International Journal of Geomechanics

This study was aimed to (1) investigate the failure surface mechanism occurring behind the wall supporting narrow backfill with experimental and numerical approaches; (2) determine the effect of internal friction angle (ϕ), friction (δ), heel length (β), foundation thickness (α), backfill inclination (ψ), and backfill width (θ) parameters on lateral earth thrust coefficients; and (3) derive finite-element analysis (FEM)-response surface method (RSM)–based lateral earth thrust coefficient equations for inverted T-type retaining walls supporting narrow backfill. The study was performed for granular backfill. A small-scale test was performed to investigate the failure surface mechanism in the narrow granular backfill. The particle image velocimetry method was employed to determine failure surface patterns for various heel lengths and backfill widths. Finite-element (FE) analyses were also performed to verify the experimental results. From the analyses, it was seen that two characteristic soil blocks formed behind the horizontally translated wall. Due to these soil blocks, the wall was divided into three characteristic regions. Lateral active earth thrust coefficients were derived for the regions. RSM was employed to derive FE-based lateral earth thrust coefficient estimation equations for each region. In this context, three design matrices were created, comprising 76 runs. In the design, parameters ϕ, δ, β, α, ψ, and θ were used as independent variables. A total of 76 FE analyses were performed for various parameter combinations. The results of the FE analyses were used as a response. From the statistical analyses, lateral active earth thrust estimation equations for narrow backfill were derived. The suggested equations were compared with the results of experimental, numerical, and analytical studies, and it was seen that the method gives reliable results. The effects of independent variables and their interactions on lateral earth pressure coefficients were examined.

The diaphragm wall is widely used in deep excavation engineering to guarantee the safety of the foundation pit and surrounding environment during excavation. An accurate prediction of the lateral deformation of the diaphragm wall is the basis of the design and optimization of the diaphragm wall structure. In this paper, we apply the third-order shear deformation plate theory to develop an analytical method for predicting the lateral deformation of the diaphragm wall to take into account the effects of shear stress distributed along the thickness direction on the lateral deformation. By using the Pb-2 Ritz method, combined with the minimum potential energy concept, the analytical solution for the deformation of the diaphragm wall is obtained under certain simplifications. Three types of boundary conditions for the bottom side of the diaphragm wall are considered to meet different geological conditions. The proposed method is verified by comparing the results predicted in this study with the monitoring data obtained from the practical pit engineering of Dinggong south road metro station of Metro Line 2 in Nanchang, Jiangxi province, China. Finally, the effects of the thickness and the embedded depth of the diaphragm wall and the lateral support patterns are analyzed, and the optimized design parameters for the diaphragm wall are also suggested. The proposed method can theoretically provide preliminary parameter optimization guidance for the design and construction of a diaphragm wall.

A thermodynamically consistent three-dimensional (3D) continuum breakage model was developed to study the effect of intermediate principal stress. Such an effect has been long overlooked in most existing constitutive models, which might result in a considerable difference in the strength estimation of crushable granular materials. Following basic thermodynamic laws, the proposed 3D breakage model was based on a new explicit mathematical expression of the original Matsuoka–Nakai failure criterion and 3D stress transformation. This proposed 3D breakage model was verified against the corresponding true triaxial constant-hydrostatic-pressure loading tests and true triaxial constant-minor-principal-stress loading tests of sand. The results confirmed that the proposed 3D breakage model accurately captures the effect of intermediate principal stress on granular crushing behaviors.

Given the prevalence of such scenarios worldwide, it is of the utmost importance for geotechnical practitioners to understand the behavior of a strip footing located over a pipe buried within a slope. This study focuses on investigating the failure mechanism of the footing-on-slope system, and a series of numerical simulations have been conducted using the material point method. The accuracy of the numerical model has been verified through comparisons with the results of model tests in the literature. The influence of the setback distance of the footing, burial depth of the pipe, and slope angle on the bearing capacity and failure modes has been analyzed in detail. The results demonstrate that the presence of the pipe reduces the bearing capacity of the strip footing. Meanwhile, the variations in the setback distance and slope angle may cause a reduction in the ultimate bearing capacity by 76% and 75%, respectively. The failure mode of the footing-on-slope system is determined by the pipe location, setback distance of the footing, and slope geometry. For shallow burial conditions, the pipe–soil interface can accelerate the development of failure surfaces. From the perspective of Mohr's circle, the principal stress deflects due to the applied pressure, resulting in different failure surface shapes. The schematic diagram of the failure mode can be used to predict the failure mechanism of the footing-on-slope system, thereby helping in the safe design of slopes with buried pipes.

