How to build large deck on sloping ground?

[Minyarra]

Hello

I am looking to build a deck at the front of our home which is a converted steel shed on concrete slab. The deck is quite large 12m x 5m overall and it sits under a pergola that has just been completed. (see attached drawings and images.)The pergola posts (150x150) are shown in pink. the dimension pointed to the posts is the height of deck from ground level. 

I am concerned mainly about the deck's height from the ground which at the lowest point would only be about 160mm and at its highest point 885mm (this is fine; I will do steps at this section).

The lowest point, being 160mm from ground to top of deck leaves only 140mm for bearers and joists, with the bearer sitting on the ground (using a 140mm bearer) and the joists flush to the top of the bearer. I expect that I need the bearer to be a minimum of 75mm from the ground.

Is that correct? 

What should I do to make this workable? 

Is a 140x45mm H3 timber bearer suitable? or should I go to a steel purlin/box section?  

Should I dig it all out to increase the space underneath? how much?

about the number of stumps 

[Nailbag]

Hi @Minyarra 

I agree with exactly @MitchellMc suggestions here. Pretty much standard practice build.

Nailbag

[Minyarra]

Thanks @MitchellMc @Nailbag @Dave-1 

super helpful. I thought the rebate would be best but wanted to reduce workload and complexity 😂😅

[Dave-1]

 @Minyarra 

I know that want, I try to ballance it with "a little bit extra is worth a factor of 3 to 4 down the track. You "know" you have done the hard yards and somehow it just feels better.

Dave

[Minyarra]

absolutely @Dave-1 

not a job I want to come back to because i compromised 

Thanks again y'all 

@EricL @MitchellMc @Nailbag @AlanM52 @Dave-1 

I will upload photos with commentary as I go. 

Currently I am digging out the holes, of which there are 44# (I think) all of which must be dug out by hand because of the ground, which is rock-laden dry clay  - literally using a straight bar and gloved hands - Auger wouldn't work because the rocks send it off-course (one rock was 65kg, but most are less than 10kg) and the post hole pincer shovel couldn't pick up the dry clay dust.