Stress-controlled Cyclic Triaxial Tests Research Articles

Use of recycled ceramic beads to enhance mechanical behavior of low-plastic clays

Due to the huge amount of annual non-renewable ceramic waste disposal, it is in favor of eco-system to reuse the ceramic waste in earthwork structures. Thus, among several different applications of ceramic components, they can also be used to replace aggregates in the core of embankment dams, waste disposal projects and to form subgrade of roads and railways. The main issues addressed in this study are the effect of the volumetric content and size of the ceramic inclusions on the mechanical properties of ceramic-clay mixtures. Using an experimental approach, strain-controlled monotonic, stress-controlled cyclic, and post-cyclic monotonic triaxial undrained tests were made. The innovation of this study is that sphere beads of ceramic with different diameters were added to low-plastic clay to explore the role of bead size on the mechanical properties of bead and clay mixtures under monotonic, cyclic and post-cyclic loadings. Thus, we tested four types of specimens consisting of merely pure clay, 80% clay–20% bead, 60% clay–40% bead, and 40% clay–60% bead. Scanning electron microscope (SEM) images were additionally used to investigate the fabric of the clay between the ceramic beads. Test results revealed, firstly, that effective friction angle and undrained shear strength increase with the bead content. Secondly, dilative behavior was captured in all the bead and clay mixtures. Further, both dynamic shear modulus and damping ratio increase with the bead content. Similarly, post-cyclic shear strength increases with the bead content. Finally, different bead diameters did not change the observed trend.

On the determination of cyclic shear stress for soil liquefaction triggering in centrifuge model test

As the dynamic shear stress acting on soil column could not be measured directly during earthquakes, the simplified procedure proposed by Seed and Idriss is used to calculate the earthquake-induced cyclic shear stress based on acceleration record of strong motion station for liquefaction triggering evaluation. However, when this procedure is applied to physical modeling such as centrifuge model test, it will result in overestimation of the cyclic loading because the acceleration is monitored in liquefiable soil deposit and the acceleration time history will be distorted due to the softening process, and the whole time history is counted regardless the triggering of liquefaction or not. To address this problem, a centrifuge model test with long duration shaking was conducted to observe the difference of acceleration response between the soil deposit and the sidewall at the same elevation. And a series of stress-controlled undrained cyclic triaxial tests of the same sand used in centrifuge model test were carried out to obtain the cyclic strength at different elevations in model ground. This study shows that the cyclic loading before the time of liquefaction occurrence instead of the whole time history corresponds to the cyclic strength well, and also the calculation based on acceleration record of sidewall is closer to the cyclic strength than that in the soil. Thus a method is proposed to determine the cyclic shear stress in dynamic centrifuge model test, which takes the acceleration record before the time of liquefaction triggering at the sidewall of laminar container instead of that of the liquefiable soil as the source data to calculate the cyclic shear stress ratio.

The Effect of Irregular Seismic Loading and Soil Density on the Liquefaction Behavior of Saturated Sand

Structures located on sandy soils can be significantly damaged by earthquake-induced liquefaction. A series of stress-controlled cyclic triaxial tests under harmonic and irregular loading under various soil densities 30%, and 50%, was conducted to evaluate the effects of irregularities and relative densities on the liquefaction characteristics of saturated sand. The irregular actual ground motions time histories obtained from six stations of the 1999 Chi-Chi Earthquake and harmonic sinusoidal cyclic loading time histories were applied to Firoozkooh #161 sand specimens, and the results were compared in terms of the type waveforms loading and relative densities. Based on the stress and energy method, the Correction coefficient is calculated for a variety of densities and types of irregular loading. The present results reveal that it is not precise to assume a single correction coefficient for all records, regardless of the complicated time-domain characteristics of ground motions. Furthermore, the results indicate that the relative soil density and the type of irregular loading influenced sand's pore pressure generation and liquefaction potential.

Experimental investigation on cyclic behavior of geogrid-reinforced coral sand from the South China Sea

The present study provides insight into the cyclic behavior of unreinforced and geogrid-reinforced coral sand. To achieve these goals, stress-controlled undrained cyclic triaxial tests were conducted on medium-dense coral sand with and without geogrid reinforcements. The influence of geogrid reinforcement, cyclic stress ratios, and confining pressure on the deformation, pore pressure, liquefaction resistance, stiffness degradation, and particle breakage characteristics of saturated coral sand was investigated. The results indicated that coral sand reinforced with geogrid increased the liquefaction resistance and decreased the rate of pore pressure accumulation. The effectiveness of the geogrid inclusion on coral sand’s liquefaction resistance was found to be more pronounced under large effective confining pressure and a smaller cyclic stress ratio. Increasing the effective confining pressure increases the liquefaction resistance for both unreinforced and geogrid-reinforced coral sand, whereas increasing the cyclic stress ratios decreases the liquefaction resistance of the unreinforced and reinforced coral sand. The particle breakage was observed to be increased with increasing the effective confining pressure and cyclic stress ratio, and the breakage was more in the reinforced coral sand than the unreinforced coral sand.

