Soil-rock Mixture Research Articles

Research on the effect of rock content and sample size on the strength behavior of soil-rock mixture

The mechanical behaviors of soil-rock mixture (S-RM) are very complicated and significantly affected by its rock block content and gradation composition. A systematic study was performed on how the strength and failure characteristics of S-RM can be affected by the rock block content, the “oversize rock block” processing methods adopted, and the sample size in triaxial tests. Both the deviator stress ratio and friction strength of S-RM increase together with the rock block content, but the former tends to decrease as the confining pressure increases. In the case of S-RM samples with the same gradation, these two parameters tend to decrease as the sample scale becomes larger. The development of deviator stress over axial strain of samples prepared using the equal quantity substitution method is similar to that of the samples of the natural gradation. During the shearing process, some large rock blocks in S-RM will be broken. The breakage mode of the rock blocks can be primarily divided into four categories according to its mechanism, namely, disintegration, corner breakage, fracturing, and breakage along bedding faces. As a result of the breakage, the strength envelope curve of S-RM follows a power function.

Failure Patterns and Morphological Soil–Rock Interface Characteristics of Frozen Soil–Rock Mixtures under Compression and Tension

Construction operations in cold regions may encounter frozen geomaterials. In construction, it is important to understand the processes by which geomaterials fail under common loading conditions to avoid accidents and work efficiently. In this work, an artificial frozen soil–rock mixture was used for uniaxial compression and indirect tension loading analysis to investigate macroscopic failure patterns and soil–rock interface crack evolution mechanisms. To further understand and compare the meso-mechanical failure mechanisms of the soil–rock interface, we used two types of rock block particles with different surface roughness for fabricating frozen artificial soil–rock mixtures. Acoustic emission (AE), ultrasonic plus velocity (UPV), and digital microscopy were utilized here to obtain the sample deformation response and analyze the morphology of the soil–rock interface. The results were as follows. From the perspective of macroscopic observation, bulging deformations and short tension cracks represent the main failure pattern under compression, and a tortuous tension crack in the center of the disk is the main failure pattern under indirect tension. From the perspective of microscopic observation, the soil–rock interface will evolve into a soil–rock contact band for the sample containing a rough rock block. The strength of the soil–rock contact band is obviously larger than that of the soil–rock interface. Three main failure patterns of the soil–rock interface were observed: a crack path through the accurate soil–rock interface, a crack path through the envelope of the rough rock block, and a crack path passing through the rough rock block. The experimental results could provide a reference for foundation engineering, especially in pile foundation engineering in cold regions.

Effects of rock block content and confining pressure on dynamic characteristics of soil-rock mixtures

Soil-rock mixture (S-RM) is a complex geo-material characterized by competent rock inclusions floating in a weaker soil matrix structure. The mechanical performances of it are significantly controlled by the interaction and properties of rock blocks and the external environment. In this study, experiments on S-RM were conducted through a large-scale triaxial apparatus for investigating the effects of rock block content and confining pressure on the dynamic characteristics of them. The features of the axial stress and strain time-histories of S-RM were analyzed. The reductions of the maximum shear modulus and the dynamic shear modulus ratio were investigated. The increase in the damping ratio was explored. And the effects of rock block content and confining pressure on the dynamic characteristics of S-RM were discussed in detail. Results indicate that there is an approximately linear relationship between the maximum shear modulus and the rock block content while an approximately exponential relationship between the maximum shear modulus and the confining pressure. The distribution of the dynamic shear modulus ratio of S-RM takes on a low dispersion at the same normalized shear strain. The unnormalized/normalized damping ratio of S-RM is increasing with the shear strain and takes on a feature of zonal distribution as a whole. Both the dynamic shear modulus ratio and the normalized damping ratio of S-RM could be evaluated by a function of normalized shear strain. The four-parameter formulas for the dynamic characteristics of S-RM were introduced for all practical purposes in the final. The results of this study can provide a reference to further understand the dynamics performance and the parameter determination of S-RM and other similar geo-materials.

