International Journal of Geomechanics

The seismic design of pile foundation–supported structures has gained significant attention in the last few decades; there have been several failures during moderate to severe earthquakes. Seismic soil–pile foundation–structure interaction (SSPSI) was reported as the governing phenomenon behind the failure mechanism of such structures. In this context, in this study an attempt is made to investigate the influence of SSPSI (mainly the inertial part of the interaction) in a pile-supported structural system with variation in dynamic characteristics and the ratio of superstructure stiffness to pile foundation stiffness (K_s/K_p) through physical model experiments using a shake table. The primary aim in this study is to assess the extent of the effect of dynamic soil–structure interaction in a scaled-down pile foundation–supported bridge structure in nonliquefied ground with the help of a simplistic normalized parameter, i.e., K_s/K_p, for different ranges of the period of the superstructure system. The study primarily highlights variation of the normalized lateral period of the whole interaction between the superstructure and pile foundation as a function of the practicable range of K_s/K_p, which may be helpful for designers in predicting the dynamic characteristics of the system as a first step in seismic design. Similarly, the trend of normalized spectral acceleration and displacement response at the superstructure and pile head with respect to K_s/K_p is presented for the prediction of design forces. Finally, an idealized case study prototype design problem is used to verify the outcome of the study and illustrates simplified design steps for pile foundation–supported structures in general.

A series of reduced-scale model tests was conducted on helical soil-nailed walls (HSNWs) under vertical surface loading to evaluate the effect of length, pattern, and inclination of nails on lateral pressure distribution in this innovative retaining structure. The particle image velocimetry (PIV) technique was also used to trace shear bands and identify the failure mechanism. PIV results showed that increasing the nail length, as well as using a square pattern and a 15° angle to install the nails, could be three effective ways to reduce the penetration depth of the slip surface and, consequently, to limit wedge failure dimensions. A linear relationship formed between the lateral pressure induced on the wall facing and the footing pressure at q < q_u. This relationship showed that a greater portion of the footing pressure was converted to lateral pressure using longer nails and installing them in a square pattern and at an angle of 15°. Considering the inverse relationship between lateral pressure behind the facing and wall displacement in HSNWs under vertical surface loading, it was concluded that both pressure and displacement criteria should be considered simultaneously to optimally select the arrangement, pattern, and inclination of helical nails.

Fully grouted bolts are widely used to reinforce road cutting slopes because of their low cost and high efficiency. However, there is a lack of research on the anchorage failure mechanism and stability assessment of bolt-supported road cutting dip slopes (BRCDSs). In this work, a progressive failure process of BRCDSs was captured via a triangular block model in the Universal Distinct Element Code (UDEC). A kinematically admissible velocity field of BRCDSs was innovatively established based on their sliding shear failure and the Mohr–Coulomb criterion. As a consequence, a novel method for evaluating the stability of BRCDSs was proposed using the upper bound theorem of limit analysis. The reinforcement effects of BRCDSs with different bolt angles were subsequently evaluated by UDEC simulations, and the obtained slope safety factors and bolt-supporting forces were compared with the results calculated by the proposed method. In addition, the influence of various parameters (bolt angle, bolt diameter, slope angle, and strengths of rock and discontinuity) on the reinforcement effect was investigated. The results obtained using the proposed method showed good agreement with those obtained via UDEC. There is an optimal bolt angle in BRCDSs, at which the bolt generates the largest supporting force and thus promotes the highest safety factor of the slope, and the value of this angle is approximated as the friction angle of the discontinuity. The safety factor of the slope also increases with the increase of bolt diameter, and the variation increasing according to a power function. The method proposed provided a convenient theoretical tool for the stability assessment and reinforcement design of BRCDSs.

The piezocone penetration test (CPTu) is a common geotechnical field test to evaluate soil properties. In interpreting the CPTu field measurements, soil drainage conditions are mostly considered completely drained or undrained; however, partial drainage conditions govern for such soils as silts or clayey sand mixtures. Previous studies show that neglecting partial drainage conditions causes incorrect estimation of soil geotechnical parameters. Most studies have been conducted using calibration chambers and centrifuge tests on clayey soils. Due to the complications in modeling the piezocone test, few numerical studies have been performed under partially drained conditions, especially on coarse-grained soils. Among the challenges of numerical modeling of CPTu, one can mention the difficulty of modeling soil structure in large strain mode and soil–water interaction behavior. In this paper, piezocone penetration tests were modeled using the advanced hypoplastic constitutive model and finite-element method. The behavior of Firoozkooh sandy soil under different drainage conditions and relative densities was analyzed. Then, the effect of cone penetration on the surrounding soils was discussed. It was shown that drainage conditions and the soil relative densities significantly affected the trend of variations in excess pore-water pressure (EPWP) generated around the piezocone.

The geometric properties of the pore space govern the electrical and hydraulic properties of soils. Recognizing this commonality, this paper proposes a new approach to determine permeability from high-frequency electromagnetic (HF–EM) measurements on geomaterials undergoing evaporative dewatering. Two materials have been used; (1) kaolin, a reference geomaterial; and (2) a dredged sediment sample from the Port of Brisbane (PoB) reclamation site (Queensland, Australia). Based on a dielectric mixture equation that integrates Archie's parameters, the evolution of formation factors as a functions of porosity have been derived. Several models that are available in the literature have been used to estimate proxies for geometric length scales for pore radii. With knowledge of the formation factors and proxies for pore radii as functions of porosity, permeabilities of the materials under saturated conditions have been estimated. The results have been compared with experimentally obtained permeabilities. Using the formation factors from HF–EM measurements and two of the proxies for pore radii (hydraulic and characteristic pore radii, as defined in this paper), a good degree of agreement with the experimental data has been obtained within a variation in one-order of magnitude for kaolin. For the PoB material, the use of hydraulic pore radius yielded the best estimation of permeability.

