Concrete repair of a jetty on the estuary of the River
Severn, where the tides are phenomenally high, presents a special
type of challenge. Jack Barfoot talks with the contractors,
Colebrand Contracting Limited.
Built in the sixties, the trestle type reinforced precast
concrete jetty supports and gives service access to a pylon
standing away from the shore immediately downstream of the Severn
Bridge at Aust in the County of Avon.
Work comprised patch repairs to the trestles, together with
repairs to, and extending the cover of, the insitu concrete plinths
at the base of the trestles.
Phil Stainton, who was in overall charge of the contract, and
Chris Walton, Manager of the South Wales office and Contracts
Manager on a major maintenance contract on the neighbouring Severn
Bridge, described the contract and its relevant aspects to me. "The
repairs themselves were fairly orthodox although the length of
defective concrete repaired at any time on a trestle leg was
confined to 2 metres, and on no more than 2 of the 4 arises, in
order to minimise the reduction of the structural integrity of the
trestle during the repair procedure. We used Nufins Nupatch
proprietary fast setting mortar and concrete repair system, and the
general procedure was that of traditionally cutting back any damage
as far as the reinforcement, which was then grit-blasted to bright
metal and primed with a Colebrand material, CXL 111. Then using a
timber shutter, the repair material was poured in from the top,
vibrated and left to set, which took about an hour to achieve".
This simple-enough-sounding operation, however, was fraught with
a number of difficulties, I learned. Programming, for example,
already on a knife-edge of dependency upon tide times and levels,
was further hindered by the limitations of working on a day-to-day
basis. Because of the very nature of the repairs, the volume of
repairs and quantities of materials required were not consistent.
It was not possible to accurately forecast the precise amount of
repair work which would be carried out in a given location. And of
course, added to this was the ever-present factor of programming to
suit changing tide tables and using the time available to the best
"On some days it was possible to achieve two working shifts, when,
say, an ebb-tide took place six am and again at six pm or
thereabouts. On another day only one shift of actual repair work
would be possible. Another factor which could be crucial was the
area where work on the jetty was actually taking place - at the far
end, in deeper water, the maximum time for repair work could be
down to one hour, but on the landward end of the jetty, four hours
could be possible within the same tidal period. These times had to
include that needed for setting of the concrete, of course. During
neap tides work could only take place at the short end, and even
there not much more than fifty per cent of the trestles were
"The plinths, or bases, to the trestle legs constituted the most
difficult part of the work. Some of these had deteriorated quite
badly, as might be expected from their situation. After digging
away the surrounding mud, cutting out the defective concrete and
affecting the normal repair procedure, we then recast and enlarged
the affected plinths to give increased cover and protection to the
reinforcement". This consisted generally of adding 150mm of
concrete to all the one-metre-square faces and tops of the plinths,
and included drilling and fixing resin-anchored dowels to assist in
providing a bond between the old and new concrete.
"The plinths were submerged for most of the time" Phil Stainton
recalled, "particularly at the far end of the jetty, where we could
only work on them during spring tides when the water was at its
lowest ebb level. Even then only a limited amount of time was
available in which to place the material and allow to set. Once the
initial set has taken place Nufins Nupatch is not affected by
immersion". He added, "Which is why it was chosen for this
Scaffolding comprised a combination of lightweight alloy staging
and conventional scaffolding. "We erected the scaffold at the
beginning of the contract". Phil Stainton said, "And left it for a
couple of tides to see how it withstood the effects of strong
current. Erection was not easy, as the support for the scaffolding
necessarily had to be the existing trestles, so we were dependent
for support on the very structure which we were repairing.
Approximately half of the fifty trestles were worked on and the
whole repair including the plinths used ten cubic metres of
material. The experimental immersion of the scaffolding, we found,
was satisfactory, and we were able to leave it in position between
shifts as we worked along the jetty".
In spite of all the difficulties, however, progress on the
contract was good, although the repair work was a little more
extensive than had been originally envisaged.
The client was CEGB, Bristol (CEGB Engineer, Mike
Contractors were Colebrand Contracting Ltd (made up of Divisions
devoted to concrete repair, painting, engineering and specialist
resins) of Goodshawfold Road, Rossendale Lancs.