For the problem that how the grouting reinforcement of a single penetrated structural plane affects the bearing capacity of surrounding rock, an approach of making rock samples with single penetrated fracture surface is proposed firstly, and the rock samples are injected with ultra-fine cement and epoxy resin, respectively. Then, the strength and deformation characteristics of the consolidated body under uniaxial (triaxial) compression are investigated with the RMT-150C experimental system. Finally, the micro consolidation mechanism of structural plane grouting is revealed by means of theoretical analysis and electron microscope scanning. It is found that the fracture surface generated under triaxial loading-unloading condition is closer to the engineering practice and meets the experimental requirements. The stress-strain characteristic curves of the specimens injected with superfine cement show phased deformation characteristics, and the curves of the specimens injected with epoxy resin are relatively smooth. For the strength characteristics, the influence of confining pressure is significantly larger than that of grouting material selection. Grouting reinforcement can improve the residual strength of rock mass. With the increase of confining pressure, the increase of improvement coefficient of residual strength is less obvious, and the peak strength of consolidated body is closer to that of intact rock. For the failure characteristics, the main fracture surface of the specimen injected with ultra-fine cement shears and slips along the original fracture surface, and the main fracture surface of the specimen injected with epoxy resin is a new penetrated fracture surface. Finally, based on the Mohr-Coulomb criterion, the relationship between the bonding force of grouting materials and the strength improvement coefficient of structural plane is established. It is found that the strength improvement coefficient of structural plane has a linear relationship with the bonding force of grout. The comparison between calculation and test results shows that the formula is reasonable and can provide reference for the optimization of deep surrounding rock support.

Dynamic load and groundwater have a significant influence on the safety and stability of engineering rock mass during underground blasting excavation. In order to study the influence of dynamic load and water content on the failure and energy dissipation characteristics of rock mass, dynamic impact tests including four impact velocities and six water content levels on red sandstone were carried out with an improved SHPB test device. By analyzing the variation laws of energy reflectivity, transmissivity, and dissipation rate under different water contents, an empirical model representing the relationship between energy dissipation characteristics and water content of red sandstone was established. Screening tests were conducted on specimen fragments, and the variation law of fractal dimension of rock fracture with water content was studied according to the fractal theory. The results show that: 1) Under the same impact velocity, the energy transmissivity and water content have an exponential function relationship and a negative correlation, the energy dissipation rate increases first and then decreases with the increasing water content, and they have a quadratic function relationship. 2) Under the same water content, the energy transmissivity is negatively correlated with the impact velocity, while the energy dissipation rate is positively correlated with the impact velocity. 3) For the specimens with macroscopic failure, the failure degree of red sandstone increases with the increase of water content, with a turning point at the water content of 1%, and the fracture fractal dimension has an exponential function relationship with water content.

Generally, synthetic fiber, mineral fiber and plant fiber are added into soil to enhance the strength and deformation resistance of soil. The unconfined compressive test and splitting tensile test of three fiber-lime-soils under freeze-thaw cycle were carried out to study the variation of compressive and tensile properties of soil with freeze-thaw times. The test results showed that the optimum fiber rates of polypropylene fiber-lime-soil, basalt fiber-lime-soil and palm fiber-lime-soil were 0.2%, 0.2%, and 0.4% respectively, whether under freeze-thaw cycle or not. With the increase of freeze-thaw times, the compressive strength and tensile strength of the three fiber-lime-soils demonstrated phased downward trend, and the failure strains of fiber-lime-soil were greater than that of lime-soil. Under freeze-thaw cycle, the compressive strength, tensile strength, and deformation resistance of polypropylene fiber-lime-soil were better than those of basalt fiber-lime-soil and palm fiber-lime-soil. The interface force and the spatial constraint between fibers and soil particles enhanced the freeze-thaw durability of soil. By comparing and analyzing the test results of three types of fiber-lime-soil, it was found that the freeze-thaw resistance of polypropylene fiber-lime-soil was the optimum.

