Shear Stress Displacement Curves Research Articles

Investigation of intralayer and interlayer shear properties of stabilized rammed earth by direct shear tests

Rammed earth construction has attracted a lot of attention because of its unique advantages of energy saving, environmental protection, low cost, easy access to materials, and suitability for living. Due to the special layered segmental ramming process, the distribution of rammed earth is superimposed, which makes the rammed earth walls have many weak horizontal interfaces. The failure mode of many post-earthquake rammed earth buildings is mainly in the form of shear failure, and the slip failure at the interface of adjacent rammed layers is particularly serious. There is a lack of research on the cracking and slip phenomena between rammed earth layers and the shear parameters between and within rammed earth layers. In this study, the interlayer and intralayer shear behavior of rammed earth were compared in four aspects: failure phenomenon, shear stress-displacement curve, cohesion and internal friction angle, and shear modulus, using normal stress, additive type, and additive content as variables. The causes of slip failure easily occurring at the interlayer interface were investigated. The interlayer interface and intra-layer shear failure laws of unstabilized and stabilized rammed earth were explored. The compressive strength was used to predict the interlayer shear strength. The shear resistance parameters of unstabilized and stabilized rammed earth were determined to provide data for future measures to enhance the optimization of the interlayer interface. The results showed that the interlayer failure of rammed earth was mainly caused by the lack of cohesion of the soil at the interlayer interface, and the interlayer cohesion was 24%-45% of the intra-layer cohesion. The interlayer interface shear strength was about 35%-84% of the intra-layer shear strength.

Variation in shear strength of soil-root system under five typical land use types on the Loess Plateau of China

Shear strength of soil-root system (SRSS) is one of the most useful dynamic properties for evaluating soil erodibility and structural stability. The adjustment of land use may strongly affect SRSS by altering soil properties and plant root system. Few studies have been conducted to investigate the effect of land use on SRSS in semi-arid region. This study was performed to quantify the variation in SRSS with five typical land use types (cropland, orchard, grassland, shrub land and woodland) and identify the related dominant contributing factors on the Loess Plateau where land use has changed greatly in the past several decades. In-situ shear tests with large shear box (25 × 25 × 10 cm) were conducted to measure shear stress-displacement curves. The strain energy calibrated to 15 % soil water content (SEc), peak shear stress (PSS) and the displacement of peak shear stress (DPS) calculated from the curves were used to reflect SRSS. The results showed that woodland had the maximum SEc (124.4 J m−2) and PSS (1048.6 kPa), whereas cropland had the minimum SEc (65.6 J m−2) and DPS (7.2 mm). Woodland, especially coniferous species and its mixed forest, might be the most suitable land use type for vegetation restoration on the Loess Plateau to enhance soil erodibility or structural stability. The variation in SRSS with land use was closely related to the changes in soil properties and root traits. PSS increased with penetration resistance, while it decreased with soil water content. DPS was positive correlated to root biomass and morphological traits. SEc was mainly affected by the direct and indirect effects of root surface area density, penetration resistance, soil organic matter content and aggregate stability. The results are helpful for understanding the mechanical mechanism of vegetation restored soils to mitigate erosion and optimizing land use type in semi-arid region.

The development and application of an interfacial cohesive zone model of the pavement interlayer considering the characteristics of adhesive materials

The interlayer adhesive material is important to ensure the integrity of a pavement structure, but its influence on pavement performance as an actual material layer is usually ignored in pavement structure design and analysis. At present, the relationship between the shear stress and slip is usually expressed in the form of a coefficient. The interlayer adhesive material is a kind of heterogeneous composite material with rheological performance. Therefore, the relationship between a force and the displacement cannot be characterized by a simple linear relationship. In this paper, we develop a method for characterizing the properties of interlayer bonding materials using an interfacial cohesive zone model. First, we perform experimental tests, that is, a pull-out test and an oblique-shear test, to determine the optimal dosage of adhesive material, and then obtain the normal stress-displacement characteristic curve and shear stress-displacement characteristic curve for the optimal dosage of the adhesive material. Then, we divide the shear stress-displacement characteristic curve into four stages. Next, based on the characteristics of normal stress displacement curve and shear stress displacement curve, we develop a cohesive zone model based pavement interlayer simulation approach that can be used to characterize the stress displacement characteristics of interlayer adhesive materials. Finally, we present the performance of the interfacial cohesive zone model using finite element software to simulate the shear test and pull-out test, and find that the simulation results are basically consistent with the experimental results. Further, we apply the model to a special case, and the results are also consistent with the actual monitoring. It shows that the proposed method is reasonable.

