The stability of excavation face in shield tunneling plays a key role for construction safety. The ignorance of soil anisotropy in most previous studies would induce inaccurate stability assessment. ...This paper studies the failure of shield tunnel face in cross-anisotropic granular media by physical model tests and discrete element simulation. Model tests were carried out on the tunnel face stability in anisotropic granular media, and initial anisotropy was generated by controlling the long axis of non-spherical particles. By conducting image analysis on the picture taken by HD camera, the failure mode of tunnel face was obtained. It consists of a sliding wedge and an overlying loosen area, and the inclination angle of sliding wedge varies with the bedding plane. The variation in limit support pressure with the intersection angle of the shield tunneling direction and the soil bedding plane was obtained. Discrete element simulation was further employed to study the tunnel face stability in cross-anisotropic granular media; the microscopic parameters were calibrated by fitting against the particle drop test and repose test. Clump particle consisted of three identical ball was used in the simulations, and its long and short axes were in accordance with rice particles. The obtained varying characteristic of limit support pressure with intersection angle from simulation is consistent with the test results, and the obtained failure mode is also similar to that of physical model test. The principal stress distribution at failure state was analyzed in the discrete element simulation, and the change of major principal stress direction from vertical to nearly horizontal in the loosen area clearly shows the formation of soil arches.
The determination of physical and mechanical properties of bauxite residue plays an important role on the stability assessment of tailing disposal dams. This paper studies the composition, physical ...properties, shear strength, and bearing capacity of bauxite residue by laboratory and field in-situ tests. The bauxite residue near the discharge point are mainly coarse particles, and the size of particle far away from the reservoir shows a decreasing trend. Through X-ray diffraction, the composition of bauxite residue is analyzed: the tailing material are mainly composed of seven main crystalline materials containing aluminum, iron, titanium, and silicon. The variation of water content with depth is measured by field tests; the water content distribution shows obvious stratification characteristic and sudden change occurs between layers. The shear vane tests show that the shear strength at a depth of more than 5 m is much larger than the material in the shallow layer close to the surface. Through dynamic penetration tests, the stratum can be categorized according to the distance from the discharge point. The bearing capacity of the soil layer close to the edge area is higher than that in the center of tailings pond. The bauxite residue shows anisotropic behavior and the mechanical property varies along horizontal and vertical directions.
Saturated tailings deposits in the upstream dam are at risk of static liquefaction due to elevated porewater pressure levels. The concept of a potential static liquefaction zone was conceived, and an ...enhanced method of slices evolving that zone was proposed. The centrifuge test modeled the aluminum tailings dam with a step-likely increasing overburden, to investigate the stress state and deformation pattern during service life. Results showed that the tailings lasted a low level of effective stress and continuous accumulation of flow deformation after the first liquefaction occurrence, implying an enduring liquefied zone under subsequent loading. For a safety design, that zone is referred to as the potential static liquefaction zone and soil inside the zone should be considered to have strength parameters from the instability line instead of the failure line. Based on this assumption, an enhanced method of slices, considering the strength reduction in the potential static liquefaction zone, was proposed. To verify the enhanced method, the centrifuge test of the liquefied tailings was simulated in GeoSlope, where a deep riskiest sliding surface, a large failure zone, and a factor of safety of less than 1 were predicted. However, the traditional Bishop method predicted a factor of safety greater than 1 for the liquefied tailings. Significant predictive differences were also observed in two other liquefied cases: the Merriespruit tailings dam and the Fundão tailings dam.
