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Brief introduction to piling.
This is contiguous piling. There is a deliberate gap between neighbouring piles. the additional concrete along the top is the pile cap. This spreads the force from the props across the centre to several piles.
Big rigs become more cost-efficient the more piles need putting in. Perhaps more than about 100.
You can see that they need a lot of space for their setup and deliveries.
What you cannot see is 600mm to 900mm of compacted hardcore all over the site so that the rigs don't sink and tip over.
Or you might have a mini rig.
You can get a mini rig into a small space and you don't need any (or else a lot less) hardcore down first because they aren't tall.
Instead of being tall, you auger down a metre, disconnect the auger, lift up the head, add another section of auger, and so on. The reverse order when the hole is complete.
You get concrete delivered toward the end of the day when you fill the holes you dug.
You need very little more kit but they are slow. Perhaps a third of the speed, which might be 3 a day. Time is money so if you piled over 100 this way your costs might become more than the big rig.
If at any depth your soil is unstable, the big rigs can have an advantage if they can auger down but not remove all the material immediately; then pump concrete through the auger core to the very bottom, then withdraw the auger and soil slowly as more concrete is pumped in. In this way, the fragile sides might be well enough supported until concrete holds them back.
There are many solutions to many problems. After you get your soil report, and if it says piles are necessary, you could ask your structural engineer for a provisional pile layout before you spend much more money. Give them to some piling companies and ask what sort of money will you need to find. If you won't be able to find enough you might stop there with some money still in your wallet.
Short 90 second video. A clever tool to clean mud off piles as you excavate.
Obviously I am well aware that steel sheet piles are an alternative as well. But they don't get chosen for basement excavations.
One reason will be that steel sheet piles are generally rented. If you don't get them out at the end, you pay the hire until you give up trying to get them out and then you have to pay the hire company the full cost of replacement.
It is easy to set a rig or an excavator with an adaption next to the sheet and vibrate it in. Vibrating and damaging neighbouring properties can be another issue. But having to stretch 10m somehow from in front of your new basement to the farthest sheet to pull it out vertically can be too difficult. That's why they get left in.
Especially if concrete was cast against steel sheet piles. That grips them even more than just soil.
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