Soft Rock Samples Research Articles

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.

Study on the failure modes of water-bearing soft rock under different low-frequency disturbance

To investigate the evolution of failure modes in soft rock under various low-frequency disturbances and varying moisture content, a creep impact dynamic disturbance loading system was employed. Experiments were conducted on soft rock samples with different moisture contents, while the loading process was monitored using an acoustic emission system. The fracture development and propagation characteristics were reconstructed using a CT scanning system. The experimental results reveal that: (1) The failure mode of dry soft rock transitions from “shear-tension-shear” with increasing initial disturbance, whereas water-bearing soft rock primarily undergoes tensile failure. (2) During disturbance, saturated soft rock exhibits significant fluctuations in acoustic emission characteristic parameters, with the initiation of larger fractures. As the initial disturbance increases, failure transitions from plastic to brittle, resembling the behavior of dry samples. (3) Microcracks in both dry and saturated soft rock predominantly range from 500 to 1000 µm, while those in naturally moist soft rock are less than 500 µm. Although the volume of primary fractures varies slightly among different states, the overall degree of damage increases with higher moisture content. These findings elucidate the evolution of failure modes and fracture development in soft rock under low-frequency disturbances, providing valuable insights for monitoring, early warning, and mitigation of dynamic disasters in deep mines.

Creep Energy Evolution of Red-Bed Soft Rocks in South China under Chemical-Stress-Seepage Coupling

The red-bed soft rocks in South China have obvious creep characteristics and are prone to engineering geological disasters such as landslide and foundation settlement under the action of rainfall, groundwater, and load. In order to reveal its creep characteristics and mechanism under complex conditions, a step-loading creep test was carried out under chemical-stress-seepage coupling, and the energy evolution law of the whole creep process was analyzed based on linear energy storage and energy dissipation theory. The results also show that the acid chemical solution has the greatest influence on the triaxial strength and creep strength, and the creep damage and energy evolution of red-bed soft rock are universal. The creep damage and total strain increase with the increase of acidity, the decrease of confining pressure, and the increase of seepage pressure. The evolution law of creep damage shows the characteristics of slow-acceleration-rapid growth, and with the increase of load level, it has obvious transfer and accumulation. After entering the constant velocity creep stage, the damage rate begins to accelerate. The proportion of instantaneous strain and creep strain in the total strain increment is about 50%, and confining pressure has little influence on their respective proportions. The instantaneous strain is more sensitive to the acidity of the chemical solution, and the proportion of creep strain increases gradually with the increase of seepage pressure. The relationship between elastic energy density and total energy density is linear. The elastic energy density and dissipated energy density in the loading stage and creep stage all increase nonlinearly with loading time. The density of dissipated energy in the creep phase is lower than that in the loading phase, but the opposite is true in the higher stress phase, and the law of energy dissipation can explain the hardening damage effect in the creep process of soft rock samples. The research results provide a new perspective for us to reveal the mechanical properties and failure mechanism of red-bed soft rocks and provide an important theoretical basis for predicting and evaluating the creep instability and long-term stability of such rocks.

Measurement, prediction, and modeling of the drilling specific energy by soft rock properties during the drilling operation

Drilling is one of the most important stages of mining operation, so, optimizing the drilling plays a very effective role in reducing the costs. The first step in drilling optimization is to measure the real-time energy. In this paper, the relationship between the specific energy and the rock properties has been investigated. For this purpose, after determining the 17 soft rock samples' properties, the drilling tests were performed, and the relationship between the specific energy and the rock characteristics including Uniaxial Compressive Strength (UCS), Brazilian Tensile Strength (BTS), porosity, hardness, and abrasivity was investigated using regression analysis. Correlation coefficients were obtained using this method, respectively, 0.727, 0.66, 0.673, 0.612, and 0.763. Also, the presented models were validated using RMSE, MAE, and ANOVA criteria. Rock properties alone can accurately predict drilling energy, allowing for real-time adjustment of drilling parameters and identification of non-optimal drilling times. This method is reliable and quick.

