To explore the strength degradation mechanism of gas bearing coal, the uniaxial compression tests were performed on coal under different initial gas pressures by using self-developed gas-solid coupling test system, and the pore and fracture structures of gas bearing coal were characterized by SEM, high-pressure mercury injection, low-temperature liquid nitrogen adsorption and micro-CT scanning system. The mechanical and non-mechanical effects of different gas occurrence states on pores and fractures were described respectively, and the internal relationship between pore and fracture failure and the loss of macroscopic strength of gas bearing coal was revealed. The results show that the degradation degree of average uniaxial compressive strength of coal increases with the increase of initial gas pressure. The pore structure and pore size distribution of coal were characterized by multi-means, and it is found that the development of fracture structure of gas bearing coal is not obvious. The proportion of isolated pores is relatively large, and the connectivity between them is poor, which is not conducive to gas seepage. The method of jointly characterizing the pore structure and pore size distribution of coals can correct the errors caused by "shielding effect" of micropores and transition pores and the errors caused by the "compression effect" of coal matrix or pore and fracture failure caused by the sample size. Adsorbed gas leads to the fracture and failure of micro-elements through non-mechanical action, mechanical action of expansion stress and gas wedge effect of free gas on coal body. The mathematical model of mechanical property deterioration and macro strength loss of gas bearing coal based on the micro view angle was established. From the model, it is found that the action of gas leads to the shift of the center of the Mohr circle to the left, the envelope of Mohr-Coulomb strength to the right, and the cohesion becomes smaller, and finally it leads to the loss of macro strength of coal.

The PHC short pile foundation is a new type of supporting structure for the power generation element of a solar power generation station. It is formed by inserting a PHC pipe pile into a hole of 2?3 m deep, followed by pouring concrete into the hole. To explore the working behavior of the PHC short pile foundation, field tests were carried out on the sand foundation under the lateral loading, coupled lateral and torsion loading and cyclic loading, respectively. According to the results of inclination of the column top, the displacement at the ground surface and the bending moment along the foundation, the force and deformation characteristics of the PHC short pile foundation and the influence of torsion were analyzed. The influence of pile cap on the inner force and deformation of foundation were then analyzed based on three-dimensional numerical simulation. The results show that the deformation of PHC short pile foundation can be divided into three phases. Torsional loads accelerate the occurrence of the soil plasticity and reduce the crack resistance of pile. The pile cap can efficiently reduce the mudline displacement with little influence on the pile deformation. Under normal ultimate load, the residual deformation after cyclic load is about 2 times of that under single load. Finally, based on the field tests and numerical simulation, the methods for obtaining the proportionality coefficient of horizontal soil resistance coefficient m and proportionality coefficient of soil shear modulus Ag under small deformation of the pile top are proposed, as well as the empirical methods for obtaining residual deformation.

To explore the time-history response laws of pile foundation under different types of seismic waves, the pile acceleration of rock-socketed single pile foundation, relative displacement of pile top, bending moment of pile body and damage of pile foundation under the action of artificially synthesized 5010 waves, 5002 waves, Kobe waves and El-Centro waves with an intensity of 0.35g were studied through the shaking table test. The test results show that the dynamic response characteristics of rock-socketed single pile foundation are related to the spectrum characteristics of input seismic waves. The acceleration response of the pile top significantly lags behind that of the pile bottom, and the peak acceleration of the pile reaches the maximum value under the action of El-Centro waves. The relative displacement of the pile top is the largest when inputting Kobe waves, and the peak value appears earliest when inputting El-Centro waves. The maximum bending moment of the pile does not exceed the bending bearing capacity of the pile foundation, and there is no damage to the pile foundation. In the anti-seismic design of pile foundation, the type of seismic waves can be reasonably selected according to the corresponding checking content, and the corresponding engineering suggestions are proposed.

In the process of hydraulic fracturing, the high-pressure infiltration of fracturing fluid on the fracture surface will cause the change of pore pressure field on both sides of the fracture, resulting in the change of mechanical characteristics of rock, and consequently affecting the propagation of hydraulic fractures. According to the macroscopic damage theory based on micro-fracture, the linear slip crack model was applied to develop the stress intensity factor model at the tip of the micro-crack considering the fluid pressure in the pores. Then the stress-strain constitutive model of saturated rock was established, and compared with the laboratory test results of saturated rock. The influence of pore pressure on the mechanical properties and damage-induced permeability was evaluated based on this constitutive model. The results show that the proposed constitutive model can better characterize the influence of pore pressure changes on rock mechanical properties. The pore pressure changes in micro-cracks have a relatively small effect on the rock elastic modulus and Poisson’s ratio, while it can greatly reduce the yield stress limit of plastic damage, weaken the compressive strength of the rock, reduce the energy loss of the plastic deformation of the rock on both sides of the fracture surface, and improve the energy utilization rate of hydraulic fracturing. When the pore pressure around the fracture is higher than a certain threshold value during fracturing process, it will promote the unsteady and rapid propagation of meso-cracks, thereby promot more cracks to be connected in series, form a complex fracture network, and enhace the rock permeability surrounding hydraulic fractures. The proposed constitutive model of saturated rock can provide support for the mathematical simulation of rock matrix damage and field practice.

