retaining wall has a completely separate foundation so as to isolate settlement effects of the reactor base. The only linkage between the two is a continuous horizontal rubber water bar. *

7General site works

7.1Flood embankments

In order to obtain an adequate supply of cooling water from natural sources many stations are located near to rivers. The locations may be on low-lying land which is subject to flooding. In such cases it may be possible to raise the whole area of the station above flood level by bringing infill from outside sources. If the level has to be raised appreciably, or there is a shortage of fill material in the district, only the area occupied by the buildings may be filled and the cooling towers pro­ tected by a flood embankment. Surplus soil arising from excavations for foundations may be suitable to form the. embankment, which will vary from some 1 m wide at the top, with slopes on each side of 1 vertical to 1.5 to 2 horizontal, the top being some 1 m above the highest probable flood level. Impervious soils make good flood banks, but sandsand gravels need to incorporate a central wall of clay, concrete or steel sheet piling to prevent the passage of water, or be surfaced with an impermeable membrane of clay, concrete, etc.

7.2 Roads

The layout of roads for a power station aims to provide satisfactory routes both for the construction traffic, and for the power station traffic once the station is in operation. Construction traffic is heavy and frequent and provides a severe test for the roads as it includes, not only heavy lorries, but tracked vehicles and large transporters. Permanent traffic may be light on certain sections of roads, but other sections will have to with­ stand the frequent passage of heavy coal and/or ash lorries, the occasional heavy load from plant trans­ porters or mobile coal handling equipment.

RoadAvidths vary from 3 m for minor roads up to 7 m for the main perimeter road. The layout must be designed with bends capable of accommodating the largest transporters. Clearance is required for these beyond the inner curve of the road at sharp bends in view of their considerable overhang. A transporter carrying a 570 MyA transformer has a total weight of approaching 3001 and is more than 30 m long with six axles at each end and with eight wheels to each axle. A road tractor at both front and rear is generally required.

The design of the joad and the depth of construction will depend on the strength of the sub-soil, its suit­ ability for compaction, and the intensity of the anti­ cipated traffic loading. The road foundations must be kept well drained by sub-soil drainage,, or the roads

built on shallow embankments if practicable. At this stage, allowance should be made for cable and pipe cables and services crossings under the roads, for the roads to cross cable reserves, and the external surface water drainage to be constructed. This drainage will prove invaluable in helping to keep the site dry and clean during construction.

The nature of the sub-soil influences the choice of road design between two types — rigid or flexible. In both types the intention is to spread a high intensity of wheel loading at the road surface, through succeeding

would be a sub-base of foiled granular material, such as sandy ballast of say 300 mm thick, depending on the strength of the ground, underlying a base of 200 mm of well-rolled brick hard core. The road surface 100 mm thick, would consist of tarmacadam, bitumen macadam or asphalt, in ascending order of cost, laid in two layers of base course and a thinner wearing course of finer material. Asphalt provides the more impermeable surface and is more resistant to traffic wear. Materials such as cement-stabilised granular soils, pulverised fuel ash, and lean mix concrete are also used for bases and sub-bases, their choice being governed by local avail­ ability and economic considerations. Flexible roads are easy to repair where individual areas become damaged or subside, but may need extensive resurfacing after use by construction traffic.

Rigid roads are generally constructed of reinforced concrete up to 300 mm thick; the reinforcement being a welded mesh fabric. The base should consist of up to 150 mm of rolled clinker ash, lean mix concrete or granular material. Rigid roads must be provided with suitable joints at intervals to allow for expansion and contraction. The surface of a good quality concrete road is easy to clean and is resistant to damage, but where uneven settlement occurs the slab will ultimately crack and effective repair is then expensive. This type of construction is often "preferable for the heavily used perimeter road.

7.3 Drainage

The water to be disposed of from a power station consists of:

• Surface water, sometimes called stormwater, which is the run-off of natural water from all the various surfaces including paved areas, roofs and unpaved land.

•Foul water, sometimes called soil or sewage, which is the water-borne human and domestic waste dis­ charged from lavatories, ablutions and kitchens.

•Plain water, which is that used for cooling and a variety of other processes inside the power station from where it is discharged to waste.

General site works

These effluents are disposed of through separate systems since they require different purification treat­ ments before discharge. Treated effluents from plant or foul water may join the surface water system at or before final discharge .or they may be routed through stilling ponds or through the cooling water outfall, the arrangement depending on the station site layout.

