Two jobs wearing one word, and which you mean changes the call
Say underpinning to an engineer and they picture a house on ground that let go, with new concrete being worked in underneath the old footing to reach something that will hold. Say it at a lumber yard in this part of Virginia and half the room pictures block being laid around a crawl space to close in the gap between the floor and the dirt. Both uses are correct. They are not remotely the same work, they are not the same price, and one of them starts with an engineer.
So when you call about underpinning, the first thing worth sorting out is which one you are describing. Is the building moving, or is it just open underneath? That answer sends the conversation two completely different directions, and it is a thirty second question on the phone.
The structural version goes in small bites, on purpose
The obvious way to deepen a foundation is also the way to drop a house. You cannot dig a trench under a bearing wall and then start pouring, because for the whole time that trench is open the wall above it is spanning across nothing. So a designed underpinning job works in short segments, spaced apart, in a sequence somebody worked out on paper. One segment is dug out under the existing footing, poured, and packed tight against the underside of the old concrete so the load transfers cleanly. That segment cures and picks up its share of the building before the next one beside it is opened.
There are other ways to get to the same place. Piers driven or drilled down to bearing with the load brought onto them through brackets, or grout injected to firm the ground itself. Which approach fits depends on how deep the good ground is, what the building is made of, and how much movement has already happened. That is a design decision made from soil information and a look at the structure, not from a photograph of a crack.
The reason to say all of this plainly is that the sequencing is the job. The concrete itself is unremarkable. The order of operations is the thing keeping the house standing while it happens.
Closing in under an existing building is a different animal
The local sense of the word is a wall built under a house or an addition that is already sitting there, usually block or brick, closing in the crawl space. Sometimes it is holding real load and sometimes it is only closing off the weather, and those two are worlds apart in what they need. Anything actually carrying the building needs a footing under it, at proper bearing, poured before a block goes down.
This is where the honest answer matters more than a tidy one. Some of that work is masonry rather than concrete, some of it needs a designer involved because it turns out to be structural after all, and some of it is straightforward. We would rather hear the specifics and give you a plain yes or no on whether we take that piece on than have you assume either way off a web page.
Something has to explain the movement before the repair gets picked
A settlement crack tells you that something moved. It does not tell you what moved, why, or whether it is still moving, and those three answers are what decide whether underpinning is the right tool or a very expensive way to treat a symptom. Plenty of movement traces back to water, and water is usually cheaper to fix than dirt is. Where downspouts empty, how the grade falls in the first few feet off the wall, and what a gutter has been doing quietly for ten years are all worth ruling out first.
We do not diagnose structural failure and we do not design repairs. What we can do is take a look, tell you what we see, and be straight with you about whether this is our work, someone else's work, or something an engineer needs to put eyes on before anybody quotes anything at all. When a contractor doing a designed repair needs concrete placed somewhere a chute will never reach, that is pumping we come out and do for them with our own crew, on our own trailer mounted line pump.