To determine the earth pressure at rest of light weight soil mixed with expandable polystyrene(EPS) particles behind a retaining wall, the model tests are conducted for remolded soil and light weight soil respectively. The distribution laws of earth pressure at rest, coefficient of earth pressure at rest and vertical settlement deformation of the filler behind retaining wall are explored under loading and unloading processes, and the mechanisms of pressure reduction and settlement deformation of light weight soil are clarified. The results show that the vertical earth pressure and lateral earth pressure of remolded soil and light weight soil increase linearly with the increasing of filling depth before loading and unloading, and the relative errors between theoretical values and measured values for vertical earth pressure and lateral earth pressure are no more than 26.35%. The effect of load on vertical earth pressure and lateral earth pressure decreases with the increase of filling depth for remolded soil. After curing, light weight soil has self-supporting property, thus the influencing depth of load on light weight soil is limited. In this test, the load influencing area depth of light weight soil is about 50% of the wall height. In the process of loading and unloading, when the filling depth is less than 50% of the wall height, the vertical earth pressures and lateral earth pressures of light weight soil increase or decrease obviously with the gradual load increasing or decreasing. When the filling depth is greater than 50% of the wall height, the vertical earth pressures and lateral earth pressures gradually converge with the gradual load increasing or decreasing, but the change range is smaller. The measured coefficients of earth pressure at rest of remolded soil and light weight soil are non-linear distribution behind the retaining wall, which are in the range of 0.27?0.74 and 0.33?0.44, respectively. The theoretical values of the coefficient of earth pressure at rest calculated by Jaky formula are basically larger than the measured values, but the difference is smaller. The vertical settlement deformation of remolded soil decreases with the increasing of filling depth, which is approximately 0?21.5 mm. The vertical settlement of light weight soil after curing has no obvious change under load, which is approximately 0?2.8 mm. Compared with remolded soil, light weight soil after curing has good buffering and self-supporting capacity, which can more effectively absorb and disperse the vertical pressure, due to the buffering effect of EPS particles and the solidification effect of cement. Light weight soil can greatly reduce the earth pressure at rest, the coefficient of earth pressure at rest and vertical settlement deformation, and meet the requirements of filling load reduction in retaining wall engineering.

The dry density and initial moisture content of compacted loess significantly affect the process of water infiltration. Taking Lanzhou Loess as the research object, the immersion infiltration tests were carried out through filling the test model in indoor. The volumetric water content and pore air pressure of the soil at different depths in the process of infiltration were tested. Then variation laws of wetting front and pore air pressure during water infiltration were analyzed. The test results suggest that when the degree of soil compaction increases, the water infiltration rate decreases, and the time of the wetting front reaching the measuring point is prolonged. The degree of soil compaction increases from 0.83 to 0.93, and the infiltration rate at different depths decreases as high as 21.7%. When the initial water content increases, the pore water connectivity is enhanced and the water infiltration rate increases. The effect of dry density on water infiltration is more significant compared with the initial moisture content. The pore air pressure changes continuously during water infiltration, and the process can be divided into two stages: rapid change and slow change. In the process of water infiltration, the maximum peak air pressure and stable air pressure are positively correlated with the initial moisture content and dry density of soil. However, the infiltration rate decreases greatly with the increase of the infiltration depth, and the bigger the dry density is or the lower the volumetric water content is, the smaller the infiltration rate is. Considering the influence of dry density and initial water content on pore air pressure, the Green-Ampt model was modified. Finally, through comparison and analysis of the measured values with the calculated values of the infiltration depth, it is found that the calculated value of the modified model is in good agreement with the measured value of the test.

