Mobilized Shear Stress Research Articles

Influence of Apparatus Scale on Geogrid Monotonic and Cyclic/Post-Cyclic Pullout Behavior in Cohesive Soils

Geosynthetics have increasingly been applied to geotechnical engineering works due to their numerous advantages, including cost-effectiveness and their significant role in sustainable development. When geosynthetics are used as reinforcement in earth structures, such as embankments, retaining walls and bridge abutments, soil–geosynthetic interface shear behavior is a critical parameter involved in the design. This paper presents a series of monotonic and cyclic/post-cyclic pullout tests carried out to examine the apparatus scale effect on the pullout response of a geogrid embedded in two different soils. To assess the small-scale equipment feasibility, comparisons were made between pullout test parameters derived from small- and large-scale equipment. The test results indicate that, under a low confining stress of 25 kPa, using a smaller-sized apparatus results in lower values of geogrid pullout resistance and maximum mobilized shear stress, but higher values of confined tensile stiffness at low strains. On the other hand, as the confining stress increases (i.e., 50 kPa and 100 kPa), the difference between the results becomes less significant and similar trends are observed regardless of the equipment type. Adopting small-scale equipment enables obtaining soil–reinforcement interaction parameters using test procedures that are less time-consuming than those associated with large-scale pullout tests. However, proper scale effect correction factors may be considered for more consistent estimates of the interface strength parameters under low normal stress values.

A novel framework for ascertaining the safety factor and critical slip surface of 3D slopes reinforced with piles

The pile is a widely used reinforcement device in geotechnical engineering. For the present paper, we propose a new solution for calculating the safety factor (SF) of the 3D slope reinforced with piles and determining the critical slip surface (CSS) in the framework of minimum potential energy method. In particular, the mobilized shear stress on failure surface is determined by the equilibrium equation of forces operating on slip body. Meanwhile, the moving direction of 3D slope is equivalent to the direction of combined force acting on slip body. Additionally, the present method is combined with the genetic algorithm to identify CSS, and a novel criterion is proposed to locate the shape and position of CSS of a 3D slope using the total potential energy of landslide system as the objective function. The validation and comparison of results indicate that the SF obtained by the introduced method are close to reference solutions, and SF shows an obvious negative correlation with sliding direction (SD). It is also found that the support effect of pile is related to the pile location and the pile density. Meanwhile, the optimal pile position depends on the shape of failure surface and shear strength parameter of soil. Moreover, the effects of some parameters on SD of 3D slopes reinforced with piles are also studied.

Pseudo-dynamic analysis of reinforced slope with anchor cables

The anchor cable is an excellent reinforcement device for enhancing slope performance subjected to earthquakes. Thus, this work combines the minimum potential energy principle with the pseudo-dynamic method and develops a novel approach to determining a reinforced slope safety factor (SF). Specifically, the mobilized shear stress on the slip surface is solved utilizing the moment equilibrium equation of the sliding mass. Moreover, the proposed scheme introduces the elastic foundation coefficient characterizing the soil’s compressibility. Subsequently, the potential energy equation is derived. Finally, the SF is calculated as the ratio of the anti-sliding moment to the sliding moment for evaluating the slope stability. The developed method is evaluated on four examples, with the results highlighting that our method is efficient and accurate. Meanwhile, we examine the influence of the variation of the axis forces on the SF when the foundation coefficient is smaller, revealing that the depth of the sliding mass is negatively correlated with the SF, while the landslide area and length of the sliding mass are positively correlated with the SF. Furthermore, this work also studies the influences of amplification factor, earthquake acceleration factor, anchored force, cohesion, and internal friction angle on the SF. Overall, this work provides suggestions for the design and construction management of reinforced slopes with anchor cables.