Influence of Meso-Structure Parameters on Wave Propagation in Soil-Rock Mixture

The macro mechanical properties of soil-rock mixture are closely related to the meso-structure features of block stones, namely, content, size, and shape. To promote the engineering application of soil-rock mixture, it is important to explore the meso-structure of the mixture, and evaluate its constitutive properties. The previous studies have shown that the wave propagation in the mixture is highly sensitive to the rock content and compaction. To clarify the meso-structure features of soil-rock mixture, this paper establishes a discrete element model of the mixture based on Particle Flow Code (PFC), investigates the wave propagation features in the model with different meso-structure parameters, and analyzes how the meso-structure parameters affect the wave propagation. The results show that: With the growing rock content, the first wave amplitude increased, while the take-off time shortened; With the growing feature size of block stone, the first wave amplitude gradually decreased, while the take-off time gradually lengthened; The soil-rock mixture containing spherical block stones had the highest first wave amplitude and shortest take-off time, while the mixture containing rectangular block stones had the lowest first wave amplitude and longest take-off time. With the growing rock content, the maximum amplitude, dominant frequency, and spectral area all exhibited an increasing trend; With the growing feature size of block stone, the maximum amplitude, dominant frequency, and spectral area all exhibited a decreasing trend.

Study on shear performance of soil-rock mixture at the freezing-thawing interface in permafrost regions

Study on shear performance of soil-rock mixture at the freezing-thawing interface in permafrost regions

Application of the discrete element method and CT scanning to investigate the compaction characteristics of the soil–rock mixture in the subgrade

The soil–rock mixture (SRM) usually contains a large amount of gravels exceeding 40 mm in size, so the traditional laboratory method cannot directly test its maximum dry density (MD), making it difficult to evaluate the compaction degree of the SRM subgrade during construction. In this paper, a numerical simulation method of the vibration compaction method for the SRM (NSM-VCM) was developed based on a discrete element method (DEM) and CT scanning. Based on the established NSM-VCM, the MD of SRMs with a maximum particle size greater than 40 mm (SRM-G) was investigated comprehensively. Based on the results of laboratory tests and the NSM-VCM, a predictive model and determination method of the MD of SRM-G were developed. Finally, field measurements were conducted to validate the laboratory investigations. The results showed that the maximum error between the MD of the SRM obtained from the NSM-VCM and the laboratory test was 0.1%, indicating that the established NSM-VCM has high predictive accuracy. The MD of SRM-G increases with an increasing maximum particle size and dosage of giant granules. Only when the soil–rock ratio is appropriate can SRM-G form a better skeleton dense structure, which is important for improving the MD and mechanical strength. The maximum error between the estimated MD and the measured MD from the field site is 1.3%, which indicates that the prediction model and method for SRM-G established in this paper have high precision. These results address the issue that the MD of SRM-G cannot be determined in a laboratory.

Influence of volumetric block proportion on mechanical properties of virtual soil-rock mixtures

It is well known that the volumetric block proportion (VBP), to a large extent, dominates the mechanical properties of the soil-rock mixture (S-RM). In this study, random convex polyhedrons were applied for simulating rock blocks. Random structural models of S-RMs were generated based on the statistical regularities of volumetric proportion, size, shape, and spatial distributions of rock blocks. Then numerical simulation of triaxial compression tests was conducted sufficiently for studying the influence of VBP on the mechanical properties of virtual S-RM. Results indicate that the VBP indeed poses a major influence on the S-RM's mechanical properties. A high VBP will always lead to a more significant improvement in the strength and deformation parameters. There is an approximate quadratic polynomial relationship between uniaxial compression strength (UCS), elastic modulus, and VBP, while a linear piecewise relationship between internal friction angle, cohesion and VBP. The formulas for the mechanical parameters of S-RM regarding VBP are finally summarized and discussed in detail.

