Effective Consolidation Stress Research Articles

Vacuum consolidation theory of dredged slurry considering nonlinear clogging behaviors

Clogging in the prefabricated vertical drains (PVDs) and soils is an inherent feature associated with vacuum treatment of slurry ground. The clogging column is characterized by nonlinear growth of volume over time and nonlinear permeability distribution along the radial direction. This study aims to develop a drain-well consolidation model for vacuum treatment of dredged slurry, with specific considerations of the nonlinear behaviors of the clogging column and PVD drainage capacity. The constitutive compression and permeation behaviors of the dredged slurry are described by extended power functions, which avoids the singularities associated with the conventional logarithmic constitutive equations when the effective consolidation stress approaches zero. Parametric studies were conducted to explore the consolidation effects while considering different combinations of parameters that govern the nonlinear behaviors of the clogging column and PVD-drainage capacity. The significance of considering the abovementioned nonlinear behaviors was demonstrated by comparing the field test results on a dredged slurry ground. Results indicate that clogging primarily affects consolidation by causing a nonlinear permeability distribution along the radial direction of the clogging column and nonlinear attenuation of PVD-drainage capacity.

Constitutive modeling of clay shales in undrained conditions and its experimental verification for Opalinus Clay

Clay shales are low-permeable sedimentary rocks that exhibit inherent cross-anisotropic stiffness, strength, and permeability. Accurate design of tunnels excavated in clay shales requires appropriate constitutive models that account for the anisotropic properties of the host rock, especially under undrained conditions. This paper presents a transversely isotropic elastoplastic-damage model for fully-saturated clay shales in undrained conditions. The model describes four deformation regimes observed during consolidated undrained experiments, i.e. (1) transversely isotropic elastic deformation, (2) anisotropic plastic deformation with cross-anisotropic peak strength, (3) semi-brittle post-peak softening due to both strength and stiffness degradation, and (4) residual strength state. The performance of the model was evaluated using available laboratory tests for Opalinus Clay, the selected host rock for underground disposal of radioactive waste in Switzerland. A large experimental data set, consisting of 18 undrained triaxial compression tests, was used to back-calculate the required model parameters. The adopted experimental data include the effective stress path and the stress–strain curves of specimens with bedding orientations between 0° and 90° with respect to the loading and effective consolidation stresses between 0.76 and 16 MPa. It is shown that the proposed model adequately captures the key material features of Opalinus Clay observed in the undrained experiments.

Hydro-mechanical behavior and microstructure evolution of red clay-bentonite backfills

Soil-bentonite (SB) cutoff walls have been extensively used as vertical engineering barriers to control groundwater flow and subsurface contaminant migration. As the pollution containment capability of the SB backfills is intimately related to the hydro-mechanical behavior, consolidation tests and hydraulic conductivity tests were carried out to investigate the effects of bentonite content on the hydro-mechanical behavior of red clay-bentonite backfills, and the microstructure evolution of backfills under vertical loading was evaluated based on mercury intrusion porosimetry (MIP) tests and field emission scanning electron microscope (FESEM) observations. The results of microstructure analysis also evaluated the influence of bentonite on the clay fabric which changed the relationship between hydraulic conductivity and void ratio. The results indicated that the void ratio and hydraulic conductivity decreased linearly with the increase of the effective consolidation stresses, and the compression index decreased with the increase of bentonite content. Meanwhile, the pore size distribution (PSD) curve of the backfill specimen almost turned into unimodal characteristics with only micropores distributed after consolidation. Additionally, the flow paths became more tortuous under compression which decreased the hydraulic conductivity. The montmorillonite aggregates in red clay-bentonite backfill formed into tortuous network structure which bound the clay aggregates together and effectively increased the tortuosity of flow paths.

