On the necessity to improve the functions and security of the ventilation systems of major importance underground structures
|Location||20th WORLD MINING CONGRESS 2005|
|Author||Nicolae Ilia؛--Iosif Andras-- Dorin Adam--Sorin Radu-- Omar Lanchava --Roland Moraru --Livia Adam|
|Holding Date||14 January 2006|
The paper deals with the theoretical basis of the assessment of vulnerability of man- made underground structures used in mining and civil purposes (such as subway networks and stations) and of their preparedness to avoid technical accidents or malevolent actions as well as the capability of their impact mitigation. On the basis of this assessment, the mitigation of the harmful effects of accidental or malevolent events affecting the quality of internal atmosphere and the prevention of critical situation issued from this kind of events is discussed at conceptual level.
Key words: ventilation system, atmosphere, security, function, underground structure
1. THE GENERAL OBJECTIVES OF UNDERGROUND AND SUBSURFACE VENTILATION
A large diversity of man made structures in underground or subsurface with different extents, purposes and with different density of people and/or potentially hazardous materials exists. From large building subsurface facilities, to tunnels, subway network and stations, electric stations, warehouses, waste deposits, until recently realized underground particle accelerator laboratory. They have in common the closed character, limited contact connection with the external environment, and the compulsory necessity to have a permanent human friendly atmosphere, realized generally by artificial ventilation. The basic objective of an underground ventilation system is clear and simple. It is to provide air flows in sufficient quantity and quality to dilute contaminants to safe concentration in all parts of the facility where personnel are required to work or travel. The manner in which Quantity and quality are defined varies from country to country , depending upon their mining history, the pollutants of grater concern, the perceived danger associated with those hazards and the political and social structure of the country. The overall requirement is that all persons must be able to work or travel within an environment that is safe and which provides reasonable comfort. This perception of “reasonable comfort” sometimes causes misunderstandings between subsurface ventilation engineers, and those associated with heating and ventilating industry for buildings. While maintaining the essential objectives related to safety and health subsurface environmental engineering has increasingly developed a wider purpose. In some circumstances atmosphere pressure and temperature may be allowed to exceed the ranges that are acceptable for human tolerance . For example, in an underground repository for high level nuclear waste, a containment drift may be sealed against personnel access after the setting of waste containers has been completed. However, environment within the drift still be maintained such that rock wall temperatures are controlled. This is necessary to enable the drift to be reopened for retrieval of nuclear waste at any time during the active life of repository.
2. FACTORS THAT AFFECT THE UNDERGROUND ENVIRONMENT
During the development of a mine or other underground facility (structure) such as subway, networks and stations, civil structures, underground structures, for special purposes, potential hazards arise from gas emissions,, dust, heat, and humidity, fires, explosions, and radiations. Fig. 1 shows the factors that may contribute towards those hazards. These divide into features that are imposed by nature and those that are generated by design decisions on how to develop and operate the facility. The major method of controlling atmospheric conditions in the subsurface is by ventilation, airflows. This is produced primarily, by main fans that are usually, but not necessarily , located on surface. While the main fan or combination of main fans handles al the air that circulates through the underground network of airways underground booster fans serve specific districts only. Auxiliary fans are used to pass air through ducts to ventilate blind headings. The distribution of air flow may further be controlled by ventilation doors, stopping, air crossings and regulators. It is often the case that it becomes impracticable or impossible to deal with all environmental hazards only by ventilation. Figure 1. includes the ancillary control measures that hat may be advisable or necessary to supplement the ventilation system in order to maintain acceptable conditions underground. The design of a major underground ventilation and environmental control system is a complex process with many interacting features. The principles of system analyses should be applied to ensure that the consequences of such interactions are not overlooked. However, ventilation and the underground environment should not be treated isolated one to other during the planning period. They are themselves integrant parts of the overall design of the mine or subsurface facility. It has been often the case that types , numbers and sizes of the machines or questions of ground stability have dictated the layout of the underground openings without initially taking into account the ventilation requirements. This will result in a ventilating system that may lack effectiveness and at the best will be more expensive in both operating and capital costs than would otherwise have been the case. Another frequent, related problem is a ventilation infrastructure that was adequate for an initial layout but lacks the flexibility to handle fluctuating demands or to be able to respond properly in emerging situations such as major accidents (fires and explosions) and /or malevolent actions such as terrorist attacks using chemicals or biologic agents.
