Shear Stress Displacement Curves Research Articles

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.

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.

Counterface soil type and loading condition effects on granular/cohesive soil – Geofoam interface shear behavior

Soil – geofoam interfaces have been studied through an extensive experimental program by performing multiple series of interface shear tests using two different granular soils (i.e. beach sand and construction material sand) and one cohesive soil (i.e. bentonite clay) as well as a soil mixture containing 75% sand and 25% clay by dry weight at distinct loading conditions (i.e. normal stresses (σ): 25, 100, 250; low, moderate, high loading conditions, respectively). Using the shear stress versus horizontal displacement curves obtained, some important engineering design parameters including peak shear stress, residual shear stress, interface sensitivity (i.e., peak/residual ratio) and displacement required to reach peak stress have been determined and the variations in those interface mechanical properties as a function of loading condition and counterface soil type have been investigated. It was shown that the peak as well as residual shear stresses increased with an increase in normal stress for all the interface systems tested. Further, the granular soil (sand) interfaces demonstrated relatively larger frictional strengths (both peak and residual) as compared to both the cohesive soil (clay) interface and the sand/clay admixture soil interface. Additionally, the higher the angularity of granular soil particles became, the larger the interface shear strengths (peak and residual), when sheared against geofoams, developed in light of experimental results attained as a result of interface shear tests on different material combinations. For comparison, the detected peak strength at average for the construction material sand, the beach sand, and the sand/clay admixture soil interfaces as compared to the bentonite clay interface were improved 59.8%, 43.4%, and 20.3%, respectively. Additionally, the detected residual strength at average for the construction material sand, the beach sand, and the sand/clay admixture soil interfaces as compared to the bentonite clay interface were improved 53.9%, 28.6%, and 15.4%, respectively.

Experimental investigation on shear strength deterioration at the interface between different rock types under cyclic loading

The shear strength deterioration of bedding planes between different rock types induced by cyclic loading is vital to reasonably evaluate the stability of soft and hard interbedded bedding rock slopes under earthquake; however, rare work has been devoted to this subject due to lack of attention. In this study, experimental investigations on shear strength weakening of discontinuities with different joint wall material (DDJM) under cyclic loading were conducted by taking the interface between siltstone and mudstone in the Shaba slope of Yunnan Province, China as research objects. A total of 99 pairs of similar material samples of DDJM (81 pairs) and discontinuities with identical joint wall material (DIJM) (18 pairs) were fabricated by inserting plates, engraved with typical surface morphology obtained by performing three-dimensional laser scanning on natural DDJMs sampled from field, into mold boxes. Cyclic shear tests were conducted on these samples to study their shear strength changes with the cyclic number considering the effects of normal stress, joint surface morphology, shear displacement amplitude and shear rate. The results indicate that the shear stress vs. shear displacement curves under each shear cycle and the peak shear strength vs. cyclic number curves of the studied DDJMs are between those of DIJMs with siltstone and mudstone, while closer to those of DIJMs with mudstone. The peak shear strengths of DDJMs exhibit an initial rapid decline followed by a gradual decrease with the cyclic number and the decrease rate varies from 6% to 55.9% for samples with varied surface morphology under different testing conditions. The normal stress, joint surface morphology, shear displacement amplitude and shear rate collectively influence the shear strength deterioration of DDJM under cyclic shear loading, with the degree of influence being greater for larger normal stress, rougher surface morphology, larger shear displacement amplitude and faster shear rate.

Mechanical properties and damage model of coral seawater sea sand concrete under compression-shear composite action

The present study investigates the mechanical properties of coral seawater sea sand concrete (CSSC) under compression-shear composite action. A total of 54 concrete specimens were designed for the shear test with varying parameters such as normal stress ratio, curing age and different aggregate types considered. The CSSC shear stress-displacement curve was obtained by observing the damage morphology of the specimen under a combination of compressive and shear stresses. The effects of different normal stress ratios on the shear strength, peak shear displacement, and damage evolution pattern of the CSSC specimen are analyzed. The results show that the width of the main cracks on the surface of the specimen gradually increases with the normal stress ratio, and the tiny oblique cracks gradually become denser. The peak shear displacement and shear strength also increase with normal stress, and the damage evolution of the concrete gradually slows down. When the curing age was increased from 7d to 14d and 28d, the shear strength of the CSSC specimens increased by 5.9% and 11.3%, respectively. In this paper, we propose two failure models that better reflect the shear strength of CSSC under the combined action of compression and shear. Finally, a compressive-shear damage constitutive model for CSSC at different normal stress ratios has been developed and the results are in good agreement with the experimental results.

