The Chiswick Flyover
The Chiswick Flyover was the first major two-level highway scheme to be carried out in the Metropolitan Area since World War II. The works extended for about half a mile and included a through road to link London's "new" western approach, the Cromwell Road Extension, to the Great West Road without interference from cross-traffic. This through road was carried over a new 400ft diameter roundabout with a 40ft wide carriageway at the junction of the North Circular Road, Chiswick High Road, the road to Kew Bridge and the Great West Road. Traffic was distributed to these roads by means of four slip roads joining the roundabout at ground level. The junction was used by at least 40,000 vehicles a day and was seriously congested at times. The scheme removed a potential bottleneck at the western end of the Cromwell Road Extension.
The through road of the flyover had dual 24ft 0in wide carriageways separated by a central island and each slip road was 24ft wide, providing two traffic lanes. The gradient of each slip road east of the roundabout was about 1 in 20 and that of the through road west of the roundabout about 1 in 29. Other gradients did not exceed 1 in 40.
The flyover was constructed through a densely built-up area but the demolition of buildings was kept to a minimum consistent with engineering requirements. The scheme also involved the diversion of many miles of underground services in the shape of G.P.O. cables, gas mains, sewers, etc.
The Contractor's programme was governed to a large extent by the requirement that construction operations should not impede the smooth flow of traffic, and the work was divided into stages, necessitating careful planning. A temporary slip road on the south side of the works connecting the Cromwell Road Extension to Chiswick High Road together with a temporary enlargement of the original small roundabout were constructed first in order to provide additional capacity for increasing traffic flow resulting from completion of sections of the Cromwell Road Extension. Traffic was diverted from this temporary road to the completed northern slip roads and the northern arm of the new roundabout to enable construction on the south side and further bridge work to proceed.
The flyover embodied two bridges:
the main bridge consisted of four freely supported square spans each of 125ft, with a width of 59ft between parapets, carrying the through road over the new roundabout and
a bridge over Wellesley Road, east of the new roundabout, 70ft single skew span and 124ft between parapets, as the slip roads met the through route at this point and four 24ft carriageways were required.
The approaches of the flyover and the section between the bridges were formed on embankments retained between mass concrete walls, brick faced, with in-situ concrete copings forming guard verges about 4ft wide to the through road and slip road carriageways. The embankment filling consisted of well graded mechanically stable gravel (hoggin) containing between 5 per cent and 15 per cent by weight of material passing a No. 200 sieve. The filling was spread in 9in layers and each layer compacted by nine passes of 8 ton smooth wheel rollers, a state of compaction corresponding to about 71 per cent air voids. Against bridge abutments and retaining walls, compaction was carried out by means of vibrating plate compactors, the degree of consolidation behind bridge abutments corresponding to about 5 per cent air voids.
Carriageway construction consisted generally of a reinforced concrete base slab 10in thick with a surfacing of 3½ in of hot rolled asphalt. Curing of the concrete was carried out by bituminous emulsion spray. Transverse joints were formed in the base slab at a maximum spacing of 120ft and a strip of light steel fabric was laid flat on the base slab at the joints before surfacing, as a precaution against possible cracking of the asphalt.
All structures of the flyover were designed for the Ministry of Transport Standard Highway Loading and for the 180 ton abnormal live load in accordance with British Standard 153 (Part 3A) 1954, Section A.
The abutments and the two easterly piers of the main bridge were supported on reinforced concrete rafts founded on a stratum of compact gravel overlying the London Clay. The western pier was carried on a narrow piled foundation owing to the proximity of an existing 48in diameter sewer at a depth of 20ft below ground level. The bridge abutments were of mass concrete, brick faced, and each pier consists of five massive concrete columns, lightly reinforced, and a heavily reinforced cap beam.
The superstructure of each span consisted of fifteen concrete beams placed side by side, each beam being precast in three sections. The end sections were about 38ft long and the centre sections 48ft long, each section weighing about 33 tons. The beam units were of inverted `T' section with a narrow top flange and with transverse diaphragms cast integral with the unit. The bottom flange of each roadway unit contains eighteen 1¾ in diameter straight ducts formed by casting in steel sheathing. The outer beams of each span were of `U' section constructed of concrete with grey Cornish granite aggregates, the exposed vertical face being lightly bush hammered.
