Soil-rock Mixture Research Articles

Study on the Influence of Seepage Conditions with Different Rainfall Intensities on the Structural Evolution of Soil-Rock Mixture Filler

The rainfall conditions cause seepage in the soil-rock mixture (SRM) filler subgrade, leading to the loss of fine particles and the change of soil structure, which eventually leads to large uneven deformation or instability of the subgrade. The particle loss test device was applied to conduct the seepage test of SRM filler, monitor the change process of permeable quality, fine particle loss and sedimentation of filler under different rainfall conditions, and analyze the evolution process of soil structure and sedimentation characteristics. The result shows that the rainfall intensity affects the permeability of the filler, and then accelerates the loss of fine particles under water migration. With the condition of the same rainfall duration, the hourly water permeability increased firstly and then gradually stabilized with the rainfall duration. The total water permeability mass, fine particle loss, and real-time sedimentation increased with the rainfall intensity. With the same rainfall condition, the maximum hourly water permeability under short-term heavy rainfall condition is about three times that under heavy rainfall condition, which is more serious for the internal erosion of SRM filler. The total water permeability mass, fine particle loss, and real-time sedimentation increased with the rainfall intensity. The stages of skeleton remodeling and relative stability are the most serious stages of filler skeleton structure damage. The sedimentation deformation of filler has hysteresis, resulting in the occurrence of sedimentation much later than the loss of fine particles and skeleton deformation. After the rainfall stops, with the loss of fine particles and water dissipation, the filler will occur secondary sedimentation, resulting in an increase in the final sedimentation as the rainfall intensity increases, making it possible for subsidence damage of the roadbed to occur both during and after the rainfall. Extreme rainfall conditions (short-term heavy rainfall) have the most obvious effect on the structural damage of the subgrade.

Centrifugal modeling test on failure characteristics of soil-rock mixture slope under rainfall

Soil-rock mixtures (SRMs) are a type of geological body with mixed soil-rock and special properties. The stability of the SRMs widely distributed on slopes is an important issue for slope safety evaluations and disaster prevention and mitigation. Based on a self-developed rainfall system, a series of centrifugal model tests with different rock contents were carried out to study the failure behaviors and failure mechanisms of SRM slopes under rainfall conditions. The digital photography technology and measurement systems were used to obtain various physical data such as the pore water pressure, earth pressure, water content, and deep displacement. The test results show that the failure mode of a SRM slope is an erosion failure. When the rock content is 0%, the slope degrades to a soil slope, and the slope failure mode manifests as a multilevel block sliding failure. The migration of fine particles is the main reason for the instability of SRM slopes. As the rock content increases, the soil skeleton gradually becomes denser, the migration of fine particles gradually decreases, and the erosion effect gradually weakens. The greater the rock content, the greater the elastic modulus and angle of internal friction of the SRM, the lower the porosity, the lower the loss of fine particles, and the more stable the SRM slope.

Experimental and DEM-Based Numerical Studies on the Shearing Characteristics of Talus-like Rock Mass.

The talus-like rock mass is a special kind of geomaterial widely distributed in southwestern China, which has induced serious engineering disasters for tunneling engineering. However, the mechanical behavior of the talus-like rock mass remains unclear as the previous studies mainly focused on similar geomaterials such as the soil–rock mixtures. In this paper, we have carried out both experimental and discrete element method (DEM)-based numerical analyses to investigate the shearing characteristics of the talus-like rock mass collected from a real project site. Large-scale direct shear tests reveal that the strength parameters increase with the block content, which is different from the traditional soil–rock mixture. A dependence has been discovered in that the specimen dilation becomes more obvious under lower normal stress and larger block content. It is also observed that higher normal stress is beneficial for crushing blocks. The force chains obtained in the DEM simulations show that distinct internal structures are generated in the rock samples with different block contents. The distribution of coordination number establishes the dependence of fabric stability on block content during shearing. Bond-break evolution reveals the tendencies of crushed particles were consistent with those of experimental tests. The findings provide a more in-depth understanding about the mechanical behavior of the talus-like rock mass, which helps to uncover the cause of the collapse of the real tunnel project.

