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Challenging chips

Environment and waste - Building a wood chip storage building for a new biomass power station has required some complex concreting, reports Adrian Greeman.

The new Stevens Croft biomass power station at Lockerbie in central Scotland is one of the first purposebuilt renewable fuel burning generation plants in the UK and will be of medium capacity. But the 60MW plant will still burn its way through a 220,000t of dried wood chips and strips each year.

Conveniently, much of this feed material is close by, in the form of offcuts and shavings from the neighbouring Jenkins turning and timber production factory. As well as using Jenkins byproducts, its operator E. on Power also plans to use local 'energy' crops in the form of coppiced or willow.

The steel frame boiler-house currently under construction by main contractor SiemensKvaerner joint venture, is a relatively straightforward looking structure, rising some 46m above the low rolling countryside with an 85m high stack alongside.

Turbine rooms and other parts of the plant will be fairly conventional, with standard concrete base slabs and platforms and steel superstructure. But the chip handling and feed building will be more complex.

The building houses a huge concrete hopper which stores dried wood chip ready for use as fuel. The hopper has two 1.2m slots along its 60m length, through which the chips are pushed onto Archimedes screw conveyors which transfer them to the furnace at a rate of 235m 3/hour. These screw conveyors are housed in preformed concrete tunnels, each with a side opening the length of the hopper.

The challenge for contractor Alfred McAlpine was to create 48m long column free slots along the length of the hopper side walls and the inner side walls of the tunnels. This was to ensure that there were no obstructions to block the flow of wood chip onto the conveyors.

As a result, the contractor has had to come up with some carefully thought out falsework and formwork sequences, worked up with major input from formwork supplier SGB.

Much of the concrete for the walls has to be supported until construction is complete.

German design consultant Envi Con's solution is to suspend the wall structure from a set of beefy angled concrete columns at 6m intervals within the outer edges of the screw feed tunnels.

The inner edges of the tunnels' top slabs are connected to the hopper sides for extra support.

Tension in the hopper support columns will be substantial and not surprisingly they are heftily reinforced. 'They are packed tight with 32mm diameter reinforcement, ' says Alfred McAlpine civil engineering agent Mark Johnston. At ground level they are tied into underground beams running across the 20m wide oor of the building to the columns opposite.

Sorting out the casting sequence and support for the sides, top and sloping front of the hopper are the contractor's main concerns as these will not be self-supporting until it is complete.

Ground beams connecting the two sides were cast in back blinded trenches together with a raised oor slab for the hopper and then lower slabs at either side for the tunnel bases. A 4m high back wall, incorporating the 1m by 600mm wide lower column sections, was also relatively conventional.

First real complexity came with the tunnel roof slab which is cantilevered from the back and cannot support itself until the top columns and sloping hopper face are in place.

With SGB, Alfred McAlpine devised a table form system for this with plywood and soldier sections supported by aluminium supports below. The tables could drop down 20mm to sit on wheels for moving along to the next position.

A form for the inclined columns came next, made up from soldier sections. Several sets were used because they must remain some days in position while a single set of front and side forms is moved on for the next column.

Then nally the top beam running between the connections between the top columns and the top of the hopper's sloping side walls will be cast. This is done as one pour using a triangular travelling form built up from Mark Two soldier sections and plywood facing with soldier sections to prop it in position.

The units have to fit between the columns and are used three at a time to allow a 12m long pour that extends just beyond each column. 'That avoids a cold joint at the column position, ' explains Johnston.

Once the concrete has achieved sufcient strength the forms can be backed out onto access platforms and moved by crane for the next pour.

At that point the props can be removed leaving the entire structure self- supporting. Steel frame elements for the hopper building are then connected into large holding down bolts which, with some juggling in the design, fit among the heavy reinforcement in the top beam.

A simple chipping building alongside will feed the completed hopper.

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