What's involved in making the base?

The base on which I construct pebble mosaics should be as firm as possible. This is usually achieved using a 2 - 4 inch / 5 - 10cm layer of well compacted hardcore. However, if you wish to drive a car over them then a good concrete base is recommended.

Whatever the base is made of, it should be 2 inches / 5cm below the ultimate level of the surrounding paving.