Decrease In Effective Stress Research Articles

Local Scour around a Square Structure due to a Runup Tsunami

The 2004 Indian Ocean tsunami caused large-scale sediment transport, resulting in substantial scour around a huge number of structures. In this study, tsunami-induced local scour around a land-based structure with a rectangular cross-section on a sand foundation was investigated with a series of hydraulic model experiments and numerical simulations. It was, as a result, revealed that local scour around the seaward corner of the structure could be evaluated with not only flow velocities on the sand foundation but effective stresses near the surface of the sand substrate, that is, flow velocities with a large Shields number resulted in a scour hole around the seaward corner of the structure during restricting force between sand particles fell down due to a decrease in effective stresses inside the sand substrate.

Cyclic shear response of fine-grained mine tailings

The mechanical response of three different types of fine-grained mine tailings is examined using data from constant-volume cyclic direct simple shear (DSS) tests. Under cyclic DSS loading, fine-grained tailings typically exhibit a cumulative decrease in effective stress along with progressive degradation of shear stiffness. The observed shear strain development due to cyclic mobility is similar in form to that of the previously observed cyclic shear response for natural silts. The cyclic resistance ratio (CRR) of laterite tailings is observed to increase with an increase in the initial effective confining stress (σ′vc). For this material, the dilative tendency due to stress densification seems to have overridden the contractive tendency due to the increase in confining stress. In contrast, the CRR of copper–gold–zinc tailings is insensitive to the initial effective confining stress, suggesting a response similar to that of normally consolidated clay. The postcyclic maximum shear strength ratio (Su-PC/σ′vc) obtained from constant-volume monotonic DSS tests is noted to increase with a decrease in the void ratio. For the fine-grained tailings considered in this study, the liquefaction susceptibility predicted using commonly used empirical criteria is not always in agreement with the liquefaction triggering determined from cyclic DSS tests.Key words: tailings, liquefaction, cyclic stress ratio, postcyclic shear strength, direct simple shear testing.

Influence of seismic cyclic loading history on small strain shear modulus of saturated sands

Small strain shear modulus G max is a key parameter together with the state of stress and shear strain amplitude for predicting the dynamic behavior of soils. Although the seismic cyclic loading on saturated soil deposits induces a decrease in effective stress and a rearrangement of the soil skeleton which may both lead to a degradation in undrained stiffness and strength of soils, only the contribution of effective stress reduction to G max degradation is considered in the Hardin and Richart equation which is widely used in seismic response analysis nowadays, and that of soil fabric change is neglected. In this paper, undrained cyclic triaxial tests were conducted on normally and isotropically consolidated saturated sands with the shear wave velocity measured intermittently by bender element during cyclic liquefaction, to study the influences of seismic cyclic loading history on small strain shear modulus G max during earthquake. And the G max values of samples without such influences were investigated for comparison. The tests results indicate that G max of sand under high amplitude seismic cyclic loading history influences is moderately lower than the corresponding value of non-cyclic loading effects at the same effective stress. Hence it is necessary to reinvestigate the determination of G max in seismic response analysis carefully to predict the ground responses more reasonably.

Experimental study of seismic cyclic loading effects on small strain shear modulus of saturated sands

The seismic loading on saturated soil deposits induces a decrease in effective stress and a rearrangement of the soil-particle structure, which may both lead to a degradation in undrained stiffness and strength of soils. Only the effective stress influence on small strain shear modulus Gmax is considered in seismic response analysis nowadays, and the cyclic shearing induced fabric changes of the soil-particle structure are neglected. In this paper, undrained cyclic triaxial tests were conducted on saturated sands with the shear wave velocity measured by bender element, to study the influences of seismic loading on Gmax. And Gmax of samples without cyclic loading effects was also investigated for comparison. The test results indicated that Gmax under cyclic loading effects is lower than that without such effects at the same effective stress, and also well correlated with the effective stress variation. Hence it is necessary to reinvestigate the determination of Gmax in seismic response analysis carefully to predict the ground responses during earthquake more reasonably.

