Residual Cohesion Research Articles

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).

Temperature driven real-time weakening and strengthening mechanisms of unconfined granite

The distinctive mechanical behaviors exhibited by granite under real-time temperature (RT) and after thermal treatment (AT) highlight a challenge in directly observing real-time cracking phenomena in RT granite within a laboratory setting. To overcome this limitation, a thermo-mechanical coupled grain-based model (GBM) was introduced for numerical investigations. The simulation outcomes reveal that disparities in mechanical responses between AT and RT specimens are attributed not merely to the count of thermally induced cracks, but rather to intricate grain-contact deformations arising from heterogeneous crystal expansions/contractions and the intricate interplay of direction-independent interactions along block edges. During elastic deformation, RT grain contacts with larger deformation reach the peak stress faster than AT contacts. Failed contacts have smaller residual cohesion under temperature with larger deformation due to the stronger slip-weakening effect. Moreover, shear displacements and grain rotations can create new interlocking, increasing granite stiffness after cooling. Consequently, the RT sample is softer to deform and fail, while the overall stress resistance of AT granite is higher due to cooling-induced micro-structure changes. These findings shed light on why the mechanical responses, including strength, stiffness, and plastic strain, of AT and RT granite are different.

Determination of Drained Residual Shear Strength Parameters of C Soil Using Ring Shear Apparatus

bstract: Shear strength parameters such as cohesion and angle of internal friction of soil are very important for the design of foundation of the structures and soil structures. Many tests such as direct shear, Tri-axial, UCC are available to determine the shear strength parameters of soil. Specifically, Residual Shear strength plays a major role in slope stability analysis and Earthquake analysis. The Residual Cohesion (Cr) and Residual angle of internal friction (Φr) are used for slope analysis. In this study, the soil used is c-Φ soil collected from Anaikatti, Coimbatore district and the residual shear strength was determined using Ring shear apparatus. In Ring shear test, the sample undergoes different stages such as consolidation and shearing for varying applied pressures of 100kPa, 200kPa and 300kPa. A plot drawn between normal stress and residual shear stress gives the residual shear parameters and the values of Cr and Φr are respectively 6kPa and 180

Analysis and verification of stress and plastic zone in surrounding rocks of hydraulic flushing borehole based on strain-softening

This study aimed to solve the problem ignored by previous research on hydraulic flushing gas extraction technology regarding the strain-softening of surrounding rock. Firstly, through the analysis of experimental data, previous studies have proved that the essence of strain-softening is that the internal friction angle remains unchanged while the cohesion decreases. According to the variation law of cohesion of surrounding rocks in a borehole, we theoretically analyzed the stress distribution of the surrounding rock considering strain-softening and determined the theoretical formula of the plastic zone radius. Subsequently, we established a numerical calculation model considering strain-softening using the COMSOL numerical simulation software. We simulated the influence of residual cohesion on the stress and plastic zone of surrounding rocks of the borehole. We found that with the decrease in residual cohesion, the peak stress transferred to the deep and the plastic zone radius gradually increased. The simulation results were compared with the theoretical values, and the errors were found to be within 10%, which verifies the model's accuracy. Subsequently, we studied the influence law of strain-softening on the stress and plastic zone of surrounding rocks of boreholes with different radii. We found that the larger the borehole radius, the greater the influence of strain-softening. The field measurement results show that the stress reduction zone of a hydraulic flushing borehole is about 10 times the borehole radius. The stress reduction zone calculated without considering strain-softening is 1.305 times the borehole radius, and that calculated while considering strain-softening is 6.663 times the borehole radius. Thus, we proved that strain-softening is an essential factor affecting the stress distribution of surrounding rocks of the borehole. When studying the gas extraction through a hydraulic flushing borehole, it is necessary to consider the strain-softening of surrounding rocks.

Predictive models for the run-out distance of clay slopes based on material point method

This paper aims to propose run-out distance predictive models for clay slopes using the material point method (MPM), which can simulate the progressive failure process of slopes considering the strain softening effect of soils. A suite of 100 ground motions is selected from the NGA-West2 database and then scaled for conducting the dynamic analysis of slopes. The permanent slope displacements (D) can be classified into two categories, namely the “un-failure” category with D smaller than 0.4 m and the “failure” category with D in the range of 10 m to 15 m. It is found that peak ground velocity (PGV) exhibits the highest correlation with D for the “un-failure” category, whereas all ground-motion intensity measures (e.g., PGV, peak ground acceleration) are less correlated with D for the “failure” category. Therefore, the run-out distance of collapsed clay slopes is more related to the failure model rather than the triggering shaking intensities. Moreover, thousands of slope models with various slope angles, slope heights (H), soil densities, and peak and residual strength parameters are developed based on MPM. The run-out distances for the slopes being collapsed are then collected. Predictive models for different slope angles are proposed, which predict the run-out distance as a function of H, unit weight, residual cohesion, and residual friction angle. The proposed models are applicable for clay slopes with slope angles in the range of 30° to 45° and H in the range of 10 m to 30 m.

