M6 Preston By-pass (J29 to J32)

In the mid-1940's, Drake had obtained the approval of Lancashire County Council to proceed with the preparation and preliminary design work for the North-South motorway through the County. It was found that the basic alignment, as envisaged in 1937, was still appropriate and only comparatively minor adjustments had to be made. It was realised, however, that economic factors made it impracticable to embark on the construction of the whole of the motorway as a single project. In 1952, therefore, consideration was given to the possibility of constructing those sections of the motorway by-passing Lancaster and Preston, in order to relieve the serious traffic congestion and reduce the number of accidents within those conurbations.

Encouraged by the preparatory work carried out by the County Council the Minister announced, in 1953, that the construction of the 8¼ mile Preston By-pass was to be included in his expanded road programme to commence in the 1956-57 financial year and that a start was to be made on the statutory procedures.

At the time, the Minister described the By-pass as a 'guinea pig' - in other words, as an experiment for all other British motorways. In view of its significance and the lessons which were learned in carrying it out, this particular project is referred to in much more detail in the Summary, than any other section of motorway.

The County Council was appointed the Minister's Agent Authority for the design and resolved to carry out the widest possible public consultations. Drake suggested that, rather than relying solely on drawings to illustrate the proposals, a large model should be constructed and, in order that the design programme should not be delayed, members of the staff keen to see the project proceeding, agreed to build the model in their own time. At subsequent public meetings which were held throughout the are, it proved to be of enormous value in explaining the scheme.

The route of the By-pass followed a line very close to the eastern edge of the built-up areas of Fulwood, Preston, Walton-le-Dale and Bamber Bridge, with only one farmhouse and three other dwellings directly affected. For many years, a corridor to provide for the future construction of the motorway had been 'protected' and, in several instances, post-War housing development within the urban areas had not been allowed to extend beyond its western boundary. Apart from terminal roundabout connections to the A6 at each end, there was to be only a single interchange junction with the A59 at Samlesbury, approximately half-way along its length.

Following the advertising of the proposals, only a few formal objections were received and these were resolved by agreement. The Minister was therefore able to confirm the various statutory Orders without a Public Inquiry. This was quite remarkable, in the light of the vociferous opposition to similar proposals which was generated elsewhere in the Country as motorway schemes became the subject of formalised and lengthy Public Inquiries, often disrupted by objectors not directly affected. Part of the explanation lies in the fact that there was close personal involvement of both the members of the County Council and the County Surveyor, in the consultation process. As a result, those affected had full confidence that their interests would be properly considered, both in the detailed design of the works and during construction.

It could be argued that, as there had been no previous experience of motorway construction in Britain, members of the public could not appreciate the full impact of the construction and the use of a road of such magnitude. If this had been so, those likely to be affected when other sections of the motorway came to be considered might have been inclined to raise strong objections. In the event, however, by adopting a similar approach, all the statutory procedures for the whole of the sixty-two miles of the M6 through the County were completed without difficulty.

Based on the experience of motorway construction and operation in Italy, Germany and the United States, various design principles had been established for use in Britain with several significant changes from those proposed in 1945, by the Chief Engineer in the Ministry of War Transport. The design speed was to be 70mph, although there was to be no 'speed limit' when the motorway was opened to traffic. Instead of lay-byes, hard shoulders 8 feet wide were to be provided adjacent to the carriageways except at bridges, where the risk of a vehicle breaking down and blocking a traffic lane was considered to be minimal. With a verge only 4 feet wide at such locations, a considerable saving was achieved in the cost of the structures. The overall width was to be 112 feet with the two-lane carriageways each 24 feet wide and a central reservation of 32 feet, in which a hedge was to be planted as a screen against the effect of opposing headlights. The limiting gradient was to be 1 in 25.

Drake had recommended two important design features but the Ministry, which was financing the whole of the cost as a 'trunk road' scheme, was not prepared to accept their introduction.