The effect of plant slope protection is strongly associated with the root architecture in soil. In this paper, the water-fertilizer combination method was used to regulate the plant root configuration, and the influence of the regulated plant root architecture on the slope soil strength was studied. In terms of the practice of highway slope engineering, a field fill slope for test was made. Vetiver was selected as slope protection species, and 9 groups of water-fertilizer combinations were designed to regulate the root architecture of plant in slope soil. After 10-months growth, the roots of plants on the slope were counted by cross layered excavation. It was found that the root content increased first and then decreased with the depth of soil layer, which accorded with Gauss curve. In control area with water-fertilizer combination, vetiver proliferated a large number of secondary and tertiary roots, which increased root content and root surface area density. The proportion of roots in the upslope direction was higher than that in the downslope direction, reaching 60%?66%. The shear test of undisturbed soil in different regions shows that the root content and root surface area density are important factors affecting the shear strength of soil. Regression analysis shows that there is a linear relationship between soil cohesion and root surface area density. The calculation based on Wu’s model shows that under natural growth conditions, the increment of shear strength of vetiver to slope soil is 5.28 kPa to 8.62 kPa, while under conditions of water-fertilizer combination, the angles between most of secondary and tertiary vetiver roots and the vertical are greater than the slope angle, which makes the shear strength of vetiver on slope soil increase by 17.59 kPa to 33.97 kPa. This research shows that water-fertilizer combination method can be used to regulate the root architecture of plants to strengthen slope soil, which provides theoretical and practical basis for improving slope soil strength and preventing slope soil erosion.

The long-term seepage of the rammed earth foundation (REF) has a great influence on the safety of the superstructure and the appearance of the ancient building. It is vital to explore the anti-seepage measures for the top of the REF. In this study, a scale model with a size of 1:18 compared with the real size of the REF was established. The rainfall infiltration tests were carried out to study the anti-seepage effect of the plain soil with compaction method and different modified soils with replacement method on the top of the REF. The results showed that: 1) The rainfall infiltration was reduced by increasing the dry density of rammed earth by using compaction method. When the dry density of REF was increased by 10%, the depth of rainfall infiltration was reduced by more than 40%, and the wetting degree was reduced by about 50%. However, using compaction method could induce damage to the superstructure and the external masonry structure of foundation. 2) When the five kinds of modified soil were used for local replacement, the anti-permeability effect of tabia was better, followed by the modified soil of sticky rice flour and straw. Finally, it is suggested to adopt local replacement of modified soil and other structural measures to improve the anti-seepage effect of REF of ancient buildings.

Study on the microstructural change of granite residual soils (GRS) during compression is important to comprehend the influence of deformation mechanism and structure characteristics on soil mechanical characteristics and to establish relations between macroscopic behavior and microstructural characteristics of weathered soils. One-dimension compression tests were conducted on Xiamen GRS and scanning electron microscopy (SEM) was used to investigate samples under different loading conditions. Structural parameters enabling the quantification of particle assemblage and pore volume, morphologies and preferred orientations of GRS were gained. This study investigated these parameters during compression procedures, then the mechanisms of compression deformation were proposed. The results indicate that the effects of cementation played an important role on GRS. The compression curve showed an obvious turning point and approached ICL gradually after the load exceeding the pre-consolidation pressure. The compression of large pores contributed to deformation of natural soils, while deformation of reconstituted soils was mainly dependent on the transformation from large pores and meso pores to small pores. Particle assemblages of natural soils showed relatively low roundness value, while after one-dimension compression test, the particle assemblage shape showed remarkable tendency towards medium roundness. Compression deformation resulted in deflection of particle assemblages perpendicular to loading, facilitating the preferred orientation. The deformation mechanism of GRS is that the microstructure constantly rearranges and reorients into a more stable and orderly structure. The research provides theoretical support for foundation deformation analysis related to GRS.

The first weighting of main roof above confined water in deep longwall face is a severe phenomenon of underground pressure, which often causes the large-scale failure at the floor area and water inrush accidents. To study the linkage effect between the floor failure and first weighting of the main roof in deep longwall mining, the fracture and failure characteristics of roof and floor during the first weighting in deep mining were analyzed by the similar material simulation test. The breakage model of the main roof and the mechanical model of floor during the first weighting were established. From the perspective of stress increment, the linkage effect between the compression failure and unloading failure at the floor area and the first breakage of the main roof was obtained. Linkage changes of the stresses and deformation at the floor area during the first weighting in different mining depths were simulated by using the discrete element software. Then the weakened control technologies of roof and floor in deep mining were provided, which mainly include the pre-split main roof for eliminating the mid-span gangue contact effect, enhancing the working resistance of supports for reducing the dynamic loads of main roof breakage, and reducing the unloading starting point and unloading level of stresses at the floor area. The results show that there is a zone of the increased compressive stresses at the floor area of contacting gangues in the middle of main roof after the first breakage of main roof in deep mining. Its displacement direction changes to downward compression after the first weighting from the upward heave before first weighting. It is also found that the position of the back foot changes to the contacting zone from the floor area in front of coal rib, the pressure arch at floor area changes from one single arch structure with the width of the ultimate span to two arches structure whose width are both about half of the ultimate span. The deeper the mining depth, the higher the increment of horizontal stresses at floor area of contacting gangues zone are, and the influence depth of vertical stresses increment are higher than that of horizontal stresses. With the increase of the maximum stress variation, the maximum deformation at floor area approximately increases exponentially, and its non-linear increases are the most serious on the vertical direction.