Undrained Triaxial Shear Tests on Hydrate-Bearing Fine-Grained Sediments from the Shenhu Area of South China Sea

Changes in undrained shear strength are important to the stability analysis of hydrate reservoirs during natural gas hydrate production. This study proposes a prediction model of undrained shear strength of hydrate-bearing fine-grained sediments based on the critical state theory. Several consolidated undrained triaxial shear tests are conducted on hydrate-bearing fine-grained samples from the Shenhu area of the South China Sea. The effects of effective consolidation stresses and hydrate saturations on the undrained shear strength are investigated. The results show that the undrained shear strength increases linearly with increasing effective consolidation stress. When the hydrate saturation is greater than the effective hydrate saturation, the undrained shear strength significantly increases with increasing hydrate saturation. The undrained shear strength of hydrate-bearing fine-grained sediments is a two-parameter function of effective hydrate saturation and a void ratio. The instability risk of the hydrate reservoir under undrained conditions is greater than that of under-drained or partially drained conditions. Furthermore, low-porosity reservoirs face more shear strength loss from hydrate decomposition yet lower risk than high-porosity ones. These results can improve the understanding of mechanical properties of hydrate-bearing fine-grained sediments under undrained conditions. This study also has implications for the design of marine structures in areas with hydrate-bearing sediment.

Accumulated plastic strain behavior of granite residual soil under traffic loading

In this study, the accumulated plastic strain (APS) of granite residual soil (GRS) under traffic loading is investigated via cyclic hollow-cylinder and triaxial tests. GRS is a special type of soil that is found widely in the hot and rainy areas of southern China. Because of insufficient understanding of the cumulative plastic deformation of GRS, how the dynamic stress amplitude, degree of saturation, effective consolidation stress, and stress path affect the APS under loading cycles is explored and analyzed. The results show that the effective consolidation stress inhibits the development of APS, whereas the dynamic stress amplitude promotes it. The APS of GRS increases with the degree of soil saturation, and its noticeable water-softening characteristics become the least favorable factor for soil deformation. The combined action of water softening and principal stress rotation helps to increase the APS significantly, and it leads to the emergence of an APS developmental curve that differs from the stable curve. Accounting for the water-softening characteristics of GRS and various stress conditions, a simple and easily adopted APS model is developed to predict the evolution of APS with loading cycles. Although specific to GRS in China, the findings of this study are expected to apply more widely to other weathered geomaterials.

Comparative Investigation on Small-Strain Stiffness Characteristics of Undisturbed and Compacted Highly Weathered Granites at Various Densities

To investigate the small-strain stiffness characteristics of highly weathered granite (HWG), a resonance column test system was used to conduct resonance column tests on highly weathered granite taken from the Lincang City, Yunnan Province, China. The effects of effective consolidation stress and structural changes is caused by remodeling process and initial dry density on the small-strain stiffness of HWG. Furthermore, the difference between the test data and other geotechnical materials were compared and analyzed. The test results show that the maximum dynamic shear modulus <math xmlns="http://www.w3.org/1998/Math/MathML" id="M1"> <mfenced open="(" close=")"> <mrow> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </mrow> </mfenced> </math> of remodeled highly weathered granite samples is greater than that of undisturbed samples once the effective consolidation stress is smaller than 300 kPa, but the opposite result is observed once the effective consolidation stress exceeds 300 kPa, and the phenomenon was also explained from a microscopic perspective based on the scanning electron microscopy (SEM). The <math xmlns="http://www.w3.org/1998/Math/MathML" id="M2"> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </math> of remodeled highly weathered granite gradually increases with the increase in dry density and effective consolidation stress. At an identical effective consolidation stress, the dynamic shear modulus ratio <math xmlns="http://www.w3.org/1998/Math/MathML" id="M3"> <mfenced open="(" close=")"> <mrow> <mi>G</mi> <mo>/</mo> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </mrow> </mfenced> <mo>−</mo> <mi>γ</mi> </math> curves of remodeled highly weathered granites at different initial dry densities are nearly consistent. Additionally, according to the test data, the mathematical model of <math xmlns="http://www.w3.org/1998/Math/MathML" id="M4"> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </math> for remodeled highly weathered granite considering effective consolidation pressure and initial dry density was established and agrees well with the test results of practical engineering cases. The range of variation in <math xmlns="http://www.w3.org/1998/Math/MathML" id="M5"> <mfenced open="(" close=")"> <mrow> <mi>G</mi> <mo>/</mo> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </mrow> </mfenced> </math> was given by the research results, which were compared with the <math xmlns="http://www.w3.org/1998/Math/MathML" id="M6"> <mfenced open="(" close=")"> <mrow> <mi>G</mi> <mo>/</mo> <msub> <mrow> <mi>G</mi> </mrow> <mrow> <mi mathvariant="normal">max</mi> </mrow> </msub> </mrow> </mfenced> </math> of weathered granite obtained from the existing research, and the results in this work can provide a valuable reference for analyzing dynamic stability of buildings in the engineering construction of weathered granite sites.

