Soft Rock Research Articles

Mechanical Characteristics of Similar Weakly Cemented Soft Rock under Directional Shear Stress Path and Modified Lade–Duncan Criterion

Mechanical Characteristics of Similar Weakly Cemented Soft Rock under Directional Shear Stress Path and Modified Lade–Duncan Criterion

Analysis of Fracture Modes and Acoustic Emission Characteristics of Low‐Frequency Disturbed Water‐Bearing Soft Rock With Different Cyclic Initial Value

ABSTRACTIn the complex geological environment of deep mining area, water‐bearing soft rock is more prone to damage and destruction by low‐frequency disturbance. In this paper, the dynamic–static combination test was conducted on the basis of uniaxial compression test by using creep dynamic disturbance impact loading system and acoustic emission technique. The test results show that with the increase of the initial value of disturbance loading, the fracture morphology of sandstone gradually changes from a single major crack to multiple cracks coexisting, and some saturated sandstones lose the bearing capacity in the process of disturbance, presenting a cone‐shaped fracture surface. The increase of the initial value of the disturbance changes the bearing capacity of the sandstone, and the peak energy of acoustic emission reaches the maximum value when the initial value of the disturbance is 80% UCS. The results of the study can provide some reference for the stability analysis of deep water‐rich soft rock mines.

Estimation of Vs30 and site classification of Bhaktapur district, Nepal using microtremor array measurement

The severe damage observed in the Kathmandu Basin, Nepal, during past earthquakes necessitates a thorough study of the seismic behavior of the basin sediments. As the shear-wave velocity is directly related to the elastic shear modulus of the material, it is essential to determine it to incorporate the behavior of the soil in the design of the structure. Hence, we determined average shear-wave velocity in upper 30 m (Vs30) of soil in Bhaktapur district in the eastern part of the Kathmandu Basin at 73 observation points, employing two methods involving the use of non-invasive microtremor array measurements (MAMs). These MAMs are widely used for determining subsurface soil characteristics by analyzing the ambient vibrations of the ground. The first method involves inversion using a genetic algorithm, and the second is a method for obtaining Vs30 directly from the dispersion curve. We found that Vs30 in the southeastern part of the study area was higher than that in other parts. Conversely, Vs30 in the western region was lower. The calculated Vs30 values were used to classify the sites. The elevated eastern and southeastern areas with high Vs30 were categorized as dense soil or soft rock, whereas the areas with low Vs30 that had suffered significant damage during the 2015 Gorkha earthquake were classified as soft soil sites.Graphical

Prediction of TBM cutter wear in heterogeneous ground under high ambient pressure

To prevent tunnel face collapse in heterogeneous ground, the applied face pressure in the excavation chamber can be increased to stabilise the tunnel face, which may hinder cutter rotation during tunnel boring machine (TBM) tunnelling. This study proposes a TBM cutter wear prediction model that considers the impact of the variation in the cutter normal force on the cutterhead owing to the high applied face pressure and low penetration rate. First, the cutter motion mode in heterogeneous ground is investigated. The cutter is found to slide when cutting soft rock based on an analysis of the field data. The evolution process of the cutter wear morphology under heterogeneous ground conditions is summarised. A method for calculating the cutter normal force when the cutter slides on the tunnel face is proposed by referring to the cutting mechanism of the drag bits. The cutter hardness is measured on-site using a portable hardness tester. A semi-empirical model considering the variation in the cutter normal force on the cutterhead is proposed to predict the cutter wear in heterogeneous ground. Wear prediction is compared with other predictive models and field data for validation, and the discrepancies between the different models are discussed. The results show that the proposed model is effective for predicting TBM cutter wear under complex geological conditions.

Deformation behaviour of strain-softening rock mass in tunnels considering deterioration model of elastic modulus

By analyzing the composite effects of the plastic shear strain and confining stress according to the laboratory test results, a model adopting a deteriorating elastic modulus for the intact rock is proposed. Soft rock such as coal and the strong rock such as granite with different quality are investigated. Geological Strength Index (GSI) is adopted to represent the integrity of rock, to be specific, from intact rock to the rock mass. Specifically, for the strain-softening rock mass, the elastic modulus is gained by introducing GSI into the deterioration model. The strength parameters are obtained by fitting GSI into Hoek-Brown failure criterion. Then, the elastic modulus and strength parameters of the rock mass are employed in the numerical procedure for solving out the rock deformation, the radii of the plastic softening and residual zones of the tunnels. The rationality of the numerical procedure is verified with the existing semi-analytical and analytical procedures. In the discussion, the influences of the critical plastic parameters and GSI, on the elastic modulus, rock deformation, radii of the plastic softening and residual zones are investigated. Five models for the variation of the modulus including the proposed deterioration model are defined. The rock mass deformation behaviours of the five models are compared. The results indicate that, in the plastic softening zone that is far away from the tunnel periphery, the elastic modulus is more sensitive to the plastic shear strain, whereas near the tunnel periphery, the elastic modulus is primarily affected by the confining stress. Regarding a linear decrease of the elastic modulus versus the plastic shear strain overestimates the elastic modulus to some extent, especially for the soft rock mass.

