Abstract:Up to now, membrane process has been adopted as one of main advanced treatment processes of landfill leachate to meet the requirement of discharge standard. However, a large amount of membrane leachate concentrate with more pollutants is produced, which will cause serious secondary pollution without proper treatment and disposal. In this work, various treatment processes of leachate concentrate from the aspects of principle, application effect, cost, and technical development were classified and discussed according to the published related studies and engineering application cases. The characteristics, existing problems, and application scope of recirculation, physical treatment (submerged combustion evaporation and mechanical vapor recompression), chemical treatment (incineration, advanced oxidation processes, and supercritical water oxidation), physical and chemical treatment (flocculation, adsorption, membrane separation, and solidification/stabilization), and coupled treatment processes with or without membrane were also reviewed. Finally, the treatment routes for different types of leachate concentrate were concluded, and the future prospect of leachate concentrate treatment was put forward, aiming to provide technical reference for further study and full-scale treatment of leachate concentrate.

Abstract:Time-averaged flow velocity and pulsating flow velocity coexist in actual turbulence. In order to study the effect of turbulent pulsating flow velocity on the physical and chemical properties of biofilm, an oscillating-grid biofilm reactor was designed which only has pulsating flow velocity with the time-averaged flow velocity being zero. The two-dimensional turbulent flow field of the reactor was measured by particle image velocimetry (PIV) at five oscillating-grid frequencies, and the effective turbulent intensities at biofilm carriers were calculated. The biomass, thickness, and density of the biofilm, as well as the composition of the extracellular polymeric substances (EPS) in the oscillating-grid reactor were measured and analyzed at different turbulent pulsating flow velocities. Research results show that changing the turbulent pulsating flow velocity would change the physical and chemical properties of biofilm. With the increase of the turbulent pulsating velocity, the biomass and thickness of the biofilm first increased and then decreased, and the maximum value was reached when the effective turbulent intensity was 1.92 cm/s. While the biofilm density and the content of protein and carbohydrates in EPS increased with increasing turbulent pulsating flow velocity, and the content of protein and carbohydrates in tightly bound EPS (TB-EPS) was more than that in loosely bound EPS (LB-EPS). The ratios of protein to carbohydrates content in LB-EPS, TB-EPS, and total-EPS were greater than one. This research can provide theoretical reference for the design and operation optimization of biofilm reactors.

Abstract:To investigate the influence of pipe materials on species of pathogens in water distribution system, a drinking water distribution system (DWDS) in a city in eastern China which has been operating steadily for eight years was taken as the experimental object. Metagenomic sequencing analysis was conducted to study the effects of pipeline materials on pathogens in 13 sets of biofilms collected from ductile iron pipe (DCIP), polyethylene pipe (HDPE), galvanized steel pipe (GSP), stainless steel composite pipe (SSCP), and steel pipe (SP). Results show that the relative abundance of pathogens in DCIP biofilm was the most, while that in GSP biofilm was the least. In addition, nine of 12 pathogens on the global priority pathogens list published by WHO were detected from the water distribution pipeline, with relative abundance ranged from 0.079% to 1.084%, indicating the severe hazard potential of biofilms in water supply pipelines to humans and the environment. Based on the relative abundance distributions of the pathogens in five pipe materials, the priority pathogens associated with the five pipe materials were found. The research results provide an effective and theoretical basis for the selection of water distribution pipe materials.