Influence of the matrix of the soil-rock mixture on deformation and failure behaviors of the slope based on material point method

In this paper, the material point method (MPM) is used to explore the influence of matrix (namely soil) in the soil-rock mixture (SRM) on the stability of SRM slope. Firstly, a typical slope model is established, and a series of circular stone blocks with different sizes are generated inside the slope, then the SRM slope model is established. Next, the gravity is linearly loaded. When it is completed, the kinetic damping is applied, and the kinetic energy of the SRM slope is set to 0 to obtain the initial state of the slope. Then, the stability of the SRM slope under different soil cohesions and internal friction angles is simulated by MPM. The simulation results show that the stability of the SRM slope is more affected by cohesion than the internal friction angle. When the SRM slope enters the large deformation stage, there are both translational sliding and rotational sliding modes in the slope. The translational sliding is mainly the soil above the slope surface, and the SRM under the slope occurs rotational sliding.

Fluid–solid coupled model for the internal erosion of gap‐graded soil–rock mixtures with different fines contents: Its verification and application

AbstractSoil–rock mixtures are widely encountered in geotechnical engineering projects. Gap‐graded soil–rock mixtures are prone to internal erosion because the fine particles are easily removed by seepage flow within the large pores between the coarse particles. Internal erosion has been the focus of geological disaster research. As concluded from different studies, the fines content (FC) of gap‐graded soils is a crucial factor in controlling soil stability. For this reason, a fluid–solid coupled model with the computational fluid dynamics—the discrete element method, with an indoor physical permeability test, was conducted to study the evolution characteristics of internal erosion of gap‐graded soil–rock mixtures of different FCs. After the erosion test, the vertical section of the sample was divided into three areas according to changes in fine particles: top, middle uniform, and bottom loss areas. The study showed that a large number of particles in the bottom loss areas are lost at first and the top loss area will enter the middle uniform loss areas. The fine particles in the middle are affected by the fine particle content, from accumulation to equilibrium to loss. Fine particle loss occurred at different sample heights. The top particle loss is the most serious, followed by the bottom and then the middle, and this is consistent with the changes, from a particle‐size perspective. An FC of 35% may be the critical value of the coarse‐fine particle skeleton structure in the preset working conditions of coarse and fine particle diameters. There are apparent ‘island’ effects in the sample where FC = 40%. This can be analysed through strong force chain analysis, from the particle‐size perspective that ‘island’ effects continue to disappear under the influence of internal erosion.

Slope stability and post-failure analysis of soil-rock-mixture using the modified 2D DDA-SPH method

Soil-rock-mixture (SRM), consisting of high-strength rock blocks and low-strength soil, is widely distributed in nature. In traditional methods such as limit equilibrium methods , this geomaterial was simplified as a uniform medium without considering the influence of rocks due to its heterogeneity and complex physical and mechanical properties. A coupling method that bridges discontinuous deformation analysis (DDA) for rock simulation and smoothed particle hydrodynamics (SPH) for soil simulation is a promising tool for SRM problems. In this study, the 2D coupled DDA-SPH method is extended to the applications of stability and post-failure analysis of SRM slopes. First, the determination of the contact spring stiffness in the contact algorithm is presented and the reasonable value is discussed. Then, to eliminate the oscillations in some simulations with polygonal rock blocks inside the soil, the contact algorithm is modified by implementing a special treatment for the contact between the SPH particles and the convex vertices of DDA blocks. In addition, the criterion based on the distribution of plastic zone is used to define the critical stable state of the slope when applying the strength reduction technique for stability analysis, and its effectiveness is demonstrated. Finally, a series of numerical analyses are carried out, and through these numerical simulations, some conclusions and suggestions can be reached. • A determination of the contact spring stiffness in the DDA-SPH coupled method was proposed and discussed. • The oscillations in the simulations with polygonal rock blocks inside soil has been eliminated by modifying the contact algorithm. • The effectiveness of the modified method has been demonstrated through several numerical simulations. • The modified method has showed advantages in modeling soil-rock-mixture materials.

