Low Effective Stress Research Articles

Influence of grain boundary structure on dissolution controlled pressure solution and retarding effects of grain boundary healing

It is widely accepted that the structure of a grain boundary undergoing pressure solution can have a strong influence on the rates at which diffusive transport in the grain boundary occurs. However, the influence of grain boundary structure on internal grain boundary dissolution rates has received little attention, despite evidence that dissolution controlled pressure solution in quartz is slower than expected assuming dissolution kinetics appropriate for free surfaces. In this paper, three hypothetical steady state grain boundary structures are defined and the influence of these structures on dissolution controlled pressure solution rates in an elastic solid are considered by deriving simple models based on internal grain boundary mass and energy balances. It is found that average dissolution rates in a rough grain boundary (island‐channel network) are slowed down by up to 13% compared to dissolution in a flat grain boundary containing a thin fluid film. This can only partly account for the discrepancy between models and experiments reported in the literature. In addition a model is derived providing a criterion or “yield stress” for pressure solution, below which the process is prevented by surface energy driven grain boundary healing (progressive reduction of the contact area filled by connected fluid). This “yield stress criterion” for pressure solution offers a further explanation for reduced rates or cessation of pressure solution at low effective stresses in nature and experiment. Using this criterion, limiting porosity depth curves are predicted for sandstones compacting by pressure solution, which show favorable agreement with porosity‐depth data for quartz sandstones.

State-dependent constitutive model and numerical solution of self-weight consolidation

A new constitutive model is developed for the compressibility behaviour of soft clay sediments at low effective stress level. Central to this model is the realisation that at low effective stress levels the void ratio–effective stress paths of soil elements are non-unique, and dependent very much on their state. Beyond a threshold stress level, these relationships become unique. These observations have been used to develop a state-dependent constitutive model for the compressibility behaviour of soft sediments. Methods to obtain the input parameters of the model are discussed. The proposed model is used to solve finite-strain one-dimensional consolidation with pertinent initial and boundary conditions. The analyses performed using a finite difference computer code predict better the observed phenomena associated with self-weight consolidation, especially during its early stages. The comparison of the predictions shows that the present model can capture the decrease in void ratio patterns, as observed in experiments at low effective stresses, and which cannot be modelled using existing constitutive relationships.

Rheology modification in mixed shape colloidal dispersions. Part II: mixtures.

We report the results of a comprehensive study of the rheological properties of a series of mixed colloid systems where the shape of one of the components has been varied systematically. Specifically we have measured the oscillatory, transient (creep) and continuous steady shear flow behaviour of a 2.5 wt% dispersion in water of a well-characterised hectorite clay modified by the addition of a series of aluminasol colloidal particles whose shape varies systematically from rod (boehmite) to platelet (gibbsite) to sphere (alumina-coated silica), all having essentially the same smallest dimension, which is similar to that of the hectorite. The particle characterisation and rheological properties of the pure components have recently been reported in Part I of this series (Soft Matter, 2007, 3, 1145). The mixtures show the same general behaviour as the pure systems, displaying a complex 'yield space' transition from an elastoviscous gel at low applied stresses to a viscous, weakly elastic, shear-thinning liquid at high stresses. The unifying theme of this work is that the addition of 0.25 wt% of the minor component in all cases results in dramatic enhancements to the dispersion rheological properties. At the same time the magnitude of this effect depends on the shape of the particles. Shear moduli, low stress viscosities and effective yield stresses all increase in the additive order rods < platelets < spheres, with enhancements for the latter being up to a factor of 500 and typically 20. At the same time the critical failure strains for the gels decreased in the same order - the strongest gels are also the most fragile in this sense. The physicochemical factors underlying this behaviour are discussed and a simple qualitative model described. While no complete explanation or model can be proposed at this stage, the study provides a quantitative model-system baseline for mixed colloidal dispersions already used for industrial applications (e.g. oilwell-drilling fluids) and suggests ways in which such fluids may be optimised and controlled.

