Best way to splice aluminum wire underground
We have approximately a 400 foot underground run from the house to a gate near the street that powers the opener and a few receptacles. The receptacles stopped working and after uncovering a plastic junction box that was buried just a few inches deep out near the gate we discovered some corrosion and water inside the box. The original installer ran several what looks to be about #6 aluminum wires and inside the box that was buried spliced the aluminum wire to solid copper conductors that then go to the 3 receptacles within about 20 feet of this box. I want to re-splice these connections and run new copper conductors from this box to the receptacles. Everything is in conduit. What's the best method for the splice since it will be underground and it looks like it may be exposed to water? I don't think there is enough slack to get the junction above ground unless I dig back and cut the conduit and move the junction back a few feet. Should I drill a hole in the jbox to allow the 1/2 inch or so of water to drain? Thanks in advance.