topped off by a sand/bitumen mix. For the larger and taller tanks piling is frequently required, as such tanks are frequently sited on poor ground close to a water’s edge.

As a precaution against the risk of spread of fire, and against considerable leakage of oil from a damaged tank, tanks are contained within earth retaining walls (called bunds) or within reinforced concrete enclos­ ures; these must be sufficiently voluminous to contain the full oil capacity of the tanks. There is no record of fire or collapse of tanks causing a large leakage at any CEGB power station. However, small spillages do occur and these arc retained within the bunds.

In general, the larger tanks hold heavy fuel oil and these are enclosed within large unsurfaced earth bunds. Rain falling within these bunded areas either drains into the ground or evaporates, whereas fuel oil solidi­ fies at normal temperature and can be removed or left to decompose. Smaller tanks normally hold light oils and these need to be enclosed within surface imper­ vious bunds which fall to a blind sump within the bunded area. The light oils separate from the rainwater and the water is pumped from the bottom of the blind sump over the bund wall and into the surface water system.

All major tankage installations need to be most care­ fully considered with respect to the oil and petroleum storage regulations and perhaps the Reservoirs Act [13].\

7.7 Ash disposal areas

The amount of preparation required for any ash disposal area will depend upon the final use to which a specific area will be put. If the area is to be landscaped by forming a contoured hill then the main problems will be the formation of terraces such that mechanical plant, e.g., tractor scrapers, caji dispose of ash in layers thin enough to ensure adequate compaction and hence avoid subsequent instability, and the provision of sufficient land drainage such that the risk of a catas­ trophic slide will be avoided.

If the area is to be used as a short term disposal facility from which the ash may be subsequently reclaimed, then it will probably be deposited in artifi­ cial or natural lagoons, the former being constructed from the ash itself. In general this form of disposal will be associated with the pumping of ash slurry and hence involves the removal and disposal of the surplus water to a river or other water course.

Large ash disposal facilities are not strictly liable to comply with the Reservoirs Act, but the CEGB has ■adopted a very similar safety control procedure. Inde­ pendent expertise is universally engaged for the design, construction and subsequent operational phase of such civil engineering facilities.

8 Methods of construction

8.1 Site clearance, access roads and construction offices

The construction of the. power station foundations is carried out in accordance with a detailed programme drawn up to provide the various foundations and general site works, in the sequence necessary to enable the building work and plant installation to proceed in accordance with the overall construction programme.

The first steps in the civil engineering work are the clearance of old buildings, hedges and other obstacles from the site, followed by any general site levelling which is necessary. This may involve only excavating soil from high areasand filling low areas with it, or excavating selected materials to form flood banks, or importing fill to raise the whole or part of the site level. To provide material for landscaping which is almost invariably necessary, the top soil should be carefully stripped off and stock-piled for re-use when construc­ tion is complete. Preparatory work for the installation of an electric power supply with ring mains follows.

The permanent main drainage of the future power station is then installed, and arrangements made to dispose of surface water from the system if the per­

the local authority sewers or, if none are available, a small sewage treatment plant will need to be built. Lavatories for construction workers will be built at various points around the site at this stage.

A new access road from the public highway to the station will be built, and an access bridge if required. The permanent roads of the station are laid down for use during the construction period, and any temporary roads necessary for the construction of the station are built. These may be removed following completion of the works. Hard storage areas for the plant contractor’s material and huts are provided by installing sub-soil drainage and then surfacing large areas with rolled hardcore and ash and making suitable temporary access roads.

A site boundary fence is built to improve security, and a car park built for the construction workers outside the boundary fence.

Site offices arc built for the CEGB’s site staff and other buildings erected such as stores, garages, con­ crete laboratory and a site canteen for all the' construc­ tion personnel. Water and electricity supplies for domestic and constructional purposes are installed. Welfare facilities, such as a site first aid centre, are provided, and in some areas where accommodation is difficult, a hostel or camp may be made available for resident workers.

Completion of the preliminary works will permit later the efficient deployment of the plant and labour required to construct both civil works and plant instal­ lation. A separate contract is frequently let for these

Civil engineering and building works

preliminary works, so that they may be put in hand while the detailed design of the foundations is under­ taken and contracts let for their construction.

8.2Underground construction

All ground is saturated with water below a certain depth depending on natural conditions in the locality. The top surface of the groundwater is called the water table, and if a hole is dug below that level water will flow into it until its surface reaches the water table, the rate of flow being largely controlled by the permeability of the ground itself. Before any deep excacations can be carried out it has to be decided how groundwater will be dealt with. Clay strongly resists the flow of water through it, silt is more porous than clay, and ordinary coarse sand is even more porous. Gravels are very porous, and water flows easily through them.

