High on the challenge list during repairs to a Victorian canal aqueduct near Walsall are not its voussoir cracks or cast iron parapet replacement, but vandals and bats. The former technical demands can be routinely tackled; the latter group is an unpredictable, unavoidable and potentially expensive intrusion that goes with the territory.
The site's two consultant ecologists may baulk at their favourite flying mammal being linked to unruly young locals.
But failure to accommodate both sets of 'visitors' could cost the project dear.
There is little direct evidence that pippistrelles or long-eareds inhabit the 157 year old structure's crevices. Nevertheless, had not the site team been proactive in encouraging them to do so, it could have run up a potential £100,000 bill through enforced delays.
And discouraging the human invaders, who regularly do try to visit the site, is currently costing around £40,000.
The three arch, brick built Grand Junction Aqueduct, carrying the Tame Valley Canal over a busy electrified suburban rail line between Birmingham and Walsall, is itself unusual - being owned by British Waterways (BW).
BW may own around 480 canal aqueducts nationwide, but relatively few of them cross rail lines. Most of Britain's canal network was built between 1760 and 1790, pre-dating the start of the railway era by some 40 years.
Routing a new railway beneath an existing canal in the mid-1800s meant that the required aqueduct was - and still is - down to the railway company to build and own.
But creators of the late arriving Tame Valley Canal, cut in 1844, not surprisingly found an existing rail line already blocking its path. So it fell to the waterway's owner to bridge the rival transport mode, promptly naming the resulting aqueduct after the rail line itself.
Water and 25,000V electrified overhead rail wires is not a happy mixture, but fortunately there are no canal bed leaks on the aqueduct repair list. Instead it features the grouting and anchoring of three, 30m long deep cracks where sandstone voussoirs have bulged free from brick arch barrelling. Above the arches, brickwork needed repointing and cast iron railings have to be repaired.
Such a list is neither unusual nor particularly taxing to contractor Morrison, holder of a three year local framework agreement with BW to maintain and repair needy structures. The contractor's current £10M annual workload throughout BW's Midland region covers some 50 projects ranging from £5,000 lock gate replacements to significant structural repairs such as the Grand Junction's £425,000 contract.
'It's more of a partnership with us, helping to tackle BW's significant repair and maintenance backlog, ' explains Tim Johnson, BW senior project engineer overseeing most of the Midlands projects. 'Our joint aim is to turn liabilities into assets.'
Just a month from completion of repairs at Walsall, Morrison's main technical concern remains temporary works. Equipment and materials are delivered to site by canal barge. And, with most of the structure below arch springing level located within the rail exclusion zone beneath, all repairs are carried out top down from an access scaffold slung beneath the arches.
But the £100,000 birdcage scaffold hangs directly over the railway and just 2.5m above its overhead power lines, so both erection and removal demand a series of track possessions.
Only half the 10 booked Saturday night possessions were needed for its erection, and the dozen strong highly co-ordinated scaffolding team is optimistic that at least two of the planned six more possessions requested for the dismantling operation can also be cancelled.
Track repairs on this busy rail route are currently under way further down the line, and Morrison is planning to piggyback on other contractors' booked possessions by reprogramming its own scaffolding removal work to suit. Engineers are 'hopeful ' such ingenuity will allow them free track access, though whether they have avoided a bill altogether has yet to be confirmed contractually.
But, at a cost of £2,500 per possession, any saving is welcome and will be immediately diverted into the vandal box.
'We do have a bit of a problem with vandals, ' admits Morrison site manager Paul Hinsley, his wry smile revealing the depth of this understatement. 'Site fencing ended up in the canal on day one and we have been busy fishing out material virtually daily ever since.'
The need to keep open both canal and towpath right through the site presents few problems.
But Walsall youth appears to have targeted this remote aqueduct as a key ingredient in gang initiation tests.
Discouraging kamikaze walks along the wrong side of newly painted parapet railings, and counteracting bolt cropper attacks on site lighting gantries, demands two permanent security guards to protect the small site. Special anti-climb railings, erected over the central railway arch, have yet to be 'tested'.
The site's second group of less certain visitors is less threatening, but equally challenging.
'We have found no direct evidence of bats using the aqueduct, but at least half our 70,000 structures nationwide are potential bat homes, ' explains BW ecologist and bat expert Mark Robinson. 'The large 300mm deep cracks here are an ideal habitat so, when we fill them in we must, by law, provide an alternative home.'
Permanent repairs involve the 75mm wide cracks being sealed with hydraulic lime, allowing minor water seepage from the structure to continue so minimising any damage from the freeze-thaw cycle. The sandstone voussoirs are then stitched back to spandrel walls with 2m long rock anchors.
But before any of this could start, the batmen moved in.
They temporarily filled the cracks for two weeks with fibre glass, inserting within the material a series of 40mm diameter hollow tubes through which any bats hibernating deep in the crevices could escape. Their new homes consist of a pair of wooden 350mm square bat boxes concealed in arch crowns.
Apparently, this narrow slit is easily sufficient for bat access - but not for birds - and the small, 150mm deep box behind can house an incredible 300 roosting bats. If the box becomes a breeding home, extra room is needed and its more permanent population would drop to around 70.
The whole bat operation costs just £2,000. But failure to consider these protected species - were they later found to be resident - could have forced cancellations of hard to renew rail possessions, triggering a potential bill 50 times larger.