Mohr-Coulomb Failure Criterion Research Articles

Seismic stability of a circular tunnel in cohesive-frictional soils using a stable node-based smoothed finite element method

This paper presents the effect of horizontal and vertical earthquake force on the stability of a single circular tunnel in cohesive-frictional soils using a stable node-based smoothed finite element method (SNS-FEM). In this study, seismic forces are computed as horizontal and vertical pseudo-static body forces arising on the soil and additional inertial forces associated with the uniform surcharge applied to the ground surface. In the upper bound limit analysis based on SNS-FEM, the soil behaviour is described as rigid-perfectly plastic materials, and plasticity deformation obeys the associated flow rule following the Mohr-Coulomb failure criterion. Firstly, the numerical results were checked against other numerical solutions in the literature. The present results agree with prior contributions, proving that the proposed approach can give efficient and reliable solutions to the stability number. Secondly, the variations of the seismic stability number with changes in the horizontal earthquake acceleration coefficient were intensively investigated for different values of soil properties, internal friction angleand the depth-to-diameter ratio of the tunnel. It is shown that the seismic stability numbers of circular tunnels reduce remarkably with the increase of horizontal seismic coefficientand the soil weight. Thirdly, the seismic stability numbers were summarised in design charts for practical use in geotechnical engineering.

Shear strength characterization of fresh MBT and MSWI wastes from a Spanish treatment facility

A laboratory shear strength characterization of the landfilled materials of the Municipal Solid Waste integral treatment plant from the Meruelo Environmental Complex in Cantabria (Spain) was performed. The materials tested come from the rejection of the Mechanical and Biological Treatment (MBT-MSW) and the slags produced in the energy recovery plant (MSWI). Laboratory characterization consisted of direct shear and consolidated drained triaxial testing. Mohr-Coulomb failure criterion parameter values were obtained and compared to reported values in the literature. In some tests, failure was not reached due to the reinforcement effect for fibrous particles; thus, the mobilized shear strength parameters for different values of axial strain were obtained. The triaxial test results showed strain-hardening in MSW-MBT but not in MSWI. Failure was reached on both materials in direct shear testing, with MSWI showing peak and ultimate strengths, whereas MBT-MSW exhibited only ultimate strength. Direct shear test obtained strength can be characterized by a cohesion of 20 kPa and a friction angle of 33° for MBT-MSW ultimate strength, while cohesion and friction angle varies from 13.4 to 29 kPa and from 38.5° to 42.3° for MSWI ultimate and peak strength, respectively. The mobilized cohesion and friction angle obtained for MBT-MSW in consolidated drained triaxial tests ranged from 15.1 to 62.7 kPa and 20° to 28.7°, corresponding to a strain level of 5% and 25%, respectively. In triaxial testing of MSWI specimens, failure was reached, and the material showed a cohesion of 51.3 kPa and a friction angle of 32.8°.

Centrifuge model and numerical studies of strip footing on reinforced transparent soils

This paper presents the results of centrifuge model tests to investigate the deformation behaviour of unreinforced and reinforced transparent soil foundations under strip loading. A digital image analysis technique was employed to obtain the soil displacement field and strain distribution of reinforcements. Two-dimensional (2D) numerical models were developed and verified using the test results. The soil was modelled as a linearly-elastic perfectly-plastic material with Mohr–Coulomb failure criterion. The reinforcement was characterised using a linearly-elastic model considering rupture behaviour. Moreover, a parametric study was conducted to investigate the load–settlement response of foundations, distribution of reinforcement tension and failure sequence of reinforcements. The experimental and numerical studies show that the results obtained from the numerical simulations are in good agreement with the results of the centrifuge model tests. The 2D finite difference model developed using the user-defined functions coded into the FLAC programme can better simulate the progressive failure of the reinforcement layers in the tests. The failure sequence of reinforcement layers is not affected by the modulus and internal friction angle of soil and the reinforcement length, but is closely related to the combined effect of spacing and number of reinforcement layers and the combined effect of reinforcement stiffness and strength.