In addition to creating environmental threats, the leakage and entrance of chemical fertilizers into soil, which is possible in various ways, dramatically changes the physical and mechanical properties of soil. Furthermore, clay soils in the vicinity of monoammonium phosphate (MAP) contamination show substantially negative behavior, thus, improving the engineering geological factors of soil has become a huge challenge for engineers. Extensive programs such as triaxial compression test, consolidation test, compaction test, Atterberg limits, and collapse tests, in addition to X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) were performed on samples containing 10%–30% MAP over 7 days, 1, and 3 months. The mentioned trend was repeated when 0.5%–3% nano A (Al₂O₃ in alpha phase), nano F (Fe₂O₃ in alpha phase), and AF in form of a nanohybrid were added to soil which had been contaminated by MAP. Gained reports demonstrated significant declines in maximum dry density, plasticity index, and shear strength of contaminated soil when compared with natural soil. Conversely, increases in liquid limit, plastic limit, consolidation settlement, as well as soil collapse index were observed. Furthermore, a decrease in the amount of minerals in the XRD results and the formation of a flocculated structure in the SEM images in MAP-contaminated soil are evident. According to microstructural analysis, adding nano A, nano F, and AF nanohybrid as an improvement phase caused remediation of the soil structure and created cementation bonds between soil particles, thereby increasing the maximum dry density and soil shear strength, and decreasing the consolidation settlement and soil collapse index. Therefore, the use of AF as an efficient additive to remediate physical and mechanical properties of soils that have been contaminated with MAP, and to decrease the geological hazards of these kinds of contamination is recommended.

As an important basis for slope stability evaluation, the permanent displacement of the slope has been widely used in the seismic design of slope engineering. This paper aims at the situation of multiple slip surfaces and multiple sliding masses in multistage loess slopes under earthquake action. After fully discussing the occurrence of two and three slip surfaces and the successive sliding of each sliding mass of the multistage loess slope under the action of an earthquake, combined with the principle of energy conservation of slope soil and Newmark slider analysis method, the calculation model of multistage loess slope sliding with multiple slip surfaces is established and the calculation method of multistage loess slope sliding with multiple sliders based on energy method is proposed. The positive and negative critical acceleration of each sliding mass is calculated according to the interaction of forces between each sliding mass, and then the influence of slope slip surface inclination and soil parameters on the positive and negative critical acceleration is discussed. Finally, the permanent displacement expression of each sliding mass is obtained by solving the energy equation. The research conclusion is drawn through the verification of an example: the results obtained by the quasi-static method of GEO-Studio (version 2012) numerical simulation software are close to each other, with a difference of 6%. If the peak value of seismic acceleration is small and the inclination of the slip surface is large, the negative critical acceleration may not be considered when calculating the permanent displacement of sliding mass of multistage loess slope under seismic action.

An innovative semianalytical solution was introduced to accurately assess the uplift bearing capacity of spread foundations in sand, which is important for the design of transmission towers. The research addresses the shortcomings of previous analyses, which relied on a 1g laboratory test and consequently overestimated uplift bearing capacities due to scale effects. To overcome these limitations, a series of centrifuge tests were conducted under stress conditions representative of field conditions. The test results led to the identification of a bilinear failure surface, effectively capturing distinct uplift failure mechanisms along the embedment depth, including a passive mode near the ground surface and an active mode downward. A novel application of genetic programming analysis, based on symbolic regression techniques, was employed to optimize the determination of parameters forming the bilinear failure surface. The net uplift bearing capacity corresponding to the bilinear failure surface was evaluated using a limit equilibrium approach combined with a slice procedure. The proposed semianalytical solution was validated with both the current test results and previously published data, illustrating its reasonable applicability and representing a significant innovation in determining the net capacity of spread foundations in sand.

Abstract

Assessment of the seismic damage of coal mine overburden (OB) dump slopes gets compromised due to negligence of the consideration of heterogeneity in the shape and size of its particles. In this paper, in situ material parameters for an OB dump slope were evaluated through multichannel analysis of surface waves (MASW) investigations performed at the Jambad opencast coal mine of India. To realistically simulate the heterogeneity of the OB dump materials, Voronoi tessellation scheme coupled with extended finite-element method (XFEM) (using RS2 software) was employed. Behavior of a double-benched OB dump slope under earthquake excitations with a wide range of strong motion parameters (such as peak ground acceleration, predominant frequency, bracketed duration, and Arias intensity) was investigated through XFEM. The influence of strong motion parameters on the seismic behavior of a double-benched OB dump slope was reported in terms of transient (during the earthquake motion) and permanent deformation (after the earthquake motion) at various key points of the slope. Finally, the levels of damage experienced by the OB dump slope under earthquake ground motions were assessed qualitatively. The study would be advantageous for the mining industry in framing the safety guidelines for OB dump slopes in coal mine areas susceptible to earthquakes.

Practical Applications

The performance of a double-benched coal mine overburden (OB) dump slope under earthquake excitations with a wide range of strong ground motion parameters (such as peak ground acceleration, predominant frequency, bracketed duration, and Arias intensity) was investigated by employing Voronoi tessellation scheme coupled with extended finite-element method (XFEM). The influence of strong ground motion parameters on the seismic behavior of the double-benched OB dump slope was reported in terms of transient (during the earthquake motion) and permanent or residual deformation of the slope at various critical points. Further, the levels of damage experienced by the OB dump slope under earthquake ground motions were assessed qualitatively. This research work may provide guidance to practicing engineers for considering the heterogeneity of the OB dump slope while assessing its seismic performance. The study is going to be useful for the design of OB dump slopes for coal mines located in a seismically active region. If the site-specific earthquake parameters are available, the study may provide important aspects of seismic design of the OB dump slope.