A New Shear Constitutive Model Characterized by the Pre-Peak Nonlinear Stage

The pre-peak shear stress-displacement curve is an important part of the study of the shear mechanical behavior of rock joints. Underpinned by the Haldane distribution, a new semi-analytical model for the pre-peak shear deformation of rock joints was established in this paper, the validity of which was verified by laboratory and in situ experimental data. Other existing models were employed to make comparisons. The comparison results show that the model has superior adaptability and is more suitable for convex-type shear constitutive curves than existing models. Besides, only one parameter was introduced to the model, which is more convenient for application. All of these imply that the proposed model is an effective tool to evaluate the pre-peak shear constitutive curves of different rock joints. The research results can provide a reference for further understanding of the shear fracture characteristics of rock materials.

Experimental and Numerical Simulation Study on Shear Strength and Secondary Particle Breakage of Coarse Sand with Different Dry Densities

In order to guarantee perpendicularity of rotary cast-in-place pile in Karst areas, filling, like sand, etc., is commonly used between the inner steel casing and the outer steel sleeve; while the outer steel sleeve is pulled out or a vertical load is applied on the top of the pile, the sand will move, contact, and push against each other, which leads to recrushing; therefore, shear characteristics and secondary breakage are a meaningful problem to study with regard to the influence of different sand densities. Through experimental direct shear tests, with different dry densities, shear stress versus shear displacement curves are obtained and internal friction angles are analyzed. The results show that internal friction angles increase along with increment of dry densities; more interesting is that the secondary breakage ratio also increases. A comparative numerical 2D model is set up using PFC2D, with lower porosity used to simulate a large dry density, and similar results are obtained, that is to say, with increase of dry densities, internal friction angles also increase.

Experimental study on interfacial damage characteristics of CRTS II slab track and CA mortar with AE and DIC techniques

Since the interface is often regarded as the weakest part of the ballastless track structure, there is a great need to investigate the interlayer bonding performance to ensure the safety of train operations for high-speed railways. In this study, the damage characteristics of the interface between China Railway Track System II (CRTS II) slab track and cement emulsified asphalt (CA) mortar (T-CA interface) under shear loading were thoroughly investigated. The interfacial deformation and damage development process during the direct shear experiments were recorded by a combination of acoustic emission (AE) and digital image correlation (DIC) techniques. Consequently, a bilinear cohesive zone model (CZM) of the T-CA interface was proposed. The results show that the AE activities are rare in the initial stage of loading, while they increase sharply when the shear loading approaches the interface bonding strength. The AE parameters can well characterize the entire process of interfacial failure. Through the DIC technique, the macroscopic deformation process was well captured and the shear stress versus relative displacement curves were derived. Subsequently, the relationship between the shear stress and relative displacement of the interface was well fitted by a bilinear CZM, of which parameters are calibrated through the experimental data. The effectiveness of the proposed bilinear CZM is validated by using a three-dimensional finite element model. This work provides a better understanding for the damage mechanism of the bond performance between CRTS II track slab and CA mortar.

Analysis of the Direct Shear Test and Microstructure of the Lunar Soil Simulant Solidified by Sodium Silicate

The lunar soil is an ideal building material for future lunar base construction. In order to improve the strength of lunar soil, basalt with similar composition to lunar soil was used to simulate lunar soil due to lack of real lunar soil, and the simulated lunar soil was reinforced with sodium silicate. In order to study the direct shear mechanical properties of the simulated lunar soil by sodium silicate solidification, a series of direct shear tests and SEM microstructure tests were carried out, and the shear stress-displacement whole process curve was obtained. Influence law of mechanical characteristic parameters such as shear strength, shear deformation modulus, peak deformation and energy dissipation was studied. The results of the direct shear test show that the shear stress-shear displacement curve of the sample changes from hardening to softening with the increase of sodium silicate content. The shear strength and shear deformation modulus of simulated lunar soil increased first and then decreased. When the sodium silicate content was 5%, the shear strength and shear deformation modulus reached the maximum, which were 560 kPa and 7224 kPa, respectively. The addition of 5% sodium silicate increases the energy dissipation coefficient of the simulated lunar soil by about 12%. The microscopic test results show that the N-A-S-H gel and AFt formed by the alkali excitation and adsorption of the sodium silicate will connect the lunar soil simulant particles into a whole network structure, thereby improving the shear strength.