Temperature effect on the axial behaviour of a pipeline buried in soft clay

Axial resistance is a key component to resist thermal buckling of an offshore oil and gas pipeline operating under a high temperature. The previous studies on the axial resistance of the pipeline were mostly performed under an isothermal condition, ignoring the temperature effect that would alter the strength of the seabed and modify the axial resistance of the pipeline. For this reason, a novel temperature-controlled experimental setup is developed in this study. Model tests were then carried out for simulating the axial interaction between clayey seabed and pipelines, which were heated to two typical temperatures (i.e., 15 and 55 °C) under the drained condition and then axially loaded under the undrained condition till failure. Additionally, the mobilized shear strain (and therefore mobilized shear stress) at the soil-pipe interfaces under the two temperatures are also investigated, based on a newly developed temperature-controlled interface shearing apparatus that has a transparent window for image recording and analysis. It is found that the pipeline with high temperature (55 °C) exhibits 20% larger initial axial soil resistance stiffness than the low-temperature pipeline (15 °C). This suggests the thermal consolidation has increased the soil stiffness around the high-temperature pipeline. Despite the increased undrained shear strength (s u) of the thermally consolidated clay around the high-temperature pipe, its peak axial resistance is 10% lower than that of the low-temperature pipe, the maximal normalized parameter of axial resistance α f max decreases from 0.90 to 0.70, when the temperature rises from 15 °C to 55 °C. This is attributed to the thermally induced suppression of the stick-slip (referred as “thermo-lubricity”) at the soil-pipe interface, which encourages the interface slippage prior to a full mobilization of s u around the pipes. Consequently, a temperature elevation from 15 to 55 °C has led to a reduction 36% (from 70% to 34%) of s u mobilization at the soil-pipe interface, which is observed in the interface shearing tests.

Finite element analysis of slope stability by expanding the mobilized principal stress Mohr's circles – Development, encoding and validation

In recent years, finite element analysis is increasingly being proposed in slope stability problems as a competitive method to traditional limit equilibrium methods (LEMs) which are known for their inherent deficiencies. However, the application of finite element method (FEM) to slope stability as a strength reduction method (SRM) or as finite element limit analysis (FELA) is not always a success for the drawbacks that characterize both methods. To increase the performance of finite element analysis in this problem, a new approach is proposed in this paper. It consists in gradually expanding the mobilized stress Mohr's circles until the soil failure occurs according to a prescribed non-convergence criterion. The present approach called stress deviator increasing method (SDIM) is considered rigorous for three main reasons. Firstly, it preserves the definition of the factor of safety (FOS) as the ratio of soil shear strength to the mobilized shear stress. Secondly, it maintains the progressive development of shear stress resulting from the increase in the principal stress deviator on the same plane, on which the shear strength takes place. Thirdly, by introducing the concept of equivalent stress loading, the resulting trial stresses are checked against the violation of the actual yield criterion formed with the real strength parameters rather than those reduced by a trial factor. The new numerical procedure was encoded in a Fortran computer code called S 4 DINA and verified by several examples. Comparisons with other numerical methods such as the SRM, gravity increasing method (GIM) or even FELA by assessing both the FOS and contours of equivalent plastic strains showed promising results.

A unified constitutive model for pressure sensitive shear flow transitions in moderate dense granular materials

Granular shear flows exhibit complex transitional regimes that are dramatically affected by the pressure level and shear stress state. New advances in granular shear tests at low pressure have enlightened the understanding of the two granular shear flow transitions: between quasi-static and moderate shear flows, and between steady-state and transient shear flows. However, a unified constitutive model to describe these two transitions is yet to develop. In this work, a simplified and unified model is proposed based on innovative triaxial shear flow tests, using two dimensionless physical variables. Model results validated against experimental data suggest that the shear flow transition between a quasi-static to a moderate Isotach type flow state is highly pressure-dependent. At extremely low pressure, the granular viscosity becomes the primary mechanism, suppressing the quasi-static mechanism even under “quasi-static” shear rates. In transient to steady state granular flow transitions, a mobilized shear stress ratio or mobilized friction coefficient between zero and the critical state ratio for consolidated granular packings is taken into consideration. This is coupled with the mechanism of granular viscosity. These findings have not been discussed before and are of great relevance to granular mechanics as well as space and earthquake engineering.

