I've got a situation where I want to put a wire in a concrete wall (I have a channel), but the one coming in isn't long enough. I cannot replace the incoming cable, so I need to join it with another one inside the channel. Said channel will afterwards be filled with cement, so I won't be able to access the joint anymore without some serious destruction. What is the best way to join the cable so that I can sleep easily knowing that there will be no problems there? If it matters, the cable will not be carrying significant loads (less than 100W max).
Added: Clarification was asked in comments. Location in world: Latvia. We're a 220V country. The incoming cable in question is marked "2x1,0" which (if I understand correctly) means two wires, each 1mm2 cross-section. Copper. It's pretty thin. The cable with which I will join it will be similar.
It will feed one LED lighting fixture (up to 50W, but probably less) and a wifi router. The location makes it unlikely that heavy loads would be attached there, however, of course, I cannot predict the future.