Bees on floor in garden. Advice please

[smashed]

Hi,
We moved into our home last summer and have been making huge changes to the garden. Bees have suddenly appeared swarming around (only about 20 at at a time) and landing on the muddy ground and either eating or digging I think.
I moved a large slab in case there was something under it but now they are just doing the same where that used to be. I have attached a few pics to help identify them / what they are trying to do.
The area did used to be full of bushes / plants last year.

Thanks for your help.

[eucera]

Hi

They are digging for mud of exactly the right consistency to take back to their aerial nests. The mud is used for making the brood cells and sealing the nest. In the world of the Red Mason bee, mud of the right sort is a rare commodity, and they may travel hundreds of meters to mine it

[smashed]

Thanks, it may explain why a lot went to exactly the same spot.
Do you know how long they are likely to be around? (I was about to pave all of this for a garden party in 10 days time).

[eucera]

Give 'em as long as you can. I suspect it is a clay they are interested in. Perhaps you could extract some... keep it moist alongside your paving. Then you'd get the best of both worlds