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2014  Vol. 36  No. 12

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Orginal Article
Abstract:
First, based on a series of tests on reinforced concrete segment joints, the rotational moment-angle models for segment joints including linear, bilinear and nonlinear models and the axial force-deformation and shear force-deformation piecewise models are summarized and proposed, and the theoretical models are also given to describe the effect of the initial torque of bolts and axial force on joint stiffness. Second, based on the bending tests on flat plate segments and arch segments with or without sealing materials, the parameter determination methods for various stiffness models and the model selection principles are proposed, and the effect of the initial torque of bolts and axial force on joint stiffness is discussed. Based on the shear tests on flat plate segment joints with or without sealing materials, the parameter determination method for three-stage shear force resistance stiffness model is proposed. The results have proved the applicability of these joint stiffness models and will promote the application of beam-joint model theory in design practice.
Abstract:
According to the working characteristics of the bidirectional reinforced composite foundation under embankment loads, the embankment and composite foundation in the equivalent reinforced range of a single pile are regarded as a typical analysis element. The large deflection circular ring thin plate is introduced to simulate the combined effect of “flexible raft” and “tensioned membrane” of the reinforced mattress. And the layered property of the foundation soil is considered based on the assumed model for the relative displacement between the pile and the soil. A mechanical model with consideration of deformation compatibility between the embankment, reinforcement and pile-soil reinforcement area is established based on the above operation, and the methods for calculating the load sharing ratio and settlement are proposed respectively. Field tests are used to verify the proposed method and the influence of the factors such as height of embankment, width of pile cap and tensile modulus of reinforcement on the position of neutral point, differential settlement and maximum tensile stress in the middle plane. The comparative results indicate the proposed method is feasible to be used in the practice.
Abstract:
According to the cyclic degradation behavior of saturated clay, a nonlinear kinematic hardening constitutive model considering cyclic degradation for saturated clay is developed in commercial finite element software. A degradation law with the equivalent plastic strain as a variable is introduced in the isotropic hardening rule. The nonlinear kinematic hardening law is adopted to describe the cyclic hysteretic characteristics of clay. Degradation of soil stiffness is considered as well. The model is validated against the test results of saturated clay under one-way cyclic loading. Based on this model, 2D numerical simulations of a single pile under two-way axial cyclic loading by FEM are conducted. The degradation of axial bearing capacity of a single pile under different cyclic loading levels, cyclic loading numbers and stiffness indices of soil are studied. Finally, this numerical method is validated by simulating the results of model tests in the literatures.
Abstract:
To obtain the spatial distribution of accelerations induced by moving loads, a coupled nonlinearly true three-dimensional numerical model for high-speed railways with a design speed of 350 km/h is established based on the multi-scale and precisely modeling technology. The model is composed of rails, fasteners, cement-asphalt (CA) mortar, reinforced concrete roadbed, upper layer of roadbed, lower layer of roadbed, embankment and foundation. The dynamic interaction between bottom of the reinforced concrete base and surface of the upper layer of roadbed is simulated using the dynamic contact algorithm. The radiation damping and elastic recovery of infinite foundation are simulated using the three-dimensional viscoelastic static-dynamic unified artificial boundary. Considering the objective influence of static stress state in the embankment before action of the moving loads on the subsequent dynamic computation and the nonlinearities of foundation soils and backfilling materials of the embankment, generation of the initial stress state of the foundation, construction of the embankment and rail system and subsequent operation of electric multiple unit (EMU) train with 8 cars are simulated using the large-scale parallel computation. The distribution of accelerations in time and space-domain for different components of rail-embankment-foundation system is summarized based on the analysis results. The advantages using solid elements to simulate spatial dynamic behaviors of rails are validated.
Abstract:
The underlying mechanism of creep deformation of rockfill dams is the particle deterioration due to environmental changes and the particle breakage under high contact stresses. A solid hardness parameter reflecting particle strength is introduced into the isotropic compress equation. In order to describe the deterioration process of rockfill particles with time, the solid hardness parameter is assumed to vary with time, following a mathematical representation of hyperbola distribution. By analyzing the data of triaxial creep tests on rockfill materials, a hyperbolic creep flow rule is established. The creep formulae for deformation are derived, and the expressions for creep modulus are proposed. Then a visco elastic-plastic constitutive model for rockfill materials is proposed considering the influence of both loading and creep deformation. Finally, the validity of the proposed model is confirmed by comparing the simulated results with the measured data of two kinds of rockfill materials.