Abstract:There are mixed water supply zones between different water plants in multi-source water supply areas, where the hydraulic water quality conditions are not stable, and water quality problems such as excessive iron release and low residual chlorine are easy to occur. The mixed water quality risk area can be predicted by using the spatial modified interpolation method of characteristic water quality index. Characteristic water quality index refers to the index with distinct identifying characteristics, which are distributed regularly in the water distribution system and have large differences in the finished water of different water plants. According to its spatial distribution, the separate water supply area and mixed water supply area of each water plant can be determined. However, there are great differences between the distribution of pipe network and the geographical space, so the correction coefficient should be introduced before interpolation to correct the characteristic index. In this study, the mixed water supply zone of H city was predicted. Results show that the spatial modified interpolation results of the characteristic water quality index conductivity and total iron concentration of H city were basically consistent, and accorded with the results of hydraulic model simulation. Online monitoring points were set in the predicted mixed water supply zone, and frequent changes in flow velocity and flow direction were observed, which conformed to the hydraulic characteristics of the mixed water supply zone. It indicates that the prediction results are correct and the method is feasible.

Abstract:In view of the vibration and plastic deformation of subgrade under high-speed train load, the 2.5D finite element method for elastoplastic subgrade was proposed. The subgrade deformation induced by high-speed train load was regarded as material nonlinearity, and the improved Mohr-Coulomb model was adopted to simulate the elastoplastic soil. Based on the 2.5D finite element method, the displacement calculated by elastic theory was regarded as tentative displacement. When the soil reached yielding according to the improved Mohr-Coulomb yield criterion, the tangent stiffness iteration method, backward Euler integration algorithm, and uniform tangent modulus algorithm were introduced into the algorithm to update the stiffness matrix, and the plastic deformation induced by high-speed train operation was solved iteratively. On this basis, a 2.5D finite element method for elastoplastic subgrade was established, where the orbit was treated as Eular beam, the modified multi-frequency train load was adopted to simulate train operation, and the viscoelstic artificial boundary was used to treat the truncated boundary of the finite element model. The closed-form solution and field measurement results were compared to verify the correctness and reliability of the proposed model. Results indicate that the model can be used to efficiently solve the vibration and accumulative deformation problems of ground induced by high-speed train load.

Abstract:To tackle the response problem of existing tunnel induced by adjacent excavation in soil-rock mixtures, numerical simulation tests based on discrete element method were utilized for investigation. By using the particle flow code software PFC2D, the micro structure of the soil-rock mixture, the tunnel lining structure, and the pit retaining structure were finely modeled, and the construction process was simulated. The influences of the rock content (w) on the responses of tunnel lining due to excavation nearby in the cases of different tunnel locations were systematically investigated. Results show that when the tunnel was located in the active earth pressure zone, with the value of w exceeded 60%, the increase in w resulted in the formation of rock skeleton around the tunnel segment, which decreased the tunnel responses induced by lateral excavation. When the tunnel was located in the passive earth pressure zone, as the value of w was lower than 75%, the excavation-induced tunnel responses started to decrease with the increase in the value of w; when the value of w exceeded 75%, such effect on tunnel responses induced by excavation above the tunnel would lose efficacy.

Abstract:In view of the stability of shallow shield tunnel excavation under seepage conditions in sandy ground with clay, a test equipment was designed and established, which mainly consists of model box, water circulation system, shield tunnel and excavation face model, saturated stratum model, and measurement system. Based on model tests, the ground settlement, saturated earth pressure, and pore water pressure in front of the tunnel were measured in the process of gradual instability of the tunnel face. Results show that under the seepage condition, the pore water pressure in front of the tunnel face increased with the increase of the clay content in soil and the volume loss of the tunnel face. Seepage could evidently increase the limit effective earth pressure at the tunnel face. The limit effective earth pressure increased linearly with the increase of clay-sand ratio. The limit collapse range of the soil was mainly dependent on the rupture angle in front and rear of the tunnel face and the transverse rupture angle, in which the rear rupture angle was not much affected by the clay content in soil and seepage. Under the condition of no seepage, the limit collapse range of the soil increased with the increase of clay-sand ratio, while under the condition of seepage, the limit collapse range of the soil decreased with the increase of clay-sand ratio. The research results improved the understanding of the stability of shallow shield tunnel excavation under seepage conditions in sandy ground with clay, which can provide reference for practical engineering and limit analysis of stability.

