Lateral Unloading Research Articles

Numerical Study on Characteristics of Stick-Slip Instability of Coal-Rock Parting-Coal Structure under Lateral Unloading

Abstract Unloading excavation can increase the possibility of rock burst, especially for coal seam with rock parting. In order to explore the evolution process of rock burst under lateral unloading, the combination of in situ measures and numerical experiments is used to study. The following four points were addressed: (1) the coal seam with rock parting easily causes the stick-slip and instability along the interface, and the process of stick-slip and instability has hysteresis characteristics; (2) the greater the degree of unloading or the smaller the interface friction angle of the Coal-Rock Parting-Coal Structure (CRCS), the more likely it is for stick-slip and instability to occur; (3) the abnormal increase of shear stress and slip dissipation energy can be used as the precursory information of the stick-slip and instability of CRCS; (4) the damage intensity of rock burst induced by stick-slip and instability of CRCS can be reduced by reducing the unloading speed or increasing the roughness of interface. The research results can be used for early warning and controlling of dynamic disaster induced by stick-slip instability in coal seam with rock parking.

Segment Thickness Design and Bearing Performance Analysis of Large Inner-Diameter Shield Tunnel under Lateral Unloading

In order to accommodate more transportation-supporting facilities, the expansion of structures’ inner diameter has become the development trend of metro shield tunnels. But for large inner-diameter shield tunnels, the segment thickness design and bearing performance characteristics of tunnels under lateral unloading are still unclear. The purpose of the research was to select the optimal segment thickness and clarify the bearing performance of large inner-diameter shield tunnels. Therefore, in this study, a 3D refined numerical model was established to analyze and determine the optimal segment thickness for a shield tunnel with an inner diameter of 5.9 m. Furthermore, a full-scale test was carried out to study the bearing performance of the shield tunnel under lateral unloading. The results showed that the maximum tunnel horizontal deformation difference between the calculation and the test did not exceed 5%, and the maximum difference in the overall structure deformation between the calculation and the test did not exceed 7%. Increasing the segment thickness can reduce the convergence deformation of the shield tunnel nonlinearly; the deformation reduction was no longer significant when the segment thickness increased to 400 mm with an inner diameter of 5.9 m. Under the lateral unloading condition, the internal force of the tunnel structure increased significantly at sections of 0°, 55°, 125°, and 190°. Compared with the normal design load stage, the maximum bending moment and axial force increased by 36% and 74.1%, respectively, in the final failure stage. There was no bolt yield during the entire unloading process, indicating that the excessive strength of the bolt could not fully play a role in the entire life cycle of the large inner-diameter tunnel structure. The failure mechanism of the shield tunnel can be described as follows: in the early stage of a load, a shield tunnel will appear with joints open and dislocated. As the load increases, cracks in different directions gradually appear near the tunnel joint. In the ultimate load stage, the shield tunnel loses load-bearing capacity, and large areas of falling blocks appear at the top and bottom of the tunnel.

Study on the Acoustic Emission Characteristics and Failure Precursors of Water-Rich Frozen Sandstone under Different Lateral Unloading Rates

The artificial freezing method is used to cross the water-rich soft rock strata in order to exploit deep coal resources. At present, studies that consider both freezing effect and unloading rate are insufficient. To study the influences of the excavation rate using the artificial freezing method on the unloading deformation and failure of the water-rich surrounding rock, we carry out mechanical and synchronous acoustic emission (AE) tests on frozen (−10 °C) sandstone samples under different lateral unloading rates. Combined with the AE signals, the stress, strain and failure process are analysed to determine the mechanical behaviours of frozen rock samples under different lateral unloading rates. The damage difference between normal temperature rock and frozen rock during lateral unloading is studied. According to acoustic emission signals, the damage relationships among acoustic emission amplitude, energy, cumulative acoustic emission energy (CAEE), stress and strain were compared and analyzed. In this paper, acoustic emission 3D positioning system is used to monitor the fracture propagation trajectory in the process of unloading confining pressure of frozen sandstone. The results show that the peak stress of frozen sandstone during lateral unloading is about 2.5 times of that at 20 °C. More than 2 AE amplitudes per second are regarded as the precursor of failure (FP), and point FP is taken as the first level warning. The CAEE of rock samples at 20 °C and frozen rock samples shows the same change law over time, increasing slowly before the FP point and exponentially after the FP point. Peak stress increases and axial strain decreases with the increase of unloading rate of frozen rock sample. The CAEE at point FP and the peak acoustic emission energy (AEE) and the CAEE at the time of failure increase when the unloading rate of frozen rock sample increases. Principal component analysis method was used to extract key characteristic energy to obtain a clearer AEE concentration area, which was defined as second-level early warning. The research results can provide guidance for freezing shaft construction to reduce the occurrence of disasters.

