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So within the footprint of the existing you will excavate lower than the footing that is there now ? Will you excavate lower than the existing footing to gain the depth, obviously to get some more head room than the crawl space there now ? If that is the case then the new footing will be lower than the existing footing, condition like that usually calls for underpinning the existing footing prior to undermining it to place a new one beside it, and technically existing should be underpinned to the same elevation which is the bottom of new footing or where you stop excavating for same. If the existing footing has a lot of weight on it and you excavate all at once, hard to say what the undisturbed soil under it will react, during construction of the new wall and with the cavity you create, what will keep that soil from displacing under the existing footing ? Is the new wall going to be load bearing and are you going to transfer the load to it, or relieve some of the load on the existing by transferring it to the new or shoring up the joists of the house ? Just thinking aloud here. As I interpret this detail you propose, you might get away with excavating and depending on the soil type, it might stay together, + if the footing is not heavily loaded, but you had best get the new footing done, 2'-0" wide x 1'-0" thick might be better, with horizontal and some vertical deformed bar (rebar) to tie into the block wall, or concrete masonry units as I call em ( C.M.U.), then get the wall up to the height of the top of the existing footing, so the annular space can be filled compacted and stabilized, could even use concrete I suppose, and although you are not supposed to have one footing close in proximity to another with one being lower then the other without underpinning it, filling that annular space would at least stabilize the existing. Footings exert displacement at 45 deg angle, so when you undermine one, it loses area to displace the loading to, concentrating more of a point load where it still is bearing on the soil, which could cause failure. Have to be careful if this is to be excavated all at once. An engineer would be good to speak with about the details of how this works. Underpinning (extending existing footing) will bring the bottom of the existing footing to the bottom of the new footing, but has to be done in sections at a time, taking a 12 foot section dividing it into (4) 3'-0" sections labeled A, B, C, D, first do A sections in each 12 foot section, then B, and so on until complete, drypack and wedge etc. Usually no more than 12 feet in one wall is allowed to be underpinned at a time. Lot of work when necessary. I'd make sure the new wall was substantial, as the existing footing will displace weight towards it, I'd use high strength block, 3000-4000 P.S.I., certified by the block mfr., with 75% filled cells (smaller cells), vertical reinforcement 2'-0" on center, with at minimum those cells with rebar being filled with 3000 psi grout. Waterproofing, typically most materials require you to wait the 28 days for the mortar to cure, just like a poured foundation you have to wait for the concrete to cure. Could be a pain to get in that space, so do a few courses, reach down and get the drain pipe in, stone and waterproofing on before topping off. Block is labor intense, not all that hard to place, but mixing mortar can be a lot of work, do a layout, use a string line with line level, start your corners both directions, soak the footer, lay bed of mortar, butter the ends of the block with mortar, place, tap into plumb and level, strike joints while still plastic (mortar not cured, wet but stiff) and repeat. I attached a masonry accessory mfr. below, you can use truss/ladder mesh in each course to reinforce the wall further, they have flashings and lots of details to look at, good products, they set the standards for masonry accessories. Does the slab on top sit on the new wall, and abut the existing wall? If so, you'll need to create a bearing surface for it, and the form under it on the existing wall, unless you jack to the house up to allow the slab to bear on both walls. Don't mind me, was trying to interpret what you describe doing there, sounds like you might want to use some care and planning as to how you will do the work. Not as critical as say a large building or structure but in theory, is similar to why underpinning is done, being residential, the loads are a lot less and you can get away with a lot more, but do use care, possibly make a sketch of it in section, show both footings, walls and slab in relation to each other, show it to an engineer or some knowledgable person, not to say you have to, but not a bad idea to let another set of trained eyes take a look, might shed some light on things.
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