Abstract:The stress states and deformation laws of the compound wall system of subway stations are quite different from theoretical calculation results, and the distribution law of earth pressure on retaining structure and the internal force of the station structure are uncertain. In this study, expanded polystyrene (EPS) foam was used to replace waterproof layer, and model tests and numerical simulations were conducted to investigate the stress states and deformation laws of compound wall and composite wall systems. The horizontal earth pressure behind the diaphragm wall, the surrounding ground surface settlement, and the structural internal force were obtained. Research results show that the compound wall technique had good load reduction effect and could effectively improve the stress state of the main structure of the subway station. The load reduction effect of the compound wall technique increased with the increase in the thickness of the EPS board or the decrease in the elastic modulus of the EPS board. However, the bending moment of the diaphragm wall would increase with the increase in the load reduction effect of the subway station structure. Thus, the load reduction effect is not always available at a higher EPS board thickness or a lower elastic modulus, but reasonable load reduction parameters of EPS board are required. The compound wall technique could induce a larger surrounding ground surface settlement than the composite wall technique. The influence area of ground surface settlement was about 0.60H to 1.25H in this study (H is the depth of the foundation pit), and the difference of the settlement between the two techniques was about 12%.

Abstract:To improve the horizontal vibration theory of tapered pile for better use, based on the Timoshenko beam model, the horizontal vibration problem of tapered pile embedded in viscoelastic foundation subjected to horizontal harmonic excitations was investigated. First, the Winkler foundation model and Timoshenko beam model were used to simulate the viscoelastic foundation and tapered pile, and the lateral coupling vibration model of pile-soil system was established. Then, the analytical expressions for the horizontal displacement, bending moment, and shear force of the tapered pile were derived by means of variable separation method and transfer matrix method. The influence of the design parameters of pile-soil system on the horizontal vibration characteristics of tapered pile was analyzed based on the obtained solutions. Finally, the results were verified by comparing with the Euler beam model solutions, and the difference between the solutions of the two models was analyzed. Research results show that the dynamic response of the pile top was little affected by the variation of the wedge angle. The dynamic response of the whole pile increased with the increase of the pile-soil stiffness ratio. With the increase of the dimensionless frequency, the horizontal displacement of the pile decreased gradually. Although there were differences between Timoshenko beam model and Euler beam model under small length-diameter ratio, wedge angle had basically the same effect on these two models.

Abstract:To optimize soil conditioning method and ensure the stability of excavation face and high efficiency of excavating, a good understanding of the mechanical behavior of conditioned soil in earth pressure balance (EPB) shield tunneling is essential. The pressurized vane shear apparatus was used to study the rheological behavior of conditioned soil from gravelly sand stratum. Results of undrained lateral confining compression tests revealed the relationship between the normal pressure and the compression strain of the foam-conditioned gravelly sand. Through vane shear tests, the shear deformation process of the foam-conditioned gravelly sand was investigated, and the peak and residual shear strength and corresponding effect factors were analyzed. Research results show that the foam-conditioned gravelly sand conformed to compressible non-Newtonian fluid. Therefore, constitutive model of compressible Bingham fluid for foam-conditioned gravelly sand was established based on rheology. The compressibility coefficient and rheological parameters (i.e., yield stress and viscosity) of the conditioned soil were obtained by fitting the test results. The pressure dependence formulas of density, peak and residual rheological parameters were proposed. The research can enhance the understanding of the mechanical behavior of foam-conditioned gravelly sand, and provide a theoretical basis for computational flow simulation to accurately reflect the macroscopic movements of conditioned soil in EPB shield tunneling.

