Face stability and required support pressure for TBM driven tunnels with ideal face membrane – Drained case

Assuming that an ideal membrane develops at the face, three-dimensional finite element simulations are employed to investigate the effects of tunnel diameter, cover-to-diameter ratio, lateral earth pressure coefficient, and soil strength parameters on the stability and displacements of the excavation face of mechanically driven tunnels in drained conditions. The relation between the face support pressure and the calculated tunnel face displacement gave the minimum face support pressure that should be applied on the tunnel face to avoid abrupt movement of the tunnel face. An equation is given for the minimum support pressure as a function of friction angle, cohesion, lateral earth pressure coefficient, tunnel diameter, and tunnel depth. The minimum support pressures are compared to the analytical solutions available from the literature. For cohesionless ground, the face support pressures obtained from the finite element analysis shows a good agreement with the values from the analytical methods published in the literature when cover-to-diameter ratio is 1. However, as the cover-to-diameter ratio increases, the values from the finite element analysis are greater than suggested by the analytical method. For cohesive ground, the pressure from finite element analysis is found to be almost always equal to or greater than the values obtained with analytical solutions.     

A survey of vehicle fires in Norwegian road tunnels 2008–2011

Norway is one of the countries that constructs most road tunnels, and there are well over 1000 in the country today. The aim of this study is to map the prevalence and describe the characteristics of vehicle fires in Norwegian road tunnels 2008–2011. The average number of fires in Norwegian road tunnels is 21.25 per year per 1000 tunnels, and the average number of smoke without fire (SWF) is 12.5 per year per 1000 tunnels. The study provides four main results. The first is that the fires generally did not involve harm to people. This has also been reported in previous Norwegian research. The second finding is that heavy vehicles are overrepresented in fires in Norwegian road tunnels. This is in line with international research. The third main finding is that the causes of road tunnel fires involving heavy and light vehicles are different. This is also in accordance with international research. The fourth important and unique finding of the study is that subsea road tunnels are overrepresented in the statistics of fires in Norwegian road tunnels.     

Large displacement stability analysis of elastic–plastic unsymmetrical sandwich cylindrical shells

The paper presents stability analysis of elastic–plastic, free supported sandwich cylindrical shell with unsymmetrical faces, loaded by longitudinal forces, transversal pressure, and shear. The J₂ incremental Prandtl–Reuss plastic flow theory constitutive relations were used in the analysis and it was assumed that geometrical strain-displacement relations are nonlinear ones. It was also assumed that the shell faces have different thicknesses and these are made of different isotropic compressible materials with linear stress-hardening. Active loading processes are accepted in the analysis, the stability equations are derived using strain energy formulation. Ritz method is used to solve the equations and an iterative computational algorithm was elaborated to get numerical results.     

Analytical solution for the excavation of circular tunnels in a visco-elastic Burger’s material under hydrostatic stress field

By excavating an underground space, the state of stress and displacement are changed in comparison to the initial state. This variation depends on the advance of the tunnel and on the rheological behavior of the hosting rock mass. In this paper, the effect of creep, in the response of the tunnel is discussed. The objective of the paper is to predict time–dependent displacement of the tunnel wall, after stopping the excavation or after installing the support system. The rock mass is assumed to be isotropic and homogenous and incompressible. The tunnel is assumed to be circular and driven in a hydrostatic stress field. The rate of excavation is considered to be infinitely large. The Burger’s body which is able to model the primary and secondary creep regions of the rock mass is applied. In such a condition, an analytical solution for predicting time-dependent deformation of tunnel wall is derived. Thereafter, the application of the proposed solution is illustrated through three examples. The examples are analyzed numerically using the finite difference code FLAC. The results obtained from numerical analyses show proper agreement to the results of proposed solution.     

