Stress Reduction Ratio Research Articles

Evaluation of the horizontal cyclic shear stress on the enclosed soil in DSM grid-improved ground by numerical simulation

Deep soil mixing (DSM) grids are widely used as effective countermeasures against liquefaction hazards. A simplified two-dimensional, nonlinear, finite element model involving DSM grid-improved ground is proposed to investigate the shear stress responses of the enclosed soil. A numerical model with several approximations of longitudinal walls was carefully validated by a centrifuge model test. The waist-shaped depth distribution of the horizontal cyclic shear stress acting on the enclosed soil (i.e., the “waist effect”) was revealed and well explained according to the characteristics of dynamic soil-grid interactions. Attention should be given to the increasing shear load sustained by the underlain layer, which may lead to an unexpected failure. A mathematical model for the shear stress reduction ratio was proposed based on a comprehensive numerical parametric analysis considering varying geometry and shear modulus of the DSM grid with a reasonable range. The proposed evaluation method, along with conventional liquefaction-triggering analysis, can be utilized for the seismic design of DSM grids in liquefaction mitigation. The previous design methods without considering the “waist effect” underestimate the overall horizontal cyclic shear stress acting on enclosed soil.

Research on bar deflection of low-stress cropping process under bending-shear coupling effect

In view of the influence of shear effect on bar deflection in low-stress cropping, which makes it difficult to control the loading die and seriously affects the surface quality of cropping bar, based on the energy balance theory in this paper, the stiffness equivalent transformation of the bar with annular V-shaped notch and dynamic crack is carried out, and the coupling effect of bending moment and shear force on the bar deflection before and after crack initiation is quantified. Based on it, the deflection characterization formula for the whole cropping process of the bar with different length-diameter (L/2R) ratios is established. The change laws of bar deflection with different L/2R ratios are analyzed by XFEM method. Especially for the bar with L/2R ≤ 2.84, it is pointed out that it is necessary to consider the influence of shear deflection on the basis of bending deflection, and its influence degree decreases with the increase of L/2R. The real-time synchronous dynamic deflection test platform is built on the cropping machine of rotary bending. The research shows that the calculated values obtained by the deflection characterization formula proposed are in good agreement with the corresponding experimental results. The evaluation parameter, maximum contact stress reduction ratio ξ(p-l)/p, is proposed to characterize the degree of surface contact stress reduction under line contact loading relative to point contact loading. It is pointed out that the variation range of ξ(p-l)/p of the bar with different L/2R ratios under the influence of deflection is as high as 49.8%-64.1%. And further application of double bearings outer ring loading is also beneficial to obtain high cropping quality.

Comparison of liquefaction behavior of granular material under SH- and Love-wave strain conditions by 3D DEM

In the study of liquefaction behavior associated with seismic loading conditions, it is often assumed that liquefaction occurs owing to the upward propagation of shear waves, despite evidence that liquefaction damage may result from or be aggravated by horizontally propagating surface waves. In this study, a series of numerical tests, based on the three-dimensional discrete element method, is performed to examine the liquefaction behavior of granular materials under Love-wave strain conditions. The response of granular packings under horizontally polarized shear- (SH-) and Love-wave strain conditions is discussed at both macro- and microscales. The simulation results indicate that, at the macroscale, the effective stress reduction ratio increases more rapidly under Love-wave strain conditions than under SH-wave strain conditions. Based on the concept of energy, the granular materials under Love-wave strain conditions can be considered more vulnerable to liquefaction than those under SH-wave strain conditions. Microscale analysis indicates the spatial rotation of the dominant direction of backbone force-chains under Love-wave strain conditions. In addition, focus here is placed on the coordination number, which represents the average contact number per particle. The difference in the degradation speed of the skeleton structure of the granular packings between the SH- and Love-wave strain conditions may not appear until the coordination number has decreased to a critical value of around 4. After the coordination number has approached approximately 3, the granular packings become unstable and soon liquefy. The minimum mean effective stress is discussed herein.