Engineering properties and optimal design of foam lightweight soil composite fly ash: An eco-friendly subgrade material

Foam lightweight soil is used frequently in engineering construction for soft soil areas due to its superior lightness and economy, but its greater cement usage will result in cost increase, resource waste, and environmental pollution. This study modified foam lightweight soil with industrial solid waste (fly ash) in place of some cement to propose an eco-friendly and useful lightweight subgrade filling material. A series of macro-micro tests were conducted to investigate the engineering properties and micro-mechanisms of foam lightweight soil composite fly ash (FLS). The results showed that the axial stress-strain and shear stress-displacement curves of FLS showed softening types. The fluidity, compressive strength, toughness and shear strength of FLS first increased and then decreased with the increasing fly ash content. The mechanical strength of 20% FLS was the optimal value for meeting the application requirements of subgrade engineering. Due to the volcanic ash and micro-aggregation effects, fly ash consumed the calcium hydroxide to produce more calcium silicate hydrate for filling the pores of sample and improving its internal structure. According to the results of numerical simulation, the settlement deformation and lateral deformation of subgrade could be better controlled by using 20%FLS as filling material compared with ordinary soil. Consequently, it is feasible to apply FLS in subgrade engineering.

On the calibration of a shear stress criterion for rock joints to represent the full stress-strain profile

Conventional numerical solutions developed to describe the geomechanical behavior of rock interfaces subjected to differential load emphasize peak and residual shear strengths. The detailed analysis of pre- and post-peak shear stress-displacement behavior is central to various time-dependent and dynamic rock mechanic problems such as rockbursts and structural instabilities in highly stressed conditions. The complete stress-displacement surface (CSDS) model was developed to describe analytically the pre- and post-peak behavior of rock interfaces under differential loads. Original formulations of the CSDS model required extensive curve-fitting iterations which limited its practical applicability and transparent integration into engineering tools. The present work proposes modifications to the CSDS model aimed at developing a comprehensive and modern calibration protocol to describe the complete shear stress-displacement behavior of rock interfaces under differential loads. The proposed update to the CSDS model incorporates the concept of mobilized shear strength to enhance the post-peak formulations. Barton's concepts of joint roughness coefficient (JRC) and joint compressive strength (JCS) are incorporated to facilitate empirical estimations for peak shear stress and normal closure relations. Triaxial/uniaxial compression test and direct shear test results are used to validate the updated model and exemplify the proposed calibration method. The results illustrate that the revised model successfully predicts the post-peak and complete axial stress–strain and shear stress–displacement curves for rock joints.

Effect of Temperature on Interface Characteristics between Compacted Recycled Asphalt Pavement and Geogrid

This study presents an experimental investigation of the interface behaviors of recycled asphalt pavement (RAP) reinforced with biaxial geogrid. A series of direct shear tests were conducted on compacted RAP specimens with or without geogrid under different normal stresses (50, 75, and 100 kPa) and test temperatures (0, 20, and 35°C) using an improved temperature-controlled direct shear apparatus. The effect of test temperature on interface shear strength and strength parameters was systematically examined. Test results showed that shear stress versus shear displacement curves of RAP specimens with or without geogrid show strain-softening characteristics. Under the same normal stress, the curve of the reinforced RAP sample with a high temperature is always below that with a low temperature, which indicates the test temperature has an adverse effect on the strength of the reinforced RAP sample. The shear strength consistently increases linearly with the increase of applied normal stress for all RAP samples. The shear strength with geogrid is greater than pure RAP under a given test temperature and normal stress. The shear strength of RAP samples at lower test temperatures is higher than that at a higher temperature. Both apparent adhesion intercept of RAP-geogrid and cohesion of RAP show a decreasing trend with the increase of the test temperature and tend to be stable with increasing temperature. Both the interface friction angle of RAP-geogrid and the internal friction angle of RAP slightly decrease with the increase of test temperature. For RAP-geogrid, the value of the apparent adhesion intercept is higher than that for sand and gravel with geosynthetics. The interface friction angle is close to that of sand and gravel with geosynthetics. Therefore, RAP material can be well used as alternative backfill materials in lieu of natural aggregates such as gravel and sand in geotechnical applications.

