I am erecting a palladium self build victorian conservatory and the base is pre-fabricated/steel base.

My problem is that I have dugged four hole for the four footings to go in. Now one of the hole is next to a drain. The soil (earth) around that hole is soft and I have already digged 1.5ft. I have not concrte in yet since I am worried about the soft soil. Someone advised me to put lentel cement block and put the concrete on the top of it.

Do you think it is a good idea? or there is any other advice you could offer?
Also, the drain which runs about 1.5m below be affcted anyway?