Civil engineering and building works

porary drainage. Underground cast iron and steel pipes are provided with protective bituminous coatings as 'h'.'y are liable to attack by groundwater. Cast iron pipvs ujc used for foul drains in buildings where extra strength is requited and lur boiler blowdown drains where the discharge may be at high temperature.

Figure 3.26 shows the connection cf a road gully to a surface water drainage system. Note that the gully acts as a silt trap and requires periodic cleaning.

7.4 Railways

Until the introduction of close coupled working, large power station sidings were necessary to accommodate coal trains. Today, coal trains with bottom opening trucks are continuously unloaded without uncoupling. Sidings are virtually eliminated except for crippled wagons and construction traffic where materials are delivered by rail. Trains are hauled by British Rail locomotives and the track, loading gauge and minimum radius of curvature have to be suitable for use by the largest and heaviest diesel locomotives.

The adoption of flat bottom running rail to British Rail standards has replaced bull head rail in most modern installations. The track is secured to timber or concrete sleepers by the use of Pandrol clips. Check rails are provided where sharp curves are unavoidable in order to distribute the lateral forces and to avoid excessive wear to the running rails.

Piped sub-soil erainage of rail tracks is frequently required in order o prevent the foundation beneath the ballast becoming partially waterlogged and weakened. This is carried out by laying open-jointed porous clay pipes or other suitable pipes, and discharging these to a main drain run, having catchpits, located parallel to the track.

CAST IRON

GULLY GRATING

AND FRAME

Chapter 3

7.5 Coal storage

The coal storage area is prepared by first levelling off the ground and then laying down a thick bed of rolled ashes or chalk, the level of which is above the highest water table. If the coal store is on an impermeable sub­ soil, some drainage should be provided, for example, the centre of the coal store base may be sloped gently to the outside and drains installed at the perimeter. These measures are necessary to prevent coal retaining con­ siderable quantities of water and thus making coal handling extremely difficult.

The coal store is generally stocked out to a maximum height of about 15 m and this gives a ground pressure of about 15 t/m2. Sometimes the storage height has had to be limited to a lesser figure due to the presence of weak sub-soils, and the consequent possibility of subsidence.

Large areas of concrete paved goal storage have been provided economically at several recent major stations by using high fly ash concrete mixes. At least 50% of the cementitious material in the mix is non-selected PFA, a material readily available at negligible cost al such localities. Such a surface has clear advantages over prepared ground surfaces whenever wheeled delivery or coal moving equipment is used extensively. Again, drainage considerations require close attention.

7.3Oil tank compounds

Oil storage tanks, which may vary in size from 6 in to 45 m in diameter and by 3 m to 15 tn high, are of welded construction. The intensity of loading is due mainly to the weight ot the contained oil and is thus moderate for the lower tanks which will generally be founded on a thick reinforced concrete raft, or on a bed of crushed stone hardcore, with a thick layer of sand