Experimental study on the prediction model of the cumulative strain of the soil–rock mixture backfill under metro train load

An indoor dynamic triaxial test was performed on the soil–rock mixture (SRM) backfill in Chongqing to study the cumulative strain characteristics of the SRM under the cyclic dynamic loading of the metro train. The effects of different consolidation degrees U, rock block contents P, and effective consolidation confining pressures were then analyzed. Results showed that when and P are the same, the lower the U, the faster will be the increase in cumulative strain. When U and P were the same, the lower the , the greater will be the increase in cumulative strain and the higher will be the cumulative strain stable value of the stability curve. When U and were the same, the lower the P, the faster will be the increase in cumulative strain. An empirical model of the cumulative strain of the SRM considering the influence of U, P, and was established on the basis of the results of the dynamic triaxial tests. The accuracy of the prediction model was verified against the actual test results. This study provides a theoretical basis for the settlement and deformation evaluation of SRM backfill subgrades under metro train cyclic loading.

Deformation Characteristics of the Shear Zone and Movement of Block Stones in Soil–Rock Mixtures Based on Large-Sized Shear Test

Soil–rock mixtures (SRM) have the characteristics of distinct heterogeneity and an obvious structural effect, which make their physical and mechanical properties very complex. This study aimed to investigate the deformation properties and failure mode of the shear zone as well as the movement of block stones in SRM experimentally, not only considering SRM shear strength. The particle composition and proportion of specimens were based on field samples from an SRM slope along national highway 318 in Xigaze, Tibet. Shear zone deformation tests were carried out using an SRM-1000 large-sized geotechnical apparatus controlled by a motor servo, considering the effects of different stone contents by mass (0, 30%, 50%, 70%), vertical pressures (50, 100, 200, 300, and 400 kPa), and block stone sizes (9.5–19.0, 19.0–31.5, and 31.5–53.0 mm). The characteristics of the shear zone deformation and block stone interactions were monitored by placing aluminum wires and dry ash in holes in the specimens. The results showed that the stone content 30% and 70% were two critical thresholds to determine the deformation characteristics of SRM. Under the conditions of high stone content and large particle size, the stones throughout the shear surface tended to extrude and roll during the shear process. The block stones around the shear surface were mainly affected by dilatancy and exhibited extrusion, particle breakage, and redistribution. The deformation pattern could be considered as be analogous to push-type shear deformation from the back to front or composite shear deformation from the front and back to the middle of the slope. It is of great importance to study the shear characteristics and deformation evolution of SRM to understand the progressive shear process of the sliding zone and the failure mode of landslides.

Numerical Investigation of the Influence of Rock Characteristics on the Soil-Rock Mixture (SRM) Slopes Stability

Soil-rock mixture is a widely distributed geological material in mountainous regions, where slope stability is an important issue. To investigate the influence of rock on slope stability, firstly, rocks in SRM are generated and then SRM slope models are simulated based on the finite difference method (FDM). The impact of various characteristics of rocks on slope factor of safety (FOS) is studied systematically by using the reduced strength method. The characteristic of rock considered include rock content, number of rock edges, aspect ratio, rock size and inclination of rock. The simulations indicate that increasing the rock content can prevent the development of high strain region and improve slope stability. However, the aspect ratio of rock has negligible impact on the mean value of FOS. For slopes with 45° slope angle, rocks with 45° angle of inclination result in smooth potential sliding surface, whereas rocks with 135° angle of inclination lead to more tortuous potential sliding surfaces. Statistical indices are utilized to analyze the results of numerical modelling.

Mechanical characteristics of soil-rock mixtures containing macropore structure based on 3D modeling technology

Soil-rock mixtures containing macropore (SRMCM) is a kind of geological material with special mechanical properties. Located in the project area of Lenggu hydropower station on the Yalong River, Sichuan Province, China, there is an extremely unstable Mahe talus slide with a total volume of nearly 160 million cubic meters, which is mainly composed of SRMCM. The study on the mechanical properties of SRMCM is of great significance for the engineering construction and safe operation. In this paper, laboratory tests and discrete element numerical tests based on three-dimensional scanning technology were conducted to study the influence of stone content, stone size, and the angle of the macropore structure on shear characteristics of SRMCM. The failure mechanism of SRMCM was discussed from a microscopic perspective. This work explains the internal mechanism of the influence of stone content, stone size, and the angle of the macropore structure on the strength of SRMCM through the microscopic level of stone rotation, force chain distribution, and crack propagation. As the macropore structure that intersects with the preset shear plane at a large angle could act as a skeleton-like support to resist the shear force, the fracture of the weak cemented surface of soil and stone in the macropore structure is an important cause of SRMCM destruction.

