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

We paid around €800 for our soil survey in 2021. There were 4 drilling points, one at each planned corner of the house. So the site plan for the house was already available. From the results of the drillings, a foundation recommendation was then made, specifically for this location of the house. I think this is always done this way and not the entire property sampled.

Thank you for the reference. Does it make sense to do the order like this? I would basically have done the survey first in order to have the necessary basis for service phase 1.

The soil survey can vary greatly in price depending on the services used. I am actually an advocate of soil surveys if they provide a calculable added value or there are justified concerns. Our survey cost nearly €3,000 (December 2021) and included only 2 boreholes or dynamic probes. I obtained 3 quotes and awarded the contract to the cheapest.

However, we built in a very confined space, under the influence of groundwater, and with a basement. 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 construction area, I would first try to find out where the soil investigations were conducted. Maybe you are lucky and fall +/- a few meters on this point and can therefore rely on the existing data. Since you are building without a basement and data is available, this is definitely worth considering. Perhaps conduct an exploratory excavation with a mini-excavator at 4/5 locations and check if the encountered soil matches the data from the report. Gravels and sands as indicated in the report can definitely be visually distinguished from peat and silts. However, if the survey actually does not cost you as much as you think, I would probably commission it after all. If you are building turnkey, the survey usually has to be provided by the builder anyway or is important for you as a contractual document.

Can you roughly say how much a main soil investigation for construction should cost per sqm or for a 650sqm plot?

It varies. But there is already a general report available, so in my opinion the chemical analysis can be omitted. Two borings plus one dynamic probing should be sufficient for a house without a basement. You will find something under Google Geologists. You can get two to three quotes if it is already roughly clear where the house will be built. Before the building permit application during the final planning phase is a good date.

I just had the invoice for the soil survey in my hand while sorting through the documents...

- Preparation of an engineering geological report flat rate 880 net
- Additional probing in the area of the infiltration system 100 net
- Calculation of the infiltration system 150 net

Prices January 2023 in Lower Saxony

Two drillings with a depth of 3m and one with a depth of 5m were made. Chemical examinations were not necessary since we have 50cm of topsoil and then pure sand down to the groundwater level at 8m depth.

Definitely a soil survey. According to the survey, my neighbors were able to build on a normal gravel bed with an 80cm frost protection layer... 10 meters further, at my place, initially piled foundations were even discussed, in the end it was "only" 3.5m deep strip foundations... we poured 60m³ of concrete just into the ground.

Especially if you have high groundwater. There can always be an area with particularly soft soil.

Definitely a soil survey. According to the report, my neighbors could build on a normal gravel cushion with an 80cm frost skirt... 10m further, at my place, initially even driven piles were discussed, in the end "only" 3.5m deep strip foundations... we poured 60m³ of concrete into the ground alone.

Especially if you have a high groundwater level. There can always be an area with particularly soft soil.

Wow. That’s quite a difference :eek: You didn’t build too far from me, did you? 60cbm for a simple slab with strip foundations for your Town & Country Flair? So for about 90sqm floor area?! Crazy. And that at the moon prices of 2022 for steel and concrete with numerous surcharges for all sorts of things?