Regarding a novel and low-carbon composite pile, the MgO-carbonated composite pile (MCP) is formed by a pervious concrete pile (inner core) at the center of a deep magnesia (MgO) mixing column (outer core) subjected to forced carbonation. The shaft resistance of the MCP is primarily governed by the interface of pervious concrete–carbonated soil. Understanding the interface’s shear behavior and strength is crucial for the design and safety assessment of the MCP. In this paper, a series of direct shear tests were conducted to analyze the shear behaviors of the pervious concrete–carbonated soil interface. The aim was to evaluate the impact of key influence factors such as carbonation time, MgO content, and initial water content on the interface friction capacity. Furthermore, a practical prediction model for interface shear strength was proposed, taking into account the aforementioned factors. The test results revealed that the peak shear strength of the specimen under 12 h of carbonation time reached 508 kPa, representing a 94% increase compared to the specimens without forced carbonation. The curves of shear stress versus tangential displacement of the inner–outer cores interface exhibited both strain-hardening and -softening behaviors. The proposed linear model exhibited excellent predictive accuracy for interface shear strength, with a regression coefficient exceeding 0.8. Based on multiple linear analysis, MgO content emerged as the most significant factor affecting the interface shear strength of the MCP, followed by vertical effective stress.

This work discussed the results from tests conducted to study the desorption and outburst characteristics of soft and hard coal under different gas conditions. The research found that gas type has a greater influence on the outburst and desorption characteristics of soft coal. Triaxial compression failure can improve the desorption rate of coal specimens, and the gas desorption rate of hard coal is more affected by triaxial compression failure. In the outburst experiments, the greater the test gas pressure, the greater the crush degree of the coal specimen, and the proportion of small-sized coal powder (<0.1 mm) increases but gradually stabilizes. Moreover, the acoustic emission (AE) signals during the outburst process were recorded, and the evolution law of gas pressure in the outburst carven was also analyzed. Finally, the changes in AE signals and gas pressure could be used to predict the damage and destruction characteristics of coal specimens well.

The aim of this study is to investigate the influence of the anisotropic variability in soil on the failure mechanism and the ultimate bearing capacity of foundations that are placed near slopes. A random adaptive finite-element limit analysis (RAFELA) with anisotropic random field modeling and the Monte Carlo simulation technique are combined. The stochastic ultimate bearing capacity for various undrained shear strengths, slope angles, normalized slope heights, distances from the slope crest, and horizontal and vertical correlation distances are analyzed. The quantitative identification and characterization of failure mechanisms, which are determined by the failure slip and passive wedge contribution, are achieved through the digital image processing of realizations from anisotropic random soil. This study showcases the diverse failure mechanisms and sliding surface sizes that arise from distinct soil patterns, which are influenced by alterations in slope geometry and soil anisotropic spatial variability. The quantitative findings highlight that alterations in failure mechanisms and slip surface sizes lead to heightened failure probabilities and significant variations in bearing capacity.

For oil and gas exploitation engineering, it is significant to study the interaction between hydraulic fracture (HF) and natural fracture (NF). An extended peridynamic (PD) model for hydraulic fracture analysis is presented, in which the PD pairwise force with the bond rotation is considered, fluid pressure is applied at fracture surfaces with broken bonds, and the critical stretch and critical relative rotation bond failure criteria are derived for Mode I and Mode II fractures of shale reservoir, respectively. Then, numerical examples of hydraulic fracturing of concrete and shale are analyzed using the PD model. The proposed model can well predict the interaction behaviors between HF and NF, and simulation results show that the larger the in situ stress difference and approach angle, the easier it is for the hydraulic fractures to cross the natural fractures.

Illite is a type of clay mineral that forms rocks. It plays an important role in the mechanical properties of the red-bed soft rock, which is significantly influenced by water. However, it remains unclear how water affects the mechanical properties of illite. An axial tensile test of illite with water content is performed using molecular dynamics (MD) simulations. Uniform tensile velocity is applied in the directions parallel (x) and perpendicular (z) to the illite. The slip and shear of this clay mineral sheet are the main cause of the softening of the red-bed soft rock because stretched illite is difficult to disturb when soaked in water. The density distribution of the interparticle solution shows that only the inner-sphere surface complexes exist in illite when it has a low water content. While in the cases of high water content, outer-sphere surface complexes exist in illite as well. The existence of water can greatly reduce the tensile strength of illite, by as much as 14%–25% (along x-direction) and 54%–84% (along z-direction) than those of the dehydrated cases, respectively, with different water film thicknesses. The destruction of tension along the x-direction is first caused by the tension fracture of the substituted tetrahedral covalent bond (At···Ob), which finally leads to the crack coalescence of the clay mineral sheet. While the tensile failure along the z-direction is mainly due to the pull off of the interparticle solution.