Numerical Modeling of Cemented Mine Backfill Deposition

In current underground mining practice, it is common to use tailings, without added cement, to fill mined-out voids (“stopes”). If fine-grained tailings are used, the high placement rates and low permeability can often result in undrained loading conditions and, hence, lower effective stress, when assessed in the conventional manner. Where cement is added, the cement modifies the consolidation characteristics in a number of ways, including increasing the strength and stiffness, reducing the permeability, and inducing volumetric changes associated with the hydration reactions leading to “self-desiccation.” As a result, conventional consolidation-analysis techniques are unsuitable for assessing the behavior. The one-dimensional mine-tailings-consolidation program (MinTaCo) has been modified, and renamed CeMinTaCo, to couple cement hydration with conventional consolidation analysis. The fundamental theory behind the modifications is presented. The model is used to undertake a sensitivity study, which highlig...

Evolution of boudins under progressively decreasing pore pressure A case study of pegmatites enclosed in marble deforming at high grade metamorphic conditions, Naxos, Greece

During ductile deformation of marbles under high grade metamorphic conditions on Naxos, Greece, pegmatites enclosed in the marbles were deformed in a brittle fashion forming blocky boudins with quartz crystallized in the interboudin zones. We studied the three-dimensional geometry of the boudins in the field and in two large samples. The first sample was serially sectioned to observe the structural and microstructural evolution; in the second sample we mapped the surface morphology of the deformed pegmatite. In profile, the boudins can be classified as symmetric torn type boudins, evolving towards asymmetric boudins with a later domino boudin component. In three dimensions, however, the morphology of the boudinaged pegma- tite is a simple set of normal faults with mode-I fractures in the fault tips and rotation of the fault blocks to accommodate the extension in the marble. Deformation history was constrained by petrology and microstructures in combi- nation with simple order of magnitude calculations of both the cooling and pore fluid pressure evolution. The dynamically recrystallized, coarse-grained calcite of the marble provides clear evidence that after the pegmatite intruded the marble and solidified, it was deformed at peak conditions of M2b metamorphism (� 670 °C and � 0.6 GPa). Such pegmatitic melts contain � 10 percent H2O which is released during crystalliza- tion. We infer that after crystallization of the pegmatite the pore fluid pressure in the pegmatite remained close to lithostatic due to the very low permeability of the surrounding marble, and the pegmatite was deformed at very low effective stress which led to brittle deformation of feldspar and mode-I fracturing of the pegmatite, forming torn type boudins. With time the pore fluid pressure slowly decreased, increasing the effective stress. Ongoing N-S extension thus resulted in slip along the quartz-filled interboudin zones and in block rotation, producing domino boudins.

Residual Shear Strength for Interfaces between Pipelines and Clays at Low Effective Normal Stresses

The residual shear strength mobilized between pipelines and supporting soils at low effective normal stresses is needed for designing stable pipelines in offshore environments. A tilt table device is used to study the effect that effective normal stress, type of pipeline coating, composition of soil, stress history, and rate of loading have on the drained residual shear strength mobilized at the interface between a variety of clays and polymeric pipe coatings. The drained residual friction angles for both the interfaces and the clays decrease substantially as the effective normal stress increases. Empirical correlations published for predicting the residual strength of clays cannot be readily extrapolated to the pipeline problem because the correlations do not cover the relatively small effective normal stresses acting on pipelines. Residual shear strengths for the interfaces range from 60 to 90% of the residual shear strength for the clay. The residual shear strength for the interface depends both on the composition of the clay and the type of pipeline coating.

The effect of pore pressure depletion and injection cycles on ultrasonic velocity and quality factor in a quartz sandstone

Passing seismic waves generate transient pore-pressure changes that influence the velocity and attenuation characteristics of porous rocks. Compressional ultrasonic wave velocities [Formula: see text] and quality factors [Formula: see text] in a quartz sandstone were measured under cycled pore pressure and uniaxial strain conditions during a laboratory simulated injection and depletion process. The objectives were to study the influence of cyclical loading on the acoustic characteristics of a reservoir sandstone and to evaluate the potential to estimate pore-fluid pressure from acoustic measurements. The values of [Formula: see text] and [Formula: see text] were confirmed to increase with effective stress increase, but it was also observed that [Formula: see text] and [Formula: see text] increased with increasing pore pressure at constant effective stress. The effective stress coefficient [Formula: see text] was found to be less thanone and dependent on the pore pressure, confining stress, and load. At low pore pressures, [Formula: see text] approached one and reduced nonlinearly at high pore pressures. The change in [Formula: see text] and [Formula: see text] with respect to pore pressure was more pronounced at low versus high pore pressures. However, the [Formula: see text] variation with pore pressure followed a three-parameter exponential rise to a maximum limit whereas [Formula: see text] had no clear limit and followed a two-parameter exponential growth. Axial strain measurements during the pore-pressure depletion and injection cycles indicated progressive viscoelastic deformation in the rock. This resulted in an increased influence on [Formula: see text] and [Formula: see text] with increasing pore-pressure cycling. The value [Formula: see text] was more sensitive in responding to the loading cycle and changes in pore pressures than [Formula: see text]; thus, [Formula: see text] may be a better indicator for time-lapse reservoir monitoring than [Formula: see text]. However, under the experimental conditions, [Formula: see text] was unstable and difficult to measure at low effective stress.

