After the publication in July 1972 of the Urban Motorways Committee report "New Roads in Towns" the proposals were reviewed. That review led to three "on-line" and two western bypass alternatives being identified as possible improvements between Roestock and Stanborough.

The public were consulted in 1974 to obtain their opinion of these alternatives and also to invite comment on the need for improvement and to elicit any other proposals. The scheme now being constructed was one of the "on-line" alternatives and it was selected as the Department of Transport's preferred scheme in 1977.

The scheme was the subject of a Public Inquiry in 1981 at which the details of the project and its effects were described, and objections to the scheme were presented, some of which resulted in amendments being made to the scheme as presented. The revised scheme is that which was built.

The aim was to create an acceptable solution to a difficult problem, which would overcome the conflicting demands of highway, environment and local needs. The overall concept had been to create a scheme which offered the opportunity to revitalise and replace the infra-structure of the District at the same time as solving the very obvious traffic problem in this largely urban environment.

Hence, a partnership involving both the public and private sectors, was brought together to bring to fruition a suitable scheme.

The Department of Transport, the Hertfordshire County Council and the Welwyn Hatfield District Council worked closely together during the project conception. The chosen scheme included, on environmental grounds, a 1150 metre long tunnel and in so doing created a redevelopment area of some 25 acres of land on the western side of Hatfield only ½ mile from the Town Centre.

In the spring of 1983, work started at Hatfield on the alterations to gas, electricity and water mains, sewers and telephone equipment, and demolition of buildings. This was necessary to enable road construction to proceed unhindered.

On April 2^nd 1984 Tarmac National Construction (now Carrilion Construction) started road construction work under contract to the Department of Transport. This work was supervised by Hertfordshire County Council.

On October 26^th 1984 W.H. Smith and Company Electrical Engineers Ltd were appointed to supply the electrical and mechanical services for the tunnel, the work being supervised by Mott Hay and Anderson Electrical and Mechanical Services.

The final link in achieving this overall Concept was the selection by the District Council of a development partner.

Numerous public utilities' services followed the route of the existing A1 through Hatfield and the new motorway is very close to the existing road and crosses it twice.

Large scale alterations were necessary to the public utilities apparatus to enable the new road to be constructed. As far as possible these alterations have been carried out in advance of the road construction. This was to minimise the disruptive effects which would otherwise have been felt both on the road construction and on public services. The alterations to the public services were co-ordinated by Hertfordshire County Council and were carried out by the various service authorities, British Gas, Eastern Electricity, Lee Valley Water Co., Welwyn Hatfield District Council (its sewerage agent to Thames Water Authority) and British Telecom.

The built-up area on the line of the motorway, between Cavendish Way and Birchwood was demolished by Kimbell Construction Ltd in advance of the road construction. This was done to avoid the potential disruption to construction work due to unforeseeable difficulties in the demolition of a large built-up area.

The demolition contract also included the erection of a temporary noise barrier along the eastern boundary of the demolition area. This barrier remained during the road construction period to screen the adjacent houses from the construction work and was removed after construction of the motorway was completed.

The motorway is entirely at or below ground level. The major cutting is from south of Cavendish Way to Birchwood, and this cutting is the site of the tunnel and its approaches. The motorway is also in cutting where it passes under Roehyde interchange.

The interchanges are on embankments above motorway level.

Excavation totals 900,000 cubic metres of which 390,000 cubic metres were placed in embankments and a further 80,000 cubic metres used as filling for landscaping and other environmental improvements, including the construction of noise mounds to protect adjacent properties from the effects of the completed motorway.

In addition to the disposal of water which falls upon the road surfaces, the scheme involves the lowering of the ground water level at the site of the tunnel.

The permanent ground water drainage system consists of a grid of filter drains below the motorway which discharging to a pump chamber behind the cast abutment of the tunnel at the Southern Sub-Station. A separate drainage system to collect water from the motorway surface in the tunnel also discharges into that pump chamber.

Water from the motorway cutting at Roehyde is also be collected in a pump chamber situated within the interchange roundabout on the west side of the motorway.

Water from the tunnel pump chamber is pumped to the Roehyde pump chamber. From there, together with the water from the adjacent motorway, it is pumped to an outfall into the Ellenbrook.