Experimental analysis and mechanical model of interaction between warm frozen silt and pre-bored grouted planted nodular pile

To reveal interaction mechanism on the interface between the pre-bored grouted planted nodular pile (PGPN) and warm frozen ground, and to compare its bearing performance with that of cast-in-place pile, shear tests on the interface of the two types of pile were carried out at negative temperature under different influencing factors through self-designed test devices. Combined with the single factor method and response surface method, the influence laws of various factors and interaction on the adfreezing force of the interface were analyzed. The micro-deformation mechanism of the interface could be revealed by the binary medium model, and volume breakage rate and Weibull distribution function were introduced to establish the constitutive equation of the interface. The results showed that the cemented soil slurry could strengthen the frozen soil around the pile, The shear zone of the interface appeared on the frozen ground around the pile rich in ice film, and the shear stress-displacement curve showed a strain softening type and included four stages. Among all the influencing factors, temperature has the most significant effect on adfreezing force. The freezing strength has an exponential relationship with temperature, and the water content and normal stress have a linear relationship with freezing strength. Compared with the traditional cast-in-place pile, PGPN pile has higher bearing capacity and more stable deformation. The proposed interface constitutive model scientifically reveals the microscopic deformation mechanism of the shear zone. The model data fit well with the test results, which could provide valuable reference for the design and calculation of the bearing capacity of the pile foundation in warm frozen ground.

Study on the cohesive shear characteristics and intrinsic modelling of the root–tailing soil interface of Amorpha fruticosa

To study the soil consolidation effect of shrub plant roots on tailings soil and to explore the frictional characteristics of plant roots on tailings soil, three experimental conditions of the root–soil interface were established by using a modified indoor direct shear instrument with binders such as liquid sodium silicate and cyanoacrylate to conduct direct shear frictional tests at the root–soil interface using the roots of the typical slope protection plant Amorpha fruticosa. The Gompertz improved curve model was established by using the relationship between shear stress and shear displacement and the trend of the root–soil interface parameter index. The results were compared between the improved Gompertz curve model and the Clough–Duncan hyperbolic model, and a two-factor coupled improved Gompertz interfacial intrinsic structure model with normal stress and cohesive strength factor was established. The results showed that the interface shear stress and shear displacement showed strain hardening characteristics at different normal pressures for cohesive strength ratios of 1.5 and 1.7 at the root–tailing soil interface. At a cohesive strength ratio of 1.6, strain-softening was observed from 100 to 300 kPa and strain hardening was observed at 400 kPa. The improved Gompertz curve model predicts the shear stress and shear displacement curves at the root–soil interface with different cohesive strengths more reasonably than the Clough–Duncan hyperbolic model, and the maximum accuracy can be improved by nearly 40%. The two-factor coupled improved Gompertz curve model can fit the shear stress versus shear displacement relationship at the A. fruticosa root–tailing soil interface.

Effect of Cementation Degree on Shear Characteristic and Failure Mechanism of Bimrocks Containing Gravels of Different Sizes

Abstract To reveal the influence of cementation degree on the mechanical properties of bimrocks with different particle sizes under different types and contents of cement, indoor direct shear tests as well as numerical tests were conducted. The variation law of mechanical properties and the development law of internal force chain distribution of bimrocks under different conditions are fully studied. The specimens with three particle sizes, two cementing types (cement and gypsum), and three cementing contents were prepared for indoor shear tests under different normal stresses. The results show that the fluctuation of shear stress-displacement curve of bimrocks with round gravels of large particle size is relatively gentle, and the fluctuation of residual strength is relatively small. Under low normal stress, the peak shear strength of bimrocks with round gravels of large particle size can be increased by 115.2% under the action of cementation. Compared with the cohesion of samples without cementation, the maximum improvement of the cohesion of the samples containing large particle size, small particle size, and mixed particle size with different cementing materials is 143.2%, 149.0%, and 141.9%, respectively. The results show that, compared with the internal friction angle of samples without cementation, the maximum increases of the internal friction angle of the samples with large particle size, small particle size, and mixed particle size for adding different contents of cementing materials are 15.7%, 18.5%, and 21.2%, respectively. Numerical tests show that, with the increase of cementation degree, the gathering of force chain is more obvious and more concentrated near the shear surface.