Abstract:The mechanical properties of sand are very complicated and directly related to its physical state. Generally, it is manifested as shrinkage of loose sand and dilatancy of dense sand, which are affected by both relative density and effective confining pressure. In order to effectively describe the shear behavior of saturated sand under different physical states, based on the theory of thermal dynamics of granular materials and the energy dissipation mechanism at granular level, a thermodynamic constitutive model was proposed combined with the dilatancy equation with state parameters. The model is simple in form, which does not involve the concepts such as yield criterion and flow rule, but it introduces the concepts of granular entropy and granular temperature to describe the irreversible deformation, and establishes a relation between the dissipation mechanism of saturated sand and the macroscopic mechanical behavior through migration coefficients and energy density functions. Thus, the model can describe the influence of relative density and effective confining pressure on the strength and deformation characteristics of saturated sand during shear process. The ability of the model to describe the shear behavior of saturated sand was verified by comparing the results of isotropic compression tests, triaxial undrained, and drained shear tests with simulation results.

Abstract:To find a more suitable soil dynamic constitutive relationship model for ground fissure sites so as to accurately study the dynamic response law of different distributions of soil layers in common sites and ground fissure sites, the soil shaking table test for ground fissure sites was designed and completed with the f₄ ground fissure site in Xi′an as an example. Then, the secondary developed equivalent linearization model, boundary surface model, and Mohr-Coulomb model were established, and numerical analysis models were compared with experimental results. Finally, ABAQUS numerical analysis was carried out on ground fissure sites and normal sites under six working conditions with the boundary surface model as the constitutive structure, and the influence of ground fissure sites distributed in different soil layers on the dynamic response of soil was explored. Results show that the soil had a large plastic deformation under high earthquake intensities, and the boundary surface model was a suitable dynamic constitutive model for soil in ground fissure sites. The seismic waves could be amplified in common sites and ground fissure sites, but the effects were different under different working conditions. The ground fissure sites all showed acceleration amplification effect, and the upper and lower wall effect was similar to the shape of “八”. The maximum influence range of the upper wall was 11 m, and the maximum value of the lower wall was 9 m. The research results can provide reference for the seismic design and engineering application of engineering structures under the ground fissure environment.

Abstract:To investigate the penetration characteristics of ball penetrometers with various geometry characteristics in two-layered clay, large deformation finite element (LDFE) analyses incorprating modified remeshing and interpolation technique with small strain (RITSS) were conducted to simulate the ball penetration process. Soil flow mechanism and variation law of penetration resistance were analyzed. The numerical model was verified by theoretical solutions and experimental results. Through parametric analyses, the effects of top layer thickness ratio, soil strength, friction coefficient between soil and ball penetrometer, and shaft-ball area ratio on penetration characteristics were discussed. Results show that in stiff-over-soft clay, the trapped soil formed underneath the ball penetrometer was carried into the bottom layer. The size of the trapped soil was the key factor affecting the trapped soil effect. The size of the trapped soil increased with the decrease of the undrained shear strength ratio of bottom clay to top clay, and the increase of the friction coefficient between soil and ball penetrometer. The shaft-ball area ratio had certain effect on the penetration resistance, but the influence was less than those of friction coefficient and soil strength. The size was not affected by the thickness of the top layer or the undrained shear strength of the bottom clay. The thickness of the trapped soil varied from 0 to 0.20D, and the width ranged from 0 to 0.50D. The effect of trapped soil increased the penetration resistance in soft bottom clay and thus led to a larger value of the undrained shear strength. Therefore, considering the effects of soil strength, shaft-ball area ratio, and friction coefficient, a modified calculation formula was proposed to obtain more accurate results of undrained shear strength of soft bottom clay.

Abstract:In order to study the mechanical properties and failure mechanism of unloading damaged surrounding rock, RMT-150 rock mechanics test system was selected to carry out tests on triaxial rock in loading-unloading process to prepare damaged rock. Combined with AEwin acoustic emission system, the uniaxial reloading test was conducted on damaged limestone, the relationship curve of stress-time-energy accumulation was tested, and the macroscopic failure characteristics of the rock were analyzed. Experimental results show that with the increase of unloading points, the damage degree and failure form of the damaged limestone were changed obviously. The failure form of the limestone transformed from brittle failure to ductile failure, and the phenomenon of expansion was less and less obvious. In the process of uniaxial loading, the variation in the cumulative value of the acoustic emission energy of the damaged rock with peak strength lower than 70% tended to be the same, which can be divided into three stages: stable stage, stable growth stage, and secondary stable stage. The distribution mode of micro-cracks in rock had an obvious influence on its macro-failure characteristics, and the micro-mechanical response of rock determined its macro-mechanical failure characteristics.

