Stress-displacement Curves Research Articles

A machine learning-based method for predicting the shear behaviors of rock joints

In this study, machine learning prediction models (MLPMs), including artificial neural network (ANN), support vector regression (SVR), K-nearest neighbors (KNN), and random forest (RF) algorithms, were developed to predict the peak shear stress values and shear stress-displacement curves of rock joints. The database used contained 693 records of peak shear stress and 162 original shear stress-displacement curves derived from direct shear tests. The results demonstrated that the MLPMs provided reliable predictions for shear stress, with the mean squared errors (MSEs) between their predicted and measured shear stress varying from 0.003 to 0.069 and the coefficients of determination (R2 values) varying from 0.964 to 0.998. The feature importance values indicate that the joint surface roughness coefficient (JRC) is the most important influential factor in determining the peak shear stress, followed by the joint wall compressive strength (JCS), basic friction angle (φb), and shear surface area (As). Similarly, for the shear stress-displacement curve, the JRC is the dominant factor, followed by As, φb, and JCS. Additional direct shear tests were conducted for model validation. The validation shows that the MLPM predictions demonstrate improved consistency with the experimental results in relation to both the peak shear stress and peak shear displacement.

In-situ direct shear test and numerical comparative research on mudstone gravel slope reinforced by polymer grouting

This study investigates the shear strength characteristics of mudstone gravel slope following polymer grouting reinforcement through in-situ direct shear test. A corresponding numerical model was developed to analyze stress distribution. The findings indicate that shear stress initially increases and then stabilizes with horizontal displacement during the pre-grouting shear test. Post-grouting, a distinct stress peak emerges on the stress-displacement curve, signifying a substantial enhancement in overall shear strength. The peak shifts with increasing vertical stress, delaying the onset of yield strength. Moreover, a direct correlation exists between shear strength and the density of grout injected. Numerical simulations reveal an H-shaped stress distribution around the shear plane. Upon severing the grouting column, the stress distribution becomes intricate, with stress appearing to circumvent the column. The discrepancy between simulation and experimental data is minimal, with the simulated internal friction angle deviating by -0.97–1.5% from the experimental values and the simulated cohesion being marginally lower, with an error range of 3.9-5%. These results offer valuable insights for engineering applications.

Study on Shear Strength of Soil-Root Systems of Different Vegetation Types.

The root systems of vegetation significantly contribute to enhancing slope stability. The shear strength of soil-root systems is a crucial parameter for assessing slope stability. This study focuses on six types of vegetation in the Yellow River Basin of China (woodland: Populus przewalskii and Broussonetia papyrifera; shrubland: Periploca sepium and Ziziphus jujuba; grassland: Artemisia hedinii and Setaria viridis), employing in situ shear tests and the Wu-Waldron model (Wu model) to investigate the shear strength of soil-root systems. The results show that the shear stress-displacement curves for P. przewalskii, B. papyrifera, and Z. jujuba are higher and steeper, with clear inflection points. The tensile strength of the roots from the six vegetation types decreases as the root diameter increases. According to the Wu model, the additional root cohesion is ranked as follows: A. hedinii > B. papyrifera > P. przewalskii > Z. jujuba > P. sepium > S. viridis. Based on the in situ shear tests, the shear strength increments are ranked as follows: Z. jujuba > B. papyrifera > P. przewalskii > A. hedinii > P. sepium > S. viridis. Overall, the additional root cohesion obtained by the Wu model in each soil layer is greater than the shear strength increment measured from the in situ shear tests. In the 0-30 cm soil layers, the soil-root systems of Z. jujuba, B. papyrifera, P. przewalskii, and A. hedinii exhibit a better shear strength, whereas P. sepium and S. viridis perform poorly. A principal component analysis reveals that the shear strength of the soil-root systems of different vegetation types is primarily influenced by the soil moisture content and root mass density. Z. jujuba, B. papyrifera, P. przewalskii, and A. hedinii are recommended for ecological restoration projects in the Yellow River Basin of China.

