the excavation for a panel, which may involve up to three vertical passes of the grab, any reinforcement is placed and the infill material inserted using one or more tremie pipes. For structural walls this material is a suitably designed concrete while for cut-off walls it may be a ‘plastic’ mixture of bentonite and cement. The method can be an extremely useful construction tech­ nique, often for temporary works, but requires close quality control at every stage. To facilitate adequate continuity of the wall, panels are constructed in a nonconsecutive sequence and ‘stop-ends’ are used to pro­ duce an appropriate profile of the first and any subsequent panels which are not being constructed against an already cast panel. However, in all cases where the wall is required to act as a structure, a capping beam should be provided to ensure continuity of structural support.

Fairly complicated plan profiles of wall are possible and the technique has been used to provide large capacity support as an alternative to piles.

8.6 Formwork and reinforcement

As soon as an area of excavation is completed and trimmed to the correct profiles, a layer of concrete usually from 75 mm to 300 mm thick but occasionally thicker, dependent on soil conditions, is laid on theexposed surface. This, known as blinding concrete, later serves several important functions. It protects the exposed subsoil from the effects of weather — drying out by wind and sun, or softening by rain — and helps to control seepage of groundwater. The subsoil is protected by it from the later activities of construction, such as pile stripping, handling and fixing of reinforce­ ment, timber, pipework, etc., and it can allow light construction traffic to cross. The shape of the underside of the foundation may be accurately formed, and the working area can be kept clean and dry for all the activities of foundation concreting to proceed.

Before concrete can be poured, the reinforcement steel has to be cut, bent and fixed, and the formwork constructed. These are major operations in the founda­ tion construction and employ a great deal of hand labour. The amount of reinforcement used throughout the whole of the foundation construction may be in the order of 10 000 tonnes, most commonly in mild steel bar ranging in size from 10 mm to 40 mm diameter and from 1 m to 20 m long. The bars may be either straight or bent into complicated shapes. The cutting and cold bending is generally carried out on site in a reinforce­ ment yard equipped with power cutters and bending machines, although specialist off-site cutting and bend­ ing operations are becoming more common for many smaller jobs in power stations. A tower or mobile crane is generally used to transport the steel around the site either in individual bar lengths or as preformed cages.

Whilst the prepared reinforcement is transported and off-loaded near the place of work., the final lifting into

Methods of construction

place of individual rods and binding together of their intersections with iron wire is done by hand. There is limited scope for prefabrication, although structural mesh reinforcement is used in roads and for lightly loaded flat areas.

Spacers, in the form of small concrete blocks, are used to maintain the clearance, or ‘cover’ between the steel and the outside of the concrete to prevent corrosion of the reinforcement. Short bars are used to space one layer of steel from another, and the top layers of steel in a foundation are held up by bars bent into ‘stools’. When completed the reinforcement forms a rigid cage. Care must be taken to ensure that space is left to allow concrete to be poured effectively around the steel and for poker vibrators to be inserted.

Buried concrete may be poured against permanent sheet piling, rough timber shuttering or occasionally against the open sides of excavation, but where the finished surface is to be exposed to view, the wet concrete has to be contained by accurately made and closely fitting timber or steel panels. The internal faces of the formwork are made from panels which are braced with walings to withstand the considerable pressure exerted by the plastic concrete.- The surface of the formwork is coated with a mould oil before the concrete is placed in order to prevent concrete adhesion to the formwork. Formwork materials are intended for re-use as many times as they remain serviceable. Formwork must not be stripped from fresh concrete' until an adequate concrete strength is developed. The shuttering also assists in preventing premature drying out which would otherwise take place due to the heat generated within the setting concrete.

Figure 3.30 shows foundations of a new reclaimer hopper at Drax power station prior to concreting.

Besides using formwork to define the permanent out­ line of the concrete, a considerable amount of rough shuttering is used to divide the foundations up into convenient sections or ‘pours’. The divisions have to take due account of the points of maximum bending and shear in the foundations when loaded. Individual pours in the main foundation areas may contain some 200 m3 of concrete, depending on the capacity of the concrete mixing and transporting plant. It should be the aim of the contractor to programme the works in order that sufficient pours, of varying size, are planned to keep the batching and mixing plant producing a steady amount of concrete throughout the construction works.

Items such as drains, cable ducts, starter bars, holding-down bolts, and pockets for bolts and steel­ work, have to be firmly fixed accurately in position before concreting can take place. It is essential that all bars are free of loose rust, oil or earth (as otherwise a satisfactory bond will not take place with the concrete) and that the inside of the formwork js clean. Before concreting is carried out these points are inspected and the whole arrangement is formally checked by contrac­ tor and engineer to see that it complies with the working drawings.

