionless Soils Considering 3D Wedge under Earthquake Loading

Ultimate soil resistance that acts on a seismically induced caisson foundation and its distribution along the caisson height have been formulated by considering passive wedge failure analysis for both dry and submerged conditions. The method for limiting the equilibrium of forces acting on a three-dimensional idealized passive wedge developed due to seismicity has been adapted in this study. Using this process, seismic inertia forces will be computed by considering the surrounding soil as a viscoelastic material that overlays a rigid stratum subjected to a harmonic shaking. For a very large value of caisson width, the comparison of this analysis with a 2D seismic earth pressure problem illustrates the compatibility of this methodology in the plane strain condition. Variation in earth pressure coefficients and their distribution for different seismic acceleration coefficients, soil properties, excess pore pressure ratios, and caisson aspect ratios will be discussed in this analysis. The reasonable agreement of the distribution of total passive pressure along the wedge height under the submerged condition with the experimental results shows the applicability of the proposed formulations as a design approach for caissons in sandy soil.

Numerical simulations of LEAP centrifuge tests for seismic response of liquefiable sloping ground

This paper presents numerical simulations of a liquefiable sloping ground related to LEAP-UCD-2017 and LEAP-Asia-2019 (Liquefaction Experiments and Analysis Projects) dynamic centrifuge model tests (Type-C phase) conducted by various institutions. The numerical simulations are performed using a pressure-dependent constitutive model implemented with the characteristics of dilatancy, cyclic mobility and associated shear deformation. The soil parameters are determined based on a series of available stress-controlled cyclic triaxial and torsional shear tests for matching the liquefaction strength curves of Ottawa F-65 sand with relative densities Dr. = 65% and 60% in calibration phase of LEAP-UCD-2017 and LEAP-Asia-2019, respectively. The computational framework for the dynamic response analysis is discussed and the computed results are presented for the selected centrifuge experiments during Type-C phase. Measured time histories (e.g., displacement, acceleration and excess pore pressure ratio) of these experiments are reasonably captured. Comparisons between the numerical simulations and measured results showed that the pressure-dependent constitutive model as well as the overall employed computational framework have the potential to predict the response of the liquefiable sloping ground, and subsequently realistically evaluate the performance of an equivalent soil system subjected to seismically-induced liquefaction.

Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model

The practical constitutive model UBCSAND2, which combines two-mobilized planes—a maximum shear stress plane and a horizontal plane within a framework of classical plasticity approach—is used to incorporate shear-induced effects in both loading and unloading as well as principal stress rotation effects. UBCSAND2 was calibrated by capturing cyclic direct simple shear (CDSS) test results of Pohang sand, which was collected from liquefied paddy fields due to the 2017 Pohang earthquake (Mw = 5.4) in South Korea. The model procedure focuses on simple shear condition because it best simulates field conditions under earthquake loading. The calibrated UBCSAND2 model is then used to assess the liquefaction-induced damages that occurred at the quay wall and backfill layer in Youngil-man port near the epicenter of the Pohang earthquake. The numerical results show that liquefaction mostly occurred in silty sand layers, in which the excess pore pressure ratio reached almost one. The estimated displacements of the quay wall and the predicted settlement of reclaimed area obtained from the analysis were in good agreement with those obtained from field measurements.

Effect of air injection on pile and pile group behavior in liquefiable soil

Soil liquefaction during large earthquakes causes serious damage to structures built on foundations. In recent years, artificially injecting small amounts of air into liquefaction-susceptible soils has been proposed as a newly developed and cost-effective liquefaction mitigation method. Nevertheless, the liquefaction response of piles through this method has not been investigated so far. A series of 1g shaking table tests were conducted in this research to evaluate the liquefaction response of pile and pile group in air-injected partially saturated soils. The results have shown that air injection effectively reduces the liquefaction-induced settlements of pile and pile group. Also, this method has relatively reduced the excess pore pressure ratio. According to visual results and bending moments in piles, the depth of a liquefied soil layer decreases in air-injected partially saturated soils. Also, the deformation of soil structure is observed to decrease by injecting air into the samples. Despite these effects, larger accelerations are transmitted to the surface of the soil and consequently to the structure, and larger bending moments are applied to the piles in air-injected partially saturated samples. It was observed that at the low confining pressure, the effectiveness of this method is not a function of air injection duration.