Helical piles have received extensive attention in recent years due to its advantages such as convenient construction, load-bearing capacity after installation, and recyclability. However, the previous research was mainly based on the premise of ignoring the installation effect. When the installation effect was considered, researches mainly focused on the degree of disturbance to the relative density of sand. The experimental study of the installation effect in clay is not enough. In response to this problem, a model test was carried out to obtain the load-displacement relationship curves of different types of helical piles. For the helical pile with a single plate, whether the installation effect is considered or not, the variation trends of the ultimate bearing capacity with the increasing of the embedment depth ratio are basically the same, but the ultimate bearing capacity decreases significantly when installation effect is considered. For double-plate pile, after the spacing is greater than 2D, the individual bearing model occurs, and each plate can play its bearing capacity independently. Increasing the number of plates can appropriately improve the bearing capacity, but when the failure model transitions from individual bearing model to cylindrical shear mechanism, the increase of bearing capacity is no longer obvious, and at this moment the critical spacing ratio (S/D)cr is between 1.5 and 2.0. Subsequently, a concept of “disturbance coefficient” is put forward to quantitatively evaluate the reduction of bearing capacity caused by helical pile installation. For the helical pile with a single plate, the disturbance coefficient varies from 0.5 to 0.6 when embedment depth ratio is greater than 4D, and increasing the spacing ratio will lead to an increase in disturbance coefficient ranging from 0.20 to 0.45. For multiple plates, the coefficient lies between 0.4 and 0.6.

Due to the complexity of the reinforced soil interface, the empirical methods are often used when laying geogrid in reinforced soil engineering construction, which largely causes the waste of geogrids and engineering safety hazards. Clarifying the influence range of the interfacial action of reinforced soil with different fillers is helpful to determine the reasonable reinforcement spacing of reinforced soil structures. In order to reveal the influence range of interface action between reinforcement and soil of different fillers, a series of pull-out tests was carried out with four types of sand and geogrid under different normal stresses. By combining with digital image measurement technology, the evolution laws of interfacial shear band thickness, particle displacement vector, peak pullout resistance, and strain of geogrid for different types of sand were analyzed. The results show that the thickness of interfacial shear band H increases with the increase of normal stress σ_v and the average particle size of sand d₅₀. Through the multivariate fitting method, the function expressions among the thickness of the interfacial shear band H, the normal stress σ _v and the average particle size of the sand d ₅₀ are obtained. In the pull-out process of geogrid, the displacement vector of sand particles is significantly different when the geogrid is taken as the boundary. The displacement vector of particles above the geogrid is significantly larger than that below the geogrid, and there is a concentration band of particle displacement vectors in a certain range above and below the geogrid. The peak pullout resistance increases with the increase of normal stress σ _v and the average particle size of sand d ₅₀. The geogrid strain of each section of different types of sand shows a decreasing trend from the front to the rear.

Predicting the initiation pressure of hydraulic fracturing is of great significance for oil and gas exploitation, in-situ stress measurement and anti-cracking design of hydraulic structures. In this paper, the discontinuous numerical simulation method of particle discrete element combined with domain-pipe flow model is applied considering the fluid-solid coupling. Based on the front-advancing method, the granular model with regular-shaped borehole is generated to analyze the meso cracking process and initiation pressure of hydraulic fracturing. According to the simulated results, the distribution of contact force chain on borehole wall is consistent with the theoretical solution on the basis of eliminating the irregularity of borehole shape. The fitted initiation pressure formula by DEM is also close to the theoretical solution. Furthermore, the difference between the fitting formula of initiation pressure and the theoretical solution is explained from the perspective of the local tensile force generated when the granular material is compressed. Finally, a method for preparing impermeable mortar samples with prefabricated boreholes is developed. The initiation pressure under different combinations of principal stresses is quantitatively studied based on the true triaxial laboratory tests, which verifies the reliability of the discrete element simulation results.

Particle impact, a new drilling technology, has been applied in drilling and gas and oil exploitation. Also, it is considerably promising as an assisting rock breakage method for the excavation of tunnels in extremely hard rocks. In this paper, an experimental study was conducted to investigate the effects of particle impact number, particle strength, and particle impact velocity on the damage and fracture characteristics of surface crater in extremely hard granite. The three-dimensional morphology, rock fragment distribution, and fracture properties in the crater were quantitatively analyzed. The results indicate that the maximum depth of the crater increases in a parabolic way, while both the volume and surficial area of the crater grow linearly with the impact number. Moreover, the crater volume first increases and then decreases with the particle impact velocity, with a critical velocity of nearly 82.5 m/s. In addition, the mechanism difference of mesoscopic fragmentation inside and outside the crater center causes the double-peak characteristics of average size of fragments. The increase of volume, surficial area, and maximum depth of the crater are in linear relation to the kinetic energy of impacting particles in the double logarithmic coordinate. The fractal dimension variation trend of internal crack distribution in main minerals around the crater under different impact velocities and numbers were obtained through image-processing method. The experimental results manifest that the damage scale of rock crater is enlarged through improving particle velocity or impact number, while the former exerts a more significant effect.