As a new grouting technology, pulsating grouting has been successfully applied to solve the problem of loose soil grouting. However, the dynamic response of slurry-soil coupling under pulsating load lags far behind the engineering practice. Based on the theory of pulsating grouting, a monitor system of pulsating grouting was designed. The response laws of sandy soil under different pulsating periodic loads were investigated by setting different pulsating periodic pressures and soil porosity ratios. Then through COMSOL Multiphysics platform and MATLAB, a program for simulating the stress-strain of slurry-soil coupling was developed. The applicability of numerical simulation was verified by comparing the existing grouting response theory with physical tests. The results show that when the pulsating pressure increases and the pore ratio of soil remains constant, the transfer rate of the pulsating stress borne by the skeleton force increases rapidly. The higher the pulsation frequency, the greater the soil stress. The fluctuating pressure will destroy the strong and weak force chain formed by the soil under constant load, resulting in the uniform stress transfer. The larger the pore ratio, the looser the soil and the higher the pulsation frequency, the more favorable the slurry migration in the soil. Compared with stable pressure grouting, pulsating grouting can lead to the stress concentration. Correspondingly, the slurry penetration and compaction caused by the stress concentration are beneficial to reduce the formation uplift displacement in the grouting process. Compared with stable pressure grouting, by which the grout is easy to split continuously along the small principal stress or formation defect, the grout diffusion induced by pulsating grouting is more controllable. In addition, the numerical simulation method provides a new idea for the slurry diffusion laws under different pulsating construction parameters, formation conditions and grouting materials. The research conclusions can provide a strong guiding significance for engineering practice.

More and more transmission lines pass through the desert area covered by aeolian sand, and steel grillage foundation has good applicability, but there are few field tests at present. In Yulin area of Shaanxi Province, southern edge of Maowusu sandy land, several groups of full-scale uplift tests of steel grillage foundation in aeolian sand ground were carried out, and the uplift bearing capacity, uplift and surface displacement, support and floor stress, upper earth pressure and other parameters were tested and analyzed. The results show that the uplift load-displacement curve of the steel grilage foundation is similar to that of the spread foundation, which can be divided into the approximate straight section, plastic transition section and linear instability section. When the cumulative uplift displacement reaches 21-23 mm, the foundation comes to the limit state. If small cracks appear on the ground surface around the top of the foundation during loading and propagate along the diagonal direction, the foundation will enter the limit state under the next level load. By analyzing the variation characteristics of the earth pressure on the upper part of the foundation, the fracture point on the uplift fracture surface of the foundation can be found, and the actual uplift angle can be calculated. The uplift bearing capacity calculated by the soil weight method in the existing codes is larger than the test value of ultimate uplift bearing capacity. It is suggested that the uplift angle should be 19.0o to 19.5o in the foundation design of relevant areas.

Using self-designed tension shear auxiliary device, we carried out a tension shear test and a compression shear test under different normal stresses between ?0.28 MPa and 3.0 MPa. We applied acoustic emission to compare and analyze the mechanical properties and the damage failure mechanisms of the specimens for both tests. The main results are listed as below. The tension shear auxiliary device can help us carry out the tension-shear test well. The peak stress varies with the normal stress non-linearly and is more sensitive to the normal tensile stress. The Hoek-Brown strength criterion can generally characterize the strength of the full stress area. The post-peak stress of the specimen in tension shear test drops severely and separates into two parts rapidly, showing the failure characteristics with more brittleness than that of the compression shear test. The morphological characteristics of the fracture plane are closely related to the direction and magnitude of normal stress. For compression shear test, the damage degree represented by the acoustic emission parameters is greater, as well as the failure degree of the fracture plane, compared with those observed in tension shear test. The frictional area and the local spalling of the fracture plane are also more obvious for the specimens in compression shear test, compared with those in tension shear test. As the normal stress increases, the duration of the acoustic emission quiet period gets longer, and the start time of the unstable crack propagation gets later. Compared with the compression one, the tension shear test shows a shorter duration time for each stage of the failure process as well as a higher failure rate. The critical points ?cc, ?ci and ?cd of the shear-stress plot and the acoustic emission parameters can represent, respectively, the macroscopic and microscopic failure process of the specimen. The critical point ?cd and the acoustic emission b value can be used as omens of rock failure.

