The outstanding feature of the 6½ mile section of the motorway south from Horwich was the construction of the Worsley Braided Interchange, connecting the M61 with the M62, A580 (East Lancashire Road) and the A666 (M) Kearsley Spur.
The first stages in the construction of the M62 between Worsley and Whitefield had begun in August 1966.
The purpose of the Kearsley Spur, which was to be constructed as part of the Interchange, was to provide a connection to the A666 Farnworth-Kearsley By-pass, an all-purpose Principal Road built to motorway -standard by Lancashire County Council. Opened to traffic in December 1967, it was later to be extended northwards by the Eastern Limb of the Bolton Inner Relief Road, built by the County Borough Council to a similar standard. The Spur was, therefore, of considerable importance in linking Bolton, and the other towns served by the A666, directly to the national motorway system.
In a heavily built-up area, such as exists in this part of the Region, the location of an interchange of this magnitude was governed by the availability of land free from development. The presence of extensive peat deposits up to 20 feet in depth on the Linnyshaw, Kearsley, Clifton and Wardley Mosses had restricted development in the past and, therefore, provided such a site.
Following detailed investigations carried out in the area, it was found that it would be possible to drain the interchange below the lowest level of the peat by means of an outfall half a mile in length. This was a critical factor in determining the vertical profile of several of the routes in order to produce the most economic design. For example, an early proposal for one particular section would have required a bridge over a mineral railway, the excavation of a substantial depth of peat and its replacement with suitable filling material. As a result of being able to lower the profile an estimated saving of over £2 million was achieved.
In controlling movement between various routes, the design was based on the principle that traffic leaving the main route does so in advance of traffic joining.
Heavily skewed crossings of link and slip roads were necessary in order to form a relatively compact free-flowing design. One of the bridges was required to carry a two-lane slip road over a dual four-lane section of motorway at a skew of 70°. The initial design envisaged through girders and a considerable depth of construction but, in order to achieve the correct sight lines, a substantial increase in width would have been required. The alternative was a change in the design speed and/or a reduction in the skew angle neither of which was acceptable.
The solution to the problem was to make the bridge span 'square' to the line of the motorway with longer structures over each carriageway. The tunnel effect was reduced by an arrangement of columns supporting the deck.
A similar design was used for two other smaller, but heavily skewed bridges within the interchange. It was estimated that the saving in the cost of the three bridges would be £1 m, and on the earthworks and drainage, £¾ m.
In the event, the design of the interchange necessitated the excavation of 1½ million cubic yards of peat. In order to avoid the movement of vehicles carrying this material on the existing road system an additional 96 acres of moss land alongside the interchange was acquired to provide tipping space. After settlement had taken place, it was the intention that the land would be made available for agricultural use.
The first advance works contract was awarded by the County Council and began in February 1967, prior to the formation of the RCU. It involved the construction of a 42" diameter outfall to Unity Brook for the purpose of, not only draining the peat area, but also the M61 itself from as far as the junction with A58 at Westhoughton 5½ miles to the north.
One year later, a further advance works contract was awarded by the RCU. Over 2 miles of channels, 80 feet in width, were formed involving the excavation of 325,000 cubic yards of peat and other unsuitable material. The haul roads, which were formed in the channels, were incorporated in the permanent motorway construction. 1½ miles of motorway drains, up to 48" in diameter, were laid.
The main RCU contract, which included not only the construction of a substantial part of the Worsley Braided Interchange, but two other two-level interchanges and a total of 26 bridges and several large retaining walls, began in January 1969.
Within the total excavation of 4½ million cubic yards of material, a heap of approximately 160,000 cubic yards of highly caustic chemical waste presented a major disposal problem in the Interchange. Deposited on Kearsley Moss over the past 90 years, the material, which gave off a very pungent odour, had polluted the surrounding ground and water courses, including the River Irwell. The River Authority stipulated that, wherever the material was tipped, measures must be taken to prevent any further nuisance. In consequence, the Contract required that this material be incorporated as a core to a motorway embankment about 30 ft high and, in order to seal the material and prevent any further pollution, it was clad with a 10 ft thickness of clay on the sides and a 15 ft thickness of clay on the top.
Shallow mineworkings and old mine shafts were encountered and these were treated in accordance with the methods used on earlier sections of motorway. Where the Kearsley Spur passes under A666, grouting techniques were adopted. The area was drilled on a 10 ft square grid and each borehole was injected with a pulverised fuel ash (PFA)/ cement grout. Some 90 tons of cement and 1500 tons of PFA was used in over 1000 boreholes,
Approximately 3½ million tons of fill were imported to make up for the deficiency of suitable material and in backfilling the peat excavation below carriageway formation level. Of this, almost 1 million tons of unburnt colliery waste was extracted from unsightly tips in the area.
The comparatively high cost of construction in this particular area was considered to be fully justified by the improvement to the environment resulting from the use of land which would not otherwise be developed and by the utilisation of waste materials.
In view of the complex traffic movements involved, lighting was provided throughout the whole of the Interchange. On the dual carriageway sections, a system of longitudinal catenary lighting was installed over a length of 7 miles. Similar lanterns were mounted conventionally on 19 miles of single carriageway link roads.
The opening of this section of motorway in December 1970 marked the completion of the M61.