Deformation and Strength Characteristics of Structured Clay under Different Stress Paths

In geotechnical applications such as deep foundation excavation, slope cutting, and tunnel construction, the stress state of the soil subjected to loading or unloading will change and undergo different stress paths with corresponding changes in its mechanical properties. Therefore, a series of comprehensive laboratory tests were conducted on the natural strong structured clay to evaluate the effects of stress paths on the stress-strain, pore water pressure-strain, deformation characteristics, shear strength behaviors, and the microscopic mechanism of the natural strong structured clay. The results show that the mechanical properties of structured clay under different stress paths are quite different and are closely related to its structure. The peak deviatoric stresses of the soil under both the compression path and the tensile path are affected by the stress path, and the order of its strength is INP > KCP > DEP. The variations of the pore water pressure reflect the shear dilation and the shear contraction characteristics of the soil under different stress paths; the pore pressure of the specimen is particularly complicated in the case of both axial stress and lateral stress unloading; therefore, the pore water pressure in the soil under unloading conditions should be monitored in real time in engineering practice. The stress path affects the total stress strength indices of the structured clay to a significant extent and the effect on soil cohesion is greater than that on the internal friction angle, and the microstructure of the specimens has been changed after shearing tests with different stress paths. Therefore, design calculations should be conducted according to the experimental mechanical parameters under the stress path experienced by the soil under the actual engineering conditions to ensure safety and stability during construction.

Fracture Characteristics of Sliding Crack in Brittle Rock: Analysis Based on an Improved Equivalent Crack Model

Under low confinement axial compression, the failure of brittle rock is mainly caused by tensile fracture. Many scholars adopt the sliding crack as an idealized model to present the intrinsic mechanism of the tensile failure but due to the complex configuration of sliding crack, its stress intensity factor (SIF) calculation has always been a difficult problem. In this study, an improved model of sliding crack is proposed; in the context of linear elastic fracture mechanics (LEFM) and weight function method, the expression of stress intensity factor (SIF) has been derived. The propagation manners of sliding crack under axial loading and lateral unloading conditions have been further analyzed. The extended finite element method (XFEM) is employed to verify the correctness of the theoretical SIF formulation and its inferences. The formula of SIF shows that a sliding crack is highly sensitive to the change of the lateral stress, which theoretically explains compressive failure characteristics of brittle rock as follows: 1) under the condition of axial compression, increasing the lateral stress has a very strong no-linear impact on the strength of brittle rock; 2) under the condition of lateral unloading, the destruction of rock is more abrupt and ferocious than that of the loading case. In order to confirm that micro-fractures in rocks are notably influenced by confining pressure, as the former theoretical fracture analysis predicted, the tri-axial compression test combined with the acoustic emission monitoring technique has been conducted on basalt samples. According to the acoustic waveform parameter method, it shows that increasing the confining pressure will greatly reduce the proportion of tension-type fractures, which indirectly proves the correctness of the sliding crack hypothesis and fracture analysis.

Prediction of Unloading Failure Strain and Undrained Shear Strength of Saturated Clays by Limit Pressure from Prebored Pressuremeter

