Showing papers in "Geomechanics and Engineering in 2021"
TL;DR: In this paper, a model of elastic rectangular cantilever thin plates was established to determine ground stress behavior under the special condition of a regenerated roof, and the critical conditions for bending and fracturing the regenerated roofs during mining were analyzed.
Abstract: To determine ground stress behavior under the special condition of a regenerated roof, we established a model of elastic rectangular cantilever thin plates. Moreover, the critical conditions for bending and fracturing the regenerated roof during mining were analysed. Meanwhile, by applying continua FLAC-3D numerical simulation, this research simulated changes in the stress and strain on a regenerated roof during mining and proposed prevention and control methods for dynamic disasters. The results show that: (1) the thinner the regenerated roof, the larger the tensile stress on the roof based on analysis using the theoretical model. Furthermore, the longer the advance distance during mining, the greater the tensile stress on the regenerated roof. (2) By analysing simulation results, during the fracturing of the regenerated roof, roof displacement firstly suddenly increases and then gradually decreases to be stable. Floor-heave-induced displacement presents a divergent state, that is, increases outwards in an elliptical manner. (3) For control of the regenerated roof, monitoring on activities of the roof should be strengthened and stress should be relieved timeously. Moreover, effective support methods should be taken to prevent development of hazards on working faces and roadways caused by the widespread behavior of the roof.
87 citations
TL;DR: In this paper, the authors modeled the tunnel system in a rock mass as a number of scaled openings in rock specimens, and the mechanical behavior of specimens having one to four horseshoe-shaped openings under uniaxial compression were investigated systematically.
Abstract: Hazardous failure phenomena such as rock bursts and slabbing failure frequently occur in deep hardrock tunnels, thus understanding the failure phenomena and mechanisms of the stress regime on tunnels is extremely critical. In this study, the tunnel system in a rock mass was physically modelled as a number of scaled openings in rock specimens, and the mechanical behavior of specimens having one to four horseshoe-shaped openings under uniaxial compression were investigated systematically. During the tests, the digital image correlation (DIC) and acoustic emission (AE) techniques were jointly employed to monitor the fracture response of specimens. After which, the stress distributions in the specimens were numerically analyzed and the stress concentration factor on the periphery of the opening was calculated. The results show that the number of openings have a significant impact on the weakening effect of rock mechanical properties. The progressive cracking process of the specimens with openings evolves from first-tensile cracks through second-tensile cracks and spalling cracks to shear cracks, and the crack threshold stresses are measured. Two failure modes are formed: shear failure and shear-tensile failure. According to the stress distribution law around the opening, the crack initiation mechanism can be fully explained. This research provides an insight to failure mechanism of hardrock tunnel.
41 citations
TL;DR: Wang et al. as discussed by the authors proposed the corresponding prominent control measures of steeply inclined coal working face, mechanical model based on coal face-support-roof system and mechanical model of coal face failure was established to reveal the disaster mechanism of rib spalling and the sensitive analysis of related factors.
Abstract: Rib spalling is a major issue affecting the safety of steeply inclined coal seam. And the failure coal face and support system can be affected with each other to generate a vicious cycle along with inducing large-scale collapse of surrounding rock in steeply inclined coal seam. In order to analyze failure mechanism and propose the corresponding prominent control measures of steeply inclined coal working face, mechanical model based on coal face-support-roof system and mechanical model of coal face failure was established to reveal the disaster mechanism of rib spalling and the sensitive analysis of related factors was performed. Furthermore, taking 3402 working face of Chen-man-zhuang coal mine as engineering background, numerical model by using FLAC3D was built to illustrate the propagation of displacement and stress fields in steeply inclined coal seam and verify the theory analysis as mentioned in this study. The results show that the coal face slide body in steeply inclined working face can be observed as the failure height of upper layer smaller than that of lower layer exhibiting with an irregular quadrilateral pyramid shape. Moreover, the cracks were originated from the upper layer of sliding body and gradually developed to the lower layer causing the final rib spalling. The influence factors on the stability of coal face can be ranked as overlying strata pressure (P) > mechanical parameters of coal body (e.g., cohesion (c), internal fraction angle (𝛟)) > support strength (F) > the support force of protecting piece (F
34 citations
TL;DR: The results show that the optimum artificial network developed in this research predicts the unconfined compressive strength of weak to very strong granites with less than ±20% deviation from the experimental data for 70% of the specimen and significantly outperforms a number of available models available in the literature.
Abstract: This paper reports the results of advanced data analysis involving artificial neural networks for the prediction of the unconfined compressive strength of granite using only two non-destructive test indexes. A data-independent site-independent unbiased database comprising 182 datasets from non-destructive tests reported in the literature was compiled and used to train and develop artificial neural networks for the prediction of the unconfined compressive strength of granite. The results show that the optimum artificial network developed in this research predicts the unconfined compressive strength of weak to very strong granites (20.3-198.15MPa) with less than ±20% deviation from the experimental data for 70% of the specimen and significantly outperforms a number of available models available in the literature. The results also raise interesting questions with regards to the suitability of the Pearson correlation coefficient in assessing the prediction accuracy of models.
