Medway Viaduct - Refurbished for the next 40
years
The River Medway poses a significant obstruction
to the transport routes from London to the Dover
ports and Channel Tunnel and with todays traffic
volumes a high capacity crossing is essential.
The problem is that the existing viaduct is simply
not up to the job, although it is not totally
a lost cause. The solution from CSM (Costain,
Skanska, Mowlem) Joint Venture working under contract
to the highways agency sounds straight forward
enough: build a new viaduct alongside the old
one, divert the traffic onto that, refurbish the
old one and then use the new viaduct for traffic
from the coast and the old one for traffic from
city, doubling the original capacity.
Concerns over potential chloride attack from
road salt penetration into the deck of the old
viaduct were one of the many problems that had
to be addressed to ensure the future life of the
structure. This would need a detailed survey covering
the entire 1km length of the viaduct, a deck area
almost 30,000m2, a huge task in itself and one
which was taken on by the Materials Testing Division
of Soil Mechanics.
The process adopted by Soil Mechanics included
non-destructive investigation, visual inspections,
material sampling and laboratory testing to identify
the areas suffering chloride attack and in need
of repair.
The first part of the process comprised non-destructive
surveying using 'half cell' potential meters to
identify areas of elevated chloride content. Once
the tarmac surface had been stripped by the main
contractor, the walk over half cell surveys were
carried out by Soil Mechanics technicians over
the entire viaduct deck in 6m squares. A colossal
task with readings taken at 0.5m intervals: 144
readings per square, well over 100,000 readings
in all.
Results from this survey enabled the areas of
potentially elevated chloride content to be delineated.
Samples from three different depths were then
taken from these areas and from a control location
in each of the 6m survey squares for laboratory
testing to determine the precise levels of chloride.
In total 8,000 chloride tests were carried out
and this required the utilisation of four commercial
laboratories to complete the testing within the
contract period.
The chloride attack survey was completed by a
thorough visual and sampling inspection of the
joints and the bridge underdeck from self elevating
platforms and specially constructed gantries.
Repair work by the main contractor to affected
areas followed however particular attention is
required to avoid damaging the existing buried
reinforcement. To ensure this was not damaged
during the repairs, Soil Mechanics provided 'Cover
Survey' information on the location of reinforcement
across the whole of the viaduct deck.
Voiding was also considered a potential problem
within the grouted pre-stressing tendon ducts.
Under the direction of Soil Mechanics, this work
was carried out by materials specialists Sandbergs.
The tendon duct surveys comprised drilling and
borescope surveys with the results being presented
as in descriptive and photographic format.
The project valued at £140k saw Soil Mechanics
working alongside the main contractors for some
four months.
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