The disintegration mechanism analysis of soft rock due to water intrusion based on discrete element method

Though many laboratory tests have been carried out on the effect of water content on the disintegration characteristics of soft rock, there are still relatively few numerical simulations that can well describe this kind of process. Based on the test results obtained by previous relevant research, the two-dimensional unconfined soft rock immersion test was numerically simulated by the discrete element method. The hydration expansion of clay particles is considered in the simulation. The breaking force and residual strength coefficient in MatDEM are selected to characterize the slaking durability of soft rock. Several groups of control sample models were set up, and the effects of crack, clay mineral content, and initial water content on the softening process of rock samples were discussed. The results show that the simulation in this paper is consistent with the process of infiltration and disintegration of soft rock after water immersion in relevant studies. The strength parameters show an exponential attenuation relationship with water content. The breaking force and residual strength coefficient of the complete sample change in a gradient from outside to inside with water content. In addition, the initial crack improves the permeability of soft rock samples and changes the distribution of water content, which has a significant effect on the slaking durability of soft rock. Soft rock samples with high clay mineral content are more prone to plastic softening. The slaking durability of soft rock decreases as the initial water content increases.

Investigation of Initial Rock Damage and its Effects on Uniaxial Compressive Strength

This study investigated the initial rock damages and their effects on the uniaxial compressive strength of soft rock (claystone) and hard rock (marble) using Computerized Tomography (CT) scanning. The soft rock (claystone) samples were initially damaged before sampling. The samples were taken from three test areas with different levels of deterioration. These areas were initially subjected to different deteriorating conditions. In Area 1, the rock was exposed to natural air, called the uncovered test area; In Area 2, the rocks were wetted for three days by watering, drying, and exposing it to the sun/wind, called a wetting-drying test area. This process was conducted by wetting and drying the rocks for three days each; and in Area 3, the rock was covered with a concrete slab (in-place poured concrete), called a concrete covered test area. These different sampling methods damaged the hard rocks (marble). CT scanning experiments showed the recorded average CT numbers of the rock samples (from the highest to lowest) are: the concrete covered area, the uncovered area, and the wetting-drying area. The test results of the soft rock samples also demonstrated that a higher average CT number corresponded to a higher uniaxial compressive strength. In addition, the average CT number for soft rock undergoing different degrees of initial damage exhibits a certain regularity. The average CT number of the soft rock confirmed induced damages by the deteriorating condition. Using a nondestructive sampling method for the hard rock led to a significantly lower degree of initial damage than the samples obtained by the conventional sampling method. The marble samples obtained by the nondestructive sampling method exhibited higher average CT numbers and uniaxial compressive strength than the samples obtained by conventional sampling method.

Modelling the behaviour of a soft rock using a kinematic hardening constitutive model

Soft rocks are of great geological and geotechnical interest, due to their mechanical and structural characteristics falling between those of a soil and those of a rock. This presents several challenges for constitutive and numerical modelling. Here we propose a calibration strategy of an advanced kinematic hardening model with structure degradation against experimental data on soft rocks, accounting for their features at the micro- and macro-scales. The experimental results show a highly structured material, with strength and stiffness being controlled by inter-granular bonding as well as by mean effective stress. The clay content of soft rock samples seems to be a dominant factor: the specimens show lower stiffness and strength when the content is higher. Accordingly, the samples have been classified into two different types (A and B). The constitutive model reproduces well both the mechanically strong and stiff behaviour of Type A as well as the more ductile mechanical response of Type B specimens observed during drained triaxial compression tests. With increasing confining pressures, the transition from brittle and dilatant to more ductile and contractant behaviour observed for both types of rocks is well captured by the adopted constitutive model.

