Deformation Response Research of the Existing Subway Tunnel Impacted by Adjacent Foundation Pit Excavation

The excavation of foundation pits is one of the most important factors causing changes to the initial stress state of its surrounding soil, thus affecting the safety of nearby existing subway tunnels. In order to study the deformation in metro lines induced by adjacent foundation pit excavation, a three-dimensional model based on an actual engineering case was established, and the deformation regulations of the retaining wall, surrounding soil, and tunnels were investigated, which also validated the model’s feasibility. Additionally, the deformation and strain response of the subway tunnel under different selection parameters of the enclosing structure and soil were studied. The results showed that, after the foundation pit excavation, the soil inside the pit underwent an uplift, the surrounding soil outside of the pit showed vertical settlement, and the retaining wall created a deformation towards the interior of the pit. Mechanical parameters of plate elements have a small influence on the deformation of metro lines. Axial strain and maximum displacement of the subway tunnel increase with the increase in the soil’s Poisson’s ratio, and on the contrary, they decrease with the increase in the m-value and G 0 , ref . The maximum responses of the subway tunnel came from changes to G 0 , ref and υ . These analysis results can be used for the safety evaluation of subway tunnel operation, design, and construction in other similar engineering settings.

Evaluation of Input Geological Parameters and Tunnel Strain for Strain-softening Rock Mass Based on GSI

The regression analysis method is being widely adopted to analyse the tunnel strain, most of which ignore the strain-softening effect of the rock mass and also fail to consider the influence of support pressure, initial stress state, and rock mass strength classification in one fitting equation. This study aims to overcome these deficiencies with a regression model used to estimate the tunnel strain. A group of geological strength indexes (GSI) are configured to quantify the input strength parameters and deformation moduli for the rock mass with a quality ranging from poor to excellent. A specific numerical procedure is developed to calculate the tunnel strain around a circular opening, which is validated by comparison with those using existing methods. A nonlinear regression model is then established to analyse the obtained tunnel strain, combining twelve fitting equations to relate the tunnel strain and the factors including the support pressure, the GSI, the initial stress state, and the critical softening parameter. Particularly, three equations are for the estimation of the critical tunnel strain, the critical support pressure, and the tunnel strain under elastic behaviour, respectively; and the other nine equations are for the tunnel strain with different strain-softening behaviours. The relative significance between the GSI, the initial stress and the support pressure on the tunnel strain is assessed.

Investigation of Stress State During Cement Hardening and its Effect on Failure of Cement Sheath in Shale Gas Wells

Consideration of initial stress state after cement hardening provides a vital basis for the prediction of cement failure, which has been overlooked in previously published methodologies partly due to the difficulties in examining this problem rationally. In the present study, the hoop stress at casing-cement interface during cement hardening is investigated experimentally based on the full-scale casing-cement sheath-formation system (CCFS) facility, which is equipped with the real-time stress-strain measurement capability. The hoop stress at casing-cement interface during cement hardening drops sharply, rather than equating with the initial annulus pressure of cement slurry. It presents a higher drawdown under higher annulus pressure and thinner casing, and a lower drawdown under elastic cement slurry and thicker cement sheath. Furthermore, an analytical model taking the effect of cement hardening into account is developed to predict the integrity of cement sheath. Reliability of the model is verified by comparison with field observations. Excellent agreements are observed. The results illustrate that the tensile cracks are likely to occur at the inner cement (inner surface of cement sheath) by the effect of cement hardening, since the hoop stress at inner cement during cement hardening drops greatly and even becomes tensile. A detailed sensitivity analysis illustrates that an elastic cement slurry with a lower elastic modulus works more effectively, which can resolve the SCP problem in shale gas wells.