This paper explores the interpretation of undrained shear strength and layer boundaries from T-bar penetration resistance profiles in layered soft–stiff and stiff–soft clay sediments. The proposed interpretation framework was based on extensive parametric large deformation finite-element analyses by varying the strength ratio between the two layers, and the relative thickness of the top layer. For soft–stiff clay deposits, the squeezing mechanism dominated the behavior when the T-bar approached the layer interface. No soil plug was trapped at the base of the T-bar. The interface can be identified as the kink point with a sharp increase in resistance profile sensing the stiff layer. For stiff–soft clay deposits, a stiff soil plug was trapped at the base of the T-bar and penetrated into the soft layer. Resistance profiles dropped before reaching the layer interface being influenced by the soft layer. A minimum thickness of 25D was needed to mobilize the ultimate resistance of the top layer. The layer boundary can be identified as 4.2D above the kink point on the resistance profile in the bottom layer. The stabilized resistance in the bottom layer increased with increasing the strength ratio owing to the thicker stiff soil plug at the base of the T-bar.

Abstract

Rock masses in engineering applications are often simultaneously soaked in a moist environment and exposed to dynamic loads with different rates. It is therefore essential to understand the mechanical behavior and failure mechanism of rocks by considering the coupled effects of water saturation and rate dependence. In this study, numerous dynamic compression tests were conducted on marble and granite under oven-dried and water-saturated conditions using a split Hopkinson pressure bar apparatus. Some indices such as dynamic compressive strength, dynamic increase factor, and dissipation energy density, were unitized to investigate the mechanical behavior and failure mechanism of saturated rocks. The test results show that, regardless of the moisture state, the dynamic strength is positively sensitive to the strain rate. The existence of water contributes to the reduction of dynamic strength and elastic modulus. The dynamic strength reduction of saturated marble decreases with increasing strain rates, while that of saturated granite remains almost unchanged. As the strain rate increases, the dissipation energy density of both types of rock increases, and the ratio of energy utilization significantly decreases. Integrated investigations on the rate dependence of dynamic strength and water-weakening mechanisms suggest that the propagation and development of cracks through the region of mineral grains are responsible for strength enhancement under increasing strain rates. The overall strength reduction of saturated marble is mainly governed by the coupled effects of pore-water pressure and viscosity. With an increase in the strain rate, the strengthening viscosity effect diminishes the strength reduction primarily caused by the increasing effect of pore-water pressure. The dynamic strength of saturated granite is mainly controlled by the rate effect, and its various water-weakening effects remain almost unchanged with an increasing strain rate.

Practical Applications

The current work investigates the coupled effects of water saturation and rate dependence on the mechanical and fracturing behaviors of two different types of rock under dynamic loading conditions. It turns out that water saturation causes dynamic strength reduction, and this reduction, which depends on the rock type, is closely related to the loading rate. For carbonate rocks, the dynamic strength reduction induced by water can be suppressed by the increasing strain rate, but this is not applicable for siliceous rocks with low porosity. The results of this work highlight to engineers that lithology and microstructure are predominant factors affecting the dynamic mechanical and fracturing behavior of rocks in a water environment. The common water-weakening phenomenon in some rocks can be inhibited by the loading rate, resulting in a conservative engineering design. However, more academic and practical investigations on the coupled effects of water saturation and loading rate are required since the rock water-weakening affected by multiple factors is a complicated topic in vague.

The present study attempts to examine the behavior of circular tunnel excavation in rock mass under anisotropic stress conditions with various postpeak behaviors. A brief review of the importance of the possible postpeak behavior of the rock mass and anisotropic field stress condition is made to highlight its importance in practical tunnel construction. A plane strain model is developed in a finite-element package to simulate the excavation response of a circular tunnel using the Hoek–Brown strain-softening model. The modeling scheme and the constitutive model are verified against published results for both isotropic and anisotropic stress conditions. The verified model is subsequently utilized to study the mechanical response of the tunnel under anisotropic stress conditions. The results are presented in terms of the evolution of the ground reaction curve, plastic zone, and radial and tangential stress distributions around the tunnel. For various stress conditions considered, a distinct variation in the displacement and yield zone is seen. In general, a higher tangential stress concentration is observed at the sidewalls in the case where the in situ stress ratio is 0.5. Formation of the yield zone begins at the sidewalls and propagates toward the crown as the degree of deconfinement increases. For stress ratios of 1.5 and 2.0, yielding begins at the crown and propagates toward the sidewall as deconfinement increases. An asymmetrical yield zone is formed because of stress anisotropy. The mechanism causing the observed behavior is discussed, and the findings’ practical implications are highlighted. The findings of this study indicate how crucial it is to take the in situ stress ratio into account when choosing the excavation sequence for underground excavations in rock mass.

Abstract

Ground response analysis (GRA) requires the use of dynamic soil property curves (DSPCs). However, because regional DSPCs are not always available, globally recorded DSPCs are utilized in regional GRA the majority of the time. For those regions where regional DSPCs, based on laboratory estimations, exist, the DSPCs are affected in a variety of ways. For this reason, inverse GRA (IGRA), performed on instrumented earthquake (EQ) data, appears to be a realistic alternative for determining in situ DSPCs. Existing IGRA studies, based on downhole EQ data from around the world, show constraints, particularly in determining damping ratio (β) properties. Furthermore, previous frequency domain IGRA studies have only detected DSPCs for the surficial layer. Whereas a few of the frequency domain IGRA studies have determined the shear modulus degradation (G/G_max) curve for multiple soil layers, they have been unable to determine the β curve for those layers. In the current study, we propose a novel frequency domain framework for determining the G/G_max and β curves for a two-layered soil system positioned between successive accelerometer levels. The proposed framework was applied to 28 EQ records from the Lotung downhole array in Taiwan to determine the G/G_max, β, and shear strain (γ) values for the top two layers from the ground surface. In addition, average DSPCs for the two layers are proposed. The effectiveness of the proposed DSPCs was tested by using them in a GRA and comparing them with recorded ground motions.