A Unified Solution for Surrounding Rock of Roadway Considering Seepage, Dilatancy, Strain-Softening and Intermediate Principal Stress

The analytic solution for surrounding rock of roadway is of significance for stability analysis and roadway support. However, analytical solution for surrounding rock of roadway which took influences of water seepage, strain softening, dilatancy and intermediate principal stress all into account did not receive much reporting. To promote research in this aspect, a mechanical model simultaneously considering water seepage, strain softening, rock dilatancy, and intermediate principal stress was established based on porous elastoplastic mechanics, and then unified analytical solution for surrounding rock of roadway was obtained. Based on an example, influences of water seepage, strain softening, rock dilatancy, residual cohesion and intermediate principal stress on surrounding rock of roadway were thoroughly investigated using single factor analysis. The obtained results are as follows: radii of plastic zones and surface displacement of roadway would increase exponentially with water pressure increasing and their magnitudes are greater than corresponding values without water seepage considered; with softening modulus increasing, peak circumferential stress location would slightly shift to deeper surrounding rock, while broken zone radius and surface displacement of roadway would increase in a decay velocity; rock dilatancy has little effect on peak circumferential stress and plastic softening zone radius, while broken zone radius and surface displacement of roadway increase linearly with dilatancy coefficient α1 increasing indicating their magnitudes are overestimated if associated flow rule is adopted; with weighted coefficient increasing, stress components in plastic zones at the same distance from roadway center would increase, while radii of two plastic zones and surface displacement of roadway are reduced, i.e., self-bearing capacity of rock is enhanced considering intermediate principal stress effect compared to Mohr–Coulomb criterion; with residual cohesion increasing, peak circumferential stress remains unchanged, while stress components in plastic zones at the same distance from roadway center would increase and radii of two plastic zones decrease significantly. The above results implicated that water seepage effect should be carefully considered for roadway stability under groundwater environment; strain-softening and flow rule of rock should be reasonably analyzed and chosen to accurately predict surface displacement and broken zone radius of roadway; rock bolt length should be increased with softening modulus increasing, while it can be decreased with intermediate principal stress effect considered; grouting measure is an effective measure to improve roadway stability. In short, the research provides a theoretical basis and some practical engineering implication for roadway support.

Mechanical properties of sandstone under hydro-mechanical coupling

Abstract Study of mechanical properties of rock under hydro-mechanical coupling. The MTS815 rock mechanics system is used to perform triaxial compression tests of water-saturated sandstones under different confining pressures and pore water pressures. The results show that the peak deviation stress and its corresponding axial strain, residual deviation stress, initial deviation stress of dilation and deformation modulus of water-saturated sandstone are related to pore water pressure and confining pressure. They decrease with the increase of pore water pressure and increase with the increase of confining pressure. Poisson’s ratio and angle of rupture of water-saturated sandstone increase with pore water pressure. Based on the effective stress principle and Mohr–Coulomb criterion, the effective cohesion and effective residual cohesion of sandstone are 11.49 and 3.13 MPa, respectively, and the effective angle of internal friction and effective residual angle of internal friction are 38.32° and 25.32°, respectively. In addition, according to the variation relationship of sandstone strength, the relative strength criterion of sandstone under hydro-mechanical coupling is established.

Study on Mechanical Behavior of Slip Zone Soils Under Different Factors—A Case Study

The mechanical properties of the slip zone soil play an important role in the evolution of the loess landslides. To further understand these characteristics, a series of ring shear tests was conducted on the slip zone soils obtained from Tianshuigou landslide, to investigate the influence of moisture content, dry density, shear rate and shear method on the mechanical characteristics of slip zone soils. The experimental results showed that: an increase in the moisture content of the slip zone loess causes a significant reduction in the residual strength. Specially, both the residual cohesion and residual internal friction angle show a deceasing tendency with moisture content. The change in the residual cohesion is more sensitive to the variation in moisture content. Additionally, a trend that strength increased with the increasing of dry density was observed, and the influence degree of dry density on the increased strength is more pronounced at low moisture contents. Thirdly, shear strength shows a negative relationship with shear rate when the shear rate ranges from 0.01 mm/min and 1 mm/min. When the shear rate increased up to 10 mm/min, a stepped shear band is developed and the strength increased. In addition, the strain-softening phenomenon was observed in the single-stage shear tests, which was not noticed in the multi-stage shear tests and pre-shear tests. The residual strength obtained in pre-shear test and multi-stage shear test is slightly greater than that in the single-stage shear test. The experimental results herein can provide an important basis for analyzing the evolution mechanism and prevention of loess landslides.

