Dilation Angle Research Articles

The role of constitutive properties on the longitudinal compressive strength of composites

The longitudinal compressive strength of fibre-reinforced composites is often a limiting factor for structural design. This paper uses micromechanical finite-element analyses to examine how the constitutive properties of the matrix, fibres, and interface affect the composite’s strength. For a typical carbon/epoxy composite, the matrix shear strength has a larger effect than the modulus; a linear-elastic perfectly-plastic approximation of the matrix shear curve may overestimate the composite’s strength by over 15%. A pressure-dependent and dilatant matrix plasticity response may strengthen the composite by 30%, considering current uncertainties in friction and dilation angles of epoxy matrices. The finite shear modulus of carbon fibres may weaken the composite by more than 10%. Considering a cohesive interface using traction–separation laws significantly affects the predicted response of the composite, even for interfaces stronger than the matrix. Opportunities for improving the characterisation, modelling and performance of the constituents in terms of matrix plasticity, fibre shear modulus, and interface response are discussed.

Investigation on mechanical behaviors and energy characteristics of deep-buried marble in a hydraulic tunnel in Southwest China

Shear behavior of rock is of great significance to the stability of deep-buried rock in transportation geotechnical engineering. A series of direct shear tests are carried out on deep-buried marble with different size and shape to focused on the mechanical properties and size shape effects of deeply buried marble, which have been less studied previously. The results show that the size and shape of the specimen have a relatively small impact on the trend of shear stress-shear displacement curve. The failure morphology at the front and rear surfaces shows a significant normal stress effect. Some properties of specimens with different sizes and shapes increase with increasing normal stress, but the dilation angle generally decreases with the increasing normal stress. With the increase of the shear plane size of intact specimen, the cohesion decreases gradually while the internal friction angle increases steadily. The difference value in peak strain between axial compression and direct shear tests is relatively small while it is huge for the peak compression and peak shear strength. The results in this study are beneficial for further understanding the shear mechanics, energy evolution process and size effects of deep-buried rock in transportation geotechnical engineering.

Seismic analysis of coastal slope stability considering strength nonlinearity and tension cut-off

Tensile stresses may lead to cracks forming at the crest of coastal slopes and subsequent slope failure, particularly under seismic load conditions. Traditional shear strength failure criteria often overestimate the tensile strength of geomaterials, and in turn the factor of safety against sliding. This paper presents an improved nonlinear analysis method for assessing the stability of slopes, while considering tension cut-off in the failure criterion as well as the detrimental effects of inertial seismic loading and pore water pressure. The method employs the principle of energy consumption to obtain solutions for the geometry of the potential sliding surface and stress distribution, while the critical slope height and the stability factor are determined by means of a particle swarm optimization algorithm. Comparison with existing methods is conducted to validate the effectiveness and accuracy of this method. Results showed that the tensile strength of geomaterials had a significant impact on the stability of the slope and its associated failure mechanism, particularly for the vertical slope. This study could avoid the assumptions of the dilatancy angle on the velocity discontinuity and critical slope sliding surface, and reflect the tensile and nonlinear characteristics of the geomaterials well.

Shear behavior of basalt fiber modified compacted red mudstone as subgrade fill material

This study experimentally investigated the shear behavior of basalt fiber-modified compacted red mudstone as a subgrade of a high-speed railway. We examined the effects of different vertical stresses, initial water contents, and fiber contents on brittleness and dilatancy. Several direct shear tests were conducted under the aforementioned conditions. The stress displacement curve tended to vary from softening to hardening with an increase in vertical stress. In the Mohr–Coulomb mode, except for the saturated state, the residual internal friction angle was greater than the peak value, whereas the residual cohesion was lesser than the peak value. The maximum cohesion occurred at the optimal water and fiber content, whereas the friction angle exhibited a downward trend with an increase in the contents discussed above. The brittleness index (I b ) was defined as the ratio of the peak to residual shear strengths. The magnitude of the brittleness index reduced with increase in the vertical stress. Moreover, the maximum value occurred at the optimal water and fiber content. Furthermore, the deformation mechanism was discussed based on the dilatancy angle(ψ). Based on the data, three different stages were proposed: Softening–Dilatancy (S1), the Hardening–Dilatancy (S2), and Hardening–Shrinkage (S3).