As a kind of meshless method, the material point method (MPM) combines the best features of the meshed method and meshless method in the calculation, and avoids the mesh distortion phenomenon of the meshed method in solving large deformation problems. In the last decades, MPM has been successfully applied in a number of large deformation geotechnical problems and multiphase flow problems. Based on the theory and algorithm of the standard material point method, two multiphase MPM formulations were studied: the two-phase single-point formulation for saturated soil, and the two-phase single-point formulation with suction for unsaturated soil. To examine the accuracy and applicability of the multiphase MPM when simulating coupled hydro-mechanical multiphase flow problems, Terzaghi’s one-dimensional consolidation problem, one-dimensional compression wave propagation problem and one-dimensional seepage problem were simulated and the results were compared to the analytical solution. The simulation results show that the numerical solution is generally consistent with the theoretical solution, and the multiphase MPM can accurately describe the hydraulic characteristics of saturated porous media and the seepage characteristics of unsaturated porous media. Finally, the multiphase MPM was used to simulate the problem of rainfall infiltration in unsaturated slope, and the change of matrix suction and the whole process of large deformation failure of the slope under rainfall infiltration were analyzed.

With the main mechanisms of energy guiding and energy dissipation, the rockfall attenuator barrier has become a flexible structure to control rockfall disaster. The problems of difficult maintenance and high cost hinder the wide application of classic rockfall attenuator barriers in the field of rockfall protection. Therefore, combining with the lower open-ended design concept, we optimize the extension length and propose an improved rockfall attenuator barrier. In this study, the energy dissipation mechanism of the improved rockfall attenuator barrier under different extension lengths was investigated, by carrying out in-situ tests of the rockfall impact. The results show that the improved rockfall attenuator barrier is effective in cleaning up the rockfall stopped in the mesh. The constraint and friction effect of the improved rockfall attenuator barrier can effectively exert its flexible energy dissipation characteristics, thus reducing the impact energy of the rockfall. When the extension length of the improved rockfall attenuator barrier increases from 3 m to 7 m, the energy attenuation rate of the rockfall increases by approximately 20%. However, the increment of the energy attenuation rate gradually decreases with the increase of the extension length. To further improve the energy dissipation effect of the improved rockfall attenuator barrier, the influence of impact positions and angles of the rockfall on the energy dissipation effect of the improved rockfall attenuator barrier was studied by numerical simulation. The simulation results show that the energy attenuation rate was the highest when the rockfall impacted the middle position of the improved rockfall attenuator barrier, and the energy attenuation rate was increased by 20% compared with the position at the side of the impact edge. The energy attenuation rate of the rockfall reached the highest at 74.1%, when the angle between the impact direction of the rockfall and the improved rockfall attenuator barrier was approximately 45o. Therefore, the energy dissipation effect of the rockfall attenuator barrier can be maximized by reasonably setting extension length, installation location and inclination angle in engineering practice.

The rock vibrations caused by blasting excavation in deep caverns under high in-situ stress conditions include explosion seismic wave and seismic wave induced by rapid release of in-situ stress on the blasting excavation surface. There is no obvious demarcation point between these two types of waves in the time domain, which brings great inconvenience to study the vibration characteristics of explosion seismic wave and induced seismic wave and its surrounding rock dynamic response alone. In this study, the variational mode decomposition (VMD) is used for the first time to separate the explosion-induced seismic waves and the in-situ stress-related seismic waves for the rock vibrations measured in a typical deep cavern. The reliability of VMD in the separation is verified before the practical application. Based on the separation signals, the frequency characteristics and peak particle velocity (PPV) attenuation laws of the two waves are investigated. The results show that VMD method can adaptively determine the number of modal decomposition of the given signal and match the optimal center frequency and limited bandwidth of each modal component, which can effectively realize the separation of the two waves in deep cavern blasting excavation. The separation signals indicate that the center frequency of the in-situ stress-related seismic waves is significantly lower than that of the explosion-induced seismic waves. Due to the seismic waves caused by the rapid in-situ stress release, the total rock vibrations are increased significantly in the lower frequency components. Compared with the explosion-induced seismic waves, the in-situ stress-related seismic waves decay with distance at a slower rate and become the predominant vibration component in the middle and far fields.

