M11 London-Cambridge Motorway
The whole 55-mile length of the M11 lies in the Eastern Region from Junction 4 with the A406 North Circular Road (there are no Junctions 1 to 3) to Junction 14 and A14/A428 north-west of Cambridge.
The need for a new motorway between London and Cambridge arose from consideration during the 1940's of the existing A11 trunk road as an important route serving traffic between London and East Anglia. The existing road at its south end started in the City of London and ran through the busy outer urban areas to Woodford in the north-east sector of London. It then ran through the heart of Epping Forest and the urban areas of Epping, Harlow, Bishop's Stortford, Stansted Mountfitchet and several other communities. All of them suffered from the heavy commercial vehicles trundling through narrow streets neither built nor intended for the volume and type of traffic.
In August 1962 the Ministry of Transport, the Rt Hon Ernest Marples, appointed W S Atkins & Partners as consultants to investigate a proposed motorway between Temple Mills in east London and Bishop's Stortford in Hertfordshire. Its purpose was twofold:
But the terms of reference did not preclude consideration of routes other than those specified. In the event, the project was subsequently extended beyond Bishop's Stortford to Stump Cross, 18 kilometres south of Cambridge, and an alignment through the Roding Valley in the London urban section was adopted in place of one through the Lea Valley (proposed initially by the Ministry and the County Surveyors concerned) in order to achieve savings in excess of £10 million.
In all, route alternatives covering 480 kilometres were examined before a draft scheme was published by the Ministry in 1966. This met with numerous objections, as a result of which the Ministry modified their proposals and republished them in December 1968. In July of the following year a public inquiry was held at Epping at which further objections to the project were voiced, not least of them being concerned with the inevitably intrusive effect of the proposed motorway on the Roding Valley, the neighbouring residential areas and their sports and recreational grounds. But the scheme did provide the most acceptable solution to a difficult problem, and the outcome proved successful. In the autumn of 1969 W S Atkins & Partners began the final design of the route.
One of the largest motorway projects ever to be engineered by a single consultant, this section of the M11 ran a distance of 63.6 kilometres northwards from Redbridge in east London to Stump Cross, where it temporarily rejoined the A11 south of Cambridge prior to the completion of the Cambridge Western By-pass. When complete, it cost over £40 million. The Eastern Road Construction Unit were responsible for designing the extension by-passing Cambridge on the west side to link it via the A14 Trunk road to the A1 for traffic to the north and East Midlands.
South of Cambridge, construction was divided into four contracts. Contract 1, mainly urban, ran from Redbridge to Loughton, Essex, a distance of nine kilometres; and 2, entirely rural, from Loughton to S Harlow (13 kilometres). Contract 3 continued across open country to just east of Bishop's Stortford in Hertfordshire (15 kilometres); and 4, still rural, temporarily concluded the motorway at Stump Cross (27 kilometres). Contract 3 was the first part to commence and be completed followed by Contracts 1 and 2. Contract 4, in two sections, 4A and 4B, commenced shortly after for completion in 1979.
Contract 1. Redbridge (J4) to Loughton (J5)
Close liaison was maintained with the local authorities, police and other interested parties during the planning and design stages. Anticipating publication in November 1972 of the Government White Paper entitled "Development and Compensation: Putting People First", the DoE in conjunction with the GLC and local authorities embarked on a detailed study of further measures that would reduce the impact of such projects on the environment. Contract 1 of the M11 was thus the first motorway constructed to incorporate the provisions of the White Paper.
Environmental considerations were very much to be desired. Contract 1, being largely urban, has undeniably affected the lives and amenities of local residents to a great extent - but it could have disrupted them entirely. No less than 27 structures comprising viaducts, bridges and subways were built on this section of the motorway. In addition, electricity, water and gas services were altered, electricity pylons and main sewers resited, and the River Roding diverted in three places. Consequently, extensive efforts had to be made to minimise the impact on residents and their amenities. Restrictions were applied on haulage routes for incoming and outgoing material; minimal use of public roads was permitted for moving excavated soil; access to the site was confined to specific locations; and most important to the local residents maximum noise levels for construction plant were established and implemented.
Other remedial measures included the siting of noise barriers, the provision of sound insulation to private dwellings in the vicinity of the motorway, landscaping motorway embankments with large-scale plantings of trees and shrubs (augmenting the GLC proposals for the Roding Valley Linear Park), the introduction of additional accesses below the motorway to permit unrestricted movement from one side to the other, and reasonable compulsory purchase and additional compensatory payments to owners and tenants whose homes or other property have been lost through or adversely affected by the construction.
