Your timber may be F7 grade but that is a strength rating and does not imply any particular durability or longevity in the ground. In fact Douglas fir, known in Australia as "oregon", is rated Durability Class 4 in ground contact according to Australian Standard 5604, Timber - Natural durability ratings. This suggests a probable life expectancy of up to 5 years. Even out of ground contact, oregon is not a highly durable timber. However, posts that are painted on all surfaces, including the ends, and supported on stirrups, will achieve a longer life above ground than in ground as long as there are no joints where water can collect. Rather than trying to jackhammer the timber out of the ground it would be simpler to cut the posts off, bring the concrete up to a little above ground level and mount the posts on stirrups attached to the concrete pads. You can give the timber some added protection by inserting boron sticks in the base of the posts - refer website here: https://preschem.com/products/timber-protection/no-rot.Oregon
Answered on 29-10-2020
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