Incorporating shear stress ratio and load cycle number in a simple permanent deformation model for unbound material

A power model (ε = A. NB) to estimate permanent strain (ε) from single-stage repeated load triaxial (RLT) tests can simulate both shakedown and incremental collapse. When the exponent B < 1, the ε versus load cycle number (N) plot concaves downwards, resembling that of a material that shakes down. When B > 1, the same plot concaves upwards, mimicking that of a material that incrementally collapses. A power model that relates A and B to the shear stress ratio (SR = 1/factor of safety), a measure of how far the RLT specimen is away from static failure, is proposed. Use of SR as an independent variable in a PD model is advantageous because it embodies the geomaterial’s gradation, physical state, suction, stress level, shear strength and margin of safety against static failure. No other parameters bar N and SR are needed, enabling the number of independent variables and hence model complexity to be kept low. This model was applied to a virgin and a recycled concrete aggregate utilizing 2 independent variables (N and SR) with A and B both functions of SR. A rational procedure to estimate the mobilized shear stress is presented and a methodology to quantify SR that considers non-linearity of the Mohr-Coulomb failure envelope and cohesion due to suction in the unsaturated RLT specimens is proposed; such considerations have been absent in previous models. For the materials tested, A was found to decrease non-linearly with increasing SR while the relationship between B and SR is quite linear. The resulting model was able to simulate the RLT test data quite well with four fitting parameters. The fitting parameters can be easily derived using linear regression on a spreadsheet and were all statistically significant with a confidence level exceeding 95%. Once B is known, the shakedown limit can be estimated by setting B < 1. An appropriate margin of safety can be incorporated on the shakedown load to limit the permanent strain.

The influence of roughness on cyclic and post-cyclic shear behavior of red clay-concrete interface subjected to up to 1000 cycles

Dynamic shear behavior of soil-structure interface is one of the most commonly problems in engineering practice, especially for the design and stability analysis of soil-structure interaction system subjected to cyclic loading. This paper presents the results of an experimental study on the interface between red clay and concrete that is subjected to a large number of cycles (up to 1000 cycles). Three concrete blocks with different surface roughnesses were fabricated and displacement-controlled cyclic tests were conducted by using a large-scale direct shear device under three different normal stresses (100 kPa, 200 kPa and 300 kPa). The effects of normal stress, roughness and number of cycles on cyclic and post-cyclic shear behavior of interface were then discussed in detail. The test results show that the red clay-concrete interface exhibits softening behavior under different normal stress levels. The mobilized shear stress decreases moderately with the number of cycles and subsequently remains constant after 500 cycles of loading. The response of interface subjected to cyclic loading shows apparent aeolotropy due to shear directions in every cycle. Interface shear stiffness and damping ratio decreases with increasing cycle numbers. The red clay-concrete interface represents an overall contraction behavior in cyclic and post-cyclic direct shear tests. The cyclic loading does not lead to degradation for post-cyclic shear strength.

Effect of density on skin friction response of piles embedded in sand by simple shear interface tests

This study focuses on a particular phenomenon related to the reduction in sand–pile skin friction with initial relative density increment from medium to dense. Frictional behaviour of a sand–pile interface is simulated using a simple shear-type device capable of inducing a constant normal stiffness condition. Sand–pile interface sliding and soil deformation components are distinguished quantitatively. The effects of initial relative density of sand, initial normal load, and constant normal stiffness on the magnitude of the pile skin friction and shear displacement at failure are examined. Results indicate that the magnitude of the mobilized shear stress at failure relies significantly on the shear stress state with respect to the inflexion point on the volume change graph, which is equivalent to the position of the peak stress ratio. Good correlations exist between results of this study and field data of several heavily instrumented piles embedded in dense to very dense sands. The presented procedure is a useful framework for establishing more accurate skin friction calculation methodologies and “t-z” curve developments of axially loaded piles.