Triaxial direct shear properties, cohesive damage behavior and shear constitutive model of sandstone under high confining pressure

The shear mechanical properties of rock under high confining stresses are required in deep underground construction projects for stability evaluation. This paper tested the sandstone shear properties under high confining pressures by a novel triaxial direct shear device with a self-balancing structure for both low and high confining pressure. The triaxial direct shear experiments on Zigong sandstone from a deep tunnel were carried out to evaluate its strength and deformability. Our experimental results suggested that the shear stress-displacement curves exhibited two distinct characteristics under different confining pressures. The damaging stage occurs initially and turns wider in the shear stress-displacement curve as the confining pressure exceeds the threshold stress. The confining pressure threshold of Zigong sandstone is observed to be 10–20 MPa. Meanwhile, a statistical damage model based on Beta distribution was proposed to describe the shear stress (cohesion contribution) during the whole shearing process, which demonstrates better prediction performance than previously existing models, especially under high confining pressures. This study provides important insights into the high-pressure induced rock shear properties and sheds light on the engineering stability assessment under high stress of tunnels.

Impact of Climate Change on the Performance of Permafrost Highway Subgrade Reinforced by Concrete Piles

Climate change has a detrimental impact on permafrost soil in cold regions, resulting in the thawing of permafrost and causing instability and security issues in infrastructure, as well as settlement problems in pavement engineering. To address these challenges, concrete pipe pile foundations have emerged as a viable solution for reinforcing the subgrade and mitigating settlement in isolated permafrost areas. However, the effectiveness of these foundations depends greatly on the mechanical properties of the interface between the permafrost soil and the pipe, which are strongly influenced by varying thawing conditions. While previous studies have primarily focused on the interface under frozen conditions, this paper specifically investigates the interface under thawing conditions. In this study, direct shear tests were conducted to examine the damage characteristics and shear mechanical properties of the soil-pile interface with a water content of 26% at temperatures of −3 °C, −2 °C, −1 °C, −0.5 °C, and 8 °C. The influence of different degrees of melting on the stress–strain characteristics of the soil-pile interface was also analyzed. The findings reveal that as the temperature increases, the shear strength of the interface decreases. The shear stress-displacement curve of the soil-pile interface in the thawing state exhibits a strain-softening trend and can be divided into three stages: the pre-peak shear stress growth stage, the post-peak shear stress steep drop stage, and the post-peak shear stress reconstruction stage. In contrast, the stress curve in the thawed state demonstrates a strain-hardening trend. The study further highlights that violent phase changes in the ice crystal structure have a significant impact on the peak freezing strength and residual freezing strength at the soil-pile interface, with these strengths decreasing as the temperature rises. Additionally, the cohesion and internal friction angle at the soil-pile interface decrease with increasing temperature. It can be concluded that the mechanical strength of the soil-pile interface, crucial for subgrade reinforcement in permafrost areas within transportation engineering, is greatly influenced by temperature-induced changes in the ice crystal structure.