The proposal to provide a 'positive' drainage system using conventional was not accepted and instead, it was decided that surface water run-off would be collected in stone-filled trenches, known as 'french drains', located either in the central reservation or behind the hard shoulders. This followed the practice adopted for the drainage of runways in the construction of airfields during the War.

On the basis of his own traffic predictions , Drake had argued that the carriageways should be constructed with three traffic lanes rather than with two lanes, which the Ministry considered to be adequate. However, a compromise was accepted whereby the overall width of the By-pass was to be increased, the bridges designed accordingly, and the central reservation made sufficiently wide to allow for the addition of a third lane to each carriageway at a later date. In due course, therefore, the carrying capacity could be increased with minimal disruption to traffic and without the need for structural alterations to bridges.

Lancashire had two main sources of materials for road construction. In the south of the County there were massive deposits of burnt red shale, which had been accumulated during many years of coal mining, while in the Clitheroe and Carnforth areas existing quarries produced high quality limestone aggregates, properly screened and graded to meet any specification. The form of carriageway construction was to be such that the maximum possible use would be made of mechanical plant and several pilot projects had been carried out on county roads in which a pre-mixed waterbound limestone macadam, to be known as 'wet mix', was developed as a material for the base.

At that time, it was common practice in the construction of new roads, particularly where major earthworks were involved, to defer the laying of the final surface until the carriageways had been subjected to traffic use. It was considered, therefore, that only a temporary surface should be laid, so that any early deformation could be remedied before completion, thereby ensuring a high standard of running surface over an extended period.

M6 Preston By-pass (J29 to J32)Taking all these factors into account, it was decided that the carriageway construction should consist of a sub-base of burnt red shale, varying in thickness from 12 to 36 inches depending on ground conditions, followed by a 9 inch thick layer of 'wet mix' and a surface of 2½ inch thick tarmacadam base course with a ¾ inch thick temporary wearing course of fine cold asphalt. The form of construction of the hard shoulders was to be little more than that of a hardened verge.

Twenty two bridges were required and great care was taken to ensure that each type was aesthetically suitable for the particular site. Designers were allowed a fairly free hand and there was a great deal of innovation with a preference for concrete construction employing differing prestressing systems. The use of concrete was also in recognition of the anticipated difficulties of providing access on a 'live' motorway for the future maintenance painting of steel structures.

The design loading for highway bridges was set out in Memorandum 577 which required all bridges, with the exception of accommodation bridges, to be capable of carrying a specified uniformly distributed load, plus a knife-edge load representing the excess of the heavy axles over the other axles in the 'Standard Loading Train'. In addition, bridges carrying motorways were required to be designed to carry 45 units of Abnormal Load which was simplified into two knife-edge loads of 15000lbs per foot, 10 feet long and 25 feet apart, with modifications for transverse members. For this loading, a permissible overstress of 25% was allowed.

In general, for spans up to about 50 feet, a desirable shallow construction depth was to be achieved using pretensioned prestressed beams placed side by side and infilled with concrete, but the Ministry required such decks to be transversely prestressed. There were no standard beam sections available, and as the beams were to be cast and stressed using the long-line system, much experimental design work was carried out.

In order to reduce the tunnel effect in instances where the motorway crossed a minor road, such that the span of the bridge was very much less than its width, light wells were to be provided within the central reservation.

For one particular single span bridge over the motorway, the deck was designed with beams 99 feet long to be cast on site and post-tensioned using the 'Magnel-Blaton' system.

Another over-bridge was required to carry 30 inch and 24 inch water mains in the footways, and the depth of construction required to accommodate them determined a single span arrangement. Steel plate girders with the deck constructed in two halves enabled the mains to be temporarily diverted in turn in order to maintain full water supply to Preston, and avoid undue restrictions to traffic.

Two major bridges were required to carry the By-pass over the River Ribble and the A59 at Samlesbury, and across the valley of the River Darwen. In both cases, steel was the preferred form of construction for the superstructure.