Evaluation of soil arching effect due to partially mobilized shear stress in piled and geosynthetic-reinforced embankment

In piled and geosynthetic-reinforced (PGR) embankment, the arching behavior determines the overburden load on piles and subsoils. Placement of geosynthetic is effective in reducing the relative displacement between pile and subsoil. When the mobilized shear stress is less than the shear strength, partially developed arching will occur. Consequently, existing analytical methods, adopting the ultimate shear strength failure criterion, need to be improved. This study developed a simplified 2D analytical method, which is based on the developing arching effect, to evaluate the load redistribution of the PGR embankment. Then, the influences of embankment height and internal friction angle, subsoil depth, ratio of pile cap width to pile clear spacing (RPC) and geosynthetic tensile stiffness on the critical height ratio, stress concentration ratio, soil arching ratio, geosynthetic tension and axial strain were investigated. This study suggests that a RPC of 1:1.0 and a one-way of single-layer geosynthetic tensile stiffness of 2000 kN/m should be considered as the sensitivity thresholds for the PGR embankment.

Implementation framework of the UIUC aggregate base rutting model

ABSTRACT This paper presents findings from a research study focused on calibrating and improving the robustness of a permanent deformation model for unbound aggregate base/subbase materials, recently developed at the University of Illinois and referred to as the UIUC rutting model. The model provides estimates for permanent strains as a function of load cycles, applied deviator stress, and the ratio of mobilized shear stress to strength. Sixteen aggregate materials were tested for permanent deformation both at their source gradations, and an engineered gradation. Based on multiple linear regression analyses, the UIUC rutting model parameters were determined to estimate the laboratory-measured permanent strains with high accuracy. Constrained and stepwise regression statistical approaches were employed to reduce variability in the regression model parameters and express the model parameters as function of material properties such as shear strength, morphological shape properties, gradation, and compaction characteristics. Results from stepwise analyses indicated that the most accurate predictions could be achieved by considering particle shape properties. Finally, a framework was established to predict unbound aggregate permanent deformation trends of flexible pavement base/subbase layers using the UIUC model parameter equations obtained from stepwise regression, along with the physical/mechanical properties of the aggregate materials determined from standard laboratory tests.

Effects of drainage condition on the stress-strain behavior and pore pressure buildup of sand-PET mixtures

Recycling and usage of Polyethylene Terephthalate (PET) in civil constructions will not only decrease the environmental challenges, but also it will result in considerable saving in the overall project costs. Introducing PET chips as a capable material for the improvement of sands, this paper presents the results of consolidated drained (CD) and undrained (CU) triaxial tests on different sand-PET mixtures. Applying four different PET contents and two relative densities, tests are conducted under a wide range of confining pressures from 50 kPa to 500 kPa and the mobilized shear stress and the pore water pressure buildup are studied. Changing in the drained and undrained dilative-contractive behavior of sand-PET mixtures in different PET contents, relative densities and confining pressures showed the multi-variable nature of the problem emphasizing on the need for the comprehensive stress-strain modeling of mixtures. Such a behavior is then assessed through proposing new predictive, evolutionary models for the shear stress and the pore pressure based on the PET content, relative density, confining pressure and axial strain. Parametric studies showed that (0.2–0.3)% PET content, also, (2–4)% axial strain are determinative ranges for the evaluation of undrained and drained shear stresses of mixtures, respectively. A three-dimensional, graphical comparison using the developed relationships introduced the (0.5–0.7)% range as the critical value for the PET content distinguishing the drained and undrained behavior of mixtures. Next, according to the sensitivity analysis, confining pressure and axial strain are, respectively, presented as the most influencing parameters on the drained and undrained, mobilized shear stresses.

Evaluation of Geomembrane Effect Based on Mobilized Shear Stress due to Localized Sinking

Installing geosynthetic reinforcement at the bottom of the embankment will provide positive support for subgrade stability and settlement control, if there is a void or a weak foundation. In routine design work, the geomembrane effect must be well estimated and the tensile strain should be precisely predicted. Conventional analytical methods often adopt the limit state method to calculate the overlying load on the deflected geosynthetic. However, this assumption does not necessarily apply to all conditions, especially when the foundation soil can provide certain resistance. In this study, a semiempirical prediction method for evaluating the geomembrane effect of the basal reinforcement was proposed, and an iterative solution for calculating tensile strains of a deflected geosynthetic was deduced. In derivation, a virtual inclined slip surface and interaction between the geosynthetic and soil were quantitatively evaluated by coupling the arching effect and the geomembrane effect. Moreover, the development of shear stress along the slip surface can be considered, as well as different segments of the basal reinforcement. Then, the proposed method was validated by two large‐scale experiments. Comparison of the results of this method with measurements and results of other analytical models confirmed that this analytical method can take good care of the varying process of the localized sinking, regarding the overlying loads on the geosynthetic and the subsequent tensile strains.