Practical Applications

It has been noted that, at present, there are limited downhole array records available. Therefore, there is ambiguity involved in applying the proposed framework presented in this paper to a larger number of records from downhole arrays. However, physical modeling of the wave propagation mechanism through layered media can be done on a smaller scale to determine dynamic soil properties in the future. Several attempts have been made to estimate the G/G_max and β based on centrifuge testing using recorded acceleration time histories at different levels in the centrifuge model. In addition, the one-dimensional propagation of shear waves could be determined in those studies by taking special measures. However, these studies were based on a stress–strain imaging method in the time domain, which has its own limitations. However, recently, a frequency domain method was used to determine the G/G_max and β from centrifuge modeling for a single soil layer. In that study, the β values obtained were scattered. Thus, the frequency domain framework proposed herein could be a viable alternative for determining the G/G_max and β of a multilayered soil profile (here, two layers) when used in centrifuge testing.

In the present study, a numerical model was established to investigate the influence of the prefabricated cracks of different dimensions on the mechanical properties and damage evolution of irregular sandstone. To this end, prefabricated cracks of different dimensions were considered in the analysis, and the real mesoscopic structure inside the sandstone was characterized using microcomputed tomography (micro-CT) scanning and digital image processing technology. Combined with the rock failure process analysis system, the effects of the prefabricated cracks of different dimensions on its macroscopic mechanical behavior and damage evolution were studied. The irregular sandstone fissure field was extracted, the three-dimensional (3D) fissure field was visualized, and the corresponding damage degree was calculated on the MATLAB platform using the modified crack volume damage method. Based on the box dimension theory and the digital image storage principle, a 3D irregular rock fracture box dimension algorithm was developed. Moreover, the fractal dimension of irregular rock failure was calculated at the mesoscale, and the corresponding relationship between the fractal dimension of the crack region and the crack volume damage degree (Du) was established. The obtained results showed that the overall energy accumulation and internal failure of the specimen with nonpenetrating cracks have the characteristics of spatial–temporal inconsistency before reaching the elastic limit point, and the failure degree decreases along the fracture surface toward the intact surface. When the total volume of cracks was 1,800 mm³, the influence of crack parameters on the compressive strength of irregular sandstone was depth > length > opening width. It was found that the damage degree of irregular sandstone has a good consistency with the fractal dimension. This study helps to understand the failure mechanism of irregular rock particles and provides a new idea to investigate the damage evolution of irregular rock particles.

Abstract

The dynamic compaction mechanism of high-groundwater-level (HGL) foundations is still unclear, which seriously affects the engineering application of dynamic compaction in the riverbed and land reclamation. In this study, a dynamic compaction model test was established to analyze the vibration acceleration of the tamper, the soil surface, and the soil interior under different depths of groundwater level. Meanwhile, the propagation of the vibration wave was also analyzed, and the energy dissipation of dynamic compaction under the HGL foundation was studied combined with the modal damping ratio (ζ). The results showed that HGL resulted in poor foundation improvement. The main effects were the energy consumption and the friction reduction between the particles when the tamper was in contact with the water, which resulted in vibration acceleration and a sharp decrease in crater depth. The crater will not maintain its cylindrical shape, and further tamping will negatively affect the reinforcement. Vibration analysis showed that dynamic compaction results in dense soil with an increased damping ratio, but when the soil is tamped at the HGL foundation, the soil tends to oscillate, and the damping ratio decreases. In terms of energy, more than 70% of energy was dissipated directly under the tamping point of the high-groundwater-level foundation.

Practical Applications

Dynamic compaction has been extended from land foundation reinforcement to artificial reclamation foundation as an economical and environmentally friendly foundation treatment technique. The groundwater level in the coastal area is usually higher than that on land, which plays a decisive role in the success of dynamic compaction. However, there are few theoretical and experimental studies on the dynamic compaction of high-groundwater-level foundations. This paper analyzed the energy transfer and consumption of the dynamic compaction method for high-groundwater-level foundations from the perspective of vibration waves. The results showed that compared with dry sand, dynamic compaction of high-groundwater-level foundations will obviously consume reinforcement energy. Dynamic compaction causes groundwater levels to rise around the tamping point. The higher the tamping energy level, the more obvious the groundwater level rise. The results can provide a reference for expanding the treatment range of the dynamic compaction method and applying the dynamic compaction method to high groundwater foundations.

Piping-seepage damage is one of the major factors leading to tailings dam failure. To explore the microscopic influence mechanism of tailings with different fine content on piping development, on the basis of the indoor permeability test, combined with the coupling method of discrete particle flow at the microlevel, through the small seepage model test, the discrete element model of the model test was further constructed. The effects of different fine particle contents on particle size composition, permeability, critical water head, fine particle migration, and porosity of samples were analyzed by an indoor small-scale seepage model and the corresponding numerical simulation study. The results showed that before the occurrence of piping, with the increase of fine particle content, the gradation constant is less than 0.074 mm, and the downward trend changes abruptly, showing a concave curve change. The permeability coefficient showed a decreasing trend, and the critical water head increased. The maximum void ratio and the minimum void ratio generally decreased first and then increased. The strong force chain decreased, and the total force chain increased; there were obvious differences in the change of porosity of different fine particle contents before, during, and after destruction. This was verified by the indoor physical model test that different fine particle contents will lead to different sand inflows in seepage dam break. In practical engineering, it is more helpful to provide a new way for the comprehensive prevention of seepage failure of tailings dam by clarifying the disaster-causing mechanism of piping and seepage failure of tailings dam from the mesostructure.

