ᐅ Load-bearing foundation structure, base layer for slab foundation
Created on: 29 Apr 2020 17:38
P
Pet1234
Hello everyone,
I urgently need your advice.
I am building a bungalow without a basement with Heinz von Heiden (GÜ).
Until today, the groundwork company was on site to prepare the foundation base layer. The plot has a slope of 90 cm (35 inches) along the 22-meter (72 feet) length facing the street. Within the actual construction area, this height difference is smaller (30 cm (12 inches)) because there was already a structure there that was covered with soil, and the foundation strips were removed by the groundwork company to a depth of about 50–60 cm (20–24 inches). The soil surveyor did not notice this and considered everything fine. I found out about it beforehand from neighbors. Towards the back, the land slopes down about 90 cm (35 inches) again.
According to the construction contract, Heinz von Heiden is responsible for a foundation cushion of 30 cm (12 inches), including excavation, plus an additional 1 meter (3 feet) around the house.
The contract states regarding earthworks, among other things:
“The existing topsoil will be removed to a depth of up to 30 cm (12 inches) within the house area and an additional 1 meter (3 feet) around it and stored laterally on the site. Subsequently, a compacted foundation cushion up to 30 cm (12 inches) thick will be installed. The foundation cushion (sand layer) will extend 1 meter (3 feet) beyond the outer edge of the base slab, corresponding to the topsoil removal area...
If local additional thicknesses result from building up greater heights or extra excavation down to a load-bearing substrate, these will be invoiced separately.
Additional thicknesses of the foundation cushion can only be determined after reviewing the soil report, establishing the house elevation, and considering local conditions.
The calculation basis for stripping the topsoil in the setback areas and any required pipe trench and frost protection excavation (depending on the chosen foundation type) is soil class 2-3 according to DIN 18300.
Deviations in soil class, soil replacement, or foundation types differing from the chosen variant are at the client’s expense.”
In the foundation section it states, among other things:
“Your house will stand on secure foundations. The ground floor slab will be constructed with concrete grade C25/30 (WU concrete) according to the structural design and founded frost-proof with frost skirts made of normal concrete.
The following assumptions underlie the structural design:
Minimum allowable stiffness coefficient 20 MN/m² and soil pressure 200 kN/m²
Maximum groundwater level at least 100 cm (39 inches) below the planned foundation or slab
Use as heated living space with interior temperatures above 10°C (50°F)
Level terrain.
The ground floor slab will have a thickness of approximately 18 cm (7 inches).”
Now to my problem.
Due to the height difference, the groundwork company submitted a separate offer, in addition to breaking up the existing strip foundations, to fill the area: delivering sandy gravel from additional excavation, installing it according to profile, and compacting it in layers.
This also applies to the terrace area, which I had excavated at the same time.
Since I currently live just a few steps from the construction site, I visit several times a day to monitor the progress.
The entire area beneath the house was filled and compacted only with sand. Because it is very dry at the moment, they brought in water after about half of the work to dampen the sand, as it would not compact otherwise.
So the current build-up beneath the house and within 1 meter (3 feet) around the planned house consists entirely of sand: first about 0/32 grain size sand (approximately half), then, because it wouldn’t compact, 0/45 was added on top.
To me, it is just a pile of sand where footprints sink right in when walking over it. There is no frost protection layer, concrete recycling, or any solid material in the lowest 30 cm (12 inches) above the clay soil as specified. The entire build-up where the land was lower, about 95 cm (37 inches), consists of dry sand. Below that is clay soil.
When I expressed my concern to the site foreman and the head of the groundwork company that this is not a load-bearing foundation for a terrace, let alone a house, they dodged the question and referred me to the construction manager. The groundwork head tried to convince me that this was “certified frost protection sand,” which was just as effective as proper frost protection. They had chosen the better 0/45 sand because the 0/32 would not hold.
At another Heinz von Heiden house I saw that reddish frost protection/sand with a higher stone content was compacted across the entire area and full depth, and it was solid and soil-like. My sand is yellow, very fine, and as I said, just a big pile of sand.
When I asked why the reddish frost protection sand was not used for my site, the groundwork manager said they would have had to travel further, and it was a cost issue. Nobody asked me! The sand pit they took the sand from is 3 km (2 miles) away. They don’t have the reddish frost protection sand there but said it could have been delivered and ordered, according to the sand pit employee.
The sand pit employee also confirmed that the sand delivered to me is not suitable as foundation material.
In the construction contract, I read that only the 1-meter (3 feet) extension of the foundation cushion—the sand layer—is specified. Otherwise, I find no information about the foundation material. But there must be DIN standards specifying what kind of foundation this type of house must rest on.
