Widen trenchfill foundation?

[pin3]

Hi, I'm in the process of building a two storey extension where foundations have been layed for a previously planned single storey extension. The two storey extension requires a number of block piers to support some steel beams. Building specs show 1 Mtre sq pads to support each pier, trouble is that one of the pads needs to go exactly where there is a 600mm trench fill foundation for the old extension. Can I just pour 400mm alonside this to widen the strip to support the pier? I'd rather not have to exract the existing concrete if possible .

Any ideas?? Thanks.

GP

 

[r896neo]

It's worth checking but i doubt BC will like that idea! trenchfill is only about 8:1 or something?