Question about soil condition building plot, single-family house planned

  • Erstellt am 2016-04-05 14:58:20

AndreasPlü

2016-04-05 14:58:20
  • #1
Hello,
we want to buy a building plot and there are questions upon questions. One concerns the soil condition. The selling municipality had an expert report made with several boreholes in the building area. Two boreholes were made near our plot; the corner was summarized as the "northern region." The result of the boreholes:
Borehole 1 (approx. 5 m from the building site): up to 0.30 m topsoil, then up to 1.70 m clay, silty, stiff, medium brown, then up to 3.50 m clay, silty, weakly organic, stiff, partially soft, medium gray-brown to dark gray-brown, then up to 4.00 m sand, silty, weakly clayey, (stream sand), partially weakly gravelly, moderately loosely deposited, medium brown, then up to at least 4.50 m gravel, sandy, weakly silty, moderately loosely deposited, medium brown. Groundwater 4.37 m.
Borehole 2 (approx. 50 m away) up to 0.30 m topsoil, then up to 0.70 m fill (silt, strongly clayey, weakly gravelly, weakly sandy), stiff, then up to 2.30 m silt, strongly clayey, stiff, gray-brown, then up to 3.80 m clay, silty, weakly organic, stiff, dark gray-brown, then up to at least 4.50 m gravel, sandy, weakly silty, weakly clayey, moderately loosely deposited. No contaminated sites.
Notes on construction in the area of the boreholes: "When building a single-story basement, strongly compressible clay soils of stiff consistency mostly have to be taken into account. Due to the high water content and the distinctly plastic composition, the structural loads should be distributed over large foundation areas in these soils, since the design bearing capacity of the soil should be assumed to be low. Possibly, slab foundations may be economical in this area."
What does this tell me? Does anyone have expertise? Is construction of a single-family house with a basement made more difficult? Are significant additional costs to be expected? Thanks for assessments!
 

andimann

2016-04-05 15:26:54
  • #2
Hi,



You want to build in the swamp....

Yes, that is possible, and yes, it will be expensive. How expensive, your general contractor/architect might be able to tell you.

Best regards,

Andreas
 

AndreasPlü

2016-04-05 15:51:31
  • #3
Thank you for the response, that sounds rather suboptimal. Do you have an idea whether we are talking about costs of €5,000 or rather €20,000? The building envelope would be about 9 by 11 meters. Which measures are sensible and what does [Plattengründung] mean? Is it generally advisable to avoid purchasing the plot?

Best regards
 

ypg

2016-04-05 16:23:53
  • #4
Have a geotechnical report made specifically for the property! That prevents surprises. The general report is not sufficient for construction at least.
 

Bauexperte

2016-04-05 16:57:04
  • #5
Rather ....

As my predecessors correctly wrote, an independent soil survey must be carried out for your property. However, the information from the selling municipality is already sufficient to make initial contact with a deep foundation specialist or provider of your choice to speculate about possible foundations. There are a wide variety of solution approaches, whose ideas would exceed the possibilities of the forum.

Best regards from the Rhineland
 

andimann

2016-04-05 19:09:35
  • #6
Hi,
my predecessors have already said it: you will only get certainty about the costs through a geotechnical survey and incorporating this survey into the house offers. However, you will probably hardly manage this before purchasing the plot (I assume you don’t have unlimited time to decide) and even a geotechnical survey can be way off. In the end, they are just some exploratory drillings; you only have certainty once the excavator has dug out the building pit.

Costs are really hard to estimate beforehand. It also depends on the house you want to build. The flat-roof bungalow without a basement will have different soil pressures than the two-story city villa with a concrete brick hipped roof.

Just to get a feel for it, I would definitely prepare for a clearly five-figure sum. Then add flood-proof basement windows including drainage of the light wells; just those last two things would have cost us around €8,000. (we then got rid of the basement windows).

Try to clarify it somehow, even if you have to commission a geotechnical survey at your own expense. If that’s not feasible, I would personally firmly plan at least €15,000-20,000 for additional foundation costs plus the same amount again as a 50% risk contingency. So in total, add €22,500-30,000 to the purchase price of the plot. If you still really like it then…

Swampy building ground can really be nasty; one of my industrial plants stands on 3,500 concrete piles going 15-20 m into the ground. It looked like they were trying to recreate Venice. Good thing we didn’t have to pay for that…

Best regards,

Andreas
 

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