Laboratory characterization of compaction and damping properties of Stone Matrix Asphalt

Stone Matrix Asphalt (SMA) is referred to gap graded Hot Mix Asphalt (HMA) that is designed to maximize rutting resistance and durability by using a structural basis of stone-on-stone contact. SMA mixture designs with gap graded aggregate gradations and polymer modified binders often have special field compaction requirements. To better understand differences in compaction behavior between SMA and dense graded HMA mixtures, laboratory tests were conducted on field collected loose samples at typical field densities and loading conditions. The goal of the first phase of this study was to investigate the growth of density and the mobilized shear stress in the specimen as a function of applied normal stress, gyration numbers, and the compactive effort. In the second phase, damping ratio tests were performed on the compacted samples at close to field compaction temperatures by applying dynamic loading at various frequencies and different confining pressures to determine the mixture damping properties. The test results indicated that SMA mixtures with the gap graded aggregate structure achieved higher densities in comparison to HMA mixtures. The frequency of dynamic loading had a significant effect on the damping ratios of the SMA mixtures and the effect of confining pressure, linked to proper lift thicknesses in the field, was found to be dependent on compaction pressure levels as well as mixture properties.

Development of a Nonlinear Model for Soft Rock and its Applications

A phenomenological model has been developed for soft rock based on the results of a series of triaxial compression (TC) tests conducted on Kobe sandstone with a very high precision measurement. From the analysis and interpretation of the test results, it has been found that small strain Young’s modulus (Ee) was a function of the major principal stress. Ee for elastic strains of soft rock was assumed to be cross-anisotropic. A damage function has been used to derive the appropriate elastic Young’s modulus when subjected to shear loading. As the basic stress–strain relation, the relationship between the tangent modulus and the shear stress level was used. The differential form of which was subsequently integrated by a 4th order Runge–Kutta solver to obtain the stress–strain relation. The model of soft rock is based on an isotropic strain hardening elasto-plastic framework which takes into account the pressure sensitivity, cross-anisotropy, degradation of Young’s modulus with the degree of mobilized shear stress and the nonlinearity of the shear stress-shear strain relationship. Although the model was developed from the analysis of the TC tests results of Kobe sandstone, it was also applied to the other types of soft rock or stiff geomaterials. Plate loading tests were conducted at a level of 61 m below the ground level at the bottom of a large excavated shaft at four locations. Finally, the model was used to simulate the plate loading test results successfully. This model was successfully calibrated with Akashi sandstone and applied in the simulation for the settlement of Akashi-Kaikyo Bridge piers. The simulations were carried out for both drained and undrained condition by changing the Poisson’s ratio. The layering information beneath the foundations were used in the FEM simulation. The use of very accurate Young’s modulus from the field shear wave velocity test was the key to the successful simulation of the settlement under bridge pier foundations.

Application of a Hydraulic Gradient Technique for Modeling the Uplift Behavior of Piles in Sand

Abstract A scale modeling technique is presented for simulating the uplift behavior of piles in sand, which satisfies stress and strain similitude with full-scale prototypes. A hydraulic gradient approach was used to increase the body forces in the scale model tests, until the stresses became representative of reasonable field-scale conditions. The associated model scaling laws that are used with this technique are presented and discussed. Eight pile pullout tests were conducted, at varying magnitudes of hydraulic gradient; one of these tests was conducted on a pile instrumented with strain gages. From the results of this study, it was concluded that the hydraulic gradient technique can be effectively applied to induce reasonable prototype stresses in a scale model, allowing for reasonable simulation of the uplift behavior of piles in sand. The hydraulic gradient technique was found to be particularly sensitive to the distribution of the hydraulic gradient in the soil profile, which is not surprising, given the nature of the scaling laws for this particular modeling technique. Analysis of the results from the pile instrumented with strain gages indicated that (1) very little shear stress was mobilized in the upper 25 % of the pile, even at applied uplift loads approaching the ultimate pullout force; (2) the largest amount of shear stress for each applied uplift force was typically mobilized at a point somewhere between 60 % and 80 % of the length of the pile; and (3) at failure, the mobilized shear stress distribution was clearly not triangular, as is postulated by commonly used pile uplift design approaches. Post-failure investigations conducted after the completion of each pullout test indicated that the failure shear surface in each of the model tests developed along the pile-soil interface.

