Is it actually the most cost effective method to pay cables in such a large tunnel?

I would imagine trench and bury or mole-digging to be a much cheaper way to install it?

AFAIK, high-voltage and high-power cables are usually over-ground for two reasons: (1) less losses with air as coolant and (2) repairs are so much easier.

Not having an accessible way to the cable is somethibg you don't want if you don't know exaclty where and what is broken (because you can't see it).

	▲	londons_explore 21 hours ago \| parent [-]
		That's why you usually lay them under roads - then you can dig a broken bit up with just a few hours notice. Generally you do know where things are broken - there are tools to identify the exact location of both broken conductors and broken insulation by reflecting high frequency signals down the cable.