Effect of Water Content and Degree of Compaction of Clay Subgrade Soil on the Interface Shear Strength Using Geogrid

Pavement performance in clay soil roads can be enhanced with the help of the geogrid concept by increasing lateral confinement, bearing capacity, and overall rigidity of the pavement. Also, geogrid is useful in minimizing vertical and lateral pavement deformations. In this study, 65 tests are performed without and with two types of geogrid, and the aim behind it is to investigate the effect of the moisture content and degree of compaction of clay soil on the interface shear strength. The number of materials used to conduct this study, included subbase granular (type B), clay subgrade soil, the Biaxial and SS2 geogrid which is used as a reinforcing material. Testing the direct shear is conducted by a large-scale direct shear manufactured device contains an upper box with side lengths of (20×20×10) cm, and a lower box with side lengths of (20×25×10) cm. Results show that for all tests the normal applied stresses are 25, 50, 75 and 100 kPa, the shear stress displacement curves for all cases follow similar trends and they are affected by normal stress, density and water content of subgrade soil and type of geogrid. Also, it is obvious that the shear stress increases by increasing the normal applied stress and degree of compaction of clay subgrade soil reaching its maximum value at 95% degree of compaction while it decreases by increasing the water content of subgrade soil. The maximum value of shear strength is observed when water content is equal to 8% (dry side) and finally, the Biaxial geogrid BX1100 (G1) is more efficient in reinforcing clay-subbase interface as compared with SS2 geogrid (G2).

Density Functional Theory Study of Adhesion Mechanism between Epoxy Resins Cured with 4,4'-Diaminodiphenyl Sulfone and 4,4'-Diaminodiphenylmethane and Carboxyl Functionalized Carbon Fiber.

The molecular mechanism of adhesion of two epoxy resins based on diglycidylether of bisphenol A (DGEBA) cured with 4,4'-diaminodiphenyl sulfone (DDS) and 4,4'-diaminodiphenylmethane (DDM) to the carbon fiber (CF) surface is investigated by employing density functional theory (DFT) calculations. The CF surface was modeled by the armchair-edge structure of graphite functionalized with carboxyl (COOH) groups. Two adhesion interfaces were constructed using the CF surface: one with the DGEBA-DDS molecule (CF/DGEBA-DDS interface) and the other with the DGEBA-DDM molecule (CF/DGEBA-DDM interface). The interfacial properties were analyzed by calculating the maximum adhesion stress (Smax) at the interface. The adhesion stress-displacement curve revealed that Smax is 1160.37 MPa, higher for the CF/DGEBA-DDS interface compared to the CF/DGEBA-DDM interface, which is 1060.48 MPa. The energy decomposition analysis showed a similar DFT contribution to adhesion stress for both interfaces, but the dispersion contribution is more significant at the CF/DGEBA-DDS interface. The crystal orbital Hamilton population (COHP) analysis revealed distinct interfacial interactions despite similar DFT contributions. Hydrogen bonding (H-bonding) between the functional groups at both interfaces including feeble OH-π interactions between the benzene rings of epoxy resins and COOH groups on the CF surface were observed. The orbital interaction energies calculated from integrated COHP, i.e., IpCOHP, revealed that the CF/DGEBA-DDS interface has six H-bonding interactions with large absolute IpCOHP values (>1 eV), whereas the CF/DGEBA-DDM interface has five. The interaction between the amine group of the DGEBA-DDM molecule and the CF surface has a large IpCOHP value among all interactions. The sulfone group being at the center of the DDS molecule and its strong surface interaction positioned the DGEBA-DDS molecule closer to the CF surface than the DGEBA-DDM molecule, enhancing dispersion interaction at the CF/DGEBA-DDS interface. Hence, the CF surface exhibits a stronger affinity toward the DGEBA-DDS molecule than the DGEBA-DDM molecule through dispersion interaction.

Influences of the normal loading condition and surface texture on interface shear behavior between sand and suction caisson wall

The mechanical behavior of the interface between soil and the suction caisson wall determines the installation behavior and bearing capacity. A series of interfacial shear tests are conducted to study the effects of the normal loading condition and the interface texture on the sand-suction caisson interface shear characteristics. The relative density of the sand used in the test is 0.9. It is found that with the increase of shear displacement, the shear stress-displacement curve under a constant normal load condition (CNL) has an obvious strain-softening trend. On the contrary, the shear stress increases linearly with increasing the shear displacement under a variable normal load condition (VNL). The stress ratio between shear and normal stress, the broken zone thickness, and the shear deformation zone thickness for a convex suction caisson wall-sand interface are significantly larger than those of the grooved interface. However, when the roughness increases to a certain value, the stress ratio between shear and normal stress would decrease. Under large displacement shearing, the degree of sand particle breakage becomes significantly higher than that under small displacement shearing. These results help understand the shear characteristics of the sand-suction caisson wall and invent the new type of the suction caisson.