Abstract:In order to evaluate the true triaxial strength properties of rocks, according to the variation of rock strength measured using conventional triaxial tests, a nonlinear strength criterion was developed based on the maximum value of deviatoric stress. The proposed strength criterion was compared with the measured strengths of 12 kinds of rocks and four typical rock strength criterions. The predicted results of the new strength criterion were close to the measured strengths of 12 kinds of rocks, and all the regression coefficients R² were larger than 0.98 and mean absolute percentage errors (MAPE,E_MAP) less than 4% (except 6.83% for No.7 rock). The average E_MAP of the 12 rocks were 2.32%, 2.43%, 5.28%, 7.39%, and 13.74%, calculated by the new strength criterion, exponential criterion, Hock-Brown (H-B) criterion, modified Mohr-Coulomb (MM-C) criterion, and Drucker-Prager (D-P) criterion, respectively. It shows that the strength criterion proposed in this paper could well predict the strength of different types of rocks, and its prediction accuracy was slightly better than that of exponential criterion, an outstanding criterion in rock mechanics, and was far better than the other three typical strength criterions. Based on the conventional triaxial strength criterion, considering the effect of intermediate principal stress, a true triaxial strength criterion was developed by introducing the intermediate principal stress parameter and Rhode stress parameter. Compared with the true triaxial test results of eight rocks, the proposed true triaxial strength criterion well reflected the variation that the maximum principal stress increased firstly and then decreased with the increase of the intermediate principal stress. The R² values of the eight types of rocks were all above 0.9, among which the R² values of five rocks were greater than 0.96. Except for rocks No.13 and No.14 with E_MAP being 7.79% and 4.84%, the E_MAP of the other six rocks were all smaller than 4%. It shows that the proposed strength criterion could well predict the true triaxial test strength of rocks, and reflect the effect of intermediate principal stress well with good universal applicability. The stress spatial characteristics of meridian plane and deviatoric plane also explained that the proposed strength criterion well reflected the effects of hydrostatic pressure and intermediate principal stress on the maximum principal stress.

Abstract:Sulphoaluminate cement and ordinary Portland cement were used to improve the frozen soil in order to ensure the stability of foundation and solve the problem of shortage of earthwork materials in permafrost area. In this study, the strength characteristics of the two types of cement were obtained by unconfined compressive strength tests at different dosages and different curing ages. The variation of free water, bound water, and constitution water were measured with the methods of centrifuge and freeze dryer in the process of cement improving frozen soil. The law of water transformation in the process of improving frozen soil was explored, and the relationship between constitution water, bound water, and the strength of improved frozen soil were established. Results show that the strength of the improved frozen soil treated with sulphoaluminate cement was obviously higher than that treated with ordinary Portland cement. Sulphoaluminate cement presented remarkable early strength characteristics. The water transformation rate of sulphoaluminate cement was faster than that of ordinary Portland cement in the early stage of curing, but the transformation amount of constitution water and bound water was low. The contribution of constitution water and bound water to the strength was more significant in sulphoaluminate cement improved frozen soil than in ordinary Portland cement. Sulphoaluminate cement is more suitable to improve frozen soil.