Mechanical Characteristics and Acoustic Emission Characteristics of Mortar-Rock Binary Medium

The stability of the interface between mortar and rock is very important in engineering construction. In this paper, the all-digital acoustic emission (AE) system is used to detect the direct shear test of the mortar-rock binary medium interface with different sawtooth angles under different normal stress states. The stress-displacement information and AE signal during the whole shearing process are extracted. The coupling relationship between stress and AE characteristic parameters is discussed. The quantitative relationship between sawtooth angle and shear strength of binary medium is established, and three AE characteristic parameters that can be used to predict structural instability are proposed. The research shows that: With the increase of the normal stress and the sawtooth angle, the shear strength of the mortar-rock binary medium increases. The relationship of that is obtained by least squares fitting. The shear stress-displacement curve is divided into five stages according to the change of deformation law. Through the analysis of AE characteristic parameters, it is found that increasing the sawtooth angle makes the AE count and AE cumulative count increase. Based on the analysis of the characteristic parameters of RA-AF, the changes of shear cracks and tensile cracks within the whole shearing process were obtained, respectively. In the process of binary medium shearing, the AE peak frequency is in the range of 120–340 kHz. Three acoustic emission parameters that can predict the macroscopic damage of binary media are obtained: the AE b value, the ratio of shear crack signals, and the number of signals with a peak frequency of 220 Hz to 320 Hz.

Shear Mechanical Properties of the Interphase between Soil–Rock Mixtures and Benched Bedrock Slope Surfaces

A series of benched excavations are typically carried out on bedrock slope surfaces to improve the stability of soil–rock mixtures (SRMs) on the fill slopes. In this paper, the shear characteristics and micromechanisms of SRMs–bedrock interphases were studied by means of large direct shear tests and discrete-element numerical simulation. In the process, this study also investigated the influences of rock block content and interface roughness by combining the direct shear test and discrete-element method (DEM) numerical simulation results. This showed that the samples exhibit obvious bulk shrinkage during shearing. As the rock block content increases, fluctuations in the postpeak shear stress–displacement curves become more obvious. Cohesion is more sensitive to roughness, while internal friction angle is insensitive to roughness. Also, the internal friction angle is very sensitive to rock block content. Moreover, the shear strength and cohesion of the interphase increase along with the superficial roughness, demonstrating a quadratic correlation with rock block content. However, this increase in roughness also increases the fracturing of rock blocks in the interphase. Within a certain interval, the shear strength of SRMs–bedrock interphases can be estimated according to a fitted empirical formula.

Experimental study on the shear behavior of frozen cemented sand-structure interface

The shear behavior of the interface between frozen cemented soil and structure is important for analyses of soil-structure interaction and foundation design involving cement grouting and ground freezing. This paper investigated the shear behavior of frozen cemented sand-structure interface. Direct shear experiments were conducted on the frozen sand- and frozen cemented sand-structure interface specimens at various sub-freezing temperatures and normal stresses. Experimental results, including shear stress-displacement curves, the adfreeze and residual strengths, and their differences between the frozen sand- and frozen cemented sand-structure interfaces, were presented and analyzed. The shear stress-displacement relationships of both interfaces display a wave pattern, and the adfreeze and residual strengths of the frozen cemented sand-structure interface were found to decrease significantly when compared with the one without cementation. On average, the adfreeze strength of the frozen cemented sand-structure interface decreases by 47% and the residual strength by 19% compared to the case without cement. As the temperature drops from −2 °C to −14 °C, the peak interface cohesion of the frozen sand case increases by 393%, while that of the frozen cemented sand case rises by 115%. Empirical formulas based on the Mohr-Coulomb model between the strengths and testing parameters, including temperature and normal stress, were proposed for both interfaces. The findings from this study help understand the impact of cement grouting on the shear behavior of frozen sand-structure interface and are significant for practices such as ground construction and foundation engineering when cement grouting is involved in ground freezing.