Temporal evolution of shear-induced dilatancy of rock fractures: controls from surface roughness and normal stress

SUMMARY Understanding the shear-induced dilatancy of rock fractures is important for assessing the permeability evolution and seismic hazard in shale and geothermal reservoirs. The displacement dependence of fracture dilation has been well studied, while the influence of slip velocity is poorly constrained. In this study, we combined displacement- and velocity-dependent aperture models to reproduce the transient shear-induced dilatancy of fractures in sandstone in 16 normal stress unloading tests. Our results show that the combined aperture model can describe the transient fracture aperture evolution during accelerating slip induced by normal stress unloading better than the model dependent only on slip displacement. Slip velocity could enhance the aperture increase on smoother fractures at lower normal stresses and higher slip velocities. Both the dilation factor and characteristic slip distance decrease with increasing normal stress and surface roughness, signifying reduced contribution of slip velocity to transient shear-induced dilatancy at higher normal stresses and surface roughness. The dilation angle increases with the increase of surface roughness, and this increase diminishes at higher normal stresses primarily attributable to more severe asperity wear. These findings highlight the importance of slip velocity in controlling the transient evolution of aperture and permeability of a rock fracture. Our study also provides constraints on the constitutive parameters in the combined aperture model for describing transient shear-induced fracture dilatancy. We suggest that it is crucial to incorporate the velocity-dependent aperture model to simulate the nonlinear evolution of fracture aperture in future analytical and numerical models involving coupled hydromechanical processes in geoenergy systems.

Assessing Mechanical Properties and Response of Expansive Soft Rock in Tunnel Excavation: A Numerical Simulation Study.

This study investigates the dynamics of moisture absorption and swelling in soft rock during tunnel excavation, emphasizing the response to support resistance. Utilizing COMSOL numerical simulations, we conduct a comparative analysis of various strength criteria and non-associated flow rules. The results demonstrate that the Mohr-Coulomb criterion combined with the Drucker-Prager model under compressive loads imposes stricter limitations on water absorption and expansion than when paired with the Drucker-Prager model under tensile loads. Restricted rock expansion leads to decreased horizontal displacement and ground uplift, increased displacement in the tunnel's bottom arch, and significantly reduced displacement in the top arch. The study also considers the effects of shear dilation, burial depth, and support resistance on the stress and displacement of the surrounding rock. Increased shear dilation angles correlate with greater rock expansion, resulting in increased horizontal displacement and ground uplift. The research study concludes that support resistance is critical in limiting the movement of the tunnel's bottom arch and impacting the stability of the surrounding rock. Additionally, the extent of rock damage during the excavation of expansive soft rock tunnels is found to be minimal. Overall, this study provides valuable insights into the processes of soft rock tunnel excavation and contributes to the development of more efficient support systems.

Study on soil pressure of loose soil in cohesive soil tunnel considering soil arch effect

Soil pressure in clay formation tunnels is closely related to soil arch effect and the development of slip surfaces. Firstly, numerical simulation software is used to simulate the actual situation of tunnel excavation, and the change rule of the slip-cracking surface of cohesive soil is analyzed. Secondly, based on the numerical simulation results and the ellipsoid theory, the pressure formula of Terzaghi loose Earth is modified considering that the principal stress trace is catenary. Finally, the calculation results are compared with the finite element calculation results to verify the rationality of the formula in this paper. The relationship between the internal friction angle, cohesion force c, eccentricity ε, looseness coefficient β, and the pressure of loose Earth is further studied. The results show that there is a gap between the sliding crack angles with or without dilatancy angle and it will affect the development form of soil arch. The slip angle decreases gradually with the increase of the buried depth ratio H/D and becomes stable when the buried depth ratio H/D≥3. Compared with deep-buried tunnels, the increase of internal friction angle in the shallow-buried tunnel is more conducive to reducing the overlying soil pressure. The loose soil pressure decreases with the increase of eccentricity ε and loose coefficient β, and the influence of eccentricity ε on loose soil pressure is significantly greater than that of loose coefficient β. Therefore, the change of eccentricity ε should be paid close attention in the project.

