Risk assessment in sustainability of dredging projects by considering uncertainties

Master's thesis in the field of civil engineering

Soil and soil mechanics trend

Abstract

Dredging projects in urban areas are always at risk. Therefore, a suitable safety margin should be considered in the design of the guard structure. Traditional slope stability assessment methods, which are usually based on empirical judgments such as the concept of confidence factor and limit equilibrium method, do not consider uncertainties. Efforts to quantify existing uncertainties and risks led to the emergence of risk-based approaches. Reliability analysis and risk analysis are developed methods to overcome this problem. In this method, the reliability coefficient is determined based on uncertainty parameters and acceptable risk level. This method can be used for risk assessment and necessary measures to control risks. By using these methods, the risk can be assessed and necessary measures can be taken to control the risk. The Monte Carlo simulation method is one of the probabilistic evaluation tools that includes many uncertainties in order to measure the safety and stability of excavation trenches. In the current research, the safety of the nailed wall was investigated using the probabilistic method of Monte Carlo simulation, and the reliability index and probability of failure were also calculated.

Key words: excavation, uncertainty, risk assessment, Monte Carlo simulation, nailing, reliability analysis, limit equilibrium

Chapter 1

General

Chapter First-General

1-1- Introduction

With population growth, the need for construction increases and this is only possible with the expansion of civil infrastructures. On the other hand, the lack of high-quality land (land whose soil has suitable mechanical properties for construction) in densely populated cities, as well as the high cost of land in some areas, has caused the construction to spread below the ground level. In some cases, due to the presence of obstacles such as the presence of buildings and underground facilities in the neighboring properties, excavations are done vertically. It is obvious that in order to prevent possible accidents, excavation operations must be accompanied by stabilization. If the safety factor [1] is not taken into account in the stabilization design of the excavation wall, it can cause irreparable human and financial losses. The occasional news of nearby buildings toppling during excavation shows the risk-taking and importance of this issue. On the other hand, the factors that are involved in determining the confidence factor also have uncertainty[2], in the sense that with the change of each of these factors, the confidence factor is also changed, as a result, the risk tolerance and risk probability[3] also change. If this issue is ignored in the designs, it can impose unwanted costs. It should be noted that very large reliability coefficients also increase stabilization costs. Therefore, the accurate and informed estimation of the reliability factor, in addition to safety aspects, can also economically reduce the cost of these projects.

Deep trenching on the Nicol Highway [4] in Singapore is an example of a huge geotechnical project that led to a disaster a few years ago. This excavation was being carried out at a depth of 30 meters and with a width of 10 to 15 meters in Marni clay, with the bracing of the diaphragm wall, when on the afternoon of April 20, 2004, the wind beam on top of it broke and the result was a 110 meter long land fall. which caused the collapse of Nicol highway in that area. Also, the large movement of soil caused an explosion in the gas supply pipes and a fire. 4 people died in this incident. Unfortunately, in recent years, similar incidents have happened in our dear country, Iran. The expansion of excavations and the possibility of such incidents show the importance of this issue. The analysis of geotechnical structures based on risk assessment[5] is a subject that has recently attracted the attention of researchers. The reason for this approach is the existence of non-deterministic parameters or uncertainties in geotechnical issues. Because the existence of uncertainties leads to unreliable designs. Therefore, the degree of uncertainty of the design and the risks caused by this design should be evaluated in order to reduce the amount of damages with risk management [6]. Risk management is the systematic application of management procedures, practices and policies to identify, analyze, evaluate, reduce and monitor risk.. Risk reduction is the use of appropriate methods and management principles to reduce the probability of an event or its adverse consequences, or both. Estimating risk and comparing it with acceptance criteria (quantitative or qualitative) is an integral part of risk management.

1-2- Statement of research topic

Researchers have always sought to quantify phenomena and use probabilistic theories to model and analyze them. The quantification of phenomena and the use of probability theories were considered in the 16th and 17th centuries. The issue of risk and its management was also raised a few years after that and it quickly received attention in various sciences, but its current form was raised after 1960, which led to the emergence of insurance. The principles of risk assessment and management have been applied more formally to urban areas at risk of landslides and slope control around highways since the 1970s. In the 1980s, and especially in the 1990s, with the introduction of quantitative methods, pipeline route risk management and especially slope risk management developed. Many researchers such as Warrens (1984), Whitman (1984), Einstein (1988, 1997), Fell (1994), Leroy (1996), Wu et al. (1996), Fell and Hartford (1997), Nadim and Lacasse (1999), Hu et al. (2000) Waltsed et al. (2001), Nadeem et al. (2003), Nadeem and Lacasse (2003, 2004), Hartford and Beecher (2004), and Lee and Jones (2004). have played a role in this development. Recently, studies have been conducted with the aim of improving risk factors, emphasizing the importance of uncertainty analysis and confirming probabilistic methods as a useful tool for decision-making. One of the important features in risk assessment is that subsequent decisions are facilitated by analyzing different risk modes (for example, cost-benefit analysis[7]). This research explains how to use probabilistic methods to describe uncertainty and risk assessment as an analytical tool for decision-making. 1-3- Design based on risk assessment [8] As stated, all engineering designs are faced with uncertainty. Uncertainty is evident in material specifications, operating conditions, engineering models, and so on. In fact, due to these uncertainties, geotechnical engineers consider the design capacity more than the required value of the project. The ratio of capacity to demand (reliability factor) is usually chosen based on experience. This method has major drawbacks. For example, this method is conservative. As a result, the overconfidence factor in the design is unknown. Although conservatism in estimating soil characteristics and forces seems reasonable. Due to the variety of uncertainties, a fixed confidence factor in different problems leads to different failure probabilities. A design based on risk assessment can cover some of the limitations of the definite confidence factor. Design based on risk assessment means trying to quantify the inherent uncertainties of an engineering problem and how to deal with them. "Design based on risk assessment" in geotechnical engineering is divided into two parts: data analysis and model structure. In the data analysis part, uncertainties are identified and quantified using statistical relationships. In the model building phase, mathematical relationships are used to evaluate the effect of uncertainties in calculations. Another result of risk-based design is quantifying the reliability of the structure. This value is called "reliability index [9]".

1-4- Comparison of traditional methods and probabilistic methods

The piedra analysis of slopes is traditionally based on determining the reliability coefficient. Geotechnical experts rely heavily on empirical judgments, such as the concept of confidence factor, to assess the stability of a suitable slope for development. The reliability coefficient of slopes is defined as the ratio of the shear capacity at the critical failure level to the shear stress on that level. In other words, the reliability factor measures the ratio of resistance that must be reduced so that the slope reaches the definite failure point. Recently, it has been found that the value of the confidence factor does not necessarily predict good slope stability performance. One of the limitations of using the confidence factor is the existence of uncertainties in soil resistance parameters. If it is possible to define the variability of the stability analysis input parameters such as adhesion, friction angle and soil specific gravity in the form of probability density, the confidence coefficient of the slope will also follow a probability density.