Abstract:
With the effects of many parameters, such as seepage, swelling and dilatation, the deformation of tunnels in swelling rocks under seepage is a complex time-dependent problem. The closed strain hypothesis is proposed. The equilibrium equation for the tunnels in swelling rocks under seepage is established. And then the elastoplastic solutions considering the effect of dilatancy angle are obtained. The answers of Yu dongming's and Hou gongyu's are the two special cases of the solutions. Accordingly, the viscoelastic-plastic creep solutions of the tunnels in swelling rocks under seepage considering the effect of swelling strain are obtained by using the modified Nishihara model. The influence of the related parameters on the time-dependent deformation of tunnel wall is analyzed, such as inflation, seepage and expansion. The results show that the time-dependent deformation presents viscoelastic-plasticity gradually with the decrease of the ratio of inner to outer water heads or the increase of the dilatancy angle. The moisture expansion coefficient α2 affects only the viscoelastic deformation of the surrounding rock, and the displacement monotonically linearly increases with it. At the early stage of excavation, the closed strain develops rapidly into displacement, which greatly influences the deformation of swelling rocks under seepage. The results can be taken as a guide for the construction and deformation control of the tunnels in swelling rocks.
Abstract:
Grouting is frequently used to control and adjust the large transverse deformation of shield tunnels in clays. However, the grouting is usually performed based on the engineering experience. The control mechanism of grouting for the tunnel transverse deformation is not clear. In this paper, the method to simulate the performance of grouting through the resulted soil expansion is proposed and validated. The effect of grouting on the large transverse deformation of shield tunnels is investigated in terms of the change of tunnel diameter and joint movement using numerical simulation. The optimization is finally carried out through the parametric study. The researches indicate that the grouting can significantly reduce the tunnel transverse deformation and joint opening, and can also produce remarkable joint offset when the volumetric strain of ground caused by the grouting is larger than 3%. The grouting firstly causes rotation and then rigid movement of tunnel segements. Thereafter, the joint opening reduces more significantly due to the segmental rotation at the beginning of grouting. The joint offset increases significantly due to the rigid movement of tunnel segments when more grouting is injected.
Abstract:
The situations of the occurrence and development of piping are different because of different soil layer structures of embankment. Based on the laboratory tests, the destruction processes of piping of multilayer embankment with three kinds of different sand layers are simulated. By changing the grain-size distribution of the underlying sand layers, the effect of sand layers with different grain-size distributions on the mechanism and process of the occurrence and development of piping is analyzed. The test results show that when the sand layer of multilayer embankment is fine sand, it bears larger water pressure, and the critical hydraulic gradient is high. Once piping occurs, the seepage discharge, erosion mass and damage range are large. The piping failure of this kind of embankment is sudden and dramatic, and prevention measures should be taken as early as possible. When the sand layer is coarse sand, the situation of piping is like that of sand gravel layer. The damage scope of piping is confined to the top of the gravel layer, and the damage depth is limited. When the sand layer is fine gravel, the critical hydraulic gradient is small, the damage extent gradually increases, and the speed of piping is quick because large amount of erosion embankment can generate obvious seepage deformation.
Abstract:
Field tests on a 50 m-high geogrid-reinforced soil embankment without concrete panel of Yichang-Badong Highway in Hubei Province are conducted. The tests include the measurements of tensile strain of geogrids, vertical earth pressure, horizontal earth pressure, layered settlement and deep horizontal displacement. The stress and deformation laws are revealed and the mechanism is analyzed. The results show that the maximum tensile strain of geogrids at different layers appears to be about 4-6 m away from the wrapped face, and the tensile strain along reinforcement has two peak values. The strain of geogrids has obvious lag in the construction period, it shrinks and rebounds after construction. The geogrids adjust the distribution of earth pressure. The measured value of the vertical earth pressure at the end of geogrids is slightly larger than the theoretical value, and it is less than the theoretical value in the middle and outside part of the slope. The horizontal earth pressure is nonlinear along the filled earth height. In the middle of the embankment, the horizontal earth pressure is larger than that at the top, and it is less than the active lateral earth pressure. There are a lot of volatilities in the layered settlement, and the value in the middle of the embankment is greater than that in other areas. The horizontal displacement decreases with depth, and it develops to a certain extent after construction.
