Initial Static Shear Research Articles

Characterization of an alluvial silt and clay deposit for monotonic, cyclic, and post-cyclic behavior

This paper presents a detailed characterization of the monotonic, cyclic, and post-cyclic behavior of two strata within a recent Holocene alluvial deposit of silty sand, sandy silt, silt, and clay. Stratum A is composed predominantly of very soft clay and very loose silt with plasticity indices ranging from 5 to 27, whereas stratum B is composed predominantly of very loose silty sand and sandy silt with plasticity indices ranging from 0 to 10. Characterization included in situ testing, undisturbed soil sampling and laboratory testing, and a field surcharge test section. Consolidation tests and monotonic, cyclic, and post-cyclic direct simple shear tests were used to evaluate the effects of varying the consolidation stress, consolidation stress history, and initial static shear stress ratio. The field and laboratory test data show distinct differences in behavior between the two soil strata, which can be related to their different index test characteristics. These results are compared with their respective behaviors predicted using common engineering correlations. The field and laboratory test data summarized herein contribute to the database and understanding of the monotonic, cyclic, and post-cyclic behaviors of low-plasticity fine-grained soils.

Strain localization characteristics of loose saturated Toyoura sand in undrained cyclic torsional shear tests with initial static shear

Strain localization, or the formation of shear bands, is a key aspect in understanding soil failure mechanisms. While efforts have been made in terms of measuring the shear band properties and the stress–strain behavior within shear bands, there are still uncertainties regarding when shear bands initiate and their influence on the development of large ground deformation. In this paper, the limiting value of shear strain, at which strain localization appears during undrained cyclic torsional shear tests with initial static shear, performed on loose Toyoura sand specimens (Dr=44–48%) up to a single amplitude of shear strain exceeding 50%, was evaluated. Non-uniform specimen deformation was observed at strain levels larger than 20%. However, the onset of strain localization could not be defined on the basis of visual observations. Therefore, the limiting values for half of the double amplitude (γDA/2) and single amplitude (γSA) shear strain, to initiate strain localization, were determined from test results based on changes in the deviator stress response and strain accumulation properties as well as changes in the strain-softening behavior during cyclic shear. It was found that γSA is a more appropriate parameter than γDA/2. Irrespective of the static shear stress level, the limiting strain value for γSA was evaluated to be in the range of 23–28% for liquefied loose Toyoura sand specimens (i.e., stress reversal and intermediate tests). Alternatively, the limiting strain value could not be properly defined when liquefaction did not occur (i.e., non-reversal stress tests), although various methods were employed.

Cyclic shear behavior and seismic response of partially saturated slopes

In order to quantitatively investigate the effects of degree of saturation on the cyclic shear characteristics and seismic response of sloping grounds, undrained cyclic triaxial tests and online pseudo-dynamic response tests were conducted on decomposed granite (Masado) soil samples. Online testing is a method of feeding soil response characteristics directly from soil samples into a modeling algorithm. In the both series of tests, soil specimens prepared under different degrees of saturation and subjected to various initial shear stresses were tested. The results obtained from the tests quantitatively confirmed the notion that unsaturated soils do not undergo large strength decrease as fully saturated ones do. The initial static shear stress and applied dynamic stress define the mode of failure of the soil. Higher saturation ratio and/or steeper slopes lead to lower cyclic resistance and larger deformation. The experimental results obtained were then used in conjunction with pseudo-static slope stability analysis and modified Newmark sliding block model to estimate the deformation of embankments which were under different degrees of saturation. The example calculations clearly showed the effect of saturation on the magnitude of seismic-induced displacements of embankments.