27 citations
TL;DR: A fuzzy logic-based multi-criteria decision-making method was proposed for susceptibility analysis and preparing the hazard zonation maps implemented in MATLAB programming language and Geographic Information System (GIS) environment as discussed by the authors.
Abstract: Due to the complexity of the causes of the sliding mass instabilities, landslide susceptibility and hazard evaluation are difficult, but they can be more carefully considered and regionally evaluated by using new programming technologies to minimize the hazard. This study aims to evaluate the landslide hazard zonation in the Tabriz region, Iran. A fuzzy logic-based multi-criteria decision-making method was proposed for susceptibility analysis and preparing the hazard zonation maps implemented in MATLAB programming language and Geographic Information System (GIS) environment. In this study, five main factors have been identified as triggering including climate (i.e., precipitation, temperature), geomorphology (i.e., slope gradient, slope aspect, land cover), tectonic and seismic parameters (i.e., tectonic lineament congestion, distribution of earthquakes, the unsafe radius of main faults, seismicity), geological and hydrological conditions (i.e., drainage patterns, hydraulic gradient, groundwater table depth, weathered geo-materials), and human activities (i.e., distance to roads, distance to the municipal areas) in the study area. The results of analyses are presented as a landslide hazard map which is classified into 5 different sensitive categories (i.e., insignificant to very high potential). Then, landslide susceptibility maps were prepared for the Tabriz region, which is categorized in a high-sensitive area located in the northern parts of the area. Based on these maps, the Bozgoosh-Sahand mountainous belt, Misho-Miro Mountains and western highlands of Jolfa have been delineated as risk-able zones.
26 citations
TL;DR: In this article, the authors explored the potential for the use of machine learning-based data driven approaches to identify the change in geology during tunnel excavation and assessed the feasibility for machine learningbased anomaly detection approaches to detect the development of clayey clogging.
Abstract: There frequently exists inadequacy regarding the number of boreholes installed along tunnel alignment. While geophysical imaging techniques are available for pre-tunnelling geological characterization, they aim to detect specific object (e.g., water body and karst cave). There remains great motivation for the industry to develop a real-time identification technology relating complex geological conditions with the existing tunnelling parameters. This study explores the potential for the use of machine learning-based data driven approaches to identify the change in geology during tunnel excavation. Further, the feasibility for machine learning-based anomaly detection approaches to detect the development of clayey clogging is also assessed. The results of an application of the machine learning-based approaches to Xi'an Metro line 4 are presented in this paper where two tunnels buried in the water-rich sandy soils at depths of 12-14 m are excavated using a 6.288 m diameter EPB shield machine. A reasonable agreement with the measurements verifies their applicability towards widening the application horizon of machine learning-based approaches.
21 citations
TL;DR: In this article, the macro-mechanical and micro-structural behaviors of dredged natural expansive clay from coal mining treated with ordinary Portland cement or hydrated lime addition were investigated.
Abstract: Expansive soil is the most predominant geologic hazard which shows a large amount of shrinkage and swelling with changes in their moisture content. This study investigates the macro-mechanical and micro-structural behaviours of dredged natural expansive clay from coal mining treated with ordinary Portland cement or hydrated lime addition. The stabilised expansive soil aims for possible reuse as pavement materials. Mechanical testing determined geotechnical engineering properties, including free swelling potential, California bearing ratio, unconfined compressive strength, resilient modulus, and shear wave velocity. The microstructures of treated soils are observed by scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy to understand the behaviour of the expansive clay blended with cement and lime. Test results confirmed that cement and lime are effective agents for improving the swelling behaviour and other engineering properties of natural expansive clay. In general, chemical treatments reduce the swelling and increase the strength and modulus of expansive clay, subjected to chemical content and curing time. Scanning electron microscopy analysis can observe the increase in formation of particle clusters with curing period, and x-ray diffraction patterns display hydration and pozzolanic products from chemical particles. The correlations of mechanical properties and microstructures for chemical stabilised expansive clay are recommended.
18 citations
TL;DR: In this article, the results of small-scale laboratory modeling of pile behavior under lateral loading, considering the parameters such as short or long, single or group, spacing and rigidity or flexibility of piles, were summarized.
