Re: Not practical
I had to lookup the pressure on Mars in the deepest canyon (which is also the deepest canyon in the solar system)
mars ground level 610 Pascal (equivalent pressure on earth would be at an altitude of ~43.1 kilometres which has an air temperature of about -12°C on earth).
mars at the bottom of Valles Marineris (~7km down) 1200 Pascal (equivalent pressure on earth would be at an altitude of ~37.4 kilometres which has an air temperature of about -28 °C on earth - thought that was totally insane when I learned it as well, colder than higher up).
earth sea level 101325 Pascal (To match that pressure on Mars would require going ~60 km below normal ground level. But at that depth the average temperature would vaporise all water and sublimate Iodine at about 473K ; 200°C ; 392°F if you assume the average ground level temperature on mars is around 223 K ; -50°C ; -58°F).
It is about twice as good at blocking radiation at the very bottom of the deepest natural canyon on Mars, but even it will provide about 1% of the protection provided by earths dense atmosphere.