Effects of initial static shear on liquefaction and large deformation properties of loose saturated Toyoura sand in undrained cyclic torsional shear tests

This study focused on the role which static shear plays on the large deformation behavior of loose saturated sand during undrained cyclic loading. A series of undrained cyclic torsional shear tests was performed on saturated Toyoura sand specimens up to single amplitude shear strain exceeding 50%. Three types of cyclic loading patterns, i.e., stress reversal, intermediate and non-reversal, were employed by varying the initial static shear level and the cyclic shear stress amplitude. The observed types of failure could be distinguished into liquefaction (cyclic and rapid flow) and residual deformation by comparing both monotonic and cyclic undrained behavior. It was found that the presence of initial static shear does not always lead to an increase in the resistance to liquefaction or strain accumulation; they could either increase or decrease with an increasing initial static shear level depending on the type of loading pattern and failure behavior. In addition, according to the failure behavior which the specimens exhibited, three modes of development of large residual deformation were observed.

Strain energy based evaluation of liquefaction and residual pore water pressure in sands using cyclic torsional shear experiments

In this study, cyclic hollow cylinder torsional tests were conducted on the reconstituted specimens of Toyoura sand in a practical range of initial density and stress states. The results were employed to evaluate the liquefaction resistance and residual pore water pressure of sand using the strain energy concept. A simple pore water pressure (PWP) model with two calibration parameters was developed for the prediction of residual pore pressure as a function of cumulative strain energy density and the capacity energy of sand. Capacity energy is defined as the cumulative strain energy that is required for liquefaction onset. Based on the results of the tests, an equation is then presented for the estimation of capacity energy in terms of relative density and initial effective confining pressure of sand. This equation is shown to work well as a state boundary curve, which can discriminate between the liquefied and non-liquefied field case histories. Several extra tests were also performed to investigate the effect of initial static shear stress on the proposed PWP model and capacity energy. The results show that initial shear stress has a minor effect on the trend of the proposed PWP model; however, it definitely affects the capacity energy. The final part of the paper aims to confirm reasonable performance of the proposed PWP model by the available observations of seismically induced pore water pressure in shaking table, centrifuge, and real site conditions.

Effect of static shear stress on the cyclic resistance of sands in simple shear loading

An experimental study of the effect of (initial) static shear stress on the undrained simple shear response of sands is presented. Liquefaction resistance of two sands, one generally contractive an...

Cyclic behaviour and resistance of saturated sand under non-symmetrical loading conditions

This paper reports findings from an experimental study that aimed to investigate the undrained behaviour of sand in non-symmetrical cyclic loading, and to clarify the role of initial static shear in liquefaction resistance. The testing programme, conducted on a standard sand under triaxial conditions, covers a broad range of initial states in terms of relative density, confining stress and initial shear stress ratio (α). Three distinct failure modes have been identified from the tests: flow-type failure, cyclic mobility and plastic strain accumulation. Of these, flow-type failure, characterised by abrupt runaway deformations without any prior warning, is the most critical, and pertains to sand in the loose state. The tests also demonstrate that the presence of initial static shear stress is beneficial to the liquefaction resistance of loose sand at low α levels, but it becomes detrimental at high α levels. In this connection the concept of threshold α is proposed, together with the use of a no-stress-reversal line for better characterisation of the effect of initial static shear. Furthermore, in the conceptual framework of critical state soil mechanics, a fairly good linear relationship has been established between the threshold α and the state parameter ψ that collectively accounts for the initial relative density and mean stress level. This relationship suggests that the threshold α decreases with increasing values of ψ, or with sand becoming looser than the critical state. It is further proposed that the concept of threshold α also applies to sand at high relative density, as long as the confining stress becomes sufficiently high. This proposal leads to a unified and consistent interpretation of the complicated static shear effect.

Analysis of pile behaviour in liquefying sloping ground

Abstract In this paper, a numerical procedure based on the finite element method is outlined to investigate pile behaviour in sloping ground, which involves two main steps. First a free-field ground response analysis is carried out using an effective stress based stress path model to obtain the ground displacements, and the degraded soil stiffness and strength over the depth of the soil deposit. Next a dynamic analysis is carried out for the pile. The interaction coefficients and ultimate lateral pressure of soil at the pile–soil interface are calculated using degraded soil stiffness and strength due to build-up of pore pressures, and the soil in the far field is represented by the displacements calculated from the free-field ground response analysis. Pore pressure generation and liquefaction strength of the soil predicted by the stress path model used in the free-field ground response analysis are compared with a series of simple shear tests performed on loose sand with and without an initial static shear stress simulating sloping and level ground conditions, respectively. Also the numerical procedure utilised for the analysis of pile behaviour has been verified using centrifuge data, where soil liquefaction has been observed in laterally spreading sloping ground. It is demonstrated that the new method gives good estimate of pile behaviour, despite its relative simplicity.