Numerical Study of GFRP-reinforced Concrete Beams with Straight and Hooked-end Bar Lap Splices

Glass fibre-reinforced polymer (GFRP) has been increasingly used as the main reinforcement in concrete structures due to its durability and resistance to corrosion. However, there is a lack of data regarding the bond behaviour of GFRP bars with 180-degree hooked ends used in flexure concrete elements. To investigate the effects of the hooked ends of GFRP bars on the bond strength with concrete, beams reinforced with GFRP bars with straight and hooked-end lap splices at the midspan were modelled using ABAQUS/Standard software. The finite element models were validated using the experimental results of four full-size concrete beams. Two beam specimens had straight-end bars, while the other two had hooked-end bars. Sensitivity analysis was performed to find the proper values for model verification, such as dilation angle and mesh density. According to the outcomes of the parametric study conducted in this research, changing the length of the bar lap splices in the range of 15 to 40 times the bar diameter had a negligible effect on the beam’s overall behaviour. Increasing the number of reinforcing bars from 3 to 5 increased the beam flexural strength by about 33%, whereas increasing the diameter of the bars from 10 to 20 mm doubled the beam strength.

Laboratory characterization of clay fouled rubber-coated ballast with and without geogrid

The utilization of rubber-coated ballast (RCB) aims to enhance the energy dissipation capacity and decrease the rate of deterioration of ballasted tracks, leading to reduced maintenance costs. Ballast fouling can affect the ballast's drainage properties, shear strength, deformation, and resilience that may ultimately lead to track misalignment. In this context, in the present study, the drainage and shear characteristics of RCB and RCB-conventional ballast mix are evaluated for various levels of clay fouling. The results of permeability tests indicated that the hydraulic conductivity (k) of RCB is more than that of conventional ballast. For example, the k of RCB is 71% higher than that of conventional ballast (fouling content = 50%). However, the k of ballast decreased significantly with the increase in the content of clay fouling. The results from the direct shear tests revealed that both RCB and RCB50% perform poorly in terms of peak friction angle (φ) when compared to conventional ballast. With the addition of clay fines, a further reduction in the shear strength is observed. For instance, the φ of unreinforced RCB, RCB50%, and conventional ballast decreases by 18.5%, 14.2%, and 10.4%, respectively, with the increase in fouling content from 0% to 50%. However, in case of geogrid-reinforced RCB50%, the peak friction and dilation angles are found to be relatively higher as compared to unreinforced conventional ballast in both clean and fouled conditions. The results of cyclic tests demonstrated that the deformation of both unreinforced and geogrid-reinforced RCB-conventional ballast samples increased while the resilience modulus (Mr), damping ratio (Dr), and ballast breakage index (BBI) decreased with the increase in fouling content. Moreover, the inclusion of geogrid significantly reduced the extent of deformation and particle breakage but enhanced the Mr and Dr when compared to unreinforced ballast. The various test results suggest that the geogrid-reinforced RCB50% performs better than unreinforced conventional ballast under both clean and clay fouling conditions. For geogrid-reinforced RCB50%, the volumetric strain (εv) and BBI are reduced by 9.7% and 69.5%, while Mr and Dr are improved by 5.75% and 176.5% as compared to unreinforced conventional ballast (fouling content = 20%). Additionally, empirical relations are developed to predict the vertical strain (ε1), lateral strain (ε3), Mr, Dr, and BBI of RCB-conventional ballast mix under clean and clay-fouled conditions.

Bond-Damaged Prestressed AASHTO Type III Girder-Deck System with Retrofits: Parametric Study

This research describes an in-depth analysis of the flexural strength of a strengthened AASHTO Type III girder-deck system with debonding-damaged strands based on the finite element software ABAQUS 6.17. To investigate the stand-debonding impact and retrofit, two strengthening techniques by the separate use of carbon fiber-reinforced polymer (CFRP) and steel plate (SP) were proposed. A detailed finite element analysis (FEA) model considering strand debonding, material deterioration, and retrofitting systems was developed and verified against relevant experimental data obtained by other researchers. The proposed FEA model and the experimental data were in good agreement. The sensitivity of the numerical model to the mesh size, element type, dilation angle and coefficient of friction was also investigated. Based on the verified FEA model, 156 girder-deck systems were studied, considering the following variables: (1) debonding level, (2) span-to-depth ratio (L/d), (3) strengthening type, and (4) strengthening material amount. The results indicated that the debonding level and span-to-depth ratio had a major effect on both load–deflection behaviors and the ultimate strength. The relationships between the enhancement of the ultimate strength and the thickness of the strengthening material were obtained through regression equations with respect to the CFRP- and SP-strengthened specimens. The coefficient of determination (R2) was 0.9928 for the CFRP group and 0.9968 for the SP group.