Pull-out creep laboratory test for soft rocks

This research introduces a laboratory creep test for measuring displacements in anchors embedded in large-scale soft rock samples (745 × 550 × 310 mm) under constant load for long periods. Here, the preliminary theoretical fundamentals that have served for the design of the equipment are detailed, together with its mechanical and geometrical characteristics, defining the test procedure. It has been tested a rock-like material composed of bentonite, cement and water, which has been geotechnically characterized by identification, strength and deformability tests. The specific manufacturing process of the test samples is also defined. Three creep tests have been carried out with durations between forty-six and seventy-five days each. An adequate interpretation of the instantaneous strain obtained in the test has been carried out, exposing two effects that must be considered: the greater ridigity on small strain and the scale effect.

Dynamic mechanical properties of jointed soft rock samples subjected to cyclic triaxial loading: a FEM-DEM-based study

In order to investigate the fatigue behavior of jointed soft rock samples with different joint dip angles, cyclic triaxial tests under different stress amplitudes and confining pressures were numerically simulated using the combined finite-discrete element method (FEM-DEM). The dynamic strength and deformation rules were revealed, including the masing effect, the ratcheting effect, the volumetric strain properties and the failure modes influenced by joint dip angles. During the first hysteresis loop, the nonlinearity of the loading curve is much stronger than that in subsequent cycles, and the damage and deformation are almost the most, which demonstrates that the effect of stress history on soft jointed samples subjected to cyclic loading is great, especially the first loading cycle. When the applied static and dynamic maximum deviatoric stress are the same, the vertical compressive stress of the dynamic case is found to be lower than that of the static case, and the vertical tensile stress exists in some local position, which may be the reason why the cracks can increase and propagate gradually when the dynamic maximum stress is lower than the static strength.

Experimental study of the effect of cooling/lubricating fluids on penetration rate in hard and soft rock drilling process

Rock drilling is the process of penetrating and crushing rocks, which is characterized by the low rate of penetration (ROP) and drilling rate because of the amount of cuttings that remain in the drilling environment and the thermal stresses generated at the bit-rock interface. Any improvement in ROP and drilling rate can be of great technical and economic value for drilling projects. ROP can be improved through the proper use of cooling-lubricating fluids during drilling. ROP is a key indicator of drilling performance and the ability to predict this rate can be very useful for mining projects. ROP prediction can also be used to estimate the total cost of a drilling operation. ROP depends on a variety of parameters, which, when available, can be used to estimate ROP. To reach a better understanding of the parameters affecting ROP and how it can be optimized, in this study, the results of 492 laboratory drilling tests were analyzed by linear and nonlinear multivariate regression in SPSS to build a number of models for ROP prediction. These tests were conducted on seven soft and hard rock samples while using six fluids as the cooling-lubricating agent. The tests were performed with a laboratory-scale drilling rig on cubic specimens at several different rotation speeds and thrust forces. The results showed that the drilling achieved significantly higher ROP in the presence of the cooling-lubricating agent. After reviewing the results, several models that were able to produce ROP predictions with the lowest average estimation error were picked as suitable choices for predicting ROP in the presence of cooling-lubricating fluids. The results of statistical tests of these models showed with over 90% confidence that they can produce highly accurate ROP estimates based on mechanical properties of the rock, physical characteristics of the cooling-lubricating fluid, and operating parameters of the drilling rig.

Experimental study on uniaxial compression after creep characteristics of similar soft rock containing cross cracks

In order to study the creep phenomenon and failure law of rock materials containing cracks, uniaxial compression test and uniaxial compression test after creep are carried out on similar soft rock samples containing different angles of cross cracks, and the differences of rock mechanical properties after two tests are compared and analyzed. Results show that creep effect increases the compactness of the internal structure of the sample, the elastic modulus improve, enhance the ability of resistance to elastic deformation, the peak strength increases, and 15° cross crack sample is more seriously damaged, 30°, 45° and 60° cross crack samples generate deeper and longer crack propagation and broken degree is lower, 75° and cross sample produce fewer cracks.