At Samlesbury, various arrangements of spans were investigated, ie two, three, four and five spans. Following discussions with the Chief Bridge Engineer of the Ministry, Mr C.S.Chettoe, a three span continuous steel box girder structure with curved soffits was chosen. At that time, this form of construction was comparatively new. Three-inch long sections of rolled steel channel welded to the top of the eight girders as shear connectors to the reinforced concrete deck slab ensured composite action. The 420 feet long deck was to be supported on mass concrete abutments with wing walls and two mass concrete piers contained within permanent sheet pile cofferdams. A steel interlocking toothed expansion joint at the Southern end of the deck was to be concreted into the deck and the curtain wall of the abutment.

The six span 474 feet long Higher Walton Bridge was designed with a superstructure of welded steel plate girders and concrete deck, carried on reinforced concrete trestle piers.

Hot rolled asphalt was specified as the carriageway surfacing on the bridges and it was assumed that this would be impervious. However, a waterproof membrane was to be provided under the side verges and the central reserve.

M6 Preston By-pass (J29 to J32)2It was decided that separate contracts should be entered into for each of the two major bridges. The major contract was for all the roadworks and nineteen other bridges. Tenders were invited by the County Council, early in 1956, and approval was given, by the Ministry, to the acceptance of the lowest bids, which enabled construction to commence in June of that year, with Drake appointed as Engineer for all the three contracts.

The provision of a bridge carrying the Preston-Longridge railway line over the motorway was to be undertaken by British Rail.

The two-year period for completion was based on the presumption that the weather would approximate to that of average summers and winters in Lancashire but, in the event, almost continuous rain fell from the start of the work.

M6 Preston By-pass (J29 to J32)3In order to deal with the massive earth-moving programme, the principal Contractor had assembled a fleet of heavy plant which included tractors, scrapers, large capacity excavators and rear-dump trucks. However, the rains of the Autumn 1956 made it virtually impossible to work the sandy clay sub-soil and the major earthworks were postponed until the following Spring. Due to the exceptional circumstances, the Contractor was granted a five months' extension of time.

Following a brief spell of fine weather early in the following year, there was a similar period of very heavy rain, and this situation was repeated in 1958. This caused the 'loss' of large quantities of excavated material which, in normal weather conditions, would have been used for embankment construction. As a result, it had to be taken to tip and replaced by imported material.

The adverse weather did not materially affect the bridgeworks' programmes. However, several issues of an unexpected nature arose during the period of construction.

The Contractors sought approval to the use of ready-mixed concrete from a plant which had been established recently some 15 miles from the site. At that time, the use of material provided in this way, was a new development in the area. Understandably, concern was expressed at the extent to which it would be possible to control the mixes, particularly, in regard to the water/cement ratio of the high strength concrete for use in prestressed elements of structures. Arrangements were made, therefore, for members of the supervisory staff to visit the plant when critical mixes were to be delivered. Not only were the materials inspected, but the moisture content of the aggregates was tested in order to ensure that the amount of added water was appropriate for the mix.

The conventional practice in the painting of steel bridges was to use a neutral colour, such as grey. Drake expressed the view that a variety of brighter colours should be introduced, for two reasons. Firstly, to enhance the appearance of the bridges and secondly, and perhaps more important, as a safety measure to ensure that drivers did not suffer from boredom while driving on the motorway.

M6 Preston By-pass (J29 to J32)4In the early stages of painting Samlesbury Bridge, and before the surfacing had been laid on the deck, it was found that in one localised area, the paint had been stripped from the face of one of the outside girders. Tests showed that rainwater seeping through a minor defective area of the deck concrete had become highly alkaline with a PH value approaching that of caustic soda. Repairs were carried out and the painting was completed satisfactorily.

An examination of concrete used elsewhere in the project indicated that this was a common phenomenon. Enquiries directed at the cement suppliers established that the cause was due to inadequate 'burning' during the manufacturing process, which arose from the high demand within the construction industry at that time. The problem was well-known in other countries, such as Denmark, where there was experience of corrosion of the steel in reinforced concrete structures. The advice which was received, was to the effect that, in most instances, the PH value of the leachate would reduce rapidly and further testing showed this to be the case.

