Cyclic Torsional Shear Tests Research Articles

The effects of frequency on damping properties of sand

In this study the effects of frequency on damping behavior of dry sand were examined. To study the effects of frequency, two mechanical models were proposed and evaluated for interpretation of the cyclic torsional shear testing results. Cyclic torsional shear tests were performed on dry sand specimens for the frequency range of 0–1–20 Hz. It is found that the frequency effect on damping properties of sand is relatively significant, and thus the damping appears to be nonhysteretic. Finally, a new combined damping model is proposed for describing the damping behavior of dry sand observed in this research.

Strain Rate Effects on Shear Modulus and Damping of Normally Consolidated Clay

Abstract A laboratory investigation into the effects of shear strain rate on shear modulus and hysteretic damping of normally consolidated clays was carried out. The effects of shear strain rate were examined at cyclic shear strain amplitudes between 10−6 and 10−3 in undrained cyclic torsion shear tests. When the frequency of loading was changed between 0.005 and 0.1 Hz, the equivalent shear modulus was insensitive to the rate of shear straining. On the other hand, the hysteretic damping increased according to the decrease in the shear strain rate. Furthermore, for shear strains less than about 2 × 10−5, the maximum stiffness was hardly influenced by the shear strain rate, type of loading, number of cycles, and the cyclic prestraining; it can therefore be characterized as pseudoelastic shear modulus. On the basis of the test results, it is concluded that when applying the results of laboratory cyclic loading tests to the analysis of in situ cyclic loading problems, the effects of shear strain rate on hysteretic damping should be properly evaluated to match the frequency of loading expected in the field.

Small Strain Shear Modulus of Clay Sedimentation in a State of Normal Consolidation

In this paper, consideration is given as to how to characterize depth-variation for the small-strain shear modulus of natural clay sedimentation, in a state of normal consolidation. A case study was carried out for a relatively uniform clay layer deposited in the Holocene era. Initially, the effects of both strain and in-situ stress levels on secant shear modulus were carefully examined in cyclic torsion shear tests using undisturbed samples, which were recovered at different depths in a test borehole. The range of shear strain examined was between 0.001% and 1%. Similar examination was made for a silty clay using reconstituted samples that were isotropically consolidated at different stress levels. On the basis of the results of these laboratory tests, together with the shear modulus from an in-situ seismic survey, the small-strain shear modulus was formulated in terms of the stress and strain levels, and linked also to undrained shear strength. Interactions of the small strain stiffness between in-situ and laboratory are discussed in depth with an attention paid to the existing aging effect in the original subsurface condition.

Transient Shear Strength of Saturated Sand under Cyclic Loading Considering Strain-Rate Effect

The shear strength of saturated sands under undrained cyclic loading is defined as transient shear strength in order to characterize its time dependency and to distinguish it from the undrained cyclic strength based on the equivalent cycle concept. It has been found from the undrained cyclic triaxial tests on two saturated Fujian sands that: (a) the transient shear strength of saturated sand is strain-rate dependent when the shear strain rate is more than 10% per second. In general, the transient shear strength of saturated sand consists of frictional resistance and viscous resistance, which can be described as τdt=τft+τηt=σt′tanϕd′+ηγ.t; (b) the angle of dynamic internal friction ϕd′ is not affected by the number of cycles, consolidation stress ratio and frequency of cyclic stress (1-20 Hz) and the apparent viscosity coefficient η almost remains constant within the range of the tested shear strain rates (10-60% per sec) for a given density; and (c) the angle θ0, formed by the phase transformation line in τ – σ plane, is nearly constant for a given sand. Furthermore, undrained cyclic torsional shear tests on hollow cylindrical specimens show that the frictional strength and the stress states at the phase transformation define two three-dimensional curved surfaces for the effective principal stress; and in addition, both of them can be described by the Matsuoka-Nakai criterion. Methods are proposed to determine the dynamic strength parameters and to correct the damping of the test apparatus system, the effect of inertia force of the sand specimen and the time lag of measured excess pore water pressure in cyclic triaxial tests with a higher frequency.

Evaluation of liquefaction resistance of sand by maximum shear modulus

Abstract Laboratory tests and in‐situ seismic cone penetration tests are conducted to evaluate the liquefaction resistance of Lanyang sandy soils. Resonant column tests and cyclic torsional shear tests are performed on remolded samples to establish the relationship between the maximum shear modulus and the liquefaction resistance. The relationship is used to predict the in‐situ liquefaction resistance. Test results show that liquefaction resistance predicted by CPT‐qc is consistent with the one predicted by maximum shear modulus. From the comparisons, the maximum shear modulus is found to be a good index in assessing in‐situ liquefaction resistance.

