At Corpach, near Fort William, the entrance lock was built on rock, but at the other end, there was of mud below the proposed site of the sea lock. Rock was tipped on top of the mud and was allowed to settle for six months before construction could begin. The ground through which the canal was cut was variable, and further difficulties were experienced with the construction of the locks, the largest ever built at the time. There were also problems with the labour force, with high levels of absence, particularly during and after the potato harvest and the peat cutting season. This led to Telford bringing in Irish navvies to manage the shortfall, which led to further criticism, since one of the main aims of the project was to reduce unemployment in the Highlands. The canal finally opened in 1822, having taken an extra 12 years to complete, and cost £910,000. Over 3,000 local people had been employed in its construction, but the draught had been reduced from in an effort to save costs. In the meantime, shipbuilding had advanced, with the introduction of steam-powered iron-hulled ships, many of which were now too big to use the canal. The Royal Navy did not need to use the canal either, as Napoleon had been defeated at Waterloo in 1815, and the perceived threat to shipping when the canal was started was now gone.