Design adaptations in a large and deep urban excavation: Case study

In this paper, design, re-design, and performance of a long-standing very deep excavation, which was originally planned to depth of 38 m, are presented. Over-digging was not planned in the original design, thus the reassessment was performed. Two main topics were followed: deepening to increase the maximum depth of an existent excavation from 38 m to 42.5 m, and feasibility for upgrading a predesigned support system from temporary to permanent support system. The geological investigations in the project site illustrated a type of stiff and cemented coarse-grained alluvium. An observational approach with additional geotechnical investigations and in situ tests was applied. Back analyses of stability of an unsupported access ramp, as well as deformation monitoring of walls, were used in order to review geotechnical design parameters that represent the full-scale behavior of the ground. Additional nails and soldier piles together with building mat foundation were implemented as a complementary lateral support in the retaining system. From an engineering point of view, by assuming a corrosion rate of 0.065 mm/a for existent rebars, according to chemical and electrical resistivity tests, the long-term performance of the revised retaining system was verified by static and pseudo-dynamic ultimate limit state analyses. Performance monitoring during the construction shows that the measured deformation is in the lower limit of the prediction.     

Hydrogeochemistry and Contamination of Trace Elements in Cu-Porphyry Mine Tailings: A Case Study from the Sarcheshmeh Mine,SE Iran

Geochemical and hydrochemical investigations were performed to understand the contamination potential of the Sarcheshmeh mine tailings. The geochemical mobility for the tailings is as follows: Cu > Cd > Co > Zn > Ni > Mn > S > Cr > Sn > As > Se > Fe = Bi > Sb = Pb = Mo. Highly mobile and contaminant elements (Cd, Cu, Zn, Mn, Co, Ni, S, and Cr), which significantly correlated with each other, were mainly concentrated in the surface evaporative layer of the old, weathered tailings, due to the high evaporation rate, which causes subsurface water to migrate upward via capillary action. The contamination potential associated with the tailings is controlled by: (1) dissolution of secondary evaporative soluble phases, especially after rainfall on the old weathered tailings, accompanied by low pH and high contamination loads of Al, Cd, Co, Mg, Cr, Cu, Mn, Ni, S, Se, and Zn; (2) processing of the Cu-porphyry ore under alkaline conditions, which is responsible for the high Mo (mean of 2.55 mg/L) and very low values of other contaminants in fresh tailings in the decantation pond; (3) low mobility of As, Fe, Pb, Sb, Mo, and Sn due to natural adsorption and co-precipitation in the tailings oxidizing zone. Speciation modeling showed that sulfate complexes (MSO₄ ⁺, M(SO₄)_(aq), M(SO₄) ₂ ^?2 , and M(SO₄) ₂ ^? ) and free metal species (M⁺² and M⁺³) are the dominant forms of dissolved cations in the acidic waters associated with the weathered tailings. In the alkaline and highly alkaline waters, trace element speciation was controlled by various hydroxide complexes, such as M(OH)⁺, M(OH) ₃ ^? , M₃(OH) ₄ ⁺² , M₂(OH) ₃ ⁺ , M(OH)_2(aq), M(OH) ₄ ^?2 , Me(OH) ₂ ⁺ , Me(OH) ₄ ^? , Me(OH) ₂ ⁺ , Me(OH)_3(aq), and Me(OH) ₄ ^? (where M represents bivalent and Me represents trivalent cations). The speciation pattern of As, Mo, and Se is mainly dominated by oxy-anion forms. The obtained results can be used as a basis for environmental management of the Cu-porphyry mine tailings.     

Second Sound in a Cracked Layer Based on Lord–Shulman Theory

In this study, a cracked layer is considered under thermal shock and is analyzed once by the Lord–Shulman theory and once by classical theory. In this way the effect of second sound of Lord–Shulman theory on a cracked layer is investigated. The Galerkin method is invoked to obtain finite element modeling of the cracked layer. The eight node rectangular element is used and the nodes near the crack tip are replaced to introduce the crack tip singularity (Barsoum element). The discretized form of Navier and energy equations are solved simultaneously in time domain by Newmark integration algorithm. The J integral formulation in dynamical thermal form is implemented to obtain stress intensity factors from the finite element solution of the problem.     

THERMOELASTICITY SOLUTION OF A LAYER USING THE GREEN–NAGHDI MODEL

The Green–Naghdi (GN) linear theory of thermoelasticity of types II (without energy dissipation) and III (with energy dissipation) for homogeneous and isotropic materials is employed to study thermal and mechanical waves in a layer. The disturbances are generated by sudden application of temperature to the boundary. The dimensionless forms of the governing equations are solved utilizing the Laplace transform method. Closed-form solutions are obtained for a layer in the Laplace domain, and a numerical inversion of the Laplace transform method is used to obtain the temperature, displacement, and stress fields in the physical time domain. Thermomechanical wave propagation and reflection from the boundary layer are investigated and the influence of the damping parameter on the temperature, displacement, and stress fields in the GN type III theory is discussed.     