Abstract Saturated clays mostly fail in lateral unloading by the reduction of undrained shear strength (Su) because of an increase in lateral unloading strain (εu), which is the most predominant factor affecting the soil mass in excavated structures. An attempt is made to extend the work of Houlsby and Withers (1988) on the self-boring pressuremeter (SBP) and Ferreira and Robertson (1994) for the full-displacement pressuremeter (FDP) and SBP, for unloading curves of the prebored pressuremeter (PBP) test for large strain (up to 41.5 %) tests in a specific type of soil (i.e., saturated lean clay, which so far have been rarely analyzed). This article presents a study to examine and explain the magnitude of strains developed in loading and unloading curves of PBP to predict the ratio of compression to unloading strain (εc/εu) and Su from unloading curves, for which open field experimentation is conducted on saturated clay deposits with a plasticity index of 10 to 16 %. The results of εc/εu and Su that were determined from the undrained PBP tests were compared with anisotropically consolidated undrained triaxial compression tests, which showed reasonable agreement. Correlations are proposed for net limit pressure versus εc/εu and Su from PBP, which offer a new approach to quantifying εu and Su by solely performing compression curves. The proposed correlations are validated by performing PBP tests at two separate sites and are further supported by comparison with the pressuremeter curves of previous studies. The variation in experimental versus predicted values for two separate sites (using the proposed correlation) is within ±10 % for a confidence interval of 95 %.

Mechanical Characteristics of Frozen Sandstone under Lateral Unloading: An Experimental Study

The lateral unloading strength and deformation of surrounding frozen rock are the key parameters for safety evaluation of frozen shaft construction. A low-temperature and high-pressure rock triaxial test system was used to simulate freezing construction, and a constant axial pressure unloading confining pressure test was carried out on frozen sandstone. The effects of freezing temperature, initial confining pressure, and unloading rate on the strength, deformation, and failure modes of frozen sandstone are studied. The main results of the study are as follows: (1) under the initial confining pressure of 20 MPa, the temperature of the sandstone decreases from 20°C to –5°C, and the peak stress and elastic modulus of triaxial compression increase by approximately 3 times. Under lateral unloading conditions, the peak stress of frozen sandstone is about 2∼3 times that of 20°C sandstone, and the peak strain of 20°C sandstone is smaller than that of frozen sandstone. The temperature of frozen sandstone decreases and the rate of increase in the peak stress of triaxial compression is slightly less than the rate of increase in the peak stress of lateral unloading. (2) The initial confining pressure of frozen sandstone increases, the growth rate of axial and radial strain increases, the radial strain dominates the failure process, and the lateral unloading strength decreases significantly. (3) The lateral unloading rate of frozen sandstone increases, the peak strength increases, and the axial and radial strain decrease. At a low unloading rate, partial creep deformation occurs. (4) The frozen rock sample undergoes tensile splitting failure under lateral unloading. According to the stress-strain curve of the frozen rock sample, the relationship between changes in the deformation modulus and changes in the confining pressure unloading amount during the unloading process of the rock sample is obtained.

Insights into a giant landslide-prone area on the eastern margin of the Tibetan Plateau, China

A good understanding of giant landslide-prone areas could greatly enhance the understanding of the formation and failure mechanisms of giant landslides. In this study, a classic giant landslide-prone area named the Diexi area located along the upstream stretch of the Minjiang River on the eastern Tibetan Plateau is adopted to analyze the failure mechanism and evolution process by detailed field investigations, Unmanned Aerial Vehicle (UAV) images and a digital surface model (DSM). The results show that among the 37 giant landslides located in the Diexi area, 18 landslides are transverse landslides (wedge failure), and the others are consequent landslides (buckling failure). All landslides blocked rivers, and some barrier lakes still remain. The Diexi area features special geological structural conditions related to the hinge section of the Jiaochang arc tectonic belt, the intersection of two active fault zones (the Songpinggou and Minjiang fault zones) and high levels of geostress. The numerous radial fissures induced by the Jiaochang arcuate belt provided lateral sliding boundaries for buckling deformation (consequent landslides) and head scarps for wedge failure (transverse landslides). The rapid incision (1.88 mm/yr) since the middle Pleistocene formed a deep gorge with steep slopes and strong lateral unloading. Frequent earthquakes and rainfall further reduced the rock mass integrity, and strong earthquakes or other factors triggered the landslides.

