Hazard Evaluation In Tube-Tunnel Construction - Assignment Example

Risk assessment Risk assessment is done at an early phase to enable the selection of a tunnel alignment. Construction project risks may be caused by the construction itself or the impact of the process of construction of the tunnel. Some construction processes are more risk and they may cause severe ground movements (Tonon et al., 2002). Such construction methods cannot be used in some sensitive structures vicinity as they may cause hazards. In the same way, hard spots like rocks, beneath some parts of a tunnel may also cause undesirable effects. Therefore, it is vital to carry out risk analysis early enough to identify the potential or possible risk due to the tunnel alignment and also identify the mitigation measures (Einstein, 1996). For a successful and effective implantation of risk management, there should be quality identification of mitigating action and active involvement, general opinion and experience of the participants that should include the owner, contractors and designers. Risk management cannot be achieved through procedure and enforcement of systems alone, but can be improved through meetings and seminars by disseminating appreciation and knowledge of the objectives of risk management throughout the organization. Risk assessment should be carried at an early stage as shown in the table below. Owner Early design stage Establish risk policy Quality risk assessment Specific (quantitative) risk assessment Project risk register Establish risk policy The main step to begin the management of risk in the construction work is for the owner to formulate risk policy which should indicate the risk objectives, the scope and the strategy of risk management (Eskesen and Kampmann, 2000). The risk policy which expresses the general objectives of risk should be used in the criteria of accepting risk that is used in risk assessment activities plan to be made. This process may include: i) Criteria for risk acceptance that will be used in qualitative risk assessment. ii) Criteria for risk acceptance that will be used in quantitative risk assessment. Each type of risk is be expressed as: The limit above which the risk will be considered unacceptable must be reduced regardless of the mitigation costs. The limit below which the risk is not required taking into account further risk reduction. The area between the lower and the upper two limits where there will be risk mitigation, as well as the mitigation measures to be implemented considering the prevailing circumstances. All risks covered are reduced to a level as low as reasonably practical (Tonon et al., 2002). The policy must emphasis minimization of the overall risk through reducing the probability of occurrence of events of large magnitude, a general statement of risk allocation between parties. Finally, a document that explains the criteria of risk acceptance should be provided (Einstein, 1996). Qualitative risk assessment Qualitative risk assessment is carried out in the early stage of design. It focuses on the on the identification of potential risk hazards to the activities that is expected to be carried out in the construction project. It covers all types of risk indicated in the risk policy. The main objective is to raise the awareness of the parties involved in the major risks in the construction, and to provide a framework for design decision making process in the early design stage (Reilly, 2000). The outcome may be used to select specific topics for further detailed analyses and in the tendering process as a starting point for the risk management. The qualitative risk assessment should be in time so that major design changes can be done early in the design phase. But it possible to update the initial qualitative risk assessment later, depending on the time plan for the design (Tonon et al., 2002). The qualitative risk assessment entails: Identification of hazards. This is done through review of operational experiences of similar construction projects from other parts of the world, the study hazards involved in the current undertaking, and discussions experienced and qualified staff within or outside the organization. Classification of hazards. Classification is done according to the occurrence frequency and the extent of the consequences in each hazard. Identification of measures to be used in risk mitigation. Risk particulars in the construction risk register giving details of risk class and mitigation measures for each hazard risk (Reilly, 2000). The beat way to carry out classification and identification of risk is through a session of brainstorming with risk screening experts that consist of technical, practical and multi-discipline members using risk analysts experience. It involves identification of all possible hazardous events that may threaten the project including low frequency risk and may be significant in the underground as they may have high possible consequence (Eskesen and Kampmann, 2000). Hazard classification and identification should take into account the causes such as: The maturity and the complexity of the technology to be applied. The conditions of undesirable ground and underground water. Management and technical skills incompetence. Human factors and errors. Insufficient coordination and communication during the construction process. A combination of unwanted events that may not be necessarily critical individually (Tonon et al., 2002). The hazards are put into different categories depending on their risk magnitude they represent. This is to provide a structure for the implementation of decisions for risk mitigation measures. Risk classification encompasses consequences and frequencies as well as risk classification in terms of basis of the frequency and consequences. This may also be included in the risk acceptance criteria (Einstein, 1996). One or different groups may be assigned to identify risk mitigation measures and may have the representatives of the groups or the major parties in the project. If the level risk does not fit into the project risk acceptance criteria, then the risk reducing actions must be identified and management decision used to implement the action must be documented and the results is record in the project risk register (Tonon et al., 2002). Essentially, in this project phase, risk mitigation will result in technical changes and possible change a working procedure. In addition, the risk reducing actions may be in the form statements or decisions that have been written in the tender document (Reilly, 2000). It is then possible to determine if the implementation of a given set of risk mitigating actions will reduce the risk to a manageable level. If this is not the case, then, other approaches should be investigated (Tonon et al., 2002). Specific risk assessment If a hazards is of specific interest for example due to the severity of the risk associated with it or due to the importance of the design decision to be undertaken, then detailed risk analysis is carried out. The analysis obtained is recorded in the project risk register (Reilly, 2000). Specific risk management may include the following: A fault tree analysis of the possible causes of the hazard that is used to analyse a single or a combined relation that precedes the event. Through the use of quantifying probabilities of events, complex events with several interacting events may be structured. An event tree analysis which describe the development of an event through possible succession to the final stage. An overall risk quantification aimed at evaluating the cost-benefit ratio of implementing mitigation measures or to provide a quantitative basis used in choosing the best alternative courses of action. (Eskesen and Kampmann, 2000) Risk Assessment Methodologies Construction damage assessments are carried out in three stages (Mair 1996). Contours are first plotted along the project corridor and the existing building along the contour maps. Maximum settlement and angular distortions have been estimated for each structure. The stages are described below. Stage 1 - Preliminary Assessment In this stage, the slope and ground settlement magnitude are estimated. They are categorized into six groups: ranging from 0 - negligible to 5 - very severe. It defines the possible degree of damage. are used to define the possible degree of damage. The table below shows the criteria used for the preliminary assessment. Classification of the building damage (Burland, 1995; and Mair, et al., 1996) Approximate slope and settlement (Rankin, 1998) 1 2 3 4 5 6 7 Risk Description Typical forms of repair Crack width (mm) Tensile strain % Slope of the ground Settlement of building (mm)3 0 Negligible Hairline crack < 0.05 1 Very slight Fine cracks, isolated slight fracture, exterior cracks 0.1 to 1.0 0.05 to 0.075 < 1:500 0.3 1:200 to 1:50 > 75 5 Very severe Major repair or complete reconstruction, walls leans, beams lose bearing, windows broken by distortion, danger of instability > 25 > 1:50 > 75 Stage 2 - Second Stage Assessment The interaction between the ground and the building is analysed through the estimation of the horizontal and shear strains on the building (Boscardin and Cording 1989). This is shown in table above. The graph below shows an alternative method based on horizontal strain and angular distortion. It provides consistent damage classification. It shows the relationship of between Horizontal Strain and Damage to Angular Distortion (Boscardin and Cording, 1989) Graph. Stage 3 - Detailed Evaluation In this stage, detailed evaluation is carried out for buildings classified in Category 3 and above. The stage establishes potential risk induced by construction of the tunnel, the risk consequences and the type of construction. References Read More