Consideration of Different Soil Properties and Roughness in Shear Characteristics of Concrete–Soil Interface

To investigate the impact of diverse soil characteristics and surface irregularities on interfacial shear strength attributes, a large-scale straight shear apparatus and particle flow software were employed to conduct interfacial shear experiments with varying soil properties and surface irregularities. The results demonstrated that, under an identical R and normal stress conditions, the clay and silty clay shear stress–displacement curves exhibited strain softening, while the silt curve exhibited strain hardening. An increase in R can markedly enhance the peak shear strength at the interface, although a critical value exists beyond which this effect is no longer observed. The Rc is primarily contingent upon the soil properties. Numerical simulations demonstrate that the internal shear displacement and deformation resulting from the diverse soil properties are distinct. Clay particles are constituted of varying-sized particle aggregates that collectively resist shear. Silt particles resist shear through interfacial friction generated by shear. The practicality of Duncan and Clough’s constitutive model for interfacial shear with roughness influence is verified, and the constitutive model under strain hardening is modified.

A novel statistical damage constitutive model of rock joints considering normal stress and joint roughness

The shear constitutive model of rock joints is of great significance to the stability analysis in rock engineering, and it is closely related to the normal stress ([Formula: see text]) and joint roughness coefficient ( JRC). However, the existing investigations seldom consider the influences of [Formula: see text] and JRC simultaneously. Therefore, a novel damage constitutive model considering the [Formula: see text] and JRC is developed in this work. In the presented model, it is assumed that the rock materials are composed of damaged and undamaged microunits, and the damage evolution law of the microunits conforms to the Weibull distribution in the shear process. Based on the proposed assumption, the constitutive relationship between shear stress and shear displacement is deduced. The evolutions of the mechanical parameters and damage variable versus [Formula: see text] and JRC are analyzed in detail. The proposed damage model that involves [Formula: see text] and JRC is verified by comparing theoretical values with the laboratory results. The results show that the damage constitutive model is in good agreement with the test results. Additionally, the influences of [Formula: see text] and JRC on the shear stress-displacement curves are studied. This work can provide a valuable theoretical method for analyzing the shear mechanical characteristics and damage evolution laws of rock joints.

Experimental investigation of permeability evolution and progressive damage of sandstone in the complete process of direct shear

Permeability evolution and damage behavior under shearing conditions are critical for reliable predictions of rock seepage characteristics during excavation and a correct assessment of their impacts on the occurrence of natural or induced hazards. However, publications concerned with this problem have been poorly issued so far. The primary objective of this study is to innovatively investigate permeability evolution and damage behaviors of sandstone in the complete process of direct shear. Several groups of red sandstone specimens are deformed in direct shearing using a newly modified servo-controlled direct shear apparatus to measure permeability evolution and monitor acoustic emission (AE) activities with damage under various normal stress conditions. The morphology of the rupture surfaces is scanned using a 3D laser scanner after tests. According to our experimental results, the locus of the shear stress-displacement curve is found to be closely associated with microcrack progressivity. The initial permeability in advance of shearing test and the peak permeability appearing after the peak shear stress decrease with the increasing normal stress. The minimal and maximum permeability values exhibit a lag-effect with respect to the turning and the peak points on the shear stress-displacement curve. Furthermore, a palpable surge is found in the interevent time function during shearing, which can be a potential indicator for early warning of catastrophic rupture of brittle rock materials under shearing or compression. The joint roughness coefficient (JRC) values in the direction of shearing deformation for each normal stress case are more significant than those in the direction perpendicular to the shear displacement. The impacts of our obtained results are potential ramifications for permeability control in rock engineering projects.

Effects of Thermoforming Parameters on Woven Carbon Fiber Thermoplastic Composites.