Comparisons of deformation characteristics of rockfill materials using monotonic and cyclic loading laboratory tests and in situ tests

The deformation characteristics of rockfill materials at very small strains were investigated by comparing the results of monotonic and cyclic loading laboratory tests with geophysical P- and S-wave logging data from the field. Using a precision linear variable differential transformer for displacement, the elastic moduli of rockfill materials at very small strains were measured in monotonic and cyclic loading triaxial tests. The laboratory test results agreed well with the field results. The shear moduli of rockfill materials from both a monotonic loading torsional simple shear test and a cyclic loading torsional simple shear test also showed good correspondence. Furthermore, the shear modulus predicted from the in situ shear wave tests in rockfill dams corresponded reasonably well with the modulus in the large-scale triaxial tests in the laboratory. Key words : deformation characteristics, embankment dams, rockfill materials, laboratory test, in situ test.

Torsional Motion Monitoring System for Small-Strain (10−5 to 10−3%) Soil Testing

Abstract For accurate measurements of stress-strain hysteresis loops at strains below 10−3%, a torsional motion monitoring system was modified to incorporate micro-proximitors. A micro-proximitor system with enlarged target arms resulted in about 50 times higher resolution in motion monitoring than previously possible. The four proximitor signals (instead of two) were measured, compared, and averaged to insure that pure torsion of the system was generated and that any bending did not enter the measurement. Ambient noise was controlled by using a low-pass filter and/or a vibration isolation table. With this new system, shear modulus was measured at strains as low as 2 · 10−5%, and hysteretic damping was measured at strains as low as 6 · 10−5%. Below the elastic threshold strain, shear modulus of dry sand measured in the cyclic torsional shear test is independent of strain amplitude. Hysteretic damping exists below the elastic threshold and is independent of strain amplitude, even though the value is quite small.

Dynamic deformation characteristics of sands and rockfill materials

Cyclic torsional simple shear (CTSS) tests and cyclic triaxial (CTX) tests were carried out to investigate the dynamic deformation characteristics of sands and rockfill materials. It was found that the shear modulus and damping ratio can be expressed as a function of shear strain, void ratio, and confining stress. Also the shear modulus in CTSS tests is larger than in CTX tests because of the influence of the intermediate principal stress. When the shear strain is increased, the shear modulus (G) and damping ratio (h) of the rockfill materials were altered at smaller strains than in sands. Key words : sands, rockfill materials, torsional simple shear, shear modulus, damping ratio.

Effects of Fabric Anisotropy of Sand on Cyclic Undrained Triaxial and Torsional Strengths

ABSTRACT In order to examine the effects of fabric anisotropy on the relationships between the cyclic triaxial and torsional shear strengths, a series of cyclic undrained triaxial and torsional shear tests was performed on three kinds of sands made by three different sample preparation methods. The test results showed that the cyclic undrained strengths varied with sample preparation methods. In particular, the effects of sample preparation methods were clearly observed in the triaxial test. The relationships between the cyclic triaxial and torsional shear strengths also varied with sample preparation methods. In addition, it was found that the effects of fabric anisotropy due to the difference in the sample preparation methods appear in the axial deformation and the developed pore water pressure characteristics during the cyclic triaxial test. In order to evaluate quantitatively the effects of fabric anisotropy on the relationships between the cyclic triaxial and torsional strengths, new index parameters were introduced, which express the degree of the anisotropy of specimens. Consequently, it was indicated that there exists a unique rule between these parameters and the ratio of torsional strengths to triaxial strengths.

The Mechanism and a Simplified Procedure for the Analysis of Permanent Ground Displacement Due to Liquefaction

ABSTRACT To study the mechanism of permanent ground displacement due to liquefaction, shaking table tests on 24 soil models were conducted. In these models, the slope of the ground surface, the thickness of a liquefied layer, the density of the liquefied layer and some other factors were changed. Permanent ground displacements did not occur at the boundary between the liquefied layer and the lower non-liquefied layer, but occurred in the liquefied layer with a constant shear strain. Relationships between the displacement and the thickness of the liquefied layer or the slope of the ground surface were also obtained. Then, vane shear tests and cyclic torsional shear tests were carried out to measure the rate of reduction of shear strength or shear modulus. By the cyclic torsional shear tests, it was clarified that the shear modulus decreases to a very small value due to liquefaction. Based on these test results, a simplified procedure for the analysis of permanent ground displacement was proposed. To confirm the accuracy of this procedure, it was applied to the models used for the shaking table tests and to typical soil cross sections of soil at Niigata City. Finally, some measures to prevent permanent ground displacement were discussed based on the shaking table tests and analyses.