In order to study the instability and failure mechanism of roadway surrounding rock under dynamic load, the stress wave propagation law and spallation failure characteristics of pre-stressed end anchorage body under different impact pressures (0.2, 0.3, 0.4, 0.5, 0.6 MPa) are studied using SHPB test system. The spallation strength and strain rate of anchorage body specimen are calculated by graphical method, and a spallation damage model of anchorage body is established and verified by high-speed camera. The results show that the peak strain of stress wave decays exponentially under different impact pressures, and the spatial attenuation amplitude and attenuation index are positively correlated with the impact pressure. The spallation of the anchorage body specimen first occurs near the free end, and the spallation thickness increases gradually along the reflected wave propagation direction. When the impact pressure is above 0.3 MPa, a new spallation occurs in the middle of every two spallation surfaces of the test piece. In the range of 18−33 s⁻¹ strain rate, the effect of strain rate is dominant, and the spallation strength is up to 69 MPa. While the strain rate is 41 s⁻¹, the spallation strength decreases to 17 MPa. The free end of the anchorage body shows spallation closure after impacted by dynamic load, but spallation closure is not observed at the spallation position at the anchor end due to the residual cohesion at the anchorage interface.

This research evaluates the potential of cement stabilized recycled asphalt pavement (RAP)/marginal lateritic soil blends as stone column aggregate instead of the traditional quarry aggregate. The undrained shear response of the blended materials at various RAP replacement ratios and effective confining pressures are investigated. The RAP replacement ratios were 10%, 30% and 50% by dry weight and ordinary Portland cement contents were 1% and 3%. It was evident that RAP replacement increased large particles and meanwhile reduced fines particles; hence the increased compactibility. Under applied effective stress lower than pre-consolidation pressure, RAP-soil blends exhibited strain-hardening behavior associated with decreased pore pressure. The strain-softening behavior in stress-strain curve for cement stabilized RAP-soil blends was diminished when RAP replacement ratio increased. The role of cementation improved the cohesion while friction angle insignificantly unchanged. The strength and stiffness of cement stabilized RAP-soil blends is mainly dependent upon the cementation bond strength and RAP replacement ratio. Shear strength improvement increased with the increased RAP replacement ratio for both unstabilized and cement stabilized RAP-soil blends, while stiffness of cement stabilized RAP-soil blends decreased due to high energy absorption of asphalt binder.

In order to study the changes of mineral composition, micro-structure evolution characteristics and their relationship with dynamic mechanical properties of granite after high temperature, 20−1 000 ℃ high temperature tests were carried out based on the Beishan granite from Gansu Province. Microscopic images of granite after high temperature treatment were obtained by means of slicing technique and polarizing microscope. The evolution of mineral composition, mineral content and micro-structure characteristic parameters of granite with temperature were studied through mineral composition analysis, micro-cracks characteristic identification and parameters calculation. Subsequently, dynamic impact compression tests of high temperature treated granite were carried out by split Hopkinson pressure bar (SHPB) system, to analyze the effects of temperature, impact rate and micro-structure characteristics on dynamic peak stress. On this basis, the quantitative relationship between micro-structure characteristics and macroscopic dynamic mechanical properties was established. The results show that high temperature has a significant effect on the mineral composition, micro-structure and impact compression peak stress of granite, and 600 ℃ is taken as the threshold temperature. The impact rate also significantly affects the impact compressive strength, and the higher the impact rate, the less the influence of temperature on the macroscopic dynamic mechanical properties. The evolution of the micro-structure of granite after high temperature is the internal mechanism of the changes of its macroscopic dynamic mechanical properties with temperature. The characteristics of micro-cracks are significantly correlated with the dynamic peak stress, and the average width of cracks is the main factor affecting the dynamic peak stress of granite.