Utilization of Different Additives in Improving Sandy Soil against Liquefaction

One of the main risks in low-densified sandy soils with the presence of water and an external force such as an earthquake is the generation of liquefaction. The influence of several types of reinforcement on liquefaction resistance, such as polypropylene fibers, geofibers, cement and polypropylene fibers with cement is shown in this study. Cyclic stress-controlled triaxial tests and cyclic strain-controlled triaxial tests were performed on saturated samples with and without reinforcements under undrained conditions. Cemented specimens were prepared with cement contents ranging from 0% to 3% by weight of dry sand and then cured for 3 days. The lengths of polypropylene fibers are 10 mm and 20 mm, respectively. The fibers were mixed with dry sand– cement mixes containing 0.50% and 1.00% by weight, respectively. Geofiber specimens were prepared in various arrangements. It was found that the liquefaction improvement factor (LIF) increased when fiber content and fiber length increased. The addition of geofibers increased the liquefaction resistance, as the number of layers increased. The addition of 3%C+1%F provided the best liquefaction resistance in this study compared with other additives. Finally, the reinforcement with cement and fibers is crucial for liquefaction resistanceof bitumen mastic should be considered beside the asphalt mixture performance and the bitumen rheological behavior. KEYWORDS: Liquefaction, Shear modulus, Cyclic stress, Geofiber, Polypropylene fiber.

Use of polyethylene terephthalate fibres for mitigating the liquefaction-induced failures

The presence of non-biodegradable plastic waste is a serious concern for the health of endangered species. The present study is based on the sustainable utilisation of polyethylene terephthalate (PET) fibres obtained from waste plastic bottles to enhance the liquefaction resistance of fine sand. After performing a series of stress-controlled cyclic triaxial tests, the cyclic behaviour of PET-fibre reinforced sand has been investigated. The application of PET fibres was found to be more satisfactory in medium dense sand than that in loose sand as observed by residual excess pore water curves. In medium dense sand with 0.6% PET-fibres, the number of cycles to reach liquefaction was about 4 times that of the unreinforced sand. Using the dynamic shear modulus (G), the degradation index was calculated for both reinforced and unreinforced soils to assess stiffness characteristics. After nearly 50 loading cycles, the value of G/Gmax increased 2.55 times with the addition of 0.4% PET fibres in unreinforced sand. Based on the results obtained, a regression model has been developed for the calculation of number of liquefaction failure cycles (Ncyc,L) in correlation with several parameters, namely, relative density (Dr), fibre content (FC) and σd/σc′ (σd = deviator stress, σc′ = effective confining stress).

Cyclic behaviour of stratified soil under liquefied states

A substantial amount of work on the cyclic behaviour of cohesionless soil has been performed after neglecting the reality of stratification existing nearby marine structures or alluvial plains. This complicated mechanism of stratified soil failure on earthquake-induced soil liquefaction is affected significantly by the presence of a less permeable soil layer. Several past studies investigated that under the action of dynamic loads, the presence of a silt layer in homogeneous sandy soil crucially modifies the pore pressure dissipation characteristics, axial strain rate and stress–strain behaviour of the whole stratified specimen. In the present study, cyclic behaviour of sand specimen interlayered with silt layer has been presented in respect of its pre-and post-liquefaction behaviour. Undrained stress-controlled cyclic triaxial tests have been performed on homogeneous sand specimens and on specimens interlayered with a silt layer, placed at H/4, H/2 and 3H/4 (where H is the height of the specimen from the top) to signify the role of placement depth. The instability and failure of such specimens have been compared using the failure line and phase transformation line. Few key observations suggested that the number of liquefaction failure cycles enhanced two times and secant modulus (under post liquefied state) improved by 4.8 times when the placement depth of the silt layer increased from H/4 to 3H/4 under similar loading conditions. Additionally, the role of initial static shear stress has been investigated at three different confining pressures in a few stratified specimens. It has been observed that failure of the stratified specimen is due to the obstruction of pore water movement caused by the presence of the silt layer. A comparison of the present work has been made with significant studies using liquefaction resistant curves to ensure the efficacy of this study.