Variation in penetrometer resistance with soil properties: the contribution of effective stress and implications for pedotransfer functions

Soil mechanical resistance to penetration by roots can potentially contribute to the spatial and temporal variability in root and shoot growth. Functions that accurately relate penetrometer resistance to soil properties are important tools for assessing the contribution of soil mechanical resistance (SMR) when the temporal and spatial variability in SMR cannot be readily measured. Although effective stress can make a significant contribution to SMR, the role of texture and compaction on the contribution of effective stress to SMR has not been explored and functions that are currently used to describe the relation between SMR, water content/potential and other soil properties do not contain terms explicitly linked to effective stress. The objectives of this study were to assess functions that included terms that would be compatible with effective stress and to subsequently develop a pedotransfer function to quantify the dependence of SMR on soil properties. Soil resistance was measured on disturbed and undisturbed soils with a range of textures, organic carbon (OC) contents and bulk density after equilibrating the soils at different water potentials ( ψ). The SMR decreased with decreasing ψ at the lowest ψ in coarser-textured soils and the effects persisted into medium-textured soils at the higher level of compaction. These effects were attributed to a decrease in effective stress. The function found to be most successful in describing SMR in both disturbed and undisturbed soils was of the form: SMR = a ∗ ( ψ b ) − c ∗ ψ where a, b and c were functions of texture, OC content and bulk density. The dependence on soil properties was different in the disturbed and undisturbed soils. Sensitivity analyses indicated that the variation in the second term with soil properties was compatible with expectations regarding increasing discontinuity in water films or the development of microcracks with decreasing ψ. The study suggested that pedotransfer functions for SMR should be adjusted to account for the possible reduction in SMR at low water contents or ψ due to decreasing values of effective stress.

Effect of penetration rate and filter location on piezocone test results

The effects of penetration rate on the piezocone penetration and dissipation test results with filter element locations were experimentally studied. Six kinds of piezocone tests were conducted for the U1 (filter element at the cone tip) and U2 (filter element behind the cone base) miniature piezocone penetrometers and the K33 specimen (mixture of 33% kaolin, 67% fine sand) underKo condition using a well-calibrated chamber system. The chamber tests were conducted for normally consolidated and heavily over-consolidated specimens at penetration rates of 0.3 cm/sec and 0.6 cm/sec. The test results were investigated with the results of the chamber test conducted at penetration rate of 2.0 cm/sec. It was observed that cone resistance and excess pore water pressure increased with the increase in penetration rate. The excess pore water pressure of U1 was larger than that of U2. The instantan eous initial drop of excess pore water pressure both at U1 and U2 were clearly identified at the very initial phase of dissipation est, and the magnitude increased with the decrease in effective vertical stress.

Compaction behavior of argillaceous sediments as function of diagenesis

The paper presents the results of laboratory compaction tests on samples of Kimmeridge clay taken from two different locations in the UK. The main objective of the laboratory tests is to determine how diagenesis affects the hydro-mechanical properties and compaction behavior of argillaceous sediments. The two Kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis. The laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity, permeability, compressibility, strength and effective horizontal stress in argillaceous materials. Another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins. The tests on Kimmeridge clay are carried out to as high as 150 MPa effective vertical stress corresponding to about 9 km in burial. The results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro-mechanical behavior of the tested materials. The differences in hydro-mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment. Other observed effects of chemical diagenesis on hydro-mechanical behavior of lithified sediments are: (1) increase in apparent pre-consolidation stress; (2) decrease in compressibility; (3) decrease in permeability; and (4) decrease in effective horizontal stress. Implications of the results to fluid flow and overpressure development in sedimentary basins are discussed.

Compaction behavior of argillaceous sediments as function of diagenesis

The paper presents the results of laboratory compaction tests on samples of Kimmeridge clay taken from two different locations in the UK. The main objective of the laboratory tests is to determine how diagenesis affects the hydro-mechanical properties and compaction behavior of argillaceous sediments. The two Kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis. The laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity, permeability, compressibility, strength and effective horizontal stress in argillaceous materials. Another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins. The tests on Kimmeridge clay are carried out to as high as 150 MPa effective vertical stress corresponding to about 9 km in burial. The results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro-mechanical behavior of the tested materials. The differences in hydro-mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment. Other observed effects of chemical diagenesis on hydro-mechanical behavior of lithified sediments are: (1) increase in apparent pre-consolidation stress; (2) decrease in compressibility; (3) decrease in permeability; and (4) decrease in effective horizontal stress. Implications of the results to fluid flow and overpressure development in sedimentary basins are discussed.