In order to study the effect of strain rate on the mechanical properties of undisturbed expansive soil, the consolidated undrained triaxial shear tests at different rates and confining pressures were conducted by the GDS triaxial test system. The variations of stress-strain curves, pore water pressure, shear strength and failure pattern influenced by strain rates were analyzed. The results show that the stress-strain curves of expansive soil show a strain harden pattern at different strain rates. With the increase of strain rate, the undrained shear strength increases monotonically. After introducing the strain rate parameter ρ _0.9, it is found that the undrained strength increases by 14.3% to 23.2% for a 10-fold change in strain rate, with an average value of 18.4%. The strain rate has less effect on the pore water pressure at a low confining pressure, and the development trend of pore water pressure changes from softening to hardening type with increase confining pressure, and the peak pore water pressure decreases with increase strain rate. The strain rate effect of undisturbed expansive soil is closely related to its fissures. The failure pattern is manifested by the coexistence of main and sub shear zones at smaller strain rates, while the failure pattern is only the main shear zone at larger strain rates. The presence of fissures or multiple shear zones enhance the strain rate effect of expansive soil dramatically.

In the past 50 years, the rainfall in the Qinghai-Tibet Plateau has shown a fluctuating increase trend, and summer rainfall is the main source of annual rainfall. The changes in rainfall will inevitably lead to change of active layer thermal-moisture dynamics. However, there are few literature reports on the effects of summer rainfall changes on the hydrothermal state of permafrost. Therefore, based on the water-vapor-heat coupling model that optimizes the surface energy balance boundary conditions, the measured meteorological data in the Beiluhe area of the Qinghai-Tibet Plateau in 2013 was used as the model-driven data to study the response mechanism of the active layer after the summer rainfall increased to 2 times. The results show that after the summer rainfall increases to 2 times, the net radiation and latent heat of evaporation increase, while the sensible heat and surface heat flux decrease. The energy exchange between the surface and the atmosphere is converted from sensible heat to latent heat of evaporation, and the overall heat used to heat the active layer is reduced by 1.4% in summer. The increase in summer rainfall leads to an increase in the transport flux of liquid water, while the transport flux of water vapor decreases. As a result, the decrease of the heat transport by conduction, the convection by water vapor, and the latent heat of water vapor diffusion is about 2.6 times of the increase of the convection by liquid water. Therefore, increased summer rainfall leads to the cooling of the soil, and the thickness of the active layer raised by 0.14 m as a whole, which has a cooling effect on the frozen soil.

Buckling instability may occur when pile foundation passes through large caves, and the critical buckling load is the ultimate bearing capacity of pile foundation. To solve the buckling problem of pile foundation crossing single layer karst cave in karst area, a total potential energy equation of pile foundation crossing single layer karst cave is established according to the principle of energy method. According to the catastrophe characteristics of pile foundation buckling, the cusp catastrophe theory is introduced, and a cusp catastrophe model of pile foundation instability under the condition of elastic embedding at the top and fixity at the bottom is established. The bifurcation set equation of the system is derived, and then by analyzing its instability conditions, the method of calculation of buckling critical load of pile foundation crossing single-layer karst cave is developed. In order to verify the rationality of the proposed method, laboratory model tests of pile foundation crossing karst caves with different heights are carried out. Some conclusions are drawn. 1) The compression failure and buckling failure are main forms. When the height of the cave is less than 6d, the foundation pile fails due to material compression failure. When the height of the cave is greater than or equal to 6d, the buckling occurs in pile foundation. The maximum displacement point is observed to be located at the midpoint of the karst cave section. 2) The critical buckling load of pile foundation decreases linearly with the increase of cave height. The proposed method is suitable for the buckling of pile foundation, and it is in good agreement with the experimental results. 3) The small cave height, thick roof, great elastic modulus of the pile foundation, and rough pile-rock interface can result in a great critical buckling load, and not vice versa.

