I want to build a basement room under existing concrete deck how do I support deck while digging out beneath? It drops off vertically would dig from that side. soft dirt. How to do this safely. thank you
You likely don't. Instead, you'd tear out your deck/patio, dig a new hole, pour a new foundation, then build a new structure on top.
You can retrofit houses with basements by lifting the entire house off the ground and then excavating below, but that'd be overkill for a deck/patio. Cheaper/safer to just remove it first, and rebuild a proper structure from the ground up.
This would be a massive job for an engineer let alone a motivated homeowner. Ideally you should have heavy earthmoving equipment at you disposal. If you are adamant about undertaking this job you must install steel I-beams (girders) under the concrete slab. The girders should be supported on either end by a cribbage of heavy timbers that are located away from the slab (this will allow space to work). You will need to install several of these supports along the length of the slab. Once the supports are in place and carrying the load of the concrete you can excavate the soil. Next setup the forms for the foundation and pour the concrete. Once cured you lay the cinder block walls to the height of the slab. This is a very general and elementary instruction to give you an idea of the work involved. You must consult with people in your area knowledgeable and experienced with this type of difficult and laborious work. To start, check in you locality for 'building movers' or 'commercial contractors'. They should be able to give you reliable answers based on experience.
The other answers cover the safety. I just want to point out it is a really really dumb idea money wise. It will cost you double to triple the amount of money to do everything below this slab than "anywhere else". What will be the flooring, how big is the room, will it have two exits, where is the plumbing or electricity?