8.7 Mixing and placing of concrete

The amount of concrete now required in typical power station foundations totals over 201) (MX) nr' and requires

is required to achieve the programmed construction targets. The choice of plant required to transport the concrete in the most cost-effective manner is dependent on the same factors, and also on the type of foundation being constructed.

The problem of continuously producing amounts of concrete of such large quantity is generally tackled by installing a central batching and mixing plant of high capacity to meet the average demand throughout the contract. This central plant is intended to operate throughout most of the life of the contract. Any extra capacity required to meet periods of maximum demand mav be obtained by establishing other plants around the periphery of the works using mobile equipment. This arrangement has the advantage that the individual smaller plants may be moved, from time to time, to follow the construction works without disrupting pro­ duction Similarly, a breakdown ol plant will not cause a serious stoppage. The travelling distance lor the concrete to individual pours may be arranged more economically, and special requirements of mix and aggregates may be more readily accommodated.

The mixers are built into a substantial arrangement containing aggregate hoppers, ‘pre-set weight' batching machines, bulk cement and PFA silos and water tanks. Large stockpiles of aggregate are established near the plant and from these the aggregate is loaded into the hoppers by means of an excavator or chain elevator. The materials arc pre-weighed and fed into the mixer pan or drum. A pan mixer consists of a large diameter open-top stationary cylinder in which paddles rotate. This type of machine produces a satisfactory concrete mix in as little as 30 s compared with the customary 1 min to 2 min for the rotating vertical drum mixer.

The main building foundations on power station sites cover a large area and are shallow in comparison to their plan dimensions. This type of construction favours distribution by pump, as it is not easily accessible to wheeled traffic. The concrete pipelines vary from lilt) mm to 175 mtn in diameter; the individual lengths of pipe ate designed to be manhandled across scaffold stagings and the joints arc quickly made and broken. Bends are available so that the pipeline can reach any part of the pour, and the breaking and re-laying of pipelines is carried out frequently as work proceeds in order to deposit the concrete exactly where required.

Mechanical pumps are most frequently used to push the concrete along the pipelines. Concrete is fed into a hopper from the mixing plant, and gravitates through an automatic valve in the bottom of the hopper into the horizontal cylinder of the pump. A steel piston oper­ ated by an electric or diesel engine forces the concrete

Methods of construction

past a second valve and along the pipeline. The pumps may be single or double cylinder. Pneumatic pumps of various types are also available. Horizontal distances of several hundred metres may be pumped and pumping heads of over 100 m have been achieved.

Hie types of aggregate and the concrete mix design have to be suited to being handled by these types of equipment; the concrete must be neither too wet nor too dty. ami the quality must be controlled to within fairly line limits to achieve success. A blockage in the pipeline due to badly mixed concrete may result in every joint having to be broken ami the concrete tipped from each pipe before it sets. The pipeline is cleared at the end of concreting by forcing a rubber ball through by compressed air. Careful calculation has to be made of the volume of concrete required and allowance made for the concrete remaining in the pipeline if wastage is to be avoided.

Concrete may also be transported about the site in spiral mixer trucks, in front-tipping dumpers, in crane skips, or in lorries fitted with large hydraulically-tipping skips holding tip to 5 m’. Temporary roads and ramps may also be required to reach the places at which con­ crete is required. It is not good practice to tip concrete from any considerable height as segregation of the materials mav occur and the reinforcement may be dis­ placed or the shuttering damaged. With deep foun­ dations, cranes are used to handle skips of concrete to the correct points of discharge; the skips are either brought to the crane full, or are filled at the crane by tipping from the lorries or dumpers. For small founda­ tions mobile cranes are often used, but for larger

to fully-compact it into the formwork and around the reinforcement. This should produce a dense concrete free of voids and hence produce a good surface finish. Vibrators may be of the internal poker type, or external type working on the outer face of the formwork. The poker type is more suitable for use in large foundation pours. They vary in size from 25 mm to 100 mm in diameter and are about 500 mm in length. They are operated by electricity, compressed air. or by flexible

the concrete is properly compacted. The depth of the concrete layers should be kept to not more than 600 mm to allow any entrapped air to escape. When using vibrators care should be taken to ensure against over-vibration thus possibly causing segregation of the concrete.

A contractor often chooses to install stationary compressor plant on site, with a galvanised steel main fitted with tapping points laid over the foundation area. A great deal of compressed air plant is required to operate the vibrators, for cleaning the face of concrete ready for the next pour, and for blowing the formwork clean of dirt and debris prior to concreting.