Pre-failure Instability Research Articles

Effect of plastic volumetric strains on shakedown of hardening granular soils

The granular soils are pressure dependent materials and substantial volume change occurs during loading. Some granular soils contract (volume decrease) when sheared and some dilate (volume increase), depending on their initial state. Owing to the plastic volume change that accompanied plastic deformation, the lack of normality rule arises and the granular soil exhibits a non-associated flow rule. As known the non normality has destabilizing effect on the soil behavior in the hardening regime (loss of positive definiteness of the second order work). The unstable behavior usually develops in association with dilation. A contracting soil displaying hardening and stable material behavior under drained conditions, may succumb to unstable flow type when its behavior becomes dilating. This paper deals with the extension of the static shakedown theorem to dilative granular soils in drained conditions, exhibiting pre-failure instability in the hardening regime, in the framework of non associated plasticity.

Sand instability under constant shear drained stress path

Experimental findings have revealed that granular soils are vulnerable to pre-failure instability when subjected to constant shear drained (CSD) stress path. Using the concept of loss of uniqueness, it is shown that two dissimilar scenarios and accordingly, two independent criteria for granular soils in loose and dense states may lead to instability under CSD. The instability criteria are then applied in association with a state-dependent elastoplastic constitutive model taking into account possibility of elastic-plastic coupling. Numerical simulations have confirmed that instability in loose sands triggers prior to complete mobilization of the critical state stress ratio. However, instability of sands in dense state instigates at peak mobilized friction angle. A nearly unique linkage between stress ratio and state parameter at the onset of instability is predicted.

Instability of sands under axisymmetric proportional strain and stress loadings

This paper compares the pre-failure instability of sands under axisymmetric proportional strain and axisymmetric proportional stress conditions. Based on a deviatoric hardening plasticity model, the critical hardening moduli under both conditions were obtained by the second-order work criterion. In the proportional strain condition, the instability of sands depends on the stress–dilatancy relationship and the imposed strain proportion ratio. Loose sand is prone to instability; dense sand maintains stable behaviour under the undrained condition, butit may become unstable in expansive loading. In the proportional stress condition, dense sand shows stable behaviour; loose sand can be stable as long as the stress proportion is at a sufficient distance from zero. A material state-dependent elasto-plasticity model was used to predict the instability of sands in triaxial tests under the proportional strain condition, constant mean stress condition, and constant shear stress condition; the predicted instability behaviour agrees well with the experimental data.

Evaluating the capability of a critical state constitutive model to predict the collapse potential of loose sand

Many catastrophic flow failures in granular soil slopes are believed to be caused by a rise in pore water pressure associated with substantial loss of soil shear strength. This failure mechanism is known as prefailure instability or static liquefaction. Constant shear (CS) and consolidated undrained (CU) triaxial tests can reproduce stress paths, in which such instability may occur before reaching the failure. In the present study, a previously proposed critical state constitutive model was first used to simulate the behavior of loose saturated sand in CU tests. It was then employed to predict the instability of loose sand subjected to the CS loading. Under such loading, loose dry sand initially experience small volume increase, and then start to contract substantially. In saturated sand, such contractions can lead to the generation of pore water pressure and sudden decrease of shear strength. The capability of the model to predict the onset of the volume contraction and collapse potential of loose dry sand was examined by comparing the model predictions with experimental results of CS tests. The comparison showed that the effect of initial void ratio, consolidation and deviatoric stresses on behavior of loose dry sand can be well predicted by the model.

Pre-failure instability of sand under dilatancy rate controlled conditions

Experimental results are presented in this paper to show that a runaway type of pre-failure instability can occur for sand under dilatancy rate controlled conditions when an appropriate strain increment ratio, dεv/dε1, is imposed. This type of instability is similar to the runaway type of instability observed for very loose sand under undrained conditions. Whether a soil element will undergo pre-failure instability depends on the difference between the strain increment ratio of the soil obtained from drained test, under a specified effective confining pressure, (dεv/dε1)s, and the strain increment ratio imposed during the test, (dεv/dε1)i, rather than the absolute magnitude of (dεv/dε1)i. Based on the experimental data obtained in this study it was found that an instability line can be determined from a series of strain path tests conducted at different effective confining pressures but with the same dεv/dε1 by joining the peak points of the effective stress paths to the origin in the q–p′ stress space. This line is similar to the instability line obtained from undrained tests on loose sand. The instability tests under dilatancy rate controlled conditions indicate that the stress ratio at the onset of instability obtained in the instability tests coincide with the peak stress ratio line. This suggests that the peak stress line can be used to predict the onset of instability under dilatancy rate controlled conditions in the same way as the use of instability line to predict the onset of instability under undrained conditions.

Instability of dilative sand

A type of pre-failure instability that occurs for medium loose to dense sand under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in the effective mean stress, it becomes unstable after the stress path crosses an instability line (IL). The IL specifies a minimum stress ratio under which instability may occur. It is not unique but changes with the void ratio of the soil and the applied effective stresses. The instability occurring under drained conditions is different from that under undrained conditions. The differences and similarities between the two types of instability are pointed out. Practical implications of the study in analysing the failure mechanisms of granular soil slopes under various drainage conditions are discussed.

Factors affecting pre-failure instability of sand under plane-strain conditions

Experimental data obtained from a plane-strain apparatus are presented in this paper to show that a pre-failure instability in the form of a rapid and sustained increase in strain rate can occur for both contractive and dilative sand under fully drained conditions. However, this type of instability is different from the runaway type of instability observed under undrained conditions, and has therefore been called conditional instability. Despite the differences, the conditions for both types of instability are the same for contractive sand. There are also other factors that affect the pre-failure instability of sand observed in the laboratory. These include the stress ratio, void ratio, sand state, load control mode and reduction rate of the effective confining stress. In this paper, these factors are discussed and analysed using experimental data obtained from undrained instability (or creep) tests and constant shear drained (CSD) tests carried out under plane-strain conditions.