Post-cyclic undrained response of sand and silt

Cyclic loading of saturated soils is often accompanied by pore pressure accumulation, the magnitude of which, has a tendency to significantly affect the post-cyclic characteristics. This study discusses the post-cyclic behaviour of fine sand and low plastic silt in laboratory testing. Different levels of excess pore pressure ratios were first allowed to develop in the samples during undrained cyclic loading. These samples were then permitted to drain and reach a new equilibrium stress state, before subjecting them to undrained monotonic loading. These findings show that post-cyclic recompression disturbed the soil's structure and changed the gradient of its original unloading-reloading line. The effect of this disturbance was greater in samples which were previously close to liquefaction. The strength improvement in sand grains was less compared to the improvement observed in silt-sized grains. The results nonetheless imply that the post-cyclic recompression strength can be related to the excess pore pressure developed prior, during the cyclic loading.

Vertical bearing capacity of a pile-liquefiable sandy soil foundation under horizontal seismic force.

In this study, research on a pile group system was conducted using shaking table tests under four working conditions: a natural foundation and pile-spacing conditions of 3D, 3.5D, and 4D (D is the diameter of the pile). The time histories of the excess pore pressure ratio and settlement were analyzed. It was determined that pile foundations improved the anti-liquefaction performance of the soil, and the effect was much greater when the pile spacing was 3D. In addition, the settlement dynamic amplification factor (SDAF) was proposed and calculated at different vibration times. The result was fitted with a linear relationship, and the correlation coefficient was relatively high. During the aseismic design of the pile foundation bearing capacity, the SDAF was multiplied by the static load, and the results showed that in the dynamic design of the pile foundation, the dynamic problem can be transformed into a static analysis, which can provide a reference for the design of the vertical bearing capacity of pile foundations under seismic force.

Comparative Study on Seismic Response of Pile Group Foundation in Coral Sand and Fujian Sand

The physical and mechanical properties of coral sand are quite different from those of common terrestrial sands due to the special marine biogenesis. Shaking table tests of three-story structures with nine-pile foundation in coral sand and Fujian sand were carried out in order to study the dynamic response characteristics of pile-soil-structure system in coral sand under earthquake. The influence of shaking intensity on the dynamic response of the system was taken into consideration. The results indicated that the peak value of the excess pore pressure ratio of coral sand was smaller than that of Fujian sand under two kinds of shaking intensities; moreover, the development speed of excess pore pressure ratio of coral sand was smaller than that of Fujian sand. The liquefaction of coral sand was more difficult than Fujian sand under the same relative density and similar grain-size distribution. The horizontal displacement, settlement, column bending moment, and pile bending moment of coral sand were smaller than those of Fujian sand, respectively. The magnification effect of column bending moment of buildings in coral sand was less than that in Fujian sand with increasing shaking intensity. This study can provide some supports for the seismic design of coral reef projects.

Evaluating the sealing potential of young and thin mass-transport deposits: Lake Villarrica, Chile

Abstract Subaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c . 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

A semi-empirical model to predict excess pore pressure generation in partially saturated sand

Past studies revealed that excess pore pressure generation due to cyclic loading is highly governed by induced strains, volumetric deformation potential of soil, number of cycles, and bulk stiffness of pore fluid. It is well established that partial saturation can significantly reduce bulk stiffness of pore fluid and consequently excess pore pressure generation during seismic loading. On the basis of that, a number of researchers have investigated induced partial saturation as an effective soil improvement technique to increase the liquefaction resistance of fully saturated soils. This paper focuses on development of a semi- empirical model to interpret the effects of partial saturation on the excess pore pressure generation in sands. In this regard, an existing strain based excess pore pressure ratio (ru) prediction model originally developed for fully saturated soils was modified to incorporate the effect of partial saturation on the excess pore pressure generation. The literature data as well as data from a series of strain-controlled direct simple shear test were used to evaluate the reliability of the proposed equation in predicting the excess pore pressure ratio in partial saturation condition.