Long‐term hygrothermal response of perforated GFRP plates with/without application of constant external loading

The use of glass fiber‐reinforced polymer (GFRP) composites is increasingly being considered in various applications where the composite is subjected to harsh hot and humid conditions. Although information on the performance of GFRP under hot and humid conditions is available, the characteristics and the response of perforated GFRP under such conditions have not been fully explored. In this article, the response of perforated GFRP plates subject to hot and humid environment is examined. The applicability and accuracy of Fick's model for establishing the amount of moisture absorption by such composites is examined, and an improved model is proposed. The article also demonstrates the influence of constant external loading on such perforated GFRP while undergoing conditioning in a hot and humid environment. Moreover, since the strength and stiffness of composites can be significantly affected by harsh environments, the degradation in the strength and stiffness of the perforated GFRP as a function of time is established. A new model is proposed wherein degradation of the strength of such perforated composites may be established as a function of time and geometric entities. The model can also account for the influence of the applied loading. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

Effect of zirconia on ablation mechanism of asbestos fiber/phenolic composites in oxyacetylene torch environment

Micron-size zirconium oxide (ZrO₂) was used to improve the thermal stability and ablation properties of asbestos fiber/phenolic composites and to reduce their final cost. ZrO₂/asbestos/phenolic composites were prepared in an autoclave by the curing cycle process. The densities of the composites were in the range of 1.64–1.82 g/cm³. The ablation properties of composites were determined by oxyacetylene torch environment and burn-through time, erosion rates and back surface temperature in the first required 20 s. To understand the ablation mechanism, the morphology and phase composition of the composites were studied by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Thermal stability of the produced materials was estimated by means of thermal gravimetric analysis, in air which consisted of dynamic scans at a heating rate of 10 °C/min from 30 to 1000 °C with bulk samples of about 23±2 mg. The thermal stability of the composites was enhanced by adding ZrO₂. The results showed that the linear and mass ablation rates of the composites after adding 14 wt% ZrO₂ decreased by 58% and 92%, respectively. The back surface temperature of a sample with 14% zirconia was 49% lower than that of pure composite. The SEM studies showed that, modified composites displayed much lower porosity than that of non-modified composite and the destruction of asbestos fibers was very low. On the other hand, it appeared that a thin melted layer of ZrO₂ covered the surfaces of zirconia-containing composites.     

Axisymmetric mechanical and thermal stresses in thick short length FGM cylinders

The exact solution of steady-state two-dimensional axisymmetric mechanical and thermal stresses for a short hollow cylinder made of functionally graded material is developed. Temperature, as functions of radial and longitudinal directions, is solved analytically, using the generalized Bessel function. A standard method is used to solve a non-homogeneous system of partial differential Navier equations with non-constant coefficients, using Fourier series, rather than potential functions method.     

Performance of spatially multiplexed MC-CDM with zero-forcing unified successive interference cancellation detection

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.     

Inverse problem of material distribution for desired fracture characteristics in a thick-walled functionally graded material cylinder with two diametrically-opposed edge cracks

This study concerns the inverse problem of evaluating the optimum material distribution for desired fracture characteristics in a thick-walled functionally graded material (FGM) cylinder containing two diametrically-opposed edge cracks emanating from the inner surface of the cylinder. The thermal eigenstrain developed in the cylinder material due to nonuniform coefficient of thermal expansion as a result of cooling from sintering temperature is taken into account. Based on a generalized method of evaluating stress intensity factors developed in a previous study, an inverse method is developed to optimize material distribution intending to realize prescribed apparent fracture toughness in the FGM cylinder. To present some numerical results, a TiC/Al₂O₃ FGM cylinder is considered and the inverse problems are solved to evaluate material distributions for two examples of prescribed apparent fracture toughness. The effect of cylinder wall thickness on the material distribution and comparison of material distributions corresponding to a single and two cracks are also discussed. The numerical results reveal that the apparent fracture toughness of FGM cylinders can be controlled by choosing the material distributions properly.     

Power system stabilizer design using hybrid multi-objective particle swarm optimization with chaos

novel technique for the optimal tuning of power system stabilizer (PSS) was proposed, by integrating the modified particle swarm optimization (MPSO) with the chaos (MPSOC). Firstly, a modification in the particle swarm optimization (PSO) was made by introducing passive congregation (PC). It helps each swarm member in receiving a multitude of information from other members and thus decreases the possibility of a failed attempt at detection or a meaningless search. Secondly, the MPSO and chaos were hybridized (MPSOC) to improve the global searching capability and prevent the premature convergence due to local minima. The robustness of the proposed PSS tuning technique was verified on a multi-machine power system under different operating conditions. The performance of the proposed MPSOC was compared to the MPSO, PSO and GA through eigenvalue analysis, nonlinear time-domain simulation and statistical tests. Eigenvalue analysis shows acceptable damping of the low-frequency modes and time domain simulations also show that the oscillations of synchronous machines can be rapidly damped for power systems with the proposed PSSs. The results show that the presented algorithm has a faster convergence rate with higher degree of accuracy than the GA, PSO and MPSO.