Seismic Bearing Capacity of Strip Footing with Nonlinear Mohr–Coulomb Failure Criterion

Many published studies on seismic conditions have focused on a pseudostatic method with a linear Mohr–Coulomb failure criterion. However, the pseudostatic method ignores the influence of time and spatial variables on seismic accelerations, and almost all soils have nonlinear strength properties rather than linear properties. Herein, a new approach was established for calculating the seismic bearing capacity that considers a linear distribution of the seismic coefficients. A nonsymmetrical failure mechanism and the nonlinear Mohr–Coulomb failure criterion were used to describe the collapse process and soil properties. A layerwise summation method was developed to calculate the work rates because the pseudodynamic method depicts the variation in seismic acceleration as well as the time and spatial variables. In this work, a first application for calculating the seismic bearing capacity of soil foundations with nonlinear strength properties was conducted using the kinematic approach of limit analysis. Based on the numerical results and discussions, the key conclusions were as follows: (1) the reliability of this approach was verified by making exact comparisons with theoretical results and lab data; (2) in general, nonlinear solutions were more conservative than linear solutions; and (3) a high value of initial cohesion c0 resulted in an increase in the seismic bearing capacity qlim, and qlim decreased when the ratio of tensile strength σt to initial cohesion c0 or the value of nonlinear parameter m increased for a given value of initial cohesion c0.

Data-driven analysis on compressive behavior of unconfined and confined recycled aggregate concretes

It is widely accepted that the inferior characteristics of recycled concrete aggregates (RCAs) reduce the mechanical properties of structural concrete and the available prediction models receive undesirable accuracies in estimating the compressive behavior of recycled aggregate concrete (RAC). To this end, this paper presents a series of data-driven analysis on unified modeling of the compressive behavior of unconfined and confined RACs. First, four reliable experimental databases containing the results of natural aggregate concrete (NAC) and RAC under uniaxial compression, RAC under triaxial compression, and fiber reinforced polymer (FRP)-confined RAC under axial compression were assembled through an extensive literature review, respectively. Based on the collected data, Mohr-Coulomb failure criterion was then employed to construct a model for estimation of the failure envelope of RAC under axial compression and different lateral active confinement stresses. Finally, unified models for predicting the compressive properties (i.e., compressive strength/peak stress and strain at peak stress) of unconfined and confined concretes incorporating natural aggregates (NAs) and/or RCAs were developed so as to provide a design-oriented procedure for material and structural assessments of both NAC and RAC. The results demonstrate that the proposed models exhibit acceptable predictions, thereby making various stakeholders gain sufficient confidence in use of RAC products.

Effect of Foundation Shape on Settlement by Numerical Method

Five prototypes arranged in three different soil conditions is presented to find out the load-settlement relationship of soil and foundation. For creating three types of condition two types of managed soils are stiff clay and soft clay. The five prototypes used here represent the condition of shallow foundation. Prototypes area being same for cases has the shape of Triangular, Square, Rectangular, Pentagonal and Hexagonal. The load-settlement behavior has been found out plan-wise using Plaxis 3D software. Geotechnical engineering software which uses finite element method and Mohr-Coulomb failure criterion following mathematical differential equational problems. Plaxis 3D software uses the properties of soil cohesion, angle of internal friction, Elasticity modulus and unit weight, elasticity modulus of the foundation to settle a relation for the interaction between soil and foundation. Load-settlement behavior for the five prototypes as tried to be found out by Plaxis 3D has represented the immediate settlement of soil on which the prototypes have been rested. The graphs also help to understand the difference in the bearing capacity and settlement for the five prototypes, each of which acted as foundations. Pressure bulb is an effect of vertical pressure which differs from shape to shape of foundation. It directly affects the intensity of pressure and the intensity of pressure affects the settlement. The bearing capacity also affects the settlement. A triangular foundation has the best bearing capacity as its settlement is the lowest. But as a triangular shaped foundation is not practically used and not ideal it is not used for structural work. This study shows that square or rectangular foundation is best for a structure due to higher bearing capacity and lesser settlement compared to the same of pentagonal and hexagonal shaped foundation. But if comparison is done between square and rectangular shaped foundation, square is the best.