The influence of structure on the behaviour of London Clay

An intensive investigation is described into the London Clay units from Heathrow Terminal 5. Intrinsic properties and composition were established, and relating the behaviour of intact and reconstituted samples allowed the effects of the clay's natural structure to be identified at all depths. Structure varied between units, but some general features emerged that have not been seen in other stiff clays. In particular, intact samples follow paths under isotropic or K0 compression that fail to provide well-defined gross yield points, or converge with the unit's intrinsic compression lines. Structure contributes to the enhanced shear strength of intact unfissured clay and affects the initial stiffness relationships. Some identifiable features of the intact units’ fabric correlated directly with their behaviour. Natural fissures within the clay, the most important mesofabric feature, had an important impact on shear strength and led to an unusual pattern of directional dependence, and particle orientation trends identified by scanning electron microscopy governed the strong elastic stiffness anisotropy. The potential for destructuration through swelling to low effective stresses was also studied and found significant for subsequent volumetric compression behaviour, but not for shearing. Hight et al. synthesise the data from this and companion papers presented by the authors, and discuss the practical consequences of the results obtained.

Velocity determination for pore-pressure prediction

Overpressured formations exhibit several of the following properties when compared with a normally pressured section at the same depth (Dutta, 2002): (1) higher porosities, (2) lower bulk densities, (3) lower effective stresses, (4) higher temperatures, (5) lower interval velocities, and (6) higher Poisson's ratios.

SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays

In assessing the performance of a deep HLW repository, the evolution of the excavated damaged zone with time is a key issue. In the framework of SELFRAC fracturing, self-sealing and self-healing processes of Opalinus and Boom clay were studied in laboratory and in situ experiments. Definitions for the terms excavation damaged zone (EDZ), excavation disturbed zone (EdZ), sealing and healing are presented. It is shown that sealing and partial healing occur and the consequences of the results for performance assessment of HLW disposal in argillaceous rocks are discussed. The results of several in situ experiments and observations at the HADES underground research facility are detailed. The origin and extent of excavation induced fractures are discussed and sealing and (partial) healing of these fractures is demonstrated. In the description of the hydraulic features of the EdZ, the anisotropic pore pressure distribution around HADES and its evolution with time are discussed. Pore pressure is influenced several tens of metres into the host rock and its evolution is influenced by the anisotropic in situ stress state and the anisotropic hydraulic conductivity of Boom clay. Around the connecting gallery, an increase of hydraulic conductivity is measured up to about 6–8 m into the host rock, outside this influenced zone values between 4 × 10 −12 m/s and 6 × 10 −12 m/s were obtained. The highest value measured (close to the gallery) was of the order of magnitude of 10 −11 m/s. The observed increase is caused by lower effective stress levels close to the gallery rather than by excavation induced fractures. Self-boring pressuremeter tests show that total stress is influenced up to 6–8 m into the host rock and material parameters such as undrained shear strength and shear modulus are influenced up to 2–3 m into the host rock. In situ seismic transmission measurements showed that the closure of a borehole influences the seismic parameters of the surrounding host rock: a decrease in seismic velocity is measured and higher frequencies disappear from the transmitted signals. After closure of the borehole, sealing of the damaged zone around it occurs, this is observed by the recovery of seismic velocity and the reappearance of higher frequencies. Fracture sealing is also demonstrated by seismic and hydraulic measurements on a reinstalled fractured clay core.