Multi-hazard modeling of erosion and landslide susceptibility at the national scale in the example of North Macedonia

Abstract Due to favorable natural conditions and human impact, the territory of North Macedonia is very susceptible to natural hazards. Steep hillslopes combined with soft rocks (schists on the mountains; sands and sandstones in depressions), erodible soils, semiarid continental climate, and sparse vegetation cover give a high potential for soil erosion and landslides. For this reason, this study presents a multi-hazard approach to geohazard modeling on the national extent in the example of North Macedonia. Utilizing Geographic Information Systems, relevant data about the entire research area were employed to analyze and assess soil erosion and susceptibility to landslides and identify areas prone to both hazards. Using the Gavrilović Erosion Potential Method (EPM), an average value of 0.36 was obtained for the erosion coefficient Z, indicating low to moderate susceptibility to erosion. However, a significant area of the country (9.6%) is susceptible to high and excess erosion rates. For the landslide susceptibility assessment (LSA), the Analytical hierarchy process approach is combined with the statistical method (frequency ratio), showing that 29.3% of the territory belongs to the zone of high and very high landslide susceptibility. Then, the accuracy assessment is performed for both procedures (EPM and LSA), showing acceptable reliability. By overlapping both models, a multi-hazard map is prepared, indicating that 22.3% of North Macedonia territory is highly susceptible to erosion and landslides. The primary objective of multi-hazard modeling is to identify and delineate hazardous areas, thereby aiding in activities to reduce the hazards and mitigate future damage. This becomes particularly significant when considering the impact of climate change, which is associated with increased landslide and erosion susceptibility. The approach based on a national level presented in this work can provide valuable information for regional planning and decision-making processes.

Experimental study on optimization of abrasive water jet process parameters

Abstract In order to improve the mining speed of coal mine roadway and fundamentally solve the problem of mining imbalance, the abrasive water jet technology is used to cut rocks in front of mechanical tools to achieve efficient improvement of coal mine roadway mining efficiency. Based on the existing high pressure pure water jet test equipment, the abrasive water jet test platform was built, and the uniaxial compressive strength of soft and hard rock was obtained by testing the rock mechanical properties. The effect of jet pressure, target distance, transverse velocity and jet impact Angle on rock cutting is studied by using control variable method. According to the single factor test analysis, L16 (44) four-factor four-level orthogonal test analysis table is selected to carry out orthogonal tests on granite and sandstone respectively. The results of range analysis and variance analysis show that the main and secondary relationships of abrasive water jet factors are as follows: transverse velocity > jet pressure > jet inclination > target distance. The optimal jet parameters are when jet pressure is 40 MPa, target distance is 5mm, transverse velocity is 2mm/s and jet inclination is 10°.

Strain-softening model for granite and sandstone based on experimental and discrete element methods

Combing macroscopic experimental method and mesoscopic numerical method, this study analyses the strain-softening behaviours of granite and sandstone. From the macroscopic perspective, the stress–strain curves of granite and sandstone under different confining pressures are studied by laboratory triaxial compression test. Variations of post-peak reduction modulus and critical plastic shear strain versus confining stress are obtained. Evolution of strength parameters at peak, residual and strain-softening stage are proposed. Then a method to develop the strain-softening model of hard and soft rocks is presented. From the mesoscopic perspective, based on the laboratory test results, the parameters of discrete element method PFC for the samples of the granite and sandstone are calibrated. Comparing the basically consistent results of laboratory experiment and numerical simulation, the feasibility of discrete element method is verified. Evolutions of mesoscopic crack propagation and mesoscopic particle displacement field in the complete failure process are analysed. Typical stresses of granite sample and sandstone sample in the failure stage are investigated. Above combined macroscopic experimental method and mesoscopic numerical method systematically analyse the characteristics of hard rock and soft rock in the strain-softening stage. Failure process and mechanical property of hard rock and soft rock are revealed at the macroscopic and mesoscopic levels. The initiation and propagation process of micro-cracks in rock are thoroughly investigated. The research results provide a scientific foundation for the analyse of strain-softening behaviour of hard rock and soft rock. The result shows that both the mesoscopic numerical method and macroscopic experimental method indicate that the failure pattern of sandstone is influenced by both confining pressure and axial stress, while granite is mainly affected by axial stress.