SHEAR STRENGTH AND DYNAMIC DEFORMATION CHARACTERISTICS OF GRAVELS

In order to investigate the shear strength and dynamic deformation characteristics of gravels, we carried out the torsional simple shear tests (TSS), triaxial compression tests (TC), cyclic torsional simple shear tests (CTSS) and cyclic triaxial tests (CTX).The results obtained can be summarized as follows.1) Shear strength in TSS and shear modulus in CTSS are larger than those in TC and CTX due to the influence of intermediate principal stress.2) Shear modulus and damping ratio are expressed as a function of shear strain, void ratio and confining pressure.3) Elastic moduli in monotonic loading test and those in cyclic loading test agree with each other in Toyoura sand and gravels.

Liquefaction Resistance of A Partially Saturated Sand

ABSTRACT Cyclic torsional shear tests on hollow cylindrical specimens were conducted to investigate the effects of degree of saturation on the undrained cyclic shear strength (liquefaction resistance) of air-pluviated Toyoura sand at a relative density of 60 percent. The tests showed that (1) the liquefaction resistance increased significantly with a decrease in degree of saturation to such an extent that the liquefaction resistance at 70 percent saturation was about three times that at full saturation; (2) the liquefaction resistance also increased with a decrease in B-value, although a near-zero B-value was required for a significant increase in the liquefaction resistance ; and (3) the cyclic shear behavior of the partially saturated sand resembled that of denser saturated sand, exhibiting high strength as well as stable deformation characteristics. Based on the above findings it is concluded that a reduction in degree of saturation can be an effective means to increase the liquefaction resistance of loose sands.

Experimental Stress-Dilatancy Relations of Sand Subjected to Cyclic Loading

Cyclic triaxial tests, cyclic torsional shear tests on isotropically consolidated specimens and cyclic torsional simple shear tests on one-dimensionally consolidated specimens, using saturated Toyoura sand, were performed under drained conditions in order to investigate the general tendencies of the stress-dilatancy characteristics under cyclic loadings. It was found that in each testing method a unique relationship between the stress ratio and the dilatancy factor (strain increment ratio) exists irrespective of void ratio and pressure level. In each testing method, the rate of dilatancy, as defined as the rate of volumetric strain increment to shear strain increment (positive for volume expansion), becomes negative by the reversal of loading direction. Then it increases continuously with shearing without showing any discontinuous behavior at the moment when the sign of shear stress changes (i.e., at the neutral stress point), and subsequently it becomes positive. It was also found that in the triaxial tests an over-consolidation stress history affects the stress-dilatancy relation only below a certain stress ratio in the first loading and has no effect in the subsequent shearing. (Author/TRRL)

On Stress-Dilatancy Equations of Sand Subjected to Cyclic Loading

ABSTRACT Based on the results of a series of cyclic triaxial tests and cyclic torsional simple shear tests on saturated Toyoura sand performed under drained conditions, stress-dilatancy equations under cyclic loadings were studied. It has been found from the experimental work that in each of the testing methods a unique relationship between the stress ratio and the rate of dilatancy (strain increment ratio) exists which is independent of void ratio and pressure level. It has also been found experimentally that the rate of dilatancy, as defined as the rate of volumetric strain increment to shear strain increment (positive for volume expansion), becomes negative by unloading after the reversing of loading direction, and it increases continuously with shearing without showing any discontinuous behavior at the moment when the sign of shear stress changes, and subsequently it becomes positive. Thus, at the same stress condition, two different rates of dilatancy are possible to exist depending on the direction of shear straining. Four different representative stress-dilatancy relations based on (i) the sliding block theory, (ii) the Rowe’s theory, (iii) the Roscoe’s energy dissipation theory, and (iv) the Taylor’s energy dissipation theory, were modified to apply to cyclic loading conditions with reversed loading directions. It was found that after these modifications some of these theories can well simulate the stress-dilatancy relations under drained cyclic conditions obtained by the tests. Finally, curves of equal plastic potential in cyclic drained simple shear were portraied on a stress plane. It is a double plastic potential function in the sense that two different plastic potential curves are passing through any given stress point, which are for two different directions of shear-straining.