Abstract:
The variation of mechanical properties of granite under uniaxial compression after being soaked in different chemical solutions (water, NaOH solution and HNO3 solution) and subjected to different freeze-thaw cycles is experimentally studied. The damage mechanism of granite subjected to chemical soaking and freeze-thaw cycles is analyzed according to the micro mechanism and chemical mechanism. In addition, a damage variable is defined in terms of which the damage degree of granite can be analyzed quantitatively. The test results show that for all the chemical solutions (water, NaOH solution and HNO3 solution), with the increase of freeze-thaw cycles, the relative Young’s modulus decreases by exponential function, the loss rate of the peak stress increases by power function, and the axial peak strain changes by Guass function. With the increase of freeze-thaw cycles, the damage and deterioration of granite in HNO3 solution is larger at a smaller number of freeze-thaw cycles than that at a larger number of freeze-thaw cycles, while an opposite phenomenon is found for granite soaked in NaOH solution. The process of freeze-thaw damage is essentially a process in which the granite is damaged gradually by thermal stresses, meanwhile, the chemical damage produced by chemical dissolution together with the freeze-thaw damage can give mutual effects on the damage and deterioration of granite.
Abstract:
For the gypsum specimens containing a set of preexisting open flaws with fixed joint continuity factor, the influences of inclination angle and spacing of joints on the strength and deformation of the jointed specimens under uniaxial compression are systematically investigated through experiments. It is found that: (1) With the increase of joint spacing, the axial stress-axial strain curve changes from single-peak curve to multi-peak one, indicating that the ductility of the specimens increases. Four types of stress-strain curves are observed, i.e., single-peak, multi-peak during softening stage, softening after multi-peak yield platform and hardening after multi-peak yield platform. (2) For the specimens with constant joint spacing, the curves of the unified Young’s modulus, the unified strength and the first peak strain with the inclination angle of joints are V-shaped, and the minimum occurs at the inclination angle of 45 degrees, while the ratio of the residual strength to the strength and the last peak strain have inverse tendency. (3) For specimens with constant joint inclination angle, with the increase of the jointing index (the reciprocal of the joint spacing), the unified Young’s modulus, the unified strength and the first peak strain decrease, while the ratio of residual strength to strength and the last peak strain increase. (4) For each joint inclination angle, the relation between the unified Young’s modulus and the jointing index and that between the unified strength and the jointing index can be expressed by the power functions. (5) The macroscopic behavior of the jointed specimens is correlated to the closure of pre-existing joints and the cracking process of the specimen matrix. The anisotropic influence of joint spacing on the strength and deformation of jointed rock masses is significant.
Abstract:
Based on the triaxial compression tests of thirty granite samples under different confining pressures and temperatures of 25~1000℃ by MTS815.02 servo-controlled testing machine,the influence of temperature and pressure on the deformation and strength characteristics is analyzed. The test results show that: (1) The quality of rock samples decreases slightly with the increasing temperature, which decreases by only 0.364% at 1000 ℃ compared to that at 25℃. When the temperature is below 600 ℃, the volume and density have no significant change, then as the temperature rises, the volume expands with acceleration and the density decreases rapidly, and the volume expands by 5.027% and the density decreases by 5.132% at 1000℃ compared to those at 25℃. (2) After high temperature, the triaxial compression stress-strain curves of rock samples have gone through the stages of compaction, elasticity, yield, destruction, softening and residual stresses. The stiffness, peak strength, shear strength, residual strength and plastic deformation of rock samples increase with the confining pressure. (3) The cohesion decreases linearly and the internal friction angle first increases and then decreases with the increasing temperature. The shear strength shows a quadratic polynomial reduced relationship as the temperature increases. The confining pressure weakens the influence of temperature on the shear strength. This work may provide the guidance for the selection of mechanical parameters of rock in the underground geotechnical engineering.
