In addition to these readings some blocks have been fitted with a large number of remote reading instru­ ments buried at strategic points as construction pro­ ceeded. Strain gauges, thermocouples and moisture cells have been used in combination for this, and give some indication of the behaviour of the block under the influence of changing load and long term heating from the turbine-generator itself.

6.4 Basement of ground floor

The choice of level for the basement of operating floor is dependent on various influences, some of which, e.g., hydraulics and layout, are discussed elsewhere in this book. The civil engineer's aim is to provide a suit­ able foundation for the main plant at the least capital cost, situated not so low as to involve expensive dewatering and not so high as to require imported fill. A cost balance can be arrived at for any given site, which is fine for producing capital estimates for the civil elements. However, other disciplines with a vested interest in station levels show more substantial revenue costs or savings, such as pumping costs and main­ tenance access. Hence the optimal level to minimise

lifetime costs needs to be calculated from a range of levels through progressive iterations.

Building and landscape architects have an. input which may also call for planning decisions before the final cost optimisation is done. The final choice from this wide parametric study is unlikely to coincide with the civil engineer’s preference and extra costs will inevitably be incurred.

The designer therefore has to accept the layout and levelsjlecided by a range of interests and must tailor his structural design to suit the engineering ground condi­ tions and the construction programme within these external constraints.

Boiler house and turbine hall foundations have been covered in the preceding sections, but the depth of basement will affect the choice of foundation from the range of buoyant, semi-buoyant, raft, piled, cylinders and contact types. For the main foundations it is not easy, or usual, to combine more than two of these methods in similar ground.

If a basement is proposed, the design must take into account the temporary stages when the excavation will heave at the bottom due to being unloaded and later when the whole basement may tend to float. Pressure relief, semi-permanent ground dewatering or ground anchors may have to be introduced.

6.5 Track hoppers

Coal-fired stations requiring some 20 000 t of coal per day need extensive bulk handling facilities for delivery, storage and retrieval of fuel. Track hoppers, into which a train can discharge its load whilst moving, form the biggest single foundation in the fuel handling system, followed by those for the boom stacker and reclaim track.

Figure 3.19 shows typical reinforced concrete hoppers which are about 70 m long and have a through­ put of over 1000 t per hour. Coal is removed from the hopper onto conveyors by paddle feeders which can travel the length of the hopper.

The track hopper extends rather more than 10 m below ground level and has to withstand considerable earth pressure. Also, if the ground water level is high, precautions have to be taken to prevent the empty hopper’s tendency to float. This may be done by increasing the dead weight of the whole construction. Another way is to extend the width of the base slab, as shown dotted in Fig 3.19(a), and use the weight of soil above the projections to counteract the tendency.

The beams supporting the track over the hopper should be as narrow as practicable so that the discharge of coal from the railway wagons is not impeded. A smooth tile or cast glass lining to the hopper face assists coal flow.

The design of hopper shown in Fig 3.19(b) provides improved handling of coals which have poor flow qualities.

6.6 Chimney foundations

The adoption of tall, multi-flue chimneys in the mid 1960s to reduce ground level concentrations of sulphur dioxide from the main boilers has resulted in structures of 200 m to 260 m in height and weighing 20 000 t to 30 000 t.

Foundations have taken several forms but generally comprise a thick reinforced concrete slab suppprted in some of the following ways:

• On driven or bored pattern piling with both vertical and raking members.

• On large diameter concrete foundation cylinders. These arc adopted when conventional pile driving might require impractically-close pile pitch or deep multiple piling where the as-built geometry would prove extremely difficult to meet the design.

•The slab is replaced by a cellular foundation where only low bearing pressures can be tolerated. The hollow cellular form allows some pre-loading of the sub-base before chimney building commences.

•As a reinforced concrete contact foundation where high bearing pressures are tolerable, such as on a high rockhead site.

Foundations for main and secondary structures

Modern design methods require that the dynamic response of tall chimneys to cross wind effects should be established, calling for study of the interaction of the chimney superstructure, the foundation and the under­ lying soils. Hence the initial choice of foundation scantlings or even forms based on static wind loadings and sized to be within allowable ground bearing pressures, may require modification in the secondary stage of design when the dynamic response is being estimated.

Two types of modem multi-flue chimney foundation are illustrated in Figs 3.20 and 3.21.

6.7Cooling tower foundations

The foundations for a cooling tower have to be considered in the light of the various components which constitute an operating tower.

Since the intensity of loading will vary in both form and magnitude from component to component, suit­ able isolation in the form of expansion joints must be provided between them to allow for differential move­ ment and settlement.

6.7.1 The cooling tower shell and shell support columns

The shell and its support columns can be carried on a discrete pier at each lower node point, i.e., the junction of each column pair at their base. Alternatively the columns can be carried on a continuous circular foun- • dation ring beam. The need for piling in either case will

the foundations, the support columns follow the basic geometry of the lower ring. As such they are raked both radially and circumferentially and the traditional X or W form results from this geometry of structure.

The need to allow in the design for the lateral thrust component of the force in these raking columns will, by analogy, necessitate raking piles where piling is con­ sidered suitable or will require a means of tying discrete footings together in the case of contact foundations (see Fig 3.22).

6.7.2The packing support structure

Since this normally comprises a square grid of columns at 6 m to 9 m centres, it is generally adequate to provide a single pile or isolated footing at the location of each column. Largely shielded by the shell from wind loadings, these foundations are virtually unin­ fluenced by horizontal loadings.

6.7.3The pond floor

This generally supports a head of water not exceeding 3 m depth and unless supported on very poor ground