Best way to fill "gap" left in floor after wall ta

[PeterRayner]

Lath and plaster wall was taken down leaving large "beam" in floor which supported the uprights. There are also also some electric cables now at floor level.
The "beam" has now been lifted out as was proud of floor which must be carpeted ( mansion flat building).
I have a hole in the floor - 6 inches wide, 4 inches deep and 15 feet long. How should I fill it?
The floor boards run up to it at right angles but are all of slightly different lengths.
The hole was filled with rubble, sand etc ( as well as the beam) - as is the floor under the floorboards. Some of this debris from gap has been removed but the sand from under boards then runs into space.
Any ideas what filler to use? I think it has to be dry - or the ceiling in flat below will get water stains? Also something that will incorporate existing debris and set hard to make smooth surface.
Any ideas as to proceed would be appreciated. Many thanks for any replies!

 

[TicklyT]

If you 'fill' the void, all that will be supporting whatever filler you use will be the ceiling of the flat below - probably not such a good idea, especially if you want to stand on it afterwards

 

[PeterRayner]

Thanks Tickly for the reply - removing the debris revealed a steel beam - not as wide/deep as the gap left by removing wooded beam ( which supported uprights of removed lath and plaster wall). I think any wet filler will stain ceiling below so presume I shall have to use some sort of wood board infill probably in different sections cut to accomodate wiring - this infil I shall have to support on wooded "blocks" - which will probably need to be wedge shaped(?) to get firm fitting between steel bean and rouble under floor boards. I can't think of anything else except I need a carpenter! Cheers!

 

[ladylola]

Screw timber to the underside of the board ends, cut wider than the gap and fixed through the face of the boards and then simply fill in with timber the same thickness as the boards. If it's being carpeted it doesn't need to be pretty.

 

[PeterRayner]

Thanks for that ladylola - great idea - I think you've got it!