Influence mechanism of structure on shear mechanical deformation characteristics of loess-steel interface.

The mechanical properties of loess-steel interface are of great significance for understanding the residual strength and deformation of loess. However, the undisturbed loess has significant structural properties, while the remolded loess has weak structural properties. There are few reports on the mechanical properties of loess-steel interface from the structural point of view. This paper focused on the ring shear test between undisturbed loess as well as its remolded loess and steel interface under the same physical mechanics and test conditions (water content, shear rate and vertical pressure), and explored the influence mechanism of structure on the mechanical deformation characteristics of steel-loess interface. The results show that the shear rate has little effect on the residual strength of the undisturbed and remolded loess-steel interface. However, the water content has a significant influence on the residual strength of the loess-steel interface, moreover, the residual internal friction angle is the dominant factor supporting the residual strength of the loess-steel interface. In general, the residual strength of the undisturbed loess-steel interface is greater than that of the remolded loess specimen (for example, the maximum percentage of residual strength difference between undisturbed and remolded loess specimens under the same moisture content is 6.8%), which is because that compared with the mosaic arrangement structure of the remolded loess, the overhead arrangement structure of the undisturbed loess skeleton particles makes the loess particles on the loess-steel interface re-adjust the arrangement direction earlier and reach a stable speed relatively faster. The loess particles with angular angles in the undisturbed loess make the residual internal friction between the particles greater than the smoother particles of the remolded loess (for example, the maximum percentage of residual cohesion difference between undisturbed and remolded loess specimens under the same vertical pressure is 4.29%), and the intact cement between undisturbed loess particles brings stronger cohesion than the remolded loess particles with destroyed cement (for example, the maximum difference percentage of residual cohesion between undisturbed and remolded soil specimens under the same vertical pressure is 33.80%). The test results provide experimental basis for further revealing the influence mechanism of structure, and parameter basis for similar engineering construction.

A New Unified Solution for Deep Tunnels in Water-Rich Areas considering Pore Water Pressure

Pore water pressure has an important influence on the stresses and deformation of the surrounding rock of deep tunnels in water-rich areas. In this study, a mechanical model for deep tunnels subjected to a nonuniform stress field in water-rich areas is developed. Considering the pore water pressure, a new unified solution for the stresses, postpeak zone radii, and surface displacement is derived based on a strain-softening model and the Mogi-Coulomb criterion. Through a case study, the effects of pore water pressure, intermediate principal stress, and residual cohesion on the stress distribution, postpeak zone radii, and surface displacement are also discussed. Results show that the tangential stresses are always larger than the radial stress. The radial stress presents a gradually increasing trend, while the tangential stress presents a trend of first increasing and then decreasing, and the maximum tangential stress appears at the interface between the elastic and plastic zones. As the pore water pressure increases, the postpeak zone radii and surface displacement increase. Because of the neglect of the intermediate principal stress in the Mohr-Coulomb criterion, the postpeak zone radii, surface displacement, and maximum tangential stress solved by the Mohr-Coulomb criterion are all larger than those solved by the Mogi-Coulomb criterion. Tunnels surrounded by rock masses with a higher residual cohesion experience lower postpeak zone radii and surface displacement. Data presented in this study provide an important theoretical basis for supporting the tunnels in water-rich areas.

Stability analysis of slope in soil with strength mobilization

Considering the strength parameters cohesion c and internal friction angle < of the soil constitutive model are mobilized non-simultaneously. By gradually applying uniform vertical loads on the top of the slope, the progressive failure process of the slope is simulated by the FLAC3D software. The failure state of the slope is judged by monitoring the maximum unbalance force in the slope system, and the slope sliding surface is determined based on the maximum shear strain increment, and the safety factor is calculated by the slope critical height ratio method. The numerical simulation shows that when the strength parameters are not synchronicity come into play, the safety factor is less than that of two cases, which consider the synchronous mobilization of strength parameters and the traditional Strength Reduction Method (SRM). Finally, the influence of residual cohesion and plastic strain threshold on the position of sliding surface and safety factor are discussed by numerical analysis.