The quality of woven carbon fiber fabric/polycarbonate thermoplastic composites after thermoforming and demolding was investigated using finite element simulation and the Taguchi orthogonal array. The simulation utilized a discrete approach with a micro-mechanical model to describe the deformation of woven carbon fabric, combined with a resin model. This simulation was validated with bias extension tests at five temperatures. The thermoforming process parameters considered were blank temperature, mold temperature, and blank holding pressure, with three levels for each factor. Optimal values for the fiber-enclosed angle, spring-back angle, mold shape fitness, and the strain of the U-shaped workpiece were desired. The results indicated that the comparison of the stress-displacement curve of bias extension tests verified the application of the discrete finite element method. Results from the Taguchi array indicated that blank holding pressure was the dominant parameter, with the optimal value being 1.18 kPa. Blank temperature was the second most significant factor, effective in the range of 160 °C to 230 °C, while mold temperature had a minor effect. Furthermore, the four quality values are dependent and have a similar trend. The best combination was identified as a blank holding press of 1.18 kPa, a blank temperature of 230 °C, and a mold temperature of 190 °C.

Experimental and numerical study on the loading rate dependent tensile behavior of carbon fiber/epoxy interface

A series of experiments and simulations were performed to explore the effect of loading rate on the tensile behavior of carbon fiber/epoxy composite interface via the fiber bundle tensile method. Varying velocities were used to test the stress‒time and stress‒displacement curves of the samples, and a high-speed camera was used to study the in situ failure behavior of the carbon fiber/epoxy interface. Increasing the loading rate from 5 × 10−6 m/s to 12.0 m/s leds to an increase in the interfacial tensile strength from 6.1 ± 0.9 MPa to 16.4 ± 0.3 MPa, an increase in the interfacial stiffness from 1.58 ± 0.3 N/m to 17.4 ± 3.1 N/m, and a decrease in the fracture displacement from 0.23 ± 0.03 mm to 0.10 ± 0.01 mm. Optical microscopy analysis revealed rougher crack surfaces at higher loading rates. The fracture mode of interface transitioned from fiber breakage and pull-out to brittle matrix cracking with increasing loading rate. The finite element method was employed to verify the effectiveness of the fiber bundle tensile method with a split Hopkinson tension bar and study the failure behavior of the interface under dynamic loading. The simulation results showed that the calculated failure stress was 20 % lower than the actual value, and the cohesive layer was found to have greater stress at the edge region. This investigation deepens the understanding of the effects of the loading rate on the interfacial tensile behaviors of carbon fiber/epoxy composites.

Stochastic fracture of concrete composites: A mesoscale methodology

The accurate prediction of concrete composites’ fracture behaviour requires precise meso-structural characterization, appropriate fracture modelling, and pivotal uncertainty assessment. Towards this goal, we adopt a dynamics approach with CT images to generate numerical concrete models using real aggregates with 30%-60% contents. An image slicing method is then developed to obtain a large number of realistic models in 2D, whose heterogeneity is characterised by aggregates, mortar and interfaces. Monte Carlo simulations of uniaxial tension are performed, incorporating cross-scale fracture evolution through a phase-field cohesive zone model. Statistical analyses reveal that the stochasticity of crack patterns and stress-displacement curves, especially post-peak softening responses, is inherently dependent on the random meso-structures. It is observed that increasing aggregate content accelerates the deterioration rate of peak stress, decreases the softening curve, and leads to more tortuous crack paths, considering the ITZ crack channels. On the other hand, the tensile strength of mortar has significant impacts on the load-carrying capacity, while the fracture energy primarily influences the ductility and has negligible effects on the peak stress and the pre-peak nonlinear part. Besides, the ratios of mortar-ITZ tensile strength and fracture energy are found to affect the crack paths. The proposed numerical framework holds promise to reveal complex fracture mechanisms of concrete with more insights into other loading or environmental conditions.

Investigation of the Shear Mechanism at Sand-Concrete Interface under the Influence of the Concave Groove Angle of the Contact Surface

A “random-type” sand–concrete interface shear test was developed based on the sand cone method, with a focus on the most commonly encountered triangular contact surface morphology. A “regular-type” triangular interface, matched in roughness to the “random-type”, was meticulously designed. This “regular-type” interface features five distinct triangular groove inclinations: 18°, 33°, 50°, 70°, and 90°. A series of sand–concrete interface direct shear tests were conducted under consistent compaction conditions to investigate the impact of varying compaction densities and triangular groove inclinations on the shear strength at the interface. Particle flow simulations were utilized to examine the morphology of the shear band and the characteristics of particle migration influenced by the triangular contact surface. This analysis is aimed at elucidating the influence of the inclination of the triangular groove on the shear failure mechanism at the sand–concrete interface. The findings indicate that: (1) The morphology of the interface significantly impacts the shear strength of the sand–concrete interface, while the shape of the stress-displacement curve experiences minimal alteration. (2) At smaller inclination angles, particle contact forces are arranged in a wave-like configuration around the sawtooth tip, resulting in a non-uniform stress distribution along the sawtooth slope. However, as the inclination angle grows, the stress concentration at the sawtooth tip diminishes, and the stress distribution across the sawtooth slope becomes more consistent. (3) Particle migration is significantly influenced by the sawtooth’s inclination angle. At lower angles, particles climb the structure’s tip through sliding and rolling. As the angle increases, particle motion shifts to shear, accompanied by a transition in friction from surface friction to internal shear friction. This leads to the formation of a wider shear band and an increase in shear strength.