A theoretical study on ground movement prediction for braced excavation in soft clay

Braced excavation in urban areas requires control of ground movements as well as wall deflection. Previously interpolation from empirical database or numerical analysis by finite element method is used for estimation of ground settlement. But these methods are relatively complex and sometimes important assumptions are invalid at different situations. A design approach for calculating wall deflection and ground settlement is proposed by Bolton and Osman (2006) assuming the plastic deformation mechanism. Here, in this paper this design method is extended by presenting displacement profiles at different zones of the mechanism with different mathematical functions on the basis of ground settlement profile suggested by Peck (1969) and reported case study of Kolkata Metro Excavation (Som, 1991). From this deformation function incremental shear strain is calculated and average shear strain mobilized in each zone is expressed in term of maximum deflection. The conventional hyperbolic stress strain relationship is used to relate mobilized shear stress and shear strain. Finally, the total loss of potential energy is equated with the virtual work done by the soil within the deformed zone in order to develop an equation which may serve as a design tool for calculating ground surface movement and wall deflection at different stages of excavation at different depth.

Numerical Simulation of the Behavior of Geocell Reinforced Sand in Foundations

This paper presents the finite-element simulations of the behavior of strip footings resting on sand beds, with different density of soil, reinforced with geocells of different dimensions. The strength and stiffness of sand confined with geocells is represented by an equivalent composite model developed from triaxial compression tests. The additional confining pressure due to geocells, calculated using hoop tension theory, is used to obtain the apparent cohesive strength imparted to sand due to geocells. The elastic modulus of the geocell encased sand is related to the elastic modulus of the unreinforced sand and the tensile modulus of the geocell material using an empirical equation. Load-settlement response of strip footings on geocell reinforced sand beds obtained from the numerical simulations are compared with the corresponding experimental results and the match is found to be good. In addition, numerical results showed that with the provision of a geocell layer, the mobilized shear stress contours b...

Shear strength components of concrete under direct shearing

Direct shear tests on concrete specimens were conducted to investigate the shearing behaviour of the material under varying normal confining stresses. Test results were analyzed to quantify the mobilization of shear strength components derived from hydrated cement paste–aggregate adhesion, contact friction, shear dilation, and cement–aggregate interlock along the sheared surface under direct shearing. It was found that each component has its unique response in terms of mobilized shear stress and shear displacement. In addition, different modes of failure observed in uniaxial compression, splitting tension, and direct shear tests were identified to delineate the Mohr–Coulomb failure envelope for shear strength of concrete material.

Compression and Shear Behavior of Mudstone Aggregates

In this paper, a staged compression-immersion-direct shear test was conducted on compacted samples of crushed mudstone aggregates and its compressive and shear behavior are discussed with a focus on cementation effects. Compression behavior of the compacted samples was influenced significantly by the compaction degree as expected, as were shear behavior and shear strength. Immersion caused an additional compression and a reduction in mobilized shear stress and in the dilatant nature during shear at low applied pressure levels. Moreover, immersion significantly reduced the peak shear strength parameter c with only a little change in phi. The compression and critical state lines of the nonimmersed and immersed specimens seem to parallel each other, and the compression line of the nonimmersed and the critical state line of the immersed form the upper and lower bounds, respectively. A gap between shear stress-void ratio lines of the specimens with and without immersion is considered to represent a combined effect of cementation retained in a crushed mudstone aggregate itself and an interlocking effect of aggregates.