Good evening everyone,
I don’t have enough knowledge about foundations and groundworks to properly assess my situation.
The soil report states the following:
- Clay
- Good load-bearing capacity
- Poor permeability
- High frost susceptibility
Now regarding the different foundation options from the soil report:
1. Frost skirt AND bedding
The frost skirt (with slab) extends down to 1 meter (3.3 feet), and the bedding under the slab should be at least 70 cm (28 inches) of frost protection material.
2. Strip footings AND bedding
At least 1.2 meters (4 feet) deep (with slab). Bedding under the slab only 20 cm (8 inches).
My groundworks contractor proposes a third option, which has also been approved structurally:
3. Bedding only
Exclusive use of frost protection material under the slab to achieve a 1 meter (3.3 feet) deep foundation.
According to the structural engineer, a plate load test is required. This is not needed for options 1 and 2. He wants this test if the bedding is deeper than 0.9 meters (3 feet).
Now I’m a bit unsure which option is actually the most sensible.
All approaches seem possible and technically sound.
Could you help me understand the pros and cons of each?
Of course, I’m talking to the tradespeople, but independent opinions from you are usually more honest 🙂
Thanks!
I don’t have enough knowledge about foundations and groundworks to properly assess my situation.
The soil report states the following:
- Clay
- Good load-bearing capacity
- Poor permeability
- High frost susceptibility
Now regarding the different foundation options from the soil report:
1. Frost skirt AND bedding
The frost skirt (with slab) extends down to 1 meter (3.3 feet), and the bedding under the slab should be at least 70 cm (28 inches) of frost protection material.
2. Strip footings AND bedding
At least 1.2 meters (4 feet) deep (with slab). Bedding under the slab only 20 cm (8 inches).
My groundworks contractor proposes a third option, which has also been approved structurally:
3. Bedding only
Exclusive use of frost protection material under the slab to achieve a 1 meter (3.3 feet) deep foundation.
According to the structural engineer, a plate load test is required. This is not needed for options 1 and 2. He wants this test if the bedding is deeper than 0.9 meters (3 feet).
Now I’m a bit unsure which option is actually the most sensible.
All approaches seem possible and technically sound.
Could you help me understand the pros and cons of each?
Of course, I’m talking to the tradespeople, but independent opinions from you are usually more honest 🙂
Thanks!
T
toxickill19 Jun 2022 12:37Hello everyone, I have a related question on this topic, maybe this way a new thread can be avoided:
We also have similar soil according to the report, and the following three options were suggested to us:
1. Excavation of 50cm (20 inches) below foundation level, compact the base and lay geotextile GRK 3, at least 0.5 m (20 inches) of soil cushion (frost-resistant mineral mixtures), then a clean layer. For frost protection, a 1 m (3.3 feet) deep frost barrier.
2. Excavation of 1 m (3.3 feet) below foundation level and at least 1 m (3.3 feet) soil cushion. In this case, the frost barrier can be omitted.
3. Use of strip foundations in clay (about 1.2 m (4 feet) deep) for load transfer, and due to the depth, no frost protection is required.
Now I am wondering which option makes the most sense. The first groundworker we consulted chose the first option in their offer. Reusing the excavated material must be evaluated after excavation begins, since two boreholes show different compositions (one with gravel and sand then clay, the other directly clay after topsoil). According to the soil expert, the clay can still be “upgraded” with lime.
As far as I have read so far, the frost barrier is quite simple and therefore not too expensive: it doesn’t have a load-bearing function but should “only” act as insulation for the area beneath the foundation slab (insulated using XPS). For me, the question is: frost barrier or thicker cushion?
Many thanks in advance for your feedback! If hijacking this thread is not welcome, please let me know 🙂
We also have similar soil according to the report, and the following three options were suggested to us:
1. Excavation of 50cm (20 inches) below foundation level, compact the base and lay geotextile GRK 3, at least 0.5 m (20 inches) of soil cushion (frost-resistant mineral mixtures), then a clean layer. For frost protection, a 1 m (3.3 feet) deep frost barrier.
2. Excavation of 1 m (3.3 feet) below foundation level and at least 1 m (3.3 feet) soil cushion. In this case, the frost barrier can be omitted.
3. Use of strip foundations in clay (about 1.2 m (4 feet) deep) for load transfer, and due to the depth, no frost protection is required.