Contract 1, let in October 1973, extended through the flood plain of the River Roding from the southern terminal with the A12 in Redbridge to a partial interchange with the A1168 in Loughton. It crossed over London's A406, one of the busiest roads in Britain, to form the Woodford Interchange, which linked the M11 with the A406 and incorporated provision for further roadworks in the future.
Six viaducts occurred at this interchange. They carried two kilometres of elevated sliproads, and ranged from a 14-span, 453 metre long viaduct to one of eight spans and 243 metres long. All occurred on horizontal and vertical curves and carried a two-lane, 7.3 metre wide carriageway. The remaining 5.6 kilometres of Contract 1 running northwards to the interchange at Loughton consisted of a dual, three-lane, 11 metre wide carriageway.
For five of the viaducts, 49 of the 55 deck spans required were formed from a total of 539 precast, pretensioned concrete M beams. A special in-situ concrete integral crosshead over the piers was developed to overcome the problems of establishing beneficial deck continuity, the variables imposed by the horizontal and vertical curvatures in the essentially standard deck spans, and the closure of the gap between the longest M beam available and the greater length optimum span utilised.
The original specification laid down conditions for transporting the beams from railhead to site. In the event, however, the contractor's alternative proposal of casting them adjacent to the site and thus avoiding severe transportation problems was accepted. But to cast so many M beams at site was a major undertaking unmatched by previous experience. Further, all beams required deflected tendons, and special equipment for this purpose had to be developed and installed in the casting beds. Beams were in two sizes, 29 metre long for standard spans and 24.5 metre long for end spans; each size was 1.2 metres deep and weighing 27 and 23 tonnes respectively.
The vertical profile of the Contract 1 motorway was influenced by the need to keep the carriageway formation above river flood level whilst maintaining a minimum height above ground level roads. Hence the viaducts. In the flood plain the embankments carrying the motorway rose to nine metres, and the only sections of cutting were at the western limit where an existing A406 cutting was widened, and a 3.2 kilometre length north of Woodford Bridge where the route curves away from the river through higher sloping ground.
Approximately 1.1 million cubic metres of predominantly London clay, renowned for its unmanageability, were excavated for compaction into embankments. Much of it came from the huge cutting which forms the Chigwell Service Area, two kilometres from the start of Contract 2 at Loughton. The motorway ran straight through what was the Chigwell Council tip and the rubbish accumulated there over 30 years had to be removed. The slopes were overdug, then backfilled with clay to stabilise them. More soil for filling came from an improvement scheme on the A406 at Waterworks Corner, close to, yet unconnected with, Contract 1 but operated by the same contractor, W&C French (Construction) Limited.
The various structures of Contract 1 comprised, in addition to the six multispan viaducts already mentioned, three overbridges and two underbridges for road crossings, five river bridges, one accommodation overbridge, three accommodation underbridges and six pedestrian subways. A new bridge carrying the London Transport Central Line over the motorway was built under an advance contract designed and supervised by London Transport. All of these structures were of prestressed or reinforced concrete.
Meanwhile, the River Roding continued to meander along the general line of the motorway in Contract 1 and at the southern end of Contract 2. To minimise the need for structures over the river and also the risk of flooding in the future, the Roding was diverted in a few places. As one of the older residents in the vicinity pointed out, this was not the first time the river had been diverted and at Woodford Bridge in Contract 1, it was diverted back into its former bed! But there was now little risk of flooding, and less possibility of damage if it should ever recur.
Contract 2. Loughton (J5) to South Harlow (J7)
Contract 2 was let in January 1975 to Dowsett Engineering Construction Ltd and traverses undulating farmland and woodland throughout its length. Its rural nature, and lack of interchanges presented ideal conditions for "rigid" pavement construction with a minimum of discipline from structural requirements. It was not surprising therefore that the contractors chose this Contract to establish themselves in the field of paving quality carriageway construction, even though the southern 1.7km was specified, without option, as flexible construction because its alignment was close to the River Roding, where settlement was anticipated.
The Specification for Road and Bridgeworks, as amended by Technical Memorandum H 10/71, issued by the DoE formed the basis of the design. The contractor elected to construct the carriageway in unreinforced concrete. The slab thickness was 280mm nominal except where the CBR was expected to be 2 per cent or less when a 305mm slab was specified.