Experimental study on the mechanical properties of Guiyang red clay considering the meso micro damage mechanism and stress path

This study investigated the macroscopic physical and mechanical properties of Guiyang red clay during surcharge loading, lateral excavation and lateral unloading with axial loading, and clarified the failure mechanism of microstructure before and after shear under different stress paths of CTC, RTC and TC. Consolidated undrained triaxial shear permeability, SEM scanning, XRF fluorescence spectrum analysis and XRD diffraction tests were conducted to simulate the actual engineering conditions. The stress–strain curve, shear strength, pore water pressure variation rule and macroscopic failure mode of soil samples under different stress paths were analysed. In addition, Image Pro Plus 6.0 and PCAS were used to study the relationship between the macro mechanical properties and micro microstructure failure under different stress paths. The stress–strain curves from CTC, RTC and TC in CU tests were different, with the peak values of shear stress under the three stress paths being P-increasing, equal P-path and P-decreasing path. Moreover, the internal friction angle and cohesion of the increasing P path were higher than those of equal P path and decreasing P path, hence, the influence of stress paths on the cohesion is greater than that of internal friction angle. The pore water pressure is strongly dependent on the stress path, and the variation characteristics of pore water pressure are consistent with the change in the law of the stress–strain curve. Under the same confining pressure in the P-increasing path, the shear failure zone runs through the whole soil sample, and the shear failure zone is significant, whereas under the condition of the P-reducing path, the shear failure angle of soil sample is about 65°, 55° and 45°, and in the equal P path, the soil sample is dominated by the confining pressure, with no obvious microcrack on the surface of the soil sample. The difference is that the distribution of pores in the path of increasing P and equal P is directional, and the anisotropy rate is small, while the distribution of pores in soil samples with shear failure and before shear is random and the anisotropy rate is high.

The Influence of Calcaneal and First Ray Osteotomies in the Contact Pressures of the Ankle Joint

Category:Ankle Arthritis; Ankle; Hindfoot; Midfoot/ForefootIntroduction/Purpose:Medial displacement calcaneal osteotomies (MDCO) and first ray plantarflexion osteotomies, such as a Cotton osteotomy, are frequently used realignment procedures for hindfoot and ankle joint valgus malalignment. Multiple studies demonstrated the effects of calcaneal osteotomies on the contact pressures of the ankle joint (CPAJ), with slight medial displacement of the center of pressure and lateral unloading of the ankle joint. However, the influence of a first ray plantarflexion osteotomy on the CPAJ is yet to be determined. In this cadaveric study, we compared the effects of calcaneal and first ray osteotomies in the CPAJ.Methods:Fifteen bellow-knee cadaveric specimens were used. Tekscan 5033 sensors were placed in the ankle joint and held with cyanoacrylate. Specimens were loaded in a servohydraulic load frame. Tension loads applied to tendons: Achilles (200N), PTT (40N), peroneals combined (44N), FHL/FDL combined (35N). Specimens were tested in intact position, isolated MDCO (6 and 10mm), isolated Cotton osteotomies (4 and 8mm) and combined MDCO/Cotton osteotomies (10mm and 8mm, respectively).Specimens were then cyclically loaded from 100N-1000N at a rate of 0.5Hz for 30 cycles while CPAJ data was collected at a rate of 20Hz. Average and maximum overall pressure data were extracted as well as the center of pressure (CoP) movement in the anteroposterior (AP) and medial to lateral (ML) directions. Data was also analyzed when divided into lateral, central, and medial areas of the contact pressure map. Groups were compared by the Wilcoxon test. P-values <0.05 were considered significant. Results: We found significant (p<0.05) and progressive decrease in the average and maximum CPAJ when comparing intact ankle (1624 and 1964kPa), MDCO (1526 and 1891 kPa), Cotton osteotomy (1370 and 1642 kPa) and combined osteotomies (1292 and 1599 kPa). Cotton (4 and 8mm) and combined osteotomies showed similar contact pressures, that were significantly lower than intact specimens, emphasizing the power of first ray osteotomies in changing the contact pressures of the ankle joint. When accounting for medial, central and lateral aspects of the joint, we found that the decrease in the pressures was only significant in the central (cotton and combined osteotomies) and lateral aspects (combined osteotomy only).No significant differences were found in CoP measurements (both AP ad ML directions).Conclusion:The results of this cadaveric study demonstrate the power of Cotton osteotomies, in isolation or combined with MDCO, in decreasing the overall CPAJ, especially on its central and lateral aspects. MDCO in isolation did not differ from intact specimens. No significant changes in the center of pressure of the ankle joint were noted following any of the performed osteotomies (combined or isolated). Our findings should guide surgeons when deciding between first ray and calcaneal osteotomies as realignment procedures for hindfoot and ankle valgus deformities, when aiming to unload the lateral aspect of the ankle joint.Figure 1.