Shear heating of a fluid‐saturated slip‐weakening dilatant fault zone 1. Limiting regimes

The one‐dimensional model of Rudnicki and Chen [1988] for a slip‐weakening dilating fault is extended to include shear heating. Because inertia is not included, instability (a seismic event) corresponds to an unbounded slip rate. Shear heating tends to increase pore pressure and decrease the effective compressive stress and the resistance to slip and consequently tends to promote instability. However, the decrease of effective compressive stress also reduces the magnitude of shear heating. Consequently, in the absence of frictional weakening and dilation, there exists a steady solution for slip at the tectonic rate in which the pressure does not change and the shear heating is exactly balanced by heat flux from the fault zone. In the absence of shear heating, dilatancy tends to decrease pore pressure and inhibit instability; more rapid slip weakening promotes instability. Analysis of undrained, adiabatic slip (characteristic of rapid slip or hydraulically and thermally isolated faults) reveals that the interaction of these effects can cause increased slip weakening to be stabilizing and increased dilatancy to be destabilizing. These counterintuitive effects are due to the dependence of the shear heating on the total shear stress (not just its change). They occur for small thermal expansivity and for material parameters within a plausible range for 0–10 km depth.

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

The effect of flow localization and shear bands development on the structure and mechanical behavior of Cu-4.98 wt.% Ti alloy

Hot compression tests were performed on solution treated (ST) Cu-Ti alloy deformed within a temperature range of 673 K - 1173 K at constant strain rate of 1.4×10 -4 s -1 and 6.9×10 -3 s -1 . Shear banding, dynamic precipitation and dynamic coarsening have operated simultaneously with different effectiveness depending on the temperature and used strain rate. At low temperature, 673 K, intergranular cracks and flow localization were found to be responsible for the material fracture at e 1 0.1 - 0.2. At 873 K and above, a flow stress maximum was followed by monotonic flow softening range. Initial hardening range has resulted from both strain and precipitation hardening. Structural observations leads to the conclusion that an interaction of localized flow and discontinuous precipitation within shear bands was responsible for effective flow stress decrease at larger strains. At high deformation temperature, the flow softening of ST samples has resulted from cellular growth and combined effect of flow localization and discontinuous precipitation within sheared area. At high strain rates, the shear banding was found to proceed the precipitation and the initial flow softening has resulted from shear bands development rather than discontinuous precipitation. During deformation at 873 K - 1073 K and low strain rate, discontinuous precipitation was observed since the beginning of the tests and the localized flow was intensified due to particles dynamic coarsening within shear bands. The structural flow localization effects were limited for overaged (OA) and hot deformed samples. During deformation above the solvus temperature, dynamic recrystallization of the material was observed for both ST and OA samples.

Deformation Characteristics of a Saturated Cohesive Soil Subjected to Increase in Pore Pressure

Landslides resulting from rain and melting snow are generally known to be caused by an increase in the ground water level within the slopes. In these cases, an element on a slip surface before failure is subjected to the increase in pore water pressure, and deforms under a drained condition. It is very important, therefore, to estimate the drained deformation characteristics of clay during this increase in pore pressure. A drained test of cohesive soils, however, requires a long-term period to complete, and the drained deformation of cohesive soils during the decrease in mean effective stress has never been made clear. In this paper, deformation characteristics of a saturated cohesive soil subjected to the increase in pore water pressure were investigated and were compared with its undrained deformation characteristics. State points (p', q) at which a given value of equivalent shear modulus Ge(= Δη / Δεs, where εs is shear strain and η is stress ratio, q / p') is the same locate in the same very narrow zone for both of the two tests stated above. Any zone for an other Ge value locates in parallel with the other zones. Therefore, the Ge-value at a given stress state is independent of stress paths. Stress points showing the same εv-value forms a contour line in the pore pressure increase tests, and the line can be uniquely defined for a given εv-value. The distribution pattern of these lines is similar to the effective stress paths obtained from the consolidated undrained tests on overconsolidated specimens with different OCR-values. As a result, drained deformation characteristics in the pore pressure increase tests may be estimated with little error from the results of rapid undrained shear tests. In the final, to investigate the deformation behavior under general stress conditions, the effects of direction of principal stress and stress induced anisotropy on the above deformation characteristics were examined using the triaxial extension and torsional shear test results, and using the results on anisotropically consolidated specimens.