Euler beam theory is always used for soil–pile analysis; however, its use is questionable for laterally loaded large-diameter monopiles. In this paper, the solution on the static impedances of monopiles under a combination of axial and horizontal loads was proposed based on three-dimensional continuous medium theory and Timoshenko beam theory. From the analysis results, the use of the Euler–Bernoulli beam theory leads to larger pile-head impedances, and the feature is more significant with lower aspect ratios and stronger pile-bottom constraints. The difference in the impedance from two theories may be up to 10%–43%. Consequently, Timoshenko beam theory is more appropriate for large-diameter monopiles, especially for monopiles with strong bottom constraints (such as rock-socketed monopiles). Meanwhile, the shear force and bending moment at the pile bottom have a considerable effect on the static impedances of large-diameter short monopiles, and only a consideration of the impedance contribution from the soil around the pile is insufficient. Furthermore, both the deflection and the bending moment of the pile increase due to the second-order effect of axial force, but the effect of the axial force on the impedance can be neglected for practical monopile designs. A simple and efficient approach to evaluate the impedance of large-diameter monopiles is desirable for designers, thus a simplified empirical equation for the pile-head static impedance was proposed for ease in calculating each component of static impedances for the large-diameter monopiles of offshore wind turbines (OWTs).

The present study pertains to the probabilistic assessment for analysis and design of uplift capacity of transmission tower foundation reinforced with horizontal anchor using the radial basis function (RBF)-based response surface method. The deterministic uplift capacities of horizontal anchors in both unreinforced and reinforced soils were obtained using a finite difference numerical scheme. For the case of reinforced soil, geogrid was used and placed on top of the anchor plate. The improvement in uplift capacity due to the inclusion of geogrid reinforcement is first discussed. Next, RBF-based response surface models were constructed using the observed uplift resistance from the deterministic numerical model. The reliability of the foundation subjected to uplift forces was assessed using the Monte Carlo simulation, considering uncertainties of the soil and geogrid properties. The influence of safety factors on the failure probability of the foundation for both unreinforced and reinforced cases was shown. Variation of the probability of failure with different coefficients of variation of input variables was also investigated. The reinforcement stiffness was the most influencing parameter, having a pronounced effect on the failure probability of the reinforced foundation. Significant improvement in the uplift capacity and reduction in failure probability of the foundation were observed when a reinforced anchor was used, showing the effectiveness of reinforcement on top of the anchor plate. The results indicated that for the same failure probability, the reinforced anchors require much lower depth compared to unreinforced anchors.

To explore the supporting performance of oblique piles (OPs) used in pit excavation, a field test was performed at a foundation excavation using OPs with various inclination angles. Then, numerical models using the finite-element method were established and validated on the basis of the field test result. A novel arrangement of oblique‒vertical alternating piles (OVAPs) was proposed as the retaining structure for excavation. The displacements of the pile top, internal forces of the pile body, earth pressures on the pile body, and soil deformation were observed and studied. The results indicated that both pile inclinations and arrangements had an effect on the supporting performance of oblique piles. Furthermore, four effects were summarized to explain the advantages and working mechanisms of OP and OVAP arrangements used in retaining structures. Finally, a force analysis of oblique supporting piles in the foundation pit was conducted and a simplified formula was derived and applied to calculate the bending moments of pile bodies under different working conditions.

Since the ground reaction curve (GRC) is a useful tool for evaluating the formation deformation and the load acting on an underground structure, it is critical to determine the GRC, especially under small strain conditions. In this paper, a new analytical model considering the progressive development of loose soil was proposed to describe GRC in trapdoor tests. The proposed model is composed of an undisturbed zone, unloading zone, failure zone, and self-weight zone. The key parameters of the proposed model were determined through the existing literature. In addition, the formulas for calculating the lateral pressure coefficient and the height of the failure zone were derived, respectively. Considering the continuity of the principal stress deflection and lateral pressure coefficient in the unloading zone, and assuming that the unloading zone is a three-hinged arch with reasonable arch axis, the load transfer expression in the unloading zone was deduced. Then, based on the limit equilibrium theory, the vertical stress acting on the trapdoor was obtained. The effectiveness of the proposed model was verified by comparing the calculation results of the proposed model with model tests, a theoretical model, and a simplified model. Finally, the influence of the key parameters of the proposed model on GRC was discussed in detail. This work provides a meaningful reference for estimating the GRC.