The three lane dual carriageway is designed to rural motorway standards with a 4m central reservation, 11.2, carriageways including hardstrips and 3.1m hard shoulders. The contractor chose to build the hardshoulders in flexible construction. The sub-base throughout was Group N (granular material complying with Clause 804 Type 2 of the specification).
Longitudinal joints were specified adjacent to lane divisions but the contractor was permitted certain choices for transverse joints. Since construction was programmed for completion within the specified summer months, the contractor elected to omit expansion joints. Transverse contraction joints spaced at 5.0m intervals, with no warping joints, were chosen. For longitudinal joints, 10mm dia. x 750mm long high yield deformed bars were required at 600mm centres whilst for contraction joints 25mm dia MS bars 400mm long at 330mm centres were specified. The concrete carriageway, was curtailed where underbridges were traversed; this assisted in avoiding possible settlement damage behind abutments.
Contract 2 was the first major motorway awarded in the south-east to Dowsett Engineering Construction Ltd., little was known of their abilities with respect to concrete carriageways which apparently resulted in a desire on their part to show that they were more than capable of competing with established contractors in this field. Good cooperation between contractor and engineer needed to be established at an early stage so that mutual understanding can prevail.
The initial decision by the contractor to invest in a new SGME rail-mounted paving train and batcher undoubtedly provided an essential pre-requisite to good construction. Inadequate, ineffective or troublesome plant only leads to interruption to paving and unsatisfactory work.
A constant high standard of supervision, both on the part of the contractor and resident engineer, was essential for smooth operation. Dowsett appointed a paving agent and sub-agent, and the supervision and inspection for the whole length of the Contract was provided by an engineer and inspector working in conjunction with respective section engineers.
This procedure eliminated differing attitudes by section engineers in the field and maintained a constant standard of supervision throughout.
(No information is available on any other aspects of this contract)
Contract 3. South Harlow (J7) to Bishop's Stortford (J8)
The main contractor for this scheme was Fitzpatrick & Son (Contractors) Ltd. The scheme consisted of dual 3-lane concrete carriageways 11.2m wide with 3m hard shoulders, and extended some 15.6km between the South Harlow and the Bishop's Stortford Interchanges. There were 2.6 km of terminal slip roads within the interchanges. A 2 km 2-lane asphalt-paved Link Road connected the northern Interchange to the A11 (now A120) northwards and formed a north eastern bypass of Bishop's Stortford. Both interchanges were omni-directional as roundabouts over the Motorway; the southern roundabout was extended westwards to intersect the A11 at grade and incorporated an additional 'filter' link for southbound traffic on the A11.
Earthworks were predominantly in Boulder Clay entailing excavation in cutting of 2½ million cubic metres and compaction of 1½ million cubic metres to form embankments. Due to the presence of water bearing gravel lenses and water permeation at excavated formation levels in cuttings it was necessary to excavate 250,000 cubic metres of more suitable filling material from local borrow areas.
The route traverses the Epping upland region. This is an area of rolling terrain requiring sensitive environmental treatment because of the high quality farmland and scattered village communities. Within these features it has been possible to achieve a direct flowing alignment with gradients not exceeding 2.7% and curvature generally above 3,250m but decreasing (for environmental planning purposes) to 1,369m in one location. The cuttings and embankments generally extend to between 4m and 8m respectively above and below the motorway level, and reach maximum depths of 11m and heights of 15m.
The excavation of cuttings and the formation of embankments required handling over 4 million cubic metres of boulder clay, gravel and silt material. Due to the high water table and water-bearing layers encountered within the clays, only about 70% of the 2½ million cubic metres excavated could be used for embankment construction. A quantity of 250,000 cubic metres was excavated from drier 'borrow' areas to complete the embankments and to replace waterlogged ground adjacent to stream crossings and beneath foundations in the cuttings. The slopes of cuttings and embankments were either 1 in 2, 1 in 2½ or 1 in 3, consistent with materials stability, and cutting stability is augmented by slope drainage in various locations.
A permitted Tender alternative of 'rigid' motorway pavement construction was adopted for this contract in combination with 'flexible' hardshoulder construction. ('Flexible' construction was adopted for all slip roads and other roads upon the Contract.)
The motorway is paved in 'unreinforced' concrete upon a sub-base of Type 2 granular (gravel) material. The construction depths vary in accordance with subgrade CBR strength values (of 3 or 5) from 275 to 200mm for the concrete and 150 to 175mm for the foundation. The entire 11.2 metre width of each carriageway was paved in a single pass of the concrete train between July 1974 and March 1975. Operations continued throughout the winter upon a cement-bound gravel-sand foundation, taking special precautions against frost damage to materials and finished work, and without serious reduction of output (summer maximum was 300 lin m per day). As the carriageways are unreinforced, joints were close-spaced at 5 metre intervals and expansion joints were introduced at 60 metre intervals in the sections laid during the winter months.