Modeling water–NAPL–air three-phase capillary behavior in soils

Existing models relating the capillary pressures and the degrees of saturation of NAPL, water, air three-phase are briefly reviewed, and their limitations are discussed. Of particular interest is their inability to describe the intrusion and emission behavior of NAPL precisely, especially when the degree of saturation of NAPL is low. A new variable μ representing the relative magnitude of NAPL pressure to water pressure and air pressure is then defined as a state parameter, and a novel water–NAPL–air three-phase model is proposed, which considers the transition phenomena between water–NAPL–air three-phase and arbitrary two-phase system through the parameter μ. The validity of the proposed model is checked by comparing the calculation results with those obtained by an ordinary model and existing experimental results. The proposed model is shown capable of properly considering the interactions among the water–NAPL–air three-phase, and predicting the actual retention behavior of NAPL in unsaturated ground.     

Axial–flexural interaction in circular FRP-confined reinforced concrete columns

External confinement by fibre reinforced polymers (FRPs) is now a widely implemented technique to strengthen reinforced concrete (RC) columns. To date, the vast majority of experiments on FRP-confined concrete have considered short, unreinforced, small scale concrete cylinders, with aspect ratios of less than three and tested under concentric axial load. In practice, RC columns invariably have aspect ratios larger than three and are subjected to inadvertent load eccentricities or combined axial–flexural (P–M) loading. Limited available research suggests that the effectiveness of FRP confinement is reduced under P–M loading. This paper presents the results of a systematic test program on circular FRP-confined RC columns of realistic slenderness under eccentric axial loads to study the mechanics and performance of these types of members. Test data are compared against theoretical P–M interaction diagrams, as well as against recently published design guidelines for P–M interaction in FRP-confined RC columns.     

Probabilistic slope stability analysis in Kahrizak landfill: effect of spatial variation of MSW’s geotechnical properties

Bulletin of Engineering Geology and the Environment - The geotechnical properties of municipal solid waste (MSW) in landfills vary considerably depending on the composition, time, and the rate of...     

What is the role of tensile cracks in cohesive slopes?

The traditional limit equilibrium method (LEM) is often used to search for the failure surface with a minimum safety factor of slope. In this method, the failure surface is considered as a shear surface, irrespective of its form. However, tensile cracks are frequently found at the outcrops of landslides. In this study, three sets of tests on small-scale landslides with different inclination angles were conducted. The test results demonstrated that tensile cracks could arise in the slope sliding process and the failure surface is composed of both a shear and a tensile fracture surface. Based on the test results, we used the improved LEM, and replaced the traditional shear failure surface by a tensile-shear coupling one, thus new tensile failure modes for slope stability analysis can be established. The safety factors of slope in different failure modes were compared, which show that when considering soil tensile failure and tensile strength less than a certain value (e.g. 15 kPa, 44 kPa and 55 kPa for linear, circular and logarithmic spiral failure surfaces, respectively), the safety factors of slope with three different failure surfaces are less than the one that did not consider the tensile failure. The most critical failure surfaces of the slope may be composed of shear and tensile damages because the tensile strength of the soil cannot be generally greater than its cohesion.     

Recent advances in fire–structure analysis

One of the recommendations of the National Construction Safety Team (NCST) for the Federal Building and Fire Safety Investigation of the World Trade Center Disaster [NIST NCSTAR 1 Final report on the collapse of the World Trade Center Towers. NCST for the Federal Building and Fire Safety Investigation of the World Trade Center Disaster, National Institute of Standards and Technology, Gaithersburg, MD, September 2005] is to enhance the capability of available computational software to predict the effects of fires in buildings, for use in the design of fire protection systems and the analysis of building response to fires. Following this recommendation, this paper presents two new interfaces in fire–thermal–structural analysis. The first interface uses adiabatic surface temperatures to provide an efficient way of transferring thermal results from a fire simulation to a thermal analysis. It assigns these temperatures to surface elements of structural members based on proximity and directionality. The second interface allows the transfer of temperature results from a thermal analysis modeled with solid elements to a structural analysis modeled with beams and shells. The interface also allows the reverse, namely the geometric updating of the thermal model with deflections and strains obtained from the structural analysis. This last step is particularly useful in intense fires of long duration, where significant deflections and strains could cause damage to insulation and displace the structure to a different thermal regime. The procedures can be used for a variety of fire simulation, thermal, and structural analysis software.     