Fractured reservoirs: An analysis of coupled elastodynamic and permeability changes from pore-pressure variation

Equivalent-medium theories can describe the elastic compliance and fluid-permeability tensors of a layer containing closely spaced parallel fractures embedded in an isotropic background. We propose a relationship between effective stress (background or lithostatic stress minus pore pressure) and both permeability and elastic constants. This relationship uses an exponential-decay function that captures the expected asymptotic behavior, i.e., low effective stress gives high elastic compliance and high fluid permeability, while high effective stress gives low elastic compliance and low fluid permeability. The exponential-decay constants are estimated for physically realistic conditions. With relationships coupling pore pressure to permeability and elastic constants, we are able to couple hydromechanical and elastodynamic modeling codes. A specific coupled simulation is demonstrated where fluid injection in a fractured reservoir causes spatially and temporally varying changes in pore pressure, permeability, and elastic constants. These elastic constants are used in a 3D finite-difference code to demonstrate time-lapse seismic monitoring with different acquisition geometries. Changes in amplitude and traveltime are seen in surface seismic P-to-S reflections as a function of offset and azimuth, as well as in vertical seismic profile P-to-S reflections and in crosswell converted S-waves. These observed changes in the seismic response demonstrate seismic monitoring of fluid injection in the fractured reservoir.

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Reply

Sonic velocity, density, and resistivity log data from 60 wells on the Norwegian Shelf have been used to investigate velocity-depth trends in sedimentary rocks as a function of sediment composition, porosity, pore-pressure, burial-history, and compaction processes. A first-order linear velocity-depth trend line has been estimated from published velocity data. The data analyzed in this study, however, show significant variations from this trend line, indicating that no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling. Lower Tertiary smectitic sediments from the northern North Sea and Haltenbanken are characterized by relatively low velocities compared to the overlying Pliocene and Pleistocene sediments, causing a distinct velocity inversion. A significant velocity increase at a burial depth corresponding to 70–100C was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales. Overpressured Jurassic sediments from Haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference. The reduced velocities may be a direct response to lower effective stresses and, thus, reduced elastic compaction. Low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy. The high velocity/depth ratio of Barents Sea sediments (after correcting for Tertiary exhumation) is explained by the burial history of the area, the subsequent thermal exposure of the sediments over time, and thus, the amount of quartz cementation.

Shear strength of geomembrane–soil interface under unsaturated conditions

Interface shear tests on non-textured geomembrane–soil interfaces were performed using a modified direct shear apparatus installed with a miniature pore pressure transducer (PPT). Using the PPT, pore-water pressure changes in the vicinity of geomembrane–soil interface were measured during shear, thus making it possible to analyse test results in terms of effective stresses. When plotted in terms of total stresses, interface shear strength envelopes, consistent with those published in the literature, were obtained. However, the results were difficult to interpret using effective stresses, particularly those obtained at high normal stresses. It was found that at low normal stress values the shear strength of a geomembrane–soil interface in terms of effective stresses could be predicted using unsaturated soil mechanics concepts. At high normal stresses, it was found that the failure mechanism changed from soil particles sliding at the surface of the geomembrane to soil particles becoming embedded into the geomembrane and plowing trenches along the direction of shear. A plowing failure mechanism resulted in the mobilization of significantly higher shear strength at the geomembrane–soil interface. It was found that at higher placement water content for the soil, near saturated conditions prevail at the geomembrane–soil interface, resulting in lower effective stresses, a shallower plowing mechanism and lower values of mobilized interface shear strength.

Interpretation and modelling of deformation characteristics of a stiff North Sea clay

An investigation into the behaviour of North Sea glaciomarine clays was carried out in which triaxial tests were conducted on both natural and reconstituted samples to assess the effects of structure. Although the tests were conventional CID tests, high quality instrumentation was used. The tests were also technically difficult both because of the very long test durations and because some of the samples were swelled back to very low effective stresses so that the effect of swelling on the influence of structure on the soil behaviour could be assessed. A "Class A" prediction of the behaviour in these tests was carried out using the BRICK model. Although the model is not designed to account for the influence of structure, it was found that its effects could be simulated by allowing the soil to have artificially high overconsolidation ratios (OCRs) so that the high undrained shear strengths resulting from structure could be modelled. Making the simple assumption that the decay of stiffness could be scaled from that of London Clay, reasonable predictions of the behaviour were made. The discrepancies between the predictions and the measured behaviour became significant only at the lowest stresses, where structure dominates the behaviour of the natural soil.Key words: laboratory testing, small strain stiffness, stiff clays, structure of soil, numerical predictions, shear deformation properties.