Evolution Characteristics of the Strength and Dilation Parameters of Gas Hydrate-Bearing Sediments and a Modeling Study

Gas hydrate has gradually become a new potential energy resource. However, some engineering and environmental problems related to the mechanical properties of gas hydrate-bearing sediments (GHBS) during gas recovery may occur. Many studies have been carried out on the basic mechanical properties of GHBS samples based on laboratory tests, but their evolution characteristics and suitable models require further research. Based on a series of data analyses of published laboratory experimental results on GHBS samples with different hydrate saturations under various confining pressures, the evolution characteristics of strength and dilation parameters were investigated. It was found that cohesion (c) increases quickly to a peak value and then decreases gradually to a residual value with an increasing plastic shear strain, and the samples with higher hydrate saturations have higher initial values, peak values, and residual values of cohesion (c). The internal friction angle (φ) increases quickly with increasing plastic shear strain and then becomes stable at a residual value for all the samples with different hydrate saturations. The dilation angle (ψ) increases from negative to positive values with increasing plastic shear strain and then becomes stable at a residual value. These characteristics are likely to be related to the compaction occurring at the early stage of compression before expansion due to dilation. In this paper, a non-linearly fitted model is proposed considering the evolution of the mechanical parameters, and the verification tests show that the proposed model can simulate the stress–strain behaviors of the GHBS samples well. This model is also adopted in the stability analysis of submarine slopes containing hydrate reservoirs. The analytical approach is developed, accompanied by the strength reduction method.

Interface frictional anisotropy of dilative sand.

Understanding direction-dependent friction anisotropy is necessary to optimize interface shear resistance across soil-structure. Previous studies estimated interface frictional anisotropy quantitatively using contractive sands. However, no studies have explored how sand with a high dilative tendency around the structural surface affects the interface shear response. In this study, a series of interface direct shear tests are conducted with selected French standard sand and snakeskin-inspired surfaces under three vertical stresses (50, 100, and 200kPa) and two shearing directions (cranial → caudal or caudal → cranial). First, the sand-sand test observes a higher dilative response, and a significant difference between the peak and residual friction angles (ϕpeak - ϕres = 8°) is obtained at even a lower initial relative density Dr = 40%. In addition, the interface test results show that (1) shearing against the scales (cranial shearing) mobilizes a larger shear resistance and produces a dilative response than shearing along the scales (caudal shearing), (2) a higher scale height or shorter scale length exhibits a higher dilative tendency and produces a higher interface friction angle, and (3) the interface anisotropy response is more pronounced during cranial shearing in all cases. Further analysis reveals that the interface friction angle and dilation angle are decreased with the scale geometry ratio (L/H). For L/H values between 16.67 and 60, the interface dilation angle varies between 9° and 4° for cranial first shearing and 3.9°-2.6° for caudal first shearing. However, the difference in dilation angle within the same shearing direction is less than 1°.

Dilatancy and Multi-Scale failure characteristics of a foliated rock under triaxial confinement unloading conditions

Controlled by the foliation structure, foliated rocks exhibit mechanical anisotropy and cause squeezing deformation and subsequent supporting structure failure during tunnel excavation. Here, the deformation and failure characteristics of a phyllite under unloading stress paths were experimentally investigated. The strength and elasticity parameters of phyllite exhibited moderate ‘shoulder’ shaped anisotropy, and specimens failed in three different modes: cross-plane shear-tensile fracture, in-plane shear sliding, and interlayer buckling, depending on the foliation angle and initial confining pressure. Meanwhile, the evolution of dilatancy angle during volume dilatancy also showed significant anisotropy, either increasing or decreasing with the increasing plastic shear strain during post-peak loading. We further discussed the mechanism of the evolution of dilatancy angle based on the multi-scale observation of the failure characteristics. Generally, interlayer buckling improved the dilatancy angle and roughness loss during shear failure resulted in dilatancy angle degradation. These findings provided evidence for a deeper understanding of the failure mechanisms of tunnel rock mass in foliated strata.