The sections were manufactured in a factory some 60 miles away and transported daily to the site where they were lifted by means of a 45 ton Lima crane and erected directly on the permanent bearings on piers and abutments and on temporary rollers supported on steel trestles on either side of the transverse joints between the sections. Each complete beam after assembly was post-tensioned by the Freyssinet system. An in-situ reinforced concrete slab was cast at the level of the top of the beams and the deck prestressed transversely by the Gifford-Udall system with cables threaded through the webs and transverse diaphragms of the beams. The anchorages were located in the inner webs of the outer `U' beams.
A loading test to failure of one complete roadway beam was carried out in order to verify assumptions made in the design, both in the elastic and ultimate conditions, and to provide information for research on prestressed concrete. The test rig was designed to be capable of applying symmetrical loads of 125 tons at two points 30ft apart. The beam failed when the applied loads reached the value of 118 tons on each jack, in close agreement with the estimated failing load. The actual failing load represents a load factor of three on the combined dead and live loading in the design.
Fixed and rocker bearings were provided alternatively on abutments and piers. The bearing plates and rockers were made of Meehanite `CB' metal, a high tensile corrosion-resisting cast iron, and the locating pins were of stainless steel. Expansion joints in the carriageways and verges above the "free end" bearings consisted of stainless steel sliding plates.
The Wellesley Road Bridge bridge was constructed in two halves separated by a longitudinal joint. The abutments were of brick faced mass concrete on reinforced concrete raft foundations. The complete deck consisted of 43 precast prestressed beams of inverted `T' section post-tensioned during manufacture by the Freyssinet system. The beams were 72ft long, each weighing about 24 tons. They were placed side by side on the bearings and in-situ concrete transverse diaphragms cast between the beams and around metal ducts passing through the beam webs. The joints between the beam flanges were filled with concrete and an in-situ concrete deck cast at the level of the top of the beams. Each half of the deck was pre-stressed transversely by the Freyssinet system.
Rubber bearings were adopted for this bridge. The outer edge beams were constructed of concrete with grey Cornish granite aggregates, lightly bush hammered on the exposed face.
The scheme opened to traffic on September 30th 1959. The consulting engineers to the Ministry of Transport and Civil Aviation for the design and supervision of the Chiswick Flyover Scheme were Messrs. Harry Brompton & Partners and the consulting architect was George Stewart. The main contractors for the flyover were Alderton Construction Co. Ltd. The precast beam units were manufactured by the Cowley Concrete Co. Ltd., Meehanite bridge bearings by the Butterley Co. Ltd. and the rubber bearings by Andre Rubber Co. Ltd. The sub-contractors for the bridge piling were Braithwaite Foundations and Construction Ltd.
The Chiswick to Langley Special Road (J2 to J5)
The Consulting Engineers for this early section of the M4 were Sir Alexander Gibb & Partners. The original scheme consisted of two 24ft wide carriageways forming an elevated viaduct 9680 ft long, commencing from the Chiswick flyover, of which the first 6300 ft is of concrete construction, and the remainder in structural steelwork. After leaving the viaduct the road widens to 36ft carriageways, passes under the Piccadilly line and continues across open ground to join with the east end of the Slough By-pass. All subsidiary roads are crossed by 15 overbridges, and access to the motorway is provided by means of slip roads at three intermediate points: 2 mile west of Chiswick flyover, at Cranford Parkway, and at the London Airport link road.
The elevated motorway section of the contract began in July 1962, and was composed of columns carrying cantilevered crossheads which in turn supported the motorway deck. With the exception of the columns supporting the lattice girder bridge across Boston Manor Road, and the adjoining cantilever spans, the in-situ concrete was provided with a high quality fair-faced finish as struck from formwork. The other four columns had a fluted perimeter.
To avoid interference with traffic below, the cantilever crossheads, and the portal beams at the first interchange, were cast within a double-lattice girder box, which cantilevered out from supporting members adjacent to the concrete columns. The contractors reported that no material damage had been caused to passing vehicles. Reinforcement in the crossheads was generally pre-fabricated and lifted into position by crane, but at the longer interchange portals it was necessary to flash-butt weld the tension bars in position.