Coseismic Stress Change and Viscoelastic Relaxation after the 2008 Great Sichuan Earthquake

Long-term stress accumulation influenced by coseismic stress changes and postseismic viscoelastic relaxation is considered critical to triggering giant earthquakes. Nevertheless, how the stress increase is interrupted by aftershocks and how it influences the megaseismic cycle remain enigmatic. In this study, based on the Mohr–Coulomb failure criterion at the nucleated segments of the 2008 great Sichuan earthquake, the stress variation associated with four M > 6 aftershocks was calculated for the period from 2010 to 2017. The results show that (1) the spatial distribution of coseismic stress change is correlated with the rupture pattern of large events and has a fundamental impact on triggering subsequent earthquakes and (2) postseismic viscoelastic relaxation leads to increased Coulomb stress accumulation at the northern and southern edges of the seismogenic Longmenshan fault, which results in enhanced fault instability and the potential for future large events.

Analysis of fault slip potential of active faults in Tangshan seismic region after the Tohoku-Oki 3.11 M9.0 earthquake based on in situ stress monitoring data

In order to ascertain the impact of the Tohoku-Oki 3.11 M9.0 earthquake on the stability of the faults in the Tangshan seismic region, we investigated the adjustment of the in situ stress field of the seismic region after this earthquake based on in situ stress monitoring data. Then, according to the Mohr-Coulomb failure criteria and Byerlee’s law, we used the FSP v.1.0 software package to calculate the fault slip potential (FSP) of the main faults in the seismic region at each adjustment stage of the in situ stress field, and to study the risk of fault activity. The research results show that 1) after the Tohoku-Oki 3.11 M9.0 earthquake, the tectonic environment of the Beijing Plain area changed rapidly from nearly EW extrusion to nearly EW extension, and this state was maintained until June 2012. After this, it began to gradually adjust to the state present before the earthquake. As of September 2019, at the depth of 100m, the maximum horizontal principal stress value in Tangshan seismic region was 7.61–7.81MPa, the minimum horizontal principal stress value was 5.30–5.50MPa, and the maximum horizontal principal stress orientation was N55.1°–59.5°E. (2) Before the Tohoku-Oki 3.11 M9.0 earthquake, the stress accumulation level of the main faults in the seismic region was relatively high with the FSP values of 30–60%. After this earthquake, the stress accumulation level of each fault continued to decrease, as of May 2013, the FSP values were mainly concentrated at 10–35%. Then, the stress accumulation level of major faults in the seismic region began to gradually increase. As of September 2019, the FSP values were mainly concentrated at 23–37%, and the stress accumulation level was still lower than the pre-earthquake state. 3) The fault activity in the central and northern parts of the seismic region was the strongest, followed by the southern part and western part, and the fault activity in the eastern part was the weakest.

Damage characterization of pouring semi-flexible pavement material under triaxial compressive load based on X-ray computed tomography

Pouring semi-flexible pavement (SFP) material is a multi-phase composite material with complex damage characteristics and strength mechanism. Therefore, revealing the shear properties and damage characteristics of SFP is essential for understanding the strength formation mechanism of SFP and guiding the design of SFP. Firstly, the effect of porous asphalt mixture (PAM) air void on the shear properties of SFP was analyzed by triaxial compressive tests, and the Mohr-Coulomb theory was applied to analyze the shear strength mechanism of SFP. Then, the pore and crack distribution characteristics of SFP specimens at different loading stages were investigated via X-ray CT. Finally, three damage parameters (DV, DC, DE) were used to describe the damage degree of SFP specimens in different loading stages. The results show that increasing the air void of PAM can effectively improve the triaxial compressive strength and compressive modulus of SFP. The Mohr-Coulomb failure criterion can accurately describe the failure behavior of SFP at 60 °C. Increasing the air void of PAM can increase the proportion of interlocking force provided by aggregate-cement contact, which in turn increases the internal friction angle of SFP. When the air void of PAM is 25 %, SFP has the largest proportion of cement-asphalt-aggregate (C-AS-A) interface area, leading to the largest cohesion force of SFP. Moreover, during triaxial compression, C-AS-A interface crack accounts for 63.47 %, which is the main failure mode of SFP. The plane porosity, volume and surface area of 3D pores and cracks of SFP specimens increase with the increase of vertical deformation. The DE can more accurately characterize the damage status of SFP.