Figure 3.24 shows a typical drainage plan where the manholes arc numbered and the levels at the bottom of the drain are given.

Surface water is normally uncontaminated and may be discharged without treatment to an adjacent storm­ water sewer, river, estuary or the sea. Drainage from vehicle hard-standing areas may also enter the surface water drains via a petrol/oil interceptor. The sizing of the drains at their point of discharge depends on. the area of the site and the extreme storm conditions considered relevant. Typically three pipes may dis­ charge into the final outfall each with a diameter of about 900 mm. Any discharge and its quality requires the consent of the statutory water authority.

Foul water pipes are sized to run at a proportional depth of 0.75. Data on discharge quantities from lavatories, etc., is given in BS5572 [11]. The total dis­ charge from a power station can normally be accommo­ dated in a pipe 225 mm in diameter. This is discharged to the local authority’s sewer if it is not too distant and has sufficient capacity. When this is not the case, a sewage treatment plant is installed within the site, sized to meet the anticipated constructional quantities, the operational quantities or both.

Discharge from the various items of plant varies greatly in its purity and quantity. The largest discharge of uncontaminated water occurs when condensers are emptied which can be at the rate of 27 m3/min for 10 minutes. This can be put into the surface water system. Likely areas o.f oil leakage are restricted by bunding so that oil does not enter the drainage system. However, drainage from floors or paved areas which may contain some oil spillage is passed through an oil/ water separator of sufficient size to allow separation to take place. Very acidic effluents from boiler washing and cleaning are neutralised on or off site or discharged to a delaying resevoir, usually the ash lagoon. Dis­ charges of acids and alkalis from the water treatment plant are neutralised in an effluent tank within the water treatment plant area, drain pipes being made of internally coated steel or cast iron or reinforced plastic. All liquid discharges from the site require the approval of the statutory water authority.

Wherever possible, drainage is by gravity at self­ cleaning velocities of flow. The slope at which drains are laid is typically such that the flow velocity is not less than 0.75 m/s when the effluent is at a quarter depth in the pipe. The minimum gradient which will produce this varies from I in 70 for 100 mm diameter pipe to 1 in 900 for a 900 mm diameter pipe.

Drainage pipes are laid in straight lines and at a constant gradient between manholes. Manholes are

provided at changes of direction and at a maximum spacing of 90 m (though a much closer spacing is usual on power stations) to facilitate cleaning of the drains when blocked. Manholes are constructed in engineer­ ing bricks, with a minimum wall thickness of 225 mm, or of precast concrete. The latter is usually cheaper, particularly for deep manholes, but brickwork is advan­ tageous where it is necessary to break into a manhole to accept an additional discharge pipe. In either case it is essential that the manhole is sufficiently large to permit cleaning to be carried out. A catchpit is similar in construction to a manhole except that the base is some 300 mm minimum below the invert level of the incom­ ing and outgoing pipes. Its purpose is to trap solids contained in the discharge. They may not, of course, be included in foul discharge lines.

A combination of gravitational and pumped flow is usual in the foul water system. Pumps are used occa­ sionally in the surface water system in remote areas or at the final discharge point depending on the initial and final topography. These higher pressure regions of drainage systems are normally constructed in cast iron or steel pipes. Concrete pipes suitably jacketed at their

General site works

joints and reinforced have been used for larger sizes. Asbestos cement pipes have been used in the past, but are now not used in CEGB stations.

In nuclear power stations any radioactive effluents from spillages and washdowns are collected by open gullies made of either stainless steel or cast iron. Borosilicate glass or cast iron pipework is used but where pipework is to be encased in concrete borosili­ cate glass is unsuitable. It is however suitable for both gravity and pumped drains. Secondary containment is obtained by providing an interspace around each drain­ age pipe and this is achieved in a variety of ways. All potentially radioactive drainage from power stations is held and monitored before final discharge or removal.

Underground drainage pipes are protected from stress by being laid on suitable supporting beds and being protected at the top and sides. The usual power station practice is shown in Fig 3.25. Further general information is given in BS8301 [12]. For the station permanent system, rigid pipes are used with either rigid or flexible joints. Vitrified clayware is used up to about 300 mm diameter with larger sizes in concrete or rein­ forced concrete. Flexible pipes are restricted to tem­

EVERY INTERNAL JOINT

MADE WITH PUSH FIT RUBBER RINGS BETWEEN SPIGOT AND SOCKET