Elastic–plastic solutions of reserved deformation for soft rock circular tunnel under high stress

The determination of reserved deformation is typically based on referring to relevant codes or the engineering analogy method, lacking a certain theoretical approach. The purpose of this study is to provide a theoretical basis for determining reserved deformation and to analyze the variation law of the surrounding rock affected by reserved deformation. Considering the reserved deformation under the condition of asymmetric load, the expression of optimal reserved deformation, the expression of support resistance reflecting the strength of surrounding rock, the strength of support material and the magnitude of in situ stress, the displacement expression of surrounding rock are derived by approximate solution based on the classical elastic–plastic theory. Numerical simulation software is used to simulate the displacement expression of the surrounding rock considering the reserved deformation and the expression of the optimum reserved deformation under the condition of asymmetric load. The results of the numerical simulation were compared with those of the analytical solutions, and the analytical results show that the errors between the two are within 12% and that the consistency is good.

Research on High-Strength Economic Support Technology for Soft Rock Roadway with Roof Drenching under Thin Bedrock Irregular Surface

Research on High-Strength Economic Support Technology for Soft Rock Roadway with Roof Drenching under Thin Bedrock Irregular Surface

Rock Mass Classification of Chalk, a UK Perspective

AbstractInvestigating the geomechanical engineering behaviour of Chalk is challenging since it is a generally soft rock formation and presents in cases a different behaviour from other competent rocks. Additionally, Chalk is widespread throughout the United Kingdom and other parts of the world. Thus, an in-depth understanding of Chalk is paramount when considered in engineering projects. In this research work, the geomechanical behaviour of Northern Province Chalk is examined. This study mainly involves characterising two significant units of Northern Province Chalk, namely Flamborough and Burnham Chalk, through intensive fieldwork and laboratory testing. Multiple scanline surveys are conducted along the cliffs of Flamborough to characterise the jointed nature of the Chalk. The samples collected are assessed in the lab to determine the physical properties of the intact Chalk. Moreover, the Chalk mass properties are compared to those estimated from empirical classification systems. Finally, the limitations of applying rock mass classification systems to Chalk have also been highlighted.

Study on the anchoring properties of new water glass-modified alkali-activated portland cement materials

This study developed a novel water glass-modified alkali-activated portland cement (WGSC) anchoring agent, characterized by low cost, rapid setting, early strength, and slight expansion. It is suitable for fully bonded anchor rod anchoring in soft rock tunnels. Using orthogonal experiments and range analysis, the optimal composition of WGSC was determined: silicate cement as the matrix, water-cement ratio of 0.3, 20 % sodium silicate with a modulus of 3.3, 2.5 % early strength agent, and 9 % UEA. The initial/final setting times were 6.72/8.63 minutes, linear expansion rates at 3 days/7 days were 0.32 %/0.54 %, and compressive strengths at 1 hour/24 hours were 16.4/28.2 MPa. Pull-out model tests demonstrated a 47.95–92.85 % increase in 24 h tensile bond strength compared to conventional silicate cement anchoring agents. Corrosion resistance tests indicated WGSC maintained a stable appearance compared to traditional agents. SEM analysis revealed a dense structure and clear network in WGSC, while XRD analysis identified microcomponents mainly as CSH, active silica (SiO2), and CASH, contributing to its dense structure and ideal expansion properties, facilitating superior anchoring and durability. This research elucidates the mechanism of WGSC, confirming its rapid setting, compressive strength, and corrosion resistance advantages, making it economically feasible and advantageous for application and promotion, thereby providing insights for the development of anchoring materials in highly deformed soft rock tunnels.

Study on the Susceptibility of Steel Arches with Letting Pressure Nodes Based on Incremental Dynamic Analysis

