Interpretation of soil investigation for construction

So it looks like they suggest a gravel bed, covered by a textile, so that the gravel doesn't run away. I wouldn't want something like that.

What would you want there?

we have a report on the soil investigation for a possible .

I’m glad to read that.

Who does a layperson usually discuss such a report with [ > ]

With the architect, in service phase 1. ...

[ < ] before buying a plot?

... or with a structural engineer and a civil engineer. But this hardly works in passing, so: don’t buy, but at most agree to buy with reservation and make it clear to the buyer that you first need a geological interpretation. From my point of view, a first point of contact for this would also be the engineering firm from which the report originated. In your place, I would commission them to create a “German translation” of it – preferably based on questions you formulate about it (whereby the question “how much does this cost” would be rather unsuitable). Above ground, rain “infiltrates” at 1*9.81 to the power of plus 1 meters per second through soil that favors infiltration comparatively at a slow-motion pace, and for the soil of this building ground even at a sloooow pace. The clay in “horizons” means that it is essentially layers, so no shafts; but of course not as homogeneous as one would put dough into a baking pan. Quaternary is a geological period which includes the present. The soil structure of the plot is therefore probably not untouched since ancient times, but some Homo sapiens might already have played there with an excavator. Furthermore, a load test with a beam similar to that of a garage foundation apparently took place here – with the result of the warning that a strip foundation could lead to a “leaning tower of Pisa” here. You see, I’m not a geologist either – and the singer’s courtesy keeps silent about my high school grades – but nevertheless the expert jargon is quite readable. Personally, I am missing two important background information for your question: namely a site plan of the plot with first of all elevation points or lines and secondly the sampling locations of the bore cores. Both are important for the interpretation of the report, and the first one additionally as a “statement” regarding the “cellar question” (which even if you are clearly against a cellar would be a valuable hint).

So, once again for your orientation. You are about to buy the plot of land and want to know if you are buying a pig in a poke.

Clay soil has its pitfalls, it is very sensitive to water, not compactable, and frost-sensitive. In winter, it is generally a disaster for earthworks... And the soil is not permeable either.

Sounds bad, but this is the case for a large part of those building in central and southern Germany... so basically nothing special or dramatic. It is also likely that the next plot in the region looks similar (unless you move to the mountain or the river).

If you, as 11ant wrote, reveal a few more details about your project, then I can gladly give you concrete (non-binding) tips.

Personally, I am missing two important pieces of background information regarding your question: namely a site plan of the property with, first, elevation points or lines and, second, the sampling points of the bore cores. Both are important for interpreting the report, and the former additionally serves as a "statement" about the property concerning the "basement question" (which, even if you are clearly against a basement, would be a valuable hint).

The property is flat, and we want to build on a slab. The property itself is being sold by a developer who wanted to build 2 single-family houses on the property. I don’t even know if I am allowed to post these excerpts here? Of course, the bore points are also marked on the report.

So, again, for your orientation. You are about to buy the property and want to know if you are buying a pig in a poke here.

Clay soil has its pitfalls; it is very sensitive to water, not compactable, and frost-sensitive. In winter, generally a disaster for earthworks... And the soil is also not permeable.

Sounds bad, but this is what a large part of those building in central and southern Germany deal with... so basically nothing special or dramatic. It is also likely that the next plot in the region looks similar (unless you move up to the mountain or to the river).

If, as 11ant wrote, you disclose a few more details about your project, I can gladly give you concrete (non-binding) tips.

Exactly – well summarized. I have posted more information. As I said: I don’t know if I am allowed to just post this since I did not commission the geotechnical investigation.

What other information would still be relevant for an assessment?

Here are the exact results of the bore cores.



Many thanks in advance to everyone who has participated here.

Regards

Infiltration is common for clay soils. I would avoid a soakaway if possible; if one is senselessly required, it should have an overflow to the sewer. It’s better to gravel generously under the driveway, especially if you install permeable paving.

Regarding the foundation, I don’t see anything particularly negative at first. A soil replacement is probably not necessary (only if weak layers are encountered). According to the report, you will need a stronger slab, which is not a big deal. It is actually recommended to have only 30-40cm of gravel under the slab.

To me as a layman, it looks okay at first, similar to our building ground. I would only build a basement with a waterproof concrete shell ("weiße Wanne") in such an area and would not skimp on the gravel. For the exterior areas, pay attention to the slope; water can flow down under the driveway in the gravel.

Hello WilderSueden,

the soil report was done by an engineering firm for geotechnics. But maybe I also misunderstand your question? Are you interested in which firm actually did it?

What would you want there?

Strip foundations. We also have unusable clay up to 2.5 meters below ground level. Simply dumping gravel, compacting it, and pouring the slab on it would not be for me. This method called "DST foundation" was proposal 1 in my soil report. That would probably have been significantly cheaper, but it had the disadvantage that you need a 60 cm cushion overhang all around, in which only very limited civil engineering work is allowed.

Proposal 2 was strip foundations down to 3 meters depth onto the load-bearing layer. That's what we did. And that was quite good, because the contractor, when laying the rainwater pipes to the cistern, started digging the trench right next to the house without asking. Not allowed with proposal 1...