Extrapolation beyond the testhole area is dangerous and must be totally at the clients risk. A situation has arisen where the client want advise on a site adjacent to a building for a parking area. It was not included in the original investigation. Any additional advice should be an extra.
The first recommendation should be testholes are required to provide the requested recommendations. We only know the soils at the testhole locations, and there may be only soils classifications that are relevant. Soil varies rapidly across a site. Fluvial deposits vary rapidly across the direction of water flow, less along the flow, and the flow is constantly changing, so the direction is changing. As sediment is placed, the channel is filled in. When old braiding and oxbows occur, peat will grow. So when we find peat, is it a channel or bog? How uniform do you expect the geology to be?
We expect that there is peat on this site. With peat, they need to be aware that movement, settlement, will occur, and that the settlement can be substantial. Grades are important when settlement is expected, and a 1.5 metre pad over the peat is require, remember that peat is associated with water, so what do we really have?
Next, as fill will be required, we will need to confirm the clay is suitable, and is consistent with the clay assumed in the fill design. Settlement of any remaining peat will occur. How much settlement? Rule of thumb 50% +_ 25% of fill depth. OK. Another one for deep muskegs-- 1/2 the depth of the fill placed. Peat is fibrosis, shearing is often not the issue, just compression of the peat. Fabric or grid can also spread the load better, but does nothing for settlement. Low weight fill has been utilized a few times. Artificial foam has been proposed to be floated, but I have never seen.
One more thing to think about.