Although three-dimensional finite-element analysis has been introduced to foundation design for years, simplified computer-based calculations based on the spring analogy of soils and structural elements are also frequently used in many engineering projects. Using coefficients of subgrade reactions and pile stiffness is a straightforward way to design and analyze a piled raft foundation. This paper discusses the determination of the coefficients of pile reactions for embedded piled raft foundations in soft clays subjected to vertically uniform loads through a series of finite-element analyses. The geologic property was assumed to be composed of homogeneous soft clays with the shear-wave velocities varying at 100–140 m/s. The foundations with a square raft ranging from 16 to 36 m and embedment depths of 8 and 12 m were studied. Load intensity approximating 1.5 times the excavated soil pressures was considered where the maximum uniform load was applied to the foundation. The foundation settlements and reactions of the piles were obtained first from a three-dimensional finite-element analysis. For design convenience, the dimensionless pile displacements, pile reactions, and coefficients of pile reactions were developed and presented in plots. The embedment depth-to-raft size ratio, pile slenderness ratio, and pile-to-pile spacing distance-to-pile diameter ratio were examined to find their optimized correlation. Finally, a design example was given to verify the proposed suggestions. We conclude that the proposed method can be an alternative to the piled raft foundation design.

This paper investigates the motion of Rayleigh wave fields produced by a seismic surface pulse in an isotropic micropolar elastic half-space and in a multilayered micropolar media. The method of potentials followed by the Hankel transformation technique is used to determine explicitly the displacement vector components and microrotation vector components. Two different cases of surface seismic pulses are taken into account and the behavior of Rayleigh waves displacement components are studied separately. A matrix approach has been implemented to obtain solutions for the wave equations to reflect the motion of Rayleigh wave fields produced in multilayered micropolar elastic media due to surface seismic loadings. The displacement solutions for the half-space are obtained in the form of integral equations, which are further numerically evaluated and a few 2D and surface plots are plotted to describe the characteristics of the solutions.

Advanced geological prediction plays an essential role in disaster prevention and safety guarantee. In this paper, by combining multiwavelet analysis with modified evidence reasoning, a new geological prediction method for surrounding rock in a tunnel considering the parameters’ coupling effect was proposed. Based on the key drilling parameters’ curves that were denoised in the No. 1 hole of the Xiaochuan tunnel in Gansu Province, China, the coupling effect of these parameters were analyzed and the power–speed ratio (PSR) concept was thereby proposed to deal with the denoised curves and obtain a PSR curve. Through continuous wavelet transform (CWT) of the PSR curve, the lithology and structure were preliminarily determined by sectional matching and optimization of multiple wavelet coefficient curves under the optimal scales and PSR curve. Further, the evidence reasoning was introduced to solve the uncertainty parts in the preceding result. To avoid misjudgment caused by the limitation of Dempster Shafer (D-S) theory in the case of complete conflict, it was modified, in which the basic probability assignment (BPA) was determined by the interval number, and a binary group was then defined by combining the traditional conflict coefficient with pignistic probability distance to measure the degree of conflict. The division result that settled by this method showed greater accuracy than tunnel seismic prediction (TSP) and the drilling core, also could give a clear division for both lithology and structure. Importantly, by applying the established method to Chengzhou tunnel, the result fitted the real condition well. This approach provides references for rock prediction in tunnels.

The mechanical properties of tunnel support structures have been widely studied by researchers, but there are insufficient studies concerning energy mechanism and mechanical properties in the failure process of concrete and rock as binary composites. Based on this, this paper first analyzed the characteristics of stress–strain curves of sandstone ordinary concrete (SNC) and sandstone fiber-reinforced concrete (SFC) in disparate confining stresses. Next, the volume strain, dynamic elastic modulus, elastic–plastic strain, brittleness index, damage constitutive model, inward friction angle, and cohesion of specimens were explored under monotonic and cyclic loads. Finally, the failure mode and triaxial loading energy mechanism were studied based on computerized tomography (CT) detection. The consequences expounded that triaxial compression failure of sandstone concrete could be divided into four stages, and the compression of the specimen changed to expansion during loading. The elastic modulus of specimens increased when the confining pressure increased and decreased with the increase of fiber. As the confining pressure increased, the plastic strain of sandstone concrete at the final failure increased. The fiber was harmful to the growth of the elastic concrete modulus and the elastic modulus first increased and then decreased. With the confining pressure increasing, the brittleness index of concrete was smaller, and the fiber could efficiently prevent the concrete from cracking. As the strain increased, the rule of change with the increase of axial strain was “S”. As the confining pressure decreased, the growth rate of the damage constitutive model rose variably. The cohesion of SFC was greater than SNC based on the analysis of the evolution of basic parameters. The pore and crack area after SNC compression failure was larger and deeper than that of SFC after compression failure. When the peak stress of specimens was maximal, the density of the elastic energy reached the maximum. The density of dissipated energy increased sharply after the peak stress.