Instability of Loose Sand under Drained Conditions

A type of prefailure instability which occurs under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in effective mean stress, it becomes unstable after the stress path crosses an instability line. The instability occurring under drained conditions is different from the type observed under undrained conditions. The differences and similarities between the two types of instability are elaborated. The practical implication of the study in analyzing the failure mechanisms of granular soil slope is also discussed.

Pre-Failure Instability Behavior of Sand in Strain Path Testing Under Plane-Strain Conditions

ABSTRACT Experimental data are presented in this paper to study the instability behavior of sand in strain path testing under plane-strain conditions. K0 consolidated strain path tests were conducted on Changi sand using a plane-strain apparatus. The test results show that the occurrence of pre-failure instability under plane-strain conditions is affected by the void ratio and the strain increment ratio. This is consistent with previous findings established under axisymmetric conditions. However, instability behavior of medium dense sand in strain path-controlled plane-strain tests can also be affected by shear band formation. In this case, on-failure instability takes place and failure is likely caused by shear band development. This observation is different from that in triaxial strain path-controlled tests where shear bands do not normally occur.

Discussion of “Possibility of Postliquefaction Flow Failure due to Seepage” by Noriaki Sento, Motoki Kazama, Ryosuke Uzuoka, Hirofumi Ohmura, and Makato Ishimaru

The discusser commends the authors for their interpretation of the mechanisms of sand flow failure, and contributes to the discussion on three separate points. The discusser is pleased that research performed over ten years ago still has current practical applications, identifies a fundamental difference between previous research on prefailure instability and the instability mentioned in the original article from July 2004, and inquires about an outcome the authors may have encountered during a reconsolidation stage in the triaxial test.

Effect of undrained creep on instability behaviour of loose sand

Prefailure instability in the form of a runway deformation can occur for loose sand under undrained conditions. The effect of creep on the instability behaviour of loose sand is studied experimentally in this note. Previous studies have established that instability can only occur when the stress state is above the instability line. However, if creep is allowed to develop under undrained conditions, instability can still possibly occur even when the stress state at which creep develops is below the instability line. A boundary that specifies the condition in which instability may be induced by undrained creep is determined based on the experimental data.Key words: laboratory test, liquefaction, sands, shear strength.

Pre-failure strain softening and pre-failure instability of sand: a comparative study

Strain softening in the form of decreasing shear resistance and instability, the inability to sustain given loads, can occur before the failure state is attained. This so-called pre-failure strain softening or pre-failure instability has been observed to occur for very loose sand in undrained tests and for dense sand in strain path controlled tests. Experimental data are presented in this paper to compare the pre-failure strain softening and pre-failure instability behaviour. It is established that the factors that control the occurrence of pre-failure strain softening and pre-failure instability are essentially the same, although the two types of behaviour are different. The similarities and differences between strain softening and instability are discussed for both loose and dense sand.

Pre-failure strain softening and pre-failure instability of sand: a comparative study

Strain softening in the form of decreasing shear resistance and instability, the inability to sustain given loads, can occur before the failure state is attained. This so-called pre-failure strain softening or pre-failure instability has been observed to occur for very loose sand in undrained tests and for dense sand in strain path controlled tests. Experimental data are presented in this paper to compare the pre-failure strain softening and pre-failure instability behaviour. It is established that the factors that control the occurrence of pre-failure strain softening and pre-failure instability are essentially the same, although the two types of behaviour are different. The similarities and differences between strain softening and instability are discussed for both loose and dense sand.

Collapse behavior of sand: Discussion

The test data that are presented in paper are of most interest. The two constant deviator stress path tests may be compared with tests done by Lade et al. (1988) and Lade and Pradel (1990) for very loose sand under undrained conditions and by Chu (1991) and Chu et al. (1993) for medium loose to dense sand under non-undrained (or strain path controlled) conditions. It should be noted that testing procedures used in and test results obtained from these tests are verv similar. All above-mentioned tests were conducted uhder a constant deviator load and decreasing effective confining stress conditions. The prefailure instability observed in these tests appears to be same type. Although authors called instability observed in constant deviator stress path tests collapse behaviour, word drained does not carry same meaning as in a drained conventional compression test. This is because pore-water pressure of sample was controlled during tests, and thus volume change of sample is not totally free. As the volume of sample for constant deviator stress (q = 125 kPa) test was essentially unchanged and remained at 0.804 before sample collapse, this test was actually conducted under an undrained condition. Therefore, 'paper by S. Sasitharan, P.K. Robertson, D.C. Sego, and N.R. Morgenstern. 1993. Canadian Geotechnical Journal, 30: 569-577.

Instability of Granular Soils under Strain Path Testing

Recent experimental studies revealed that when the drainage condition of loose sand is switched from drained to undrained it can manifest runaway instability prior to attaining the failure stress state. To study this type of unstable behavior (referred to as prefailure instability) under more general conditions, the strain-path testing technique was used to constrain the strain development of a triaxial sample along a constant strain-increment-ratio path. Medium-loose to dense samples were used in this study. Provided certain conditions are met, prefailure instability can occur even for a dense sample. The factors affecting the occurrence of instability are established from test results. Two instability conditions are established for predicting of instability under a general stress-controlled condition. These two conditions require input parameters that are relatively simple to obtain yet can accurately predict the occurrence of instability. For the special case of undrained condition, these two instability conditions degenerate to those proposed by previous researchers.