Abstract: This paper summarizes the results of small-scale laboratory modelling of pile behavior under lateral loading, considering the parameters such as short or long, single or group, spacing and rigidity or flexibility of piles. The head of piles was fixedly connected to the cap. In addition, the PIV method has been used to examine the effect of the mentioned parameters on the failure mechanism and pile-soil interaction more accurately. The results show that the short piles have a rigid movement, the displacement of the surrounding soil has occurred along the total length of the pile and the piles rotate around a point but the long piles have a flexible movement at the part of the pile length. It seems that the group effect be more obvious for long piles than short piles. Also, the effective depth of total soil displacement vectors around the trail pile is more than the lead one in long pile group, while this depth for trail pile is less than the lead pile in short pile group. Due to the sharper angles of total displacement vectors around the trail pile, the intensity of soil shear strains around the trail pile is greater than the lead pile.
17 citations
TL;DR: In this article, a steel-concrete composite support system (SCCS) was proposed for tunnel construction in soil. And the bearing characteristics and construction performance of SCCS were systematically studied using a three-dimensional numerical model.
Abstract: The sufficient early strength of primary support is crucial for stabilizing the surroundings, especially for the tunnels constructed in soil. This paper introduces the Steel-Concrete Composite Support System (SCCS), a new support with high bearing capacity and flexible, rapid construction. The bearing characteristics and construction performance of SCCS were systematically studied using a three-dimensional numerical model. A sensitivity analysis was also performed. It was found that the stress of a π-shaped steel arch decreased with an increase in the thickness of the wall, and increased linearly with an increase in the rate of stress release. In the horizontal direction of the arch section, the nodal stresses of the crown and the shoulder gradually increased in longitudinally, and in the vertical direction, the nodal stresses gradually decreased from top to bottom. The stress distribution at the waist, however, was opposite to that at the crown and the shoulder. By analyzing the stress of the arch section under different installation gaps, the sectional stress evolution was found to have a step-growth trend at the crown and shoulder. The stress evolution at the waist is more likely to have a two-stage growth trend: a slow growth stage and a fast growth stage. The maximum tensile and compressive stresses of the secondary lining supported by SCCS were reduced on average by 38.0% and 49.0%, respectively, compared with the traditional support. The findings can provide a reference for the supporting technology in tunnels driven in loess.
16 citations
TL;DR: In this article, the effects of impact loading on the rock tunnels, constructed in different region corresponding to varying unconfined compressive strength (UCS), through finite element method has been carried out.
Abstract: The present paper has been carried out to understand the effects of impact loading on the rock tunnels, constructed in different region corresponding to varying unconfined compressive strength (UCS), through finite element method The UCS of rockmass has substantial role in the stability of rock tunnels under impact loading condition due to falling rocks or other objects In the present study, Dolomite, Shale, Sandstone, Granite, Basalt, and Quartzite rocks have been taken into consideration for understanding of the effect of UCS that vary from 285 MPa to 20703 MPa The Mohr-Coulomb constitutive model has been considered in the present study for the nonlinear elastoplastic analysis for all the rocks surrounding the tunnel opening The geometry and boundary conditions of the model remains constant throughout the analysis and missile has 100 kg of weight The general hard contact has been assigned to incorporate the interaction between different parts of the model The present study focuses on studying the deformations in the rock tunnel caused by impacting load due to missile for tunnels having different concrete grade, and steel grade The broader range of rock strength depicts the strong relationship between the UCS of rock and the extent of damage produced under different impact loading conditions The energy released during an impact loading simulation shows the variation of safety and serviceability of the rock tunnel
15 citations
TL;DR: In this article, the authors investigated the seismic response of a shallow foundation resting on dry silica sand using the linear elastic (LE) model, elastic-perfectly-plastic (EPP) model and hardening soil with small strain stiffness (HS small) model.
Abstract: The time-history finite element analysis is usually used to evaluate the seismic response of shallow foundations. However, the literature lacks studies on the influence of the soil constitutive model complexity on the seismic response of shallow foundations. This study, thus, aims to fill this gap by investigating the seismic response of shallow foundation resting on dry silica sand using the linear elastic (LE) model, elastic-perfectly-plastic (EPP) model, and hardening soil with small strain stiffness (HS small) model. These models have been used because it is intended to compare the results of a soil constitutive model that accurately captures the seismic response of the soil-structure interaction problems (which is the HS small model) with simpler models (the LE and EPP models) that are routinely used by practitioners in geotechnical designs. The results showed that the LE model produces a very small seismic settlement value which is approximately equal to zero. The EPP model predicts a seismic settlement higher than that produced using the HS small model for earthquakes with a peak ground acceleration (PGA) lower than 0.25 g for a relative density of 45% and 0.40 g for a relative density of 70%. However, the HS small model predicts a seismic settlement higher than the EPP model beyond the aforementioned PGA values with the difference between both models increases as the PGA rises. The results also showed that the LE and EPP models predict similar trend and magnitude of the acceleration-time relationship directly below the foundation, which was different than that predicted using the HS small model. The results reported in this paper provide a useful benchmark for future numerical studies on the response of shallow foundations subjected to seismic shake.