Shear dilation occurs during the failure of geomaterials subjected to shear stress. To account for the dilatancy and nonlinearity of geomaterials, a nonlinear upper-bound analysis method was proposed for evaluating the stability of homogeneous slopes based on the coaxial nonassociated flow rule. The rotational failure mechanism of a homogeneous slope was established within the kinematic approach of limit analysis, using the Davis approach to convert the nonassociated flow rule into an associated one. By applying the variation principle, ordinary differential equations of the potential sliding surface and its corresponding stress were derived, which were then solved using a fourth-order Runge–Kutta method in conjunction with appropriate boundary conditions. Furthermore, the balance equation was derived from the virtual power principle, and the critical height of the slope was calculated using a particle swarm optimization algorithm. The strength reduction technique was then introduced to determine the factor of safety of the slope. The accuracy and effectiveness of the proposed nonlinear upper-bound variation method for evaluating the stability of homogeneous slopes under nonassociated flow rule and nonlinear failure criteria were verified when compared with existing studies and techniques, such as the finite-element limit analysis and the finite difference method. This study accurately reflects the nonlinearity and dilatancy of geomaterials and avoids assumptions regarding the sliding surface and its corresponding stress, making it a valuable reference for future research.

This study proposes an analytical solution for heat transfer in energy soldier piles, considering the convective boundary conditions and temperature distribution differences in the angular direction. The problem is solved by using the separation of variables method and the Fourier–Bessel series. The proposed solution is validated by comparing the results with field test data. The field test results indicate that air convection significantly impacts temperature when the pile segment is buried shallowly. At the measurement point far away from the excavation depth (10.0 m), the temperature difference during the thermal response tests (TRTs) before excavation is 14% of the temperature difference after excavation at that point. The temperature changes for the measuring points buried in soil and the bare measuring points near the excavation face do not exceed 3°C. Heat transfer analysis using this analytical solution reveals that convective heat transfer from the surrounding environment affects the pile temperature. The further the energy pile is from the air, the more the soil temperature increases; however, the soil temperature decreases as the convective heat transfer coefficient increases. Larger diameter piles are more easily affected by this phenomenon and reach stable temperatures more slowly. The thermal influence range is 1.5–2.0 m, which decreases as the convective heat transfer coefficient of air increases. To ensure the supporting effect, burying the pipeline on the soil side and reducing the diameter of the energy soldier pile can effectively improve thermal performance.

Abstract

In excavation engineering, most of the retaining structures are subjected to asymmetrical loads, including in asymmetrical excavation and asymmetrical overloading. However, a theoretical solution to, and deformation-controlling method for, this problem has remained elusive to date. In this work, a semianalytical solution for determining the deformation of retaining structures subjected to asymmetrical loads was derived based on the principle of minimum potential energy. The retaining structures were viewed as flexible, and the overall deformation of the retaining system was evaluated by our proposed theoretical model. This model was verified by comparing it with practical monitoring data. Subsequently, the theoretical solution was applied to analyze the stress and deformation characteristics under the conditions of asymmetrical excavation and asymmetrical overloading, which can be controlled to ensure the safety of a project. Finally, because of the requirement for a small amount of deformation in the foundation pit retaining piles, the earth pressure on the retaining piles is close to the static earth pressure. Reducing the length of the retaining pile on one side had a greater influence on the deformation on that side, but less of an influence on the other side. Thus, the proposed model can be used for optimizing the asymmetrical design of a retaining structure in order to balance the economy and safety of a project.

Practical Applications

Our design theory can be applied to a variety of engineering projects. A typical case is the Guangzhou Baiyun District Comprehensive Transportation Hub Project in China. The method was used to analyze the status of the subway foundation pit, offering a new design scheme to reduce the design length of the retaining piles, which resulted in great economic benefits in its practical application. The findings provided in this paper can be applied to the engineering of any long, narrow foundation pit that can be simplified into a two-dimensional plane–strain calculation model, being used to realize more accurate deformation control over the foundation pit envelope. The core of this calculation method is that it can accurately describe the magnitude of the earth pressure. The distribution of the earth pressure is worthy of further study.

The pressure-settlement response and failure mechanism behavior of two interacting rigid rough identical footings resting on geocell-reinforced sand is investigated using finite-difference software, FLAC3D. The geocell is modeled with the actual shape considering the inclusion of the interface element. The numerical simulation is validated with experimental investigations and then parametric studies are conducted on the geocell geometry at different spaces between footings, simultaneously. A mathematical formulation is proposed to obtain the improvement factor using response surface methodology as a subdivision of the design of experiments method. Comparison between the simulated results with performed test results and previous investigations shows an excellent agreement. The derived mathematical expression shows the optimum values of 0.1 and 0.35 times the footing diameter for embedded depth and height of geocell-reinforced soil, respectively. The maximum edge-to-edge distance of twin footings with no influence on each other is approximately 3.5 times the footing diameter while the maximum improvement factor is obtained when two footings are beside each other with no clear distance.

Strength characteristics of graded gravels are essential in the construction of roadway and railway substructures. Traditional constitutive models, primarily nonlinear elastic and plastic types, fall short in accurately capturing the strain-softening properties of such materials. To address this limitation, the current study introduces a statistical damage model designed to outline the stress–strain behavior of densely compacted graded gravels in transport infrastructures. Utilizing medium-sized triaxial tests, the model examines variations in strength and deformation parameters in relation to compaction levels and incorporates a unique damage-softening index (DSI) along with a threshold axial strain to improve accuracy. The study establishes that the DSI and threshold axial strain effectively regulate stress–strain relations in the postpeak segment, the model’s statistical parameters and threshold axial strain can be precisely determined through the introduction of DSI, and the model closely aligns with experimental data across multiple compaction levels. These findings are especially relevant for engineering design in the context of roadway and railway construction and indicate potential for further refinement, such as the incorporation of loading rate considerations.