An Improved Method of Vertical Slices to Determine Seismic Passive Earth Pressure and Its Point of Application

This paper presents a new approach for calculating seismic passive earth pressure, its point of application, and the approximate stress-state within the rupture wedge. The method of vertical slices combined with the limit-equilibrium (LE) technique was used as the framework for the analysis. While the available LE analyses used the rupture surface of predefined shapes, in this approach, the rupture surface is derived through the analysis. First, the mobilized soil behind the wall was split into thin vertical slices and the rupture plane of every slice was calculated by enforcing the Mohr–Coulomb failure criterion. Thereafter, by adding those rupture planes consecutively, starting from the wall toe, the passive rupture surface was achieved. The inclination of the rupture planes is guided by the soil friction angle, wall friction angle, seismic acceleration, and slice-interface friction angle. The distribution of slice-interface friction angle across the rupture plane was presumed to obey a power function. The exponent of the power function, the seismic passive earth pressure, and its point of application were derived concurrently by satisfying the static equilibrium conditions of the slices and applying a boundary condition. The results show that the outline of the rupture surface is guided by the slice-interface friction angle, and the shape is curvilinear for rough walls. With increasing seismic acceleration, the passive pressure decreases, and the point of application moves predominantly downward for loose sands. The obtained results were compared with those of previous numerical and experimental studies and were found to show good agreement, thus demonstrating the validity of the proposed method.

Macromechanical Failure Criteria: Elasticity, Plasticity and Numerical Applications for the Non-Linear Masonry Modelling

Sometimes it is difficult to choose the most appropriate failure criterion for the problem analyzed. For brittle materials, attention must be paid to the availability of experimental data and the calibration of the representative parameters, within the chosen failure criterion. The work herein presented, starting with an overview on machromechanical failure criteria, analysed in the Haigh-Westergaard Stress Space, investigates the suitability of Mohr-Coulomb, Drucker-Prager and Concrete Damaged Plasticity failure criteria of masonry structures, underlining their specific characteristics and implementation in FEM simulations. The Pavia Door Wall experimental campaign under pseudo-static cyclic test is considered as benchmark study. The results of the experimental tests are compared with a FE model developed with ABAQUS computer program considering several failure criteria and equivalent frame approach. Among the investigated failure criteria Concrete Damaged Plasticity is able to capture the actual behaviour of the masonry walls under monotonic excitation. In particular, thanks to the adaptability of the Guo’s model in the definition and calibration of the uniaxial behavior, the model suitability in catching the variation of the cohesion and the evolution of the damage is better in comparison with the other addressed failure criteria.

Identifying plausible historical scenarios for coupled lake level and seismicity rate changes: the case for the Dead Sea during the last 2 millennia

Abstract. Studies of seismicity induced by water level changes in reservoirs and lakes focus typically on well-documented contemporary records. Can such interactions be explored on a historical timescale when the two data types suffer from severe uncertainties stemming from the different nature of the data, methods and resolution? In this study, we show a way to considerably improve the correlation between interpolated records of historical Dead Sea level reconstructions and discrete seismicity patterns in the area, over the period of the past 2 millennia. Inspired by the results of our previous study, we carefully revise the historical earthquake catalog in the Dead Sea to exclude remote earthquakes and include small local events. For addressing the uncertainties in lake levels, we generate an ensemble of random interpolations of water level curves and rank them by correlation with the historical records of seismic stress release. We compute a synthetic catalog of earthquakes, applying a Mohr–Coulomb failure criterion. The critical state of stress at hypocentral depths is achieved by static poroelastic deformations incorporating the change in effective normal stress (due to the best-fit water level curve) superimposed on the regional strike-slip tectonic deformations. The earthquakes of this synthetic catalog show an impressive agreement with historical earthquakes documented to have damaged Jerusalem. We refine the seismic catalog by searching for small local events that toppled houses in Jerusalem; including all local events improves the correlation with lake levels. We demonstrate for the first time a high correlation between water level changes and the recorded recurrence intervals of historical earthquakes.

