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Too little is known of William Evans, described as 'an exceptionally competent and enterprising contractor'. It appears that he was the son of a building contractor, John Evans of Oldham, who had been awarded the contract for the abutments for the Conwy Railway Bridge. However, John Evans died shortly afterwards, and the contract was reassigned to his son William.
1819 William Evans born in Oldham, son of John Evans and his wife Susannah Scholes Evans 
1821 Mary Ann Cheetham born in Rochdale
1841 William Evans married Mary Anne Cheetham in Manchester
1841 William Evans 20, agent, and Mary Ann Evans lived in Prestwich cum Oldham
1842 William was an engineer at the time of his son, Herbert's, birth in Royton
1851 Contractor for public works, living in Chatteris, Cambridgeshire with Mary Anne Evans 30 (b. Rochdale), Herbert Evans 8, Thomas Evans 6, Eliza Evans 4
1851 Won the contract to build the Boyne Viaduct
c.1855 Evans was bankrupted by the contract for the Boyne Viaduct, as a result of serious problems in reaching bedrock for one of the piers.
William Evans had died by 1858.
Another source indicates that William Evans was born in Oldham c.1815, the son of John Evans, a railway contractor, and married Mary Ann Cheetham (b.26 February 1821 in Rochdale) on 27 May 1841. After William died his widow emigrated to Australia.
Tenders for the ironwork of the Conwy bridge had been received from a number of major companies, including Fairbairn, Hick, Rothwell, and James Lillie, but none were acceptable. Surprisingly, William Evans decided to bid to undertake iron work as well as the masonry, and his bid was accepted (to include construction of six pontoons for floating the tubes).
1851 'THE BOYNE VIADUCT
On Saturday the directors of the Dublin and Belfast Junction Railway Company let the contract for the entire of the works the Boyne Viaduct to Mr. William Evans, of Cambridgeshire. This gentleman was the contractor who executed the entire works of tho Conway Tubular Bridge on the Chester and Holyhead Railway, and who is now carrying on the extensive drainage works Cambridgeshire, where has now upwards of three thousand workmen employed. It is the intention of Mr Evans to erect workshops on the ground adjacent to the site of the intended viaduct at Drogheda. The amount of his contract is sixty-eight thousand pounds. The tender of Messrs Strapp and Cowper, of Glasgow and Dumfriesshire, was next in amount, the difference between the two being 520l; and next in amount was the tender of Messrs Dickson and M'Kenzie, of Wellington, Salop. The viaduct is to be constructed on the plan of Sir John Macneill, the eminent engineer-in-chief to the line. ....'
Evans was bankrupted by the Boyne contract, as a result of serious problems in reaching bedrock for one of the piers. See below.
1855 'Assignees of Evans v. the Belfast Junction Railway Company.
Mr Fitzgibbon, Q C, with whom was D C Heron, moved, on the part of the plaintiffs, that engineers and measurers, on their behalf, might have liberty to examine the works and machinery of the Boyne Viaduct. The action was brought by the assignees of the contractor for the construction of the bridge over the Boyne. The work was contracted for at a sum of 68,000l. Counsel stated that in the course of the work, Mr Evans, the contractor, found that he had been so much deceived by certain plans which were furnished to him to be made bankrupt by the misrepresentation. On seeking for the foundation for one of the piers of the bridge he expended a sum of 90,000l beyond that which the plans exhibited to him led him to suppose the undertaking would cost. The action was brought for the recovery of 21,500 for work done by Mr Evans, and for the value of planks and machinery which the company look possession of when discharging him from the contract. The defendants, relying on a deed which, on the representations made him, had been signed by Mr Evans took up the contract from him after he had expended nearly 10,000l on the work. The defendants, in their defence pleaded this deed to which plea replication had been put in that the deed was obtained by fraud and misrepresentation. The defendant traversed the fraud, and demurred to the plea, The fraud as alleged by the plaintiffs, consisted in furnishing to Mr Evans what purported to be a drawing of vertical section of the River Boyne, derived from proper bearings and examinations of the place. The deed upon which the defendants relied contained a clause by which Mr Evans covenanted to build a pier of the bridge upon a stratum of rock which was represented by the drawing to exist at a certain depth. At the time of the execution of the deed Mr Evans believed that this representation regarding the stratum of rock was correct, but when be sought for it that it did not exist at all. After sinking thirty feet the cost of every foot after was 1,000l. The consequence was that Mr Evans became bankrupt.— It was necessary to measure the works which Mr Evans executed, and inspect such of the machinery as the company seized, for the purpose of putting a value them. Counsel submitted that the 57th section of the procedure act gave the court authority to direct that the inspection should be made.
Mr Macdonogh, Q.C , with whom was Mr Norman, resisted the motion. Counsel stated that it was because he had failed to perform his contract that the defendants had taken work out of the hands of Mr Evans, to whom a sum of money had been paid more than would cover the work done by him. The allegation of fraud having been practised was quite untrue. It was unlikely that any conclusion could be come to by an inspection, in consequence of the long interval of time that had elapsed since the work was taken from the defendant in 1853, it would be impossible to mark out the rest of the structure, which was now completed, that portion of the work which had been executed by Mr Evans, beside the plaintiffs had inspected the premises at the time when Mr Evans became bankrupt a fact which they now concealed.
The order made by the court was, that the inspection applied for should be made by respectable persons at the expense of the plaintiffs; that, in the event of the defendants getting a verdict, they should have the costs of the present motion against the plaintiffs, and that the inspection should not take place until after the arguing of the demurrer.'
William Evans had died by 1858.