Numerical modeling of logged wellbore breakouts using cohesion-weakening frictional-strengthening models

The wellbore stability analysis based on the simplified models such as the elastic-perfect plastic or the elastic-brittle models results in the unrealistic prediction of the wellbore damage zone characteristics and consequently underestimate or overestimate the required mud pressure respectively. To overcome this weakness, the advanced elastic-plastic models considering the full rock failure mechanisms in the post-peak region are reviewed briefly in this paper. The damage zone surrounding a wellbore located in a carbonate oil field in a southwest of Iran is analyzed based on the different advanced models. The objective is to study how well the numerical simulation results based on the advanced models agree with the logged fallouts with respect to location, depth, and shape of damage zone around the wellbore. An evaluation of which advanced model can best capture the actual rock behavior especially in the post-peak region is carried out by comparing the simulation results with the logged fallouts around the wellbore.The depth of the damage zone and transverse extension angle around the studied wellbore predicted well based on a kind of the cohesion-weakening frictional-strengthening (CWFS) model are 3 cm and 84.3° respectively. The simulation results of this model derived from the tests on the carbonate rocks are in more agreement with the logged breakouts around the studied wellbore drilled in the carbonate rocks like limestone. Moreover, according to the key input parameters sensitivity study carried out in this paper, decrease in the residual cohesion and critical plastic strain parameters result in the larger simulated damage zone. On the contrary, increment of the initial friction angle and drilling mud pressure leads to the smaller simulated breakout around the wellbore. As higher dilation angle is utilized in the numerical model as input parameter, the larger simulated breakout is created around the wellbore.

Research and analysis on residual shear strength of surrounding rock base on elastic-plastic softening model

The excavation of caverns caused the unloading of the surrounding rock, which decrease the strength of the rock mass. In this paper, an ideal elastic-plastic stress-strain softening model is used to analyse the shear strength sensitivity of the surrounding rock during cavern excavation, in order to research the relationship between deformation, stress, plastic zone and the strength parameters. The results show that the values of each shear strength parameter have a significant effect on the distance of the plastic zone, and the sensitivity of the plastic zone distance shows that the influence of peak internal friction angle, residual internal friction angle, residual cohesion and peak cohesion decreased successively.

An analytical solution for stresses and deformations of tunnels in a non-uniform stress field based on strain-softening model and Mogi-Coulomb criterion

Abstract In this study, a mechanical model for tunnels excavated in a non-uniform stress field is developed. A new strain-softening model simultaneously considers the weakening of cohesion and internal friction angle is proposed. Then, an analytical solution for the post-peak region radii, stresses, and displacements is deduced. Taking a tunnel in Taoyuan coal mine as an engineering example, the post-peak region radii, surface displacement, and stresses distribution are determined. The effects of the horizontal-to-vertical stress ratio, intermediate principal stress, residual cohesion, and residual internal friction angle on tunnel deformations are discussed. The results show that the post-peak region radii and stresses distribution around the tunnel varies with direction due to the non-uniform stress field. The post-peak region radii and surface displacement are larger with consideration of intermediate principal stress. Tunnels surrounded by rock masses with a higher residual cohesion and internal friction angle experience lower post-peak region radii and surface displacement.

Experimental study on the effect of basalt fiber on the shear behavior of cemented sand

In this study, a series of triaxial tests was performed on a cemented sand improved by adding various contents of basalt fiber. The binding and interfacial interaction between the fiber and cemented sand matrix and the failure model of the interfacial interaction at the sliding surface were examined using scanning electron microscopy (SEM). The triaxial tests show that as the fiber content was increased, the cohesion and residual cohesion first increased and then decreased and the brittleness index decreased, whereas the internal friction angle and residual internal friction angle exhibited no clear variation patterns. Under the optimum fiber content of 0.75%, the cohesion is 406.1 kPa, and the residual cohesion is 145.4 kPa. Compared with cemented sand, the increase is 74.6% and 148.0%, respectively. The SEM tests show that the failure model of the interfacial interaction on the sliding surface included essentially the sliding and pull-out of fibers. The fiber was found to improve shear behavior through the interfacial effect and to affect the failure model through its bridging effect in binding sand particles.

