I would plan on using the crane/bucket, 6' is going to be a stretch to get it to come down the shoot, even with a front discharge truck.
I've been on jobs where we've built fill ramps to increase height.
As for dropping the mix you need some kind of an elephant trunk or tremie for your bucket.
I agree with all of this & will add:
- IMO, 4' high + 10' of chute = 8"+ slump.
- 6' high + 10' of chute = you need a ramp to get the truck higher.
- When in these situations on the past, we've called the supplier to send a truck out (usually returning from another job empty) at their leisure to do a trial fit. An experienced driver can usually tell you right away if it's going to work or not. This is obviously easier to do when you have a good working relationship with the supplier. It also let's the dispatcher know from the driver how long they may be tied up on a strange pour.
- Avoid a rear discharge at all cost. It's been years since I poured with one, but as I recall, there discharge height is quite a bit lower.
- If these columns have any amount of steel in them, your going to have some segregation no matter how far you drop the crete.
- Use a vibrator to consolidate when worried about segregation.