3. A FRAMEWORK FOR A SYSTEM APPROACH TO TERORISM RELATED RISK ASSESSMENT AND MANAGEMENT
When modeling terrorist related risk on homeland systems, knowledge of the associated architectural framework including its characteristic state variables is essential. In fig. 2, threats from terrorist attacks constitute a key input. These threats can be understood and modeled only when we identify and understand the societal environment, the geopolitical dynamics within which the terrorist networks are operating and are energized, the characteristics and limitations of the threatened infrastructure. The causal relationship among these inputs and outputs than enable the building of models that can predict the efficacy risk management policy options. An essential factor for sound decision–making is identification of risks at a sufficient level of detail. This will enable effective strategic and tactical planning and design of the safety system which will operate within the underground structures. The entire process of risk assessment and management is an objective and subjective issues, in the risk assessment stage the analyst often attempt to answer the following three questions:
- What can go wrong?
- What the likelihood that it will go wrong?
- What are the consequences?
Answers to these questions helps risk analyst to identify, measure, quantify, and evaluate risks and their consequences.
Risk management builds on the risk assessment process by seeking answers to a second set of three questions:
- What can be done and what options are available?
- What are the trade -offs in terms of all costs , benefits and risks?
- What are the impacts of adopted decisions on future options?
The generic process for managing risk shown in figure 3 is a management process. It can be applied at any stage in the life of a program, project or activity . It should be applied where possible at the beginning of any major new project or change in operational environment. The risk management process can be applied at all levels, namely strategic, operational and tactical. A further sound practice is to avoid the temptation of leaving out elements of the risk management process. All too frequently managers , when confronted with problems which demand quick solutions , succumb to the imperative of haste and move directly from risk identification to treatment of the risk. By omitting consideration of the context and the performance of proper analysis and prioritization it appears probable to implement costly ill-conceived and inadequate prevention and mitigation systems.
4. GENERAL CAPABILITIES NEEDED TO IMPROVE THE FUNCTIONS AND SAFETY OF THE VENTILLATION SYSTEMS IN ORDER TO MITIGATE THE VULNERABILITY OF UNDERGROUND STRUCTURES
Taking into account the wide potential range of risk factors which can act in the underground environment, we will refer in the following considerations only to the hazards induced by toxic chemicals and explosive materials. Chemical continue to be weapons of choice for terrorist attacks. They are readily available and have the potential to inflict significant casualties. The most plausible use of chemicals as weapons is in attacking aggregations of people in enclosed spaces with narrow exit ways (e.g. subways) in way that would cause disruption to crucial infrastructure services or render them unusable. The ventilation system could be used itself to spread, for instance to toxic gases. A wide variety of chemicals including many of common use could be used as weapons. There are three major classes of such chemicals:
- Chemical weapons developed by states for military use. Some biological and radioactive agents can be considered as chemical weapons because the response to attacks involving them would be similar (e.g. botulinum toxin, enterotoxin and dispersible radioactive materials);
- Toxic industrial chemicals that are produced , transported and stored in large quantities in the civil economy
- Explosives and highly combustible materials.
Underground utility tunnels or sewers could be used as conduits for releasing toxic , flammable or explosive materials. Chemicals could disperse through these systems and eventually emerge from manholes, drains and other openings. Another potential dispersal mechanism is a subway system. Materials in the subway tunnel could be pumped through the city by the trains –a particularly effective method for delivering powder-like materials as anthrax, but it might also work for spreading chemical agents.