In order to explore the contact law of asperities in the joint during shearing process, three-dimensional scanning and direct shear tests were carried out on the tensile joint surface. The contact area and contact angle of asperities in the joint surface shearing process were analyzed by 3D point cloud tracking technology, and a shear stress model was established based on the contact law of asperities. In this process, the following conclusions were obtained: 1) In the shear process, with the increase of shear displacement, the contact area gradually decreases, and the contact points change from scattered distribution to centralized distribution. The area of a single contact point gradually increases, and the number of contact points gradually decreases. Normal stress has a significant impact on the contact area. The larger the normal stress, the larger the contact area. 2) Before the peak value of shear stress, most of the contacting asperities in the upper rock block are in a “climbing” state, and after the peak value of shear stress, some contacting asperities are in a “deviating” state. Before the peak of shear stress, most of the asperities contacted can provide shear resistance, while after the peak, some asperities cannot provide shear resistance. 3) The contact angle approximately presents a normal distribution and the average contact angle first increases and then decreases with the increase of shear displacement. 4) The verification results of the model in this paper show that the model can not only describe the shear strength of joint surface under different normal stress conditions, but also reflect the relationship between shear stress and shear displacement, which proves the rationality of the model. The model in this paper provides a new calculation method for accurately predicting the shear strength of joint surface.

The penetration angle of pick is one of the key factors in the design of tooth arrangement system of double-wheel trench cutter. Based on the continuum-discontinuum element method (CDEM), a numerical model for picks to penetrate the rock at different angles is established, and the elastic-damage-fracture fracture process of rock mass is realized by the fracture energy constitutive model. Nine groups of impact penetration rock breaking simulations are carried out within the pick’s effective angle range of 50º to 90º. The influence of penetration angle on the rock breaking performance of pick is studied, and the influence of penetration angle on the geometric arrangement of pick is discussed. The geometric arrangement principle of the pick of the tooth arrangement system is summarized. The results show that when the penetration angle decreases from 90º to 50º, the rock mass changes from tension and tension-shear fracture to shear and tension-shear fracture, the average penetration force and crushing degree of pick increase, and the leaping invasion characteristic of pick decreases significantly. Type I picks with penetration angles of 75º−90ºand type III picks with penetration angles of less than 55º play an auxiliary role in rock breaking, while type II picks with penetration angles of 55º to 75º can produce a wide range of horizontal non-penetrating cracks in the rock mass and play a major role in the process of breaking rock mass in the tooth arrangement system. Two type II picks have successively penetrated into the rock mass between the better broken sections of the rock mass, two type II picks forming the mechanical model of the basic rock breaking unit, and several basic rock breaking units are arranged according to the sinusoidal form, thus forming a design method of tooth arrangement system. The research results provide a theoretical basis for perfecting the design method of the tooth arrangement system of the double-wheel trench cutter and breaking the technical barriers in the design method of the tooth arrangement system of the double-wheel trench cutter abroad.

Earthquakes are frequent in Northwest China, and the earthen sites in this region have been adversely affected by seismic activity for a long time. In order to study the dynamic characteristics of the earthen site soil treated by enzyme-induced calcium carbonate precipitation (EICP), earthen site soil samples with initial dry densities of 1.55, 1.65, and 1.75 g/cm³ were subjected to EICP treatment, and a control group without EICP treatment was set up. Dynamic triaxial tests under different confining pressures and vibration frequencies were carried out. The results show that under the same dynamic stress, the dynamic strain of the experimental group is smaller, the energy dissipation is less, and the damping ratio is smaller compared with the control group. After EICP treatment, the dynamic strain could be reduced with the increasing of the dry density, confining pressure, and vibration frequency but this effect diminishes in turn. The dynamic constitutive relation is in line with the Hardin model, and the influence of dry density, confining pressure, and vibration frequency on the model parameter a (dynamic elastic modulus) gradually decreases. Microstructure analysis show that after EICP treatment, calcium carbonate precipitates and adheres to the surface of soil particles, filling the pores, and cementing with the soil particles to form a dense structure, as a result, the strength of the soil structure is improved.

The soil-water characteristic curve shows hysteresis when boundary conditions such as temperature and water head change cyclically. To simulate the hydraulic properties of the unsaturated soil under temperature cycle, this paper proposes an empirical model to reflect the hysteresis effect. The model assumes that the curve shape of each scanning curve is similar to the corresponding boundary curve, therefore, using the equations of the two boundary curves, the model is capable of predicting all the scanning curves without introducing new parameters. In this paper, a soil column test using kaolin clay as the medium and dry-wet cyclic driven by temperature is designed. In the impermeable soil column, the water bath is used to make an end of the soil change temperature cyclically. The results show that the pore water will migrate from the high temperature to the low; after the temperature drops to the initial state, the distribution of water content will not completely return to the initial state, showing a hysteresis effect. Using the hysteresis model in numerical calculation can reflect the hysteresis in the water migration process in soil. The existing results of sand and silt also verify the effectiveness of the proposed model. This model is easily to calculating and programming, and is capable of being applied to engineering problems such as geological disposal, heating pipeline design.