プレート境界断層が海溝から地震発生深度にかけて発達する際の流体挙動

Fluid behavior during the evolution of the plate boundary fault (pbf) from a trench to seismogenic depths is the central problem when evaluating the relationship between fluids and seismicity in subduction zones. Ocean Drilling Program Legs 190 and 196 at the toe region of the Nankai accretionary margin reveal that fluid-filled dilatant fractures and underconsolidated underthrust sediments lead to an elevated fluid pressure in and below the pbf, respectively. The pbf with elevated fluid pressure extends down-dip to 35 km, resulting in the absence of seismic behavior at shallow depths and mechanical decoupling between accreted and underthrust sediments. Underconsolidated underthrust sediments are primarily caused by rapid tectonic loading compared to the rate of fluid escape in underthrust sediments and secondarily by a lowpermeability cap due to the compactively deformed pbf. Fluid-filled dilatant fractures represent the overconsolidate state within the pbf, which is caused by the generation of high fluid pressure after compactive deformations.The exhumed plate boundary rocks (i.e., tectonic melange) in the Shimanto accretionary complex indicate that the underthrust sediments became rocks due to dewatering, pressure solution, and other diagenetic reactions, thus acquiring elastic strength. The pbf in the upper part of the seismogenic depths was weak due to elevated fluid pressure, this facilitated the downward step of the pbf and the underplating of underthrust rocks. The pbf under low effective stress was unlikely to nucleate the instability; however the fluid-related repeated deformations, which probably reflect the seismic cycle in the subduction zone, could be recorded. The coseismic deformations were attributed to hydraulic implosion breccias, injection of ultracataclasite, and fluid inclusion stretching in the pbf. Implosion breccias suggest rapid depressurization associated with the passage of the rupture through dilational jog. Other deformations represent shear heating and fluidization along the narrow ultracataclasite layer, which could enhance the propagation of instability at the pbf in the upper parts of the seismogenic depths.

準静的多孔質弾性論に基づく地盤・岩盤と間隙水の相互作用と地球科学的意義

The theory of poroelasticity-one of the theories addressing deformation-fluid flow coupling processes-which can be used to explain processes occurring in the geosphere, is described. Both undrained and drained conditions are explained in some detail to make it easier to understand both coupled and uncoupled processes under this theory. Then, the following three examples of applications of this theory to earth sciences are presented ; 1. redistribution of strains due to an earthquake and temporal changes of displacement and pore fluid pressure ; 2. pore pressure fluctuations due to cyclic loads such as atmospheric pressure and earth tides ; and, 3. the effects of pore pressure change on in-situ stress in a sedimentary basin. Finally, extending this theory to anisotropic materials is discussed, and the importance of anisotropy under low-effective stress conditions is emphasized.

Shear strength of cohesionless soils at low stress

Mobilisation of shear strength in shallow translational slips occurs at very low effective stress. Accordingly, a series of direct shear box tests was performed to characterise the strength of colluvium, under drained conditions, at vertical effective stresses between 5 and 20 kPa. Results from in situ tests on undisturbed block samples of moist soil are compared with laboratory tests on dry reconstituted specimens of the soil matrix only. The in situ tests determine a mean value of peak or maximum angle of shearing resistance between 58° and 64°, and a mean value at large displacement between 46° and 52°, with 46° believed representative of shearing at constant volume. Mobilisation of similar angles of shearing resistance, between 46° and 48°, at large displacement in laboratory tests on the 25 mm minus fraction indicates that the soil matrix is controlling strength. The values at large displacement are attributed to mineralogy, grain shape and angularity, and grain size distribution of the soils. Observations of the angle of repose corroborate the measured values at large displacement. The peak or maximum values are consistent with the phenomenon of stress dilatancy and, although very high, compare well with limited data reported for coarse granular soils at very low effective stress.

