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Edinburgh City By-Pass
In brief the project itself was not unusual based upon a typical Design and Build Contract
The works continued the basic format of a previous scheme, which improved and widened the A720 Trunk Road to the West of Edinburgh.
The Client was The Scottish Executive
represented on site by Scott Wilson Consulting Engineers.
Like the previous scheme the successful Main Contractor, Barr Construction Limited, and Designer, The Babtie Group had elected to construct a Vertical Concrete Barrier and slot drain along the central reservation of the road, the shoulders were strengthened and the whole existing flexible road provided with an asphalt overlay.
Space for the widening was gained by reducing the width of the central reserve to an absolute minimum and reinforcement of verge batters.
The project value was 6.0 million pounds and the works were to be completed within 30 weeks.
The road footprint available to the Design team dictated that only a 950mm central reservation was available to house the slot drain and a single concrete barrier.
The width of the base of a standard Vertical Concrete Barrier is 650mm leaving only 300mm for the slot drain.
Unlike the previous adjacent improvement scheme or for that matter, any previous project in the UK to date, the design required that the single barrier profile varied along the length to suit the differing levels of the two adjacent carriageways.
Where the adjacent Westbound and Eastbound carriageways are designed to be constructed at the same level a standard profile Vertical Concrete Barrier symmetrical about the centre line provides both carriageways with the required minimum 800mm high wall and associated haunch toe detail.
However where the adjacent carriageway levels differ, in order to maintain a 800mm high wall adjacent to the higher of the two carriageways, the barrier height adjacent to the lower carriageway must be increased, resulting in a non symmetrical barrier profile.
The maximum level difference experienced was 350mm but this occurred in both possible cases. For example, in places the Westbound carriageway was 350mm higher than the adjacent Eastbound carriageway whereas in other locations the opposite was true the Westbound carriageway was 350mm lower than the adjacent Eastbound carriageway.
We realised that this project required a double-sided variable barrier mould, possibly the most complex paving mould imaginable.
Slipforming variable barrier follows the same basic principle as any other section:- the passage of a conforming plate over consolidated concrete. The subtle difference is that the conforming plate profile is constantly changing shape!
Both entire sides of the conforming plate and hopper are capable of independent movement. In order to ensure that this movement occurs only in the intended plane the sides and hopper are supported in a heavy steel box chassis.
Only the roof of the mould remains in a fixed position, which is dictated by the paver. The sensor mounted to the paver dictate the line and level of the mould roof and therefore the top of the barrier wall.
The position of each side relative to the top of the wall is controlled automatically by a separate sensing and hydraulic system.
The sensors, 2 for each side of the barrier, run along the surface of the carriageway adjacent to the barrier haunch.
Whilst the stage of construction at the time of our works allowed us to take levels of existing adjacent carriageway variable barrier could as easily be paved fully free-standing where the level of each side and haunch is taken from sensors running on 2 additional guidance wires.
The relatively short time between our being awarded the Sub-Contract for the slipform works and our required start on site precluded our preferred option to purchase the appropriate mould from our original equipment manufacturer, Gomaco. Whilst they may have been able to fabricate the mould the time taken to ship would have caused unacceptable delay.
We decided that the mould should be designed and fabricated in house by our own Engineering Division who have considerable experience of slipform mould manufacture including slab paving, barrier, canal and slot drain. In all over 250 moulds have been designed and fabricated. We had not however fabricated a variable profile barrier mould.
We realised from the onset that to attempt to design, construct and operate such a complex mould without any experience would embark us upon a rather lengthy learning curve. We sought the help of Gomaco.
Both Gomaco International in the UK and Gomaco Corporation in the United States provided the invaluable benefit of their considerable experience. We visited the factory and sought the answers to our numerous questions.
Design and fabrication took us around 12 weeks and involved input from all corners of the company.