The weak mechanical properties of dredged soil can be improved by using cementing agents. Comprehensive investigation of the efficacy of stabilizing methods, including Portland cement (PC) and alkali-activated ground granulated blast-furnace slag (CaO-GGBS), cementing agent content from 12% to 16%, and curing methods in different initial moisture content of soils, organic matter content, and soil types was carried out. Experimental results showed that dredged soils have stronger properties using CaO-GGBS than PC. Maximum strength was found by using the highest CaO-GGBS content of 16%. The increase of organic matter content can weaken the enhanced properties of dredged soils stabilized by CaO-GGBS. The highest shear strength of specimens stabilized by CaO-GGBS occurred in dredged soils with 20% sand while the greatest yield strength in e-log p′ space was found in dredged soils with 30% sand. A modified constitutive model based on the framework for cemented geomaterials proposed by Gens and Nova has been developed. The ability of the model to reproduce the mechanical behavior capturing strength development with curing period was explored.

Drilling with prestressed concrete pipe cased piles (DPC piles) introduces a novel methodology in the domains of energy conservation and environmental protection, utilizing nondisturbed, high-strength, prestressed concrete pipe piles with diameters ranging from 800 to 1,400 mm. The distinctive construction method intrinsic to DPC piles creates an annular gap, roughly 10 mm thick, between the pile shaft’s wall and the drill hole’s wall. Grouting within this pile–soil gap constitutes an effective strategy to recover and augment the skin friction of DPC piles. Although an innovative design, the grouting mechanism and its implications on the load-bearing attributes of DPC piles are still under investigation. To explore this, on-site experiments and laboratory physical model assessments were performed, focusing on DPC piles’ load-bearing performance. The principal findings are as follows: (1) DPC piles serve as friction end-bearing piles, where a DPC pile of 1.0 m diameter consistently exceeds a load-bearing capacity of 20,000 kN; within this, lateral friction accounts for 67.84%–72.85%. The primary failure mode of DPC piles is identified as shear failure at the contact interface between the grouting stone and adjacent soil. Remarkably, in comparison with cast-in-place piles and pile-driven pipe piles, DPC piles demonstrate load-bearing capacity increments of 33.42% and 23.16%, respectively. (2) Both the coverage extent of the pile area by grouting stone and the per-unit-area shear strength of the pile–slurry–soil contact interface have substantial impacts on the load-bearing performance and load transmission characteristics of DPC piles. (3) After grouting, the ability of DPC piles to withstand upper loads in silty clay layers exceeds that in sandy soil layers; moreover, their vertical load-bearing capacity within silty clay layers outperforms their capability in sand layers. (4) The presence of voids within the pile–soil gap or infiltration of parts of this gap by sand may alter the grout’s flow trajectory, consequently changing the thickness of the grouting stone and the area covered by it. These variations underlie the effect on the skin friction of DPC piles. (5) And, the primary aspect contributing to the enhancement of shear strength at the contact interface through postgrouting resides in the increase of cohesion or the internal friction angle at this interface.

The Hoek–Brown failure criterion has been widely applied to predict the strength of rock masses, demonstrating its relevance in diverse geotechnical contexts. This paper presents a novel 3D numerical implementation of the Hoek–Brown criterion in a finite-element limit analysis code. The proposed formulation is unique in its ability to produce strict upper and lower bounds for 3D problems, providing more accurate and reliable predictions of failure mechanisms compared to previous formulations. The validity of the formulation is demonstrated through comparisons with known analytical solutions or other authors’ numerical solutions. Furthermore, the proposed numerical tool is used to determine the stability of shallow circular tunnels in rock masses, highlighting its practical applicability in engineering design.

An empty hole in tunnel blasting and excavation serves as an auxiliary free surface and compensation space for blasting rocks, significantly influencing the blasting effects. The straight-hole cut blasting with an empty hole in a railway tunnel project was studied by theoretical analysis, numerical simulation, and field application. The influence law of two key technical parameters, the empty hole diameter and the distance between the cut and empty holes, on the blasting effect under the action of multihole blasting, was analyzed using ANSYS/LS-DYNA R11. The research results show that the final forming effect of the blasting cavity is closely related to the diameter of the empty hole and the distance between the cut and empty holes. When the empty hole diameter is twice the cut hole diameter, and the distance between them is 2.5 times the empty hole diameter, the cut blasting effect is the best. The blasting design and application of the straight-hole cut blasting with an empty hole on the site were carried out based on the theoretical analysis and numerical simulation results. The smooth tunnel after blasting, the average linear overexcavation value of about 9.3 cm, and the high contour-forming quality verified that the numerical simulation and blasting scheme was rational. The research findings can serve as a valuable reference for the blasting design of similar tunnel projects.

The mining fractures in overlying strata are the channel of gas migration, which is closely related to the occurrence of coal mine gas disaster accidents. In this paper, eight groups of discrete element three-dimensional models considering the effect of the hard rock proportional coefficient (HRPC) are established for the rock structure characteristics of a composite roof, and corresponding numerical experiments are carried out to clarify the fracture evolution mechanism of different HRPC effects on the composite roof of the coal seam. Then, the multifractal figure processing program is applied to the analysis of the nonuniformity of the fracture’s distribution and the size difference between fractures in the composite roof of the coal seam. Furthermore, the effects of HRPC on the multifractal characteristics of mining fractures in the composite roof are discussed, and the distribution characteristics and development degree of mining fractures of different HRPC models after mining are quantitatively characterized. According to the quantified values, the evolution rules of overburden fractures in each stage of coal seam mining can be clearly analyzed.