EFFECT OF MANUFACTURING ON THE TRANSVERSE RESPONSE OF THERMOSET COMPOSITES

This study presents a finite element (FE) based virtual test procedure to investigate the effect of the manufacturing process on the transverse composite response. Computational models of the composite microstructures are generated and analyzed in commercial FE solver Abaqus supplemented by user-written subroutines. Several realizations of the composite microstructure with random fiber arrangement are analyzed assuming appropriate initial stress state and material definitions. The virtual test procedure is established to define the evolution of process-induced in-situ matrix properties through direct and inverse process modeling approaches. Subsequently, the composite microstructures are virtually tested in transverse tension to predict the transverse composite properties by implementing progressive damage models. In order to quantify the effect of manufacturing on the transverse composite response, predictions from the two approaches are compared to a third case which assumes an initial stress-free state and neglects the effect of processing conditions on the in-situ matrix properties. Variations of ±5% in average strength and 18% in standard deviations are observed with respect to ideally cured RVEs. It is established that process modeling in necessary to optimize the residual stress state and improve composite performance.

Deformation Failure and Support Test of Surrounding Rock in Deep Arched Roadway with Straight Wall

The deformation and failure of the uphill roadway on the 3rd horizontal track in the No. 6 Mine of Pingdingshan Coal Group was taken as the engineering background. The similar simulation material of the roadway surrounding rock with quartz sand as the aggregate, cement as the cementing agent, and gypsum powder as the regulator was selected. Through mechanical tests on 25 sets of specimens with different proportions, the best proportion of similar simulated materials for simulating the deformation and failure of the surrounding rock of the roadway was obtained. Later, a large-scale deep mine roadway simulation test system independently developed by the company was used to carry out the roadway deformation and failure test. First, load the test body to the set initial stress state, and then carry out the full-face excavation and unloading of the roadway; finally, load it in the vertical direction until the roadway wall is damaged. It can realize the actual effect of simulation of roadway deformation and failure under the path of “high stress + internal unloading + stress adjustment.” The results showed that after the deep roadway is excavated with preload and high stress, the surrounding rock deformation, failure, and instability of the roadway mainly experience 3 periods: the first period is the period of uniform deformation of the roadway surrounding rock, the second period is the development period of the roadway surrounding rock slab structure, and the third period is the period of instability of the roadway surrounding rock slab structure. Combined with the time period of deformation and failure of the surrounding rock of the roadway, the damage scope of the surrounding rock and the actual situation of the site engineering. A step-by-step combined roadway repair and support plan of “bolt mesh + shotcrete + full-face hollow grouting anchor cable” with hollow grouting anchor cable as the core was determined. The stability of the repaired roadway has been significantly improved, ensuring the long-term use of the roadway.

3D crustal stress state of Germany according to a data-calibrated geomechanical model

The contemporary stress state in the upper crust is of great interest for geotechnical applications and basic research alike. However, our knowledge of the crustal stress field from the data perspective is limited. For Germany basically two datasets are available: orientations of the maximum horizontal stress (SHmax) and the stress regime as part of the World Stress Map (WSM) database as well as a complementary compilation of stress magnitude data of Germany and adjacent regions. However, these datasets only provide pointwise, incomplete and heterogeneous information of the 3D stress tensor. Here, we present a geomechanical–numerical model that provides a continuous description of the contemporary 3D crustal stress state on a regional scale for Germany. The model covers an area of about 1000×1250 km2 and extends to a depth of 100 km containing seven units, with specific material properties (density and elastic rock properties) and laterally varying thicknesses: a sedimentary unit, four different units of the upper crust, the lower crust and the lithospheric mantle. The model is calibrated by the two datasets to achieve a best-fit regarding the SHmax orientations and the minimum horizontal stress magnitudes (Shmin). The modeled orientations of SHmax are almost entirely within the uncertainties of the WSM data used and the Shmin magnitudes fit to various datasets well. Only the SHmax magnitudes show locally significant deviations, primarily indicating values that are too low in the lower part of the model. The model is open for further refinements regarding model geometry, e.g., additional layers with laterally varying material properties, and incorporation of future stress measurements. In addition, it can provide the initial stress state for local geomechanical models with a higher resolution.