Several techniques which can be employed to improve the functions and security of the ventilation systems of major importance underground structures are discussed below.
Sensors and operational systems for detection and identification of chemical agents
Improved and expanded use of sensors must play a major role in preventing catastrophic events and, if they occur, in minimizing their consequences.
Possible applications include the following:
· Improved sensors to detect explosives and chemicals.
· Sensors to help provide sensitive and rapid warning for the protection of fixed sites. For example, sensors for ventilation systems capable of detecting deviations from normal conditions and monitoring of chemical and biological agents could be coupled to rapid shutdown procedures.
Figure 1: Factors that feature in the creation and control of hazards in the subsurface environment
· Portable sensors to allow first responders to assay levels and types of hazard at a distance, without themselves becoming casualties.
· Sensors to be used in mapping the extent of a volatile agent and to guide civil authorities in controlling population movements.
· Sensors to assess the level of contamination following an event and, more importantly, to determine when a site is safe and can be return to normal function.
In addition, sensor systems will require a number of different subsystems, including sample collection and processing, sophisticated signal processing and amplification of the transduction events.
Data processing networks
In many cases, efforts to prevent undesired events will involve large data streams. It is important to be able to efficiently process the data for useful information, so as the quickly distinguish patterns of actual threats from noise or natural events and to adopt the appropriate measures and decisions concerning the ventilation’s system operation. Advanced computational techniques, identification of interdependencies among the ventilation systems elements and development of software and data-analysis capabilities that make use of latest developments in computer hardware are also required. The ability to test the system against real-time data to determine model fidelity for particular structures and interdependencies between different infrastructures is critical.
Locating in strategic points of the ventilation system improved filters, absorbents, scrubbers and membranes for chemical decontamination and restoration of function.
In most cases the impact of a major accident or of a chemical attack would be limited to the harm done at the time of occurrence. Contamination by a volatile agent (e.g. sarin) presents the problem of removing a toxic vapor without releasing in into the outside environment. Further research is needed to identify more effective technologies for removal of contaminants from air. These technologies are likely to be specific for the contaminants involved and for the media in which they are dispersed. Chemical, biological or nuclear contaminants may be present as aerosols (particles of solid or liquid) in air, or they may be homogeneously dispersed as gaseous contaminants in air. Particles may be removed by filtration, with the specific technology depending on particle size. Several technologies are available. Improved high-efficiency and low pressure-drop filter systems (such as HEPA – high efficiency particulate air) could be extremely useful in rapidly treating large volumes of particle-contaminated air. Homogeneously dispersed contaminants may be removed by absorption, chemical reaction/neutralization or selective membrane filtration. Absorbers and filters can be used both to prevent toxic chemicals from entering a facility through the ventilation system and to decontaminate a structure after an undesired eventAnother area of need is for better methods to contain and neutralize clouds of airborne toxic materials such as ammonia, chlorine, hydrogen fluoride, hydrogen sulfide and sulphur dioxide. Work to date has shown that large quantities of water must be sprayed in the air to “knock down” any significant portion of such airborne chemical clouds. In addition to the potential threat of contamination, ventilation systems are vulnerable to physical damage that could easily lead to disrupt of service. While not necessary catastrophic, this disruption could have serious effects on the economy and on public confidence. The systems need to be highly redundant so that failure of one or more components does not lead to a major disruption.
1. High Impact of Terrorism , Proc. Of a Russian-American Workshop, National Academy Press, Washington D.C., http: //www.nap.edu/ books/ 0309082706/html/R1.html
2. David W. Siegrist Advanced technology to counter biological terrorism, A presentation to the international conference on threats in the technological age Holon, Israel march 18, 1998
Figure 2: A framework for systems approach to counterterrorism
(ASCE Journal of infrastructure system, vol8, No.2, June, 2002)
Figure 3: The structure of the risk management process