Mojtaba Mahmoodian

Stochastic method for predicting risk of slope failure subjected to unsaturated infiltration flow

The increase of rainfall and rise of groundwater level can cause frequent failures of unsaturated soil slopes that result in catastrophic landslides. This paper proposes a stochastic method for predicting the risk of failure of an infinite soil slope subjected to unsaturated infiltration flow. Stochastic models for shear stress and strength at an arbitrary plane of an infinite slope are developed. The accuracy of the proposed method is verified with the Monte Carlo simulation method. The merit of the proposed method lies in its analytical form, which can easily facilitate practical applications. Furthermore, a risk-based sensitivity analysis is undertaken in this study to identify the factors that are the most random and that affect the slope failure most significantly. It is found in this study that the developed stochastic models can capture the randomness of all factors that contribute to slope failure. It is also found that the slope inclination angle (), soil air entry value (b), water table rise from matric suction (yb), dry soil unit weight (d), and soil-specific constant (a phi) are important random variables in the accurate prediction of slope failure. The paper concludes that the proposed stochastic method can serve as a tool for geotechnical engineers to predict the risk of slope failure with accuracy and ease.

Effect of temperature and acidity of sulfuric acid on concrete properties

Concrete corrosion caused by sulphuric acid attack is a known phenomenon in sewer systems, resulting in significant economic losses and environmental problems. However, there is a scarcity of reported laboratory simulations and experimental work investigating the contributing factors controlling the corrosion. In this EPSRC (Engineering and Physical Sciences Research Council, UK) funded investigation the effect of temperature and the acidity of sulphuric acid solution on concrete specimens extracted from brand new concrete sewers has been investigated. In this investigation the concrete samples are submerged in three sulphuric acid solutions (pH = 0.5, 1 and 2) for 91 days under different temperatures (10oC, 20oC and 30oC). Mass loss and compressive strength of the concrete specimens were tested and recorded at 7, 14, 28, 42, 56 and 91 days providing interesting data for visualising the changes taking place in the concrete samples (change in properties) during the time of immersion. The results revealed that samples overall mass increased at the early stages of the corrosion process. It also was observed that the overall mass of the samples decreased significantly at the later stages of the testing process with respect to the acidity of the solutions used. Although the change in temperature did not have a significant effect on the compressive strength of the tested samples, rise in temperature however, had considerable effect on the mass loss of the concrete samples which were immersed in the most aggressive solution (i.e., pH=0.5 and temperature = 30oC) at 91 days. This research clearly demonstrated a high correlation between the acidity of the solution and the rate of corrosion with respect to time.

Structural integrity of corrosion-affected cast iron water pipes using a reliability-based stochastic analysis method

Abstract A significant proportion of current water networks are composed of cast iron pipes. The occurrence of failures of this type of pipe is increasing due to ageing and progressive deterioration (i.e. corrosion). This necessitates the completion of structural integrity assessment and reliability analysis of in-service cast iron water pipes. The failure of a cast iron pipe can be characterised by fracture, when the stress intensity factor exceeds the toughness of the pipe material. In this research, a stochastic model for the stress intensity factor is developed and a time-dependent analysis method is employed to quantify the failure probability so that the time for the pipe to be failed and, hence, requiring repairs or replacement is determined with confidence. For a comprehensive structural assessment, two types of stresses (hoop and axial) are considered for two cases of corrosion (external and internal). A case study is provided to illustrate the application of the proposed method. In the system failure analysis, it is found that hoop stress is the dominant stress, since it can result in external corrosion, the effects of which are known to be of greater significance than those caused by internal corrosion.

Stochastic failure analysis of defective oil and gas pipelines

Abstract Determination of pipe residual strength is essential to predicting the service life of corrosion-affected oil and gas pipelines and to instigating maintenance and repairs for the defected pipeline system. In this chapter, reliability methodologies for failure analysis of corrosion-affected oil and gas pipelines are presented. The residual strength is considered as the critical parameter that causes failure when it is less than operating pressure of the pipe. A stochastic model for residual strength is developed and time-dependent methods are employed to quantify the probability of failure so that the time for the pipeline to be failed and hence requiring repairs can be determined. Analytical and numerical techniques and gamma-distributed degradation model are introduced and developed for failure assessment of oil and gas pipelines. The methodology can help pipeline engineers and asset managers in making decisions with regard to the safety of corrosion-affected oil and gas pipelines both for assessment of existing systems and for designing new pipelines.

Structural failure assessment of buried steel water pipes subject to corrosive environment

Abstract It is essential to predict the lifetime of buried pipelines since they are not easily accessible for inspection. In this study a time-dependent, non-linear state model has been introduced for the structural analysis of corrosion affected steel water pipes, stressed by external forces. Using limit state concept, the simultaneous effect of externally applied loading and material corrosion are considered through failure modes. A non-linear corrosion model is used to simulate the loss of pipe wall thickness during the operation period. In order to take the uncertainty associated with the design and environmental variables into account, a Monte Carlo simulation technique has been adopted using MATLAB. A parametric sensitivity analysis is also carried out to measure the effectiveness of each parameter on the probability of pipe failure. Results obtained for a steel water pipeline in Eastern Sydney are presented and discussed.