Now I am wondering which option makes the most sense. The first groundworker we consulted chose the first option in their offer. Reusing the excavated material must be evaluated after excavation begins, since two boreholes show different compositions (one with gravel and sand then clay, the other directly clay after topsoil). According to the soil expert, the clay can still be “upgraded” with lime.
As far as I have read so far, the frost barrier is quite simple and therefore not too expensive: it doesn’t have a load-bearing function but should “only” act as insulation for the area beneath the foundation slab (insulated using XPS). For me, the question is: frost barrier or thicker cushion?
Many thanks in advance for your feedback! If hijacking this thread is not welcome, please let me know 🙂
Hello Toxickill,
Without knowing the exact ground structure, I think option 3 makes the least sense because strip footings are significantly more expensive than frost skirts, and a certain amount of soil replacement is still necessary (removing topsoil and refilling with appropriate material).
For the other two options, it’s unfortunately not possible to generalize, as material and disposal costs vary regionally. It’s important to clarify whether the excavated soil can remain on your property or needs to be removed. The volume of excavation should not be underestimated (50 cm (20 inches) excavation on an area of about 100 m² (1,076 ft²) means 50 m³ (1,765 ft³) of soil, which corresponds to roughly 100 tons and about 4 to 5 truckloads). At 1 m (3 feet) excavation depth, this would double accordingly. A waste analysis is required beforehand, otherwise no landfill (or similar facility) will accept the soil. If the material can remain on-site, the issue effectively resolves itself.
In general, foundations with a frost-protected base are simpler since everything can be done in one work step. With frost skirts, after creating the base layer, a trench must be dug for the skirts using a mini excavator. Also, option 2’s 1.0 m (3 feet) thick base layer provides additional security for the foundation (1.0 m crushed stone > 0.5 m crushed stone).
So I would initially consider:
1. What happens with the excavated soil at 0.5 m (20 inches) and 1.0 m (3 feet) excavation depths
2. What is the cost of the frost protection material (2 tons correspond to about 1 m³ (35 ft³))
3. What are the costs for the frost skirts
Then compare both options accordingly. I will leave out the labor effort for now (option 2 requires more excavation and installation, option 1 involves the frost skirts).
I hope this helps somehow.
Best regards
PS: In my opinion, option 1 does not require frost protection material because of the frost skirts.
Without knowing the exact ground structure, I think option 3 makes the least sense because strip footings are significantly more expensive than frost skirts, and a certain amount of soil replacement is still necessary (removing topsoil and refilling with appropriate material).
For the other two options, it’s unfortunately not possible to generalize, as material and disposal costs vary regionally. It’s important to clarify whether the excavated soil can remain on your property or needs to be removed. The volume of excavation should not be underestimated (50 cm (20 inches) excavation on an area of about 100 m² (1,076 ft²) means 50 m³ (1,765 ft³) of soil, which corresponds to roughly 100 tons and about 4 to 5 truckloads). At 1 m (3 feet) excavation depth, this would double accordingly. A waste analysis is required beforehand, otherwise no landfill (or similar facility) will accept the soil. If the material can remain on-site, the issue effectively resolves itself.
In general, foundations with a frost-protected base are simpler since everything can be done in one work step. With frost skirts, after creating the base layer, a trench must be dug for the skirts using a mini excavator. Also, option 2’s 1.0 m (3 feet) thick base layer provides additional security for the foundation (1.0 m crushed stone > 0.5 m crushed stone).
So I would initially consider:
1. What happens with the excavated soil at 0.5 m (20 inches) and 1.0 m (3 feet) excavation depths
2. What is the cost of the frost protection material (2 tons correspond to about 1 m³ (35 ft³))
3. What are the costs for the frost skirts
Then compare both options accordingly. I will leave out the labor effort for now (option 2 requires more excavation and installation, option 1 involves the frost skirts).
I hope this helps somehow.
Best regards
PS: In my opinion, option 1 does not require frost protection material because of the frost skirts.
Hello,
the answer referred to the question from Sunday, so it wasn’t that long ago 😉
When removing clay soils, you should assume at least 2 t/m³ (2 tons per cubic meter). Of course, this also depends on the moisture content. Dry soils are somewhat lighter, while wet soils are significantly heavier.
For gravel, it naturally depends on the source rock. Basalt is usually around 2 t/m³ (2 tons per cubic meter), while recycled material tends to be closer to 1.5 t/m³ (1.5 tons per cubic meter), though there are variations here as well. I would generally use 2 t/m³ (2 tons per cubic meter) for gravel, as this makes calculations easier and the material needs to be compacted densely during installation.
the answer referred to the question from Sunday, so it wasn’t that long ago 😉
When removing clay soils, you should assume at least 2 t/m³ (2 tons per cubic meter). Of course, this also depends on the moisture content. Dry soils are somewhat lighter, while wet soils are significantly heavier.