When soft rock tunnels pass through fractured fault zones, they are particularly susceptible to extrusion and large-scale deformations, especially during seismic events. To address these challenges, this study introduces an innovative yield-support steel arch design featuring a circumferential letting pressure node at its core. This design delivers incremental support resistance within the deformation zone and a susceptibility curve is applied to evaluate the damage probability of the steel arch with a letting pressure node under seismic loading conditions. Measurements of the surrounding rock pressure and structural forces on the steel arch with the letting pressure node were conducted at the Baoshan Jewel Mountain Tunnel in China. The field experiment results revealed a 23% reduction in the surrounding rock pressure and an 11% decrease in the internal forces of the support structure. These findings demonstrate the successful application of the letting pressure node-supported steel arch in mitigating large deformations in soft rock environments. Additionally, using finite element software ANSYS 2022, a seismic time-history analysis was conducted, employing the relative deformation rate of the letting pressure node steel arch as the damage index and the peak ground acceleration (PGA) as the strength parameter to generate the incremental dynamic analysis (IDA) curve. According to the susceptibility curve derived from the incremental dynamic analysis, at the design ground motion level of 8 degrees, the letting pressure node steel arch has a 94% probability of exceeding its normal service life limit and experiencing damage. The findings of this study offer a novel approach to addressing large deformations in soft rock tunnels. The proposed susceptibility curves for steel arches with letting pressure nodes provide a robust foundation for predicting the damage probability of yielding support structures under seismic conditions.

A first-principles study of the effects of temperature on the atomic and electronic structures of Mg-montmorillonite with H2O molecule adsorption

The interaction between water and soft rock rich in clay minerals is one of the main factors causing damage in soft rock tunnel engineering. The effects of temperature on the atomic structure and water absorption capacities of clay minerals cannot be ignored. In the present study, the influence of temperature on the microstructure of Mg-montmorillonite and the thermal stability of the Mg-montmorillonite (010) surface were calculated based on density functional theory. Furthermore, the adsorption properties of H2O molecules within different coverages (0 < Θ ≤ 1.0 ML) on the (010) surface were investigated in the temperature range of 0–900 K. First, the volume of Mg-montmorillonite expanded significantly with increasing temperature along the z-axis direction. The surface energy of the Mg-montmorillonite (010) surface decreased with increasing temperature, indicating that the high temperature weakened the interatomic interactions on the surface. Moreover, the calculations demonstrated that H2O molecules could still be stably adsorbed on the Mg-montmorillonite (010) surface at different temperatures, and the order of stable adsorption configurations for a single H2O molecule was hollow > bridge > interlayer bridge > top sites. With increasing coverage of H2O molecules, the adsorption energies increased for the top sites, while decreased for the bridge, hollow, and interlayer bridge sites at high temperature. In addition, the changes in atomic structure and electronic characteristics during water absorption were further explored by calculating the ICOHP, PDOS, and lattice relaxation.

Investigation of Reasonable Reserved Deformation of Deep-Buried Tunnel Excavation Based on Large Deformation Characteristics in Soft Rock

Investigation of Reasonable Reserved Deformation of Deep-Buried Tunnel Excavation Based on Large Deformation Characteristics in Soft Rock

Study on Failure Characteristics and Acoustic Emission Laws of Rock-like Specimens under Uniaxial Compression

Complex underground conditions make it challenging to conduct extensive coring, and it is difficult for laboratories to carry out a large number of rock mechanics experiments due to the limited number of cores. Rock-like specimens are commonly used in the laboratory to replace coal-rock specimens. In this paper, rock-like specimens with different proportions are produced to study the mechanical properties, failure characteristics, and acoustic emission laws of rock-like specimens under uniaxial compression. The results show that when the rock-like specimen does not contain gypsum, the stress of the specimen decreases rapidly after reaching the peak stress, which is similar to the mechanical properties of hard brittle rock. When the rock-like specimen contains gypsum, the stress of the specimen decreases slowly after reaching the compressive limit, and the failure of the specimen is gentle, which is similar to the mechanical properties of soft rock. When the specimen lacks gypsum, the strength of the rock-like specimen is larger, and the strength of the specimen is positively correlated with the proportion of cement. When the specimen contains gypsum, the strength of the rock-like specimen decreases sharply, and the strength of the rock-like specimen is negatively correlated with the proportion of gypsum. The maximum acoustic emission ringing count increases with higher cement content but decreases with increased gypsum content. The cumulative count change of acoustic emissions approximately underwent four stages, aligning well with the four stages of uniaxial compression failure observed in typical rock. The research results have important reference value for the selection of rock-like materials to replace the original rock materials for laboratory research.