Analysis of Geocell-Reinforced Stone Column-Supported Embankment Considering Soil-Structure Interaction

Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. The effect of the basal geocell layer on the performance of the stone column-supported embankment was evaluated in this paper by carrying out time-dependent coupled analyses considering the geocell-infill soil interactions. The various parameters, such as arching, stress concentration ratio, surface settlement, column bulging characteristics, and the influence of various types of infill materials and drainage blanket thickness on column-supported embankment behavior, were analyzed. The load transfer mechanism was quantified using the term stress concentration ratio, and with the use of geocell mattress above stone columns, the stress concentration ratio increased by 1.5 times that of ordinary stone columns. Also, it was observed that compared to ordinary stone column-supported embankment, the combination of stone columns with geocell-sand mattress resulted in a further reduction of the foundation settlement by 15%. Analysis results showed that the arching behavior is not predominant in geocell-reinforced columnar structures. The various analytical methods like Guido et al. (1987), Low et al. (1994), and Abusharar et al. (2009), which were developed for geosynthetic reinforced columnar embankments, were found to significantly under-predict the stress reduction ratio values with geocells.

Study on arching effect in the embankment over pile - reinforced soft soil

The paper employes 3D numerical modeling to analyze the soil arching mechanism within embankment by FLAC3D code, based on the finite difference method (FDM). To consider the pile group effect, the 3D mesh of four pile has been created. Related to the constitutive models, the embankment is used Mohr - Coulomb model, the soft soil is represented by modified Cam - clay model, and footing and piles are employed by elasticity model. The numerical results focus on the soil arching phenomena in terms of stress distribution on piles and soft soil, the stress concentration ratio and the stress reduction ratio. Additionally, the axial force along pile and the settlements of embankment, soft soil and pile are studied.

Reinforcement Effect of a Concrete Mat to Prevent Ground Collapses Due to Buried Pipe Damage

This study described a ground reinforcement effect of a concrete mat, in order to apply a concrete mat for ground subsidence restoration of an open cut. A concrete mat can prevent the expansion of a cavity and relaxation area underground due to buried pipe damage when the buried pipe is in use. An experimental study was conducted to analyze the stress distribution characteristics of an underground area by ground reinforcement of a concrete mat. In addition, a numerical analysis was performed to estimate the range of underground reinforcement of a concrete mat. As an experiment results, the maximum stress reduction ratio of the concrete mat in the underground was 28.5% to 30.9%, which means the reinforcement effect of the concrete mat, according to the installation depth of the concrete mat. The finite element analysis (FEA) results showed that the installation depth of the concrete mat differed in various scenarios, in order to secure the reinforcement effect of the concrete mat according to the load conditions (point and uniform load). Therefore, the reinforced depth of a concrete mat should be determined by the load type on the surface.

Magnetic resonance imaging T1 and T2 mapping provide complementary information on the bone mineral density regarding cancellous bone strength in the femoral head of postmenopausal women with osteoarthritis

BackgroundSince bone mass is not the only determinant of bone strength, there has been increasing interest in incorporating the bone quality into fracture risk assessments. We aimed to examine whether the magnetic resonance imaging (MRI) T1 or T2 mapping value could provide information that is complementary to bone mineral density for more accurate prediction of cancellous bone strength. MethodsFour postmenopausal women with hip osteoarthritis underwent 3.0-T MRI to acquire the T1 and T2 values of the cancellous bone of the femoral head before total hip arthroplasty. After the surgery, the excised femoral head was portioned into multiple cubic cancellous bone specimens with side of 5 mm, and the specimens were then subjected to microcomputed tomography followed by biomechanical testing. FindingsThe T1 value positively correlated with the yield stress (σy) and collapsed stress (σc). The T2 value did not correlate with the yield stress, but it correlated with the collapsed stress and strength reduction ratio (σc/σy), which reflects the progressive re-fracture risk. Partial correlation coefficient analyses, after adjusting for the bone mineral density, showed a statistically significant correlation between T1 value and yield stress. The use of multiple coefficients of determination by least squares analysis emphasizes the superiority of combining the bone mineral density and the MRI mapping values in predicting the cancellous bone strength compared with the bone mineral density-based prediction alone. InterpretationThe MRI T1 and T2 values predict cancellous bone strength including the change in bone quality.

Microstructure Evolution and Softening Mechanism During Hot Deformation of Cu–0.19Cr–0.1Ag Alloy

Hot deformation tests of an upward continuous cast Cu–0.19Cr–0.1Ag were performed on a thermal simulation testing machine with various strain rates (0.01–10 s−1) at different deformation temperatures (750–950 °C) to investigate the microstructural evolution of hot deformation and the softening mechanism. From the results, a flow stress reduction ratio diagram was constructed. The diagram was divided into four regions with different softening mechanisms according to the stress–strain curve. Dynamic recrystallization was accompanied by two nucleation mechanisms: continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization, and the effect of CDRX decreased with increasing strain rate.