Seismic Performance of Soil-Ash and Soil-Ash-Foundation System

In this study, a 3D seismic response of soil deposit, soil-ash deposit and soil-ash-foundation system was investigated. Homogeneous sand deposit of 80m × 9m × 20m was initially analyzed. A pond ash layer is on top of the sand deposit with varying thicknesses and the efficiency of the pond ash layer on the sand deposit was evaluated for its best suitability. The optimum sand-ash deposit overlain by a shallow foundation has been analysed under the excitation of the Nepal (Mw:7.8) and North East India earthquake (Mw:7.5). A seismic response analysis was performed using finite element software PLAXIS3D. The finite element model adopted for the present study has been validated using 1D nonlinear ground response analysis programs. e.g. DEEPSOIL and Cyclic1D. Results of the response analysis have been determined in terms of acceleration, displacement, excess pore pressure, and excess pore pressure ratio. It was observed that, the sand-pond ash-foundation system experienced liquefaction when excited under the Nepal earthquake motion whereas it is safe against the North East India earthquake.

Predicting blast-induced liquefaction within the New Madrid Seismic Zone

The Turrell Arkansas Testing Site (TATS) is located in Northeastern Arkansas, United States. The site consists of approximately six meters of overconsolidated clay underlain by loose liquefiable sand deposits. Numerous traditional (rotary-wash boring with sample collection) and advanced (seismic piezocone soundings) geotechnical investigations have been performed at this site. A controlled-blasting testing program was also performed at the TATS to determine the blasting layout (the appropriate amount of explosive charges, the detonation delays, and the charge spacing), and to verify induced liquefaction of the soil deposit at a testing site located within the New Madrid Seismic Zone (NMSZ). The results obtained from the installed transducers and the pre- and postblast cone penetration tests (CPT) are discussed. Although, the CPT were performed when the excess porewater pressures were dissipated, a review of CPT profiles after blasting showed no evidence of increase of cone tip resistance and sleeve friction. Due to the small amount of explosive charge weight that was used, the excess porewater pressure ratio values only increased above the unity at the depth of 11.30 m. A review of the existing empirical models used to predict blast-induced porewater pressure responses and liquefaction is presented. A new empirical model that accounts for the in-situ soil properties, to estimate the excess pore pressure ratio, was developed and is presented herein.

Study of mechanism and theoretical model of seabed destruction caused by gas hydrate dissociation

Gas hydrate decomposition is an important cause of submarine geological hazards. Understanding the mechanism of overlying seabed failure caused by gas hydrate decomposition is important to engineering practice and geological hazard prediction. In this paper, an observational device is designed to simulate the process of seabed failure caused by gas hydrate decomposition and the evolutionary process of seabed failure under different seabed thickness and slope conditions is compared and analysed. The mechanism of seabed damage is also discussed. In addition, based on the seabed failure mechanism, a computational model for the influence of hydrate dissociation on seabed stability is established. It is found that in shallow or steeper strata, the seabed will be destroyed by shear slip; however, when the stratum is thicker or the slope is gentle, the seabed surface first collapses, then is deformed and destroyed. The excess pore pressure ratio of the seabed in critical failure state is only related to the seabed gradient. The larger the gradient, the smaller the critical excess pore pressure ratio.

Performance evaluation of a MICP-treated calcareous sandy foundation using shake table tests

Calcareous sandy foundations are susceptible to liquefaction when subjected to dynamic loading such as seismic or wave loading, which could cause severe damage to the superstructure and incur significant financial losses. Therefore, reducing the liquefaction potential of calcareous sand is of considerable significance. Microbially induced calcite precipitation (MICP) treatment is a relatively new method to improve the liquefaction resistance of calcareous sand, and it has the advantages of incurring small disturbances, low grouting pressure, less environmental harm, long transmission distance and a remarkable effect pertaining to the improvement of the engineering properties of the soil. However, few studies have evaluated the application of MICP treatment to calcareous sandy foundations. In this study, several shake table tests were conducted to evaluate the seismic performance of MICP-treated calcareous sandy foundations. Special attention was paid to the shear wave velocities, natural frequency and shear strength of the calcareous sand during MICP treatment. Additionally, the horizontal accelerations, pore water pressures and vertical settlements of calcareous sand before and after the treatment were compared. After MICP treatment, the shear wave velocity, natural frequency and shear strength of soil were improved, which indicated that the strength and stiffness of the model were considerably improved. During the shaking, the excess pore pressure ratio and surface settlement of MICP-treated model were significantly decreased. The liquefaction resistance of the MICP-treated model demonstrated greatly improved. However, the surface accelerations of the model strengthened by MICP were amplified. The test results indicate that use of the MICP method can significantly improve the strength, stiffness and liquefaction resistance of calcareous sand.