Initial Static Shear Effect on Cyclic Liquefaction Behaviour of Sand

Evaluation of soil liquefaction potential plays a key role in geotechnical earthquake engineering design since liquefaction is a major cause of earthquake-induced damages. While the evaluation procedure works well for free-field level ground conditions, the existence of initial static driving shear stress under earth dams or buildings complicates the issue. To address the insufficiency in current understanding, this paper presents an experimental study of the initial shear effect on the liquefaction resistance of sand, focusing mainly on the dependence of the effect on the initial state and sand type. A large number of cyclic triaxial tests have been conducted on two standard silica sands under a wide range of initial shear stress, density and confining pressure. Three distinct cyclic responses are identified to be dependent on the initial state and degree of stress reversal. The test results indicate that while the initial shear effect on loose sand can be beneficial or detrimental, that on dense sand is...

The influence of an initial simple shear deformation on long-wave motion in an elastic layer

Long wave dispersion in an incompressible elastic layer subject to an initial static simple shear deformation is investigated. Long wave approximations of the dispersion relation associated with zero incremental traction on the faces are derived for both low and high-frequency motion. Comparison of approximate and numerical solutions is shown to provide excellent agreement over a surprisingly large wave number range. Within both the low and high-frequency regimes, the approximations are employed to establish the relative asymptotic orders of the displacement components and hydrostatic pressure. In the high-frequency case, the in-plane component of displacement is shown to be asymptotically larger than the normal component; motion is, therefore, essentially that of thickness shear resonance. The influence of this specific form of initial deformation is, therefore, seemingly minor in respect of longwave high-frequency motion. However, in the long-wave low-frequency case, considerable differences are noted in comparison with both the classical and previously published prestressed cases. Specifically, both the normal and in-plane displacement components are of the same asymptotic order, indicating the absence of any natural analogue of either classical bending or extension.

Study on seismic behavior of nonplastic silt by means of ring-shear apparatus

The influence of static shear stress on undrained cyclic behavior of nonplastic and low-plasticity silts has been studied by means of undrained cyclic torque-controlled ring-shear tests. The cyclic and post-cyclic behavior of silty soils assumed on sliding surface were investigated to assess the liquefaction potential and cyclically induced deformation of silty slopes. Six different initial static shear stresses corresponding to slope angles from 0° to 25° were examined. To better understand undrained cyclic behavior of silt governed by a change in clay content, three different mixtures were achieved by mixing of nonplastic silt with 0%, 10%, and 20% of commercially available clay. These tests were conducted to simulate field conditions prior to earthquake with initial static shear stresses corresponding to slopes and those with no initial static shear stresses of level grounds. The gradual loss of mobilized undrained cyclic shear resistance after failure and pore water buildup in relation to a number of cycles was observed. The undrained response of the soil to cyclic shear stress loading with the constant amplitude revealed the significant effect of the initial static shear stress on the excess pore water pressure generation and post-failure shear resistance. Test results showed that an increase in the initial static shear stress at the given initial effective normal stress is associated with an increase of mobilized shear resistance at its peak state; however, the actual resistance to liquefaction diminished for both nonplastic and low-plasticity silts. During both cyclic and post-cyclic stages of loading, distinctly different types of shear deformation were identified. In order to evaluate mobility of landslides, a modified conventional brittleness index for seismic loading, \(I_{{\text{BS}}} = {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} \mathord{\left/ {\vphantom {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}\), was proposed and used to characterize unlimited deformation of silts.