The results of the soil investigation provide the level of the groundwater and the types of soil which lie below. Choice of the best method to be adopted to keep the excavations dry and stable enough to work in can be assessed from this data. For excavations above the water table and for conditions of low flow through clays or silts, it is sufficient to introduce ordinary suction pumps or lifl-and-force pumps as the excava­ tion proceeds. This equipment will also remove such surface rain water as finds its way into the excavation. This method is often sufficient for the main foundations excavation for a power station to proceed in open cut, i.e., with the sides sloped back to maintain stability. For deeper excavations this is seldom practicable and the stability of the sides has to be maintained with ring main dewatering on berms surrounding the excavation and deep-well point dewatering to local foundations below the main level.

Other techniques are available for unusual or con­ fined conditions but these are not yet of general appli­ cation or economic. They include freezing, ground con­ solidation techniques and grouts, vibro-stabilisation and explosive compaction, and are each suited to parti­ cular ground and site conditions.

More generally applicable methods of deep construc­ tion are described as follows.

8.2.1 Cofferdams

Figure 3.27 shows a diagrammatic arrangement for the construction of a typical closed sheet pile cofferdam used for a construction in water; a similar arrangement would also be suitable for a construction on land. Sheet piles are obtainable in various sections and lengths and have interlocking clutches along their edges, so that when driven they form a watertight wall. Diesel, steam and compressed air hammers are used for driving sheet piles, and electrically-driven vibrators can be used for driving and extracting piles. A ‘silent’ hydraulic system is available, albeit expensive, if noise or vibration needs

Chapter 3

to be minimised. The sheet piles are driven down into an impervious layer, or into a firm foundation such as chalk or hard ballast taking care not to drive so hard as to decouple the clutches at the bottom.

Excavation, or dewatering in the case of an offshore cofferdam, proceeds in stages so that the various levels of bracing can be assembled. In large cofferdams the long struts may need supporting at mid-span by tem­ porary piles called 'king' piles. The bottom bracings are spaced more closely together to take the increased load witji increased depth. Excavation and bracing is con­ tinued until the foundation level is reached, when con­ creting of the permanent structure follows in the normal way. Piles may be driven below the excavated level if needed. If the sheet piles are to form part of the permanent structure, concrete is poured against them, and the temporary bracings removed as work progresses. This practice is not recommended, how­ ever, for should there be any ground pile movement or any flexing of the sheet piles when removing struts serious damage can be caused to the recently poured concrete structure. The sheet piles are otherwise driven wide of the permanent structure, so that as it rises within the cofferdam, load from the sheet piles can be progressively transferred to the concrete walls by short struts and wedges. The space between the sheet piles and the concrete is finally backfilled or flooded and the piles extracted.

Considerable temporary works design is necessary to place the cofferdam walings and struts so that the main vertical reinforcement is not fouled and the rising shutters can sail past walings until concrete is just below them. Folding wedges between the w aling and the sheet piles can then be taken out and the waling and strutting removed. Thus load need not be transferred to green concrete. To achieve this it is often necessary lor the permanent works to be substantially re-designed to accommodate the complex temporary works.

This form of construction is widely used for such structures as pumphouse, intakes and outfalls, ash plants, and the deeper parts of large open cut excava­ tions. Conveyors, cable ducts and culverts are fre­ quently constructed within continuous sheet pile coffer­ dams-with long modules of piles, strutting and shutter­ ing being fleeted forward. Intake and outfall structures in calm or shallow water lend themselves to construc­ tion within a circular cofferdam of piling or diaphragm walling. In these types cross-bracing can be eliminated by using reinforced concrete circular walings in com­ pression to support the cofferdam wall, leaving the centre free of obstructions.

The diaphragm wall or bentonite trench wall tech­ nique makes use of the properties of bentonite and bentonite clay suspensions in water to produce pump­ able liquid/gels with specific gravities above 1.0 and some shear strength equivalent. This fluid is used to support the sides of an excavation until digging is complete, when it is displaced by reinforcing cages and concrete carefully tremied in from the bottom. The

displaced bentonite is then cleaned and pumped else­ where for re-use. Excavation is done by machine work­ ing through the fluid and when working in shallow water it is usual to provide a temporary artificial island from which to excavate. Walls made in this manner have been incorporated in permanent works with little or no surface treatment.