Mechanical properties of soft soils experiencing lateral unloading under initial excess pore water pressure

The mechanical properties of soft soils were investigated through triaxial lateral unloading tests under different initial excess pore water pressures and confining pressures. The lateral unloading stress–strain, strength parameters, and initial unloading moduli of soft soils are presented and discussed in this study. The results show that soft soils abruptly undergo unloading failure. The lower confining pressure and the higher initial excess pore water pressure can result in intensive unloading failure behavior. The unloading strength of soft soils linearly decreases as the initial excess pore water pressure increases. The stress–strain curve is characterized as strain hardening. The cohesion and initial unloading moduli of soft soils drastically reduce when the initial excess pore water pressure exceeds 20 kPa. The conclusions obtained in this study can provide a theoretical reference for the design and numerical analysis of underground spaces in soft soils.

Constitutive Model of Lateral Unloading Creep of Soft Soil under Excess Pore Water Pressure

Presented in this paper is a study on the lateral unloading creep tests under different excess pore water pressures. The marine sedimentary soft soil in Shenzhen, China, was selected in this study. The results show that the excess pore water pressure plays a significant role in enhancing the unloading creep of soft soil. Higher excess pore water pressure brings more obvious creep deformation of soft soil and lower ultimate failure load. Meanwhile, the viscoelastic and the viscoplastic modulus of soft soil were found to exponentially decline with creep time. A modified merchant model and a combined model of the modified merchant model and plastic elements are used to simulate the viscoelastic and the viscoplastic deformation, respectively. Therefore, a lateral unloading creep model of soft soil is developed based on the modified merchant model. The accuracy and applicability of this model were verified through identifying the parameters in the model. Research results are of particular significance to the numerical simulation of underground space excavation in soft soil areas which considers the effects of excess pore water pressure.

The Influence of Calcaneal and First Ray Osteotomies on the Contact Pressures of the Ankle Joint

Category:Ankle, HindfootIntroduction/Purpose:Medial displacement calcaneal osteotomies (MDCO) and first ray plantarflexion osteotomies, such as a Cotton osteotomy, are frequently used realignment procedures for hindfoot and ankle joint valgus malalignment. Multiple studies demonstrated the effects of calcaneal osteotomies on the contact pressures of the ankle joint (CPAJ), with slight medial displacement of the center of pressure and lateral unloading of the ankle joint. However, the influence of a first ray plantarflexion osteotomy on the CPAJ is yet to be determined. In this cadaveric study we compared the effects of calcaneal and first ray osteotomies in the CPAJ.Methods:Fifteen bellow-knee cadaveric specimens were dissected to expose the ankle joint and isolate the flexor and peroneal tendons. Tekscan 5033 sensors were placed in the ankle joint and held in place with cyanoacrylate. Specimens were loaded in a servohydraulic load frame. The following loads were applied to the tendons: Achilles (200 N), PTT (40 N), peroneals combined (44 N), FHL/FDL combined (35 N). Ankles were tested in an intact position, after isolated MDCO (6, 8, 10 and 12 mm), isolated Cotton osteotomies (4, 8 and 12 mm) as well as combined osteotomies (10 mm and 12 mm, respectively). Specimens were then cyclically load from 100N-700 N at a rate of 0.5 Hz for 30 cycles while CPAJ data was collected at a rate of 20 Hz. Average and maximum pressure data were extracted as well as the center of pressure (CoP) movement in the AP and ML directions.Results:There was a significant (p<0.05) and progressive decrease in respective maximum and average contact pressures of the ankle joint when comparing intact ankle (1608 and 1312kPa), calcaneal osteotomy (1291 and 1034 kPa), Cotton osteotomy (1165 and 962 kPa) and combined osteotomies (1134 and 903 kPa). Cotton osteotomy and combined osteotomies showed similar contact pressures. Regarding CoP measurements of the ankle joint, native ankle and MDCO demonstrated similar positionings in the sagittal and coronal planes. Cotton and combined osteotomies caused a significant shift of the CoP anteriorly and laterally when compared respectively to the intact/MDCO and MDCO ankles.Conclusion:The results of this study demonstrate that the Cotton osteotomy has a greater effect on the contact pressures of the ankle when compared to the MDCO. There is an overall decrease in the maximum and average pressures as well as a deviation of the center of pressure toward the anterior and lateral aspect of the ankle joint. These findings should guide surgeons when deciding between first ray and calcaneal osteotomies as realignment procedures for hindfoot and ankle valgus deformities.