Cyclic loading response of loose air-pluviated Fraser River sand for validation of numerical models simulating centrifuge tests

Cyclic loading response of loose Fraser River sand was investigated, as input to numerical simulation of centrifuge physical models, using constant-volume direct simple shear tests conducted with and without initial static shear stress condition. Although the observed trends in mechanical response were similar, air-pluviated specimens were more susceptible to liquefaction under cyclic loading than their water-pluviated counterparts. Densification due to increasing confining stress (stress densification) significantly increased the cyclic resistance of loose air-pluviated sand, with strong implications for the interpretation of observations from centrifuge testing. The stress densification effect, however, was not prominent in the case of water-pluviated specimens. The differences arising from the two specimen reconstitution methods can be attributed to the differences in particle structure and highlight the importance of fabric effects in the assessment of the mechanical response of sands. The initial static shear stresses appear to reduce the cyclic shear resistance of loose air-pluviated sand in simple shear loading, in contrast to the increases in resistance reported on the basis of data from triaxial testing. Data from laboratory element tests that closely mimic the soil fabric and loading modes of the centrifuge specimens are essential for meaningful validation of numerical models.Key words: liquefaction of sands, air-pluviation, cyclic loading, direct simple shear testing, specimen preparation, fabric.

Direct Simple Shear Testing for Post-Cyclic Degradation in Stiffness of Nonplastic Silt

Abstract Using a sequential procedure of cyclic and postcyclic direct simple shear tests, the stiffness degradation characteristics of nonplastic silt are studied during and immediately after cyclic loading by direct simple shear (DSS) tests. The results from cyclic and postcyclic DSS tests were interpreted using methods modified from those adopted for cyclic and postcyclic triaxial tests. In particular, the effect of initial static shear stress (ISSS) on postcyclic behavior is considered for nonplastic silt. The findings obtained from the sequential DSS testing on nonplastic silt are: (i) during cyclic loading the tendency of the stiffness to decrease with increasing normalized pore pressure is different for plastic and nonplastic silts, (ii) since the relation between equivalent stiffness ratio and single amplitude shear strain for nonplastic silt does not filt the Hardin-Drnevich Model, new modified relation is needed, (iii) the larger the ISSS is, the faster the decrease in stiffness ratio becomes, and (iv) the decrease of stiffness ratio for nonplatic silt is less marked than that for plastic silt when plotted against the normalized pore pressure after cyclic load.

Undrained cyclic shear response evaluation of sand based on undrained monotonic ring shear tests

The equation of motion of an undrained torque-controlled ring shear apparatus is formulated in a Newmark sliding block manner to evaluate the undrained cyclic shear response of a saturated clean sand with initial (static) shear stress. The computational procedure employs the shear resistance–displacement curve from undrained monotonic shearing as an estimate of the undrained yield strength under cyclic loading conditions. The numerically assessed cyclic response of sand in the ring shear apparatus is compared with the experimental data from undrained cyclic ring shear tests. The results of this study enhance the accuracy of a sliding block method for the assessment of seismically induced undrained displacements on shear surfaces in a saturated cohesionless soil using the experimental data from undrained monotonic shearing.

Stress-Strain Behavior of Compacted Sandy Material Under Cyclic Simple Shear

A conventional direct shear apparatus was modified to provide drained simple shear mode during cyclic loading. Several tests were performed in order to investigate the seismically induced deformation, stress-strain behavior and cyclic residual shear deformation of unsaturated and relatively dense sandy earth structures. This paper reports the effects of different parameters such as number of cycles, initial static shear stress, soil type, degree of compaction, consolidation time and stress, creep time and aging, amplitude of harmonic and irregular cyclic loading and loading rate on the cyclic stress-strain behavior and residual deformation accumulation of the samples. Based on the test results, concepts of Masing’s rule are modified to model the stress-strain behavior of the material during the cyclic simple shear.

