But Mark didn't know that Sami would be absent to supervise for a year. Or did he?...
Sami had made the planning application to fell the trees several years previously and, as was his habit with most jobs, progress had been slow. Construction sites dotted around the house often lay incomplete for years. It is likely that Sami had discussed his plans with contractors in his employ. We know for example that his landscape gardener, AH, knew about the site and something of Sami's intentions.
So the claim of no building knowledge is indeed a lie
Knowlege doesn't equate to practical experience. Mark had been present during previous construction work at the house, even acting as a hod-carrier for his dad on occassion, but had never actually taken the lead before. His lack of practical experience is evident in the amateur workmanship evidenced in the layer of concrete. Remember that this was very different from the professional layers underneath.
The final root barrier had already "gone to plan" with three thick "professionally laid" slabs of mortar and offending trees removed, so all Mark needed to do was to backfill with soil from the original hole, just like he did after his own unnecessary ready-mixed concreting job.
When Mark arrived in November, the site appeared to have been prepared with a professional foundation, but it was clearly incomplete and remain far too deep to simply backfill. The previous two sites had been filled up to a particular height in the brickwork, and although he lacked the practical experience, he knew from this basic prior knowledge that this was what was required here too.