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.