In order to improve the reinforcement effect of microbial induced calcite precipitation (MICP) technology on sand, the test of MICP-treated sand reinforced by pozzolan was designed and carried out considering the porous structure and activity characteristics of pozzolan. The enhancement effect and enhancement mechanism of pozzolan on the MICP-treated sand were systematically analyzed by integrating macroscopic physical and mechanical tests and microscopic tests. The results showed that: 1) Pozzolan could significantly improve the bacteria fixation rate and cementation material production in the process of sand microbial reinforcement, and the optimal amount of pozzolan content was around 10%, which increased the bacteria fixation rate by 118.28% and the cementation substance production by 29.55% compared with the conventional MICP. 2) The addition of pozzolan greatly improved the compressive strength and resistance to deformation of bio-cemented soil. Under different confining pressures, the compressive strength of the bio-cemented sand increased by 52.26%−62.96%, and the strain at failure increased by 100.00%−112.58%. 3) After the addition of pozzolan, the pore size and void ratio of the bio-cemented sand were decreased obviously, and the overall compactness and impermeability performance were further improved, with the void ratio decreased from 20.12% to 14.17% and the permeability coefficient reduced by an order of magnitude. 4) The enhancement mechanism of pozzolan on MICP-treated sand mainly included three aspects: for one thing, the pozzolan played a good filling effect between the sand particles, which significantly reduced the large pores between the particles and increased the compactness of the sample; for another, the good adsorption of pozzolan effectively increased the content of bacteria in the sample, which increased the production of calcium carbonate and enhanced the uniformity of distribution; on the third hand, the active substances in pozzolan participate in the reaction to generate gelling substances and calcium carbonate crystals to form a composite gel, which could further enhanced the cementation properties and compactness of the bio-cemented sand.

Reinforced soil abutment is widely used in various engineering constructions because of the good composite performance. The study of its working performance under actual conditions is of great significance to design and popularization. Reinforced soil abutment is mainly subjected to traffic loads in service. In this study, an accelerated stress test of large-scale model was carried out to investigate the working performance of reinforced soil abutment under traffic loads, and to analyze the deformation characteristics of the abutment and the strain of the reinforcements under different loading levers and numbers of loading cycles considering some factors such as spacing and stiffness of the reinforcement. The results indicate that the overall deformation of the reinforced soil abutment tends to converge under traffic loads. The settlement at the top of the abutment, the lateral displacement of the facing and the strain of the reinforcements are much less than the threshold suggested by the current specification. The deformation exhibits stepped variations over loading time and the convergent tendency differs under various conditions. Increasing the stiffness of reinforcement and reducing the spacing of reinforcement are beneficial to improve the ability of deformation resistance of the abutment, while the stiffness of reinforcement plays a more important role in deformation control. In addition, the quantitative relationship between the maximum lateral strain at the facing and the maximum vertical strain at the top of the abutment is not two times, and the quantitative relationship between the two needs to be further studied.

The dynamic response of rock in high stress state under low-frequency disturbance load are important factors affecting the stability of interlayer strata during upward mining in residual mining area. Brazilian splitting test on sandstone in high stress state under dynamic and static load coupling was carried out using the self-developed dynamic and static load coupling test machine. The effect of low frequency disturbance load amplitudes on the mechanical response characteristics of high static stress sandstone was analyzed. The results show that: 1) The sandstone in high stress state will not be destroyed in the process of small disturbance load, but the disturbance damage will reduce the tensile strength of rock specimens. Therefore, the appropriate reduction factor should be introduced into the upward mining stability analysis based on the static mechanical parameters of interlayer rock. 2) The fatigue life of sandstone decreases with the increase of the amplitude of the disturbance load. The average number of dynamic load cycles before failure of the sandstone subjected to the load the amplitude of 5.0, 7.5 and 10 kN is 157, 70 and 3, respectively. 3) The “turbulence” describing the fluid state is introduced to visualize the fast input and output of additional energy in the process of disturbance load. The additional energy flow in the state of “turbulence” weakens the orderliness of sandstone crack propagation, and induces large-scale crack propagation at the moment of sandstone failure. The research results can provide certain reference for the stability analysis of upward mining in residual mining area.