The inner surfaces of the steel box girders of Samlesbury Bridge were to be painted, but it was considered that some means should be provided for measuring the humidity of the interior. A tap was, therefore, fitted on each of the access manhole covers to enable air samples to be extracted. It was found, however, that during periods of changeable weather the expansion and contraction of the air within, was so great that the girders could be heard to 'breathe'. Meanwhile, consideration was being given to controlling the humidity by placing silica gel within the girders in order to reduce the frequency of maintenance painting. This form of treatment was found to be successful, by completely sealing the boxes with the fitting of air-tight gaskets to the covers and the removal of the taps.

In view of the innovative nature of the design of many of the bridges, arrangements were made for the Ministry to carry out the test-loading of several of the completed structures prior to the opening of the By-pass. The results were used in the development of future design standards.

M6 Preston By-pass (J29 to J32)5 Extensive landscaping was carried out, not only within the limits of the motorway but also on adjoining land acquired after negotiations with the owners, and in the areas where unsuitable excavated material had been tipped. This was followed by an extensive tree planting scheme and the provision of hedges along the boundaries and within the central reservation.

It had been decided that lighting was only needed at the Samlesbury Interchange and at the terminal roundabouts. Lighting schemes were designed in accordance with the appropriate standards in use at the time. However, towards the end of the construction period, when most of the lighting columns in the interchange had been erected, the Minister visited the site. On viewing the Interchange from the south escarpment of the Ribble valley, he expressed concern at the effect on the surroundings of the large number of columns in such a small area and immediately ordered their removal - a further indication, even in those days, of the interest in minimising any adverse effect on the environment of the area.

M6 Preston By-pass (J29 to J32)6On 5th December 1958 the Preston By-pass, Britain's first motorway, was opened to traffic. In view of the national importance of the event the ceremony was performed by the Prime Minister, Harold Macmillan, and a granite plinth marking the occasion was erected at the Samlesbury interchange. A copy of the opening brochure can be seen on the on the internet.

In January 1959, however, a small amount of frost damage affecting the carriageways occurred as a result of an exceptionally rapid thaw, when the temperature rose from 8°F to 43°F within a period of thirty-six hours. This situation had arisen primarily because of the exceptionally wet weather during construction, which meant that there was still a high water table in the formation below the carriageways. As the hard shoulders were not paved and, therefore, were permeable, surface water run-off was able to drain directly into the formation, thereby exacerbating the problem. Furthermore, the burnt red shale used in the sub-base had not been screened or graded and, where it contained an excess of fine material, capillary action took place, drawing ground water to the surface. In the circumstances, the thin temporary surface of the carriageways proved to be inadequate to resist the effect of the freeze/thaw cycle when subjected to heavy traffic.

In view of the fact that there was no speed limit, it was decided, for safety reasons, that the motorway should be closed temporarily to enable repair work to be carried out. This led to much criticism, which failed to recognise that, under a financial regime requiring the maximum economy in design and construction of a type of road new to Britain, it was probably inevitable that problems would arise.

Because of the importance of food production, at that time. It was necessary to restrict the acquisition of agricultural land to the absolute minimum. Based on the original site investigation, it was considered that the cuttings could be formed with slopes as steep as 1 in 1½ but, due to the nature of the material, surface slips occurred both during construction and after the motorway had been opened to traffic. The remedial work was both costly and disruptive.

Many other important lessons had been learned and it was considered vital that the experience gained in the preparation, design, construction and operation of this first section of motorway should be taken into account in carrying out future schemes. It was clear that attention needed to be given to a number of aspects, particularly in respect of the following:-

1.The extensive public consultation carried out throughout the whole process, had not only assisted in the smooth passage of the statutory procedures but also in dealing with complaints during the period of construction.