TL;DR: Different feature selection methods have different effects on the performance of LSA machine learning models, and the best FS-ML model is the recursive feature elimination (RFE) optimized RF, and RFE is an optimal method for feature selection.
Abstract: Machine learning models have been widely used for landslide susceptibility assessment (LSA) in recent years. The large number of inputs or conditioning factors for these models, however, can reduce the computation efficiency and increase the difficulty in collecting data. Feature selection is a good tool to address this problem by selecting the most important features among all factors to reduce the size of the input variables. However, two important questions need to be solved: (1) how do feature selection methods affect the performance of machine learning models? and (2) which feature selection method is the most suitable for a given machine learning model? This paper aims to address these two questions by comparing the predictive performance of 13 feature selection-based machine learning (FS-ML) models and 5 ordinary machine learning models on LSA. First, five commonly used machine learning models (i.e., logistic regression, support vector machine, artificial neural network, Gaussian process and random forest) and six typical feature selection methods in the literature are adopted to constitute the proposed models. Then, fifteen conditioning factors are chosen as input variables and 1,017 landslides are used as recorded data. Next, feature selection methods are used to obtain the importance of the conditioning factors to create feature subsets, based on which 13 FS-ML models are constructed. For each of the machine learning models, a best optimized FS-ML model is selected according to the area under curve value. Finally, five optimal FS-ML models are obtained and applied to the LSA of the studied area. The predictive abilities of the FS-ML models on LSA are verified and compared through the receive operating characteristic curve and statistical indicators such as sensitivity, specificity and accuracy. The results showed that different feature selection methods have different effects on the performance of LSA machine learning models. FS-ML models generally outperform the ordinary machine learning models. The best FS-ML model is the recursive feature elimination (RFE) optimized RF, and RFE is an optimal method for feature selection.
TL;DR: In this paper, the effects of far-fault ground motions on the nonlinear seismic settlement and shear strain behavior of Oroville EF dam are determined and evaluated in detail using FLAC3D software based on finite difference approach.
Abstract: Structural design of the vertical displacements and shear strains in the earth fill (EF) dams has great importance in the structural engineering problems. Moreover, far fault earthquakes have significant seismic effects on seismic damage performance of EF dams like the near fault earthquakes. For this reason, three dimensional (3D) earthquake damage performance of Oroville dam is assessed considering different far-fault ground motions in this study. Oroville Dam was built in United States of America-California and its height is 234.7 m (770 ft.). 3D model of Oroville dam is modelled using FLAC3D software based on finite difference approach. In order to represent interaction condition between discrete surfaces, special interface elements are used between dam body and foundation. Non-reflecting seismic boundary conditions (free field and quiet) are defined to the main surfaces of the dam for the nonlinear seismic analyses. 6 different far-fault ground motions are taken into account for the full reservoir condition of Oroville dam. According to nonlinear seismic analysis results, the effects of far-fault ground motions on the nonlinear seismic settlement and shear strain behaviour of Oroville EF dam are determined and evaluated in detail. It is clearly seen that far-fault earthquakes have very significant seismic effects on the settlement-shear strain behaviour of EF dams and these earthquakes create vital important seismic damages on the swelling behaviour of dam body surface. Moreover, it is proposed that far-fault ground motions should not be ignored while modelling EF dams.
TL;DR: In this article, the Mohr-Coulomb material model has been considered to simulate elastoplastic nonlinear behavior of different rock types, i.e., Basalt, Granite and Quartzite.
Abstract: Tunnel provide faster, safer and convenient way of transportation for different objects. The region where it is construction and surrounding medium has significant influence on the overall stability and performance of tunnel. The present simulation has been carried out in order to understand the behaviour of rock tunnel under static loading condition. The present numerical model has been validated with the laboratory scaled model and field data of underground tunnels. Both lined and unlined tunnels have been considered in this paper. Finite element technique has been considered for the simulation of static loading effect on tunnel through Abaqus/Standard. The Mohr-Coulomb material model has been considered to simulate elastoplastic nonlinear behaviour of different rock types, i.e., Basalt, Granite and Quartzite. The four different stages of rock weathering are classified as fresh, slightly, moderately, and highly weathered in case of each rock type. Moreover, extremely weathered stage has been considered in case of Quartzite rock. It has been concluded that weathering of rock and overburden depth has great influence on the tunnel stability. However, by considering a particular weathering stage of rock for each rock type shows varying patterns of deformations in tunnel.
TL;DR: In this paper, the authors used the Brillouin Optical Frequency Domain Analysis (BOFDA) for in-situ monitoring of an offshore PHC pipe pile under axial load.
