What are advantages of deep strip foundation?

Narrow strip or Deep strip footing

This type foundation is used for normal soil and normal loads which just need 0.9m dept cheep in cost construction.

The essential feature of the narrow strip foundation is that the trench is too narrow to be dug by labours working in trench. It depends for its success in the ability of a mechanical excavator, such as a light tractor-mounted back-actor with a narrow bucket to dig the trench, which must be self-support until it can be backfill with concrete.

It can't be economical be use in very soft clays or water bearing sand which support by close timbering.

From Lewis Telfer