Experimental study on rock drilling vibration of PDC bit in interbedded formations

Mastering the vibration characteristics of PDC bit, especially in interbedded formations, can help to explore reasonable bit failure control measures, achieve bit optimization and drilling optimization. This work used single rocks to construct different types of vertical and horizontal interbedded rocks with different interbedded properties, the experimental study obtained the bit vibration characteristics under single rock, sudden-changed rock, and alternating rock conditions. The vibration acceleration of a single rock is consistent with the rock strength. The acceleration increases with the increase of weight-on-bit and rotary speed, the influence of weight-on-bit is greater than that of rotary speed. The maximum amplitude of accelerations always occurs in the low frequency range of 0∼300 Hz, the difference between rocks is mainly reflected in the high frequency range greater than 400 Hz. When the drill bit suddenly crosses from soft rock to hard rock in the vertical interbedded formation, the acceleration increases rapidly, causing direct impact. When the drill bit suddenly crosses from hard rock to soft rock, the impact and acceleration attenuates slowly, which could cause fatigue damage to the drill bit and induce high-frequency vibration. The rapid and significant change in depth-of-cut is the fundamental reason for the impact generated when the bit crosses through the interlayer, and both the maximum and minimum depth-of-cut occur in the transition region. PDC bit alternately encountered soft and hard rocks in the horizontal interbedded formation, causing intensified vibration. The acceleration is consistent with rock difference. The influence of weight-on-bit on acceleration increases with the increase of rotary speed for horizontal interbedded formation with small rock differences. However, with the increase of weight-on-bit, the influence trend and degree of rotary speed on acceleration remain basically unchanged. The increase in weight-on-bit /rotary speed weakens the effect of rotary speed/ weight-on-bit on acceleration in horizontal interbedded formations with significant rock differences. The research results illustrate that the reason for the abnormal bit failure in interbedded formations is the frequent changes in depth of cut caused by rock mutation and alternation, and the resulting impact. This provides theoretical guidance for the implementation of effective measures such as reasonable cutter arrangement design, pertinent setting of auxiliary structures, and appropriate adjustment of drilling parameters.

Analysis of macroscopic, microcosmic, and microscopic characteristics of red-bed soft rock in Central Yunnan and its mechanical response in construction

Analysis of macroscopic, microcosmic, and microscopic characteristics of red-bed soft rock in Central Yunnan and its mechanical response in construction

A Numerical Study of the Mechanical Behavior of Jointed Soft Rocks under Triaxial Loading Using a Bonded Particle Model.

In order to master the strength and deformation characteristics, including the macro-micro failure mechanism of soft rock samples with penetrating joints under triaxial loading, a series of numerical triaxial tests have been carried out. The strength and deformation characteristics, failure modes, crack propagation, distribution of force chains, and the influences of joint dip angles and confining pressures have been analyzed and compared with the laboratory test results. The results show that (1) the residual strength ratio of jointed rock samples generally increases first and then decreases with the increase in joint dip angles under the same confining pressure and reaches the maximum value around 23-24°. Poisson's ratio increases with the increase in the confining pressure or the joint dip angle. The elastic modulus increases with the increase in the confining pressure and decreases with the increase in the joint dip angle. (2) The jointed rock samples with different joint dip angles compact with relatively small volumetric strains and then dilate up to failure with relatively large volume expansions. Lower confining pressure and smaller dip angles will lead to a more pronounced dilation phenomenon and less obvious volume shrinkage rules. (3) The low-angle jointed rock samples all exhibit the X-type shear failure. The jointed rock samples with a joint dip angle of 45° exhibit hybrid failure with both slippage and shearing, which are controlled by both the matrix and the joint. (4) The change in the number of cracks includes three stages: the slow crack initiation stage, rapid growth stage, and crack coalescence stage. The total number of shear or tensile cracks all decrease with an increase in the joint dip angles, with the number of tensile cracks being approximately twice that of shear cracks. The tension cracks are mostly horizontal, and the shear cracks are mostly vertical. (5) The number of force chains shows a decreasing trend after the cracks begin to grow. The jointed rock samples for the intact, 15° and 30° cases all form a main force chain during the failure process, while there is no main force chain for the 45° case.

On the Initial Fabric of Naturally Occurring and Reconstituted Weakly Cemented Geomaterials

The understanding of naturally occurring materials such as clay, sand, hard and soft rocks under a common theoretical framework has been a topic of persistent research interest. Over the past few decades, various sample reconstitution techniques have been developed in the literature to mimic in situ conditions, and to parse carefully the influence of various components in a cohesive-frictional geomaterial such that their behavior can be folded into the broad ambit of a continuum mechanics framework. The initial fabric of natural rock specimens is compared with reconstituted cemented sand samples using X-ray computed tomography (XRCT) scans. The efficacy of laboratory reconstitution techniques in replicating the initial microstructural features of natural rocks is evaluated here. Additionally, discrete element method (DEM) protocols which are often employed in generating cohesive granular ensembles are employed here and compared against the naturally occurring and artificially reconstituted fabric. A significant difference is observed in the grain boundaries of reconstituted and naturally occurring rocks. Additionally, the arrangement of particles, the orientation of grain contacts, and their coordination number are examined to assess the efficacy of laboratory-reconstituted specimens at micro-length scale.