Three-dimensional mesoscale computational modeling of soil-rock mixtures with concave particles

Soil-rock mixtures (SRMs) are the main unfavorable geologic bodies in Southwest China. This paper presents a novel mesoscale computational modeling study of SRMs with concave aggregates. An efficient 3D mesoscale SRM generation method is proposed by combining the Gilbert–Johnson–Keerthi (GJK)-based collision detection technique, the border placement algorithm and the particle position selection method. A periodic mesh is generated based on the mesh mapping technique. A numerical homogenization analysis of an SRM with a large number of elements is realized, and the estimated parameters are validated by the experimental test results. The results indicate that SRMs with concave aggregates have a higher elastic modulus than those with convex aggregates. This method is helpful for predicting the physical properties of SRMs and has promising applications in engineering.

Study of hidden factors affecting the mechanical behavior of soil–rock mixtures based on abstraction idea

Because of spatial variability and complex compositions, the mechanical test results of natural soil–rock mixtures (SRMs) are often discrete and lack reproducibility, which has greatly restricted the practical application of the experimental findings. The objective of this study was to examine the general mechanical behavior of SRMs under the influences of some hidden factors (e.g., structural parameters, parent rock type and weathering degree). To that end, the abstraction idea was adopted to prepare purified SRM samples. Large-scale triaxial tests were performed on these purified materials. On this basis, the influences of three structural parameters on the mechanical behavior of SRMs were studied. Moreover, the relationship between the shear strength and parent rock type and that between the shear strength and the spatial distribution of rock blocks were quantified. Some additional intrinsic behavior was distinguished from individual experimental phenomena through the comparative analysis of the test data in this study and those reported in the literature. The results show that the hidden factors had significant influences on the mechanical behavior of SRMs. A greater saturated uniaxial compressive strength of rock blocks generally led to a larger shear strength of SRMs. According to the significance of their influences on the shear strength parameters of SRMs, the structural parameters are ordered as: the gradation of rock blocks, the initial dry density of sample and the spatial distribution of rock blocks. The deformation and failure feature of SRMs were considerably affected by the spatial distribution of rock blocks and shear rate. And the shear strength parameters of SRMs were mainly influenced by the content of grains between 40 and 60 mm. The findings of this study would provide useful guidance for engineering practice.

Hydro-mechanical simulation of the saturated and semi-saturated porous soil–rock mixtures using the numerical manifold method

Dynamics of the widespread saturated and semi-saturated soil–rock mixtures (SRMs) are of importance in practical engineering. In this paper, a numerical manifold model is presented for hydro-mechanical simulation of the saturated and semi-saturated SRMs. In the case of the semi-saturated SRMs, the model is based on the generalized Biot theory involving immiscible two-phase wetting and non-wetting fluids in deformable porous media. The wetting, non-wetting fluid pressure and skeleton displacement are chosen to be primary variables which are related to the wetting and non-wetting saturation and permeability by experimental relationships. For the mechanical problem, the material interfaces between soil and rock are deemed discontinuous by imposing a stick–slip contact constraint using an augmented Lagrange multiplier approach. For the hydraulic problem, the material interfaces are continuous for the fluid pressure and flux fields. Within the framework of the numerical manifold method (NMM), the discretized model including the interfacial discontinuities always can be established using the triangular mesh and the lumped mass representation is always available to increase computational efficiency. Besides the benchmark problems of saturated and semi-saturated porous media, two examples of soil–rock foundation and slope are performed to demonstrate the versatility and robustness of the model and to investigate the hydro-mechanical responses of the porous SRMs.