The concrete viaduct deck consisted of precast prestressed inverted T-beams laid side by side, which were lifted into position by a mobile gantry straddling the crossheads which travelled forward on temporary bridge beams ahead of each completed span.
Each prestressed beam was supported on natural-rubber bearing pads. These were ½ in thick at the fixed end; and 2½ in. thick at the free end, which was of bonded rubber-to-steel construction designed for a longitudinal movement of ¼ in.
Gaps between the flanges of the T-beams were filled with expanded polystyrene prior to concreting, to encase transverse reinforcement threaded through holes left in the web. The in-situ deck was supported on Asbestolux permanent shutters with polyurethane foam strips forming a grout seal. To avoid propping of the T-beams and also a temporary overstress, the deck was concreted in lengths of 15 ft at a time. An asphalt surface was applied to the finished deck.
The structural steel viaduct through Boston Manor Park, spanned 153 ft and 183 ft between pairs of tapered concrete columns. Main beams were a pair of plate girders internally braced and stiffened, using BS968 steel, and site-welded to provide continuity, thus avoiding bolt and rivet heads. These in turn supported variable-depth plate girders to give the required cross-fall to the concrete deck slab, which was designed as a composite structure under live loading.
All steel was protected by shot blasting in the works, and had been metal sprayed and etch-primed to receive finished site painting. Erection of steelwork was by travelling Scotch derrick working ahead of itself. Main beams were supported on bonded rubber-to-steel bearing pads bolted to the concrete columns, and on temporary trestles prior to welding of the joints, which occurred at approximately one-fifth span.
The bridge carrying Southall Lane over the motorway was of four-span post-tensioned hollow-box construction, utilising precast soffit slabs which incorporated vertical starter bars for the in-situ webs. These slabs were curved in two directions to suit the profile of the bridge and supported on temporary scaffolding. To minimize site formwork, thin precast slabs were also used to support the in-situ top deck. Concrete hinges were formed at top and bottom of the intermediate piers and at one end of the bridge the supporting piers, which are buried in the approach embankment, were encased in concrete pipes to provide for longitudinal movement.
The Simpson Lane bridge carrying this road over the airport link motorway, was of 3-span continuous post-tensioned double-box construction, supported on pairs of opposed inclined struts to provide a clear span over the full width of the motorway. Here again, precast soffit slab units were supported on temporary scaffolding, with the webs containing the tensioning cables cast in situ. All in-situ concrete was provided with a fair-faced finish frequently matching the same high quality of the precast factory-made units.
The motorway was carried over Cranford Parkway. At the eastern approach, the embankment was formed utilising inert domestic refuse, consolidated in layers by a combination of grid rollers, vibrating equipment, and finally by heavy smooth roller. As with the remainder of this section of the motorway, this embankment was surfaced with an 11 in thick concrete carriageway.
The end of Contract 4, took the motorway from the County Road to the Slough By-pass. The north carriageway was constructed throughout of 10½ in thick asphaltic bound sub-base, supporting a 2½ in thick asphalt base course and 1½ in thick asphalt wearing course with precoated drippings rolled in. The 36-ft-wide carriageway was bordered by precast concrete blocks with an exposed white calcined flint surface, and the hard shoulder was of 11 in thick lean-mix concrete, finished with 2 in bitumen-bound carpet and a surface dressing.
The south carriageway was treated as an experimental road, and was of varying types of flexible construction throughout its length. In a typical section, it was of the same overall thickness of construction as the north carriageway, but the sub-base was of bitumen-bound gravel. In order to achieve an economy, the precast concrete borders were dispensed with but it had been found necessary to extend the sub-base into the central reservation, and also to strengthen the edge with a concrete haunch prior to laying the base and wearing courses. To provide the same surface appearance, white calcined flint borders were applied to the margins.
The contractors employed on the various contracts were Marples Ridgeway, Kier, Christiani & Nielson, Holland Hannen & Cubitts, and Costain.