Abstract

Polyurethane (PU) cutoff walls have increasingly been used in earth dam reinforcement projects due to their excellent antiseepage properties, strong deformation capabilities, ease of construction, and environmental friendliness. However, the parameter sensitivity of earth dams with PU cutoff walls under stress–seepage field coupling has not been revealed. In this study, a three-dimensional finite-element (3D FE) model was developed based on the Jiulong Reservoir Dam prototype, and the accuracy of the model was verified through a scaled model test. Using the 3D FE model as a foundation, a series of two-dimensional finite-element (2D FE) models were then constructed to conduct a parametric study of earth dams with PU cutoff walls. The thickness, density, and position of the PU cutoff wall were selected as the sensitive factors, while the safety factor, impermeability, deformation characteristics, and von Mises stress of the earth dam and cutoff wall were selected as the evaluation indicators. The mechanical and seepage characteristics of the dam were analyzed under the condition of different sensitive factors. Furthermore, the sensitivity of the sensitive factors was ranked based on the orthogonal test program. The research results of this study can provide a theoretical basis for the optimization design method of PU cutoff walls in earth dam reinforcement projects to a certain extent.

Practical Applications

Polyurethane is a kind of building material synthesized by the chemical reaction of isocyanate and polyether. Its lightweight nature, toughness, and impressive mechanical strength make it an ideal choice for various engineering projects including but not limited to dam seepage prevention, airport pavement repair, and underground pipeline filling. This study utilizes an orthogonal experimental method to establish finite-element models of an earth dam featuring a polyurethane cutoff wall under the coupling effect of stress and seepage. By analyzing the mechanical and seepage characteristics of the dam under different sensitive factor conditions, the research results demonstrate the impact of these factors on the performance of the earth dam, providing a theoretical basis for the design method of polyurethane cutoff walls to a certain extent.

The mechanical responses of tunnels with stiff and yielding linings were predicted using an analytical method. A theoretical model of a stiff lining-supported tunnel was established, which considered the effects of tunnel face advancement and lining installation delay. The deformation process in the yielding lining can be divided into three stages based on the elastic–plastic–stiff deformation characteristics of the yielding elements. The circumferential shortening of the yielding lining during Stage 1 was caused by the elastic deformations in the segmental linings and yielding elements. The theoretical stiffness of the yielding lining at this stage was determined, and analytical solutions were derived from the theoretical stiffness in the mechanical model of the stiff lining. The yielding lining deformation during Stage 2 was induced by plastic compression of the yielding elements, and the lining resistance pressure remained unchanged. The yielding lining was reduced to a stiff lining during Stage 3, and analytical solutions for the tunnel displacement and lining resistance pressure during Stages 2 and 3 were provided. The analytical solutions for a stiff lining-supported tunnel were compared with those provided in previous studies, and an analytical model of a tunnel that used a yielding lining was applied to an actual tunnel. The effectiveness and reliability of the solutions were verified by comparing them with the application results. Theoretical analysis showed that tunnel deformation could be controlled using a stiff lining, because the increasing rate of pressure is considerably less than that of the lining bearing capacity. However, a 1.4 m lining thickness was required to prevent lining damage. The yielding lining resistance pressure decreased from 3.46 to 0.68 MPa when the lining was 0.2 m thick, and the yielding displacement increased from 50 to 150 mm. The maximum stress in the lining decreased by 56.95 MPa when the yielding lining was displaced by 50 mm, and the thickness increased from 0.2 to 0.5 m. The application of a stiff lining could be considered for rocks that exhibit weak or average deformability; however, attention should be paid to the yielding lining in rocks that exhibit strong rheology.

Fracture permeability is one of the critical factors affecting thermal production in hot dry rock reservoirs. Mechanical creep can cause temporal reduction of fracture permeability. However, the study solely on mechanical creep is limited, particularly under high confining stress. In addition, a physics-based stress- and time-dependent permeability model is essential for predicting the in situ geothermal production. This work aims to study the mechanical creep on the time-dependent fracture permeability. Long-term flow tests through single fractured granite samples under constant loading (20, 35, and 50 MPa, respectively) and stepwise increased loading (20 → 35 → 50 MPa) were conducted. The influence of the loading stress on the creep rate and the influence of the time on the permeability damage were quantitatively investigated. Based on the experimental data, a permeability model considering both stress and time effects was established based on viscous–elastic mechanics. According to the study, we obtained the following conclusions: (1) A higher constant confining stress can result in larger creep deformation, a larger damage ratio of hydraulic aperture (e_h), and a longer duration of rapid reduction of e_h. (2) The previously accumulated creep deformation can affect the subsequent time effect on the temporal evolution of e_h when the loading stress changes, causing e_h rapid reduction stage to weaken or disappear. (3) The transient creep behavior of e_h can be described by the Kelvin creep model, and the maximum damage caused by the creep deformation is almost linearly proportional to the loading stress. The increase in stress caused by the bridging effect between adjacent contact asperities can dramatically reduce the creep rate. (4) The established permeability model can effectively predict the permeability with change in both stress and time considering the effect of accumulated creep deformation on the subsequent creep deformation, and it can be easily implemented in numerical simulation.

Abstract

The water retention characteristics of geomaterials directly influence the hydrological processes for the subsurface flow regime and surface runoff. Despite its utmost importance, most of the soil–water retention models were simply developed based on the retention data of pure water. However, natural processes, such as acidic rainfall or dissolved ions in the pore fluid and the undeniable role of industrialized pollution, result in a certain impurity in the pore water, which is reflected as an osmotic suction component. Therefore, the main goal of this study is to introduce a novel soil–water retention model that considers the role of osmotic potential. The proposed model is developed from a previous model by embedding two new parameters that have clear physical meanings. The parameters are the pore fluid salinity index (PSI) and the modified effective stress that account for the influence of the osmotic potential on the desorption rate and air entry suction, respectively. The model is benchmarked and validated against four experimental data sets that have been rigorously measured and reported by various research groups. The results reveal the robustness of the model when capturing the retention behavior of soils that are exposed to different salt concentrations and species for the matric and total suctions.