Exploration of damage evolution in marble due to lateral unloading using nuclear magnetic resonance

Multi-scheme lateral unloading confining pressure tests on marble specimens were conducted by using an MTS 815 electrohydraulic servo testing machine, and nuclear magnetic resonance (NMR) tests were performed on the unloaded specimens for the first time to investigate the mesodamage evolution characteristics of the unloading rock. The change laws of stress-strain curves, rock porosity, and NMR parameters during the failure process were obtained. The results indicate that the rock sample exhibits evident plastic strain softening and shear dilation characteristics during the unloading process, and that the deformation modulus, Poisson's ratio, and axial compression strength decrease continuously with a decrease in the confining pressure. Given the different levels of unloading points, there are differences in the failure characteristics. When the unloading intensities were lower than 90%, the microfissures inside the sample extended uniformly, and the internal damage to the marble was mainly caused by the increases in the number of small pores. When the unloading intensities exceeded 90%, interpenetrating fissures developed and extended rapidly, and rock damage was mainly caused by the increase in the number and size of the large pores. The spectrum peaks area of large pores play a dominant role in the development of marble porosity. Furthermore, the development process of microcracks and fractures in rock specimens is dynamically indicated by NMR imaging.

Plane strain evaluation of stress paths for supported excavations under lateral loading and unloading

Excavations are generally analyzed as plane strain problems. The shear strength parameters determined from the triaxial conditions are not representative of the real field conditions. Therefore, plane strain tests on reconstructed silty clay were performed using a modified apparatus to measure the stress-strain curves directly. Three stress paths were tested, where the lower shear strength and the failure strain were measured for tests with unloading in the minor principal stress direction and compared to tests with loading. A set of hyperbolic constitutive models was developed to fit the experimental data at different stress paths. The stress-strain-strength characteristics and the deformation parameters were determined for these tests. A displacement-dependent earth pressure model was then proposed for retaining walls due to excavations, and then it was calibrated against the measured soil response. In the end, a case history of bored pile walls subjected to excavations was illustrated. Calculations using the proposed approach and the standard Mohr-Coulomb model were compared. The analysis using the standard Mohr-Coulomb model was found to underestimate the maximum wall deflection by 15%.

Further Modified Duncan-Chang Constitutive Model Considering Intermediate Principal Stress with Lateral Unloading for Silty Clay

Duncan-Chang model describes the soil stress-strain relationship more accurately when the soil will be loaded. However, it will be inaccurate when the soil is in the unloading process, such as the pit excavation. In order to dispose of properly the Duncan-Chang constitutive model describing accurately the stress-strain relationship of the silty clay under the unloading condition so as to achieve the purpose of engineering application. This paper developed a further modified Duncan-Chang model, which consider intermediate principal stress, lateral unloading process, the parameter λ was introduced to the model; The result by taking the true triaxial test data of a typical silty clay shows that when λ=0.45, a new modified unload Duncan-Chang model for silty clay is effective in solving the problem of descripting unload stress-strain relationship test curves of the silty clay. And for the results further showed it is not suitable for using the sum of the intermediate principal stress σ2 and third principal stress σ3 is equal to 0.5 instead of the intermediate principal stress σ2 through the finite element calculation to the silty clay pit excavation.

Nonlinear simulation of tunnel linings with a simplified numerical modelling

A high-efficiency simplified modelling approach is proposed for investigating the nonlinear responses of reinforced concrete linings of shield tunnels. Material and geometric nonlinearities are considered in the analysis of the lining structures undergoing large deformation before ultimately losing the load-carrying capacity. A beam-spring element model is developed to capture the force-transfer mechanism between lining segments and radial joints. The developed model is validated by comparing analyzed results to experimental results of a single-ring lining structure under two loading conditions: the ground overloading and the lateral unloading respectively. The results show that the lining structure under the lateral unloading due to excavation on the both sides of the tunnel is more vulnerable compared to the case of ground overloading on the top of the tunnel. A parameter study is conducted and results indicate that the lateral pressure coefficient has the greatest influence on the behaviour of the lining structure.

