Steelwork well advanced on unit No. 1
safety against flotation are: ignoring backfill including specified depth of backfill
: 1.05
: 1.25 In both cases the factor of safety must be realised under either normal working conditions (water level + 8.50 P.D.) or abnormal working conditions (water level + 16.00 P.D.)
Plant and finishes
When the placing and jointing of units is under way a start will be made on installing plant and applying finishes. Major items of plant are: Ven- tilation fans and ducting; lighting; traf- fic control equipment; fire-fighting and emergency equipment and toll col- lection equipment.
Fans will be housed in the two ven- tilation buildings, one on either side of
PRIES
the harbour. Fresh air will be forced through the ducts above and below the running tunnels. It will pass into the running tunnels through grilles and dis- place contaminated air which will be exhausted through the portals (a 'semi- transverse' ventilation system).
At maximum output the ventila- tion plant will be capable of supplying fresh air at about 2.6 million cu.ft/ min. which corresponds to an average of about 100 cfm/ft/lane of tunnel.
The tunnel will be monitored to check the CO concentration to ensure that it does not exceed the permissible peak value of 400 ppm or a mean value, measured over 30 minutes, of 250 ppm. In practice it has been found that visibility is more critical than CO concentration and is generally the cri- terion by which the plant is operated.
Lighting will be provided by a con- tinuous line of fluorescent tubes posi- tioned in the centre of the ceiling units in each running tunnel. The general level of illumination during the day will be 15 lumens/sq.ft. Dimming equipment will be provided so that this can be varied as required. Lighting at the portals will be modified so as to assist the eye in adjusting to changing levels of illumination. Louvres will be provided externally for the same pur- pose.
Traffic control will be by means of traffic lights, illuminated signs, closed- circuit television and traffic patrol- men. Apart from lights and signs with- in the tunnel other signs will be sited on the approach roads to signify whe- ther or not the tunnel is open to traf- fic. The traffic patrolmen will control traffic within the limits of the tunnel and ensure that tunnel by-laws are complied with.
Fire fighting and emergency equip- ment will consist of charged firemains with twin outlet hydrants at 175ft. centres, fire points with alarms and ex- tinguishers at about 175ft. centres, emergency telephones at 175ft. cen- tres and a special recovery truck fitted with fire-fighting and all other neces- sary equipment for dealing with an emergency in the tunnel and capable of towing any disabled vehicle likely to be encountered.
The tunnel walls will be lined with glazed tiles as these are durable, easy to maintain and can be washed down. The walls of the south ramp will also be tiled while the walls of the north
Batching plant
Far East BUILDER, June 1970
33