At the southern interchange, a new length of dual carriageway principal road 1.1 miles long, and known as the Horwich Link, was to connect with the A673 and provide a northerly access to Bolton.

Major earthworks were necessary and 42 bridges were required. It was considered, therefore, to be essential to carry out the following advance works to deal with a number of difficult problems, likely to cause major interference with the programming of construction in the two main contracts which were to be awarded.

A watercourse known as Pearl Brook passed under Crown Lane on the centre line of the motorway and lay within the motorway land-take for a distance of approximately 350 yards. The motorway at this point is on embankment with a maximum height of 35 feet and due to the poor nature of the ground it was considered necessary for the embankment to be constructed early to allow any settlement to take place. The Pearl Brook Diversion consisted of the construction of two reinforced concrete box culverts connected by an open concrete paved channel of a total length of 550 yards.

The diversion of the aqueduct carrying the main water supply from Rivington Lakes to Liverpool passed under the line of the motorway consisted of the laying of twin 42" diameter steel pipes surrounded with concrete and with chambers at each end.

The connection of M61 with the Preston By-pass section of M6 at Bamber Bridge involved the construction of Blacow Bridge which carried the north-bound carriageway of M61 over M6.

The Thirlmere aqueduct which carried over 40,000,000 gallons of water daily to the Manchester area passed under the motorway. The diversions consisted of the laying of twin 72" diameter steel pipes surrounded with concrete.

The first of the advance works contracts awarded by the County Council began early in 1967. However, a major change took place on the 1 April of that year when the first Road Construction Unit (RCU) covering the North West of England came into being, with Drake appointed as its Director, on secondment from the County Council. In accordance with the terms of the partnership agreement between the Minister and the County Council, the Lancashire Sub-Unit of the RCU was established by the allocation of appropriate members of the staff of the County Surveyor's Department. This ensured continuity in the preparation and design of the project, and in the supervision of construction.

Alternative tenders were invited for three different types of carriageway construction. For both Contracts, flexible construction with a base of 8" of dense bituminous macadam proved to be the cheapest with the same Contractor successful in each case.

Work began on the southerly of the two Contracts on the 1 January 1968 followed by a start on the other Contract 5 months later. Both were scheduled for completion in November 1969.

A prominent feature along the line of the Motorway was the 100 year old Botany Railway Viaduct, which had carried the Chorley-Cherry Tree railway line over the Leeds-Liverpool Canal at Chorley. This viaduct had an overall length of 385' and the width between spandril walls was 28'6". It consisted of nine masonry faced semi-circular arches each of 33' clear span carried on 4'6" wide masonry piers, and was demolished on the 10 November 1968 by means of explosives.

A total of 51 houses had to be demolished and of these 44 in the Botany Brown area of Chorley were due for demolition by 1970 under a re-development scheme. Arrangements were made with the Housing Authorities concerned for the re-housing of tenants where required.

Soils encountered along the route consisted of boulder clay of varying plasticities, sand, silt, peat, shale and gritstone. Over a length of 800 yards immediately north of Red Moss Railway Bridge, a deposit of peat with an average depth of 13 feet was dug out and replaced with granular materials, before the motorway embankment could be formed.

At Gale Moss a deposit of water-logged peat, reaching a maximum depth of 42', was encountered, and was dealt with by a combination of excavation and displacement techniques to replace the peat with granular material. Test bores were taken through this granular material to prove that the removal of the peat below the formation was complete.

Horwich Link to PrestonAt Whittle-le-Woods, the route of the motorway passes through a valley between hills, from which gritstone has been quarried. This valley was found during site investigation to be an old glacial channel and, although rock appears on both faces of the motorway cutting, peat, silt and other soft materials were found at motorway formation level. These conditions, combined with a water table which was virtually at existing ground level, made excavation extremely difficult.

Shallow mine workings were found in the cutting near Nick Hilton Lane and where the top of the 'seam' was within 17 feet of finished motorway level, all material was excavated out to the bottom of the 'seam', and back-filled with granular material. Where the depth was greater, a 8" thick double reinforced concrete raft was provided extending 4' beyond the outer edges of each carriageway.

Eight old mine shafts were exposed and these were proved for the depth of fill. The shafts were then covered either with a substantially reinforced concrete slab at rockhead level, or with a thick concrete plug. One of these shafts proved very difficult to locate due to dredgings from the canal overlying it to a depth of approximately 30 feet.

Approximately 6 million tons of material were excavated and more than 5 million tons of fill were imported to make up the deficiency of suitable material required to form the embankments and backfill the peat excavations.

The imported material was mainly obtained from four sources, the most important being Healey Nab lying adjacent to the line of the motorway. By excavating the top of this hill to a depth of 30 feet some 2½ million tons of rock were obtained with the advantage that direct access to the line of the motorway could be obtained without haulage vehicles having to use existing roads. This was an important factor in enabling the Contractor to obtain planning permission and with strict conditions concerning reinstatement, the resulting appearance of the hillside was very little different from the original.

Another source of material was an old tip at Blackrod from where some 300,000 tons of burnt colliery shale was obtained. A further example of the construction of a motorway improving the environment, by using waste materials.

Whereas twenty of the bridges had to be constructed to carry the motorway over roads, railways, a canal, rivers, footpaths and streams, sixteen carry other roads over the motorway and four carry footpaths over the motorway.

The bridges carrying the motorway are generally of one or three spans, whilst those over the motorway are mainly four-span, the elevations being necessarily asymmetric in a few instances. The unusual elevations add interest to the appearance of the motorway.

Piled foundations were necessary for 21 of the bridges. To suit the differing design requirements, prestressed concrete, reinforced concrete, steel girders and preflexed girders were used to form the decks. For most prestressed and steel deck bridges, standard I-section beams were used.

The motorway was opened to traffic on the 28 November 1969.