The Pavement Quality Concrete accords with the requirements of Section 1000 of the DoE Specification for Road and Bridge Works and was constituted as follows for normal working conditions: A/C 6.65.1., W/C 0.46, Air Content 3.5% average. The gravel aggregate was obtained from pits in the Chelmsford area. Grading is to 40mm maximum size with 35% sand.
All other roadways upon the contract, the slip roads and the motorway hard shoulders, were paved in hot rolled asphalt over a lean concrete foundation. The shoulders were slurry sealed.
There were 28 structures, including bridges of various types, in the contract. Most of the M11 bridges were built to accord with standard 'type' designs. All bridges in Contract 3 were standardised 'types' of which 3 apply to road overbridges, 2 to road underbridges and one for accommodation overbridges. In this manner a visual interest is maintained by varied appearance with individual sites defining the particular 'type' requirement.
Road curvature defined the use of three or four span overbridges and the skew and width of road underpasses define which of the two types of underbridges were constructed at individual sites. The 4-span overbridges were further designed as type alternatives - as constant depth spans or with variable depth motorway spans more ideally suited to carriageway superelevation requirements than where the road alignment is straighter. Within these type designs varying finishes were applied together with varied forms of pier construction to provide further visual contrasts-pier shapes, grit blasting and column faceting are examples. Cutting slopes beneath side spans were specially finished, The two underbridges types were designed for either large or small skew road crossings by single 2 lane carriageways. The 'tunnel' effect sometimes encountered has been overcome by a 3 span structure for the latter type of crossing.
A single clear span is provided for all farm accommodation bridges. Two footbridges - examples of four M11 design types - were provided upon this contract.
There were 25 bridge structures of various types comprising 10 overbridges, 3 underbridges for road crossings, 5 accommodation overbridges, 5 underpasses for farm accesses and 2 footbridges. All were constructed in prestressed and reinforced concrete, and in accordance with standardised 'type' designs applicable for the whole of the M 11 Project.
Two small skew underbridges were built to carry the motorway over single-carriageway side roads. The widths of these bridges had a dominating effect, and undesirable dark tunnels would have been very evident if designs with a single span and abutment walls had been adopted. A continuous three-span deck on column piers provided a better solution by opening up the motorway embankment 'slot', so improving both appearance and natural lighting. However, the flat (1 to 2½) embankment slopes would have required considerably larger outer spans flanking the carriageway span. Such an unfavourable span relationship is uneconomic and visually unsettling.
Another unusual feature was the inclusion of a light-well grillage in the central reservation of the deck, to alleviate further the tunnel effect. Open wells would have required vehicle parapets which would have been potentially hazardous where they ended next to the fast lanes. The grilles are of reinforced concrete and designed to withstand errant vehicle loading.
An unconventional feature of the 38-metre-span accommodation bridge was that it was constructed of reinforced concrete cast in place, yet lost little in slenderness against the more conventionally prestressed footbridge of lesser span. This feature was achieved by the use of hollow construction and heavy overhanging end sections.
There were two differing footbridges in Contract 3 and, as in most motorway projects, considerable design effort was expanded on these lightly loaded structures to provide an interesting and attractive appearance.
Construction throughout was of concrete cast in place, prestressed in the deck and reinforced in the piers and stairs. The vee piers were supported on concrete hinge bearings over cast-in-place reinforced concrete piled footings.
The overbridge decks were conventional, continuous four-span slab structures of reinforced concrete cast in place. The outer verge piers were completely different from the middle pier in the central reservation. This came about because of the differing load conditions on the central and outer piers set up by the adoption of symmetrical articulation with expansion joints at each end of the bridge.
The use of expansion bearings at the tops of the outer piers was found to be less economical than the use of built-in circular columns, slender enough to offer minimal resistance to deck movements, and providing little more than vertical support. The central pier had then to be designed to resist the majority of the traction and braking effects on the deck and required a solid or pierced wall structure, thus contrasting strongly with the outer piers. These central piers had dowelled rubber hinge connections at deck level and were designed in three basic shapes to offer variety along the motorway.