Stress-based forecasting of induced seismicity with instantaneous earthquake failure functions: Applications to the Groningen gas reservoir

In this study we use the Groningen gas field to test a new method to assess stress changes due to gas extraction and forecast induced seismicity. We take advantage of the detailed knowledge of the reservoir geometry and production history, and of the availability of surface subsidence measurements and high quality seismicity data. The subsurface is represented as a homogeneous isotropic linear poroelastic half-space subject to stress changes in three-dimensional space due to reservoir compaction and pore pressure variations. The reservoir is represented with cuboidal strain volumes. Stress changes within and outside the reservoir are calculated using a convolution with semi-analytical Green functions. The uniaxial compressibility of the reservoir is spatially variable and constrained with surface subsidence data. We calculate stress changes since the onset of gas production. Coulomb stress changes are maximum near the top and bottom of the reservoir where the reservoir is offset by faults. To assess earthquake probability, we use the standard Mohr-Coulomb failure criterion assuming instantaneous nucleation and a non-critical initial stress. The distribution of initial strength excess, the difference between the initial Coulomb stress and the critical Coulomb stress at failure, is treated as a stochastic variable and estimated from the observations and the modelled stress changes. The exponential rise of seismicity nearly 30 years after the onset of production, provides constraints on the distribution of initial strength. The lag and exponential onset of seismicity are well reproduced assuming either a generalized Pareto distribution, which can represent the tail of any distribution, or a Gaussian distribution, to describe both the tail and body of the distribution. The Gaussian distribution allows to test if the induced seismicity at Groningen has transitioned to the steady-state where seismicity rate is proportional to the stressing rate. We find no evidence that the system has reached such a steady-state regime. The modeling framework is computationally efficient making it possible to test the sensitivity to modeling assumptions regarding the estimation of stress changes. The forecast is found robust to uncertainties about the ability of the model to represent accurately the physical processes. It does not require in particular a priori knowledge of the location and orientation of the faults that can be activated. The method presented here is in principle applicable to induced seismicity in any setting provided deformation and seismicity data are available to calibrate the model.

Extension of 3-D coupled DDA-SPH method for dynamic analysis of soil-structure interaction problems

Soil-structure interaction problems are an important issue in the research field of geotechnical engineering. Understanding and modeling the soil-structure interaction problems is thus critical. Such interaction problems involve the mechanical behaviors of both soil and structure as well as their complex interaction. Discontinuous deformation method (DDA) is appropriate for modeling mechanical behavior of structure, while smoothed particle hydrodynamics (SPH) offers a feasible technique for describing the mechanical behavior of soil material. A coupled DDA-SPH method can integrate the advantages of both DDA and SPH. For three-dimensional (3-D) dynamic analysis of the soil-structure interaction problems, the original 3-D coupled DDA-SPH method must be extended, which is the primary purpose of this paper. In the extended 3-D coupled DDA-SPH method, an elastic-plastic soil constitutive model associated with the Drucker-Prager yield criterion is implemented. The interaction between soil and structure is realized through a newly proposed coupling scheme based on a penalty method using the Mohr-Coulomb failure criterion. Furthermore, the extended 3-D coupled DDA-SPH method is validated through the investigation of four experiments. The simulation results suggest that the extended 3-D coupled DDA-SPH method can accurately simulate the large deformations and post-failure behavior of soil-structure interaction problems.