Comparative Analysis of Load Responses and Deformation for Crust Composite Foundation and Pile-supported Embankment

Ground improvement using artificial crust composite foundation, consisting of stabilization of soft clay and composite foundation, is an effective technique for the treatment of deep soft soil layers under infrastructure embankments. In this study, the load responses and settlement performance of this improvement technique were investigated using two centrifuge model tests to compare the variations of the vertical deformation, pore water pressure, axial force of the piles and tensile stress at the bottom of the artificial crust in the crust composite foundation with those in pile-supported embankment. The results of centrifuge model tests showed that the load responses and settlement performance of artificial crust composite foundation was different from the pile-supported embankment, which displayed mainly that the final middle settlement of crust composite foundation can be reduced by about 15% compared with those of pile-supported embankment with the same length of pile and construction cost. The deformation of the crust with the characteristics of the plate was found based on the change of the tensile stress. Additionally, the excess pore water pressure in the crust composite foundation was lower owing to the stress diffusion effect of the crust during the loading period and the dissipation rate of excess pore water pressure was slower due to lower permeability of the crust at the same loading period. Eventually, the axial force of the middle piles was reduced. At the same time, the boundary stress was functioned with the crust, the axial force of the side piles was improved. The comparison of measured and calculated results was carried out using the stress reduction ratio, the result shows that the bearing capacity of the subsoil in the crust composite was improved.

Estimation of arching effect in geosynthetic-reinforced structures

An analytical method is proposed to estimate arching effect in geosynthetic-reinforced structures. The method integratedly considers the stresses and deformations of soils and geosynthetics, and variation of settlement with depth in overlying soil. The stresses and deformations of soil and geosynthetics can be obtained considering equilibrium and settlement models of overlying soil, membrane effect of geosynthetics, and one-dimensional compression model of underlying soil. The method is verified using a field test, the stress reduction ratio (SRR) is precisely estimated. The results of parametric study indicate that the method has extensive applicability and is helpful for rational design of geosynthetic-reinforced structures.

Numerical Simulation of Quenching and Pre-Stretching Residual Stress in 7085 Aluminum Alloy Plate

The finite element method (FEM) was used to study the quenching and pre-stretching residual stresses of the 7085 aluminum alloy plate, combining with experimental measurement. Quenching residual stress simulation results showed that the stress of the rolling-direction and transverse-direction in the center were 16.5MPa and 9.2MPa, respectively. The stress of the rolling-direction and transverse-direction on the surface were-33.8MPa and-40.4MPa, respectively. The quenching residual stress simulation results were in good agreement with the X-ray diffraction experimental results. As the pre-stretching ratio increases, the quenching residual stress reduction ratio increases. The ratio of 2.0~3.0% pre-stretching can effectively release the quenching residual stress in aluminum alloy plates. After 3.0% pre-stretching, the stress of the rolling-direction and transverse-direction in the center were 0.9MPa and 1.8MPa, respectively. The stress of the rolling-direction and transverse-direction on the surface were-3.6MPa and-7.1MPa, respectively. The quenching residual stress reduction ratio reached above 80%.

Case Studies of Reinforced Piled High-Speed Railway Embankment over Soft Soils

AbstractSoil arching in the reinforced piled embankment is investigated in this paper on the basis of two case studies of high-speed railways constructed in the eastern coastal region of China. The majority of the overburden load was found to have been transferred from the soil to the pile because of soil arching in the piled embankment. In addition, the critical height was approximately 2.6–3.8 times the net pile spacing. The adapted Terzaghi method, the BS8006 method, the Hewlett and Randolph method, EBGEO, and a simplified analytical method used to analyze the reinforced piled embankment were discussed and verified by the two cases. It was found that the adapted Terzaghi method was consistent with the measurement for the stress reduction ratio, with an error of 2.0–38.0%. The simplified analytical method predicted the maximum geogrid strain with the error of 6.7–33.2%, and assessed the maximum settlement at the subsoil surface with an error of 4.4–51.0%.