High and steep sandstone slopes along highway line are at high risk of disasters such as landslides, cracking of support structures, and so on. The monitoring, early warning, and emergency response ...of such slope disaster face enormous challenges, especially during the rainy season. In this paper, intelligent monitoring, early warning and forecasting system were carried out for the high steep sandstone slope with a transmission line tower at the slope crest along the highway under construction in Guangxi, China. The automatic monitoring data, emergency rescue program and rescue effect were analyzed, and emergency rescue measures for high steep slope protection were taken. The research results show that timely access to disaster warning information can effectively support the analysis of disaster causes and the evaluation of disposal programs. Deep-hole monitoring of deformation characteristics can determine the stable state of slopes, and the tangent angle warning criterion can be used for early warning and prediction of high steep slope landslides. By analyzing the location of the sliding surface and taking timely emergency disposal measures such as layered counterpressure method and micropipes, the landslide activities can be effectively controlled to prevent further acceleration of slope collapse. This study can provide an important reference for the monitoring, early warning, forecasting and emergency rescue of sandstone slopes along highways under construction.
Understanding of the failure mechanism and limit support pressure of a shield tunnel face under seepage condition is important in engineering design and construction. Nine physical model tests, i.e. ...three tests in dry sands, three tests in submerged sands under undrained condition, and three tests under seepage condition were carried out. A rigid plate was set in front of the tunnel face to maintain stability at the initial state. By moving the plate backward from the soil, the displacement and earth pressure curve of the tunnel face was obtained. The earth pressure dropped sharply to a constant value which corresponds to the limit support pressure required to stabilize the tunnel face. The limit pressure was found to increase with the cover-to-diameter ratio in a shallow tunnel, and it turned to be irrelevant to cover-to-diameter ratio in a deep tunnel. The ratio of limit pressure and initial earth pressure decreases with the cover-to-diameter ratio, indicating the effect of soil arching in stabilizing tunnel face stability. Under the same water level condition, the limit pressure of tunnel face with seepage is larger than the one without seepage. The limit support pressure under seepage condition is about 70% of the earth pressure at rest; it is about 50% under undrained condition and is about 15% in dry sand. The flow line around the tunnel face was traced by infused pigment, and the distribution was shown to be irrelevant with water level. The image captured by a HD camera during the test was analyzed by PIV (particle image velocimetry) analysis; the soil particle movements and the distribution of shear strain showed the failure mechanism of tunnel face. The failure mode is a combination of a wedge with slip arc and a prism. Comparing with the failure mode in dry sands and in saturated sands below water table undrained condition, the inclination angle of the wedge block under seepage condition is much smaller.
•Shield face stability model is proposed for layered ground with seepage flow.•Collapse mechanism includes wedge with inclined angle varying between soil layers.•Limit equilibrium is established by ...method of slices and considering seepage force.•Stability model is validated by 3D stress-seepage coupled FEM.
This paper presents a limit equilibrium model for the shield face stability with special emphasis on the effects of ground heterogeneity and seepage flow. The model considers a new wedge mechanism whose inclination is capable of changing across soil layers to account for the spatial variation of soil mechanical properties. The limit equilibrium of the mechanism is established by using the method of slices, which considers horizontal arching without the need for arbitrary assumptions on soil stress distribution. The deterioration effects of groundwater flow are included by evaluating seepage force in the limit equilibrium based on an analytical hydraulic head field. To assess the proposed model, three-dimensional coupled pore fluid flow and stress finite element simulations are conducted. The above two methods are compared under different combinations of ground profiles, soil parameters, and hydrogeological conditions. These comparative analyses show the suitability of the limit equilibrium model. The influences of how horizontal arching is considered on the computed necessary face pressures under complex geological and hydrogeological conditions are explored.
This work presents a viscoplastic localization criterion to detect quasi‐instantaneous (i.e., load‐induced) and delayed (creep‐induced) strain localization in rate‐dependent solids. The study is ...based on the theory of controllability and a viscoplastic description of the mechanical response. Analytical precursors of unstable states are defined through systems of ordinary differential equations (OEDs). The use of the proposed criteria is illustrated at the material point level through a set of strain localization analyses simulating active strain localization of a porous rock. In addition, full‐field finite element simulations of compression tests conducted under various pressures are reported to demonstrate the role of local unstable viscoplasticity in the spontaneous propagation of deformation bands under stationary boundary conditions. The study shows that the viscoplastic localization criterion maintains a negative sign as long as the behavior is unstable, that is, the rate of deformation is accelerating. The sign switch coincides with the transition to decelerating deformation. The analyses revealed that pulses of overstress always emerge in correspondence with the growth of unstable behavior, and the peak matches the transition to stable behavior. The local responses recovered from full‐field analyses were consistent with those observed in analyses at material point level and the predictions of the presented theory.