Bioinspired soft-hard interfaces fabricated by multi-material additive manufacturing: A fracture mechanics investigation using essential work of fracture

This study employs the Essential Work of Fracture (EWF) concept to evaluate the interfacial fracture toughness of bioinspired interfaces between soft and hard polymer phases. The experimental framework utilizes a model material system with Polylactic Acid (PLA) as the hard phase and Thermoplastic Polyurethane (TPU) as the soft phase. Employing the Fused Filament Fabrication (FFF) technique, bioinspired sutural interfaces are created, characterized by interpenetrating soft and hard protrusions. The modulation of a critical parameter in the FFF process enables the variation of interpenetration length of protrusions at the interface, thereby achieving a spectrum of interfacial strength and toughness. The determination of essential work of fracture from double edge notch tension samples, with varied ligament sizes, allows for the specific EWF measurement. This specific EWF is found to align with the initiation value of plane strain interfacial fracture toughness obtained through the Double Cantilever Beam (DCB) test. Therefore, the proposed approach asserts the elimination of the need for complex interfacial fracture tests, such as DCB. A noteworthy discovery is the establishment of a correlation between specific plastic energy dissipation and protrusion length. This correlation suggests an increased plastic dissipation with longer protrusions within the investigated bioinspired interface. Due to this heightened plastic energy dissipation, the shape of the interfacial nominal stress-displacement curves demonstrates substantial dependence on interface morphology, shifting from a triangular to a trapezoidal shape as protrusion length increases. These findings offer valuable insights into the mechanics of bioinspired interfaces, presenting a more efficient and nuanced approach to characterizing their fracture properties.

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).

Numerical Analysis for Shear Behavior of Binary Interfaces under Different Bonded Conditions

The shear performance of the binary interface formed by mortar and rock cementation is a key factor influencing the stability and safety of basic engineering projects related to livelihood and economy since concrete has become one of the most widely used materials in engineering. Therefore, it is an urgent practical problem to further explore and clarify the shear failure mechanism of the mortar–rock binary interface. In response to the current research objective focused on fully bonded interfaces, this paper constructed binary interface models with different bonded conditions to perform direct shear experiments using numerical simulation methods, and the effect of bonded conditions on the shear behavior of the mortar–rock binary interface was analyzed. The results indicate that the bonded conditions have a significant influence on the shear mechanical behavior of the mortar–rock binary interface, which is mainly reflected in the stress-displacement curve characteristics, the shear strength, the fracture development and the stress distribution state. The research findings are of great theoretical significance for the further study of shear mechanics at the mortar–rock binary interface and of great practical significance for safe construction, resource conservation and disaster warning.

Shear behavior of cement-stabilized silty clay exposed to low-temperature curing

Cement-stabilized soils are widely used in civil engineering applications. However, they inevitably encounter low-temperature curing conditions, particularly in cold regions. In this study, shear experiments were conducted on cement-stabilized silty clays with different dry densities, cement contents, curing ages and temperatures. The factors influencing the shear performance of cement-stabilized soils were analyzed. The results showed that the peak value of the shear stress-displacement curve of the cement-stabilized silty clay increased with the vertical pressure, and the failure patterns for soils with and without cement significantly differed. Generally, the soils without cement underwent ductile failure, whereas the cement-stabilized soils experienced brittle failure. The shear strength and cohesion of the cement-stabilized soils increased with cement content, dry density, curing age and temperature. In addition, the ice and hydration products significantly influenced the internal friction angle of the cement-stabilized soils. An optimal cement content for silty clay was determined to obtain the largest internal friction angle, which ranged from 12% to 15%. Furthermore, at the curing temperature of −2 °C, the edge-face contact form accounted for the majority with a relatively high porosity, but the morphology of C-S-H changed from a sheet-like form to reticulate structure when the curing temperature increased to 22 °C. However, the contribution of the ice crystals to the shear strength was less significant than that of the hydration products. This study provides insights into the mechanical and microstructural properties of cement-stabilized soils in cold-region geotechnical construction.