Investigating the Macro-Micromechanical Properties and Failure Law of Granite under Loading and Unloading Conditions

The excavation of rock significantly changes the initial stress state of rock slopes, which makes rock in complex loading and unloading conditions. However, the failure mechanisms and macro-micromechanical properties of rock under loading and unloading conditions are not very clear. This study investigates the macro-micromechanical properties and failure law of granite under loading and unloading conditions through traditional laboratory tests and particle flow simulations. Granite specimens are taken from Shuichang iron mine, and stress unloading experiments are designed based on the engineering practices. The stress-strain curves and failure modes under different confining pressures and unloading paths are obtained to analyze the granite properties from the viewpoint of the macroscopic mechanism. Moreover, numerical models are established in PFC software. The microcracks developments, failure characteristics, and energy evolutions under loading and unloading conditions are obtained and discussed. Results show that compared with the loading tests, the brittle failure characteristics of specimens under unloading tests are more obvious. When the confining pressure reduces to about 66% of the initial confining pressure, the specimen loses its load-bearing capacity and destroyed due to the lateral expansion. For loading tests, an inclined plane can be produced as the main failure surface. While for confining pressure unloading tests, there are many damage zones parallel to or intersecting with the main failure surface.

THE INFLUENCE OF LATERAL PRESSURE ON THE DEFORMATION CHARACTERISTICS OF THE SOIL

В статье приведены результаты экспериментальных исследований коэффициента бокового давления в условиях компрессии и деформационных характеристик Е и ν (модуль общей деформации и коэффициент Пуассона). По существующим методикам расчета осадок оснований, коэффициент Пуассона рекомендуется принимать постоянным в зависимости от вида грунта. Неточность такого подхода иллюстрируется проведенными авторами исследованиями по определению коэффициента бокового давления, который зависит от физических свойств (плотности, влажности, гранулометрического состава и др.) и начального напряженного состояния грунта. Показано влияние способа определения коэффициента бокового давления и соответственно коэффициента Пуассона при расчете деформационных свойств грунтов и границы применения обобщенного закона Гука. В реальных условиях, распределение напряжений от внешней нагрузки и сжатие по центральной оси близко к условиям компрессии, но общий процесс деформирования основания обусловлен как боковыми так и вертикальными деформациями, которые характеризуются коэффициентом поперечного расширения ν. The article presents the results of experimental studies of the lateral pressure coefficient under compression and deformation characteristics E and ν (modulus of total deformation and Poisson's ratio). According to the existing methods for calculating the settlement of foundations, it is recommended to take Poisson's ratio constant depending on the type of soil. The inaccuracy of this approach is illustrated by the studies carried out by the authors to determine the lateral pressure coefficient, which depends on the physical properties (density, moisture, particle size distribution, etc.) and the initial stress state of the soil. The influence of the method for determining the lateral pressure coefficient and, accordingly, Poisson's ratio in calculating the deformation properties of soils and the limits of application of the generalized Hooke's law is shown.In real conditions, the distribution of stresses from an external load and compression along the central axis is close to the compression conditions, but the general process of deformation of the base is caused by both lateral and vertical deformations, which are characterized by the coefficient of transverse expansion ν.

An Experimental Study on the Deformation and Strength Characteristics of Q3 Loess under a Plain Strain Anisotropic Consolidation Condition

To study the strength, deformation characteristics, and intermediate principal stress of intact loess, vertical loading stress tests with different initial stress state k value were conducted under different confining pressures. Plane strain tests were carried out by the transformed true triaxial apparatus of Xi’an University of Technology. The study shows that loess tends to be in a plastic failure state in different confining pressures and k values, and the stress-strain relationship curve is of a hardening type. Results reveal that loess lateral and volume deformations have nonlinear relationships with its vertical deformation, and volume deformation shrinks in the process of shearing. The effect of confining pressure on soil deformation is greater than k value. The intermediate principal stress coefficient decreases with the increase of the confining pressure and transforms from increasing to decreasing with the increase of k value (ranging from 0.2 to 0.4). In brief, the loess failure strength is closely related to k value, confining stress, and spherical stress state. When k value increases, cohesion effect reduces, whereas internal friction angle increases linearly. The influence of k value on soil strength and deformation is closely related to confining pressure.