The next problem I see is with the installation of the continuous frost skirt. According to the groundwork company, this skirt only goes about 60 cm (24 inches) deep; it is a special formwork or something similar from Heinz von Heiden.
On the right side of the plot, there is load-bearing clay soil at 40 cm (16 inches) depth.
On the left side under the house, as mentioned, the 95 cm (37 inches) of built-up sand means the frost skirt, provided the loose soil does not fall back in during excavation, would be constructed in the fill sand! This can’t be right!
Today at noon, a tester commissioned by the groundwork company was there to check compaction. Everything was, of course, compacted and considered stable!
I raised my concerns about the frost skirt sitting in the sand. He said the frost skirt depth should be between 80 and 100 cm (31 to 39 inches), depending on the elevation. Our elevation is 215. How does Heinz von Heiden justify only going 60 cm (24 inches) deep?
Thank you in advance for your opinions.
By the way, next Wednesday the shell construction company will arrive to build the frost skirt and the base slab within two days.
I urgently need your advice.
I am building a bungalow without a basement with Heinz von Heiden (GÜ).
Until today, the groundwork company was on site to prepare the foundation base layer. The plot has a slope of 90 cm (35 inches) along the 22-meter (72 feet) length facing the street. Within the actual construction area, this height difference is smaller (30 cm (12 inches)) because there was already a structure there that was covered with soil, and the foundation strips were removed by the groundwork company to a depth of about 50–60 cm (20–24 inches). The soil surveyor did not notice this and considered everything fine. I found out about it beforehand from neighbors. Towards the back, the land slopes down about 90 cm (35 inches) again.
According to the construction contract, Heinz von Heiden is responsible for a foundation cushion of 30 cm (12 inches), including excavation, plus an additional 1 meter (3 feet) around the house.
The contract states regarding earthworks, among other things:
“The existing topsoil will be removed to a depth of up to 30 cm (12 inches) within the house area and an additional 1 meter (3 feet) around it and stored laterally on the site. Subsequently, a compacted foundation cushion up to 30 cm (12 inches) thick will be installed. The foundation cushion (sand layer) will extend 1 meter (3 feet) beyond the outer edge of the base slab, corresponding to the topsoil removal area...
If local additional thicknesses result from building up greater heights or extra excavation down to a load-bearing substrate, these will be invoiced separately.
Additional thicknesses of the foundation cushion can only be determined after reviewing the soil report, establishing the house elevation, and considering local conditions.
The calculation basis for stripping the topsoil in the setback areas and any required pipe trench and frost protection excavation (depending on the chosen foundation type) is soil class 2-3 according to DIN 18300.
Deviations in soil class, soil replacement, or foundation types differing from the chosen variant are at the client’s expense.”
In the foundation section it states, among other things:
“Your house will stand on secure foundations. The ground floor slab will be constructed with concrete grade C25/30 (WU concrete) according to the structural design and founded frost-proof with frost skirts made of normal concrete.
The following assumptions underlie the structural design:
Minimum allowable stiffness coefficient 20 MN/m² and soil pressure 200 kN/m²
Maximum groundwater level at least 100 cm (39 inches) below the planned foundation or slab
Use as heated living space with interior temperatures above 10°C (50°F)
Level terrain.
The ground floor slab will have a thickness of approximately 18 cm (7 inches).”
Now to my problem.
Due to the height difference, the groundwork company submitted a separate offer, in addition to breaking up the existing strip foundations, to fill the area: delivering sandy gravel from additional excavation, installing it according to profile, and compacting it in layers.
This also applies to the terrace area, which I had excavated at the same time.
Since I currently live just a few steps from the construction site, I visit several times a day to monitor the progress.
The entire area beneath the house was filled and compacted only with sand. Because it is very dry at the moment, they brought in water after about half of the work to dampen the sand, as it would not compact otherwise.
So the current build-up beneath the house and within 1 meter (3 feet) around the planned house consists entirely of sand: first about 0/32 grain size sand (approximately half), then, because it wouldn’t compact, 0/45 was added on top.
To me, it is just a pile of sand where footprints sink right in when walking over it. There is no frost protection layer, concrete recycling, or any solid material in the lowest 30 cm (12 inches) above the clay soil as specified. The entire build-up where the land was lower, about 95 cm (37 inches), consists of dry sand. Below that is clay soil.
When I expressed my concern to the site foreman and the head of the groundwork company that this is not a load-bearing foundation for a terrace, let alone a house, they dodged the question and referred me to the construction manager. The groundwork head tried to convince me that this was “certified frost protection sand,” which was just as effective as proper frost protection. They had chosen the better 0/45 sand because the 0/32 would not hold.
At another Heinz von Heiden house I saw that reddish frost protection/sand with a higher stone content was compacted across the entire area and full depth, and it was solid and soil-like. My sand is yellow, very fine, and as I said, just a big pile of sand.