Experimental and Modeling of Residual Deformation of Soil–Rock Mixture under Freeze–Thaw Cycles

Projects in seasonal frozen soil areas are often faced with frost heaving and thawing subsidence failure, and the foundation fill of most projects is a mixture of soil and rock. Therefore, taking soil–rock mixture with different rock contents as research objects, the residual deformation of soil–rock mixture under multiple freezing–thawing cycles is studied. In addition, the deep learning method based on the artificial neural network was pioneered combined with the freezing–thawing test of the soil–rock mixture, and the Long short-term memory (LSTM) model was established to predict the results of the freezing–thawing test. The LSTM model has been verified to be feasible in the exploration of the freeze–thaw cycle law of a soil–rock mixture, which can not only greatly reduce the period of the freeze–thaw test, but also maintain a high prediction accuracy to a certain extent. The study found that the soil–rock mixture will repeatedly produce frost heave and thaw subsidence under the action of freeze–thaw cycles, and the initial frost heave and thaw subsidence changes hugely. With the increase of the number of freeze–thaw cycles, the residual deformation decreases and then becomes steady. Under the condition that the content of block rock in the soil–rock mixture is not more than 80%, with the increase of block rock content, the residual deformation caused by the freeze–thaw cycle will gradually decrease due to the skeleton function of block rock, while the block rock content’s further increase will increase the residual deformation. Furthermore, the LSTM model based on an artificial neural network can effectively predict the freezing and thawing changes of soil–rock mixture in the short term, which can greatly shorten the time required for the freezing and thawing test and improve the efficiency of the freezing and thawing test to a certain extent.

Shear Characteristics of Gravel Soil With Different Fillers

In recent years, China has established many large water conservancy projects in the western mountainous areas. However, dam foundation excavation produces a huge amount of waste rock mixture. A soil-rock mixture with unique structural characteristics can easily cause geological disasters, such as collapse, landslide, and debris flow following an earthquake, rainfall, and engineering disturbance. Therefore, research on the strength and shear characteristics of the soil-rock mixture is beneficial to avoiding soil-rock mixture disasters. In this study, the soil-rock mixture in the slag dump site of Jinping Hydropower station is taken as the research object. The influence of different fillers on the gravel soil shear characteristics is investigated through a large-scale triaxial test of coarse-grained soil. The slope stability of the slag dump site is analyzed using Midas GTS. Then, the most unfavorable sliding surface of the slag dump site is discovered and the slope stability coefficient under the most unfavorable conditions is calculated. The results lay a scientific foundation for the design and construction of a slag dump slope.

Modeling of soil–rock mixture landslides with the generalized interpolation material point method

The new numerical model for studying the dynamic evolution of soil–rock mixture landslides is presented in this article. The numerical model based on the generalized interpolation material point method analyzes a simplified slope. The gravity is linearly loaded, and the linear elastic model is used to update the stress to obtain the initial state of the slope. A small soil cohesion is set to trigger the slope sliding until the equilibrium state is reached again. During this period, the elastic–plastic material model based on the Drucker–Prager criterion is adopted for soil and stones. The differences in dynamic evolution between the homogeneous soil slope and soil–rock mixture slope are studied. Under the same stone content, the influence of the size and shape of stone on the dynamic evolution of slope is studied.