Failure mode transition in Opalinus Clay: a hydro-mechanical and microstructural perspective

Abstract. The way rocks deform under changing stress conditions can be described by different deformation modes, which is fundamental for understanding their rheology. For Opalinus Clay, which is regarded as a potential host rock for nuclear waste, we investigate the failure mode as a function of applied effective stress in laboratory experiments. Therefore, we performed consolidated undrained triaxial tests at different effective consolidation stresses from 2.5 to 16 MPa, in which samples were loaded parallel to bedding, and analysed the deformation structures using ion-beam polishing and electron microscopy. With increasing effective confining stress, the results show a transition from brittle-dominated to more ductile-dominated deformations, localising in distinct shear bands. Both effective stress paths and microstructural analysis indicate a tendency towards less dilation in the shear zones for higher effective stresses. Triaxial test results suggest a non-linear failure envelope. The non-linearity of the failure envelope is associated with decreasing dilation with increasing effective stress accompanied by changes in microstructural deformation processes, which explain the decreasing friction angle. For the first time, we can verify that the observed non-linear failure envelope is due to the gradual transition from brittle- to more ductile-dominated deformation on the microscale controlling the bulk hydro-mechanical behaviour of Opalinus Clay.

The deformation behaviour of an anisotropically consolidated kaolin clay under lateral cyclic loading

The soil behind wharfs or around pile foundations for wind turbines may be subjected to lateral cyclic loading. A series of cyclic triaxial tests were performed on a kaolin clay to study the response of the soil to lateral cyclic loading. Cyclic cell pressure was applied under a constant total vertical stress condition. The deformation behaviour of the clay under undrained condition is presented and discussed under different cyclic stress amplitudes, initial static deviatoric stresses and effective consolidation stresses. The results indicate that, the samples prepared under isotropic consolidation condition experienced axial extension, while those prepared under anisotropic consolidation condition exhibited axial compression under cyclic loading. There exists a static deviatoric stress ratio (between zero and 0.15) at which pure excess pore water pressure accumulates without axial strain development. Under anisotropic consolidation condition, with the increase of static deviatoric stress ratio, the deviatoric component of direction of strain accumulation increases in comparison to volumetric (excess pore water pressure) portion. The flow rule of Modified Cam Clay model (MCC) is revised to describe the direction of strain accumulation with the consideration of anisotropy of the clay. The predicted direction of strain accumulation is comparable with the observed values. Highlights The deformation behaviour of a kaolin clay under lateral cyclic loading is analysed. Zero axial strain accumulated at the static deviatoric stress ratio of about 0.1 (rigorously between 0 and 0.15). The flow rule of MCC is modified to describe the direction of strain accumulation considering the anisotropy of the clay.

Improved Vacuum Preloading Method Combined with Sand Sandwich Structure for Consolidation of Dredged Clay-Slurry Fill and Original Marine Soft Clay

An improved vacuum preloading method combined with a sand sandwich structure (SSS) was developed to facilitate the consolidation of dredged clay-slurry fill and original soft marine clay for land reclamation. In this method, a novel sandwich structure is formed as a sand layer introduced on to original soft marine clay prior to the placement of reclaimed, dredged clay-slurry. This assists in transmitting the vacuum pressure to both the reclaimed, dredged clay-slurry and the original soft marine clay, through prefabricated vertical drains, with little vacuum loss, and the sand layer thereby improving the efficiency of the vacuum preloading. Field pilot tests were carried out on improved ground both with and without using a SSS in order to demonstrate the capacity of the improved vacuum preloading method combined with the SSS. Field measurements were performed to evaluate the ground settlement, degree of vacuum loss, effective consolidation stress, shear strength, and bearing capacity. The test data showed that better consolidation results were achieved using the improved vacuum preloading method combined with the SSS.