Normalized Nondegrading Behavior of Soft Clay under Cyclic Simple Shear Loading

The Masing rules, often used in modeling the behavior of soils under cyclic loading, are unreliable when there is significant cyclic degradation of the soil-strain curves. In the present paper it is shown that degradation under simple shear loading conditions may be attributed, mainly, to decrease in the mean effective stress. The influence of decreasing mean effective stress on the parameters of Iwan's series-parallel model, which obeys the Masing rules, is considered. The slip stresses of the model elements are shown to degrade proportionally to the decrease in mean effective stress, while the small strain shear modulus appears to be invariant to this change. Incorporation of slip stress dependence on mean effective stress into Iwan's model leads to the concept of a normalized backbone curve that is nondegrading, and to predicted cyclic simple shear behavior consistent with the Masing rules. The nondegrading backbone curve concept is shown to be consistent with experimental results obtained in torsional...

Contributions of Compaction and Aquathermal Pressuring to Geopressure and the Influence of Environmental Conditions (1)

In their discussion of our paper (Luo and Vasseur, 1992), Miller and Luk (1993) present interesting arguments leading to the conclusion that, in some circumstances, the aquathermal pressuring effect may be appreciable in the development of overpressure sin sedimentary basins. This contradicts our conclusion that, in actual geological environments, the aquathermal factor has no practical significance. Miller and Luk (1993) argue that we did not take into account possible unloading effects. Indeed, we implicitly assumed that the sediment is submitted to effective stresses with a monotonous increase. In such circumstances, we showed that the sediment compressibility is large enough to prevent the aquathermal effect from being important. But Miller and Luk stated that a much smaller value of sediment compressibility should be used when the effective stress becomes less than it has been previously. This would drastically increase the relative importance of the aquathermal pressuring effect. The key point, therefore, is in what circumstances such an effective stress decrease may occur. These circumstances are discussed.

Influence of Rainfall on Movement of Fractured Zone Landslide

The relationship of displacement of fractured zone landslide and rainfall is investigated on basis of the observational data. In the majority of cases, the displacement increased in proportion to rainfall. In Nuta and Honmura landslide, the displacement is correlated to the change of ground water level. This fact means that the decrease of effective stress is a primary factor of the char ge of displacement. As the landslide movement has an immediate connection with rainfall or ground water level, the prediction of landslide displacement is possible by the measurement of rainfall of ground water level.

CYCLIC STRESS‐STRAIN RESPONSE OF 2 1/4Cr‐1 Mo STEEL AT ELEVATED TEMPERATURES

AbstractThe temperature dependence of the cyclic stress‐strain response of 2 1/4 Cr‐1 Mo steel has been studied in the temperature range 20–550°C. Fatigue hardening‐softening curves and cyclic stress‐strain curves were obtained. A more detailed analysis was based on the statistical theory of the hysteresis loop and the concept of internal and effective stresses. A stress‐dip procedure allowed the separation of the contribution of internal and effective stresses both in dependence on temperature and during fatigue life in high temperature cyclic straining. The high effective stress contributes to the fatigue resistance of the steel. In high temperature fatigue both the internal and the effective stress decrease with cyclic straining.

UNDRAINED SHEAR OF SOFT CLAY AFTER A CYCLIC STRESS HISTORY

Effects of undrained cyclic stress-strain history on clay behavior were studied by cyclic triaxial compression tests. An apparently-overconsolidated clay produced by undrained cyclic loading decreases its undrained strength because of the decrease in effective stress in clay. while a clay subjected to drained cyclic loading gains the strength similarly to the long-termed consolidated clay. Undrained strengths of a reconstituted marine soft clay were predicted by a method which was formerly extended from the empirical relation existing between overconsolidated and normally-consolidated clays. In terms of the effective stress analysis, the critical state parameter in clay has nothing to do with undrained cyclic loading histories.