Influence of Stone Shape Factor on the Pull-Out Resistance of Geogrid in the Soil-Rock Mixture

In this paper, the particle shape factor of the stone in the soil-rock mixture was used as the specific research object of the geogrid pull-out test, combining the random generation method and evaluation criteria of rock blocks in the soil-rock mixture, and a feasible method for classifying stones according to their shape factors is presented. By comparing the mechanical behavior of soil-rock mixture composed of rock blocks with different rock material shape factors in the geogrid pull-out tests under different working conditions, we studied its changing laws. The research shows that with the increase of the shape factor of the stone, the pull-out resistance of the geogrid gradually decreases. When the stone paved in the model box changes from a single layer to a double layer, the pull-out resistance of the geogrid increases, and there is a disturbance range in the contact interface between the geogrid and the stone; stone blocks 5 cm away from the geogrid surface can still have an indirect effect on the pull-out resistance of geogrids; the pull-out resistance of geogrid in clay is greater than that in the sand; and an empirical expression for predicting the pull-out resistance of geogrids is obtained. This empirical expression helps to explain the influence mechanism of stone shape on drawing resistance of geogrid.

Numerical investigation of the failure mechanisms of soil–rock mixture slopes by material point method

A new technique for establishing material point method (MPM) models of soil–rock mixture slopes was proposed to accurately analyze the large-deformation failure mechanisms of soil–rock mixture slopes. First, natural rock block contours were obtained through a digital image processing technique, followed by analyzing and reconstructing the rock block contours with the help of the discrete Fourier transform theory. Next, The discrete models of soil–rock mixture slopes for MPM were constructed by combining the stochastic placement algorithm with the digital image processing technique. Moreover, using the MPM, the influences of the content, the size, and morphology of rock blocks on the failure mechanisms of the slopes were investigated in detail. The results reveal that the content, features, and spatial distribution of rock blocks significantly influence the slope failure characteristics. Owing to the existence of rock blocks, the plastic zone of a slope at failure becomes heterogeneous and complex, showing multiple sliding regions and failure modes.

Dynamic modelling of soil-rock-mixture slopes using the coupled DDA-SPH method

Soil-rock-mixture (SRM) slopes widely exist, and their failure may cause serious damage to infrastructures and endanger human lives. Thus, it is important to study the mechanical behaviors of SRM slopes. Analyzing these behaviors is complicated and typically involves soil-rock-structure interactions. Fortunately, numerical methods can provide adequate descriptions of these behaviors. Discontinuous deformation analysis (DDA) is a powerful method that can be used to simulate the behavior of solid blocks, including rocks and structures. Smoothed particle hydrodynamics (SPH) can effectively simulate the behavior of fluid flows and soil materials. To take advantage of both methods, the coupling of DDA and SPH is investigated in this study, where the coupled DDA-SPH method is used to study the mechanical behaviors of SRM slopes. Two verification tests are investigated to demonstrate the accurate calculation of the interaction force and impact force using the coupled method. A series of simulations are then performed considering different shapes, sizes and contents of rocks and their distributions in SRM slopes. Results show that the deformation of the SRM slope is proportional to the roundness but inversely proportional to the sorting coefficient of the rock blocks. Additionally, the deformation of the SRM slope with rocks distributed along the potential sliding surface is much smaller than that with rocks distributed in the sliding body. In addition, the runout distance of an SRM landslide and the maximum impact force on nearby buildings increase with higher rock contents, which is more destructive to nearby structures and human lives.

Study on Shear Creep Characteristics and Creep Model of Soil-Rock Mixture Considering the Influence of Water Content

Water content has a significant effect on the creep properties of soil-rock mixtures (SRM). Multi-loading shear creep tests are carried out on SRM samples with different water contents. The test results show that deformation gradually increase with increasing water content, while long-term strength gradually decrease with increasing water content. The deformation mechanism shows that increasing of water content causes the change of rock particles at on the shear surface from fracture to rotation. Based on the creep test results, a modified Burgers model considering the water content is proposed by the empirical relationship between the parameters of the traditional Burgers model and the water content. And the results predicted by the modified Burgers model agree well quite well with the experimental creep data.