For gravel, it naturally depends on the source rock. Basalt is usually around 2 t/m³ (2 tons per cubic meter), while recycled material tends to be closer to 1.5 t/m³ (1.5 tons per cubic meter), though there are variations here as well. I would generally use 2 t/m³ (2 tons per cubic meter) for gravel, as this makes calculations easier and the material needs to be compacted densely during installation.
T
toxickill3 Dec 2022 11:47@Cronos86: First of all, thanks for your response! Excavation work has now started, and the earthworks contractor (who only excavates, compacts, and installs the pipes but does not pour the concrete slab) wants to use a 58cm (23 inches) thick base layer.
Background: Since the insulation and concrete slab require 37cm (15 inches) and the blinding layer is about 5cm (2 inches), this would result in a frost depth of 1 meter (3.3 feet) below the base layer. Around the concrete slab, he plans to backfill with frost protection gravel to a width and depth of 1 meter (3.3 feet), essentially creating a gravel frost apron. In his opinion, this satisfies the structural requirement of a 50cm (20 inches) base layer as well as the 1 meter (3.3 feet) frost protection distance from ground level, and it is also the most cost-effective solution. He could also use a full 1 meter (3.3 feet) thick base layer but advises against it due to the higher costs. The soil is quite clayey and is close to a brook that presumably ran over the property a long time ago.
Do you consider this approach viable? I plan to call the geotechnical expert on Monday to get their opinion as well, but I would appreciate your assessment and any tips!
Background: Since the insulation and concrete slab require 37cm (15 inches) and the blinding layer is about 5cm (2 inches), this would result in a frost depth of 1 meter (3.3 feet) below the base layer. Around the concrete slab, he plans to backfill with frost protection gravel to a width and depth of 1 meter (3.3 feet), essentially creating a gravel frost apron. In his opinion, this satisfies the structural requirement of a 50cm (20 inches) base layer as well as the 1 meter (3.3 feet) frost protection distance from ground level, and it is also the most cost-effective solution. He could also use a full 1 meter (3.3 feet) thick base layer but advises against it due to the higher costs. The soil is quite clayey and is close to a brook that presumably ran over the property a long time ago.
Do you consider this approach viable? I plan to call the geotechnical expert on Monday to get their opinion as well, but I would appreciate your assessment and any tips!
Hello,
Regarding frost protection, your approach sounds good as long as the foundation slab ends up flush with the surrounding ground.
In your first post, you mentioned a static requirement of 70 cm (28 inches). Make sure to review the geotechnical report! In the end, a compaction test must be carried out on the gravel base.
Alluvial clay can be quite problematic, especially with the current weather conditions.
Moisture makes the soil soft and muddy, and frost also has a negative effect.
In general, work needs to proceed quickly. Excavate and immediately lay a layer of gravel. The first layer of about 20 cm (8 inches) should only be compacted statically (pressed down). If vibratory compaction is applied right away, the clay will react and become soft.
The excavation pit should not be driven over, not even with a mini excavator, until the gravel base is completely laid.
Since I’m not familiar with the geotechnical report, it’s best to check the exposed clay foundation layer. If it’s too soft, muddy, or water-saturated, it’s better to bring in a professional and possibly add an extra 10 cm (4 inches) of gravel base underneath.
Good luck!
Regarding frost protection, your approach sounds good as long as the foundation slab ends up flush with the surrounding ground.
In your first post, you mentioned a static requirement of 70 cm (28 inches). Make sure to review the geotechnical report! In the end, a compaction test must be carried out on the gravel base.
Alluvial clay can be quite problematic, especially with the current weather conditions.
Moisture makes the soil soft and muddy, and frost also has a negative effect.
In general, work needs to proceed quickly. Excavate and immediately lay a layer of gravel. The first layer of about 20 cm (8 inches) should only be compacted statically (pressed down). If vibratory compaction is applied right away, the clay will react and become soft.
The excavation pit should not be driven over, not even with a mini excavator, until the gravel base is completely laid.
Since I’m not familiar with the geotechnical report, it’s best to check the exposed clay foundation layer. If it’s too soft, muddy, or water-saturated, it’s better to bring in a professional and possibly add an extra 10 cm (4 inches) of gravel base underneath.
Good luck!
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