but the emphasis must be on control of the design and specification at the construction stage.

A prior knowledge of the characteristics of the water and the environment both physical (silt, etc.) and biological (fouling, etc.) is essential at the planning stage, as well as any changes that a thermal and water flow shock to the area may trigger.

10Harbours and jetties

10.1General

All .coastal and estuarine stations have seaborne fuel and other deliveries and exports as a valuable option, especially those with either overor under-developed hinterland infrastructures. Apart from pipelines, sea transport often offers the most economical method of servicing a station; and once the decision is taken to provide a berthing facility, it then becomes a more logical and economic option for other materials. Exports such as ash or unprocessed fuel, sections of the power station and its plant, both incoming and out­

going, can clearly be considered to be potential water­ borne loads.

The jetty and its connections to the shore need to carry all cargo handling equipment, conveyors, pipe­ lines, etc., all services needed by vessels and must have space for access and manoeuvring by fire, ambulance and other services. Overall dimensions are governed largely by long term considerations such as source and quantities of fuel, size and types of ship, turnround time and varieties of other anticipated cargo. These factors depend on commercial decisions, but they control the minimum laden draught and hence the possible location of a jetty and its structural form. t

Figure 3.35 shows the West Thurrock coal-fired power station where coal is received by collier unloading at the riverside jetty.

Landarms can be. over or under water, or non­ existent if deep water is close inshore. They may also double as cooling water inlet or outlet pipes if this suits the layout with the intake, outfall or pumphouse form­ ing part of the jetty or quay structure. Interference with shipping movements needs to be avoided by correct

233

Fig. 3.35 West Thurrock coal-fired station

(see also colour photograph between pp 242 and pp 243)

jetties and Harbours

placing and direction of discharge and intake structures and their flows.

Development of harbour facilities needs to be inte­ grated with other marine works and sea defences, and to be superimposed on the background topography and current pattern so as to cause minimum disturbance to the natural balance of silt and littoral drift movements. Often this can only be done by physical models and by reference to similar sites elsewhere. Mathematical models may be helpful for specific parts. Both types of model need hydrographic information from site to enable building and validation.

10.2 Types of harbours and jetties

Free-standing jetties in deep water present totally different engineering problems to those posed by wharves or quays suitable for shallower-draught vessels. Opportunities for cheap wharf construction are virtually limited to redevelopment areas in Western Europe. Basic types of construction arc described here but it should be remembered that with large power stations the logistics of programme construction, plant and techniques available may affect marine works more than pure cost.

The simplest type of wharf is illustrated in Fig 3.36. Here the river bank is supported by a line of sheet piles with their toes driven well below the ultimate dredged level of the river bed, and with their heads supported

WALL OF STEEL

SHEETPILING

Fra. 3.36 Section of wharf with sheet piling retaining wall

by walings held back by tie-rods to anchor blocks set well back in stable ground. This type of wharf is most suitable for low ranges of tide and shallow draught vessels, due to the limited earth pressures which can be supported.

Figure 3.37 illustrates a type of wharf used where deeper berthing is required. The stability of the bank is again maintained by a sheet pile wall and by a rein­ forced concrete sub-deck and retaining wall supported on piles. The bulk handling plant is carried on a beam ancislab deck supported partly on piles, and on its out­ side edge by a row of cylinders. These arc spaced at intervals of about 10 m and besides supporting the front of the wharf, also resist the impact loads from ships berthing. In this instance, piles are driven down through the base of the cylinders into the chalk, and the cylinders are braced from the sub-deck and concrete retaining wall by reinforced concrete trusses. Replace­ able timber fenders and rubbing timbers are fixed to the face of the wharf and cylinders to minimise impact loads. The berth needs to be dredged, but the river bed can be left at its natural profile behind the row of cylinders.

TRACK FOR

COALING CRANE

Fra. 3.37 Section of wharf with concrete piles and cylinders

Figure 3.38 shows a cross-section of a wharf taken in a position where the sub-deck has been replaced by coarse sareen chambers. The cylinders, fendering $nd deck are indicated, with one of the travelling cranes. The river bed at the wharf’s frontage has been dredged to give sufficient depth of water at all stages of the tide for laden vessels to remain afloat. Consequently they can enter or leave the berths at any time.

Deeper berthing facilities may call for the construc­ tion of free-standing jetties far enough off-shore to be on the edge of the deep water channel. A reinforced

concrete deck of beam and slab construction is sup­ ported on tows of raking and vertical piles, or on two or three rows of cylinders. The cylinders are connected across the jetty by deep beams. The junctions between beams and cylinders are heavily reinforced, as are the cylinders, so that a rigid structure is produced. This removes the need for deep cross-braces between beams and cylinders, and the consequent difficulty of working between tides. The cylinders may be taken down to final foundation level or may have piles driven down through their bases to take the vertical and berthing loads.

A typical tendering system may consist of hollow steel piles driven between the front row of cylinders and supported off the deck by buffers. Timber protec­ tion can also be provided, fixed to the steel piles, and spanning between them. This type of jetty may also have intake or outfall chambers built integrally with it.

The free-standing deep water jetty needs to be con­ nected to shore for virtually everything that passes to or from it. Because of the high cost of this link it is better to limit its capability to essentials, the largest of which is usually fire appliances, and to use conveyor and pipe­ line housings as structural members.

10.3 Construction of harbours and jetties

Whether the site is in a remote area or an urban area

struction programme to relieve traffic or open up isolated sites for deliveries of construction equipment. This pressure usually means that jetties are designed for quick construction rather than engineering inno­ vation, while construction depends heavily on the weather at site and the floating plant available. Wharves and harbours are not so amenable to the use of flexible or prefabrication techniques and usually take longer to construct and require more site-produced concrete. Hence jetties are usually the first choice if their sole purpose is to service the power station.

The necessary major plant; floating cranes, pile frames, batchers, dredgers and tugs must be known to be available at the site before a specific design is adopted. Alternatively, contractor designs should be invited from tenderers who have known access to suitable plant.

Mobilising marine construction equipment is likely to be a major part of the cost so standard sizing of piles,