The design of a cofferdam is a difficult exercise because of soil mechanics not lending itself to precise analysis, and failures have occurred. Failure may be due to the mechanical failure of the lower struts, or in a plastic soil the unbalanced earth pressures may cause the sheet piles below the bottom strut to buckle and

the bottom of the excavation to heave up. Unbalanced hydrostatic head may cause water and fine material to1 flow under the toes of the sheet piles, and the bottom of the excavation to ‘boil.* Particularup care is needed

when there is a large range of tide or groundwater outside which must be balanced during all stages of construction. Groundwater lowering, as described in Section 8.3 of this chapter, may be sufficient to deal with this problem. The out-of-balance forces may also be overcome by flooding the cofferdam, excavating under water with grabs to below foundation level, and placing a layer of mass concrete under water before pumping out for construction to continue. This mass

concrete must be thick enough to balance the forces likely to cause failure, and to form in effect a per­ manent bottom strut and seal to the cofferdam. Il may also be tied to underlying rock and anchored down if there is any danger of the cofferdam becoming buoyant.

Injection grouting techniques may be employed to increase the strength of the soils and reduce their permeability. Depending on the nature of the soil, the materials used vary, examples are cement grout, special clays (sometimes mixed with cement), PFA, bitumen compounds, and a variety of chemicals. These are so effective in certain soils that excavation within a grout curtain has been carried out without recourse to sheet piling.

In very large cofferdams, which may be required to enclose the entire area of bulk excavation for a power station, it is impracticable to support struts right across such a large area. In this case the cofferdam walls are tied back with steel rods or cables to anchors in firm ground, or berm is left immediately at the edge of the foundations to support the wall. When the permanent foundation has been built sufficiently close to the sheet piles, the berm is removed and replaced by raking struts braced off the foundations.

The depth to which construction can proceed in cofferdams is limited according to soil conditions and it may be necessary for shafts to use an upper cofferdam to get a seal at rockhead and then sink a second cofferdam through it.

Three other methods for forming deep foundations are now discussed and they are equally suitable for construction on land or through water.

8.2.2 Compressed air or pneumatic caissons

A steel or concrete working chamber is constructed at ground level, its shape corresponding to that of the completed structure. The working chamber walls are sloped to form a cutting edge at the maximum external dimensions and work continues on the permanent rein­ forced concrete walls above the working chamber roof. Once the walls are high enough to provide sufficient weight and strength, excavation commences under the working chamber roof through access shafts which will later carry the air and muck lock shafts. The ground under the cutting edge is broken out and the whole structure settles and follows the excavation down until water makes it necessary to put the working chamber under air pressure when the air shafts, man locks and muck locks are installed. Where possible they should be installed to full height as the opportunity to lift them may not occur.

Excavation and concreting then continue under air with the same problems as those discussed in Section 8.2.1 of this chapter, until founding level or suitable foundations are reached. Often the weight of the caisson is insufficient to overcome skin friction from the ground and kentledge is added to the caisson as

blocks, sand or water. Alternative methods of easing the sinking are to inject a bentonite lubricating film at the cutting edge, or ‘blowing down’ by removing the workers, raising the air pressure and dropping it suddenly. This, whilst effective, involves some loss of control over where and at what level the caisson is less than the length of its shortest side into the ground. Also, behaviour of the rapidly decompressed ground can be unpredictable, leading to muck being forced up to the air shafts.

When foundation level is reached, the bearing sur­ face is cleaned up, the working chamber filled with

is about 25 mm to 30 mm below high water because of the human limitations in compressed air. Final position, level and plumb of a caisson must allow reasonable tolerances due to the inherent uncertainties and so all sluice gate guides, plummer blocks and other plant related items built into the concrete must have adjustment.

Caissons built on tidal river banks always move towards the river (often 1-2 m) and will tend to lean towards it. Allowance should be made for this when setting out the working chamber. For work in water the cutting edge, working chamber and sufficient wall to provide freeboard are floated into position and sunk with as much working plant installed as possible. Other plant must either be afloat or on temporary piled structures outside the area affected by caisson sinking.

8.2.3Monoliths

Monoliths are not often employed where techniques for ground stabilisation can be used. I lowever thev provide simple, but not quick, ways of installing quay walls ami harbours which are intended to be dredged later and which may include CW intakes and outfalls.

A cutting edge is assembled at ground level on temporary foundations and the permanent concrete walls built up for a height of three to four lifts. The soil is grabbed out from below the cutting edges and the monolith descends under its own weight. Excavation is halted'from time to time while extra sections of wall are built up. The process is continued until foundation level is reached, excavation being carried out under water when water-bearing strata are encountered and kent­ ledge .added when required.

The monolith is finally plugged with mass concrete placed under water. Divers using high pressure water jets and air lift pumps are used to clear the slurry from the foundation level. The monolith is then dewatered and the reinforced concrete work completed.

The permanent structure has to be made strong enough to withstand the additional stresses involved in this type of moving construction. In a large structure temporary bracing walls may have to be built and later removed. Pumphouses have been successfully con­ structed using this method, their ‘egg crate’ internal