Abstract

Collapsible and expansive soils cover almost one-fifth of the Earth's land surface. The key feature of these categories of problematic soils is the considerable volumetric strain upon saturation under constant stress. However, most studies on saturated hydraulic conductivity of soils assumed no volumetric tendency of the specimen subjected to water saturation. As a result, a significant error may arise from this assumption in collapsible and expansive soils. In accordance with the standard testing protocols, no modeling approaches regarding the hydraulic conductivity can consider the significant changes in void ratio caused by the saturation-induced collapse or expansion phenomena. Therefore, this study aims to introduce a new modeling framework for the saturated hydraulic conductivity of problematic soils with a marked volumetric tendency upon saturation. Accordingly, 36 tests are conducted on two typical soil types representing collapsible and expansive soils with three levels of saturation-induced volume changes and a wide range of void ratios. All tests are conducted in a low-compliance double-cell twin-burette triaxial permeameter. Afterward, new modeling frameworks based on the Kozeny–Carman model are developed to account for the collapse and swell potentials. The results reveal unique functions between the saturated hydraulic conductivity and the reliable void ratio for both collapsible and expansive soils. Eventually, the performance of new models is validated against some independent data sets retrieved from the literature and discussed.

Practical Applications

Collapsible and expansive soils cover about one-fifth of the Earth’s surface. These soils are characterized by high deformability once a near-saturation condition is approached. As a result, the corresponding hydraulic conductivity can be subject to sever variations because of significant microstructural modification induced after inundation or hydration. However, most of the measuring devices and modeling frameworks fail to take into account the saturation-induced volume changes of problematic soils in estimation of permeability versus void ratio function. Therefore, the significance of volumetric strain during the saturation procedure was first revealed and highlighted through conducting 36 permeability experiments in a low-compliance triaxial permeameter on both collapsible and expansive soils, with various deformation potentials in response to full saturation. Afterward, new models pertaining to both types of soils were developed based on the Kozeny–Carman model with two additional physics-based parameters reflecting the deformation tendency of a given soil. Indeed, the merit of the new modeling framework is the incorporation of collapse and swell potentials into the void ratio function, which can be easily measured with a conventional oedometer cell. Eventually, the proposed modeling approach was validated against measurements conducted in this study as well as several data sets retrieved from the literature.

Liquid nitrogen (LN₂) fracturing is beneficial to the development and utilization of geothermal energy. In this paper, red sandstone was heated from room temperature to different temperatures (25°C–800°C) and then cooled with LN₂. After attaining room temperature, nuclear magnetic resonance (NMR), uniaxial compression, and acoustic emission (AE) tests were conducted, and results were compared for different samples. The results showed that with an increase in quenching temperature difference, the volume of micropores decreased gradually, while the volume of fine pores, mesopores, macropores, and total porosity increased, resulting in the reduction of the compressive strength. A higher quenching temperature difference also reduced the sample’s total time to fail, and the failure mode was transformed from single inclined shear failure to conical failure. This is because the thermal stress (caused by the rapid cooling of LN₂) expanded the original cracks of the sandstone, leading to high porosity and low compressive strength of the heat-treated and quenched samples.

This paper aims at proposing a calculation model to deal with the face stability of shield tunnels in unsaturated soil. Based on the existing results of centrifugal test and numerical simulation, a new failure mechanism is proposed, which combines the logarithmic spiral with upper loosening arch mechanisms. The failure mechanism considers the influence of the soil arching effect and the logarithmic spiral height coefficient. Formulas for calculating the loosening earth pressure of unsaturated soil are deduced. Subsequently, this model of loosening earth pressure is introduced into an analytical model to compute the limit support pressure on the tunnel face. Compared with other height coefficients and the degree of soil arching effect, when the height coefficient is 0.95 and the degree of soil arching effect is 10%–30%, the limit support pressure proposed in this paper is in good agreement with the centrifugal test results. Finally, comparisons are performed with the proposed model and classical models as well as typical centrifuge model tests. The comparative analysis shows that the proposed theoretical model is in good agreement with the numerical simulation. The outline of the failure zone by the theoretical model in this paper is in good agreement with the centrifuge results.

The inclined retaining wall has been demonstrated to be an effective and efficient retaining structure for limiting deformation during excavations. However, a design method regarding wall deformation and soil movement of excavations with inclined retaining walls is lacking. The mobilizable strength design (MSD) method has been used to provide reasonable deformation results for excavations with cantilever retaining walls. The deformation of an excavation is calculated based on the moment equilibrium of the strain field, and the deforming regions are divided by straight zero-extension lines (ZELs). The boundary of the deforming region of an inclined retaining wall is observed to be curved by performing centrifuge tests. Hence, a logarithmic spiral-type function of the ZEL is fitted to the generated numerical data. Based on the fitted function of the ZEL trajectory, the stress distribution on the retained side of the inclined retaining wall is modified, and the wall deformation is calculated within the framework of the MSD. Finally, the soil movement vectors at the ground surface, deduced by the fitted ZEL function and calculated wall deformation, are used to evaluate the influence of excavation on adjacent buildings.