Edge punching shear of waffle slabs subjected to moment parallel to the slab's free edge

The test results of five one-tenth scale micro-concrete waffle slabs subjected to edge punching shear in the presence of moment transfer parallel to the slab's free edge are presented. Although the observed punching failure mechanism of the waffle slabs was found to be very similar to that of a flat slab, the shear capacity was reduced because some of the potential failure surface is lost when it extends into the waffle section. Since the current codes of practice do not specifically consider the punching shear mechanism of waffle slabs, comparisons with test results revealed that the application of various codes overestimated the punching capacities of waffle slabs. An upper-bound plasticity theory model was developed to predict the punching shear failure load. The proposed model assumes concrete to be a perfectly rigid plastic material with the modified Mohr–Coulomb failure criterion. The model showed good agreement with the test results.

Impact of Tohoku-Oki 3.11 M9.0 Earthquake on the Fault Slip Potential of the Active Quaternary Faults in Beijing City: New Insights from In Situ Stress Monitoring Data.

In order to ascertain the impact of the Tohoku-Oki 3.11 M9.0 earthquake on the stability of the faults in the Beijing Plain, we investigated the adjustment of the in situ stress field of the Beijing Plain after this earthquake based on in situ stress monitoring data. Then, we analyzed the stability of the five main faults in each adjustment stage of the in situ stress field based on the Mohr–Coulomb failure criteria and Byerlee’s law. Finally, we studied the fault slip potential (FSP) of the main faults under the current in situ stress field. The research results show that (1) after the Tohoku-Oki 3.11 M9.0 earthquake, the tectonic environment of the Beijing Plain area changed rapidly from nearly EW extrusion to nearly EW extension, and this state was maintained until June 2012. After this, it began to gradually adjust to the state present before the earthquake. As of September 2019, the tectonic environment has not recovered to the state present before the earthquake. (2) The ratios of shear stress to normal stress on the fault plane of the fault subsections in the three time periods before the Tohoku-Oki 3.11 M9.0 earthquake, 6 June 2012 and 8 September 2019 were 0.1–0.34, 0.28–0.52, and 0.06–0.29, respectively. It shows that the stress accumulation level of faults in the Beijing Plain area increased in a short time after the earthquake and then gradually decreased. (3) Under the current in situ stress field, most of the subsections of the five main faults have a low FSP (<5%). The areas with high FSP are mainly concentrated in the central and southeastern parts of the Beijing Plain, including the Nankou-Sunhe fault, the northern section of the Xiadian fault, and the areas where the five faults intersect.

Dynamic Soil–Structure Interaction Effects in Buildings Founded on Vertical Reinforcement Elements

Pile foundation is an effective technique to support buildings in the presence of soft soil and seismic areas. More recently, the rigid inclusions system has also been utilized for founding buildings. Both systems increase the bearing capacity of the soil and allow reducing the total and differential settlements in the structure. However, the study of these systems in a complete and accurate way implies the consideration of the soil–structure interaction (SSI). In order to investigate the impact of different pile toe conditions (including the placement on hard soil, an anchorage and floating piles) in the response of mid-rise buildings, numerical models with a 5-storey frame building founded on the inclusions system (soil–inclusion–platform–structure) are analyzed and compared with the pile system (soil–pile–structure). Fully coupled finite difference numerical models were developed using Flac 3D. The influence of the dynamic characteristics of the structure was considered analyzing buildings with different heights (3 storeys to 7 storeys). The linear elastic perfectly plastic model with a Mohr–Coulomb failure criterion is used to represent the behavior of the soil. Values of the maximum lateral displacements, of the inter-storey drifts and of the shear forces distribution in the buildings, as well as the rocking of the foundation, are presented. Concerning the foundations, efforts and displacements are compared for the different systems. The results show that the type of support condition influences the seismic response of the building and the efforts and displacements in the rigid elements, depending on the foundation system. The efforts at the toe level in the rigid elements are highly influenced by the support conditions, but there is only a slight influence from the head connection.