Abstract: Brillouin Optical Frequency Domain Analysis (BOFDA) is a distributed fiber optic sensing (DFOS) technique that has unique advantages for performance monitoring of piles. However, the complicated production process and harsh operating environment of offshore PHC pipe piles make it difficult to apply this method to pile load testing. In this study, sensing cables were successfully pre-installed into an offshore PHC pipe pile directly for the first time and the BOFDA technique was used for in-situ monitoring of the pile under axial load. High-resolution strain and internal force distributions along the pile were obtained by the BOFDA sensing system. A finite element analysis incorporating the Degradation and Hardening Hyperbolic Model (DHHM) was carried out to evaluate and predict the performance of the pile, which provides an improved insight into the offshore pile-soil interaction mechanism.
TL;DR: In this paper, the authors investigated the local stress variation in the process of borehole drainage and provided a reference for the stress concentration caused by borehole flooding, which can be potentially utilized in the optimal arrangement of drainage boreholes in underground mining.
Abstract: Pre-drainage of groundwater in the roof aquifer by boreholes is the main method for prevention of roof water disaster, and the drop in the water level during the drainage leads to the variation of the local stress in the overlying strata. Based on a multitude of boreholes for groundwater drainage from aquifer above the 1303 mining face of Longyun Coal Mine, theoretical analysis and numerical simulation are used to investigate the local stress variation in the process of borehole drainage. The results show that due to the drop in the water level of the roof aquifer during the drainage, the stress around the borehole gradually evolved. From the center of the borehole to the outside, a stress-relaxed zone, a stress-elevated zone, and a stress-recovered zone are sequentially formed. Along with the expansion of drainage influence, the stress peak in the stress-elevated zone also moves to the outside. When the radius of influence develops to the maximum, the stress peak position no longer moves outward. When the coal mining face advances to the drainage influence range, the abutment pressure in front of the mining face is superimposed with the high local stress around the borehole, which increases the risk of stress concentration. The present study provides a reference for the stress concentration caused by borehole drainage, which can be potentially utilized in the optimal arrangement of drainage boreholes in underground mining.
TL;DR: In this article, uniaxial compression tests were conducted on sandstone-CGFB composite samples with different interface angles, and their strength, acoustic emission (AE), and failure characteristics were investigated.
Abstract: Uniaxial compression tests were conducted on sandstone-CGFB composite samples with different interface angles, and their strength, acoustic emission (AE), and failure characteristics were investigated. Three macro-failure patterns were identified: the splitting failure accompanied by local spalling failure in CGFB (Type-I), the mixed failure with small sliding failure along with the interface and Type-I failure (Type-II), and the sliding failure along with the interface (Type-III). With an increase of interface angle β measured horizontally, the macro-failure pattern changed from Type-I to Type-II, and then to Type-III, and the uniaxial compressive strength and elastic modulus generally decreased. Due to the small sliding failure along with the interface in the composite sample with β of 45° , AE events underwent fluctuations in peak values at the later post-peak failure stage. The composite samples with β of 60° occurred Type-III failure before the completion of initial compaction stage, and the post-peak stress-time curve initially exhibited a slow decrease, followed by a steep linear drop with peaks in AE events.
TL;DR: In this article, the impact force change characteristics of gravel side support structures during gangue heaping can provide useful information about roadway stability in a new non-pillar-mining approach, where the coefficient of restitution and Janssen model are introduced into the theoretical analysis.
Abstract: The force change characteristics of gravel side support structures during gangue heaping can provide useful information about roadway stability in a new non-pillar-mining approach—noncoal pillar mining with automatically formed gob-side entry (NMAFG). Considering the dynamic shock and static stacking phenomena during gangue heaping, the coefficient of restitution and Janssen model are introduced into the theoretical analysis. Analytical results show that the impact force decreased with increasing gangue heaping height under dynamic shock, while under static stacking, the gangue extrusion force first increased sharply, then increased slowly and stabilized, and the final force was unrelated to the gangue heaping height. Field monitoring was conducted to verify the rationality of the pattern obtained from theoretical analysis. The gangue support structure lateral stress from field monitoring can be divided into two periods. In Period I, the peak value at the lower monitoring point was greater than that at any other point. The lowest sensor was subjected to the greatest impact, at 59.09 kN. In Period II, the stress value first rapidly increased, then slowly increased and stabilized. The final force was unrelated to the gangue height. The sensors at #2 (highest position), #4 (middle position), and #6 (lowest position) measured 31.91 kN, 44.82 kN and 38.19 kN, respectively. The analysis confirmed the variation characteristics of the impact force and extrusion force.
TL;DR: In this article, the effect of biopolymer treatment on clayey sand at different dosages and curing periods was investigated and triaxial tests were conducted on treated soil samples.