Framework of vehicle–bridge–wind dynamic analysis

This paper presents a framework of dynamic analysis of coupled three-dimensional vehicle–bridge system under strong winds. A general formulation of this system is introduced to simulate a series of vehicles consisting of different numbers and different types of vehicles running on bridges under hurricane-induced strong winds. Each vehicle is modeled as a combination of several rigid bodies, axle mass blocks, springs, and dampers, considering wind and road roughness loads. With this vehicle–bridge model, coupled dynamic analysis of vehicles running on bridges is conducted with a numerical example. Effects of driving speeds on the dynamic performance of the vehicles as well as the bridge are discussed. It is found that the driving speeds mainly affect the vehicle's vertical relative response while they have insignificant effect on the rolling response of vehicles. Vehicle's absolute response is dominated by the bridge response when wind speed is high, while it is dominated by road roughness when the wind speed is low. Detailed accident analysis of vehicles on bridges under strong winds will be reported in an accompanying paper.     

Component analysis of some chemical parameters influencing the stability of DDVP in sea‐sediments along the east coast of India

The component analysis of some chemical parameters such as half‐life period, pH, salinity, total organic carbon (TOC), clay mineral concentration (Kaolinite, Illite, Montmorrillonite) and the elemental concentrations (Al, Fe, Ti) showed that only three components collectively retained 98% of variation of the original data. The first component carries maximum co‐variance with salinity and TOC, the second component is due to clay‐montmorrillonite, and the third component is attributed to TOC and Fe. The first component accounted for 69%, the second 28% and the third for 3% of total variation. The impact of TOC on the stability of pesticide, DDVP is much more pronounced by its negative covariance with the half life period as revealed by the first principal component.     

Effect of polyurethane on the stability of sand–clay mixtures

Three types of polyurethane were synthesized from mixtures of toluene diisocyanate, polyethylene glycol and polypropylene glycol for use in soil stabilization to improve the erosion resistance. The three polyurethanes were tested at different aqueous concentrations and sand:clay mixtures at weight ratios of 1:1, 1:3 and 1:5. The results of the rainfall simulation, unconfined compression and direct shear tests showed that the polyurethanes improve both strength and erosion resistance significantly. The observed improvement in soil erosion resistance is attributable to the physico-chemical interaction of the long-chain macro-molecules of polyurethane with the clay fraction of the soil.     

Thermal gradient–dependent fracture behavior of ice-rich frozen soils

Bulletin of Engineering Geology and the Environment - Fractures in frozen soils are strongly associated with thermal gradient–dependent heterogeneity and significantly affect frost heaving...     

Effect of soil–infilled joints on the stability of rock wedges formed in a tunnel roof

The use of an equivalent continuum for a rock mass is not always suitable for situations, where the failure is structurally controlled by discontinuities as in the case of wedges in the tunnel roof. In these instances, discontinuum approaches are usually preferred. Rock joints that are filled with soft infill are likely to be the weakest planes in a rock mass, having a dominant influence on its overall shear behaviour. In this case, the joint material model adopted for the discontinuities should be able to describe important mechanisms, such as asperity sliding and shearing, post-peak behaviour, asperity deformation, and the effect of the soft infilling. The latest version of a soil–infilled joint model is discussed here. It describes more comprehensively than previous models the occurrence of dilation and compression with lateral displacements, and also represents the hardening mechanism related to asperity interference as observed in the laboratory that cannot be readily captured by the existing joint models. An analytical approach for the analysis of rock wedges structurally controlled by soil–infilled joints and a numerical simulation based on a metro station collapse which occurred in Brazil in 2007 are presented.     