Assessing the geometry of defect waviness from borehole data

Within the large, open cut, iron ore mines of the Pilbara region of Western Australia, defect shear strengths often control the slope design where bedding dips shallowly to moderately out of the pit slope. The presence of metre to decametre scale open folding or waviness in these units can contribute to the friction angle of bedding shear strengths, potentially allowing for steeper slope angles and improved economics for the deposit. Traditionally, waviness affecting defect shear strength is assessed from surface mapping, bench mapping or qualitatively from observations in core. Surface mapping of bedrock is often not possible due to detrital cover or a lack of suitable outcrop, while bench mapping is inherently conducted perpendicular to the direction of sliding risk. The use of downhole data from boreholes drilled into the slopes circumvents these issues. The method presented here involves assessing characteristic downhole wavelength, inter-limb angle and amplitude of folding from defect orientation data interpreted from borehole televiewer imaging. The downhole wavelength and defect orientations are transformed to a true down-dip wavelength, dilation angle, and estimated amplitude in the direction of sliding risk. The calculation of down-dip wavelength is critical for assessing the applicability of the associated dilation angle to the scale of the slope and failure mechanism in question. The adoption of defect shear strengths that include a waviness contribution to the friction angle allows for implementation of steeper slope angles in structurally controlled slopes.

Bearing Capacity of a Shallow Foundation above the Soil with a Cavity Based on Rigid Plastic Finite Element Method

Based on the Rigid Plastic Finite Element Method (RPFEM), this study investigates the performance of the footing on the soil with a cavity. The RPFEM is used in plane strain conditions and necessitates only a few materials to predict the bearing capacity: the unit weight of the soil, the cohesion, the shear resistance angle, and the dilation angle. Considering diverse soil types, including cohesive and intermediate soils, the findings are presented through dimensionless 2D charts in which the horizontal axis X and vertical axis Y are normalized to parameters R and H, representing the horizontal and vertical dimensions of the plastic mechanism beneath the footing in the absence of the soil cavity. Analyzing geometric factors such as footing width B and cavity characteristics (shape, size, and location), the study reveals that the farther the cavity, the less it impacts the footing performance. The distribution of the normalized bearing capacity across the (X, Y) space elucidates the expansion and variation of the influence zone. Equations incorporating the mentioned geometric parameters and soil shear strengths are proposed and verified with data in the literature. In cohesive soils, the influence zone predominantly extends vertically, following the expansion of the slip surface in no void condition. Conversely, for intermediate soils, the zone of influence exhibits a dependency on the shear resistance angle, resulting in an extension in one direction more than the other. Illustrating typical failure mechanisms, the study delves into detailed discussions to enhance comprehension of how the cavities affect the bearing capacity of the footing.

A Finite Element Modeling Approach for Analyzing the Cyclic Behavior of RC Frames

Numerous approaches and inputs for nonlinear finite element analysis used for modeling reinforced concrete systems under cyclic loads have been outlined. Practitioners usually take into account a number of significant factors when modeling, for example deciding upon the constitutive relations, the connection among concrete and steel reinforcement, and the mesh sensitivities, in order to accurately simulate the nonlinear response of reinforced concrete under cyclic loads. Therefore, it is essential to thoroughly assess popular modeling approaches and develop a reliable modeling approach with reliability, stability, and consistency. A modeling technique and practical guidelines for nonlinear finite element analysis of reinforced concrete frames subjected to cyclic loading are described in this study, taking into account the influence of parameters such as dilation angle, stiffness recovery, friction coefficient, and mesh sizes. The hysteretic force–displacement behavior and load-carrying capacity of reinforced concrete frames subjected to cyclic loading have been compared using numerical and experimental curves. Discrete fracture paths are specified in the model’s geometry to achieve an accurate depiction of the stress distribution and an adequate fit of the hysteretic behavior. The proposed modeling approach provides a precise representation of the stress distribution and hysteretic behavior while keeping computational costs reasonable.