Influence of water and rock particle contents on the shear behaviour of a SRM

Abstract The contents of both water and rock particles are important factors affecting the mechanical strength of a soil–rock mixture (SRM) filled subgrade in the western mountainous area of China. Therefore, the purpose of this paper is to study the mechanisms of reconstituted landslide deposit samples with different water and rock particle contents by analysing the characteristics of shear strength, volumetric strain and ‘jumping’ phenomenon via large-scale direct shear tests. The results show that the influence of water content on shear strength is greater than the influence of rock particle content under a lower normal stress, and the results are reversed in the case of a higher normal stress. The effect of water content on the equivalent cohesion is bigger, especially for the sample with a high rock particle content. The friction angle of the specimen with same water content increases with the increasing rock particle content, but when the number of rock particles increases to a certain extent, there is a little effect on the friction angle. However, the friction angle decreases with increasing water content at the same rock particle content. Specimens with the same rock particle content change from dilation to compression with increasing water content. Finally, the continuous stage of the ‘intense jumping’ at different water content has been analysed. The ‘jumping’ phenomenon of samples with low water and rock particle content will first strengthen and then weaken the samples with increasing normal stress.

Strain-softening failure mode after the post-peak as a unique mechanism of ruptures in a frozen soil-rock mixture

A large number of traffic systems in China pass through a large area of frozen soil, most of which contains frozen soil–rock mixtures (FSRMs). For a profound understanding of the mechanical behaviours and damage mechanisms of geotechnical and geoenvironmental projects, it is necessary to understand the stress–strain characteristics and failure modes of FSRMs. This study focuses on the uniaxial compression strength (UCS), Young's modulus, and complete stress–strain curve under loading, considering the post-peak behaviour. More than 250 FSRM specimens were tested for UCS evaluations, while considering the effects of different freezing temperatures (−5, −10, −20, −30, and − 40 °C), initial water contents (15.0, 25.0, and 30.0%), and volumetric block proportions (0, 30, 40, 50, and 100%). The stress–strain curve of the FSRM was characterised by six distinct stress levels and deformation stages, different from those of rock or soil in the normal state. Particularly, the turning point from volume compression to expansion was observed after the post-peak, at which superficial cracks were initiated. Ice crystals provided ductility of the FSRM and maximised its load capacity to prevent premature cracks on the FSRM surface. The block stone hindered the linear propagation of cracks. The failure patterns of the FSRM for the UCS evaluation could be classified into four types. The UCS was influenced by mainly the strength of the mixture composed of ice crystals and soil particles and friction and occlusal strength between blocks. Notably, the gradual thawing of the FSRM might cause the largest deformation at the bottom of the active layer in summer and autumn, according to the experimental results. Thus, it is very important to understand the strength characteristics, deformation stages, and failure patterns associated with the geomechanical behaviour of the FSRM for engineering design and applications in a permafrost region.

Mathematical cover refinement of the numerical manifold method for the stability analysis of a soil-rock-mixture slope

An improved NMM (Numerical Manifold Method) with mathematical cover refinement (RNMM) is adopted to investigate the stability of a SRM (soil-rock-mixture) slope. In the RNMM, refined mathematical meshes are deployed at specified domains where stress concentration may occur. Compared to the TNMM (Traditional NMM), the computational efficiency of the RNMM is better for slope stability problems. Besides, the recently proposed scheme called MLC (mathematical cover systems with multiple layers) is adopted to further improve the computational efficiency of the RNMM. Based on the proposed RNMM+MLC (RNMM with MLC), stability analyses about two typical examples including a soil slope and a SRM slope are investigated. The numerical results demonstrated that: 1) The FOS (factor of safety) based on the proposed RNMM is better than that based on the TNMM; 2) The FOS based on the RNMM + MLC is almost the same to that based on the RNMM, but the computational efficiency of the RNMM + MLC is better than that of the RNMM; 3) The distribution mode of plastic zones in a SRM slope differs a lot from a soil slope.