Liquefaction potential of reinforced sand with plastic wastes

Granular soil liquefaction, due to developed pore water pressure during undrained cyclic shear of saturated soils, is regarded as a common phenomenon under earthquakes loading. This phenomenon results in the huge damage to infrastructures. Various reinforcement materials have been successfully implemented with particular attention to use waste materials to satisfy design specifications and also reducing the adverse environmental effects. This paper investigates the possibility of using waste plastic fibers as a reinforcement material to mitigate liquefaction potential and pore pressure generation of reinforced sand under cyclic loading. For this purpose, 42 stress-controlled cyclic triaxial tests were conducted on Babolsar sand, reinforced by polyethylene terephthalate (PET) and polypropylene (PP) fibers with fiber contents of 0.25%, 0.5% and 1%, under confining pressures of 50, 100 and 200 kPa, and with cyclic stress ratios (CSR) of 0.2 and 0.35. Results revealed that the addition of these waste plastic fibers could significantly increase the liquefaction resistance of Babolsar sand, and also, with an increase in the waste content, the number of cycles leading to liquefaction increased. Adding wastes decreased the pore water pressure generation, and this effect was more pronounced with an increase in the waste content.

Influence of stress history on the cyclic behavior of compacted soils in the frozen state: Deviator stress history

Observation of the cyclic behavior of compacted soils in a frozen state is required for assessing the dynamic stability of subgrades or embankments induced by earthquakes or traffic in cold regions, in which the compacted frozen soils have often experienced deviator stress history before applying cyclic stress. Therefore, strain-controlled monotonic triaxial tests followed by stress-controlled cyclic triaxial tests were carried out on compacted frozen soils to investigate the influence of deviator stress history on the cyclic behavior of such soils. The results shows that nonlinear and accumulated behaviors are represented in the stress-strain response of frozen soils regardless the amounts of residual strain induced by stress history. Although the volume fluctuations along with the general path are observed in the cyclic loading sequence, the general path seems consistent with volume changes during the monotonic loading stage, which potentially indicates that the plastic flow characteristics of this frozen soil are independent of the axial loading models (monotonic or cyclic loading). Moreover, an increase in the residual strains induced by stress history led to a decrease in the axial strain accumulation rate and to an increase in cyclic stiffness due to strain hardening induced by preloading. A previously proposed model for predicting the changes in the accumulated strain with the cyclic loading process was found to be applicable to compacted frozen soils regardless of the amounts of residual strain induced by stress history.

Laboratory-Based Correlation between Liquefaction Resistance and Shear Wave Velocity of Sand with Fines

This paper presents the results of a laboratory investigation into the effect of non-plastic fines on the correlation between liquefaction resistance and the shear wave velocity of sand. For this purpose, undrained stress-controlled cyclic triaxial and bender element tests were performed on clean sand and its mixtures with non-plastic silt. It is shown that the correlation between liquefaction resistance and shear wave velocity depends on fines content and confining effective stress. Based on the test results, correlation curves between field liquefaction resistance and overburden stress corrected shear wave velocity for sand containing various contents of fines are derived. These curves are compared to other previously proposed by field and laboratory studies.

Responses of frozen, fine-grained soils due to multiple packets of cyclic stress with variable amplitude and an empirical prediction model

A suite of stress-controlled cyclic triaxial tests were performed on frozen, fine-grained soils to assess the characteristics of the accumulated and cyclic behaviours under packets of cyclic stress with variable amplitude. The influence of cyclic stress sequences is highlighted, and the results of the variable amplitude cyclic tests indicate that the applied cyclic stress amplitude sequence is of importance regarding the final accumulated deformations. A step change is observed in the accumulated strain when the applied cyclic stress amplitude is beyond that of the previous stress package, while a decreasing level of cyclic stress below the previous loading package results in slight additional strain accumulation. The results of these tests also indicate that the cyclic stiffness is dependent on both the past accumulated strain and the current cyclic stress amplitude. For frozen soil, a higher past accumulated strain and lower cyclic stress levels yield a higher cyclic stiffness. An empirical approach for representing the response of frozen, fine-grained soils under multiple packets of cyclic loading is proposed and then verified by the test data. The results show that the proposed empirical model is able to extract and predict the accumulated and cyclic behaviours under multiple packets of cyclic loading.