Theoretical and experimental investigation of a shear failure model for anisotropic rocks using direct shear test

This study proposes a theoretical model for predicting the shear strength of anisotropic rocks in which failure is dependent on the inclination of the inherent structural weak planes. For this purpose, triaxial and direct shear tests were conducted on dried and saturated shale samples. Samples were cored at different orientations (β) to the inherent structural weak planes, i.e., bedding planes. To verify the anisotropic behavior of the samples, triaxial tests were performed on samples with various inclinations of the bedding planes. The experimental results demonstrated a U-shape relationship between the compressive strengths and the inclination of the bedding planes, which is in accordance with Jaeger's model of the plane of weakness (JPW). Accordingly, apparent strength and elastic properties, such as Young's modulus, Poisson's ratio, cohesion, and internal friction angle were all observed to be dependent on β, which verified the mechanical anisotropy of the shale samples. Many of the existing failure criteria consider the impact of bedding inclination on failure strength based on uniaxial and triaxial conditions. However, it is also important that the failure modes of anisotropic rocks are evaluated during shearing. For this purpose, a theoretical model was derived to relate shear strength to β following the Coulomb-Navier criterion. The relationship between shear strength and β was plotted and illustrated in an Inverted-U shape. Direct shear tests were conducted to validate the proposed criterion. Plotted experimental data confirmed the shape of the geometry determined by theoretical analysis. From the results, the maximum shear strength occurred when β was between 30° and 60° in direct shear tests. To obtain a more comprehensive understanding, changes in the shear stress-displacement curve and dependence of the friction angle, to the bedding plane orientation, β, were evaluated and indicated shear anisotropy behavior.

Analytical model of skin friction for bored pile socketed into weak rocks in ice-rich permafrost regions

Bored piles constructed in permafrost regions often need to be socketed into weak and weathered rock strata, such as marl stratum. When these strata are ice-rich, the meltwater caused by construction will form an ice film at the pile-rock interface during refreezing, leading to a reduction in pile bearing capacity. By analyzing the shear mechanism of pile-rock interface with ice film, an interface shear model was proposed in this paper. Based on this model, the load transfer analytical method of rock-socketed bored piles was improved. Finally, the effects of pile-rock interface roughness, ice filling degree, and residual friction angle on skin friction were analyzed. The results show that the shear stress-displacement curve of the ice-containing interface unit exhibits two peaks and can be divided into four stages: ice film shear stage, ice film extrusion stage, concrete-rock relative sliding stage, and interface residual sliding stage. Increasing either the dilation angle or half-wavelength can improve the interface's shear resistance when only one variable is altered. Furthermore, the peak shear strength of the interface decreases as the filling degree increases. The interface residual shear stress is positively correlated with both the residual friction angle of weak rock (ϕu−r) and ice (ϕi−r), and increasing the proportion of ice in the residual sliding surface will lead to the reduction of residual shear stress.

Experimental study on the effects of triangular groove inclination angles on the mechanical behavior of sand–concrete interfaces

The mechanical properties of pile–soil interfaces directly affect the bearing capacity of pile foundations. In this study, the effects of triangular groove inclination angles on the mechanical properties of sand–concrete interfaces were investigated. Random rough concrete surfaces were obtained through slurry intrusion testing, and the roughness of concrete surfaces was evaluated based on the sand cone method. According to the roughness of random concrete surfaces, five regular concrete surfaces with different triangular groove inclination angles were constructed. A series of direct shear tests between (i) sand and random concrete surface, (ii) sand and regular concrete surface, as well as (iii) sand and smooth concrete surface were performed. Moreover, a direct shear test of natural sand was conducted. Lastly, a shear strength model considering the triangular groove inclination angle of sand–concrete interfaces was established, and the shear failure mechanism of interfaces was discussed. The results firstly indicated that the shear stress-displacement curves of random rough concrete surfaces were ideal elastic-plastic curves; and the regular rough concrete surfaces were found to incur strain softening under a normal stress of 300 kPa. Secondly, the shear strength of sand–concrete interfaces increased with the increase in groove inclination angle of concrete interfaces. Thirdly, sand was found to achieve the highest shear strength, while sand–smooth concrete interfaces proved to attain the lowest. Lastly, the internal friction angle value of random concrete interfaces was shown to range between the values of concrete surfaces with triangular groove inclination angles of 70° and 90°.

