I need to cut a trench in my garage floor in order to install a 110mm soil pipe for a new WC. The house is 12 years old and the garage floor is 4" reinforced concrete with a DPC under. The main issue is how to make good the DPC on completion?
The best method is to cut out the required width of channel right through, break it up and remove it. Then make a second cut maybe 4 inches wider on both sides but only cut this to 3 inches deep or 3 1/2 if you dare. Then break this part off with a bolster and it should hopefully break off but not damage the dpm. then you have 4 inches of exposed undamaged dpm which you can lay in a new piece lapping and taping the joint with buytl adhesive tape or denso tape.
A few words of caution.
Make sure you cut the channel wide enough so your vibrating plate will fit into to compact the trench. It is VITAL that you make the it wide enough to dig the trench without undermining the surround concrete floor.
Also make sure you break out and remove the centre section of concrete before making the secondary cuts or the extra 4 inch section will probably break off as you try to remove the middle.
Lastly when replacing the concrete drill some steel pegs (off-cuts of rebar) into the existing slab half way up and sticking out 4 inches as this will reduce the cracking between the old and new.
Good luck and be cautious and gentle or you can easily get into a situation where you have to keep chipping off more and more to get an undamaged section of dpm.