The male workers, aged between 22 and 56, were arrested on site after a raid last week.
Six had overstayed their visas and two had entered the UK illegally – the other was a failed asylum seeker.
The men were employed through a subcontractor working on the project, which has not yet been identified.
A spokesperson for the Home Office said: “Illegal working is not victimless. It defrauds the taxpayer, undercuts honest employers and cheats legitimate job seekers out of employment opportunities.
“Our enforcement visits are intelligence-led and timed to maximise their effectiveness. We would urge members of the public with information about suspected immigration abuse to get in touch.”
Investigations are ongoing to identify the employer of illegal workers so further action can be taken.
The employer is liable for a financial penalty of up to £20,000 per illegal worker, unless evidence of legal pre-employment checks is produced. The total potential fine is £180,000.