Review of existing design methods for geosynthetic-reinforced pile-supported embankments

Embankment construction over soft foundation soils is a challenging task for geotechnical engineers due to the undesirable characteristics of soft soils, such as excessive settlements and low bearing capacity. Among the various ground-improvement methods available for overcoming these undesirable characteristics, geosynthetic-reinforced pile-supported (GRPS) embankments are considered to be a reliable solution suitable for time-bound construction projects and difficult ground conditions. Various researchers have introduced methods to design GRPS embankments based on different load transfer mechanisms. However, among design engineers, there is uncertainty regarding the applicability of these design methods. This paper investigates the load transfer mechanism of GRPS embankments using two-dimensional and three-dimensional finite element analyses, and currently available design methods are compared with the results of the finite element modelling. A comparison of the design methods was carried out using the stress reduction ratio, the geosynthetic tension and pile efficacy, considering different pile diameters and spacing, and embankment heights, which govern the currently available design methods. Based on these model results, the inconsistencies in the currently available design methods are identified and discussed in detail.

Stress analysis of clamped-free grid composite cylindrical shell under various end loadings

Stress analysis of cylindrical grid-stiffened composite shells was conducted under transverse loading, pure bending, torsion and axial compression under clamped-free boundary condition. Electrical strain gauges were employed to measure the strains in transverse loading case to validate the finite element analysis which was conducted using ANSYS software. Good agreement was obtained between the two methods. It was observed that stiffening the composite shell with helical ribs decreased the average equivalent Von Mises stress on the shell. The reduction of the stress seemed to be higher in the intersection of two ribs. It was also seen that the stress reduction ratio was higher when the structure was under bending compared to torsion and axial compression. The reduction ratio was approximately 75% in pure bending in the intersection point of the ribs, while it was approximately 25% in torsion. Therefore, it is concluded that the presence of the ribs is more effective under bending. Failure analysis was done using Tsai-Wu criterion. The ribs were observed to result in maximum and minimum increase in the failure load of the structure under transverse bending and torsional loading, respectively.

A simplified model to analyze the reinforced piled embankments

It is an economic way to use the piled embankment for the construction of embankment over soft soil. The combination of piles and reinforcement can effectively reduce the differential settlement at the surface of embankment. The paper presents a simplified model for analysis of an embankment of granular fill on soft ground supported by reinforcement and piles. This model is based on consideration of the arching effect in granular material proposed by Hewlett & Randolph. The vertical equilibrium of the unit body at the center of pile caps immediately below the reinforcement is established. The refinements of the model are that the failure mechanisms of the arch both at the crown and at the pile cap were considered, three-dimensional situation was taken into account for reinforced piled embankment, calculation of the vertical stress carried by the subsoil due to arching effect and reinforcement for multi-layered soil was proposed. Using the simplified model, the influence of embankment height, one-dimensional compression modulus of subsoil, tensile stiffness of reinforcement on stress reduction ratio (SRR) and tensile force of reinforcement is investigated. It is found that the model can be used to assess the relative contribution of the reinforcement and subsoil. The results show that subsoil gives a major contribution to overall vertical equilibrium, while the reinforcement gives obvious contribution at relatively large settlement. The inclusion of the reinforcement can reduce the vertical stress acting on the subsoil. The simplified model is then evaluated by three case studies. The results of this model show good consistence with these cases.

EXCESS PORE WATER PRESSURE ACCUMULATION AND RECOMPRESSION OF SATURATED SOFT CLAY SUBJECTED TO UNI-DIRECTIONAL AND MULTI-DIRECTIONAL CYCLIC SIMPLE SHEARS

This paper is to investigate the effect of cyclic shear direction (or phase difference) on the accumulation of excess pore water pressure during cyclic shear and on the recompression after cyclic shear. Several series of uni-directional and multi-directional cyclic simple shear tests under undrained condition were carried out for normally consolidated Kaolin. From the test results it is shown that the accumulation of pore water pressure and the post-cyclic settlement increase with the shear strain amplitude and the phase difference. The values of the shear strain amplitude at which the effect of cyclic shear direction is most significant, decrease with the increase of the number of cycles. The change of the void ratio in the recompression stage increases approximately in proportion to the logarithm of the stress reduction ratio and is not affected by the shear strain amplitude. For multi-directional cyclic shear, this change of the void ratio depends on the phase difference and the number of cycles. The cyclic recompression indices in the recompression stage were obtained for uni-directional and multi-directional cyclic shears. The cyclic shear induced settlement can be calculated by using these indices.