Practical Applications

The soil water retention curve (SWRC) is one of the main hydraulic features of unsaturated soils, which has been extensively used when simulating the transient multiphase flow, soil deformation, and shear strength characteristics. Due to its essence and application when solving a wide range of geoenvironmental problems, numerous models have been developed to date. However, the majority of the existing models rely on the simplifying assumption that pure water is the pore fluid, which is contradictory to the nature of several natural and human-induced phenomena, such as acidic rainfall and contamination. Therefore, the main goal of this study was to introduce a new solute-dependent fluid retention model. Then, the modeling framework was validated against comprehensive fluid retention data sets from several research groups. The new framework proved robust when predicting fluid retention as a function of the solute concentration in a simplified yet precise manner. The proposed model, which is an extended form of a previous model, is easy to implement and requires two additional parameters. In conclusion, the new model has the potential to be implemented in numerical codes, constitutive models, and analytical solutions to analyze and design certain geoenvironmental problems in the presence of contamination.

Abstract

A novel deep learning method based on the two-dimensional convolutional neural network (2D-CNN) was proposed to predict the horizontal and vertical convergences of high-speed railway tunnels considering the spatial variability of soil Young’s modulus. The input and output of the neural network were the soil Young’s modulus random field and tunnel convergence, respectively. The coefficient of determination (R²) and the relative error of the predicted results were determined to evaluate the prediction performance and extrapolating ability of the proposed CNN model. The prediction accuracy increased with increasing scale of fluctuation (SOF) from 10 to 60 m as the R² increased. Two prediction data sets with 10,000 samples (per set) were generated to illustrate the model, where the R² values were greater than 0.99. Also, the relative errors of the limit values of 90% and 99% exceeding the probability between the CNN-predicted and random finite difference method (RFDM)-calculated convergences were within 0.64%. The computational efficiency was significantly improved by 2,371 times with satisfactory accuracy. The trained CNN model showed excellent extrapolation ability in solving cases with an anisotropic random field and variation of COV. Results indicated that the proposed CNN model is a promising surrogate of RFDM with Monte Carlo simulations to analyze tunnel convergence considering soil Young’s modulus in an isotropic random field.

Practical Applications

The spatial variability of soil parameters is commonly believed to have a significant influence in assessing tunnel reliability. Traditional probabilistic analysis of tunnel deformation was generally conducted by time-inefficient random finite-element/difference methods with Monte Carlo simulations. Recently, machine learning methods are vastly applied in geotechnical engineering with the rapid development of computational techniques, aiming to improve calculation efficiency. This study develops a two-dimensional convolutional neural network-based model to predict tunnel convergence with consideration of soil spatial variability. The input and output of the surrogate model are the soil Young’s modulus random field and tunnel convergence, respectively. The coefficient of determination, mean square error, and relative error are used to evaluate the prediction performance. The surrogate model is trained by isotropic random field data sets and performs excellent extrapolation ability on the data sets of the anisotropic random field. It suggests that the proposed model can conduct a probabilistic analysis of tunnel convergence in spatially variable soil with high accuracy.

The foundation and dam body below the seepage-free surface are in a saturated seepage state for a concrete-face rockfill dam (CFRD) built on a deep alluvium foundation. The seepage and stress coupling effect could influence the dam deformation behavior. This study focuses on the seepage–creep coupling behavior of the CFRD on alluvium. The Drucker–Prager (D–P) plastic and time-hardening creep models are used to describe the instantaneous and creep deformations of rockfill and alluvium materials. The back propagation (BP) neural network method is adopted to invert the material creep model parameters. The dam and foundation seepage process is described using Signorini’s type variational inequality method. A seepage–creep coupling analysis method for CFRDs on alluvium is proposed on the basis of the momentum conservation principle and the Kozeny–Carman equation. The deformation mechanism and evolution process of CFRD on alluvium are investigated whilst considering the seepage–creep coupling effect. The influence of the seepage effect on the dam’ long-term deformation is discussed.

In this paper, the smear effect and the sand cushion that modifies the permeability of the top boundary are considered equivalently as semipermeable boundaries, to obtain an axisymmetric free-strain consolidation model for unsaturated soils capable of allowing for coupled radial–vertical flow under time-dependent loading. The semianalytical solution is then solved with finite Hankel transform and Laplace transform. This is also verified by comparison with two special cases of axisymmetric consolidation solutions in unsaturated soils. After analysis of the parameters, it is concluded that as the radial or vertical semipermeability factor increases, there is less impediment to the dissipation of excess pore pressures (EPPs); and when it takes values within 0.1–50, it can be used to simulate boundaries with arbitrary permeability from permeable to impermeable. The EPP distribution at different points and times reveals that the effect of the air- and water-phase semipermeability factor on the consolidation of unsaturated soil is different. Furthermore, the contour plots of EPP distribution under free-strain assumption provide a decent reflection of the smear effect and the alteration of the top boundary permeability due to the sand cushion.