This paper investigates the effect of temperature variations on the creep and stress relaxation behavior of clay samples from London Bank Station. The independent and coupled effects of strain rate and temperature on one-dimensional (1D) stress–strain and stress relaxation responses were investigated based on a series of temperature-controlled constant rate of strain (CRS) compression–relaxation tests carried out at fast, intermediate, and slow displacement rates and over 20°C–55°C. The temperature effect on the creep index ( $C_{\alpha }$) was investigated based on a series of temperature-controlled multistage loading (MSL) oedometer tests. The results of the CRS compression–relaxation tests showed that with the increase in temperature, the coefficient of stress relaxation ( $R_{\alpha }$) decreases for samples that were loaded at fast and intermediate prerelaxation displacement rates ( $\dot{\upsilon }$); however, it increases for samples loaded at the slow prerelaxation displacement rate. A decrease in $\dot{\upsilon }$ by a factor of 10 (i.e., from 0.010 to 0.001 mm/min) causes the $R_{\alpha }$ values to reduce by 55%–11% with the temperature increase. The increase in temperature caused an increase in $C_{\alpha }$ that were obtained from the MSL tests. The maximum value of $C_{\alpha }$ increased by 18% from 35°C to 45°C and by 37% from 45°C to 55°C. The temperature effects on other conventional parameters that included the preconsolidation pressure, and the compression and swelling indexes (C_c and C_s) were comparable with the findings reported in the literature. Comparing $C_{\alpha }$ that were obtained from the MSL tests and $R_{\alpha }$ that were obtained from the CRS tests supports the validity of $R_{\alpha }=C_{\alpha }/C_c$ correlation for thermally influenced saturated reconstituted clays and that the time-dependent soil parameters could be obtained from relatively fast CRS compression–relaxation tests as an alternative to conventional time-consuming oedometer tests.

This study focuses on the nonlinear flow characteristics of a single rough-walled fracture in a marble sample subject to various confining pressures (5.0–50.0 MPa) and deviatoric stresses (0–60.0 MPa). First, a series of seepage tests were conducted to investigate the impacts of fracture dilation and inertial effect on the nonlinear flow behaviors. Then, the competitive effects of fracture dilation and inertial effect on the transition of the nonlinear flow regime were theoretically evaluated. Finally, a novel seepage model incorporating fracture dilation was proposed based on the Goodman hyperbolic closure deformation relationship. This model was further validated through a comparison with both experimental observations and classical theory. The investigation reveals that variations in confining pressures and deviatoric stresses induce a substantial reduction in flow rate by factors of 5 and 2 magnitudes, correspondingly. Notably, the influence of the inertial effect diminishes as the fracture dilation effect becomes more pronounced, particularly evident under conditions of lower confining pressures. The proposed model considering the fracture dilation effect accurately predicts the relationship between flow rate and pressure conditions. Moreover, it demonstrates superior performance compared with the Forchheimer equation. This study can provide guidance for underground engineering practices when encountering high confining pressure, high deviatoric stress, and high water pressure.

Offshore wind turbines (OWTs) are used as renewable energy sources to generate electricity. Offshore, wind, and wave loads act as transverse loads on these turbines. Furthermore, the topography of the ocean is not always flat, varying from steeper to more gentle slopes. The passive resistance offered by the soil is reduced owing to the presence of the slopes. A finned pile is a unique rigid hollow steel tube with fins on both sides of the monopile used as the foundation for OWTs, increasing lateral resistance, ease of operation, and cost savings. A comparative study was undertaken based on experimental and numerical investigations on both monopile and finned piles placed at different infinite slopes (Flat, 1V:5H, 1V:3H, 1V:2H, and 1V:1H), pile positions (c = 0, 2.5, 5, and 7.5 Dp), and ground conditions. Three different densities of the soil (35%, 55%, and 75%) were analyzed to show how the behavior of the pile–soil stiffness affects the lateral load capacity. A numerical investigation was undertaken using PLAXIS 3D (version 1.6) finite-element software. The fin lengths and widths were varied to obtain the optimum fin size. An e/Dp (eccentricity to diameter) ratio of 12 was maintained for the experiment and numerical analysis. The displacement and rotation results of the monopile and finned pile were compared. The result shows an increase in lateral load capacity by 15%–40% when the monopile was replaced with a finned pile considering different ground conditions. In addition, the distance from the crest where the slope effect is minimum was found to be 7.5 Dp.

Abstract

This paper establishes a fully analytical equation for the relative hydraulic conductivity of soils through a probabilistic porous network model. Under this approach, a parameter, accounting for the proportion of pores of a certain maximum size forming continuous paths of saturated elements between the boundaries, is obtained and used to compute the hydraulic conductivity of soils. This method avoids the necessity of the pore-scale model approach required by computational networks to obtain the hydraulic conductivity of soils at different water contents. Additionally, constraints related to computing time, memory size, and the effect of the size of the network on the results are avoided. Furthermore, the network can consider single-, double-, or triple-structured soil. The theoretical and experimental comparisons indicate that the capillary flow regime can account for the hydraulic conductivity of sandy soils for suctions below 0.1 MPa without the need for a different flow regime as the film flow. Finally, all parameters required in the relative conductivity equation can be obtained directly from the pore-size distribution or by fitting the numerical with the experimental soil water retention curves of the material.