Water Vapor Adsorption and Main Controlling Factors of Deep Shale

Water is one of the causes of softening of soft rocks in deep strata. Water absorption in soft rock with high clay content may cause strength softening and deformation. In this paper, soft rock samples from three deep strata are investigated, and the stratigraphic characteristics such as stratum physical parameters, mineral composition, and pore structure are systematically analyzed. The characteristics of water vapor adsorption curves of deep strata soft rock are obtained by water vapor adsorption experiments. The authors analyzed the main factors controlling water vapor adsorption capacity of the rock samples. The experimental results show that the water vapor adsorption capacity increases with increase in humidity. When the humidity exceeds 0.8, the adsorption of water vapor by soft rock increases rapidly. The adsorption capacity of shale is significantly higher than that of conglomerate samples, and the difference between coarse conglomerate and fine conglomerate samples is insignificant. It is shown that water vapor adsorption correlates positively with clay mineral content, particularly minerals with high specific surface area. Under low humidity the temperature has little effect on the adsorption of soft rock water vapor. As the humidity increases, a rise in temperature can significantly increase the water vapor adsorption capacity The results also show that the soft rock water vapor adsorption capacity correlates negatively with initial moisture content, porosity, and permeability. This study is of great significance for understanding the hydrological characteristics and catastrophic processes of deep strata soft rocks.

Benthic foraminifera of the Maastrichtian sediments from the Maliboğazi (Kalecik, Ankara) and Irmak (Kırıkkale) areas in Türkiye

The investigation focuses on benthic foraminifera and their paleoecology, particularly on the Orbitoides genus in the Maaastrichtian sediments from the northern and western Ankara City (Turkey). The hard and soft rock samples from two measured sections, namely as Malibogazi (N Ankara) and Daryaka-Irmak (W Ankara, Kirikkale) were evaluated. More than one hundred equatorial and axial sections of the Orbitoides genus were prepared. The benthic foraminifera of the Maastrichtian sediments are as follows: Dorothia sp., Marssonella sp., Lenticulina sp., Saracenaria sp., Oolina sp., Cibicides sp., Gavelinella sp., Orbitoides apiculatus, O. gruenbachensis, O. medius, Pseudomphalocyclus sp., Omphalocyclus macroporus, Lepidorbitoides sp., Sirtina sp., Sulcoperculina globosa, S. vermunti, Siderolites calcitrapoides, Laffitteina mengaudi, Laffitteina sp. In the Malibogazi area, biometric data based on the internal and external parameters of the Orbitoides genus present reliable interpretations for paleoecological approaches of the Maastrichtian time. The calcareous siliciclastics comprise abundant Orbitoides apiculatus and O. gruenbachensis species. We have identified two ichnospecies as Maeandropolydora osmaneliensis and Curvichnus semorbis in the Orbitoides tests. It is thought that meandered microboring activity are related to parasitic life modes while cave shaped ones are associated with hermit type life modes. Orbitoides bearing siliciclastics overlies reefal sediments including biostromal limestones with rich rudist fauna, Thalassoniodes rich carbonates and massive, thick bedded limestones in the Maliboğazi area. In contrast, siliciclastics indicating open sea sediments outcrop widespreadly in the Irmak area. The obtained stratigraphical, sedimantological and paleontological data show that northern part of the Ankara was shallower than western part of the Ankara during the late Maastrichtian.

Analysis of the Displacement Field of Soft Rock Samples During UCS Tests by Means of a Computer Vision Technique

The measurement of rock sample displacement during laboratory testing is generally carried out by means of instrumental devices, which are capable of detecting average or local sample displacements, but are subjected to various error measurements; instrumental errors especially increase when localization takes place in the rock sample and macro-cracking develops. Photogrammetric techniques and, more recently, computer vision techniques based on non-contact digital image change detection propose an interesting alternative in this field, since they allow for detecting, with high precision, the visible displacement field of the rock sample external surface. This work is aimed at presenting the results of the application of an advanced computer vision technique to the assessment of the evolving displacement field of soft calcarenite samples subjected to uniaxial compression test. The corresponding results confirm that the technique is capable of detecting, with high level of accuracy, both the pre-failure displacement evolution, when continuity conditions still exist in the sample, and in the post-failure state, when large fissuring occur and a clear failure mechanism develops in the sample. A comparison between the results obtained from the technique here proposed and those resulting from a more conventional digital image correlation technique is also provided, highlighting a clear improvement in terms of accuracy of the images and capability of detecting the failure mechanisms of the samples.