In order to explore the simulation method of layered soft rock in model experiment, relying on the indoor test results of samples selected from a layered soft rock tunnel of Jiuzhaigou-Mianyang Expressway, the optimal ratio of barite powder, quartz sand, gypsum powder, talc powder and water was determined by many tests to simulate soft rock matrix. The film with holes was used to simulate the adhesion of weak surface of bedding, and the porosity was determined by direct shear test. And direct shear and uniaxial and triaxial tests were carried out on the samples with different bedding angles and thickness to reflect the anisotropy. The optimal ratio of soft rock matrix simulation is 0.55:0.15:0.07:0.06:0.17. Barite powder plays a decisive role in strength and failure deformation. Barite content is too low for easy crushing and too high for producing top-to-bottom penetration cracks. The film with 30% porosity has the best effect on simulating bedding. The strength of soil sample changes in U-shape with the layer angle, and the strength decreases with the decrease of the thickness of layers (no less than 2 cm). In the direct shear test, the shear plane of 45º bedding is inclined to the direction of bedding, while the shear plane of 90º bedding leads to cracks and fractures on both sides of the shear plane. In uniaxial and triaxial results, for 0º bedding, small angle inclined cracks generate, for 45º bedding inclined cracks perpendicular to the bedding plane and secondary cracks arise, and for 90º bedding vertical splitting along bedding plane emerges. The optimal layer thickness is determined to be 3 cm after compared with the field results. The direct shear test of layer angles and thickness is consistent with the results of uniaxial and triaxial tests, revealing the process of pore compaction closure-elastic-plastic expansion crack failure-creep of soil samples.

In the downward drift cut and fill mining method, the stability of the backfill roof is very important to the safety of the mining process. However, the calculation of the superimposed load of the slicing and backfill is always a difficulty in the stability analysis of the roof. In this paper, an equilibrium differential equation of the roof is derived and its theoretical solution of the static load of the roof is obtained by fully considering the engineering practice such as the load of the mining rock, the dip angle of the ore body, the staggered arrangement of the adjacent layered roadway, and the contact between the backfill and the surrounding rock. By combining with the mining process, a mechanical model of “multi-span beam” is established, and the theoretical calculation formula of tensile stress of roof is obtained. The four important theoretical factors affecting stability of roof were analyzed: the upper load of roof σ _v, the roadway span l, the thickness of 1:4 backfill h, and the tensile strength of backfill itself [σ _t]. In order to fully consider the influence of static load and blast load on the roof, FLAC^3D is used to simulate the orthogonal calculation of roof stability under the influence of multiple factors. According to the simulation results, the influence of various factors on the roof tensile stress is analyzed, and the stability evaluation model of roof under the influence of multiple factors is established by using the method of multiple nonlinear regression. The model is applied to the field practice of a copper mine test stope, showing good practicality.

The existence of underground shallow gas will pose significant hazard and risk to the investigation, construction, and subsequent operation of subway projects. In this study, the probabilistic method based on geostatistics and reliability analysis is used to analyze the resistivity piezocone penetration test data of the Tangxinline station site of Hangzhou Metro Line 7. Regression analysis is performed to remove the trend term of resistivity data, and the variogram function is used to describe the anisotropy of the resistivity residuals in the vertical and horizontal directions. Kriging interpolation is used to obtain the optimal linear unbiased estimation of non-sampling points. By considering the resistivity background values of different soil types, three-dimensional probability maps of the subsurface shallow gas are obtained using the first-order reliability method. The results show that the resistivity residual has significant anisotropy, and the autocorrelation distances in vertical and horizontal directions are 5.1 m and 55.6 m, respectively. In the vertical direction, the shallow gas is distributed in the vicinity of 25 m and the depth interval of 30−35 m, and in the horizontal direction, it is mainly distributed around RCPTU1 point. The results of this paper can provide basis for shallow gas monitoring plan, exhaust hole layout and other treatment measures.