Shear modulus and damping ratio of clay soil under repeated freeze-thaw cycles

The dynamic properties of soil deposits subjected to dynamic loading, such as the shear modulus G and material damping ratio D, are important parameters in ground response seismic analysis. In seasonally frozen regions, these properties can be significantly affected by microstructural changes that occur during freeze-thaw cycles. The current study evaluated the dynamic properties of clayey soil exposed to freeze-thaw cycles. Dynamic triaxial testing was conducted to determine the influence of mean effective consolidation stress, cyclic stress ratio, loading frequency, and freeze-thaw cycles on G-γ and D-γ curves. Scanning electron microscopy (SEM) was carried out to investigate microstructural changes in the clay soil fabric. The results indicate the freeze-thaw process has an important effect on dynamic properties of the soil. The dynamic shear modulus increases with increasing effective confining pressure, loading frequency, and confining pressure, and decreases with increasing number of freeze-thaw cycles. Development of more voids between clay particles after ice lens formation during these cycles results in an increase in the damping ratio, but this trend decreases with increasing confining pressure. Increasing the loading frequency increases or decreases the damping ratio depending upon the mean effective confining pressure and number of freeze-thaw cycles.

Quantitative shear strength–consolidation stress–void ratio interrelations for reconstituted clays

Isotropically and anisotropically consolidated undrained triaxial compression shear tests are conducted on 87 specimens of eight kinds of clays reconstituted over a wide spectrum of initial water contents. These data in conjunction with those of 55 specimens previously published by the authors and their research team are used to investigate the shear strength–compressibility interrelation for reconstituted clays. The effects of the initial void ratio and the consolidation stress path on both the consolidated undrained shear strength–consolidation effective stress relationship and the compression curves in terms of the void ratio against the consolidation effective stress are investigated. It is found that the consolidated undrained shear strength–void ratio relationship is independent of the initial void ratio and the consolidation stress path, and a quantitative expression is proposed for determining the relationship as a function of the void ratio at the liquid limit. Meanwhile, the compression curves of reconstituted clays at different values of the initial void ratio and the void ratio at the liquid limit from different consolidation stress path tests can be normalised into a unique line with the application of the intrinsic compression framework. Accordingly, a quantitative expression can be established to determine the relationship between the consolidated undrained shear strength and the effective consolidation stress. The validity and accuracy of the proposed equations are discussed based on data from independent researchers and from the research team.

A Parametric Study on the Evolution of Cyclic Clay-Pile Interface Friction for Large Numbers of Cycles

The present experimental study investigates the influence of cyclic displacement amplitude and effective consolidation stress on the evolution of mobilized local shaft friction along piles submitted to large number of cycles (up to 105 cycles). Two-way cyclic displacement-controlled tests were performed on an instrumented pile-probe installed and loaded in a calibration chamber. Tests were performed on reconstituted specimens of saturated clay to examine the shaft friction evolution in the soil-pile interface during cyclic loading. Displacement-controlled static tests were also performed before and after the cyclic loading in order to quantify the influence of cyclic parameters on post-cyclic static response. It was found that the amplitude of cyclic displacement and the initial state of stress have an influence on the evolution of local friction during cyclic loading. The degradation rate of local friction increased for larger cyclic displacement amplitudes whereas with increasing the effective consolidation stress, the degradation rate decreased. The application of displacement- controlled cycles resulted in a modification in the behaviour of the interface. A significant peak of static friction followed by strain softening was observed during post cyclic static tests, which was not the case for pre-cyclic static tests. The peak value of friction obtained upon post-cyclic static loadings found to be more important for higher values of applied displacement amplitude during cyclic loading. Finally, a brief synthesis of the results is given.

Effect of Shredded Rubber on Undrained Shear Strength of Fine-Grained Sands

In many urban cities due to the enormous growth in the number of vehicles, scrap rubber disposal has been a critical environmental problem. In recent years, important research efforts have been dedicated to investigating the use of scrap rubbers in civil engineering applications, like recycling or reuse of scrap rubbers is the preferred possibility from a waste management perspective. This study furthers the knowledge of the undrained shear strength in clean and mixed well-sorted sands (different shape properties) with waste sand-sized rubber. For this purpose, shredded rubber content varies from 0 to 25% and initial effective consolidation stress of 100 to 300 kPa. Thirty-six undrained shear tests (CU) were conducted using a triaxial apparatus. On randomly mixing sands with shredded rubber, a dilative and strain-hardening and softening behavior governed, and undrained strength generally decreased. The results indicated that undrained shear strength is augmented with the growth of the effective mean stress. Peak index and Young’s modulus decreased, and flow potential increased with shredded rubber content increment in composite specimens. The undrained shear strength parameters (qpeak and qf) across 200 and 300 kPa effective mean stress were higher in the sand with a large angularity (low rr and rs). Further, a decrease in particle shape scale (rs and rr) ratios caused an increase in peak index and elasticity modulus and reduced occurrence in flow potential.