Effects of freeze–thaw cycles on the shear stress induced on the cemented sand–structure interface

Freeze–thaw alternation and freeze–thaw cycle erosion are the main reasons for the failure of the cemented soil–structure interface in cold regions. In this study, the direct shear and cyclic direct shear tests of cemented sand–structure interface were conducted in the frozen state (−10 °C) and thawed state (25 °C) at a 300 kPa normal stress under different freeze–thaw cycles (i.e., 0, 1, 5, 10, 15, 20, 25, 30). The experimental results for the shear stresses of the cemented sand–structure interface are presented, and the mechanism of shear stress formation and change was examined through mesoscopic analysis. Variations in shear stress with different freeze–thaw cycles were consistent. Typically, 10 was identified as the critical freeze–thaw cycle number for adfreeze strength, and the hyperbolic model of shear stress–displacement curve in the thawed state was obtained. The peak shear stresses of different freeze–thaw cycles gradually decreased as the shearing cycle number increased until entering a stable stage in frozen state and were close to each other in the thawed state. The internal damage of cemented sand and the formation of ice crystals on the surface increased with the increase in freeze–thaw cycle number. Hence, internal change in cemented sand under freeze–thaw cycles is the fundamental reason for alterations in shear stress. The results of this study shed light on shear stress complexity at the cemented sand–structure interface under freeze–thaw cycles and provide references for practical engineering.

Experimental and theoretical investigation on compression-shear properties of high-strength engineered cementitious composites (HS-ECC)

High-strength engineered cementitious composites (HS-ECC) has broad application prospects due to its excellent mechanical performance and durability. Construction materials in structural members are susceptible to combined compression-shear stress. Therefore, the mechanical properties of HS-ECC under the combined stress are experimentally investigated by a servo-controlled biaxial testing machine. The effects of normal stress ratio on the compression-shear properties are systematically studied, and mortar specimens serve as control group. The experimental results show that the shear stress versus shear displacement curves of HS-ECC are able to be divided into four stages, namely linear ascending, non-linear ascending, rapid descending and stable stages. Compared with mortar specimens, the shear strength and residual shear strength of HS-ECC increase by 5–21% and decrease by 33–42% respectively. HS-ECC has a friction coefficient of 0.53, which is smaller than that of mortar specimens and concrete. When the normal stress ratio increases, the dilation angle of HS-ECC decreases at first and then remains basically unchanged. The damage evolution of HS-ECC is delayed when compared with that of the mortar specimen. Finally, a damage constitutive model is proposed to describe the compression-shear properties of HS-ECC. It is found that the predicted results agree well with the experimental results. This study is of great significance to understand the compression-shear properties of HS-ECC and promote its structural applications.

Shear strength of soil-root system under different vegetation restoration modes on the Loess Plateau of China

The soil-root system shear strength (SRSS) is a useful dynamic parameter for evaluating soil detachment capacity by flowing water and slope stability. The changes in soil properties and root traits induced by vegetation restoration likely affect SRSS considerably. Nevertheless, few in-situ shear tests have been conducted to quantify the effects of restoration mode on SRSS in semi-arid region. This study was conducted to quantify the impact of restoration mode on SRSS of 0–50 cm soil layer at sites of one slope farmland (maize) and five vegetation restored lands (abandoned farmland, korshinsk peashrub, black locust, Chinese pine, and the mixed forest of amorpha and Chinese pine) and further to identify the related dominant contributing factors on the Loess Plateau. SRSS was reflected by the strain energy calibrated to 15% soil water content (SEc), peak shear stress (PSS) and the displacement of PSS (DPS) determined from the measured shear stress-displacement curves. The results showed that SEc decreased with soil depth (0–50 cm) for all vegetation restored lands. PSS increased with soil depth, except for slope farmland and abandoned farmland. DPS also decreased with soil depth as a whole. The mean SEc of 0–50 cm soil layer of different vegetation restored lands were 2.29–2.67 times greater than that of slope farmland. Chinese pine with a mean SEc of 138.3 J·m−2 was the best restoration mode to enhance soil erodibility. Soil water content significantly impacted SRSS and it decreased logarithmically with soil moisture. Plant roots could improve the ductility of soil, making shear failure with a great displacement. Root surface area density was the most sensitive parameter to reflect the effect of root system on soil strain energy. These results are helpful for understanding the mechanical mechanism of vegetation restored soils to mitigate erosion and to enhance slope stability in semi-arid region.

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.