Cyclic softening and thermally activated deformation of titanium and zirconium

Cyclic softening in commercial purity zirconium and titanium corresponds principally to a decrease in effective stress and to an increase in screw dislocation mobility. This result indicates that the thermally activated deformation of these metals is not controlled by the overcoming of individual interstitial solute atoms by dislocations as usually proposed.

Modelling and minimization of membrane penetration effects in tests on granular soils

In experimental research into the constitutive properties of granular materials the phenomenon of membrane penetration is one of the major sources of error. For instance, in drained isotropic loading and unloading tests on medium loose fine sand the errors in the volume change due to membrane penetration can be of the order of 20% and 40% respectively. In the first stage of an undrained triaxial compression test on the same sand, when the effective isotropic stress decreases, the deviator stress can become 50% greater owing to membrane penetration; because of this a tendency towards lique faction might remain obscure. It is shown that such errors can be reduced by a factor of 5 if the proper membranes are used and if the remaining membrane effect of restraint is accounted for properly. To this end the membrane should consist of much stiffer material than latex rubber, the membrane thickness should be of the order of the mean grain size and the ratio of the mean grain size and the sample diameter should not be too large (0·003 in the case considered). For medium loose fine quartz sand the optimum value of Young's modulus for the membrane should be of the order of E≃5×107 N/m2 so that the membrane is about 40 times stiffer than latex rubber. Dans la recherche scientifique sur le comportement mécanique des sols granuleux le phénomène de la pénétration de la membrane est une des sources importantes d'erreur. Par example en cas des essais de chargement et déchargement hydrostatiques avec drainage sur un sable fine assez lâche l'erreur de la variation volumétrique par suite de la pénétration de la membrane peut monter 20% et 40% respectivement. En cas des essais de compression triaxiale sans drainage sur le même sable la tendance à liquefaction restera obscure; avant que la pression effective isotrope minimale est attenu le deviateur des contraintes peut être 50% plus grand par la pénétration de la membrane. Il est montré que cettes erreurs peuvent se réduire à un facteur de 5 pourvu que les propres membranes soient utilisés et pourvu que la contrainte de soutien de la membrane soit tenu en compte proprement. A cet effet il faut, que les membranes soient composés de matériau plus rigide que caoutchouc, que l'épaisseur soit à peu près aussi grande que le diamètre moyen des grains et que le rapport du diamètre moyen des grains et du diamètre d'échantillon soit limité (dans le cas présent=0·003). Par example le module d'élasticité de la membrane doit être environ de E ≈ 5 × 107 N/m2 pour un sable de grains fines de quartz et d'une compacité assez lâche; donc approximativement 40 fois plus rigide que cauotchouc.

A Method of Analysis of Earthquake-Induced Liquefaction of Horizontally Layered Sand Deposits

When a saturated sand is subjected to earthquake excitation under undrained conditions, excess pore water pressure is generated, and the effective stress decreases. This eventually leads to sand liquefaction. It is important to analyze the liquefaction phenomena, particularly for the earthquake-resistant design of structural foundations. Although .many experimental studies on liquefaction have been performed, their results have not well implemented in developing analytical models. In this study, the dynamic response of horizontally layered soil is numerically analyzed, using the stress-strain relation of sand and the equation of motion for two-phase mixtures consisting of an incompressible fluid and an elastic-plastic solid. As the mixture of pore water and soil particles is assumed to be incompressible, effective stress analysis is appropriately applied. In the case of a horizontally layered soil deposit, deformation should be horizontally confined during earthquakes. In this study, therefore, the horizontally confined condition is considered. By using this condition, stress path particular to the liquefaction process can be obtained analytically. The effect of the dissipation of pore water pressure due to seepage on the response of the ground is also examined. It is concluded that liquefaction greatly influences the dynamic response of ground.