502.1. General 1. Pits, shaft tunnels and headings shall at all times be kept ventilated to maintain an atmosphere fit for respiration and free from oxygen deficiency, potentially explosive or noxious gases and dust, whether present naturally or otherwise.
Ventilation shall also be used to maintain a safe working temperature.
2. The Contractor shall take proper precautions to ensure that the Works are kept in a safe and workable condition throughout.
In all tunnelling operations the Contractor shall comply with the relevant recommendations of BS 6164 Code of practice for safety in tunnelling in the construction industry and HSE Guidance Note EH 40 Occupational exposure limits.
3. The Works shall be undertaken in a way that ensures compli-ance with The Best Practice Guide for Occupational Exposure to Nitrogen Monoxide in a Tunnel Environment by the British Tunnelling Society.
4. Where more than one pollutant is present any adverse inter-action between them shall be identified and mitigated.
5. In underground workings and in confined spaces the air breathed by persons shall contain not less than 19% of oxygen by volume.
6. Smoking is forbidden in tunnels, headings, pits or shafts and all confined spaces.
7. In rock excavation all drill holes shall be wet drilled.
502.2. Ventilation systems
1. The Contractor shall agree ventilation proposals with the Engineer. Agreement shall not relieve the Contractor of his obligations under the Contract.
2. Proposals shall include but not be limited to the types of fan employed, siting arrangements where appropriate, the power supply and the fan performance data, together with duct characteristics.
3. In forcing systems, fans shall normally be placed on the surface.
4. If booster fans are to be employed by in-line staging, they shall be of an approved flameproof (FLP) construction and a mon-itoring system shall be installed so that the status and condition of such fans can be monitored at all times.
5. The inlet to any surface forcing fan shall have unobstructed access to fresh air. It shall not be in the vicinity of a storage site for oil, chemical or diesel drums. The fan shall also be
sited so that it cannot draw in internal combustion engine fumes or gas from charging batteries.
6. Provision shall be made for the fan to be run continuously whether persons are within the underground works or not.
7. Where a fan has been stopped and restarted, the condition of the air shall be tested before personnel enter the tunnel. If only forcing surface-mounted fans are employed, the ventila-tion system should be restarted and run continuously ensuring that any plugs of oxygen-deficient, flammable or noxious mixtures of gas are flushed out. Care should be taken that workmen do not encounter any plugs of these gases on re-entry to the tunnel. The Contractor should take into account that air residence time in long drivages can be several hours and that layered gases of different densities are difficult to disperse, especially where the gradient of the tunnel changes.
8. The outlet of the duct shall be kept as close to the face as is practicable, designed to avoid turbulence and creation of dust and not more than 10 m away.
9. Where dust is being produced by the tunnelling system, exhaust ventilation shall be used to extract such dust from the working area.
10. Tunnelling shall not continue more than 10 m from the shaft or pit unless positive ventilation has been established.
502.3. Monitoring 1. Atmospheric monitoring equipment shall be positioned at each working face, inbye of each airlock, and also within 20 m of the tunnel entrance when the tunnel has advanced 250 m or more.
Monitors shall also be provided every 500 m along the tunnel.
Monitoring equipment shall be capable of continuously moni-toring the levels of potentially explosive gases, toxic gases and radioactive gases as appropriate and the oxygen content. The equipment shall give both visual and audible warning of the presence of potentially explosive, radioactive or toxic gases and where the oxygen content falls below safe working levels.
An immediate and effective means of communicating warnings to the surface shall be installed. The atmospheric monitoring system shall be a fixed system supplemented by portable mon-itoring equipment as necessary, except in small tunnels where the use of portable equipment only shall be permitted at the dis-cretion of the Engineer.
2. Each working shaft and the full length of all tunnels shall be mon-itored continuously in accordance with Clause 1 of Section 502.3 for the presence of explosive or noxious gases or lack of oxygen.
Records shall be kept of monitoring results. Should the workings be found to contain explosive or noxious gases above the level set out in BS 6164 or HSE guidance document EH40, or oxygen con-tent below the level set out in BS 6164, all work shall stop and the Works shall be evacuated until a safe atmosphere is established.
502.4. Start-up ventilation
1. If the ventilation system is for any reason not in operation for a period greater than 2 hours, a start-up procedure shall be
invoked. This requires that the shaft and tunnel shall not be re-entered until one complete air change in the tunnel has taken place and the tunnel atmosphere shown, by monitoring, to be safe.
2. Persons re-entering after shutdown must carry instruments to detect for the presence of dangerous gases and the sufficiency of oxygen, and these must be used continuously during re-entry.
502.5. Checking and inspection
1. During each shift, the following checks shall be made:
(a) The fan or fans shall be checked for heat, unusual noise and vibration. The results shall be reported and remedial action taken if required.
(b) The ventilation ducting shall be checked for damage and the joints checked for integrity. The results shall be reported and remedial action taken if required.
(c) The atmospheric monitoring system shall be checked at both local and remote stations and the results recorded.
2. The air flow quantities shall be checked at both the face and 20 m from the shaft bottom on a weekly basis. These figures shall be recorded and compared with the calculated flows.
Any shortfall shall be made good.
3. The ventilation records shall be maintained and be made available for inspection by the Engineer.
502.6. Ventilation failure
1. In the event of ventilation equipment failure all personnel shall be withdrawn from the underground workings.
2. In the event of ventilation equipment failure, where a tunnel boring machine is in use, it shall automatically be stopped and isolated until the ventilation is restored.
502.7. Ventilation after breakthrough
1. After tunnel breakthrough, ventilation facilities designed to ensure safe atmospheric conditions throughout the tunnel system shall be installed.
503. Lighting
503.1. General 1. Floodlighting on the site surface shall be adequate for the safe operation of the site. It shall be shrouded where necessary to ensure the light is directed to areas within the site, and to avoid nuisance.
2. Lighting in the tunnel shall extend the full length and not be less than that required for safe working and access.
3. An alternative source of power and emergency lighting system shall be provided to allow emergency securing operations and evacuation safely in the event of a primary power failure. An adequate number of hand lamps shall be located at key points underground.