The Baihetan hydropower plant is one of the most important backbone energy points of power transmission from west to east in China’s energy layout. In the initial stage of hydropower plant construction, some in-situ stress data were obtained by using conventional methods. However, with the continuous excavation of underground cavern, significant brittle failures caused by high stresses were observed in the surrounding rock. Therefore, the study on the stress field of surrounding rock mass is highly important for the long-term stability control of underground caverns. The borehole wall was scanned by the high-precision ultrasonic downhole television imaging system to obtain the logging image, and the widths of breakouts WBO was extracted by WellCAD software. The stress polygon based on Mohr-Coulomb criterion and Anderson theory and the breakout failure method based on Wiebols-Cook criterion were integrated and utilized to limit the estimation of stresses value, the characteristics of rock mass stress field in the project area were analyzed. The results show that the orientation of the maximum horizontal principal stresses in the engineering area was about N30.15°E, and the magnitudes of the major principal stresses were Sh=11.62±0.34 MPa (horizontal minimum principal stress), SV=14.45±0.33 MPa (vertical principal stress), SH=25.88±5.61 MPa (horizontal maximum principal stress) respectively. This stress state is favorable for the strike-slip faulting(Sh

The existence of cavities behind the segments is one of the common disease phenomena in shield tunnels. The cavities not only affect the static mechanics behavior of soil and segment, but also directly affect the dynamic response of the tunnel, which could aggravate the tunnel’s seismic damage. The existing related researches all use the deterministic analysis methods and lack the quantitative evaluation method in terms of probability perspective. Taking a shield tunnel in a rail transit section as an example, considering factors such as the location and size of the cavity, site conditions, and the incident direction of seismic wave, a large number of nonlinear dynamic time-history analyses are conducted based on the soil-tunnel-void interaction using an increment dynamic analysis method. Combined with the theory of tunnel seismic vulnerability, the seismic vulnerability is studied for the shield tunnel with the cavity disease. It is found that: the size of the cavity, the site conditions and the incidence direction of seismic wave have an important influence on the tunnel seismic vulnerability. As the cavity size increases, the plastic deformation of the soil adjacent to the cavity increases substantially, the eccentricity extent of the segment section increases, and the load-bearing performance decreases, and the affected area is about 3?5 times the cavity size. The cavity behind the segment increases the vulnerability of the tunnel structure. The increase of vulnerability is more obvious with the void size increasing. The impact of the cavity on the structure vulnerability varies with different locations of the cavity. When the cavity size is relatively large, the impact extent shows the following trend with a descending order: the side wall, shoulder, and crown cavity. As site condition worsens, the tunnel vulnerability increases and the influence of the void on tunnel vulnerability also enhance. Under the transverse and vertical incident directions of seismic ground motions, the tunnel vulnerability increases nonlinearly with the increasing of the cavity size. Although the damage probability under vertical ground motion is less than that of transverse ground motion, the increasing extent is significantly larger and more sensitive to the cavity size. It is worth to noting that for the three cavity locations and the two incident directions of ground motion, the larger cavity size may significantly affect the seismic performance of the tunnel structure.

The spatial variability is an inherent uncertainty of soils. The random field theory is used to represent the spatial variability of soils, and the random field discretization is performed by the Karhunen-Loève (KL) expansion method. Using the slope upper bound analysis based on the discrete mechanism, the discretization results of the internal friction angle random field at each point in the space are considered when generating the velocity discontinuity surface. The upper bound analysis, shear strength reduction technique, bisection searching, and sequential quadratic programming method are combined to solve the safety factor of slopes. The first-order reliability method (FORM) and subset simulation (SS) are employed for slope reliability analysis. Given the characteristics of SS and the shear strength reduction technique, an optimization algorithm coupling the two is proposed to improve computational efficiency. By calculating and analyzing an earth slope, the similarities and differences between FORM and SS based on the KL expansion method in solving the slope reliability index and failure consequence are clarified. The influence of the variation coefficient of soil strength parameters on the slope reliability index and failure consequence is investigated, providing a theoretical basis for risk analysis and prevention of slopes.

The interaction of super-large section chamber group in deep and close-distance condition will lead to stress concentration and wide failure range of surrounding rock, especially under dynamic disturbances. In this paper, numerical simulation software FLAC3D is used to establish the calculation model based on the field condition of coal gangue separation system in Longgu Coal Mine. The deformation and failure evolution of chamber group under different chamber spacing and dynamic loads are studied by using built-in dynamic module. The simulation results show that: With the decrease of chamber spacing, the deformation and failure degree of surrounding rock gradually increases, and the overall failure and instability occur eventually. Compared with the static load, the range of critical spacing under dynamic disturbance is enlarged by 33.3%?50%. Meanwhile, the response of anchored surrounding rock is gradually intensified with the dynamic load strength increase, and the critical strength of failure and instability is about 4.0?4.5 MPa. Based on the elastic-plastic mechanics and elastic wave theories, the mechanical model of anchored surrounding rock under dynamic and static loads is established. The failure and instability criterion are obtained. The anchored surrounding rock can be divided into three states: overall stability, static failure and dynamic failure. On this basis, the analytical expression of critical distance between failure and instability is presented. Finally, in-site calculation and field monitoring verify the rationality and feasibility of the theoretical analysis. This study provides a reference for layout design and stability control of super-large section chamber group.

