Peat in soil - pile foundation, well foundation, vibro stone columns

Hello! We were lucky enough to reserve a plot of land again that really appeals to us. Unfortunately, the soil report we commissioned revealed peat in the ground. Since the peat is not very deep, the potential general contractor thought that soil replacement might be the most financially attractive option for a secure and stable foundation. We contacted every company that came up in our internet search for a quote, but received only one offer, which amounted to nearly 75,000 euros. Since the groundwater level is relatively high, soil replacement might not be a sensible option after all? Now we are looking at alternatives, we have found various methods for pile foundations, drilled foundations, and vibro stone columns, but we are absolutely overwhelmed in initially assessing which of the methods could be suitable, what the most sensible option would be, and also in searching for a suitable company, so far all we have is browser searches. Through this post, we hope for some information to help us classify everything and initiate the appropriate steps. I am attaching the soil report and thank you very much to everyone who takes the time to have a look! A shallow foundation with reinforced reinforcement might be possible, but everything we have read so far sounds like "just don’t do it". Are we right in thinking that?

Actually, I would be inclined to say tough luck, peat in the soil means expensive, let’s look for another plot. That’s what we have been doing in recent weeks as well; we are on every waiting list we found, we have looked at various plots, and the search area with "Schleswig-Holstein" was not exactly small. But we keep coming back mentally and in conversation to this plot, this new development is simply located exactly in the area where we would most like to live, we really like the town and the surroundings. It is just uncertain whether we can get the foundation on a reasonable financial basis, and we would be very grateful for any help!

Best regards,
Kerstin

Hi,

unfortunately, I see it the same way, the high groundwater level makes soil replacement uneconomical. Here, a shoring (preferably sheet piles) and a closed water control would be necessary, followed by excavation and backfilling...

You have basically already recognized the alternatives correctly and they are "mentioned" in the report. You either accept cracks or choose a deep foundation. If I were to build a modern house today, I would not take any risks, especially since all further damage during construction is blamed on the foundation...

A deep foundation is somewhat more expensive (I estimate >30,000 € for a single-family house, but my figures are a few years old) but it is safer.

The alternatives:
Pile foundation: reinforced concrete piles are driven into the sand. I guess 5 - 6 m depth... values must be specified by the geotechnical engineer and the structural engineer must then calculate the number. Additional investigations may be necessary, water analysis (concrete aggressiveness) and possibly a pressure sounding to determine skin friction and tip resistance (costs around 1000 €).
The expensive part of all deep foundations is usually the site setup and often already around 15,000 €.

Personally, I am a fan of vibro-compaction piles. A "hole" is rammed into the ground here and then gravel/crushed stone is gradually filled and compacted from above. If needed, a binding agent can also be added (CMC piles). Once the construction area is basically packed full with this, a gravel cushion is placed on top to distribute the loads.

Well rings: well rings are brought to the target depth and concreted if necessary. Here it depends on the earthworks contractor.

All 3 variants achieve the goal and in my opinion are feasible for you.

I would contact the geotechnical engineer again to see if he can also offer foundation recommendations for the other variants.

Then get offers. Known companies are Menard (CMC piles) and Keller Grundbau.

Good luck!

Then your potential general contractor should get his ass moving and have the structural engineer do the calculations. Of course, the effort has to be paid for, obviously. But he knows what kind of house you want, the structural engineer calculates the loads, and then(!!) you also get a meaningful offer from, for example, the pile foundation specialist. Then you call him, and things get moving. If the general contractor won’t agree to this in advance, I would look for another one...

Thank you very much for your answers and the info!

Cronos, that was really super helpful, thank you! We have now received various offers for different piles. But comparing them is at least impossible for us. Completely different specifications and of course all just estimated costs. Then there would be the reinforced reinforcement in the base plate. Currently, I would say that we have no idea where the price will end up. We have an offer for the pile foundation that is under 10,000 euros and one that is almost 40,000 euros, and the other offers fall in between.

There is another meeting this week with the potential GU (Thanks Rübe!). Well, we’ll see if we can at least get some info on the structural engineering of the house so that the offers can be more precise. Can you actually create a thread here in the forum to evaluate offers?

Without signing the GU contract, we don’t get the floor plans. But the size of the house would be adapted to the floor plan (half a meter more here, a meter less there). We had to determine the positioning of the house for the soil survey so that drilling can be done accordingly, and that was already hair-pulling without knowing what dimensions the house will actually have. If the house were smaller, larger, or different in shape, then the soil could again have a peat lens at a deeper depth and the foundation would have to be different, or is that a misconception? The structural engineering of the actual house would then provide the appropriate info for the civil engineering companies. But the structural engineering is only calculated once the contract is signed and the floor plan is created. Without at least a good estimate of how high the costs for the foundation would be, we cannot calculate the financing (not more precisely than before) and decide whether to buy the plot. That’s a bit maddening. With unlimited financial means no problem, a huge buffer and it’s all good. But for that, we really should have bought a lottery ticket.

We also contacted the soil surveyor again and asked whether we could get foundation recommendations for the other foundation options. Now we’re waiting for a reply!

Without signing the contractor's contract, we won't get any floor plans. However, the house size would be adjusted to the floor plan (half a meter more here, a meter less there). We had to determine the positioning of the house for the soil survey so that drilling could be done appropriately, and that was already hair-pulling without knowing what dimensions the house would actually have.

It's quite simple: there is a planning contract and that's it. Currently, you as the builder are in a better position than a general contractor; I would make that very clear to him. You are currently guessing blindly; it has also happened that peat lenses change in size, and the soil surveyor only caught the edge. I also had a case where 3 points were absolutely fine and at the 4th there was peat up to 6 meters. Ultimately, everything must be specified for the financing of the house as well. By signing a house construction contract, you are basically issuing a blank check without knowing what it costs, etc.

Yes, that is true, just signing first and then planning everything can go seriously wrong ... we would of course prefer to know the exact costs for everything down to the cent before construction begins, but that's wishful thinking.

What is a planning contract? I've never heard of that, it was never offered to us.

An update ... we have received an offer for the foundation with vibrating displacement columns that simply sounds great! According to the company, a foundation with a normal slab without extra reinforcement is then possible, so really founding as if you had perfect soil. The settlement is supposed to be even, so no damage to the house can occur and the settlement itself is greatly reduced. Another advantage is that the columns function like a drainage system. And then the cherry on the cake, the pressure sounding, which would have been necessary for the piles, is not required with the foundation using vibrating displacement columns here, as confirmed by the soil expert.

We are now including this offer in the financing planning.