FEM simulation of simply supported GLARE plates under lateral indentation loading and unloading

In this article, a 3-D finite element modeling procedure is implemented, in order to predict the static loading and unloading response of thin circular simply supported GLARE plates, subjected to lateral indentation by a hemispherical indentor. ANSYS software is used and a non-linear analysis is employed with geometric and material non-linearities. The modeling procedure is applied to GLARE 2-2/1-0.3 and GLARE 3-3/2-0.3 simply supported plates with 40mm radius, and to a GLARE 2-2/1-0.3 simply supported plate with 50mm radius. The load–deflection curve during loading and unloading, and the permanent plate deflections of the GLARE plates are calculated for different maximum loading indentation levels. It is found that the permanent dent depth of the GLARE plates is increased as the maximum loading indentation increases. It is revealed that for the same maximum loading indentation, when the GLARE plate radius is increased, the permanent dent depth is reduced. It is also found that the unloading load–deflection curves, corresponding to different maximum loading indentation levels of each GLARE plate, are approximately parallel. Based on this finding, an approximate graphical method to obtain unloading curves is proposed. The role of the metal volume fraction to the loading–unloading response of the fiber–metal laminates is also investigated.

Effects of lateral unloading on the mechanical and deformation performance of shield tunnel segment joints

Unloading during lateral excavation widely occurs in existing subway shield tunnels. Previous studies have focused on the overall stress and deformation of existing tunnels caused by nearby unloading. However, the stress and deformation state of tunnel segment joints have yet to be considered. This study considered the non-continuity of the shield tunnel lining and the interactions among tunnel segment, surrounding rocks and ballast bed. A hybrid model of a shield tunnel was established based on 3D nonlinear contact theory. The mechanical and deformation properties of the segments and joints of an existing shield tunnel under the influence of lateral excavation of the foundation pits were studied. Unloading during lateral excavation caused the cross section of the shield tunnel to generate vertical convergence and shift horizontally towards the foundation pit. An opening and dislocation in the joint, which caused the waterproof ability of the joint to decrease sharply, were observed. Meanwhile, stress at the segment joint increased sharply and caused local cracks in the segment lining. Axial and shearing force on the joint bolt also increased significantly. Based on existing subway regulations, the calculation results were combined to establish a deformation control standard for existing shield tunnels under lateral excavation. The rate of vertical convergence of the lining should be less than 3.68‰, and the rate of horizontal shift of the axis should be less than 0.53‰.

A combined geomorphological and geophysical approach to characterising relict landslide hazard on the Jurassic Escarpments of Great Britain

The Jurassic Escarpment in the North York Moors in Northern Britain has a high density of deep-seated relict landslides but their regional hazard is poorly understood due to a lack of detailed case studies. Investigation of a typical relict landslide at Great Fryup Dale suggests that the crop of the Whitby Mudstone Formation is highly susceptible to landslide hazards. The mudstone lithologies along the Escarpment form large multiple rotational failures which break down at an accelerated rate during wetter climates and degrade into extensive frontal mudflows.Geomorphological mapping, high resolution LiDAR imagery, boreholes, and geophysical ERT surveys are deployed in a combined approach to delimit internal architecture of the landslide. Cross-sections developed from these data indicate that the main movement displaced a bedrock volume of c. 1×107m3 with a maximum depth of rupture of c. 50m. The mode of failure is strongly controlled by lithology, bedding, joint pattern, and rate of lateral unloading. Dating of buried peats using the AMS method suggests that the 10m thick frontal mudflow complex was last active in the Late Holocene, after c. 2270±30calendar years BP.Geomorphic mapping and dating work indicates that the landslide is dormant, but slope stability modelling suggests that the slope is less stable than previously assumed; implying that this and other similar landslides in Britain may become more susceptible to reactivation or extension during future wetter climatic phases. This study shows the value of a multi-technique approach for landslide hazard assessment and to enhance national landslide inventories.