The effect of wind pressure on surface erosion of soft rocks in arid regions

The wind acts as a land-shaping agent in desert and coastal regions, with more significant effects on soft, porous rocks than on massive rock formations. Air-suspended particles degrade rock surfaces, form cavities, and produce drift channels; however, abrasions that have been formed without the impacts of suspended particles are occasionally observed on very soft rock surfaces in areas exposed directly to the wind. The effect of wind without abrasive particles on soft and porous rock samples was examined using a newly designed wind tunnel. Samples were exposed to various wind speeds of 6–9 m/s to determine the most efficient wind speed. Ignimbrite, mudstone and sandstone specimens, all of which possess high porosity and low strength, were tested in the wind tunnel under a constant air temperature for 1 year. During that time, changes in sample weight were measured at weekly intervals. The specimens started to decrease in weight after losing surface moisture, with the highest weight loss observed in the samples with high porosity and low strength. Air pressure from the wind caused tension on rock surfaces. Surface strain measurements were carried out to determine the effects of wind pressure on weight loss. The maximum strain (250 μ) was recorded on mudstone samples with high porosity and low strength. A good relationship was detected between surficial strain and weight loss. The air pressures on the sample surfaces caused by the wind forces were measured with a new sensor. The air pressure values measured on the surface oscillated between −56 and 133 Pa.

Effects of microfabric on drillability of rocks

Quantifying the rock microfabric is one of the most important research interests and challenges in the field of rock drilling and excavation. In this study, the relationship between quantitative parameters of the rock microfabric and the rate of top-hammer drilling was studied. For this purpose, a drilling operation was performed on five soft rock samples and three hard rock samples and drilling rates were recorded. Thin sections from the rock samples were prepared and micrographs of the sections were reviewed using AutoCAD software and digital boundaries of the grains were clarified and drawn. After preparation and filtering of the digitized thin sections, four basic properties of each grain, including maximum diameter, minimum diameter, area and perimeter, were calculated with the help of the software. These parameters were then analyzed statistically and used to obtain six indexes including equivalent diameter, grain compactness, shape factor, aspect ratio, interlocking index and grain size homogeneity index. Finally, the microfabric and drilling rate data were subjected to regression analysis and the significance of the relationships between the grain parameter values and drilling rate were explored. Drilling rate was found to increase with increasing equivalent diameter and shape factor. In contrast, drilling rate decreased with increasing grain compactness, aspect ratio, grain size homogeneity and grain interlocking index. Grain compactness showed the most significant relationship (R 2 = 0.8) with drilling rate among the studied parameters.

Dynamic Behaviours of a Single Soft Rock-Socketed Shaft Subjected to Axial Cyclic Loading

The soft rock was simulated by cement, plaster, sand, water, and concrete hardening accelerator in this paper. Meanwhile, uniaxial compressive strength tests and triaxial compression tests were conducted to study the mechanical properties of simulated soft rock samples. Model tests on a single pile socketed in simulated soft rock under axial cyclic loading were conducted by using a device which combined test apparatus with a GCTS dynamic triaxial system. Test results show that the optimal mix ratio is cement : plaster : medium sand : water : concrete hardening accelerator = 4.5% : 5.0% : 84.71% : 4.75% : 1.04%. The static load ratio (SLR), cyclic load ratio (CLR), and the number of cycles affect the accumulated deformation and cyclic secant modulus of the pile head. The accumulated deformation increases with increasing numbers of cycles. However, the cyclic secant modulus of pile head increases and then decreases with the growth in the number of cycles and finally remains stable after 50 cycles. According to the test results, the development of accumulated settlement was analysed. Finally, an empirical formula for accumulated settlement, considering the effects of the number of cycles, the static load ratio, the cyclic load ratio, and the uniaxial compressive strength, is proposed which can be used for feasibility studies or preliminary design of pile foundations on soft rock subjected to traffic loading.

