Lerwick harbour's clear waters make it relatively easy for divers to carry out visual surveys of sheet piling all the way from the surface to the seabed, some 9m below. What they discovered last summer was enough to give the port authority nightmares.
They were looking for the classic signs of 'steel eating bugs' - colonies of microbes whose aggressive by-products can eat through steel at an alarming rate. Soft organic orange patches identify the microbe colonies, under which is a slimy black layer hiding bright, pitted steel. In the cold Shetland waters the divers found the orange blight almost everywhere they looked.
'We checked the whole 1.4km of sheet piles and 200 bearing piles and discovered all of it was affected by microbiological corrosion to a greater or lesser extent, ' reports Andy Sanderson, Lerwick office manager for consultant Arch Henderson.
'Generally, the older Larssen piles dating from the 1950s were only affected around the splash zone and had lost less than 30% of section at most. However, around 500m of Frodingham piles dating from the late 1970s were extensively holed and pitted from the splash zone right down to the sea bed.'
Although not yet penetrated, one run of piles dating from 1993 had lost half its section - equivalent to 1mm of steel being eaten away every year. In all cases the attack was concentrated away from the pile clutches; on the webs of the Frodingham piles or on the flanges of the Larssen pile.
There was some good news.
'One stretch was completely clear of biological attack, ' Sanderson says. 'This was where sacrificial anodes had been fitted in 1994.'
Repairing the badly holed 0.5km stretch of quay wall, where there was significant backfill loss, was the port authority's top priority. Arch Henderson was charged with the task of coming up with repair options. Sanderson freely admits it was an uphill struggle at first.
'It was difficult to obtain information on what techniques had been found to work in this situation, ' he says. 'Obviously complete replacement was an option if a cost/benefit analysis showed that it could be justified.
But failing that we had to consider other methods.'
Many possible techniques with an established track record could be ruled out almost immediately. Welding on steel plates across the inpans and backfilling with concrete, usually from inside a limpet dam or cofferdam, had worked well when corrosion was confined to a narrow zone near the low water mark.
But at Lerwick repairs were needed right down to the seabed, and the engineering team believed this option would be impractical and expensive.
Similar considerations, plus the risk of mechanical damage, seemed to rule out the use of high performance paint or coating systems.
'We also considered refacing the quay by pouring concrete behind a line of new, sacrificial sheet piles, ' says Sanderson.
'This has the advantage of low long term maintenance, but it's still rather costly.'
Shetland's remoteness was another important factor. Construction costs are up to 15% higher than UK mainland levels.
But there were also problems when specialist contractors from the South were used on projects in the islands. A repair option that used as much local resources and expertise as possible was needed.
Working with local diving contractor Ocean Kinetics the Arch Henderson team came up with a dual approach. The piles would be repaired and strengthened by infilling the damaged in-pans with high performance concrete.
Undamaged steel would benefit from cathodic protection.
Sanderson says this option had many benefits and only one obvious drawback. 'It would cost less than half as much as refacing the quay and cause less disruption. But it would need long term maintenance of the cathodic protection system.'
This was felt to be a price worth paying. After a successful trial last autumn, work on the main contract began in June this year. Ocean Kinetics managing director John Henderson says that the key to the success of the repair operation is the set of six special shutters that Ocean Kinetics developed for the contract.
'These bolt onto the outpans after we've welded on shear tangs and installed the reinforcement cages. In practice this has turned out to be the trickiest bit, as the piles aren't exactly uniform or parallel.'
A special concrete mix is then pumped to within 50mm of the capping beam (see box, page 27). Arch Henderson is currently developing the design of the passive cathodic protection system with a specialist consultant, and installation is scheduled to start in November. Concreting operations are due to finish in early February.