Fracture is one of the most common failure modes of materials and components and greatly restricts engineering design. Understanding of the crack propagation and evolution of rock and other engineering materials is of great significance to engineering construction. For the current numerical methods there are more or less limitations when analyzing the evolution of cracks, such as the grid dependence of the crack path, the difficulty to deal with crack bifurcation and merging by the classic fracture criterion. In recent years, the phase field method (PFM) has been widely used in simulating crack growth. A phase field numerical manifold method (PFNMM) makes use of the advantages of the phase field method in simulating crack propagation and those of the numerical manifold method (NMM), and is proposed for crack growth in rock. The implementation details of the proposed numerical model are presented. Several benchmark examples, including notched semi-circular bend test and Brazilian disc test, are adopted to validate the proposed numerical approach. After that, the multi-crack propagation process with different rock bridge inclination angles under uniaxial compression is simulated, which is in good agreement with the results derived from laboratory and PFC. And the results indicate that the PFNMM has broad application prospects in simulating crack growth of rock.

The conventional quasi-three-dimensional technique has a high efficiency in the simulation of regional groundwater flow in multilayered subsoils, with the assumption that groundwater flow is essentially horizontal in the aquifers and vertical in the aquitards. This assumption requires a two orders of magnitude permeability contrast between the adjacent soils, which is not conducive to a wide application of the conventional technique. In this study, calculation theory is extended to be applied to regional groundwater flow in general multi-layered subsoils. Firstly, the partial differential equation governing two-dimensional horizontal flow in each soil is derived with the average hydraulic head as the main variable and considering the infiltration at the upper and lower interfaces. The velocity continuity conditions at the soil interfaces provide the coupling condition among the soils. Then, the vertical variation of the hydraulic head in individual soil is obtained by quadratic interpolation between the average head and the head at the upper and lower interfaces. A quasi-three-dimensional finite element model is presented based on the extended calculation theory. Each soil is discretized with the same two-dimensional horizontal grids, and the fluid mass balance is established at each node. The extended quasi-three-dimensional technique has a higher accuracy and greater applicability, as it can overcome the limitations of the conventional technique, and simultaneously get a reasonable simulation of the horizontal and vertical flow in each layer without sacrificing the calculation efficiency. The extended technique provides an efficient and competitive scheme to simulate regional groundwater flow in multilayered subsoils, because it is convenient to solve free surface problems and deal with singular problems of underground structures, and compute a high-precision three-dimensional representation of the hydraulic head distribution.

Brittle failure is usually observed during excavation of surrounding rock, while the simulation results of brittle failure often vary with numerical methods. Therefore, it is highly important to study the applicability of different numerical methods to simulate brittle failure characteristics of surrounding rock. Firstly, in the continuous method, the calculation formula of residual stress in the improved stress drop model is deduced, which removes the correlation of the change of strength parameters in the traditional stress drop model, and is verified by simulated uniaxial compression test. Then, the continuum method (method 1) and the continuum-discontinuum method (method 2) were used to simulate brittle failure characteristics of circular tunnel surrounding rock under hydrostatic pressure. Finally, taking Taipingyi tunnel as an application example, two methods were used to simulate brittle failure characteristics of circular tunnel surrounding rock under non-hydrostatic pressure. It is found that under hydrostatic pressure, the results obtained by both methods are similar to those obtained by the field observation or laboratory experiments, while the failure zones obtained by method 1 is larger. Under non-hydrostatic pressure, although the result obtained by method 1 is similar to that obtained by field observation of surrounding rock failure of the Taipingyi tunnel, the results obtained by method 2 are closer to that by the field observation. This phenomenon could be explained as follows. In method 1, the surrounding rock is still a continuous after failure, which is conducive to stress transfer between rock blocks, thus facilitating the development of failure zones; while in method 2, the surrounding rock is converted into a discontinuum after cracking, and the contact and friction forces reflect the interaction between rock blocks and consume the system energy, thus limiting the development of cracking zones.