If the level risk does not fit into the project risk acceptance criteria, then the risk reducing actions must be identified and management decision used to implement the action must be documented and the results is record in the project risk register (Tonon et al., 2002). Essentially, in this project phase, risk mitigation will result in technical changes and possible change a working procedure. In addition, the risk reducing actions may be in the form statements or decisions that have been written in the tender document (Reilly, 2000).

It is then possible to determine if the implementation of a given set of risk mitigating actions will reduce the risk to a manageable level. If this is not the case, then, other approaches should be investigated (Tonon et al., 2002). Specific risk assessment If a hazards is of specific interest for example due to the severity of the risk associated with it or due to the importance of the design decision to be undertaken, then detailed risk analysis is carried out. The analysis obtained is recorded in the project risk register (Reilly, 2000).

Specific risk management may include the following: A fault tree analysis of the possible causes of the hazard that is used to analyse a single or a combined relation that precedes the event. Through the use of quantifying probabilities of events, complex events with several interacting events may be structured. An event tree analysis which describe the development of an event through possible succession to the final stage. An overall risk quantification aimed at evaluating the cost-benefit ratio of implementing mitigation measures or to provide a quantitative basis used in choosing the best alternative courses of action.

(Eskesen and Kampmann, 2000) Risk Assessment Methodologies Construction damage assessments are carried out in three stages (Mair 1996). Contours are first plotted along the project corridor and the existing building along the contour maps. Maximum settlement and angular distortions have been estimated for each structure. The stages are described below. Stage 1 - Preliminary Assessment In this stage, the slope and ground settlement magnitude are estimated. They are categorized into six groups: ranging from 0 - negligible to 5 - very severe.

It defines the possible degree of damage. are used to define the possible degree of damage. The table below shows the criteria used for the preliminary assessment. Classification of the building damage (Burland, 1995; and Mair, et al., 1996) Approximate slope and settlement (Rankin, 1998) 1 2 3 4 5 6 7 Risk Description Typical forms of repair Crack width (mm) Tensile strain % Slope of the ground Settlement of building (mm)3 0 Negligible Hairline crack < 0.05 1 Very slight Fine cracks, isolated slight fracture, exterior cracks 0.1 to 1.0 0.05 to 0.075 < 1:500 0.

3 1:200 to 1:50 > 75 5 Very severe Major repair or complete reconstruction, walls leans, beams lose bearing, windows broken by distortion, danger of instability > 25 > 1:50 > 75 Stage 2 - Second Stage Assessment The interaction between the ground and the building is analysed through the estimation of the horizontal and shear strains on the building (Boscardin and Cording 1989). This is shown in table above. The graph below shows an alternative method based on horizontal strain and angular distortion.

It provides consistent damage classification. It shows the relationship of between Horizontal Strain and Damage to Angular Distortion (Boscardin and Cording, 1989) Graph.

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