2.In order to ensure that as much suitable excavated material as possible is used in the construction of embankments, the responsibilities of the Contractor in carrying out the earthworks should be more clearly defined in both the Specification and the Bill of Quantities. In that respect, it is incumbent on the Contractor to employ appropriate types of plant, having regard to the ground and weather conditions.

3.Great care should be taken in the design of the slopes of cuttings and embankments, having regard to the characteristics of the materials involved.

4.The carriageway sub-base material should be of properly screened and graded material complying with a clearly defined specification.

5.The surface-water drainage system was unsatisfactory.

6.The principle of laying a temporary surface on the carriageways of motorways should not be perpetuated.

7.It soon became evident that continuous hard shoulders would have been a valuable asset:

* as a means of access for emergency vehicles when a breakdown or accident occurred which brought traffic to a halt, and
* for use as additional traffic lanes when it was necessary to close the normal running lanes for maintenance purposes.

8.The form of construction of the hard shoulders was inadequate, as it was not unknown in jacking-up a heavy vehicle, for the jack to be driven into the surface instead of lifting the vehicle.

9.The explosive growth of traffic, which occurred within a short period after opening, demonstrated the need for all future motorways to be constructed with dual three-lane carriageways at the outset, unless there are very strong reasons to the contrary.

10.Irrespective of the type or surfacing, bridge decks should be fully waterproofed.

11.Hedges are unsuitable for use as any form of barrier in the central reservation, particularly due to the effect on growth arising from winter gritting operations. In view of the liabilities of the Highway Authority for the maintenance of the motorway boundary 'fencing', doubts were expressed at the suitability of hedges for this purpose.

12.Light-wells in the central reservation of underbridges are a potential hazard.

13.There was a favourable reaction to the use of different colours in the painting of steel bridges.

Following the frost damage to the carriageways, local repairs were carried out and immediate steps were taken to improve the drainage system. The final 4 inch thick hot-rolled asphalt surface was laid within 12 months, much earlier than originally intended. In 1963, the hard shoulders were reconstructed and paved, their distinctive red surface giving a contrasting colour as an aid to drivers.

A safety barrier was erected in the central reservation in replacement of the hedge and to prevent out-of-control vehicles crossing from the opposing carriageway.

After only eight years, traffic flows had increased to such an extent that it became necessary to add a third lane to each carriageway by reducing the width of the central reservation, as originally intended.

Following several failures during the construction of box girder bridges in Britain and abroad in the late 1960s, national concern was expressed at the adequacy of this type of design. Temporary lane closures were, therefore, applied to all such bridges until an independent check had been carried out. It is pleasing to note that Samlesbury Bridge was one of the few which did not require any strengthening.

Despite its problems, the construction of the By-pass undoubtedly gave good value for money compared with later motorways designed and constructed to higher standards.

The Road Plan had included a proposal for a northern by-pass of Preston, to connect with the M6 at Broughton and provide a motorway to Blackpool and the Fylde Coast. In the early 1960's, representations were made to the Ministry of Transport for an early start to be made on its construction, and preliminary preparation and design work was carried out by the County Council. However, as a result of the general increase in traffic, particularly on M6, and with the prospect of further increases arising from the development of the Central Lancashire New Town, it was decided to undertake a Study of the need for improved East/West communications throughout the whole of the Preston area.

The Study Report, published in September 1969, concluded that, on the basis of predicted traffic flows, a full network of by-passes, to be known as the 'Preston Box', would be required by 1980. In addition to the Northern By-pass, which merited the highest priority, it was considered that both Southern and Western By-passes were necessary.

The preparation, design and construction of the Northern By-pass, subsequently numbered M55, is referred to elsewhere in this Summary. The crossing of the River Ribble by a Western By-pass subsequently became more viable by the closure of Preston Dock to large merchant vessels. By the early 1990's considerable progress, in terms of design and public consultation, had been made in respect of both the Southern and Western By-passes, which were then the responsibility of the Department of Transport (DTp).