Abstract:
The ground movements caused by urban shallow tunnelling can be passed to the ground surface and then form surface settlements. It is necessary to accurately predict the ground movements before tunnelling for effectively controlling the ground settlements. Based on the complex variable function method, the series forms of complex potential functions in z-plane are first obtained using the inverse mapping function. By taking the derivative of analytical functions proposed by Verruijt with Cauchy-Riemann equations, explicit analytical solutions for the stress and displacement induced by shallow tunnelling are obtained. The explicit solutions are intuitional and easily used by engineers, which have smaller amount of calculation than the solutions proposed by Verruijt. Moreover, by improving the previous researches on deformation modes of shallow tunnels, three deformation modes and two deformation ratios are put forward. Finaly, the explicit solutions are compared with the field test data of two tunnel projects, and the accuracy and practicability of the proposed explicit solutions are proved.
Abstract:
The chemistry of pore water obviously affects the hydro-mechanical behaviour of clay, but the quantitative evaluation of pore water effect needs to be further conducted. The oedometer tests on kaolinite and Ca-bentonite mixtures saturated with different concentrations of NaCl solution are performed to investigate pore water chemical effect on the hydro-mechanical behaviour of remolded clay, where the osmotic suction is adopted as the characterization parameter. The results indicate that the compression index exponentially decreases with the osmotic suction, while the swelling index is almost constant. The compression behaviors after pre-yielding can be normalized in the Iv-lgσv system proposed by Burland, but it is not suitable before pre-yielding. There is a negative correlation between the secondary consolidation coefficients and the osmotic suctions, furthermore, the ratio of the secondary consolidation coefficient to the compression index decreases with the osmotic suction. The coefficient of Cke/Δlgkv to depict the hydraulic conductivity is related to the initial void ratio and can be expressed as a function of the osmotic suction, which extends the understanding of this parameter.
Abstract:
The retaining wall constructed with soilbags is relatively flexible and may deform under the action of the lateral earth pressure. The distribution and value of the lateral earth pressure are quite different from those of a rigid retaining wall. Model tests are performed on a retaining wall constructed with soilbags to investigate the deformation mode of the wall and the failure mode of the back-filled soil. The distribution of the earth pressures along the height of the wall and the horizontal transfer within the retaining wall are investigated. An equation for calculating the earth pressure on the retaining wall of soilbags is derived from the force equilibrium of a differential element. The earth pressures calculated by the derived equation agree roughly with the experimental measurements. The frictions between the soilbags are experimentally investigated, and are used to explain the horizontal decrease of the earth pressures within the retaining wall.
Abstract:
Based on the acid-base proton theory, the erosion mechanism after heavy metal pollutants infiltrating into rock is discussed. The extension performances of lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn) are analyzed by using the convection-diffusion model. By way of combining the viscoelastic model and the damage monomer model, a 3D extended-fissile model is established for heavy metal contaminated rock. The deformation rules of invaded surface are discussed under different extension rates. The model fitting results are verified by the data of axial static load tests. The results show that different heavy metal pollutants have different invasion and extension performances in rock, and there is a good agreement between the experimental stress-strain curves of contaminated rock with different lithologies in the same test conditions and the fitting results of the combination model.
Abstract:
Field test data show that the installation of HDPE pipes results in considerable additional strain and stress on the pipes. However, it has not been well addressed in previous studies. In this study, the 0.6 m-diameter HDPE pipes are used in the field tests to investigate the relationship among the hoop strains, vertical and horizontal deflections and thickness of soil cover. It is found that the hoop strains generated in the construction phase (i.e., installation of HDPE pipes) have a good linear relationship with the deflection and the thickness of soil cover. This finding is then validated by a series of finite element numerical simulations. Based on the FE analysis, an empirical relationship is proposed to predict the hoop strains on the HDPE pipes generated in the construction phase. The availability of the proposed method is validated by comparing the predicted values with the field measured ones reported in the previous studies. The results demonstrate that the proposed method has merits of satisfactory accuracy and easy use.