•Face stability model is formed for shield tunnel in ground with confined aquifers.•Tunneling-induced seepage flow shows distinct pattern from that under phreatic water.•Analytical seepage flow model ...is combined with a multilayer wedge mechanism.
Confined aquifers with high fluid pressures pose severe risks for the stability of the face of earth-pressure-balanced shields. This study presents a method to compute the limit face pressures in the layered ground with confined aquifers. Seepage analyses based on the finite element method (FEM) are conducted to investigate the underground water flow patterns in the above ground due to shield tunneling. These analyses show: (1) the presence of confined aquifers enhances the hydraulic gradient around the tunnel face; (2) the hydraulic heads display distinct distribution patterns in the confined aquifers and adjacent aquitards; (3) the hydraulic gradients exhibit abrupt jumps at the aquifer-aquitard interface. These characteristics are in strong contrast to the distribution functions assumed in existing studies. Therefore, a new analytical flow model is proposed based on the hydraulic fields obtained from the seepage analyses. An approach to evaluate necessary face pressures is constructed by combining the flow model and a limit equilibrium model (LEM). By comparing against the results from drained FEM analyses under seepage flow, this paper shows that the proposed LEM can reasonably represent the influences of confined aquifers on the critical face pressures and the collapse mechanism of soils in front of the tunnel face.
•The self-consolidating CTG was prepared to fill voids of pre-placed CGs skeleton.•The relationship between the rheology of CTG and the filling ability was investigated.•The strength efficiency, ...cost, and embodied CO2 of CGFB were analyzed.
Aiming at the utilization of regional mining wastes including iron tailings sand and coal gangue (CG), a novel coal gangue-filled backfill (CGFB) is proposed in this paper, which is achieved by casting the self-consolidating cement-tailings grout (CTG) into pre-placed CGs skeleton. The mixture proportions of self-consolidating CTGs were first designed based on the rheological properties by adding viscosity enhancing agents (i.e. hydroxypropyl methylcellulose and polyacrylamide) and polycarboxylate superplasticizer (PCE). As a consequence, the static stabilities and filling abilities of fresh CTGs were also strengthened. The filling ability of fresh CTGs is a flowability-related parameter and can be reflected by the surface bugholes of hardened CGFBs. The results indicate that the use of CTG with flow diameter > 25 cm and flow time < 15 s is suitable for filling voids in pre-placed CGs skeleton. With the use of optimized self-consolidating CTGs with high W/C ratios (i.e. 1.0 and 1.2) and mass fraction of 70%, CGFBs show advantages in strengths and strength efficiencies over the conventional cemented tailings backfills. Moreover, owing to the utilization of high-volume solid waste, CGFBs bring benefits of low costs and low environmental impacts.
Abstract
To investigate the stress state of soil during the instability of the hole wall (hole shrinkage) in saturated clay, the theory of cylindrical cavity contraction in an infinite medium is ...applied, utilizing the modified Cam-Clay (MCC) model. By employing intermediate variables, the issue of cylindrical cavity contraction is reformulated into a first-order ordinary differential equation in Lagrangian form, with three effective stress components and pore pressure as the unknowns. In MATLAB, the stress conditions at the elastoplastic boundary serve as the initial values for solving the initial value problem of the differential equation system. After determining the correlation between the stress distribution around the cavity and the diameter reduction ratio, the analysis proceeds to evaluate the internal support pressure required for stabilizing the hole wall according to various stability criteria. This pressure is then integrated into the hole wall stability coefficient formula within the soil pressure model, facilitating the evaluation of hole wall stability. The effectiveness and accuracy of the proposed semi-analytical elastic–plastic solutions were subsequently confirmed through comparison with finite element analysis results, underscoring its relevance for addressing hole wall stability challenges.
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