Soft Sediments: Wave-based Characterization

Soft sediments are frequently encountered in geotechnical practice, forming natural deposits or manmade mixtures. These high water content soil-water mixtures involve soils with high specific surface and experience relatively low effective stresses, hence the role of electrical interparticle forces is enhanced and nonstandard soil-like behavior may be observed. Testing such materials presents some unique challenges not faced in soils with a stronger granular skeleton. This experimental study of soft sediments involves kaolinite- electrolyte mixtures. Two test sets are conducted. The first one involves index tests and the second set of tests explores the combined utilization of elastic and electromagnetic waves to assess soft sediments and to monitor their evolution. The kaolinite mixtures tested in both sets of measurements were prepared using fluids of different ionic concentrations to create different soil fabrics. The mixtures change behavior when the ionic concentration of NaCl exceeds ,0.1 mol/ L, indicating internal changes in fabric formation. While the liquid limit is sensitive to fluid-particle interactions, it is less sensitive to fabric effects than viscosity or sedimentation tests. The maximum porosity a soil may attain is determined by the particle slenderness; in the case of kaolinite, a skeleton capable of transmitting a shear perturbation is observed at water content as high as six times the liquid limit. At this incipient skeletal condition, minor differences in shear wave velocity hint to the stiffness of different fabrics. High frequency permittivity is a good indicator of water content and can be effectively used to monitor the evolution of soft sediments. On the other hand, the evolution of the effective conductivity varies with pore fluid concentration: for high ionic concentration fluids, the effective conductivity increases as porosity increases, yet, the opposite is true for low ionic concentration fluids due to the contribution of surface conduction. In a wide range of water contents @0.6 liquid limit ~LL! ,w, 6L Lg, the shear wave velocity is primarily determined by the water content or porosity, and fabric appears to have a limited impact. Test procedures and results can be used to develop parallel field characterization tools and interpretation guidelines.

Laboratory Investigation on Rate-Dependent Properties of Sand Undergoing Low Confining Effective Stress

ABSTRACT Experiences of past earthquakes show that many structural damages occur due to liquefaction-induced ground deformation. Accordingly, the prediction of liquefaction-induced ground deformation plays a major role in mitigation of damages. Even though numerical methods are developed in three-dimensional way to predict liquefaction-induced deformation, lack of understanding of mechanical properties of liquefied sand makes the prediction unrealistic. Therefore, employing a hollow cylinder torsion shear apparatus, the present study aims to investigate the large deformation of liquefied sand which is tentatively considered to be rate-dependent. Since it is impossible to run experiments on specimens with null effective stress, tests were conducted at very low effective stresses such as 5, 10, and 15 kPa and possibility for extrapolation of data to zero effective stress was attempted in order to study mechanical properties of liquefied sand. In this study, specimens with low effective stress were further sheared by stepwise monotonic axial compression in drained manner and the shear stress was found to be composed of frictional as well as rate-dependent viscous components. The results suggested that viscosity of sand increases with the increase of the mean effective stress. Note that a triaxial extension test and two tests on a special light grain material (Styrofoam) were conducted to investigate viscosity at low effective stress such as 3 to 4 kPa. Noteworthy was that effects of relative density on viscosity were not significant. The test conducted with pore water, pore air, and vacuum revealed that viscosity of liquefied sand was contributed by pore fluid and particle collision during flow. Furthermore, it was suggested that increase in the fines content leads to lower viscosity.

Liquefaction Properties of Toyoura Sand in Cyclic Tortional Shear Tests Under Low Confining Stress

A series of undrained cyclic torsional shear tests was conducted to investigate liquefaction properties of Toyoura sand under low confining stress. Hollow-cylindrical specimens were prepared by air pluviation at a relative density of about 55%. After being saturated, they were anisotropically consolidated to an effective vertical stress of 9.8, 29.4 and 98.1 kPa, while keeping the effective horizontal stress to be half of the vertical one. They were subjected to undrained cyclic torsional shear without allowing any vertical displacement of the top cap. The amplitude of the cyclic shear stress was kept constant with correction for the effects of membrane force on the measured values. The liquefaction resistance was found to increase with the decrease in the confining stress, while correction for the effects of membrane penetration did not significantly affect this tendency. Analyses of the test results revealed that the liquefaction properties were affected by the mobilization of shear resistance under extremely low effective stress states and the stress level-dependency of shear modulus that was normalized by the confining stress. Finally, by introducing a concept of apparent increase in the effective mean principal stress, a simplified procedure to estimate the liquefaction resistance under low confining stresses was proposed.