I agree with gordonl; this is related also to the authority requirements in the country you are working in.
I had a similar experience in Qatar (Gulf area), where the authority required no oil pit for the 11/ 0.415kV oil filled transformers. Only the floor level of the transformer room is to be filled with aggregate. Any oil leakage from the transformers will go through the aggregate. In other countries they require oil pit for the oil filled transformers.
In France, the common practice is to construct a u shape oil pit around the transformers.
In Saudi Arabia, the authority (scico) requires a rectangular shape oil pit below the oil filled transformers.
In Angola, they require the same as in Saudi Arabia.
In case you have more than one transformer you may need to construct dedicated oil pit for each transformer and a collecting oil pit for the whole number of transformers and this to be constructed outside the transformer room.
Elecme