Effect of grain size on undrained anisotropic behaviour of sand–fibre composite

Abstract This study investigates the dependency of steady states on anisotropy in unreinforced uniform angular silica sands and those reinforced with micro-synthetic fibres. In doing so, the principal stress orientation varies from 15° to 60° for an intermediate principal stress ratio of 0.5 and 1.0 and the initial effective consolidation stress of 200 kPa. Twenty-four undrained torsional shear tests are conducted using a hollow cylindrical torsional shear apparatus. On randomly mixing sands with fibres, dilative and strain-hardening behaviour attains prominence, and the undrained strength generally improves. Anisotropy in the samples decreases and distortion of the deviatoric strength envelope is produced by the addition of fibres to the host sand. The results show that fibre reinforcement contribution growth by the increase of the major principal stress direction. This phenomenon is attributed to the change in the loading combination of the sample from compression to torsion. The contribution of the fibre to the strength in all principal stress directions is higher in the sand with smaller median grain size (D50). Furthermore, increasing the median grain size caused the stress-strain curves to be adjacent to one another and increased specimen anisotropy. The flow potential decreases and begins to follow an inverse relationship with the principal stress direction.

Triaxial extension and tension tests on lime-cement-improved clay

This paper presents the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement-improved very soft clay. The main objective of these tests was to investigate the material strength and stiffness properties for stress conditions similar to those expected on the passive side of excavations where a retaining structure is supported by Deep Mixing columns. The different stress paths to failure were obtained by varying the directions of the major and minor principal stresses in a conventional triaxial test cell. The undrained tests conducted at low consolidation stresses, corresponding to depths of approximately 0–10 m below the ground surface, revealed significant differences in undrained strength depending on the directions of the major and minor principal stresses, indicating anisotropic material behavior. Based on the undrained triaxial test results, the relationship among the undrained strength, the effective consolidation stress and the over-consolidation ratio (OCR) is presented for different stress paths to failure.The experimental data from the drained tests show that a failure surface comprised of a shear failure function based on the Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement-stabilized clay.

Undrained Shear Strength and Monotonic Behavior of Different Nonplastic Silts: Sand-Like or Clay-Like?

The common assumption in geotechnical engineering research and practice has been that nonplastic silts behave sand-like, whereas, as silts become more plastic, they start to behave clay-like. To date, there are practically no studies focusing solely on various nonplastic silts and their monotonic response, perhaps because of the solid belief that nonplastic silts behave sand-like in all aspects. A series of constant volume direct simple shear tests were conducted on three different nonplastic silts. Because the specimens were reconstituted to have loose grain structures, volumetrically contractive tendency was observed for all the silts with relatively small effective stress friction angles. It was observed that Ko compression lines and critical state lines are parallel to each other for all three silts, which is considered to be a clay-like behavior in literature. It was shown that undrained shear strengths can be normalized with the effective vertical consolidation stress, which is also considered to be a clay-like behavior in literature. Normalized values are compared with the ones obtained for plastic silts compiled from previous studies. Normalized undrained shear strength values for plastic silts in literature were seen to be typically greater. Possible reasons such as density effect and strength anisotropy were discussed.

Influence of Depositional Method on Dynamic Properties of Granular Soil

The behaviour of granular soil is mainly dependent upon the nature and arrangement of particles, which in turn affect fabric anisotropy. It has been known for many years that depositional method plays an important role in the results of laboratory testing of granular soils. Despite extensive studies on the dynamic properties of granular soil, previous research has lacked a quantitative study of the effects of depositional methods on the dynamic properties of the granular soil. The primary objective of the current study was to evaluate the effect of fabric anisotropy on the dynamic properties of granular soil for practical applications. The influence of depositional method, mean effective consolidation stress, shear strain amplitude and relative density on the maximum shear modulus (Gmax), shear modulus curve (G-γ), normalized modulus curve (G/Gmax-γ) and damping curve (D-γ) of pure sand, pure gravel and sand-gravel mixtures is investigated. The results of the tests indicate that depositional method has a significant effect on Gmax that this effect was more pronounced for the pure sand specimens than for the pure gravel or sand-gravel mixture specimens. However, the depositional method had no significant effect on the shear modulus at large shear strain amplitudes. The effects of depositional method on the G/Gmax-γ and D-γ curves are dependent on effective confining pressure and relative density.