Rotation of Principal Directions in K0‐Consolidated Clay

Monotonic and cyclic torsion shear and cubical triaxial tests were performed to study the influence of rotation of principal stress directions on the behavior of K0‐consolidated clay. The stress‐strain and pore‐pressure relations were clearly affected by stress rotation. Strength and pore‐pressure developments occurred faster in monotonic tests without stress rotation, but the effective friction angle was not affected by initial K0‐consolidation or stress rotation. In cyclic tests, two‐way stress rotation produced more rapid degradation, higher pore pressures, and larger strains than in tests with one‐way stress rotation and tests without stress rotation. Monotonic tests after cyclic loading showed that the effective stress failure condition was not influenced, but the stress‐strain behavior was affected by hardening due to cyclic loading.

DYNAMIC DEFORMATION CHARACTERISTICS OF SANDY SOILS UNDER LOW CONFINING PRESSURE

Dynamic modulus of rigidity and damping coefficient of sandy soils under low confining pressures (less than around 20kPa) were investigated in the present study. Series of cyclic simple shear tests in which horizontal inertia force is applied on the surface of the specimen by use of a shaking table apparatus were performed on Toyoura sand and a compacted sandy soil. Some features associated with deformation parameters such as their dependencies on confining pressure and shear strain amplitude were examined, and the results were compared with those of cyclic triaxial and torsional shear tests conducted under routine high confining pressures in order to discuss characteristic differences and correlation between parameters obtained in low and high stress ranges.

Effect of Initial Shear on Cyclic Behavior of Sand

Drained and undrained cyclic torsional simple shear tests are conducted for saturated Ottawa sand with and without initial static shear applications. It was found from the experiment that when the tests were conducted under uniform cyclic shear strains, both pore water pressure generation in undrained conditions and the volume changes in drained conditions were affected very little by the level of the initial static shear applications. The strain method, which uses shear strain parameters as determinative parameters, has been proven to be very useful for liquefaction and cyclic volume change analyses.

Analysis of Wave-Induced Liquefaction in Seabed Deposits of Sand

To explore the nature of cyclic stress alteration in the seabed deposits of sand due to travelling waves, two-dimensional stress analysis was made for homogeneous elastic half-space subjected to a series of harmonic loads moving on its surface. The analysis indicated that changes in shear stress occur in such a way that, while its amplitude is maintained constant, the directions of the principal stresses rotate continuously. With a view to simulating such stress changes in the laboratory test, a series of cyclic triaxial torsion shear tests was conducted on loose specimens of sand. The test results indicated that the conventionally defined cyclic stress ratio is reduced by about 30% if the rotation of the principal stress directions is involved in the cyclic loading. The magnitude of wave-induced pressure at the seabed was determined on the basis of the linear theory of water wave propagation. This water pressure fluctuation was applied over the seabed, which is assumed to be an elastic half-space, to determine the cyclic stress ratio induced within the seabed deposit. The cyclic stress ratio thus determined is compared against the cyclic stress ratio causing liquefaction and cyclic mobility in which the continuous rotation of principal stress directions is taken into account. The procedures as above are integrated into a simple methodology to perform liquefaction analysis under wave loading conditions.

Effects of Overconsolidation and K0, Conditions on the Liquefaction Characteristics of Sands

When overconsolidation is induced in a horizontal deposit of sand by the removal of surcharge pressure or by other events, the lateral deformation is prevented and the coefficient of earth pressure at rest, K 0 , increases. To evaluate effects of the overconsolidation ratio as well as increased K 0 -value on the liquefaction resistance of sand deposits, several series of cyclic torsion shear tests were carried out on samples of clean sand overconsolidated to different degrees under various K 0 -conditions. The results of these tests were summarized in an empirical formula. This formula is used to estimate the cyclic strength of sand at any given OCR and K 0 -values, based on the cyclic strength for isotropically and normally consolidated samples.

Stress Conditions and Stress Histories Affecting Shear Modulus and Damping of Sand under Cyclic Loading

To evaluate the effects of various static stress conditions and stress histories on shear modulus and damping of sand under cyclic loadings, a comprehensive series of cyclic torsional shear tests on hollow cylindrical specimens of a clean sand was performed. All the tests were performed and the constant value of mean principal stress = 1.0kgf/cm2 (about 100 kN/m2) under fully drained condition. From the test results, the followings were found. For a wide range of shear strain amplitude (γ = 5 × l0-5 ~ 5 × l0-3), the effect of stress ratio on shear modulus is minor in the triaxial compression case but considerable in the triaxial extension case. Initial shear stress decreases shear modulus especially for the triaxial compression case. Although shear modulus is affected by various static stress conditions, its strain-dependency is insensitive to those static stress conditions. The effects of stress ratio and initial shear stress on damping ratio are less important than shear strain amplitude γ and mean principal stress p. It was found that the effects of overconsolidation history and previous larger cyclic shear stress history on G and ƞ are also less important than γ and p.