Back analysis of long-term stability of a 92 m span ancient quarrying cavern

Long-term stability of large-span caverns is a challenging issue for design and construction of underground rock engineering. The Heidong cavern group consisting of 21 caverns was constructed about 1400 years ago for quarrying in massive Cretaceous tuff. The cavern No. 5 of the Heidong cavern group is characterized by an unsupported span up to 92 m, with the overburden thickness of only 3–25 m. To analyze its long-term stability, a detailed investigation was conducted to obtain its geometry and rock mass characteristics, and to monitor surrounding rock displacements. Based on field survey and laboratory tests, numerical simulations were performed using the finite difference code FLAC^3D. The analysis results revealed that for the long-term stability of the cavern No. 5, some major factors should be carefully considered, such as cavern excavation method in hard massive rocks, site investigation using trial pits, tools like short iron chisel and hammer for manual excavation, geometric dome roof, and waste rocks within abutment or on the floor. The highlights of the technologies obtained from this large-scale ancient underground project can provide reference for other similar project excavations in practice.     

Safety monitoring and stability analysis of left abutment slope of Jinping Ⅰ hydropower station

Safety monitoring and stability analysis of high slopes are important for high dam construction in high mountainous regions or precipitous gorges. In this paper, deformation characteristics of toppling block at upper abutment, deforming tensile rip wedge in the middle part and deep fractures are comprehensively analyzed based on the geological conditions, construction methods and monitoring results of left abutment slope in Jinping I hydropower station. Safety analyses of surface and shallow-buried rock mas...     

Behavior of noncircular tunnels excavated in stratified rock masses – Case of underground coal mines

The amount of tunnels excavated along stratified/sedimentary rock masses in Quangninh coal mine area,Vietnam, is gradually increasing. Rock mass in Quangninh is characterized by beddings between rock layers. The behavior of stratified rock masses surrounding the tunnels depends on both the intact rock and the beddings between rock layers. The main characteristics of stratified rock masses that need to be considered are their heterogeneity and anisotropy. Depending on the dip angle of rock layers, movements and failure zones developed surrounding the tunnels can be asymmetrical over the vertical axis of tunnel. This asymmetry causes adverse behaviors of the tunnel structures. The objective of this study is to highlight convergences and yielded zones developed in rock masses surrounding noncircular tunnels in Quangninh coal mine area using a finite element method. The presence of bedding joints is explicitly simulated. The numerical results indicated that with the increase in dip angle of bedding joints, the stress asymmetry over the tunnel vertical axis increases. It gradually leads to an asymmetry of the failure zone surrounding the tunnel. An increase of rock mass quality means a decrease of rock mass sensitivity to the discontinuities. In addition,a dip angle of the bedding joints of approximately 45° could be considered as the critical angle at which the rock mass mechanism changes between sliding and bending.     

Soil–water–air fully coupling finite element analysis of slope failure in unsaturated ground

In this paper, a program of the finite element method (FEM), named as SOFT, using a finite element–finite difference scheme (FE–FD) for soil–water–air three-phase coupling problems, has been developed based on a rational and simple constitutive model for unsaturated soil proposed by Zhang and Ikariya (2011). In the program, similar to the works by Uzuoka et al. (2009) and Oka et al. (2010b), the FE–FD formulation in saturated condition of soil–water coupling problem, proposed by Oka et al. (1994), has been extended to unsaturated condition in soil–water–air fully coupling scheme, taking the saturation as a state variable. In order to verify the availability of the proposed numerical method, triaxial tests on unsaturated silty clay under fully undrained and unvented conditions, conducted by Oka et al. (2010a), are firstly simulated by the proposed method. The development of pore air pressure and pore water pressure measured in the specimen can be reproduced well by the proposed method. Furthermore, model tests on slope failure in unsaturated Shirasu, carried out by Kitamura et al. (2007), are also simulated by the same numerical method. From the simulation it is known that the slope failure behavior of the model ground observed in the tests can be described, on the whole, with satisfactory accuracy