DYNAMIC PROPERTIES OF RIGID POLYURETHANE FOAM IN CYCLIC TRIAXIAL TESTS

The aim of this study is to clarify the dynamic properties of rigid polyurethane foam. A method for renovating a deteriorated bridge as a lightweight embankment was proposed. The space underneath a bridge is filled with polyurethane to support the upper structure. In this case, the upper structure of the bridge is considerably heavier than the polyurethane. Therefore, it is important to examine the seismic behavior of this new lightweight embankment. However, the dynamic deformation characteristics of polyurethane have not been clarified previously. In this study, stress-controlled cyclic triaxial tests based on JGS0542-2009 are used to evaluate the effect of the confining stress and the presence of a rigid layer on the dynamic properties of rigid polyurethane foam. As a result, the shear modulus of rigid polyurethane foam increases with increasing confining stress regardless of the absence of the rigid layer. The shear modulus of polyurethane with the rigid layer is lower than that of polyurethane without the rigid layer. The value of shear modulus of rigid polyurethane foam was measured in the range of approximately 1.6~3.2MPa. Moreover, the stiffness degradation and of rigid polyurethane foam are in good agreement with the Hardin-Drnevich model.

Dynamic behavior of cement-stabilized organic-matter-disseminated sand under cyclic triaxial condition

Organic matter–disseminated sand (OMDS) is a problematic soil widely distributed in Hainan Province, China. OMDS is usually stabilized using cementitious materials to improve its bearing capacity. However, there is a lack of understanding of the dynamic characteristics of cement-stabilized OMDS. In this study, a series of unconsolidated undrained stress-controlled cyclic triaxial shear tests were conducted on OMDS samples stabilized by cement and lime. The cyclic triaxial behavior of cement-stabilized OMDS under various confining pressures, curing periods, and cement contents was studied. Through data fittings, the maximum elastic modulus (Emax), normalized elastic modulus (Ed/Emax), and maximum dynamic damping ratio (λmax) were obtained. Then, the effects of cement content, curing time, and initial confining pressure on Emax, Ed/Emax and λmax were systematically analyzed. Finally, the effectiveness of cement stabilization on the dynamic behavior of cement-stabilized OMDS was discussed by comparing the data with that of un-cemented OMDS.

Effect of prior small to moderate seismic events on monotonic undrained shear strength of sand

In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global densification in the sample is observed. The results of this paper will help assess the change in static slope stability after a single or multiple small to moderate events occurred without causing initial instability.

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.

Stress-strain behavior and liquefaction strength characteristics of Ottawa F65 sand

The stress-strain-strength response of soils is of significant interest to development and calibration of realistic constitutive models that can be used in numerical simulation of geotechnical engineering problems. An extensive characterization of Ottawa F65 sand along with various monotonic and cyclic tests conducted during the course of the Liquefaction Experiments and Analysis Projects (LEAP) are presented here. The specimens in these tests were prepared using a meticulous sample preparation technique to facilitate their consistency and repeatability. Monotonic drained and undrained triaxial tests shed light on the steady-state (critical state) of Ottawa sand, while stress-controlled and strain-controlled cyclic triaxial and direct simple shear tests provide key information on the cyclic stress-strain behavior and liquefaction strength of this soil. The triaxial tests identify the liquefaction strength of the soil at different densities, while the direct simple shear tests evaluate the effect of overburden pressure on the cyclic response. The results of these experiments are also compared to the experimental results available in the literature. The data obtained from the cyclic triaxial and direct simple shear tests were further analyzed by plotting the computed shear modulus degradation and damping curves. The results show how the soil stiffness degrades as cyclic shear stress is applied for soil at different densities and confining stresses. It can be seen that the rate of shear modulus degradation increases with the increase of confining stress and decreases with the increase of soil density. The damping curves consistently show an increasing in damping ratio until a shear strain of about 0.05%, followed by a plateau at about 20–25% damping ratio for shear strain between 0.05 and 0.5%, and ending with a decrease in damping until reaching a final damping ratio of about 10%.

Liquefaction Characteristics of Sand With Polypropylene Fiber at Low Confining Stress

The present study evaluates the liquefaction characteristics of Ottawa sand with polypropylene fiber using cyclic triaxial test. A series of stress-controlled cyclic triaxial tests were performed at 34.47 kPa (5 psi) effective confining pressure. Specimens of clean Ottawa sand, and sand containing 0.05, 0.075, 0.1, and 0.3% polypropylene fiber by dry weight of sand were tested at 0.2, 0.25, 0.3, and 0.4 Cyclic Stress Ratio (CSR). Results show that a significant improvement in liquefaction resistance when the polypropylene fiber content exceeded beyond 0.075% at 34.47 kPa effective confining stress.