When I asked why the reddish frost protection sand was not used for my site, the groundwork manager said they would have had to travel further, and it was a cost issue. Nobody asked me! The sand pit they took the sand from is 3 km (2 miles) away. They don’t have the reddish frost protection sand there but said it could have been delivered and ordered, according to the sand pit employee.
The sand pit employee also confirmed that the sand delivered to me is not suitable as foundation material.
In the construction contract, I read that only the 1-meter (3 feet) extension of the foundation cushion—the sand layer—is specified. Otherwise, I find no information about the foundation material. But there must be DIN standards specifying what kind of foundation this type of house must rest on.
The next problem I see is with the installation of the continuous frost skirt. According to the groundwork company, this skirt only goes about 60 cm (24 inches) deep; it is a special formwork or something similar from Heinz von Heiden.
On the right side of the plot, there is load-bearing clay soil at 40 cm (16 inches) depth.
On the left side under the house, as mentioned, the 95 cm (37 inches) of built-up sand means the frost skirt, provided the loose soil does not fall back in during excavation, would be constructed in the fill sand! This can’t be right!
Today at noon, a tester commissioned by the groundwork company was there to check compaction. Everything was, of course, compacted and considered stable!
I raised my concerns about the frost skirt sitting in the sand. He said the frost skirt depth should be between 80 and 100 cm (31 to 39 inches), depending on the elevation. Our elevation is 215. How does Heinz von Heiden justify only going 60 cm (24 inches) deep?
Thank you in advance for your opinions.
By the way, next Wednesday the shell construction company will arrive to build the frost skirt and the base slab within two days.
F
fach1werk7 Jun 2020 09:30Yes, exactly, K1300S.
Good that you mentioned it, Andre! You and hopefully also Immermehr can certainly contribute valuable input.
Best regards
Gabriele
Good that you mentioned it, Andre! You and hopefully also Immermehr can certainly contribute valuable input.
Best regards
Gabriele
Hello,
I saw in your pictures that you have the same civil engineer as we do. We are also building with Heinz von Heiden—a bungalow as well.
The ground was all natural clay soil, class 4. However, a full 70cm (28 inches) deep bedding layer with geotextile was installed in our case, so that all areas have the same strength. I found the civil engineers competent, and Mr. N. was also experienced with several years in the business. Frost protection gravel (yellow) was installed as well. At first, it looks odd, but after compaction it is definitely load-bearing.
Our foundation bedding became fully saturated due to about 120 liters (32 gallons) of rain per square meter in January 2020. After about 2 months of pumping to drain it, we were able to demonstrate load capacity in several bearing plate tests. After that, the ground slab was poured.
I saw in your pictures that you have the same civil engineer as we do. We are also building with Heinz von Heiden—a bungalow as well.
The ground was all natural clay soil, class 4. However, a full 70cm (28 inches) deep bedding layer with geotextile was installed in our case, so that all areas have the same strength. I found the civil engineers competent, and Mr. N. was also experienced with several years in the business. Frost protection gravel (yellow) was installed as well. At first, it looks odd, but after compaction it is definitely load-bearing.
Our foundation bedding became fully saturated due to about 120 liters (32 gallons) of rain per square meter in January 2020. After about 2 months of pumping to drain it, we were able to demonstrate load capacity in several bearing plate tests. After that, the ground slab was poured.
Vicky Pedia7 Jun 2020 16:54fach1werk schrieb:
Vicky Pedia, come on, please pull yourself together, it feels like the situation we had and that was really difficult for us. Maybe there is still a way to adjust things so it works better for these clients.Well, here I am!Vicky Pedia7 Jun 2020 17:05Pet1234 schrieb:
By chance, it was discovered that where the foundation cushion is only 45 cm (18 inches) thick and thus at its minimum thickness, the clay soil beneath the cushion over an area of about 5x5 meters (16x16 feet) is no longer load-bearing and has become saturated. The soil now needs to be replaced down to a depth of 80 cm (31 inches). Replacing or improving the soil (with lime) is the right approach given the shallow layer thickness. However, this doesn’t mean the soil is completely “no longer load-bearing.” Clay is always a critical soil type.
I will bite the bullet and hire an independent geotechnical expert to perform dynamic probing tests. Let’s see what the results show.
[/QUOTE]
That’s not a bad idea! Three to four probe tests should be enough. You absolutely need to have the results interpreted by a geologist, as the raw data won’t make much sense to you. The tests will indicate how many hammer blows with a standardized weight are required to reach a defined depth. Generally, the higher the number of blows, the better the soil conditions. Whether “better” is actually good enough must be assessed by the geologist.