Volumetric behavior of an unsaturated clayey soil-rock mixture subjected to freeze-thaw cycles: A new insight

The volumetric behavior of clayey soil-rock mixtures under freeze-thaw action is very important for the design of anti-frost heave in cold-region engineering. However, existing studies mainly focus on the effect of freeze-thaw cycles on the global volume change of fine-grained soils, and few attempts have focused on mixed soils with coarse particles such as clayey soil-rock mixtures. In this study, a series of laboratory freezing-thawing tests were conducted to investigate the effects of the freeze-thaw cycles and rock content on the global volume change, diameter change, and height change behaviors of cylindrical soil-rock mixture samples. The results demonstrate that: 1) the volumetric behavior of the samples under freeze-thaw cycles is closely related to the rock content, which is mainly because different rock contents dominate the different skeleton structures. 2) For samples with 0%, 10%, and 30% rock content, the global volume of compacted samples shows shrinkage upon freezing and expansion after thawing. However, the global volume of the sample with 50% rock content began to shrink after three freeze-thaw cycles. 3) Compared with the samples not subjected to freeze-thaw weathering, the height of samples with 0%, 10%, and 30% rock content shows shrinkage upon freezing and expansion after thawing no matter what the number of freeze-thaw cycles is. However, the diameter and height of the sample are always shrinking, regardless of whether they are frozen or thawed. 4) Upon freezing, the shrinkage of the sample's global volume and diameter tends to decrease linearly with the increase of rock content, but the shrinkage of the sample's height is not sensitive to the change of rock content. Upon thawing, the global volume and height of the sample exhibit a trend from expansion to contraction with increasing rock content, but the shrinkage of the diameter of the sample is nearly the same no matter what the rock content is. Furthermore, a computed tomography (CT)-based mesoscopic structural characteristic and two idealized schematic diagrams well illustrated the evolution mechanism of the volumetric behavior of an unsaturated clayey soil-rock mixture subjected to freeze-thaw cycles.

Feasibility Study on the Bamboo Grid Instead of Geogrid for Soil-Rock Mixture Subgrade Reinforcing.

To promote the application of the bamboo grid in the soil–rock mixture subgrade in mountain areas, the mechanical properties of bamboo reinforcement were investigated in this study, and the reinforcement effect and interface characteristics of uniaxial/biaxial bamboo grid on the soil–rock mixture under different vertical loads was comparatively analyzed. The results show that the tensile force (2% elongation) of the bamboo reinforcement is 50.21 kN/m, and its average tensile strength is 236.01 MPa. Moreover, bamboo reinforcement has excellent shear and flexural properties. In general, the reinforcement effect of the biaxial bamboo grid on the soil–rock mixed subgrade is better than that of the uniaxial bamboo grid. In the case of using a uniaxial bamboo grid, its pull-out curve is generally a strain-softening type. As for the biaxial bamboo grid, due to the existence of bite force, its pull-out curve usually presents a strain-hardening type. Compared with the uniaxial bamboo grid, the friction coefficient of the reinforcement–soil interface using the biaxial bamboo grid is higher, and the interfacial shear stress is increased by 72.2–91.2%.

Effect of Particle Combination Ratio of Soil Rock Mixture on Shear Strength

With the increase of fine-grained soil content, the properties continue to transform to fine-grained soil, which will affect the permeability and strength of the soil. Even if the content of silt and clay in fine-grained soil is not high, it will significantly impact soil properties. When the content of coarse particles is low, the nature of soil depends on the fine particles in the soil. When the content of coarse particles gradually increases, and the coarse particles are overhead, the nature of soil only depends on the coarse-grained soil, and the influence of fine particles on the nature of soil gradually weakens. To reveal the interaction mechanism between coarse and fine particles in high moisture content clayey gravel, this project conducts a systematic experimental study on the high moisture content clayey gravel, takes 0.5mm as the particle size limit, takes the separation of coarse and fine particles in the soil sample as the control test, and systematically tests the shear strength of the separated fine particles; systematic shear strength tests were also carried out on the separated coarse particles. In addition, shear strength tests were carried out on soil samples with different fine particle content.