An Experimental Study on Measurement Methods of Bulk Density and Porosity of Rock Samples

Density and porosity are fundamental and important physical properties of rocks in various geological problems, and affect the other physical properties. Therefore, measurements of density and porosity of rock samples are important investigation items in both geo-science and geo-engineering areas. Several measurement techniques of the density and porosity are available and being applied currently. To ensure the data quality and to conduct its quality assessment, comparison of measurement results by different measurement techniques is necessary since the techniques are based on different principles and test procedures. In this study, we collected eight types of rock samples including a gabbro, a granite, four sandstones, a welded tuff and a mudstone as study materials, and also prepared several metal specimens for the experimental comparison. The porosities of the eight rocks covered a very wide range from 0.3% to 50% approximately. We employed three methods (caliper, buoyancy and helium-displacement pycnometer) to measure volumes of regularly-shaped specimens and to determine their bulk densities and porosities. As a result, the three techniques yielded almost same bulk densities and porosities for all the specimens. In addition, we also applied mercury intrusion porosimetry to measure density and porosity as well as to determine pore size distribution of the rock samples. Porosity values obtained by the porosimetry method were underestimated in the case of high-porosity (soft) rock samples and overestimated for the very low-porosity rock samples. Ability to determine pore size distribution, however, is a very important advantage of the porosimetry method.

Geomechanical and water vapor absorption characteristics of clay-bearing soft rocks at great depth

The geological and physico-mechanical properties characterization of deep soft rocks is one of the critical scientific issues for deep soft rock engineering. In the present study, X-ray diffraction (XRD) analysis, scanning electron microscope (SEM), and mercury intrusion porosimetry experiments were carried out to investigate the mineral compositions, microstructure and porosity characteristics of the 13 clay-bearing soft rock samples collected from a deep coal mine in China. Water vapor absorption and uniaxial compressive experiments were also performed to examine water absorption characteristics and water-induced strength degradation effect of the investigated deep soft rock samples. The results show that the dominant mineral components in mudstone, coarse sandstone and fine sandstone samples were calcite, quartz and clay respectively. The contents of clay minerals in all samples were relatively high and ranged from 12.3% (N-4) to 56.5% (XS-1). Water vapor absorption processes of all the soft rock samples follow an exponential law which is very similar to the water vapor absorption behavior of conglomerate samples reported in our earlier study. Correlation analyses also suggested that there were good positive correlation relationships between water absorptivity and clay minerals for both mudstone and sandstone samples. Furthermore, it was found that vapor absorption was not correlated with the porosity for mudstone, however, positive correlation relationship was found between them for sandstone. Correlation analysis between UCS, modulus of elasticity and water content demonstrated that both of them tend to decrease with the increase of their water content due to water absorption.

ESTIMATING UNSATURATED HYDRAULIC PROPERTIES OF SOFT ROCK BY THREE DIFFERENT FLOW MODELS

Several countries utilize the sedimentary soft rock for nuclear waste disposal because it has a potential to use as a host rock. Therefore, the analytical researches on the unsaturated flow of this type of rocks have been increased to assure the safety of the disposal. In this study, the applicability of three models; Brooks-Corey/Burdine (BC), Campbell (CB) and van Genuchten (VG) that have been used commonly for soil was studied, by applying these to one dimensional flow induced by evaporation in six disk shaped soft rock samples. The best parameters in each model were inversely estimated by adopting genetic algorithm (GA). It was clearly found that the best parameters in BC and CB models can be easily estimated and the measured evaporation change could be well analyzed. However, the parameters in VG model could not be estimated easily. It was found that BC and CB models are more robust than the VG model for the analysis of unsaturated flow in soft rock.