The apparent cohesion caused by matric suction makes the mechanical properties of unsaturated sand significantly different from those of dry sand. In order to study the soil arching effect of the unsaturated sand tunnel, trapdoor tests with different water contents and buried depths were carried out. The time-varying characteristics of soil arching evolution under different working conditions were revealed by analyzing the failure mode of sand and the change law of earth pressure in the process of baffle falling, and the influence of water content and buried depth on soil arching effect was expounded. Meanwhile, the distribution mode of earth pressure above the baffle was analyzed based on the arc arch theory of large principal stress trace and considering the suction between particles. The discrete element numerical simulation was carried out based on PFC (particle flow code) adhesive rolling resistance linear model, and the soil arching effect under different working conditions was analyzed from the micro perspective. The results show that the failure mode of dry sand develops rapidly from triangle to trapezoid, and the failure mode of unsaturated sand is triangle and related to water content. The earth pressure changes in three stages and the earth pressure decreases to the extreme value and then rises when the sand is dry. The extreme value of soil pressure in the unsaturated conditions is greatly reduced compared with that of dry sand. The soil pressure is less affected by the burial depth when the water content is larger. Cracks appear at the edge of the loosened area, and natural arch is formed after the local collapse. The numerical simulation shows that with the baffle falling, the direction of principal stress deflects obviously. The contact force chain changes from the loose area to the stable area from weak to strong. The soil pressure distribution of model test and numerical simulation is consistent with the theoretical analysis. The results show that the porosity of dry sand is consistent with the soil pressure, and the porosity increases rapidly when the cracks appear in the water bearing condition, while the contact fabric changes obviously with the water content.

The seismic response time-domain analysis of the complex sites (complex sites-city buildings coupling) under far-field earthquakes requires the free-field seismic wave motion input at the computational truncation boundary. In order to avoid memory waste for storing time histories of boundary nodes and low parallel computing efficiency caused by frequent I/O’s, it is necessary to construct the time-domain simulation method of free-field wave matching the time-domain analysis of seismic response. For the seismic response time-domain analysis method based on the Legendre spectral element, the corresponding free-field wave time-domain simulation method for SH wave in layered media under obliquely incident plane wave is developed. First, the SH wave simulation in layered media under obliquely incident plane wave is clarified. Then the Legendre spectral element method and explicit integration schemes is adopted to establish the space-time decoupling discrete scheme for such a problem. The accuracy and stability of the proposed scheme is verified through analytical examples. Moreover, the proposed scheme is applied to compare the crust amplification effects during the seismic waves propagation from the seismic source to the bedrock site. The results show that the amplification effects in the two regions are systematically different in the low frequency range, and tend to be the same in the high frequency range. The results explain qualitatively the problem that the predicted response spectrum value over long-period and relatively long-period through the next generation seismic attenuation relationship are inconsistent with Wenchuan earthquake records. Finally, taking the analysis of mountain topography effect as the example, the proposed scheme is applied together with the Legendre spectral element method and the transmitting boundary condition to near-field external source wave simulation.

At present, the viscoelastic method is widely used in the dynamic analysis of concrete faced rockfill dams (CFRDs), basically forming a method framework: the viscoelastic model for calculating the dynamic response and the permanent deformation model for calculating the permanent deformation. Then the dam safety is evaluated through both the dam dynamic response and the permanent deformation. However, when the current method is used in the dynamic analysis of the CFRDs, there is an obvious defect, i.e., it cannot consider the influence of permanent deformation on the stress of the concrete slab during the strong earthquakes, which may cause serious errors when applying the method in CFRDs with obvious permanent deformation. Therefore, it is necessary to improve the current method. This paper suggests to adopt the strategy of “divide first and combine later”. Firstly, the dynamic displacement and permanent displacement of the dam are calculated separately, then the two are added as the actual displacement of the structure, and finally, the strain and stress of the structure are calculated based on this actual displacement. This paper took the Yulong Kashi CFRD as an example to illustrate the necessity of the improvement.

To study the influence of the underground tubular structure on the seismic subsidence of shallow raft foundations in the soft soil site, a method for analyzing the seismic subsidence of the shallow raft foundation in soft soil was established based on the FLAC3D finite difference analysis platform and vibration softening model. By comparing the simulated and measured values of seismic subsidence and damage of typical building foundations in soft soil of Tanggu Port in Tianjin during 1976 Tangshan earthquake, the rationality of the proposed method was verified. Numerical simulation was carried out for analyzing the influence of different burial conditions and cross-sectional dimensions of square-sectional (for example) underground tubular structures on seismic subsidence of shallow raft foundation. Results show that the underground tubular structure has a more significant impact on the seismic subsidence of the superstructure with a lighter self-weight, affecting the design and construction of workshop structure. The seismic subsidence decreases with the increase of the relative size of the underground tubular structure, and tends to be steady after it exceeds the width of the foundation. Seismic subsidence increases with the increase of the burial depth of the underground tubular structure, and eventually tends to be the result without underground tubular structures. The seismic subsidence initially increases with the increase of the burial distance between the underground tubular structure and building, and decreases afterwards. In terms of seismic subsidence control, the distance from the underground tubular structure to the existing building needs to be controlled.