Contract 4. Bishop's Stortford (J8) to Stump Cross (J9)
Contract 4 was split into two sections. Contract 4A (valued at £8m) extended northwards from the Bishop's Stortford interchange (junction 8) to near Quendon and was undertaken by Sir Alfred McAlpine (Southern) Ltd. The contractor for Contract 4B (valued at £9.8m), which extended from Quendon to the Stump Cross interchange (junction 9) was Holland Hannen and Cubitts (Civil Engineering) Ltd (later bought out by Tarmac (now Carrilion Construction)).
Both contracts, totalling 24 km, included dual two-lane concrete carriageways 7.3m wide with 3.5m hard shoulders. A further 0.7km long link connected the M11 motorway with the Cambridge Western Bypass.
Earthworks were predominantly in boulder clay and chalk, with this latter material being confined to Contract 4B and constituting approximately 50 per cent of the total excavation in that contract. Due to the presence of water-bearing gravel lenses and water permeation at formation level in cuttings it was necessary to excavate 400,000 cubic metres of more suitable filling material from local 'borrow' areas.
The route traverses an area of rolling downland, requiring sensitive environmental treatment because of the high quality farmland and scattered village communities. To take account of these constraints, and at the same time achieve a flowing alignment, radii generally between 1312m and 2755m were used in conjunction with gradients not exceeding 3.8 per cent. The cuttings and embankments reached maximum depths and heights of 11.5m respectively below and above motorway level. The Department of Transport later carried out tree planting and other landscape work, and the contracts included extra deep soiling at selected locations for this purpose.
The excavation of cuttings and the formation of embankments required the handling of over 3.9 million cubic metres of boulder clay, gravel, silt and chalk material. Due to the high water table and water-bearing layers encountered within the clays, only about 70 per cent of the three million cubic metres excavated could be used for embankment construction and this was supplemented by 400,000 cubic metres of material excavated from drier 'borrow' areas to complete the embankments and to replace waterlogged ground adjacent to stream crossings and beneath the road formation in cuttings. The slopes of cuttings and embankments varied from 1:1½ to 1:3 consistent with materials stability, with cutting stability being augmented by slope drainage in various locations.
There were 29 structures, including bridges of various types, in the two contracts. Most of the M11 bridges were built to standard 'type' designs, and therefore many of the bridges on contracts 4A and 4B were similar to designs already built on the completed sections of the motorway. In this manner a visual interest is maintained by varied appearance with individual sites defining the particular 'type' requirement.
Except where horizontal curvature prevented the use of a central pier, motorway overbridges were normally four span, being either constant depth spans or with variable depth motorway spans more suited to super-elevated carriageways. Within these 'type' designs varying finishes were applied together with varied forms of pier construction to provide further visual contrasts. Underbridge types were either rectangular or, in order to counter the 'tunnel' effect, had sloping walls.
Farm accommodation, foot and bridleway bridges occured in several different forms to suit the individual locations and to further increase the 'interest' factor.
The Cambridge Western Bypass (J8 to J14)
On the M11, between junctions 10 and 11, a Public Inquiry was held at Cambridge between February and July 1972, a total of 72 days, which considered Cambridge Western Bypass (the length of M11 north of Stump Cross), Cambridge Northern Bypass (A45, now numbered A14) and A604 Improvement Girton - Godmanchester (now numbered A14). The report indicates the many complex issues involved. These included the reliability of traffic predictions, location and layout of interchanges, effect of the motorway on the internationally important Radio Astronomy Observatory of Cambridge University, and many other diverse issues.
The Inspector submitted his report on the Public Inquiry on 30 April 1973, to the Secretary of State for the Environment, Rt Hon Geoffrey Rippon. He recommended that the proposals be made as drafted, subject to various modifications and further reconsideration of interchanges.
This picture is a view of construction work on the M11 at Junction 10 viewed during an air show at Duxford in the summer of 1979. In the foreground is the contractor’s camp with a vast assemblage of graders, lorries, and an aggregates washing and grading facility. Further away lie some contractor’s living and office accommodation and museum car parks. As compensation for severance of the runway, the Imperial War Museum subsequently had the land used for this construction site cleared and a vast display hall for a variety of aircraft built as “accommodation work”.
Designed for the ERCU as a £9.8m contract, construction of the Cambridge Western Bypass length of M11 started in the summer of 1977 and opened to traffic in February 1980, with Contractors Bovis for the length of 8 miles north of Stump Cross and Amey Roadstone for the remaining 6 miles to Junction 14. Project direction was again by the Eastern RCU, but with design and supervision of construction by its Bedfordshire Sub-Unit.