A Monte Carlo Approach to Estimate the Stability of Soil–Rock Slopes Considering the Non-Uniformity of Materials

A soil–rock slope is a heterogeneous slope composed of soil and rocks that is widely distributed throughout the world. In order to accurately analyze the slope stability of soil–rock mixture, based on a Monte Carlo algorithm (fuzzy-based method), a symmetrical stability analyzing method for soil–rock slopes is proposed, considering the dispersion of strength of soil–rock mixtures. In analyzing it, the numerical model is symmetrical to the real soil–rock slope in geometry and material properties. In addition, the effect of rock content to slope stability was studied by this symmetrical method. The specific work of this paper is as follows: (1) The acquisition method of random number series for the Monte Carlo algorithm and the method of slope stability analysis, using the Monte Carlo method, are introduced. (2) According to in situ samples and remade samples, the strength characteristics of soil–rock mixtures were measured with different rock contents, which proved the scatter of strength of soil–rock mixtures. (3) Based on the measured strength parameters of soil–rock mixtures and the slope landslide, the reliability in analyzing results and superiority in calculating time of using the Monte Carlo method to analyze stability of soil–rock slopes are detailed. (4) The stability of soil–rock slopes with different rock content is discussed with the Monte Carlo method, and it is concluded that with the increase of rock content, the stability of a soil–rock slope decreases first and then increases, and the minimum safety factor is acquired at 20% rock content. (5) Based on a large number of calculation examples, the applied situations of the Monte Carlo method to analyze stability of soil–rock slopes are detailed according to sampling results and rock size.

Resilient Properties of Soil-Rock Mixture Materials: Preliminary Investigation of the Effect of Composition and Structure.

The physical composition and stress state of soil-rock mixture (SRM) materials have a crucial influence on their mechanical properties, and play a vital role in improving the performance of subgrade. To reveal the resilient behavior and mesostructure evolution of SRM materials, triaxial tests and discrete element method (DEM) numerical analysis have been carried out. In the triaxial test section, the mechanical response of SRM materials was investigated by preparing samples under different stress states and physical states and conducting triaxial tests on samples. Simultaneously, a new irregular particle modeling method was developed and applied to the discrete element modeling process to analyze the mesostructure evolution of SRM materials under cycling loading. First, a cyclic triaxial test of SRM material is performed on the SRM material, and the effects of bulk stress, octahedral shear stress and rock content on the resilient modulus of the SRM material are analyzed. It is revealed that the resilient modulus increases with increasing bulk stress and rock content, and decreases with increasing octahedral shear stress. Based on a new resilient modulus prediction model, the relationships among the rock content, stress state and resilient modulus are established. Then, based on an improved DEM modeling method, a discrete element model of the SRM is established, and the influence of rock content on coordination number and mesostructure evolution of the SRM is analyzed. The results show that in SRM materials, the increase of crushed rock changes the mesostructure of the SRM material. With the increase of rock content, the internal contact force changes from “between soil and rock” to “between rocks”, and the skeleton formed in the rocks gradually develops overall stiffness. Under the condition of low stress, the anisotropy of the SRM material is mainly caused by the shape and grade distribution of crushed rock. The induced anisotropy caused by the change of stress state has little effect on its mechanical behavior, which may lead to the greater dispersion of multiple SRM test results.

Site Characterization of Soil-rock Mixture Sedimentary Stratum Based on HVSR Analysis in the Chinese Loess Plateau

The soil-rock mixture sedimentary stratum is a compound with complex and loose topography, of which the structure is difficult to detect by the ordinary geophysical method. There is a need for a convenient, efficient and effective geophysical method to detect site effects in this area. This paper is an application of the S wave velocity profile inversion for the soil-rock mixture sedimentary stratum, using HVSR (Horizontal to Vertical Spectral Ratio) analysis of ambient noise by some three-component observations in the Chinese Loess Plateau. We carried out the measurement using three nested circular arrays and data recording systems with a spectrum expansion circuit. Inversion of the HVSR curves was performed by a three-layer model. Results of geological observation reveal that the upper part of the sedimentary stratum is Quaternary strata containing a large amount of humus and loess, the middle layer part is the stratum of the loose gravel and the under part is completely weathered granite with homogeneous lithology and fewer rocks. Interpretation results are consistent with previous drilling data, providing a valid geophysical basis for evaluating the stability of the soil erosion and designing a reasonable water and soil erosion control scheme.