Compressibility and hydraulic conductivity of sand-attapulgite cut-off wall backfills

Soil-bentonite cut-off walls have been used widely to control pollution in landfills but their antifouling property (their ability to prevent contaminants in landfills from polluting the surrounding environment) decreases significantly over time due to a variety of factors (e.g. contaminant concentrations). In recent years, attapulgite has been considered as a backfill material for cut-off walls, but relevant studies are lacking. In this study, the compressibility and hydraulic conductivity of sand-attapulgite backfills were investigated using consolidation and hydraulic conductivity tests. In these tests, attapulgite comprised 10%, 20%, 30%, 40%, 60%, 80%, or 100% (dry weight) of the backfills. The results showed that (1) the compression (Cc) and swell (Cs) indexes of the backfills increased linearly with increasing attapulgite content (Ap); (2) both the consolidation coefficient (Cv) calculated by the Casagrande and Taylor methods and the hydraulic conductivity (ktheory) calculated according to Terzaghi consolidation theory decreased with increasing attapulgite content. In the case of an effective consolidation stress σ′<100 kPa, ktheory <10−9 m/s when Ap≥30%, which was supported by the hydraulic conductivity tests. Two methods were developed based on laboratory data, for predicting the hydraulic conductivity of sand-attapulgite backfills. We conclude that the use of sand-attapulgite backfills applied to cut-off walls as substitutes for soil-bentonite backfills is technically feasible.

Strength and consolidation characteristics of clay with geotextile-encased sand column

Installing sand columns in clays is a common ground improvement technique used to treat soft soils, and the inclusion of geotextile to encase the sand column can further improve the performance of the reinforced clay composite. In this work, the shear strength and consolidation characteristics of clay, clay with an ordinary sand column (OSC) and clay with a geotextile encased sand column (GESC) were investigated by carrying out laboratory direct shear and oedometer tests to study the response of the reinforced clay composite under both lateral and vertical loading conditions. It was found that the OSC and GESC significantly increased the shear strength of the clay, mainly by improving the friction angle, with little impact on the apparent cohesion. Furthermore, the OSC and GESC reduced the compression index, swell index, and recompression index of the clay to some extent. The secondary compression indexes obtained from the reloading stage were lower than those from the initial loading stage, indicating that over-consolidated clay has a lower secondary compression index than normally consolidated clay. The coefficients of consolidation showed an upward trend as the effective consolidation stress increased. In addition, the compressibility and hydraulic conductivity tended to decrease with increasing effective consolidation stress.

Cyclic threshold shear strain in pore water pressure generation in clay in situ samples

The threshold level of the cyclic shear strain required for pore water pressure generation in clay samples is examined through the results of torsional hollow cylinder cyclic shearing tests according to JGS 0543-2009. The study confirms the previous results, namely, that the threshold cyclic shear strain is dependent on the effective consolidation stress and plasticity index (Ip). The average and standard deviations in the estimated threshold strain levels are 0.038 ± 0.023% (Ip < 30, σ′c ≦ 100 kN/m2), 0.047 ± 0.016% (Ip < 30, σ′c > 100 kN/m2), 0.079 ± 0.028% (30 ≦ Ip < 5 0), and 0.143 ± 0.041% (Ip ≧ 50). As was found in past research, the levels of threshold strain for pore water pressure generation for clay are larger than those for clean sand. An increase in pore water pressure is only observed when the stiffness is reduced to around 80% of its initial value. This delay occurs because there is a difference between the cyclic threshold strain of the pore water pressure generation, γtp, and the cyclic threshold strain of the stiffness degradation, γtd. Since the test procedure of JGS 0543-2009 is a standard scheme in the practical design process, it is expected that more data will become available in the near future which will allow for further discussions on threshold strain.