Decking foundations

[poppyspringer]

I am planning a dec for an area of the garden that gets very wet in the winter due to a high water table. My plan is to use cardboard forms to make concrete pillars (550mm high by 283 mm diameter) to then set posts into and the beams will go onto the posts with the joists on top of the beams...

The top of the concrete piers will be clear of ground level by about 200mm so hopefuly no timer at all will ever be in contact with ground water...I hope

I have a few questions:

1. The ground is solid chalk 200mm below ground level and I only plan to go down another 150 mm into the chalk. Is this deep enough and should I line the hole with some scalpings/hardcore?

2. At the moment if I were to dig a hole for the piers to this depth the bottom 200mm would fill with water. I plan to dig a sump for a sump pump (thats a whole new topic that I also need help on...!), but my question is:
Can I put the forms into the holes and pour concrete in; will the concrete displace the water?

3. final question: what mix of concrete should I use

Any help appreciated...

[thedoctor]

Yes the holes are plenty deep enough, scalpings wont be needed on chalk, yes the concrete will replace the water but it would be better if you could dig a slightly deeper hole nest to each post hole to drain the water while you concreted. See our mixing concrete project.