Abstract: Soil stabilization is widely used to favourably amend the soil behaviour. The use of biopolymers to treat soil is not only an eco-friendly but is also a sustainable approach. Biopolymers, xanthan gum and guar gum are used to augment the strength of clayey sand. Xanthan gum is anionic while guar gum is non-ionic. Triaxial tests were conducted on treated soil samples to understand the effect of biopolymer treatment on clayey sand at different dosages and curing periods. Shear strength parameters –angle of internal friction and cohesion increases appreciably on treating soil with xanthan and guar gum for all dosages investigated, though angle of internal friction decreases with the curing period in case of xanthan gum treated soil. Xanthan gum performs better in enhancing the strength and deformation behaviour of the soil compared to guar gum. There is a substantial gain in early strength but as the curing period increases further, the rate of increase in strength is marginal. The deformation modulus at failure also increases with the biopolymer content. The reduction in post-peak strength of treated soil is sudden and drastic indicating brittle behavior. The energy absorption capacity of the biopolymer treated soil increases with increase in biopolymer content and curing period. The strength gain in soil can be ascribed to the formation of hydrogels that are cementitious in nature. Strength is also improved through the ionic / hydrogen bonds that are formed by biopolymer addition.
TL;DR: In this paper, the calculation formulas for mechanical parameters of bolt-reinforced rocks are provided using the homogenization method and the supporting characteristic curve is divided into three stages with proposing the corresponding stiffness equations.
Abstract: Attempt has been proved to be effective that surrounding rock reinforcement is emphasized simultaneously considering displacement release in weak rock tunnels. In this study, the calculation formulas for mechanical parameters of bolt-reinforced rocks are provided using the homogenization method and the supporting characteristic curve is divided into three stages with proposing the corresponding stiffness equations. The mechanical model for bolted-reinforced rock-yielding supports interaction is then established and coupled solutions for displacements and stresses around tunnels considering bolt reinforcement and yielding effects are provided. Furthermore, parametric investigations on the influences of rockbolts and yielding supports are carried out. Results show that (1) rock displacement gradually decreases as the rockbolt length increases. However, when rockbolt length becomes large enough, the further improvement of rock displacement will not be obvious by still increasing their length. (2) Both rock displacement and plastic zone of tunnel decrease with the increase of rockbolt radius. There exists the highest utilization of rockbolts corresponding to a certain rockbolt radius. (3) Also, the rock displacement and plastic zone of tunnel decrease as installation density of rockbolts including circumferential space and longitudinal space increases. Under the condition prescribed, this decreasing trend becomes sharper and the improvement is more evident. (4) Larger yielding displacement or stiffness parameter leads to smaller support pressure, but greater plastic radius of tunnel. The optimal yielding displacement and stiffness parameters need to be determined through a comprehensive investigation combining rock properties, support characteristics and tunnel design requirements.
TL;DR: In this article, the porosity and void included in constituent is described by three different distribution models through the beam thickness and the governing equations are obtained by using Lagrange's equations and solved by finite element method.
Abstract: This paper concerns with forced dynamic response of thick functionally graded (FG) beam resting on viscoelastic foundation including porosity impacts. The dynamic point load is proposed to be triangle point loads in time domain. In current analysis the beam is assumed to be thick, therefore, the two-dimensional plane stress constitutive equation is proposed to govern the stress-strain relationship through the thickness. The porosity and void included in constituent is described by three different distribution models through the beam thickness. The governing equations are obtained by using Lagrange's equations and solved by finite element method. In frame of finite element analysis, twelve-node 2D plane element is exploited to discretize the space domain of beam. In the solution of the dynamic problem, Newmark average acceleration method is used. In the numerical results, effects of porosity coefficient, porosity distribution and foundation parameters on the dynamic responses of functionally graded viscoelastic beam are presented and discussed. The current model is efficient in many applications used porous FGM, such as aerospace, nuclear, power plane sheller, and marine structures.
TL;DR: In this article, a generalized analytical expression has been presented for calculating the vertical load on a conduit buried under a sloping ground. But, the analytical results for specific soil parameters have been compared with the results extracted from a commercial software PLAXIS 2D, for a developed numerical model and an independent study.
Abstract: Conduits are commonly installed below the ground for utility conveyance around the world. Vertical load on a buried conduit is an important parameter that needs to be known to ensure its safe design and installation. Consideration of soil arching in load calculations helps achieve a more realistic and efficient design. In the past, considering the arching effect, the design charts have been presented for use by practicing engineers to calculate the vertical load on the conduit buried below the level ground. There are currently no design charts for calculating the vertical load on the conduit buried under a sloping ground. In this paper, an attempt has been made to present the derivation of a generalized analytical expression considering that the soil mass overlying the conduit has a sloping face and the arching phenomenon takes place. The developed generalized expression has been used to present some design charts considering specific values of slope geometry, soil properties and burial depths. Furthermore, analytical results for specific soil parameters have been compared with the results extracted from a commercial software PLAXIS 2D, for a developed numerical model and an independent study.
TL;DR: In this article, the strength and failure mechanism of red sandstones with combined defects were investigated by uniaxial compression tests on red sandstone with different crack angles using two-dimensional particle flow code numerical software, and their mechanical parameters and failure process were studied and analyzed.
