Hello,
we now have the following "conflict" to resolve.
We are building a 14 x 14.5 m (46 x 48 ft) bungalow. The soil report indicates the following layers:
Layer 1: Topsoil 20 cm (8 inches)
Layer 2: Sand, highly silty, soft to stiff, gray-brown
Layer 3: Sand, highly silty, humus, soft to stiff, dark brown to black-brown
Layer 4: Sand, low silt content, partly silty, medium dense, ...
Recommendation: remove 1 to 1.1 m (3.3 to 3.6 ft) and refill with compacted gravel or a similar material.
Now, a groundworks contractor who has seen our plans pointed out that he was not informed about our planned fill. We actually need to build up an average of 1 m (3.3 ft).
He suggested that it would be sufficient to just strip off the topsoil (and possibly some centimeters of layer 2) to achieve a 1.4 m (4.6 ft) gravel layer underneath the foundation slab. If necessary, the foundation slab could be made a bit thicker and the reinforcement adjusted accordingly. This would save us money.
However, our structural engineer insists that everything must be removed as recommended in the soil report.
Now we are uncertain. Both are professionals.
We will consult other groundworks contractors to see their opinions.
However, this involves a significant amount of money. Has anyone here had experience with a similar case?
we now have the following "conflict" to resolve.
We are building a 14 x 14.5 m (46 x 48 ft) bungalow. The soil report indicates the following layers:
Layer 1: Topsoil 20 cm (8 inches)
Layer 2: Sand, highly silty, soft to stiff, gray-brown
Layer 3: Sand, highly silty, humus, soft to stiff, dark brown to black-brown
Layer 4: Sand, low silt content, partly silty, medium dense, ...
Recommendation: remove 1 to 1.1 m (3.3 to 3.6 ft) and refill with compacted gravel or a similar material.
Now, a groundworks contractor who has seen our plans pointed out that he was not informed about our planned fill. We actually need to build up an average of 1 m (3.3 ft).
He suggested that it would be sufficient to just strip off the topsoil (and possibly some centimeters of layer 2) to achieve a 1.4 m (4.6 ft) gravel layer underneath the foundation slab. If necessary, the foundation slab could be made a bit thicker and the reinforcement adjusted accordingly. This would save us money.
However, our structural engineer insists that everything must be removed as recommended in the soil report.
Now we are uncertain. Both are professionals.
We will consult other groundworks contractors to see their opinions.
However, this involves a significant amount of money. Has anyone here had experience with a similar case?
D
Doc.Schnaggls19 Jul 2016 09:24Hello,
I would also recommend consulting the geotechnical engineer again.
Even if it means spending a few more euros for a new (partial) report, it would be worth it to me for the certainty that I won’t face structural issues later on.
Regards,
Dirk
I would also recommend consulting the geotechnical engineer again.
Even if it means spending a few more euros for a new (partial) report, it would be worth it to me for the certainty that I won’t face structural issues later on.
Regards,
Dirk
I highly value the opinions of experienced civil groundworkers, as they often have a lot of knowledge and have completed many projects. However, they are neither structural engineers, geotechnical experts, nor construction engineers. Their recommendations are based on experience and solid know-how. This is usually very valuable, but ultimately no one will assume liability for it, so I would be very cautious. Even the groundworker cannot guarantee what will happen in 20 years. Sand layers are always somewhat tricky. Basically, they are very strong under pressure and generally not a concern. But once water becomes involved, things can get critical. Therefore, it’s not possible to say from here whether your groundworker is right or not. In the end, you bear the full risk if you do not follow the soil report.
Personally, I am also surprised by how much soil replacement is done nowadays. I can hardly remember the last time I saw an excavation without at least 40cm (16 inches) of soil replacement. Much of this, in my opinion, is increased perceived safety or double insurance for the architect or structural engineer. Still, in many cases, the costs are manageable. The groundworker only benefits from this to a limited extent, as this is not typically a highly profitable item and requires additional equipment on-site (mini excavator, compactor roller, etc.).
What strikes me about your data: If you are digging 2.1m (7 feet) of substructure under your house, have you considered including a basement? This could be cost-effective and perhaps even accommodate the garage. It always pains me to see large amounts of expensive fill piled up just to build bungalows on top.
Personally, I am also surprised by how much soil replacement is done nowadays. I can hardly remember the last time I saw an excavation without at least 40cm (16 inches) of soil replacement. Much of this, in my opinion, is increased perceived safety or double insurance for the architect or structural engineer. Still, in many cases, the costs are manageable. The groundworker only benefits from this to a limited extent, as this is not typically a highly profitable item and requires additional equipment on-site (mini excavator, compactor roller, etc.).
What strikes me about your data: If you are digging 2.1m (7 feet) of substructure under your house, have you considered including a basement? This could be cost-effective and perhaps even accommodate the garage. It always pains me to see large amounts of expensive fill piled up just to build bungalows on top.
We would need a waterproof concrete basement. Costs would be over 50,000 euros. I also don't want a cellar. I really hate those things.
The geotechnical engineer says that excavation is still necessary because of the third soil layer. It is partly quite rich in humus. Could the second layer then be compacted and put back in?
The geotechnical engineer says that excavation is still necessary because of the third soil layer. It is partly quite rich in humus. Could the second layer then be compacted and put back in?
No, because compacted sand cannot be properly consolidated!
A basement was just an idea. I like basements, but of course, it's a matter of personal preference.
Then you will probably have to bite the bullet, unless you get your excavation contractor to provide written assurance that you don’t need soil replacement and that they will be liable if necessary 😉
A basement was just an idea. I like basements, but of course, it's a matter of personal preference.
Then you will probably have to bite the bullet, unless you get your excavation contractor to provide written assurance that you don’t need soil replacement and that they will be liable if necessary 😉
Evolith schrieb:
Our structural engineer now says that everything needs to be removed, as recommended in the geotechnical report.
Now we are uncertain. Both are experts.
We will consult other groundworks contractors and see what they say. Of course, the structural engineer follows the geotechnical report because that elegantly shifts the liability to the geotechnical engineer. And the geotechnical engineer avoids liability by including a clause somewhere in the terms and conditions, which somehow passes the blame to the contractor performing the work.
That leaves the groundworks contractor, who in case of any issues will naturally not remember this recommendation.
In the worst case, this could involve hundreds of thousands (of currency), so no one wants to be held responsible. Consequently, EVERYONE officially agrees only to what someone else has already recommended (who in turn shifts the liability elsewhere).
Conclusion: in the end, you need to decide how much replacement you want. You only really see during excavation which soil layer actually has enough load-bearing capacity to support the building. A single borehole at one location tells you very little. That’s how it was done for us: a large excavator started digging and after five minutes, at 1.1 meters (3.6 feet), it hit undisturbed soil. That was then recorded as the best possible foundation soil, and that was it.
The truth probably lies somewhere close to the geotechnical engineer’s recommendation and somewhat further from the groundworks contractor’s suggestion. The contractor is happy to dig a hole only 1 meter (3.3 feet) deep—that’s not much of a challenge. In the end, you will probably have to bite the bullet so that the house stands properly.
And again: these people are only liable on paper for their statements! Years of lawsuits, numerous experts, personal financial ruin in between, no insurance covering such cases, etc.—and in the end, the responsible party is suddenly insolvent, or the whole mess ends in a settlement where, at best, you recover the costs of the experts. You may be told otherwise here because some people make their living from exactly this. The only guaranteed winners are the experts and lawyers.
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