Introduction:
Underground structures are characterised by the fact that their outer walls are tightly joined to the soil. Those being deeper than one floor beneath the ground normally have neither celler windows nor outside of the use of the vertical passages such as stairways and elevators.
The inaccessibility of underground structures due to smoke generation in the event of fire results in some unique problems and difficulties. Primary among these are:
a) Difficulty in venting smoke, heat and toxic products of combustion.
b) Difficulty in fire fighting and evacuating occupants.
c) Difficulty in making any appraisal of the fire conditions or about whereabouts of the fire itself.
d) Difficulty in communication between personnel below ground and also between persons on the surface and below ground.
e) Difficulty in effectively applying extinguishing media.
f) Unusual congestion and restriction of movement in the interior of the underground space.
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Some peculiarity of fires in underground structures due to restricted supply of air are also noticed. These are:
i) These fires generate dense smoke and carbon monoxide due to reduced supply of air/oxygen in the fire zone and the retained hot gases pyrolize the combustible materials without subsequent complete combustion. Thus heat, smoke and poor visibility make the work of fire fighters more difficult and often prevent them for quickly locating the seat of fire.
ii) The burning rate of materials in fire area may, however, reduce to such an extent due to oxygen hunger, that it changes into a true smouldering fire.
iii) Reversely there is a danger of combustible gases flaring up at once, as soon as air gets into the room.
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These are the main reasons why fire fighting in underground floors is one of the most difficult task of the fire fighters.
The problems and difficulties stated above supported by inaccessibility in the underground structures eliminates any thought of partial fire protection. Protecting occupants and properties in underground structures demands maximum utilisation of good fire protection.
2 Constructional Features:
Some essential features to be considered for construction of underground structures are as follows:
a) The use of combustible constructional materials and other than class- A interior finish should be prohibited.
b) All underground floors shall be compartmented with area not exceeding 500m² by a separation wall with atleast 2 hours fire rating. Where this is not possible the spacing of sprinklers shall be suitably reduced.
c) At least 2 hours fire rating encloses types staircase shall be made at ground level only from open air and in such position that smoke from any fire in the basement shall not obstruct any exit serving upper floors of the building.
d) Building services such as electrical sub-station, electrical boiler room, etc. in underground floors shall comply with the provision of Indian Electricity Acts and Rules.
e) In multistoried underground structures, intake ducts may serve all basement levels, but each basement levels and basement compartment shall have separate smoke ducts having the same fire resistance rating as the compartment itself.
f) Lift shall not normally communicate with the underground floors. If, however, lifts are in communication, the lift lobby of the underground floors. If, shall be pressurized with self-closing doors in accordance with relevant standard in their respect.
g) Adequate means of escape from the underground structures shall be provided in accordance with relevant codes and standard specification.
3.Smoke Venting :
Undoubtedly smoke and hear represent the greatest impediment of effective rescue and fire fighting operations. Therefore measures improving the visibility of the seat of fire deserve first priority. This requirement may be met by providing effective means of venting.
Opening of doors and windows (where available) may provide for natural smoke removal in the form of grills, breakable stall board lights, or pavement lights or by way of shaft, but this not usually lead to a complete control of smoke flow.
A mechanical ventilation system is the only means that garauntees reliable removal of smoke. Mechanical ventilation system combined with extinguishing systems have essential advantages over natural venting. If the underground floor(s) of a building is supervised by smoke detectors and protected by automatic sprinkler systems, the following control is generally recommend:
a) Upon response of a smoke detector, switch off the ventilation system.
b) Upon release of sprinkler installation, switch on smoke or heat vents.
4.Fire Protection arrangements:
a) Automatic sprinklers must be installed in all underground structures having floor area exceeding 200m², excluding any area to he used for electric substation, air condition plant room, diesel generator room, etc.
b) Other manual fire fighting equipment such as stand pipe and hose systems, hose reels, fire extinguishers, etc. shall be installed in the underground floor(s) in accordance with relevant standard for quick extinguishments of fire in its incipient stage.
5.Fire detection and alarm systems:
a) In the underground structures, automated warning systems with smoke detector is best suitable for early detection without human intervention. The system shall fulfill the requirements of standard specification in this respect. This system may be coupled with the operation/stopping of venting system, sprinkler installations, etc. by suitable design techniques.
b) Manually operated electric fire alarm system with break-glass type call boxes at suitable locations shall also be installed in accordance with relevant standard specification.
6.Important points of consideration:
a) Drainage of water from sprinkler discharge or from hose stream can be serious problem in the underground floors and therefore, requires early planning of removal water.If drainage is provided for reasons other than fire protection, it may be possible to use one drainage system for all purposes, provided it is designed to handle the expected maximum flow.
b) Natural illumination is limited or non-existence in the underground structures. This issue has an impact upon egress and emergency operations within the basements. As such, effective artificial lighting from two separate source of supply shall be ensured in the underground structures, specially the escape routes.
c) Iluminated directional and exit signs shall be displayed prominently with broad and bright letterings in conspicuous positions to guide peoples in coming out from the underground floors in case of any emergency.
d) Fire notices/orders shall be prepared befitting for underground structures and shall be displayed in prominent and lighted positions for informing people what to do in case of fire or emergency.
e) Good communication system between underground floors and ground floor control room shall be established to facilitate fire fighting operation and also to achieve quick evacuation of the floor.
f) For keeping all fire fighting and smoke venting systems in perfectly working condition, periodical inspection, checking and testing of all such equipments and installations shall be done by competent person (s) in this matter.
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