Abstract: In this study, the strength and failure mechanism of red sandstones with combined defects were investigated by uniaxial compression tests on red sandstones with different crack angles using two-dimensional particle flow code numerical software, and their mechanical parameters and failure process were studied and analyzed The results showed that the mechanical characteristics such as peak strength, peak strain, and elastic modulus of the samples with prefabricated combined defects were significantly inferior than those of the intact samples With increasing crack angle from 15° to 60°, the weakening area of cracks increased, elastic modulus, peak strength, and peak strain gradually reduced, the total number of cracks increased, and more strain energy was released In addition, the samples underwent initial brittle failure to plastic failure stage, and the failure form was more significant, leading to peeling phenomenon However, with increasing crack angle from 75° to 90°, the crack–hole combination shared the stress concentration at the tip of the crack–crack combination, resulted in a gradual increase in elastic modulus, peak strain and peak strength, but a decrease in the number of total cracks, the release of strain energy reduced, the plastic failure state weakened, and the spalling phenomenon slowed down On this basis, the samples with 30° and 45°crack-crack combination were selected for further experimental investigation Through comparative analysis between the experimental and simulation results, the failure strength and final failure mode with cracks propagation of samples were found to be relatively similar
TL;DR: This paper develops a convenient approach for deterministic and probabilistic evaluations of tunnel face stability using support vector machine classifiers using SVM-KNN classifiers.
Abstract: This paper develops a convenient approach for deterministic and probabilistic evaluations of tunnel face stability using support vector machine classifiers. The proposed method is comprised of two major steps, i.e., construction of the training dataset and determination of instance-based classifiers. In step one, the orthogonal design is utilized to produce representative samples after the ranges and levels of the factors that influence tunnel face stability are specified. The training dataset is then labeled by two-dimensional strength reduction analyses embedded within OptumG2. For any unknown instance, the second step applies the training dataset for classification, which is achieved by an ad hoc Python program. The classification of unknown samples starts with selection of instance-based training samples using the k-nearest neighbors algorithm, followed by the construction of an instance-based SVM-KNN classifier. It eventually provides labels of the unknown instances, avoiding calculate its corresponding performance function. Probabilistic evaluations are performed by Monte Carlo simulation based on the SVM-KNN classifier. The ratio of the number of unstable samples to the total number of simulated samples is computed and is taken as the failure probability, which is validated and compared with the response surface method.
TL;DR: The influence of ground motions on the seismic response of utility tunnels was investigated in this article, where a series of small-scale shaking table model tests were carried out under uniform excitation in the transverse direction.
Abstract: The influence of ground motions on the seismic response of utility tunnels was investigated. A series of small-scale shaking table model tests were carried out under uniform excitation in the transverse direction. Different peak accelerations of EL-Centro and Taft earthquake waves were applied. The acceleration responses, earth pressure, seismic strain, bending moment and structure deformations were measured and discussed. The results showed that the types of earthquake waves had significant influences on the soil-structure acceleration responses. However, the amplitude of the soil acceleration along the depth showed consistent variation regardless of the types of earthquake waves and tunnels. The horizontal soil pressure near the top and bottom slabs showed obviously larger values than those at other depths. In general, the strain response in the outer surface was more significant than that on the inner surface, and the peak strain in the end section of the model was larger than that in the middle section. Moreover, the bending moment at the corner points was much larger than that at middle point, and the bending moment was greatly affected by both input accelerations and seismic wave types. The opposite direction of shear deformation on the top and bottom slabs presented a rotation trend of the model structure.
TL;DR: In this article, an electrical resistivity monitoring system was designed to collect multi-channel data in real time to detect grout penetration and evaluate the grouting performance for such a waterproof efficiency in single rock fracture.
Abstract: In this study, a new approach using electrical resistivity measurement was proposed to detect grout penetration and to evaluate the grouting performance for such as waterproof efficiency in single rock fracture. For this purpose, an electrical resistivity monitoring system was designed to collect multi-channel data in real time. This was applied to a system for grout injection/penetration using a transparent fracture replica with various aperture sizes and water-cement mix ratio. The electrical resistivity was measured under various grout penetration conditions in real time, which results were directly compared to the visual observation images of grout penetration/distribution. Moreover, the grouting success status after the curing process was evaluated by measuring the electrical resistivity in relation to changes in frequency in fracture cells where grout injection and penetration were completed. Consequently, it was determined that the electrical resistivity monitoring system could be applied effectively to the detection of successful penetration of grouting into a target area and to actual field evaluation of the grouting performance and long-term stability of underground rock structures.
TL;DR: In this article, the authors studied the relationship between crack development and cavity group initiation and found that an increase in the cavity group inclination angle can facilitate the initial damage degree of the rock and weaken the crack initiation stress.