Although piezocone penetration test (CPTU) is widely used in engineering geological exploration, it has undeniable deficiencies in detection accuracy of shallow gas-bearing strata. Based on CPTU penetration principle, the shear deformation characteristics of soil at cone shoulder during penetration are analyzed. Furthermore, using the consolidated undrained triaxial shear test of dense gassy sand with different initial saturations, the response law and difference of excess pore pressure in soil at cone shoulder are discussed when CPTU probe penetrates into saturated and gas-bearing strata. The results show that, the presence of gas can significantly affect the shear deformation characteristics of sand, and with the increase of gas content, the shear volumetric strain increases, and transitions from shear dilatancy to shrinkage; however, the sensitivity of pore pressure response in soil decreases, leading to a non-negative characteristic phenomenon, which can be regarded as an important symbol of gas-bearing strata identification. According to the characteristics of shallow gas reservoir distributed in Jiangsu and Zhejiang coastal plain in China, a fine identification method of gas-bearing strata based on in situ multi-function piezocone penetration test is developed. In this method, the “dual control” index composed of silt or fine sand soil property identification and non-negative characteristics of excess pore pressure is used for preliminary judgment, and then combined with multi-functional auxiliary parameters for further inspection and correction. The validity of identification method is preliminarily verified in a practical engineering, and the identification accuracy of multi-layer gas-bearing strata is thereby improved.

The classical Green-Ampt infiltration model is simple in calculation and widely used in the study of soil infiltration. However, there exist some certain theoretical assumptions for the obvious dry-wet separation interface in the treatment of wetting front in the model. In view of this, based on the layered model of saturated zone-unsaturated infiltration zone and combined with Darcy’s law, a theoretical formula of the thickness relationship between the saturated zone and the unsaturated infiltration zone during infiltration was deduced in this paper and a theoretical model of water profile was proposed, which can eliminate the traditional assumption of the thickness relationship between the saturated zone and the unsaturated infiltration zone. On the basis, an improved Green-Ampt model considering the shape of soil moisture profile and the equivalent parameters in the unsaturated infiltration was established. The model was verified by experiments and finite element numerical simulation. The results show that the model has high accuracy and good adaptability to different types of soil. Meanwhile, the model is simple to solve with clear physical meaning parameters, and is convenient for engineering application.

The boundary element method (BEM) and discrete element method (DEM) are combined to calculate the internal stress and breakage paths of brittle granular materials. DEM is exerted to simulate the interaction between particles and the contact forces on each particle. Then, the calculation of the stress distribution inside the particle is conducted using BEM, during this procedure, the non-static or dynamic equilibrium of a particle is taken into consideration via treating the acceleration of particles as constant body force. Meanwhile, the body force leads to a domain integral in boundary integration equation (BIE), to avoid BEM losing the traditional advantage, i.e., dimension reduction, the domain integral is treated by the line integration method (LIM) and transformed into boundary integrals. In order to improve the computational efficiency of BEM, for particles with similar geometric shape, a random particle can be used as the template particle in the granular system, only the calculation of coefficient matrices of the template particle in the local coordinate system is needed, then, coefficient matrices of the rest particles can be obtained by mapping the solutions between the local and global coordinate systems. After the stress field is obtained, the Hoek-Brown criterion is applied to estimate whether particles are “broken” or not. Additionally, the breakage path that is assumed as a straight line can be obtained based on the least-squares fit.

It is difficult to monitor the underground tunnel lining in an all-weather manner due to its concealment. An intelligent monitoring method of tunnel lining based on self-sensing FRP (fiber reinforced plastic) bar embedded with optical fiber is proposed in this paper. The optical fiber sensor is embedded in the fiber reinforced plastic to form self-sensing FRP bar, which is installed in the tunnel lining, to monitor the stress state of the tunnel lining in a real-time and all-weather manner. The monitoring results can show the circumferential stress of the tunnel lining and predict the risk of crack generation. The intelligent monitoring method is used to monitor Chentang tunnel in Guangzhou-to-Shantou high-speed railway. The main factors affecting the safety of lining structure during the period from lining construction to operation are surrounding rock pressure, temperature difference stress caused by hydration heat and stress caused by concrete drying shrinkage. The temperature difference stress caused by hydration heat can lead to a large tensile stress on the inner side of the lining. Therefore, reasonable construction measures need to adopt to avoid cracking caused by temperature effect. The proposed intelligent monitoring method is capable of monitoring the tunnel lining stress-strain state in real time, for a long period of time and in all weather. This self-sensing system will continuously and timely reflect the risk of tunnel lining cracking during the tunnel operation, providing a technical guarantee for the operational safety of high-speed railroad trains.