Effects of water content and interface roughness on the shear strength of silt–cement mortar interface

Although the shear behaviors of silt–structure interfaces are critical in engineering practice, they have not been extensively investigated systematically. In this study, the influence of the silt water content and interface roughness on the shear behaviors of silt–cement mortar interfaces is investigated. Forty–eight silt–cement mortar structure interface tests and 18 silt direct shear tests were carried out. The results indicated that as the water content of the silt gradually increased to saturation, the interface shear stress–displacement curves changed from strain softening to strain hardening, and the interface shear strength decreased significantly. The shear strength of the rough interface was found to be significantly greater than of the silt, and the shear strength of the smooth interface was found to be lower than that of the silt. The interface shear strength gradually increased with the surface roughness, but the increase tended to be gentle. A large shear deformation was observed, and this increased with the decrease in soil water content and the increase (up to a point) in interface roughness. A model of the interface shear strength between unsaturated soil and structures considering the influence of water content and interface roughness was established and verified.

Experimental study on the shearing and crushing characteristics of talus-like rock mass

Talus-like (T-L) rock mass is a special kind of engineering material in the nature, which owns a mixed fabric of coarse rock blocks and fine grinded rock powder. The basic mechanical properties of T-L rock mass are difficult to know due to its special composition and structure. This paper carried out a series of large-scale direct shear tests on the T-L rock mass collected in site considering four normal pressure levels. The shearing characteristics and the associated block crushing phenomena were analysed. The shear strength of the T-L rock mass increased as the normal pressure increased. The T-L rock mass had a large internal friction angle but a small cohesion. The shear stress-displacement curves can be divided into three stages: material densification, shear failure and plastic flow. The T-L rock mass showed obvious dilatancy under a low normal pressure of 50 kPa, but behaved contractively when the normal pressure became larger than 50 kPa. This indicates that the T-L rock mass may have a loose mixed fabric structure, which can be easily compressed under large normal pressures. The sieving analyses showed that the grading curve of T-L rock mass shifted upwards after shearing, suggesting that external shearing under different normal pressures led to particle crushing and changed the grain composition. The particle crushing mainly happened to the grains larger than 10 mm, which caused the portion of grains larger than 10 mm to decrease and the portion of grains smaller than 5 mm to increase. Nonetheless, the portion of grains in the size range of 5-10 mm remained approximately unchanged. Three breakage indices were adopted to quantify the crushing characteristics of the testing materials, which all increased with increasing normal pressures.

Three-dimensional stress-controlled cyclic behavior of the interface between gravel and steel

Three-dimensional (3D) stress-controlled cycling is arguably reflective for soil–structure interfaces under in-situ loading conditions. The 3D stress-controlled cyclic behavior of a gravel–steel interface and its influencing factors were explored by conducting several series of tests. Distinct tangential displacement is induced and shows a noticeable coupling effect when subjected to 3D cycling of shear stress. The stress-controlled behavior differs significantly from the displacement-controlled behavior, and presents obvious 3D features which are influenced by the shear path, shear stress amplitude, and unequal stress amplitudes. The shear path affects the magnitude and relationship pattern of the performance parameters, while the shear stress amplitude primarily impacts the magnitude. The shear stress-displacement curves exhibit elliptical and hyperbolic formations in circular and linear shear paths, respectively. The peak reversible normal displacement remains constant with cycling in two-way shear paths, but is negligible in one-way shear path. Increased shear stress amplitude results in magnified tangential displacement and migration, peak shear flexibility, irreversible and peak reversible normal displacements, and decreased non-coaxial angle. The critical shear stress amplitude is approximately 240 kPa, at which the peak shear flexibility remains constant with cycling. Induced aeolotropy of the interface occurs in the reversible normal displacement subjected to unequal stress amplitudes.