Preliminary study on double-sided direct shear test of microbial mortar

A new method of strengthening masonry structures of ancient architecture has been proposed, which involves the use of microbial grouting to generate mineralized mortar. To study the shear mechanical properties of microbial mortar, a comparative experiment was designed with 9 groups of specimens, including acid washing method to test the amount of calcium carbonate, penetration method to test compressive strength of mortar, double-sided direct shear method to test shear strength of mortar and get shear stress-strain curve. According to the experimental results, when the brick joint is thick, more bubbles remain at the contact between the brick and the mortar, reducing the saturation of the grouting liquid and affecting the amount of calcium carbonate generated, thereby affecting the shear strength of the mortar. When the brick is placed vertically, the contact area between the brick and the mortar is on the side, and it is not easy for bubbles to remain, so the amount of calcium carbonate generated is high, and the corresponding shear strength of the mortar is high. Reducing the concentration of calcium solution and implementing bidirectional perfusion can slow down the deposition of calcium carbonate and blockage of perfusion pathways, thereby improving the homogeneity of the carbonate level. A well-graded filling of sand particles is an effective measure to improve the binding efficiency of calcium carbonate.These measures not only increase the production of calcium carbonate, but also result in higher compressive and shear strength.The shear stress-displacement curve of microbial mortar specimens with damagedmortar layer exhibits strain-softening behavior, while the specimens with inter facial damage show strain-hardening behavior.The shear strength of specimens with interface damage is lower than that of specimens with mortar layer damage, so measures should be taken to avoid interface damage.

Study on Mechanical Model for Postpeak Shear Behavior of Rock Joints Based on Degradation Characteristics of the 3D Morphology

The 3D morphology of the joint surface significantly influences the shear behavior of the jointrock. Constant normal load (CNL) direct shear tests with different shear displacement were conducted to understand the shear stress changing with joint roughness and damage degree during shear. The rough joint specimens were prepared using 3D scanning and printing techniques, and shear tests with different normal stresses and shear displacements were performed. Four different parameters and the damaged area quantitatively described by the image binarization and box dimension were calculated and compared to study the roughness evolution of joint surfaces. The experimental results demonstrated that the roughness parameter and shear stress decrease and approach constant values with increasing shear displacement. A JRC degradation model was presented based on regression analyses to evaluate the JRC values of rock joints under various displacements to replace it in the JRC–JCS model. Additionally, a new postshear behavior modeling was proposed for rock joints based on surface degradation characteristics under various initial joint roughness coefficients (JRC0) and normal stress. The stress–displacement curves resulting from the proposed modified model work well in predicting the postpeak stress–displacement curve, which can prove the effectiveness of the postpeak shear behavior modeling.

Large-scale field test on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking: A case study

With the rapid advancement of urbanization, research on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking has a high engineering application value. In this study, a large-scale field direct shear test apparatus and the corresponding test method were designed and proposed to investigate the shear properties of the concrete-silty soil interfaces in pipe jacking. A series of in-situ direct shear tests were conducted at five normal stress levels under four interface contact conditions. The results show that the shear stress-displacement relationships of the interfaces conform to the typical elastic-plastic constitutive relations, the shear strength-normal stress relationships of the interfaces can be well fitted by the Mohr-Coulomb failure criterion, and the interface shear properties are influenced by soil properties, normal stress, lubrication conditions and stagnation time. Without lubrication, the concrete-silty soil interfaces have a shear stress-displacement curve shape similar to that of the test results obtained from indoor direct shear tests, but their ductility and shear strengths are lower. Under lubrication conditions, the paraffin wax and thixotropic slurry can both effectively diminish the interface friction angle, thereby reducing the concrete-silty clay interface friction coefficient by 62.8% and 72.2% and reducing the concrete-silty sand interface friction coefficient by 60.8% and 59.2%, respectively. Based on the macroscopic and microscopic images observed in the tests, the lubrication mechanisms of the lubricants were analysed. The research results can provide a reference for the prediction of jacking force and construction disturbance in adjacent pipe jacking projects.