All water from the motorway and all-purpose roads between Roestock and the north end of the tunnel discharges into the Ellenbrook. However, discharge to the brook has to be limited to not more than 75 litres per second. To achieve this all water first passes into a balancing pond, either via the pumps at Roehyde or directly from gravity drains.

The capacity of the balancing pond is 1,200 cubic metres and the outfall from the pond is designed to pass a flow of 75 litres per second when the pond is full.

Water from the northern approach to the tunnel collects in a pump chamber adjacent to the north portal of the tunnel; discharge from the pumps is piped some 1,600 metres to an outfall into the River Lea. The motorway and all-purpose roads at the northern end of the scheme discharge by gravity to the River Lea.

To prevent pollution of the water courses by oil or other chemicals which might be spilled on the highway, Ellenbrook is protected by the balancing pond which is designed to act as an oil trap. It also has a penstock by which the outfall can be closed if pollutants which are miscible with water should be spilled on the highway.

The River Lea is protected by an oil trap which will also have a penstock to close the outfall.

The 1150m long tunnel is a cut-and-cover structure constructed east of the existing A1 between Cavendish Way and the Green Lanes Roundabout. The tunnel accommodates the full motorway formation width with a continuous separating wall along the motorway centre line. Raised walkways along both sides of each carriageway accommodate the extensive cabling and service ducts required for tunnel services and motorway communications.

The construction of the tunnel is of traditional reinforced concrete, the roof, walls and foundation bases being continuous to form a two bay portal. The tunnel is founded on the glacial gravels with spread footings, but where additional heavy loads from the development structures affect the central wall its foundations are strengthened locally by insitu bored piles founding in the hard chalk sub strata.

It is estimated that nearly 10,000 tonnes of reinforcement and over 80,000 cubic metres of concrete were used to construct the tunnel.

Services include power supplies, lighting, ventilation, environmental control, plant monitoring, pumping, communications, fire fighting, security and traffic surveillance. Automatic control systems will continuously monitor and report to central control and maintenance on the operational state of the plant and information transmitted will be stored on computer disk.

The lighting system for each bore comprises two sub-systems. Basic lighting comprises a near continuous row of twin 65 watt fluorescent Iuminaires over the centre of each of the near-side and off-side lanes. Boost lighting comprises 135 watt and 180 watt low pressure sodium luminaires arranged over catch entry and exit for optimum visual adaptation.

The tunnel is longitudinally ventilated by 52 jet fans in each bore, located above the walkways at roof level and automatically controlled.

Panels at 39m intervals on the nearside walkway, provide emergency telephone facilities connected to police control, and automatic fire hose reels. Radio aerials for emergency and maintenance services are mounted in the tunnel roof.

Closed Circuit television System is provided in each bore of the tunnel and is remotely controlled from Police Control at Hertfordshire County Police HQ at Stanborough.

Sensor loop systems in the carriageway surface at intervals throughout the motorway detect vehicle movements. The data obtained is used to provide the police control room with information of traffic flow patterns and potential problems.

Apart from the tunnel, there are 12 other structures in the scheme plus extensive retaining wall construction through Green Lanes Roundabout.

There are 6 subways, 2 footbridges and 4 large road bridges over the motorway. All of the structures are of traditional reinforced concrete except for the OIdings Interchange bridge where the bridge was built on line with the necessity to keep the Al open and running. This requirement precluded temporary shuttering for an insitu concrete deck, thus precast prestressed concrete `U' beams lifted into position under a short-term temporary closure of the A1 formed part of the main span of the bridge.

 Redevelopment

The route of the new motorway necessitated the demolition of over 200 properties, many of them commercial, but on completion the land above the tunnel will again be available for development together with areas immediately adjacent to the tunnel line which have been acquired by WHDC, so bringing the entire area into public ownership.

On June 12th 1984 the Welwyn Hatfield District Council unanimously selected as their development partner the Carroll Group of companies who put forward the futuristic Park Plaza plan.

With the comprehensive leisure theme which is the underlying feature of the scheme the focus of the proposed development was a 200,000 sq ft. glass covered shopping area situated over the southern section of the tunnel and containing a variety of leisure orientated retail units. Included in this totally new retail concept is an ice skating rink, restaurant, exhibition and activity areas. Covered ways link this centre to a 100-bedroom prestige hotel, enclosed garden centre and further open leisure activity areas. The proposed scheme also contains a small residential zone and a prestige office development at the northern end of the site.