Abstract: To study the evolution mechanism of cracks in rocks with multiple defects, rock-like samples with multiple defects, such as strip-shaped through-going cracks and cavity groups, are used, and the crack propagation law and changes in AE (acoustic emission) and strain of cavity groups under different inclination angles are studied. According to the test results, an increase in the cavity group inclination angle can facilitate the initial damage degree of the rock and weaken the crack initiation stress; the initial crack initiation direction is approximately 90°, and the extension angle is approximately 75~90° from the strip-shaped through-going cracks; thus, the relationship between crack development and cavity group initiation strengthens. The specific performance is as follows: when the initiation angle is 30°, the cracks between the cavities in the cavity group develop relatively independently along the parallel direction of the external load; when the angle is 75°, the cracks between the cavities in the cavity group can interpenetrate, and slip can occur along the inclination of the cavity group under the action of the shear mechanism rupture. With the increase in the inclination angle of the cavity group, the AE energy fluctuation frequency at the peak stress increases, and the stress drop is obvious. The larger the cavity group inclination angle is, the more obvious the energy accumulation and the more severe the rock damage; when the cavity group angle is 30° or 75°, the peak strain of the local area below the strip-shaped through-going fracture plane is approximately three times that when the cavity group angle is 45° and 60°, indicating that cracks are easily generated in the local area monitored by the strain gauge at this angle, and the further development of the cracks weakens the strength of the rock, thereby increasing the probability of major engineering quality damage. The research results will have important reference value for hazard prevention in underground engineering projects through rock with natural and artificial defects, including tunnels and air-raid shelters.
TL;DR: In this paper, the authors used piezoelectric ring actuators to measure the shear wave velocity of unreinforced, fiber, cemented, and fiber reinforced cemented Toyoura sand.
Abstract: Several additives are used to enhance the geotechnical properties (e.g., shear wave velocity, shear modulus) of soils to provide sustainable, economical and eco-friendly solutions in geotechnical and geo-environmental engineering. In this study, piezoelectric ring actuators are used to measure the shear wave velocity of unreinforced, fiber, cemented, and fiber reinforced cemented Toyoura sand. One dimensional oedometer tests are performed on medium dense specimens of Toyoura sand-cement-fiber-silica flour mixtures with different percentages of silica flour (0-42%), fiber and cement (e.g., 0-3%) additives. The experimental results indicate that behavior of the mixtures is significantly affected by the concentration of silica flour, fiber and cement additives. Results show that with the addition of 1-3% of PVA fibers, the shear wave velocity increases by only 1-3%. However, the addition of 1-4% of cement increases the shear wave velocity by 8-35%. 10.5-21% increase of silica flour reduces the shear wave velocity by 2-5% but adding 28-42% silica flour significantly reduces the shear wave velocity by 12-31%. In addition, the combined effect of cement and fibers was also found and with only 2% cement and 1% fiber, the shear wave velocity increase was found to be approximately 24% and with only 3% cement and 3% fibers this increased to 35%. The results from this study for the normalized shear modulus and normalized mean effective stress agree well with previous findings on pure Toyoura sand, Toyoura silty sand, fiber reinforced, fiber reinforced cemented Toyoura sand. Any variations are likely due to the difference in stress history (i.e., isotropic versus anisotropic consolidation) and the measurement method. In addition, these small discrepancies could be attributed to several other factors. The potential factors include the difference in specimen sizes, test devices, methods of analysis for the measurement of arrival time, the use of an appropriate Ko to convert the vertical stresses into mean effective stress, and sample preparation techniques. Lastly, it was investigated that there is a robust inverse relationship between α factor and β_0 exponent. It was found that less compressible soils exhibit higher α factors and lower β_0 exponents.
TL;DR: In this article, a new hydraulic conductivity-void ratio equation was derived by introducing the concept of effective void ratio and breakage index, which integrated the hydraulic conductivities equation with different particle sizes over a wide range of consolidation pressures.
Abstract: Particle size of tailings in different areas of dams varies due to sedimentation and separation. Saturated hydraulic conductivity of high-stacked talings materials are seriously affected by void ratio and particle breakage. Conjoined consolidation permeability tests were carried out using a self-developed high-stress permeability and consolidation apparatus. The hydraulic conductivity decreases nonlinearly with the increase of consolidation pressure. The seepage pattern of coarse-particle tailings is channel flow, and the seepage pattern of fine-particle tailings is scattered flow. The change rate of hydraulic conductivity of tailings with different particle sizes under high consolidation pressure tends to be identical. A hydraulic conductivity hysteresis is found in coarse-particle tailings. The hydraulic conductivity hysteresis is more obvious when the water head is lower. A new hydraulic conductivity-void ratio equation was derived by introducing the concept of effective void ratio and breakage index. The equation integrated the hydraulic conductivity equation with different particle sizes over a wide range of consolidation pressures.