Post-liquefaction pore pressure dissipation in sand under cyclic stress triaxial testing

Soil liquefaction has conventionally been studied as a pure undrained condition subject to cyclic shearing in the laboratory. However, in the field, ground settlement and expulsion of pore water in the form of sand boils are frequently observed, which conventional triaxial and simple shear apparatus are unable to replicate. In this study, a novel insight of coupled pore pressure generation and dissipation in liquefiable clean sand is discussed with the use of a modified cyclic triaxial testing set-up. Stress-controlled cyclic triaxial tests with controlled pore water drainage far lower than the soil's permeability were carried out to investigate ‘near-perfect undrained’ conditions, which are more representative of field conditions. Comparison of results from three sets of stress-controlled cyclic triaxial testing were carried out: (a) conventional shearing under an undrained condition; (b) shearing under controlled drainage; and (c) undrained shearing until liquefaction, and thereafter continual shearing under controlled drainage. The critical cyclic shear stress ratio where excess pore pressure dissipation dominates excess pore pressure generation is identified from the coupled cyclic shear loading and pore water drainage tests. In addition, this study also noted a critical void ratio where the soil is no longer susceptible to liquefaction even under considerable shearing amplitudes. These findings may offer guidance on the tolerable cyclic shear stress and minimum degree of densification to avoid occurrence of soil liquefaction in the field.

Assessment of Dynamic Response of Cohesionless Soil Using Strain-Controlled and Stress-Controlled Cyclic Triaxial Tests

Soils subjected to earthquake motions undergo random variations in stress, strain and frequency during the entire period of shaking. The spatial and temporal complexity of the strong motions is commonly addressed through simplistic strain-controlled or stress-controlled cyclic loading tests as a part of laboratory investigations. Such a methodology is utilized and reported in the present study for assessing the dynamic response of cohesionless sand obtained from River Brahmaputra, India. In order to assess the dynamic response and liquefaction potential of the cohesionless Brahmaputra sand, both stress-controlled and strain-controlled cyclic triaxial tests were performed on reconstituted cylindrical specimens prepared at different relative densities (Dr) ranging from 30 to 90%. The specimens were subjected to varying effective confining pressures (σ′c: 50–200 kPa), shear strain amplitudes (γ: 0.015–4.5%) and cyclic stress ratios (CSR: 0.05–0.3). For all the tests, the frequency of the applied harmonic loading was maintained at 1 Hz. The magnitude of excess pore-water pressure (PWP) generated during successive loading cycles of the strain-controlled tests was found to be considerably lower than that generated in a stress-controlled test. The strain-controlled test reveals a reduction in the development of excess PWP with the increase in confining pressure and relative density, thereby indicating a decrease in liquefaction potential with increasing confining pressure. The stress-controlled test highlighted that based on a particular CSR, an increase in confining pressure results in the requirement of higher deviatoric stress and smaller numbers of cycles are required to initiate liquefaction. Although apparently misleading, it is imperative that during earthquake, same deviatoric stress is applied on the entire soil deposit. Thus, for specimens at larger depths having higher confining pressure, the CSR value is smaller, and thereby successive deeper layers require more number of stress cycles to liquefy. Hence, the study revealed that both stress-controlled and strain-controlled tests can be successfully used to assess the dynamic properties and liquefaction potential of cohesionless specimens. Based on the findings, the developed cyclic shear stress ratio (CSR) = 0.5 and cyclic shear strain amplitude (γ) = 0.5% are considered as the limiting conditions to achieve the onset of liquefaction through strain-controlled and stress-controlled approaches, respectively.

Experimental Research on Effect of Fines Content on Liquefaction Properties of Sand

Liquefaction is a phenomenon mainly occurred in saturated fine grained soils under major earthquakes causes tremendous loss to infrastructure. From the literature it has been observed that liquefaction not only occurs in fine sands but also occurs in sands containing some amount of fines particles, which are of less than 75µ in size. Unfortunately there is no clear conclusions given as how effect the fines content on liquefaction resistance of sandy soils. In order to solve above mentioned problem this study was undertaken through stress-controlled cyclic triaxial tests to know the effect of fines content on liquefaction resistance of sandy soils. In this study the program of experimentation was done on base sand and sand mixed with four different combinations of fines like 10%, 20%, 30%, and 40% of fines with base sand by weight.. The main parameters changed in this work were percentage fines and shear stress ratio (CSR ), where the observed parameter was amount of pore water pressure and cycle of loading.. The result showed that, rate of pore water pressure generation during cyclic loading was largely affected by limiting silt content and density index. The trend observed as amount of pore water pressure is increased more than base sand with adding of fines content up to 20%, later the trend observed as reverse. And also noticed that more CSR value increases the pore water pressure generation and decreases the cyclic resistance