Before I get started on this entry, a disclaimer. Installing the posts is probably the most critical part of the entire deck building process. In my case I am building a deck that is very close to the ground so I don't really have to do anything special structurally in my government area. However, your area may be different and the height of your deck may be very different to mine. Please contact your local council/shire/district to find out what you need to do in your particular case.Under no circumstances should you blindly copy what I have done here.
Due to the extremely hot weather I decided to tackle the main part of the deck (3.6m x 3.1m) today and leave the smaller 2.2m x 1m section in front of the garden shed for another day. First task was to layout the site with a series of string lines. As usual I used the edge of the paving as a datum as I know that the garden shed is not square to the house. I laid the first string line 100mm in from the edge of the pavers, the next parallel to this one 1560mm in and the final one at the full 3100mm. At right angles to these I laid another three string lines using the fence line as a datum. These were at 100mm, 1895mm, and 3600mm from the fence line. At all times I ensured squareness of the lines by measuring from the line back to the fence line.
Our house is built on a cut and fill site. Basically this means the the high side of the site is dug out and the soild placed on the low side and compacted to provide a level site. The upshot of this is that below about 5cm, the compacted clay soil is extremely hard. To help digging the holes I excavated a shallow pit at each string intersection and filled it up with water and left them for an hour for the water to soak in and the soil to soften. When fully absorbed I was able to easily excavate 30cm x 30cm x 40cm deep post holes with a post hole shovel.
I had already decided when building the deck to put the posts in long and then cut them off to level after they were set into the ground. I knew that none of the posts would extend more than 45cm from the soil so I decided to cut all the posts at 90 cm. This was done by marking the posts with a set square all way round and then cutting from each side with my 185mm circular saw with the cut depth set to 50mm. If you have a bigger circular saw or a big mitre or sliding compound saw you may be able to cut them with one pass.
I used some scrap wood nailed to the posts to support them and got them plumb using a post level. Once I had positioned them at the intersection of the string lines they were ready for concreting. In my case I decided to use rapid set concrete covered with tamped earth. This just meant ensuring the hole was damp to stop water absorption, putting in the correct amount of water (in my case 2.2 liters, your case may be different) and emptying a bad of rapid set concrete evenly around the post. Within an hour the posts were set enough to be able to be worked on.
Not as much as I wanted to. I started working on the posts at 12:30pm and by 3:00pm the heat was really affecting me. I found out later that night it was 37°C (98.6°F) in the shade while I was working. By 3:00pm I was having trouble walking in a straight line despite having drunk several liters of water and being fully covered from the sun. I got 3 of the 9 posts in that I had intended, plus the holes dug for three more, and all nine posts cut to length.