Meanwhile, a Study carried out in 1984-86 drew attention to the need for the widening and improvement of the Preston By-pass Section of M6 between the junctions with the M61 (J30) and the M55 (J32).

In 1987 following several multiple fatal accidents, the DTp included the project in the Trunk Road programme contained in the White Paper "Policy for Roads in England". Rendel Palmer and Tritton (RPT) were commissioned to appraise the situation, prepare an appropriate scheme and process it through to completion by acting as the Engineer for the Contract.

Although the whole length of the original Preston By-pass between Bamber Bridge (J29) and Broughton (J32) had been widened to three lanes in 1967, the hard shoulders had remained discontinuous at all the bridges. The number of lanes available, therefore, during maintenance operations, or when dealing with accidents, was inadequate to cope with the volume of traffic, up to a maximum of 140,000 vehicles a day.

The section between J30 and J32 is a common link for both East-West traffic (M61-M55) and North-South traffic (M6) and is particularly overloaded when tourist traffic to and from the Lake District and Blackpool coincide.

The DTp considered 'speed' to be of the essence in preparing and carrying out the scheme and initial target dates proposed a start on site in 1991-92, but it was agreed that even an accelerated programme would take somewhat longer. In the event the following dates were achieved:-

* 1989 - Public Consultation and Exhibition,

* 1991 - Public Inquiry, and

* March 1993 - Main Contract let.

In July 1993, the Minister announced that further work on the Southern and Western By-passes would be deferred until traffic conditions on the widened section of the M6 could be assessed. However, it was decided that the route of the By-passes 'should be protected for planning purposes'.

The approved scheme widened the M6 between J30 and J32 to a dual four- lane motorway and added extra lanes to both M6 and M61 at J30. Lighting, gantry signs and signals, CCTV and upgraded motorway communications were provided throughout.

Consideration was given, in the traffic studies, to the contiguous scheme for the Blackburn Southern By-pass section of the M65, joining south of J29, and the possible extension of it via the proposed Southern and Western By-passes.

It was decided that although dual four-lane carriageways might not, in later years, be always free from congestion, the recommendation of anything greater than this could not be justified.

The original DTp requirement was to maintain three lanes in one direction and two in the other, during construction. This was later increased to three in each direction and greatly influenced the design.

The options for widening a live motorway are,

1. 'Symmetrical' - where width is added on both sides

2. 'Asymmetrical' - where width is added on one side only and

3. 'Parallel' - where a new section of road is constructed off-line.

The approved scheme was a combination of all three options. From the M61 overbridge at J30, the widening was all to the west (Asymmetrical). The alignment came back to cross Samlesbury Bridge on line and then the widening swung across to the east (Asymmetrical), with a section of new carriageway off-line (Parallel) under Longridge Road before rejoining the existing alignment at J32.

M6 Preston By-pass (J29 to J32)7The scheme required the demolition of all 11 overbridges, their replacement by eight new structures, and one underbridge. The four remaining underbridges were widened and strengthened.

During the design process the Commission for New Towns appointed RPT to examine the possibility of providing a new junction, (J31a), between J31 and J32 to give access to their proposed Preston East Employment Area. A half junction was eventually approved with south facing slip roads only, and the underbridge and stub slip roads were constructed as part of the M6 widening contract. The junction has since been completed and opened to traffic.

Ground conditions proved more difficult than expected and earthworks were suspended over one winter. This meant that the Contractor's accelerated programme would not be achieved, but the dual four-lane carriageways came fully into use in August 1995.

Two bridges with discontinuous hard shoulders remain south of J30. These could be widened using M65 and M6 for traffic diversions.

The difficulties in carrying out the work were considerable. The Engineer, the Contractor and the Police exercised great skill in dealing with the traffic during the progress of the work but it was inevitable that serious disruption would occur, from time to time. This was reflected in the very high cost of the project and reinforces the view expressed at the outset that it would have been preferable, and more economic overall, to have embarked on the construction of the Southern and Western By-passes first, before carrying out the M6 widening.