21 Members visited the Limehouse Link road on a dull but mild 17th March.
The Limehouse Link is 1.8 Kms of road with 1.5 Kms in tunnel, connecting the City of London with the Isle of Dogs in the London Enterprise Zone. The four lane highway widens to a grade separated underground junction with speed change lanes and slip roads.
From arrival all 21 Members were firm supporters of the new road after spending a half an hour on the coach in dense traffic travelling the last half a mile to the site on the existing road.
After a welcoming cup of coffee and an inspection with our guide from the Consulting Engineers, Sir Alexander Gibb and Partners, of maps and pictures of construction, we were shown a 20 minute video film of various aspects of the construction work. We then visited parts of the site to see construction currently in progress.
The area through which the road had to pass contained several docks and waterways, some existing roads, numerous Statutory Authority Services, and a number of large blocks of flats. The decision to route the road underground was taken to minimise future disruption of this area and studies of soils dictated a ‘top down’ method of construction. The design and documentation involved the production of 850 drawings and 18 volumes of tender documents. The investigations, design, public enquiries, various planning permissions, tender and award were completed in 40 months, a remarkable achievement for such a complex project. The Contract was awarded to a consortium of Balfour Beatty and Fairclough,
Before construction could commence, residents of flats to be demolished had to be rehoused and residents temporarily rehoused of flats adjacent to the site, who would be severely affected. Severe noise and dust limitations were written into the contract and monitored during construction. To avoid disruption to approach roads, all materials, plant and equipment were required to be brought in and removed by river barges.
Completion to a tight timetable was essential to meet traffic requirements for the Canary Wharf Development and this together with limitations imposed by the restricted site, the need to utilise only river traffic, limitations on noise and requirements to maintain services and existing road traffic placed considerable burdens, on the Contractor, additional to those entailed by the complex nature of the work.
Five depots were set up along the Thames to handle transhipment of materials and equipment, including materials from North Sea dredging, and to dispose of spoil. On site the Contractor constructed a £2.5 million handling facility for this river traffic amounting to some 3.5 million tons of material and as a location for a central concrete batching plant for the 400,000 tons of concrete pumped to all parts of the site, temporary bridges were built across the site for pedestrian and vehicular rights of way and to carry existing services..
The ‘top down’ construction as designed comprised:-
a) after clearing the site, construction of 1.2m by up to 24m deep disphragm walls by excavation in trench in bentonite, installation of reinforcement cages and concreting. In the six lane wide section of the tunnel, central bored piling was installed as temporary props to the roof during construction.
b) well points were established to dewater between the walls and these had to be carefully designed to avoid disturbance to adjacent ground.
c)excavation between the diaphragm walls down to about 4m to cast the roof. Dropping the diaphragm walls dundnq excavation. The roof, except where it would be under water in the Limehouse Basin, was 2.25m. thick, designed to allow structures up to 5 stories in height to be built over the tunnel in the future. Temporary openings were left in the roof for access.
d) excavation below the roof to form the tunnel using tracked front end loaders operating from the temporary openings. During excavation the diaphragm walls were propped using 1.3m diameter horizontal props at 4m centres.
e) concreting of the base slab, central wall and wall linings.
The Contractor proposed several changes to this construction procedure and some were accepted which benefitted the project in time or cost. The most significant change was in construction across the Lime-house Basin, an area of water into which the Regent’s Canal discharged, and which was to be retained for amenity reasons. Here he constructed the tunnel box by ‘bottom up’ conventional means in dewatered ground between temporary sheet pile cofferdams.
We saw the closing stages of this construction in the Limehouse Basin where lm of fill was being placed over the completed roof as a protective lay before removing the temporary props and piling and allowing water to return over the completed tunnel. We were also able to see at one end of this construction, the floor of the tunnel where the roof had not yet been cast, and to appreciate the depth of the tunnel some 20m below ground level. Further east we looked along the roof towards the widened section where work was in progress to concrete the final temporary openings.
In a two hour visit to a construction project costing over £200 million one cannot expect to receive more than a few glimpses of the planning, organisation, effort and expertise necessary to bring it to a successful conclusion. At least we will have a little more appreciation than the thousands of commuters who, in about 2 1/2 minutes, will eventually drive through this, section of road.