Shake Table Tests on Residual Deformation of Sandy Slopes Due to Cyclic Loading

The evidence of earth structure damages due to earthquakes and the need for study on relatively dense and unsaturated earth structures such as river dikes and road embankments motivated this research work. A series of shake table tests were performed in order to provide the needed experimental data for this study. During the experimental program, two types of soils, i.e. Toyoura sand and a sample from natural deposits of sandy material were used. A rectangular laminar box was mounted over a rigidly fabricated slope and subjected to horizontal shaking. Sweep tests and Swedish sounding tests were also performed in order to measure the material properties. The results of these tests show the effect of different parameters such as amplitude of cyclic loading, number of cycles, initial static shear stress, confining pressure and shear stress history (loading-unloading, over-consolidation and creep time) on shear deformation of unsaturated slopes during and after cyclic loading. The outcomes of the test results are used as a basis to develop a new single degree of freedom model of mass and nonlinear spring over a slope. Stress-strain behavior of the soil is defined as a nonlinear stiffness function of a spring. Time dependent input acceleration is considered as the base excitation and the induced inertia force is superimposed on the initial static shear stress. The numerical solution of the equation of motion of the model predicted the residual deformation of the system at the end of shaking. Several cases were analyzed and compared with the test results.

Relating Kα to Relative State Parameter Index

The effect of an initial static shear stress on the undrained cyclic loading resistance of saturated sand, as observed in laboratory testing, is often expressed in terms of a correction factor, Kα. Prior research has demonstrated that Kα depends on the state of the sand, as commonly represented by its relative density and confining stress conditions. This dependence of Kα on state is intuitively consistent with critical state concepts, but the implementation of this dependence in practice has been hindered by the absence of simple methods for estimating state and its effects without having to perform an extensive program of laboratory testing. In this paper, a simple index for representing state is derived from Bolton’s (1986) relative dilatancy index and shown to provide a reasonable and practical index for describing the variation of Kα with both relative density and confining stress.

Cyclic strength and deformation of crushable carbonate sand

Cyclic triaxial tests have been carried out on a skeletal carbonate sand from the west coast of Eire. Results are presented for undrained cyclic shear tests on samples with 80% of relative density consolidated under both isotropic and anisotropic conditions. Failure was defined as a 5% double amplitude cyclic strain and a 5% peak axial strain for stress-reversal and non-reversal stress conditions, respectively. Using this definition the cyclic strength for isotropically consolidated samples was affected by the confining pressure although the angular platey nature of the sand resulted in higher cyclic strengths than for a comparable silica sand. For anisotropically consolidated samples the cyclic strength increased with increasing initial static shear stress while on the other hand the cyclic strength normalised in the usual way with respect to the initial confining pressure decreased as this pressure increased.

Strain Rate and Preshear Effects in Cyclic Resistance of Soft Clay

Cyclic constant-volume direct simple shear tests were performed on intact specimens of a sensitive clay reconsolidated to the in-situ stress state resulting in an overconsolidation ratio (OCR) of 2.2. The tests were conducted for different values of initial static undrained shear stress. The results confirmed that the high strain rate associated with cyclic loading has a significant effect on the cyclic resistance of soft clays. For up to 12 cycles, the strain-rate effect fully compensates the degradation of shear strength associated with cycling. The presence of an initial static shear stress decreases the cyclic resistance but increases the total undrained shear resistance, due to the partial or total disappearance of stress and strain reversal. The results indicate that for such a clay, the static undrained shear strength provides a conservative estimate in a pseudostatic analysis.

Undrained Cyclic Shear Behaviour of Normally Consolidated Clay Subjected to Initial Static Shear Stress

A series of undrained cyclic triaxial compression tests has been performed on a high plasticity marine clay. Testing was performed using various combinations of initial static and subsequent cyclic shear stress on isotropically and anisotropically consolidated specimens. Additionally, monotonic triaxial compression and extension tests were also performed on the same clay with the same initial condition. The cyclic shear strength was considered first, and then, the accumulated shear strain was investigated related to the effective stress ratio at the peak cyclic stress. Based on the experimental results, a semi-empirical model is proposed for evaluating the development of pore pressure and residual shear strain during cyclic loading. The model successfully explains the behaviour of clay subjected to various magnitudes of initial static and subsequent cyclic shear stresses.