Upgrading work on a bridge over the busy M20 that takes traffic from London south into Kent shows how civil engineering decisions have to be made, according to Peter Chong, divisional director at consultant Peter Brett Associates.
'You look at what your constraints are, and then gradually narrow down the options, ' he says.
The constraints in this case are many: work must be done over a busy motorway without disrupting it; the connecting road must also be kept running; headroom is limited; and the post-tensioned concrete design of the original bridge produced an unusually slim structure, requiring a complex match from the extension.
The project's aim is to expand part of the roundabout junction between the M20 and the A228, which is being widened to dual carriageway and extended by Kent County Council as part of significant development of the West Malling area. The $6.5M (ú3.5M) junction work falls under a separate contract to the main ú47M widening scheme, though Birse Civils is main contractor for both.
Two curved bridges carry the A228 across the M20, with slip roads linking to the motorway.
Traffic demand requires the western bridge to be widened from two lanes to four.
'The old bridge is a posttensioned voided slab with an unusually slim depth-to-span ratio, ' says Chong. 'The deck is just 1.37m deep despite crossing some 40m.
'It is probable that it was done as a green field project when it was built about 35 years ago and they had the space for substantial falsework support.' To match that design alongside the bridge would require more in situ works and falsework ? out of the question over the busy M20.
'The headroom we have is not too brilliant; it needs at least 5.7m, ' Chong says.
'So we looked at demolition and a completely new structure but that would also have been very disruptive.' With options narrowed, the answer had to be a pre-made structure that could be dropped into position with limited possessions of the motorway.
Precast pre-stressed concrete beams were examined but eventually ruled out because they needed greater depth than the old bridge.
That left a composite structure using steel beams to achieve the slimness, and a concrete deck which could be poured in place and stitched to the old bridge. Total steel and concrete thickness is 1.3m, whereas a basic design would use about 2m depth.
Even that structure would be unusual, using a less than optimum amount of steel to achieve the narrow deck height.
'You have to use six closely spaced shallow beams ? they are very squat ? rather than four deeper ones, ' says Chong.
Additionally, the inner beam has to be curved 'in sympathy' with the curve on the original bridge.
To make room for the new bridge, one of the first tasks when work began in April was to demolish a cantilevered side section of the original deck.
This was nibbled away with hydraulic shears, 'leaving lots of mangled steel protruding', according to Chong. The exposed reinforcement can be straightened by heating and bending he says. Being mild steel there is no issue with changing bar properties.
The exposed reinforcement will eventually help form the stitch between the new and old structures, aided by further dowell bar connections. Holes for these are being drilled into the old concrete at present.
New concrete abutments were formed either side of the motorway, forming the end points of the simply supported span. Steel was lifted in during short Saturday night and Sunday morning possessions. 'We have been able to keep the M20 and the A228 open most of the time apart from that, ' says Chong.
The new deck, in a conventional C40/50 concrete, will be poured over glassfibre reinforced concrete permanent formwork panels between the steel beams in a few weeks.
First, a number of small precast cantilever panels have to be positioned to form the outer parapet on the new bridge. These were made in nearby Sussex.
Once the new deck is ready, a final stitching operation will be carried out. It is important to let the new structure settle and to wait for concrete curing and load effects to even out before making the stitch, says Chong.
'The contractor will probably wait long enough to transfer some trafc to the new bridge, ' he says. This will allow the lanes closer to the stitch to be left empty, reducing immediate vibration when the pour is made.
The new structure has been designed as far as possible to match its responses to the old, so the joined deck should behave as one bridge, with comparable stiffness and strength.
Work on the junction nishes in the autumn.