Experimental Study on the Shearing Behaviour on the Interface between Coarse Sand and Concrete under High Stress

The shear behaviour on the interface between soil and structure is a research hot point. Based on the RMT-150B rock mechanics test system, a series of high-stress direct tests were performed on the coarse sand under the condition of different moisture contents and concrete substrates with different rough and hardness. The results showed that the shear stress-displacement curve and volumetric strain-displacement curve of the interface under high stress could be fitted by a hyperbolic model; the ultimate shear strength and initial shear stiffness of the interface both increased with the normal stress while the shear stiffness decreased with the shear displacement. The crushing rate of the coarse sand particles on the interface increased with the normal stress. After the range analysis for the influencing factors of the interface’s shearing behaviour, it was shown that for the ultimate shear strength, their sequence of influencing degree was normal stress, the roughness of interface, moisture content, and hardness of concrete base; for the initial shear strength, the sequence was normal stress, moisture content, interface roughness, and basal hardness. As for dry sand, the possibility of relative particle crushing was higher than that of sand with a moisture content of 8%, and a peak of crushing occurred when the moisture content was 16%.

Study on the mechanical properties of outwash deposits with random structure method

Abstract Outwash deposit is one of the major threats to the safety and stability of railway transit engineering in the Sichuan–Tibet area. In this paper, different samples of outwash deposits were prepared based on the random structure method, and a series of large-scale direct shear tests were carried out under different normal stresses. The influence of stone content and the spatial distribution of stone blocks on the physical and mechanical properties of outwash deposits were studied. The results show that with the increase of stone content, the shear stress-displacement curve changs from strain softening to strain hardening. The shear zone is larger, and its shape is more tortuous due to the movement and rotation of stone blocks. The internal frictional angle increases linearly while the cohesion diseases with the increase of stone content. The main influence of the spatial distribution of stone blocks on the mechanical properties of outwash deposits is the peak stress. The shear zone is mainly determined by the distribution of the stone blocks near the shear zone.

Behaviour evaluation of a gravelly soil–geogrid interface under normal cyclic loading

Reinforced soil structures are often subjected to both static and dynamic loads. However, experimental data concerning the interaction of soil–geosynthetic interfaces under normal dynamic loads are scarce. This paper reports on a series of large-scale direct shear tests that are used to investigate the shear behaviour of a gravelly soil–geogrid interface under normal cyclic load (NCL) conditions. The results show that the shear stress magnitude under NCL conditions is generally between the shear stresses under normal static load (NSL) conditions corresponding to the upper and lower bounds of the NCL. The upper and lower envelopes of the shear stress–displacement curve increase with increasing initial stress and amplitude. The enhancement coefficient ([Formula: see text]) is used to describe the degree of influence of the additional cyclic load on the shear stress of the interfaces. The relative time shift between the shear stress ratio ([Formula: see text]) and the normal stress ([Formula: see text]) is nearly a constant of 0.5 cycles, and the influence of the initial stress on the stress ratio is opposite to that of the amplitude. Finally, the volume change response is discussed under different normal load conditions.

An experimental and analytical study of rate-dependent shear behaviour of rough joints

Understanding the shear behaviour of rough joints is of great significance for dealing with rock engineering problems. In many cases, rock joints are often subjected to dynamic loadings, which are usually caused by explosions, impacts or earthquakes, etc. Until now, the dynamic shear characteristics of rock joints have not been well understood. In the present study, we investigate joint shear behaviour at different shear rates and develop a rate-dependent constitutive model of rough joints. Testing results shows that the shear/frictional strength of joints with meso-roughness (i.e., planar joints) is independent on the shear rate, while the strength of joints with macro-unevenness (i.e., saw-toothed joints) increases with the shear rate. Under identical boundary conditions, the residual strength of joints with macro-unevenness approximates to the kinetic frictional strength of planar joints. Based on the testing results, the shear stress-displacement curve of rough joints was divided into four phases, i.e., linear elastic phase, shear hardening phase, shear softening phase and residual strength phase. A viscous joint model was thus proposed and validated through comparison with laboratory measurements. This new model correlates the static joint shear stiffness with dynamic one, and has the ability to describe the shear stiffness evolution during the whole shearing process. Based on the proposed model, a hypothesis about shear rate effects of rough joints was introduced and then verified through numerical modelling. The findings in this paper could facilitate better understanding the dynamic behaviour of rough joints and be useful for analyzing rock engineering problems with discontinuous rock masses.