The mould cost around ten times the price of a standard non-variable mould and weighed in at around 6 tonnes compared to a standard non-variable mould at around 1 tonne
Our works on site were programmed to be undertaken within a 3-week window within which we had to allow for periods of non productivity due to inclement weather. As you will appreciate this is a little difficult to accurately foresee in Edinburgh in Autumn.
Our works involved the slipforming of approximately 1,800 linear metres of 150mm bore slot drain and a similar quantity of variable barrier.
Two Gomaco Commander III four track paving machines were mobilised one to pave the slot drain and one for the barrier.
The Main Contractor had to maintain at least 2 lanes of traffic in both directions through the works. It was decided to run both pavers on the Eastbound carriageway mounting the moulds on the right hand side.
This enabled our works to progress in the same direction as traffic flow and did not involve the turning around of our concrete supply trucks.
The slot drain was paved first, interestingly against the edge of the existing Westbound carriageway – this is the opposite of normal “scab-on” type construction.
Production of up to 600 linear metres per shift was achieved. Although we believe this could easily be exceeded on a project where greater lengths were available.
Contraction cuts were sawn transversely as soon as practicably possible at 5 linear metre intervals. Outfalls were paved through and a gap provided by sawing completely through the slot drain section and simply lifting out the surplus length. The carrier pipe work was then raised and a grating installed.
Barrier construction followed on, utilising the second paver. Linear production was somewhat less than that of the slot drain at around 250 linear metres per shift or 120 cubic metres.
As with all concrete barrier the section included 5 T10 longitudinal reinforcement bars which were fed into the front of the paving mould. These bars were supplied in 12 metre lengths, overlapped 300mm and welded at either end of the lap. The bars were located down the centre line of the barrier at 150mm centres. The position of the bars relative to the top of the wall did not vary.
The overall height of the barrier wall was maintained within the Specified range of +20 –10mm at any given point. Surface regularity was maintained within a range of +/-10mm over a 3 metre length. These tolerances provided the Client with a flat barrier wall.
The works were completed within 17 working days including eight transition units cast between fixed forms connecting the slipformed barrier to pre cast units at cross over locations and existing insitu barrier cast around bridge piers.
Grade C35 concrete was supplied by Tarmac Topmix Northern as detailed below:
| Cement Portland Blast Furnace – 60%PC & 40% GGBS | 360kg / M3 |
| 20-5mm Limestone Aggregate | 946kg / M3 |
| 5mm Sand | 731kg / M3 |
| Washed Building Sand | 168kg / M3 |
| Air Entrainer | 5.5% |
| Polypropylene Fibres | 0.9kg / M3 |
We used exactly the same concrete mix for both slot drain and barrier production. Albeit that the workability was increased from 20mm to around 30mm when paving slot drain.
Air testing of the concrete was rigorously enforced with a test being carried out on each delivery. There were no failures.
Again transverse construction cuts were sawn at 5 linear metre intervals as soon as practicably possible in the set concrete. The cuts were aligned with the cuts previously placed in the slot drain.
A secondary wider cut was placed to allow the contraction joints to be sealed.
A project is soon to be carried out on the M25 motorway, which includes the use of a wider Vertical Concrete Barrier that permits lighting columns to be fixed to the top of the barrier wall, such that the barrier wall not only acts as a safety restraint system but also as the base for the street lighting column.
If this principle was to be developed to include a variable wider profile the costly twin barrier detail which exists between Junctions 8 and 11 of the M25 could be superseded.
Variable barrier provided one of two final pieces in the puzzle. The other piece is a workable Specification and set of drawings. It was some 12 years ago that the first successful trial of a slipformed barrier took place, we are still awaiting production of the Specifications and drawings.
Little wonder that the steel safety fence manufacturers continue to enjoy almost total market domination.
Little wonder that we the tax payer continue to fund the high cost of steel barrier maintenance.
Little wonder that we the motorist see on our televisions pictures of vehicles which have breached the central reservation steel barrier and crossed over on to the opposite carriageway causing serious injury and loss of life.
Little wonder that our country’s Highway Designer Engineers consider steel first and concrete second.