Practical Applications

The determination of the hydraulic conductivity function of soils is required for a number of geotechnical problems: the flow of contaminants, the flow of water in slopes and cuts to define the variations in the factor of safety against the infiltration of water, the flow of water in expansive soils for the design of optimal foundations, and the behavior of buffers of bentonite pellets around the radioactive repositories of nuclear waste during wetting–drying cycles. With the present development, it is possible to define the relative permeability function from the retention curves of soils and the use of a probabilistic porous model. When the retention curves are fitted, the proportion of continuous paths of saturated pores of certain maximum size connecting the flow boundaries can be extracted. This procedure results in a fully analytical solution and a more expeditious method for the determination of the hydraulic conductivity of soils subjected to different suctions.

An equation for the filter cake growth rate for shield tail grouting in water-rich fine sand was established with the water–cement ratio and grouting pressure, and the average acceleration of filter cake growth a_r was defined. A filter cake formation test device and shield tail grouting similarity model test system were developed, and the model was verified for different water–cement ratios and grouting pressures. Under certain conditions, the greater the grouting pressure was, the greater the filter cake growth rate and thickness, the shorter the filter cake growth duration, and the smaller the surface settlement. Under other conditions, the greater the water–cement ratio was, the smaller the filter cake growth rate and thickness, the longer the filter cake growth duration, and the smaller the surface settlement. The grouting pressure should be increased and the water–cement ratio of the grout should be reduced to ensure that the filter cake has a certain thickness, forms rapidly, and reduces surface settlement. The recommended water–cement ratio of grout for grouting at 0.4 MPa in water-rich silty fine sand with a porosity of 0.36 is 0.8–0.85. These results are relevant to shield tail grouting in water-rich fine sand layers.

A pseudostatic coefficient is required for pseudostatic analysis of mechanically stabilized earth (MSE) walls, but there are no clear criteria for its selection. The current study assessed the horizontal pseudostatic coefficient (k_h) for MSE walls by considering the effects of a tiered configuration and reinforcement type. A series of shaking table tests were performed on reduced-scale models of integrated and two-tiered MSE walls reinforced by metal strips and geogrids to determine the failure threshold acceleration, axial force distribution along the reinforcements at the failure stage, and geometry of the slip surfaces. Physical models were simulated using the limit equilibrium horizontal slice method to estimate the value of k_h required to establish slip surfaces and reinforcement forces similar to those observed in shaking table tests. Based on the results, the horizontal pseudostatic coefficient was introduced as a function of the maximum horizontal acceleration, offset distance, and reinforcement type.

Large deformations of soft rocks often occur during the excavation and support of the Chuxiong section tunnel in the Yunnan Central Diversion Project. The physical characteristics and creep mechanical properties of the red-bed soft rock were analyzed by combining engineering experience, laboratory tests, and scanning electron microscopy. Furthermore, the creep parameters of the red-bed soft rock were obtained. The issues related to large deformations of deeply buried tunnels with soft and weak rock masses were investigated. Based on construction engineering practice and numerical simulations, characteristics of extrusion damage of deeply buried tunnels with soft and weak rock masses were analyzed. Countermeasures were implemented to solve the problems of large deformations, which can be used as references for the construction of similar soft rock tunnels in the future.

Both cyclic loading on soil and intermittent periods come into play with the passing of trains. However, the existence of both cyclic deviator stress and cyclic confining pressure has already been observed in the stress field. In this paper, two types of cyclic triaxial tests, i.e., continuous cyclic loading and intermittent cyclic loading, were conducted to study the deformation behaviors of soft clay. The effects of cyclic confining pressure, duration of intermittent period (i.e., intermittent time), and loading cycles were analyzed. Compared to continuous cyclic loading, the intermittent period has a substantial influence on the deformation behavior of soft clay: the strain produced by intermittent cyclic loading is larger than that produced under undrained conditions, but less than that generated under partially drained conditions. The increment of accumulated axial strain corresponding to each loading stage under various factors was compared: an increase in intermittent time and loading cycles leads to greater degradation of the increment of accumulated axial strain, while the greater cyclic confining pressure corresponds to lower the increment of accumulated axial strain. The differences in the increment of accumulated axial strain for various loading cycles and cyclic confining pressures are significant in the cyclic loading period of the first loading stage, and can be ignored in subsequent cyclic loading periods. Besides, an empirical formula is provided to calculate the total accumulated axial strain caused by intermittent cyclic loading, and the predicted results accord well with the measured data.

To improve the swelling–shrinkage characteristics of expansive soil, excessive amounts of cement are often used to solidify it. This study aims to solidify expansive soil by using industrial graphene (G) and graphene oxide (GO)-modified cement to avoid the problem of environmental pollution resulting from the excessive use of cement. A series of unconsolidated-undrained triaxial and microstructure tests were conducted to investigate the engineering properties and micromechanisms of G and GO composite cement-solidified soils. Testing results showed that the deviatoric stress–strain curves of G and GO composite cement-solidified soils showed softening types. The compressive strength, toughness, shear strength, and ductility of cement-solidified soil were significantly improved by adding both G and GO. G and GO not only acted as a template to promote cement hydration and generate regular hydration products but also played a role in pore filling. Consequently, it is feasible to apply G and GO composite cement-solidified soils in practical projects.