Here are some photos of typical looking utiltiy tunnels-
Some general technical info on medium voltage cable application-
See this link for temperature derating factors from one cable manufacturer. Consult your manufacturer for exact numbers-
For cable tray and other support systems-
Some articles found at IEEE-
Gregory, B., Choices and considerations for the selection of power cables in tunnel installations. Cables in Tunnels (Ref. No. 2000/070), IEE Seminar , 2000 Page(s): 8/1 -8/9
Schroth, R.G.; Obst, D., Long distance tunnels for the installation of 400 kV XLPE cables. Cables in Tunnels (Ref. No. 2000/070), IEE Seminar , 2000 Page(s): 4/1 -4/4
Mendez, J.M., Sr.; Monteys, J., Design of power and communication cable installations to minimise interference in tunnels. Cables in Tunnels (Ref. No. 2000/070), IEE Seminar , 2000 Page(s): 6/1 -6/6
Anders, G.J., Rating of cables on riser poles, in trays, in tunnels and shafts-a review. Power Delivery, IEEE Transactions on, Volume: 11 Issue: 1 , Jan 1996 Page(s): 3 -11
Ohata, K.; Sakurai, S.; Ichiyanagi, N.; Shibata, K.; Inoue, T., Transient temperature rise of tunnel for power cables
Power Delivery, IEEE Transactions on , Volume: 5 Issue: 2, Apr 1990 Page(s): 745 -752