if air losses are high, otherwise condensation of ex­ panding air cuts visibility in the tunnel and interferes with progress and setting out.

A high pressure supply is required for air tools, boogee pans and grouting. Water, electricity and com­ munications are also needed at the face, together with safety monitoring equipment for methane and carbon dioxide, and Budenberg-type individual air lock press­ ure gauges.

The passage of men and materials into and out of the tunnel is through air locks, which are chambers with double airtight doors in which the air pressure can be raised or lowered without affecting the pressure inside the tunnel. Air locks are either bolted to the top of the working shaft or are formed with double bulkheads in the tunnel.

Tunnels in soft ground are normally driven using a circular steel shield. This is fitted with a cutting edge and a skin plate and is driven forward by jacks reacting on the most recently fixed tunnel lining. A short head­ ing is first dug by hand or compressed air tools in front of the shield, the shield is moved forward on the jacks and the excavated material is removed. A further ring of lining is erected inside the skin plate when the jacks arc withdrawn.

The structural tunnel lining may be formed with ribbed cast iron or concrete segments. The meeting faces are a close fit, but the joints are caulked with lead or plastic to make the lining watertight. The segments have screwed plug holes through which grout is pumped to ensure full contact between the segment and the soil outside. When the lining is complete, caulked and grouted, air pressure is reduced to atmospheric

surface.

As with culverts, the cooling water tunnels and shafts require a smooth surface to the lining, and bends have to be properly designed to eliminate hydraulic losses. The size of tunnel has to take into account the designed velocity of flow, but in some cases may also depend on the size of tunnelling shield available or on an economic decision being taken to standardise the intake and out­ fall tunnels.

The CW tunnel is completed by forming the shaft at the outboard end connecting it to the source of cool­ ing water. This shaft can be sunk independently, raised from the tunnel or dropped in a cofferdam to meet the tunnel. Numerous ingenious methods of doing this have been devised (see Fig 3.34), but it is difficult to devise a method which allows the shield to be recovered and the tunnel to be dewatered on completion.

As a consequence of efforts to avoid the risks inherent in shield driven tunnels, the development of submerged tube tunnels and pressure balanced drilling mud shields has been rapid, but neither can be said to be an all purpose answer in any ground or water conditions.

Direct cooled circulating water systems

9.5 Submersible cooling water structures

In parallel with the incentive to avoid risky, unpredict­ able and labour intensive tunnelling work has been the growth of the capital plant industry capable of operat­ ing in shoaling tidal water. The range of dredgers, tugs and floating cranes available and the experience gained in estuarine road tunnel construction makes their appli­ cation to offshore structures for cooling water the next logical step.

Plant currently available can dredge to 20 m depth, provide 12 000 kW of towing capacity per unit and lifting capacity of over 50001 per hook; and the development of more powerful equipment is underway.

it and back-filling has become much simpler than at­ tempting to do the work piecemeal under the sea, even though the weather is still a major risk to plant, trench, units and men.

Intake and outfall structures can be built in the same way, although the tunnel element design requires detailed analysis or modelling to ensure that they are stable under tow or when being lowered. In the per­ manent condition, if the tunnel is required to be dewatered, it is often the positive buoyancy which constrains the final .design.

With the floating of large capacity plant coming available and dredging costs falling, it is becoming more attractive to consider floating ever larger sections of CW systems, or indeed other modules for station construction, and bringing them to site by water.

9.6' Maintenance considerations

Civil engineering works associated with the cooling water system should be designed to be maintenance free for the first generation of plant. Exposed metal surfaces, screens, gates and valves will need inspection, and if any form of cathodic protection is incorporated it will need regular inspection and maintenance.

Apart from this routine servicing of metal surfaces it should be possible with good design to achieve a balance of water velocity, chlorination and concrete quality control so as to limit maintenance to inspection only, unless the cooling water is particularly aggressive or biologically active.

Access for inspection used to involve the need to dewater, which induces large stress changes to the structure. However, with the use of underwater scan­ ning and film records these can be avoided and the system designed for a more consistent stress regime. This may not be a total advantage for, if inspection shows remedial work is needed, it must then be done underwater and access gained underwater. Again, modern equipment can usually be adapted to do this,