Location of Tension Cracks at Slope Crests in Stability Analysis of Slopes

Over the conventional limit equilibrium method and limit analysis method, the finite element method is advantageous, especially for slopes involving complex failure mechanisms where the critical slip surfaces cannot be represented by log spirals and other similarities. In the presence of tension cracks at slope crests, however, the finite element method encounters difficulties in convergence while handling Mohr–Coulomb’s yielding surfaces with tensile strength cut-off. Meanwhile, the commonly used load-controlled method for the system of nonlinear equilibrium equations is hard to bring the slope into the limit equilibrium state. The two drawbacks drag down the finite element method in more extensive applications. By reducing the constitutive integration of plasticity with non-smooth yielding surfaces to the mixed complementarity problem, the convergence in numerical constitutive integration is established for arbitrarily large incremental strains. In order to bring the slope to the limit equilibrium state, a new displacement-controlled algorithm is designed for the system of nonlinear equilibrium equations, which is far more efficient than the load-controlled method. A procedure is proposed to locate tension cracks. Corresponding to the Mohr–Coulomb failure criterion with and without tensile strength cut-off, the failure mechanisms differ significantly, while the difference in the factor of safety might be ignorable.

Another look at the stability of unsaturated soil slopes considering nonuniformity and nonlinearity

Natural soils in site are mainly unsaturated under evaporation or infiltration effects, which cause nonuniformity along depth or present nonlinearity in strength properties even for uniform soil strata. Conventional upper-bound analysis is unable to handle the above issues, requiring alternative approach such as the discretization-based kinematic analysis to be adopted. Nonlinear matric suction profile along depth is theoretically formulated under one-dimensional steady flow. Within the context of extended Mohr Coulomb (MC) failure criterion, total cohesion is explicitly expressed for unsaturated soils. Meanwhile, an extended Power-law failure criterion is proposed to consider nonlinear characteristics of uniform soils in unsaturated zones. Based on these two failure criteria, upper-bound analyses of unsaturated clay slopes’ stability are evaluated by slope safety factor (FoS). Optimal upper-bound FoS solutions are sought by a strength reduction technique and iterative algorithm. Effects of clayey soils’ non-uniformity, nonlinearity and some other influential factors on unsaturated slope stability are investigated, which is of engineering significance for sound design and reliable assessment of unsaturated slopes. One of key findings is that stability of unsaturated soil slopes enhanced by their unsaturated soil strength is substantial and should be utilized to derive a more economical design or construction of temporary slopes in practice.

Analytical stress analysis method of interbedded coal and rock floor over confined water: a study on mining failure depth

Deep mining commonly causes redistribution of the stress-strain field for coal seam floor strata. The study of this distribution law was of great significance to analyzing the characteristics of floor deformation and failure and predicting the risk of floor water inrush. In this paper, the damage of a coal seam floor with strong water pressure in the Junggar Basin in western China was examined, and the stress analytical formula for every point of floor confined aquifer was constructed using a stress model in the direction of coal seam strike. Combined with Mohr-Coulomb failure criterion, the maximum depth of floor failure was 17.5 m. According to geological and stratigraphic data of the stope floor, a 3D numerical model of fluid-structure coupling was built. Simulation results showed that under a 1.5 MPa water pressure, the maximum floor failure depth was 18 m. The internal stress of the coal-rock interface and soft rock increases significantly, and the weak coal seam intensifies the transfer of stress and strain at depth to some extent. Water pressure had a certain influence on floor deformation and failure. The joint simulation results of stress-seepage-fracture were closer to the real stope measurements. In the boreholes, two types of sensors, distributed fiber and electrodes were arranged. When the coal floor broke, the compressive strain increased with a peak of 4152 με. Meanwhile, the apparent resistivity of the rock seam reached 500–700 Ω•m. The comprehensive analysis showed that: the floor failure depth in the monitoring area of the 61,303 working face was −18 m, and the floor failure disturbance depth was −35 m. The results presented in this contribution can provide reference for future examinations of coal and rock interbedded floor failure over deep confined water.