So I pulled the trigger on a triumph c7000. I know I'm going to have to cut into my old pad and pour some new spots. Debating between underpinning, side drilling the pad and connecting with rebar or digging a deep hole and making a new freestanding pylon.
first: why? what do you know about your current slab that suggests it's inadequate? might be worth drilling some sample holes and possibly getting it tested - a few hundred bucks vs all of the work of ripping it out and pouring new.
for a 7k lift, (indeed, 9-10k too), a reinforced 4" slab in good condition is generally considered fine. the triumph is no different in that regard.
if you still need to - when I looked into doing this, I found this was a typical recommendation - note this is for a standalone slab-free install, which is how I'd treat it if I couldn't trust my floor, personally.
Q: How large "stand alone" concrete pad is needed for an
outdoor lift?
A: 2-post lift:
The concrete should be twice the required thickness of a "normal" concrete
floor for the lift's weight capacity. The "slab" should extend at least 24
inches past the outside of each column (side to side) and one foot (front
to back) longer than the end of the extended lift arms when moved to their
"fully extended" position.
I've seen similar recommendations with smaller footprints to be "keyed" under the existing slab, so you basically pour a wedge shape under the current slab. you could drill into the existing slab and use rebar - but I'd rather key it under, myself.
if you are unsure of the thickness and PSI of your concrete and want to
install a 2 or 4 post lift you should pour concrete pads. The
basic rule is that you need the pads to be 12" wider on each side from the base plates of the lift. For a 2 post, you'll need to go down at least 2 feet.
For a 4 post, you'll need to go down at least 6" New concrete must also
cure at least 28 days.
most slab thickness recommendations tend to run around this area - thicker for lower psi concrete:
* 7,000-10,000lb Two Post Car Lift 4" min. 3000 PSI concrete
* 12,000-18,000lb Two Post Car Lift 6" min. 3000 PSI concrete
* 7,000-40,000lb Four Post Car Lift 4" min. 3000 PSI concrete