Abstract:
In the current design of geosynthetic and pile supported (GSP) composite foundation, a pre-assumed soil arch height is always utilized in load sharing calculation, and the contribution of subsoil resistance is weakened in supporting the embankment fill. To improve the above two defects, the soil arching effect and the geomembrane effect are analyzed, and then the quantitative evaluation methods are presented. In the derivation, the pile-soil differential settlements at pile top and toe are considered, and the arch height can be calculated according to the relative pile-soil displacement. Simultaneously, the neutral point is adopted in skin friction analysis, and hence a formula for the load distribution is obtained based on the mobilized shearing stress developing from bottom to top. This method can take good care of the interaction between the embankment fill, pile-reinforced area and lower underlying layer, which can consider the penetration of pile tops, interaction between piles and surrounding soil and penetration of pile ends. According to the continuity condition of stress and displacement, a load effect solution of GSP composite foundation is obtained. The present method is validated to be reasonable by comparing the analytical solutions with the FEM results and the monitoring data, and can be adopted by engineers when it comes to the situation in presence of the bearing stratum at pile tip with certain stiffness.
Abstract:
The conventional methods for slope stability analysis are mainly based on the theory of saturated soils and the stable seepage situation and difficult to reflect the influence of rainfall and water level drawdown on slope stability. Actually quite a number of slope instabilities are caused by transient unsaturated seepage. Based on the effective principle of unsaturated soils by Bishop and the theory of two-stress-state-variable mechanism of the shear strength by Fredlund, the water pressure at any time is introduced into the balance equation as the volume force and the effective stress fields satisfying the conditions of equilibrium, stress boundary, yielding and stress discontinuity are constructed. By using the idea of nonlinear programming, a lower bound analysis model for the slope stability subjected to transient unsaturated seepage is established, and the corresponding program is compiled. Through numerical examples of impoundment drawdown and rainfall infiltration analysis, the correctness of the proposed method is verified, and a further understanding about the law of slope instability subjected to transient unsaturated seepage is obtained.
Abstract:
Micropiles are common reinforcements often used for slope stabilization under emergency conditions. In many circumstances they are extensively installed to cover a large area. A flow chart was suggested to calculate the factor of safety for micropile-reinforced slopes according to the shear strength reduction technique. Coupled analyses are performed for micropiles in a homogeneous slope with and without a thin weak layer. It is shown that the position of the micropiles has a significant impact on the failure mode of the reinforced slope for a homogeneous slope, indicating that the conventional design method based on uncoupled analysis in which the pile response and slope stability are considered separately is inaccurate. For relatively small embedment of micropiles, the response of micropiles is dominated by rigid-body rotation without substantial flexural distortion. The critical embedment depth to achieve fixed conditions at the base of the micropiles is found to range from 1.5 to 2 times the length above the sliding plane. Truncation of micropiles likely increases the capacity of the reinforcement system, and this benefit is particularly important for the slope with a thin weak layer. The largest truncation length of micropiles for homogeneous slopes and the slope with a thin weak layer is about 1/4 and 1/2 times the length above the sliding plane, respectively. The portion of truncated micropiles can be filled with soils and compacted in the standard procedure to decrease the cost in practice engineering.
Note
Abstract:
To study the rockburst nucleation and evolution process in deep-buried tunnels for Jinping Ⅱ Hydropower Station, a microseismic real-time monitoring system is established to monitor diversion tunnel No. 3 during TBM excavation. Based on a great deal of microseismic monitoring data, the characteristics of spatiotemporal distribution, activity rate, exhausted energy, energy density and apparent volume of microseim are comprehensively considered before occurrence of rockbursts. What’s more, the initiation, propagation, extension and inter-coalescence mechanisms of microcracks during progressive nucleation process of rockbursts at Jinping Ⅱ Hydropower Station are revealed. The spatial relationship between evolution rules of microseismic activities and rockbursts is discussed. The monitoring results show that precursory microcracking exists prior to most rockbursts, which can be captured by the microseismic monitoring system. The event rate, microseismic released energy and accumulated apparent volume have their different increase tendencies before rockbursts. The energy source of some rockbursts is composed of local energy and transfer energy, namely Edrive source =Elocal+Etransfer, and a portion of rockbursts can be induced by certain adjacent violent rockbursts at the same time. It is feasible to monitor and predict rockburst events by using the microseismic monitoring technology for deep-buried tunnels.