Numerical study on uplift capacity and behavior of plate anchor considering different dilation angles

Numerical study on uplift capacity and behavior of plate anchor considering different dilation angles

Influence of Moisture Content on Some Mechanical Properties of Wheat

The loads generated inside agricultural silos under static and dynamic conditions depend on the mechanical properties of the materials stored inside them. Silo calculation methodologies are based on these mechanical properties. Although it is known that moisture content greatly influences the values reached by these mechanical properties, only a few studies have been conducted to determine them. The present work determines the angle of internal friction, the apparent cohesion, the dilatancy angle and the apparent specific weight of wheat when subjected to different moisture contents. Direct shear and oedometer assay devices were used. In addition, a climatic chamber was used to moisten the wheat samples used in this work. From the different assays conducted, it could be observed that the values of the angle of internal friction, the apparent cohesion and the apparent specific weight were like those found in the literature. However, no values of the dilatancy angle of wheat as influenced by moisture content were previously reported. The values obtained here for this parameter are within the range of those specified for dry wheat samples. Finally, higher apparent specific weight values were observed as moisture content increased up to 13.4%, then decreasing at a moisture content of 15.5%. This was not expected according to the results stated by some authors, although others reported a similar tendency. The values here provided can be used in silo load calculations involving numerical methods for modeling technological processes.

Study of hydro-mechanical behaviours of rough rock fracture with shear dilatancy and asperities using shear-flow model

The geometric properties of fracture surfaces significantly influence shear-seepage in rock fractures, introducing complexities to fracture modelling. The present study focuses on the hydro-mechanical behaviours of rough rock fractures during shear-seepage processes to reveal how dilatancy and fracture asperities affect these phenomena. To achieve this, an improved shear-flow model (SFM) is proposed with the incorporation of dilatancy effect and asperities. In particular, shear dilatancy is accounted for in both the elastic and plastic stages, in contrast to some existing models that only consider it in the elastic stage. Depending on the computation approaches for the peak dilatancy angle, three different versions of the SFM are derived based on Mohr-Coulomb, joint roughness coefficient-joint compressive strength (JRC-JCS), and Grasselli's theories. Notably, this is a new attempt that utilizes Grasselli's model in shear-seepage analysis. An advanced parameter optimization method is introduced to accurately determine model parameters, addressing the issue of local optima inherent in some conventional methods. Then, model performance is evaluated against existing experimental results. The findings demonstrate that the SFM effectively reproduces the shear-seepage characteristics of rock fracture across a wide range of stress levels. Further sensitivity analysis reveals how dilatancy and asperity affect hydraulic properties. The relation between hydro-mechanical properties (dilatancy displacement and hydraulic conductivity) and asperity parameters is analysed. Several profound understandings of the shear-seepage process are obtained by exploring the phenomenon under various conditions.

Investigating effects of tire geo-scrap reinforcement on shear strength characteristics of railway ballast using large-scale direct shear test

ABSTRACT The utilization of geogrids used to ballast reinforcement has been widely investigated by researchers in the technical literature. This technique aims to enhance the load-bearing capacity of ballast while mitigating its degradation by minimizing breakage and settlement. This research proposes a novel approach to enhancing ballast shear behaviour in a large-scale direct shear test through the use of geogrids composed of waste tire strips, referred to as ‘geo-scraps’. In this study, the geo-scrap performance on ballast shear reinforcing was investigated to find the best laying depth in the ballast layer and the optimum aperture size of grids. To this aim, the geo-scraps were positioned at four different depths of the ballast (6, 12, 18, and 24 cm) and then the shear tests were carried out on geo-scraps with three different aperture sizes (5 cm x 5 cm, 10 cm x 10 cm, and 25 cm x 25 cm). All the large-scale shear tests were conducted under normal stresses of 50, 100, and 150 kPa. The findings of laboratory tests indicate that the 5 cm x 5 cm aperture positioned 12 cm from the bottom of the box yields the most desirable results for the ballast in terms of enhancing shear strength, augmenting the internal friction angle, and reducing the dilatancy angle. Compared to the non-reinforced (NR) state, the shear strength and internal friction angle experienced respective increases of 20% and 7%, while the dilatancy angle decreased by 30%. These outcomes show that the geo-scraps appropriately reinforced the ballast in terms of shear behaviour.