Abstract:To study the correlation between ground surface deformation and influential factors in permafrost regions, the Qinghai-Tibet Engineering Corridor (QTEC) from Xidatan to Amdo was chosen as research object. The small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) technique was applied to obtain the ground surface deformation of the study area. Empirical models were used to calculate the influential factors of the study area, including volume ice content, mean annual ground temperature, and active layer thickness, with the help of geographic information system (GIS) technology. Simple and partial correlation analysis methods were used to analyze the correlation coefficient between ground surface deformation and influential factors. Results show that the ground surface annual deformation rate ranged from -33 mm/a to 15 mm/a, and the study area was mainly dominated by settlement, with the mean ground surface annual deformation rate in the whole study area about -13 mm/a. In the areas where the volume ice content was greater than 30% and the mean annual ground temperature was higher than -1 ℃, the ground surface deformation was closely related to the influential factors, with the partial correlation coefficient greater than 0.8 and the P value less than 0.05. In the areas where the volume ice content ranged from 10% to 30%, the mean annual ground temperature was between -2 ℃ and -1 ℃, and the active layer was thicker than 3 m, the ground surface deformation had a stronger relationship with the influential factors that the partial correlation coefficient was about 0.4-0.8 and the P value was less than 0.05. In general, the ground surface deformation had strong positive correlation with volume ice content and mean annual ground temperature that the mean partial correlation coefficient was about 0.75 and 0.70 respectively, and there was a moderate positive correlation between ground surface deformation and active layer thickness that the mean partial correlation coefficient was 0.42.

Abstract:For the in situ reduction of landfill sludge, the reduction effect of freeze-thaw process combined with vacuum preloading was studied. Samples were taken from a landfill in Shanghai. First, five freezing temperatures (0, -6, -11, -14.5, and -18.5 ℃) were set during the freeze-thaw treatment of sludge to analyze the effects of freezing temperature on specific resistance of sludge and critical water content. Then, a model test was conducted to investigate the variation of displacement, dewatering rate, settlement, volume reduction ratio, water content, and unconfined compressive strength under different working conditions by comparing single vacuum preloading with freeze-thaw treatment combined with vacuum preloading. Results show that the freeze-thaw treatment could effectively reduce the specific resistance of sludge and improve its dehydration performance, and the optimum freezing temperature was -11 ℃. The sludge after freeze-thaw treatment showed a good drainage consolidation effect in the process of vacuum preloading, and the optimum treatment temperature was consistent with the results of sludge specific resistance test. The minimum water content of the treated sludge reached 54.0%, the unconfined compressive strength was up to 42.84 kPa, and the maximum reduction ratio was 40%, which achieved the purpose of sludge reduction.

Abstract:To further explore the failure mode and mechanical mechanism of reservoir-reactivated landslide, a new test device was designed and developed, the main body of which is composed of several permeable boxes, and it can simulate sliding surfaces with different geometric shapes. The sliding zone can be multi-stage softened by injecting water into different permeable boxes, so that the retrogressive-slide process between adjacent sliders caused by the rise of the reservoir water level can be realized. Through the techniques of digital photography, digital image processing, and transducers measurement, a series of important test data were obtained, such as volume water content, pore water pressure, horizontal displacement of slope, and evolution characteristics of the trailing edge surface. Moreover, the failure mode of the landslide body was discussed. Test results show that the significant decrease in shear strength of sliding zone soil after saturation was an important reason for the occurrence of landslide. The pore pressure variation at the sliding surface was the main inducing factor for the revival of the old landslide. The increase in pore water pressure at the sliding zone occurred simultaneously with the increase in the landslide displacement, and there was a good correlation between the strength degradation of sliding zone soil and the deformation of sliding body. The slope deformation area was divided into three parts: strong and weak deformation area and traction area. The slope deformation range was about 1 to 1.5 times the length of the unstable sliding zone. With the increase in the length of unstable sliding zone, the deformation zone of the slope increased, and the influence on the traction deformation of the back stable slope decreased. The trailing edge rupture surfaces were mostly in the shape of folded slip surface, and the experimental values of the inclination angle of the trailing edge surface were significantly affected by the position of the unstable sliding section and the thickness of the sliding body. The research results provide important basis for the understanding and controlling of landslide disasters in reservoir area.