Is a soil survey necessary when a geotechnical report is available?

The soil report can vary greatly in price, depending on the services utilized. I am actually an advocate for a soil report if it provides a calculable added value or there are justified concerns. Our report cost just under €3,000 (December 2021) and included only 2 boreholes or dynamic probing. I obtained 3 quotes and awarded the contract to the cheapest.

However, we built on a very confined space, under the influence of groundwater and with a cellar. And just for the groundwater lowering, the groundwater level was necessary to calculate the water inflow into the excavation pit.

If there is a comprehensive report for your building area, I might first try to find out where the soil investigations were conducted. Maybe you’re lucky and fall +/- a few meters on that spot and can therefore rely on the existing data. Since you’re building without a cellar and data is available, it’s certainly worth considering. Perhaps conduct an exploratory excavation with a mini excavator at 4/5 spots and check whether the encountered soil matches the report’s data. Gravels and sands, as indicated in the report, can in any case be visually distinctly differentiated from peats and silts. However, if the report does not actually cost you as much as expected, I would probably commission it after all. If you are building turnkey, the report usually has to be provided by the builder anyway or is important for you as a contract-relevant document.

How did the other builders found their buildings in the development area? This graphic provides information about which soil types were found at which depths and in what layer thickness. In my opinion, it looks really good from a depth of 1.5m. It is quite possible that one or two additional work steps might still be necessary above that for the foundation.