Abstract:
Based on Wumengshan No. 2 railway station tunnel with four tracks, numerical simulation is performed to comparatively study the differences of loosened zone between multi-partition excavation and full face excavation. Based on the field monitoring data, the evolution of loosened zone by multi-partition excavation is analyzed. The results indicate that: from the viewpoint of thickness of loosened zone, the full face excavation makes it greater than the multi-partition excavation does, with the maximum value being 12.6% greater and the minimum value being 41.2% greater. From the viewpoint of distribution of loosened zone, the full face excavation makes it generally symmetrical, but the multi-partition excavation does not. Besides, for the multi-partition excavation, the thickness of loosened zone decreases from the crown to the side wall, but for the full face excavation, the thickness has a distribiotion of “crown > arch toe > arch shoulder > side wall”. For the multi-partition excavation case, the loosened zone near the earlier excavated part will increase owing to the later excavation of other parts. This influence of “up and down position” is greater than that of “left and right position”. Furthermore, for the left and right position, the loosened zone near the later excavated part is little greater than that near the earlier excavated part. This makes it clear that the earlier excavation has deteriorating effect on the later excavated part. The final loosened zone is the result of excavation interaction of all parts, so it is highly process-related.
Abstract:
The automatic monitoring system for safety of earth-rockfill dams based on Internet of Things is established. Both the hardware and the software for nodes of sensors, routers and coordinator node are designed by using the wireless sensor network chip CC2530 and GPRS module. The wireless sensor network and geographic information system are integrated. The all-weather real-time monitoring, control and data acquisition of a series of parameters including dam deformation, seepage, stress and strain as well as environmental and meteorological observations are achieved. Therefore the intelligent and scientific management of safety of earth-rockfill dams is realized with the help of the expert knowledge base.
Abstract:
Using the vertical-torsional coupling shear apparatus, a set of bi-directional cyclic loading tests on saturated sand under isotropic consolidated condition are conducted. Based on the existing definitions of dynamic strength, the effects of phase difference of bi-directional dynamic load (β) and ratio of bi-directional dynamic load amplitude (λ) on dynamic strength and pore pressure of sand are studied. The test results show that the dynamic strength of liquefaction of saturated sand considerably relates with both β and λ. The existing definitions of dynamic strength have obvious limitation. The phase difference of bi-directional dynamic load (β) and the ratio of bi-directional dynamic load amplitude (λ) have significant influences on the growth rate of pore water pressure, while they have no significant influence on the development model for the normalized pore water pressure.
Abstract:
The analytical layer element solution for axisymetrically transversely isotropic multilayered soils is adopted to solve the interaction between the rigid circular plate and the foundation. Then based on the displacement continuity conditions between the soil surface and the rigid circular disc, the integral equation of an axisymmetrically loaded rigid circular disc on transversely isotropic multilayered soils is developed. The integral equation is solved numerically, and the contact stresses between the rigid circular disc and the subsoil are obtained when the rigid circular disc is given a value of displacement. This solution is implemented into a computer code for numerical analysis, and the influences of parameters of transversely isotropic foundation, thickness and stratification of the soils on the contact stresses are analyzed.
Abstract:
The conventional method for deformation monitoring of earth-rock dams based on a single point cannot acquire enough information. It doesn’t consider the position and the relationship between points. It cannot accurately monitor the real and three-dimensional deformations of the earth-rock dams. Owing to their complex structure and non-rigid character, the earth-rock dams exhibit irregular deformation when subjected to external force. So a new method for dam deformation monitoring based on three-dimensional laser scanning point cloud data is proposed. First, the absolute orientation is implemented for the point cloud data series collected at different time, allowing that the post-possessing can be performed in a unified coordinate system. Then, the cross-section point cloud data series at the same location collected at different time are compared and analyzed to figure out the dam deformation and displacement in horizontal and vertical directions. Jiangang Reservoir in Zhengzhou City is chosen to verify the accuracy and efficiency of the proposed dam deformation monitoring method. The experimental results show that the proposed method is highly accurate and real-time, and can meet the requirements of dam deformation analysis.