Deep soil mixing and jet grouting are in situ ground improvement techniques that are often used to enhance the stability of soft soil slopes for excavation and waterway projects. Although researchers have reported spatial variability in cement-treated ground, the literature on the influence of spatial variability on the stability of treated soil slopes is limited. In this study, the stability of a cement-treated soil slope is analyzed through two-dimensional random finite-element analysis, taking into consideration spatial variabilities in strength and unit weight of a cement-treated soil mass. Spatial parameters such as the mean strength µ_qu, coefficient of variation COV, horizontal scale of fluctuation SOF_h, and vertical scale of fluctuation SOF_v are varied, and the equivalent design strengths and strength reduction factors are calculated. Furthermore, strength reduction factors for isotropic (SOF_h/SOF_v = 1) and anisotropic (SOF_h/SOF_v = 7.5) spatially correlated random fields are calculated. The results show that an increasing value of SOF_h has a worsening effect on stability up to a particular value, that is, approximately 2.0 times the slope width B, beyond which the influence does not appear to be significant. A strong logarithmic relationship between the SOF_h/B ratio and the computed strength reduction factors is observed. Finally, a modified equation for the strength reduction factor is proposed; in this equation, the effect of the scale of fluctuation is considered by introducing an influence factor R_SOF. For SOF_h/B ratios ranging from 0.02 to 0.7, R_SOF varies from 1.0 to approximately 2.0, indicating a significant effect of the scale of fluctuation. However, for SOF_h/SOF_v ranging from 1 to 7.5, R_SOF varies slightly.

The dynamic response of a composite lining tunnel with an isolation layer under blast-induced cylindrical P waves is investigated by using the wavefunction expansion method. The spring model is introduced to simulate the imperfect interfaces between the structural layers of tunnels. Numerical calculations are carried out based on the Xianglu Mountain Tunnel in Southwest China to illustrate the influence of the source-to-tunnel distance, the degree of interface imperfection, and the shear modulus and thickness of the isolation layer on the dynamic stress concentration factor (DSCF), the radial velocity scaling factor (RVSF), and the hoop velocity scaling factor (HVSF) of the final lining. It is concluded that dynamic stress and peak particle velocity in the final lining are strongly affected by the source location, especially under the condition of a small source-to-tunnel distance and a relatively low incident frequency. The DSCF, RVSF, and HVSF for blasting P waves, the frequency of which is below 200 Hz, agree with the corresponding results for far-field plane P waves if the source-to-tunnel distance surpasses 50 times the inner diameter of the tunnel. The interface imperfection tends to weaken the DSCF in the final lining, and its effect on RVSF and HVSF is frequency-dependent. The isolation layer can significantly reduce the dynamic stress and particle velocity response in the final lining.

An experimental simulation of cyclic and postcyclic pullout resistance for cement-grouted soil nails in the sand is presented for the first time. Occasionally, soil nails in excavations and slopes are affected by cyclic loading due to road or railroad traffic. Hence, this research aimed to study the effect of cyclic loading on the cyclic and postcyclic pullout resistance of soil nails. A nail pullout set has been built that is able to apply static and cyclic loadings in force or displacement control conditions. The effects of different parameters on interface resistance are examined, including static and cyclic loadings levels, grouting pressure, overburden pressure, number of cycles, and frequencies. It is concluded that soil nails’ postcyclic pullout resistance is generally more than the static pullout resistance. The results show that by increasing the cyclic loading level threefold, the postcyclic pullout resistance is decreased nonlinearly by about 35%; while with the double increasing in overburden pressure or grouting pressure, the postcyclic resistance is increased linearly nearly 15% and 35%, respectively. The results also show that for the selected range of the number of cycles and frequencies, the postcyclic pullout resistance is almost constant; in other words, observations show that when the number of cycles or frequencies increases, the postcyclic pullout resistance decreases very slowly.

Abstract

The safety factor (SF) in riverbank stability problems for noncohesive soils is treated mathematically in this article to unravel its inherent bimodality, as stipulated by the cusp catastrophe technique in a hyperspace. The developed methodology may be contrasted with traditional approaches delineating the SF space to failure and operational states. The emerging three states are a significant shift from traditional treatments overlooking bimodality. The mathematical treatment presented in the technical note incorporates classic soil equations for noncohesive soils into the cusp catastrophe technique, with clear formulations at multiple levels of a potential function, including the energy level. The integrated mathematical expressions use soil properties to express lower order properties such as catastrophe flags, for example, bifurcation sets, hysteresis, and bimodality delineating sudden and gradual change in the state of a system. These equations show that even the SF of operational states depends on soil properties and gravity, and therefore a safe use of the SF requires a deep knowledge of the SF hyperspace.

Practical Applications

Practical applications of the new mathematical development presented in the technical note may be viewed in three steps. In Step 1, the tacit nature of the cusp catastrophe bimodality of safety factor in riverbank stability problems needs to be tested through experimental data. The laboratory tests and fieldwork can be designed to encompass the full range of cases comprising: (i) the failure region; (ii) the operational region; and (iii) their bimodal zone. In Step 2, the existence of the three cases is verified by various applications to underpin the dependency of the safety factor on soil parameters. In Step 3, this new knowledge is realized by wide applications to gain an insight into behaviors of safety factors in wide-ranging problems such as natural slopes, channels, embankments, riverbanks, and levees.