Estimation of transverse ground surface settlement induced by DOT shield tunneling

Double-O-Tube (DOT) shield tunneling has attracted increasing attention because it offers cost-efficiency in underground construction. Prediction of DOT shield induced ground surface settlement is crucial to underground construction in densely populated urban areas since excessive settlement could trigger potential damage to existing structures. Hitherto, there have not been many studies on this aspect, as only three regions: Japan, Shanghai and Taipei have adopted this tunneling technique. The transverse ground surface settlements induced by the closure of tail void of a DOT shield that formed a portion of the Taoyuan International Airport Access Mass Rapid Transit system in Taipei have been used and back-analyzed using the empirical equation that assumed the settlement profile over a tunnel to be a Gaussian distribution curve and the finite element (FE) analysis. The results showed that the ground surface settlement trough of the DOT tunnel could be conveniently and reliably estimated using the empirical equation, while the FE simulated result was unsatisfactory. Nevertheless, the FE simulation revealed that the ratio of the maximum ground surface settlement to the depth of tunnel axis is linearly proportional to the stress reduction ratio of the ground.

Analysis of geosynthetic reinforcement in pile-supported embankments. Part III: Axisymmetric model

ABSTRACT: In the first part of this study, the geosynthetic reinforcement in a pile-supported embankment was modelled as a thin plate, and in the second part it was modelled as a cable net. Three-dimensional analyses were conducted. A notable feature of the results was the existence of spikes in strains and stresses near the edges of the pile caps, particularly near corners. The computational effort was significant. In this last part an axisymmetric model is analysed, which is simpler, and yields approximate results. It is most appropriate for circular pile caps, but can also be used to estimate displacements for square pile caps. The geosynthetic is modelled here as a membrane with linear stress–strain response, and the governing equations allow for large strains and slopes. The finite difference method is utilised, and the total energy is minimised to obtain equilibrium shapes. For two cases, the effects of Poisson's ratio, soil stiffness, stress concentration ratio, average embankment stress, area replacement ratio and membrane stiffness on the maximum vertical displacement, maximum strains and net stress reduction ratio are determined. A Rayleigh–Ritz procedure is also carried out. Finally, determination of the magnitude of the applied stress on the geosynthetic and the effect of its distribution are discussed.

Analysis of geosynthetic reinforcement in pile-supported embankments. Part I: 3D plate model

ABSTRACT: Geosynthetic reinforcement over a grid of piles in soft soil can help transfer loads to the piles, and reduce settlements. This first part of a three-part study utilises a three-dimensional thin-plate model of the geosynthetic. The von Kármán theory, which includes the effects of bending and stretching, is used. The geosynthetic layer is represented using a linear stress–strain relationship. Owing to symmetry, a one-eighth triangular section of a square unit cell is considered, and the pile caps have either a square, diamond or circular shape. The soil between the pile caps and the geosynthetic, and the soft soil between the piles, are modelled as linear foundations with specified stiffnesses. The embankment stresses over the piles and over the soft soil are also specified. The numerical procedure involves finite differences and minimisation of the total energy. For two sets of parameter values, the effects of the shape of the pile caps on the displacements, strains and net stress reduction ratio are investigated. Sharp spikes in strain and stress occur at the edges of the pile caps. The second paper in this study adopts a three-dimensional cable-net model of the geosynthetic, and an axisymmetric model is analysed in the final paper.

Analysis of geosynthetic reinforcement in pile-supported embankments. Part II: 3D cable-net model

ABSTRACT: In this second part of a three-part study, the geogrid reinforcement is modelled as a three-dimensional cable net. Square pile caps are considered. Again, symmetry is utilised, and numerical results are obtained by minimising the total energy. The cables are connected at nodes, which are represented as spherical joints with variable rotational stiffness. Nodal displacements are computed, and strains and stress resultants can then be calculated from them. The net stress reduction ratio is also found. The effects of variations in the following quantities are determined: the size of the pile caps; the stress on the geosynthetic over the soft soil between the piles; the stiffness of the soft soil; the spacing between geogrid nodes; the orthotropy of the geogrid; and the bending stiffness (modelled by rotational springs) at the nodes. In addition, the effect of rotation of the geogrid by 45° is analysed, in which each rib is parallel to one of the diagonals of the unit cell. Also, the case of two geogrid layers is treated. As for the plate model, large spikes in strain and stress occur near the corners of the pile caps, and three-dimensional effects are important.