Seismic fragility analysis of a High Arch Dam-Foundation System based on seismic instability failure mode

Seismic stability of concrete arch dams has always been a critical issue in seismic safety evaluation. In this paper, a high concrete arch dam is selected as a case study for the seismic fragility analysis based on instability failure mode. A comprehensive approach considering the opening and closing of contraction joint, probable sliding rock mass of dam abutments, the interface between the dam and foundation as well as the effect of foundation radiation damping is presented. The random and uncertain parameters containing the friction coefficients and cohesions are generated with Latin hypercube sampling method (LHS). The approximate IDA is performed, and the slippage and sliding area ratio are chosen as the engineer demand parameters (EDP). Different damage levels are identified by the slippage-based rule and sliding area ratio-based rule respectively according to their corresponding overall mean IDA curves. In addition, seismic fragility curves are developed for the defined damage levels. The results reveal that the damage levels defined based on different EDP-based rules could be different, while the ultimate capacity of the concrete arch dam reflected by selected EDPs are basically the same. Sliding area ratio-based rule can better reveal the whole process of sliding development of the arch dam abutment and the influence of residual cohesions. The existence of residual cohesion delays the sliding development and enhances the seismic stability of arch dam-foundation systems. Furthermore, a comparative analysis is performed based on the extended limit states. Seismic fragility analysis is of great significance for quantitatively evaluating the seismic performance of concrete dams from the probability point of view.

Laboratory evaluation of shear strength properties for cement-based grouted coal mass

Cement-based grouting has been widely used in underground coal mining to improve the overall quality of highly fractured rock masses. Due to the difficulties in obtaining characteristic specimens from highly fractured coal rocks, there are very limited laboratory studies investigating the effect of grout on the strength of coal. The purpose of this paper is to address this gap by examining the effects of grouting on fractured coal in terms of overall shear strength. Two failure modes associated with coal and grouted coal specimens have been identified. The results show the shear strength improvement of grouted coal specimens as indicated by the improvement of their peak and residual cohesion as well as friction angle when the amount of reinforcing grout increases. Linear regression models were constructed for the prediction of shear strength parameters based on grout percentages. These models can be used as a guideline for the preliminary design of pre-grouting applications.

Study on Friction Characteristics of Root-soil Interface of two Types of Soil

Through quick direct shearing test, friction characteristics of root-soil interface between Caragana root and two types of soil, sandy soil and silt soil, is studied. The results show that: shear properties and residual shear properties of root-soil interface between Caragana root and two types of soil are both in the line with the Mohr-coulomb theory. The root-soil interface had significant differences in friction coefficient, cohesion, and residual cohesion between sandy soil and silt soil under the same dry density and same saturation of soil. Silt soil root-soil interface friction coefficient, residual friction coefficient, cohesion and residual cohesion were 1L94%,6.69%,100.98%,46.37% more than that of silt soil. Silt root-soil interface residual friction coefficient and residual cohesion were lower than friction coefficient a1nd cohesion, sandy root-soil interface was opposite.

Moisture content effect on the ring shear characteristics of slip zone loess at high shearing rates

The residual strength of slip zone loess is strongly affected by the moisture content. To explore such an effect, slip zone loess obtained from Baqiao landslide was used in ring shear tests under moisture content ranging from 6% to 29%. Laboratory tests were utilized for the determination of the stress–displacement relationship and residual strength parameters for the effect of moisture content in loess. The experimental results showed that the residual strength generally decreases with increasing moisture content. In addition, for a given shearing rate, the residual strength at any moisture content is approximately proportional to the normal stress. The stress fluctuation of shear stress was found to be greater with high shearing rates. Furthermore, the effect of moisture content on the residual strength parameters (cohesion and friction angle) was presented: the residual cohesion of slip zone loess was observed to increase with moisture content to a certain limit that approaches the plastic limit, above which it decreases. Also, the friction angle of slip zone loess was found to decrease with increasing moisture content. Additionally, due to the increase of the moisture content, the scratch color on the shear surface changed from the original color yellow to brown and the thickness of the shearing band of samples reduced. Finally, the mechanism of the influence of moisture content on the macroscopic morphology of the shear surface was also analyzed and illustrated. The study may provide a basis for understanding the shear behavior of slip zone loess under different moisture contents, as well as guidelines for loess landslide stability prevention and prediction.

Stability analysis of strain-softening slopes based on the gravity increase method

With traditional slope stability analysis methods, it is difficult to accurately describe the progressive failure process and dynamic variation law of slope stability. The strain-softening constitutive model was therefore used to simulate the progressive failure process of a strain-softening slope based on the gravity increase method (GIM), with the displacement interface employed to determine the sliding surface. A sensitive analysis of the characteristic parameters within the softening stage was then conducted. The results are as follows: There are similar space-time evolution laws of residual strength factor and shear strain increment, with failure starting from the slope toe and extending gradually. The sliding surface of strain-softening slopes is located between that of the slope with peak strength and the sliding surface of the slope with residual strength. The stability coefficient shows an exponential growth trend with the increase of residual cohesion and residual plastic shear strain threshold, with a positive linear correlation between the residual friction angle and stability factor. The residual friction angle is the most sensitive factor in slope stability, followed by the residualcohesion, with the residual plastic shear strain threshold being the least sensitive.