Mechanism of shear strength deterioration of soil-rock mixture after freeze–thaw cycles

The growth and melting of internal ice crystals reorganizing the pore structure and internal skeleton of soil-rock mixture (S-RM), which leads to the deterioration of the S-RM shear strength after freeze–thaw cycles and its strength characteristics after the cycles are different from the normal temperature one. Due to unclear S-RM strength deterioration mechanism after freeze–thaw cycles, stability of S-RM cutting slopes in cold regions cannot be effectively evaluated. In this paper, particle flow code (PFC) simulation as well as direct shear and nuclear magnetic resonance (NMR) tests were carried to study strength degradation behaviors and pore structure changes of S-RMs containing various amounts of rock after freeze–thaw cycles. The results show that the shear strength and internal friction angle of S-RM are no longer positively correlated with the rock content after the cycles, and the strength parameters will decrease at rock contents of above 45%. Based on the simulation test, shear band thickness variation after the cycles was quantitatively evaluated, and an innovative method to obtain the fluctuation value of shear failure surface was proposed. It is found that the change laws of shear band thickness and failure surface fluctuation value with rock content after the cycles are consistent with that of shear strength, and they all reach the maximum value when the rock content is 45%. Fractal theory was introduced for quantitative evaluation of changes in S-RM pore structure after the cycles, and combined with the strength parameter attenuation, the strength deterioration mechanism of S-RM was revealed: The deterioration of shear strength in samples containing low rock content is mainly due to changes in the contact form between particles caused by internal inclusion structure formation after the cycles. The deterioration of S-RM with rock content of 55% and 65% is mainly due to attenuation of the internal skeleton effect caused by the appearance of overhead structures. The internal pores and skeleton structure of the sample with 45% rock content have little change, so the attenuation changes of the strength parameters and the undulation value of the failure surface are minimal.

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.

A coupled discrete element material point method for fluid–solid–particle interactions with large deformations

Fluid–solid–particle systems such as water–soil–rock mixtures are very common in many natural processes and engineering applications. However, modelling these complex interactions is still very challenging due to their multi-scale nature. Here, we present a hybrid model that combines the advantages of the two-phase (solid and liquid) Material Point Method (MPM) on handling continuum materials with large deformations and the capability of the Discrete Element Method (DEM) on simulating the mechanical behaviours of rigid particles. Moreover, in our model, the DEM has the ability to deal with non-spherical particles. A new collision detection approach is presented in detail, and a unified DEM style contact force model is proposed to couple MPM with DEM by considering momentum exchanges between particles and continuum phases. The proposed model is validated by several numerical benchmarks, including (1) flows around a cylinder, (2) a block sliding on an inclined plane, (3) projectile impacts a granular medium, and (4) sphere impacts and penetrates into wet granular material. The results match well with analytical solutions and experimental observations, demonstrating this model’s capability to efficiently solve complex fluid–solid–particle interactions with large deformations. Finally, an example of column collapse is simulated to show future potential applications of the proposed method.

Study on Physical and Mechanical Characteristics of Shear Band in Jinpingzi Landslide Region II

The shear band is an important portion of a landslide. Investigating the shear band’s physical characteristics is one of the most significant challenges for landslide stability analysis and control. A systematic study of shear band in Jinpingzi landslide region II was conducted in this paper. The microscopic composition, deformation, strength, and creep characteristics are investigated by various experiments. In addition, CT visualization technology is utilized to monitor and scan the failure process of the shear band in real-time, as well as carry out studies on failure mechanisms. Furthermore, a constitutive model of the shear band is established as an alternative to the conventional Duncan and Chang model, which does not have this model’s limitations. The associated research will broaden understanding of such a medium’s properties, but it will also be highly useful for stability analysis, deformation prediction, and prevention of such soil-rock mixture landslides.