Hi @kiranraob4u
H4 treated pine is great for you purpose. Treated pine is chemically treated all the way through so a cut timber is still protected 100%. Termite wont touch them wood rot in 25 years may occur under ground. Sealing the underground ends touching the dirt in bitumous paint will preserve them longer should you wish but added expense, like $55 so not feasable for a smaller project.
Yes a ryobi 165mm circular saw will cut a 100mm round allthough you will need to cut it from the front and the back make sure the whole log supported for the cut including the off cut. Cut it on the lawn if you dont have trestles. Timber that large are like small trees and will move and splintter as you cut through them as the cut is almost complete and can fall and trap a fast moving blades spining and can cause seriuus injury and kick backs at the user. Your cuts will rarly align 100% so bury that rough end into the ground saves time.
Before you cut them lets work out what depth under ground will suit your application.
Now the general rule of thumb is depending on if the timbers will be under any side ways load. These guides assume yes under load.
A- Sand 1/2 in sand ie 3 meter length 1.5m is below sand
B- Dirt 1/3 of the post length needs to be in the ground and that leaves 2/3 sticking out thats in general dirt. So thats 1 metre in the dirt 2M out.
C- Heavy compacted clay ground 1/4 length. thats approx 750mm under ground and 2.25M out.
D - Concrete Might get away with 1/5 thats 600mm in the cement and 2.4meters out.
So you asked if 500mm into the ground on a 3M post That means 2.5M above ground into cement.
ANSWER is cut your posts 3.1m long and cement 600mm down on the post length (draw lines around post at 600mm as s a guide.)
Dig your hole 700mm deep to allow 100mm of concrete below your post this forms a cement shroud around your post giving it further life span.
I mean put 100mm of concrete into the hole then your post then cement all around the post.
If your post is too deep Lift it up a bit the concrete will flow underneath, If the post to high tap it down witha lump hammer or a brick to the line drawn on the post.
,
