ᐅ Garage on a slope / basement, L-shaped retaining walls, sloping backfill?
Created on: 17 Jul 2014 19:43
T
tabaluga
Hello and thank you for reading. I’ll try to describe our problem/challenge as precisely as possible. Sorry if it gets 2-3 sentences longer, and thanks again for taking the time:
We found a really beautiful sloped plot for our house and are already deep into the planning phase with a structural engineer. The house design is almost finished, with only a few minor details giving us trouble—if it weren’t for the garage/carport issue.
The land drops about 2.50 meters (8 feet) down from the street and then runs quite flat. Because of this, we have to build with a basement (walkout basement). From the street side, the ground floor is level, but from the garden side, it’s on the first floor, and the basement is at ground level. So far, everything fits. However, we still need to plan the garage or carport. It should be located between our house and the neighbor’s garage, who built right on the property line (building encumbrance exists). Legally, everything is settled—we’re allowed to build right up to the boundary, and the neighbor agrees to extend the encumbrance for us to build slightly offset.
The challenge is that we somehow need to position our garage “on the first floor,” and the solution shouldn’t cost an extra €20,000 or so (basement construction). Since we have enough space inside the house, we don’t need extra rooms, not even for garden tools or similar. That’s why we’d ideally like to “simply raise the ground level” and then place the carport on top.
Our builder initially suggested attaching a carport to the house with a shared roof (the house’s structure allows this) and supporting the outer side on isolated footings/piers down to the natural ground. In between, a simple fill of soil would be added, stabilized toward the garden and the neighbor with a retaining wall (such as gabions).
We also considered using L-shaped concrete blocks (L-blocks) to retain the soil on the neighbor’s garage side (since it has a basement, we’d need to hold back the soil so the neighbor’s garage isn’t damaged), and also on the garden side.
However, our builder has now withdrawn his suggestion (gabions and isolated footings/piers) because his structural engineer says it’s uneconomical—you would apparently need to compact the fill layer by layer if cars are parked on the carport. In the end, it would cost about as much as building a basement. The engineer also rejects the L-block idea because that would require a very heavy foundation.
We have found this structural engineer quite difficult to work with and can’t really imagine it’s that complicated to build. Maybe a professional or someone with a similar problem is reading and could offer a tip?
If our excavation company removes the soil anyway (we only have about 60-100cm (2-3 feet) depth) and prepares a base on which our basement and house (weighing about 100-120 tons on 80 m² (860 sq ft)) can stand, then it shouldn’t be a problem for them to also prepare another 36 m² (6x6 m/20x20 ft) area nearby so that the L-blocks can be placed and supported there. We would use blocks with a height of 300 cm (10 feet), extending 6 meters (20 feet) to the south (nothing behind there because the garden begins) and 6 meters (20 feet) to the east (toward the neighbor’s garage for retaining). The blocks weigh about 2 tons (2.2 U.S. tons) per meter. If we then attach a carport to the house and mount it directly on the L-blocks on the other side, it should be rock solid. In the garage, there could be about 6 tons (6.6 U.S. tons) of cars at worst (two large SUVs; currently we drive a smaller car weighing around 1.5 tons (1.65 U.S. tons)).
Can heavy L-blocks and common soil fill with paving on top actually support this weight without compacting and collapsing if left un-compacted?
Wouldn’t a concrete slab on isolated footings down to natural ground be significantly cheaper than building an entire basement room (6x6 m/20x20 ft), especially since that basement would also require a finished floor?
Are we overcomplicating things, or is the structural engineer maybe just being overly cautious?
I’m grateful for any advice, even links. This garage issue is stressing us out quite a bit at the moment, and we can’t believe we’re the first owners of a sloped plot who want to build a garage without a basement.
Best regards and many thanks in advance from the little dragon, tabaluga
We found a really beautiful sloped plot for our house and are already deep into the planning phase with a structural engineer. The house design is almost finished, with only a few minor details giving us trouble—if it weren’t for the garage/carport issue.
The land drops about 2.50 meters (8 feet) down from the street and then runs quite flat. Because of this, we have to build with a basement (walkout basement). From the street side, the ground floor is level, but from the garden side, it’s on the first floor, and the basement is at ground level. So far, everything fits. However, we still need to plan the garage or carport. It should be located between our house and the neighbor’s garage, who built right on the property line (building encumbrance exists). Legally, everything is settled—we’re allowed to build right up to the boundary, and the neighbor agrees to extend the encumbrance for us to build slightly offset.
The challenge is that we somehow need to position our garage “on the first floor,” and the solution shouldn’t cost an extra €20,000 or so (basement construction). Since we have enough space inside the house, we don’t need extra rooms, not even for garden tools or similar. That’s why we’d ideally like to “simply raise the ground level” and then place the carport on top.
Our builder initially suggested attaching a carport to the house with a shared roof (the house’s structure allows this) and supporting the outer side on isolated footings/piers down to the natural ground. In between, a simple fill of soil would be added, stabilized toward the garden and the neighbor with a retaining wall (such as gabions).
We also considered using L-shaped concrete blocks (L-blocks) to retain the soil on the neighbor’s garage side (since it has a basement, we’d need to hold back the soil so the neighbor’s garage isn’t damaged), and also on the garden side.
However, our builder has now withdrawn his suggestion (gabions and isolated footings/piers) because his structural engineer says it’s uneconomical—you would apparently need to compact the fill layer by layer if cars are parked on the carport. In the end, it would cost about as much as building a basement. The engineer also rejects the L-block idea because that would require a very heavy foundation.
We have found this structural engineer quite difficult to work with and can’t really imagine it’s that complicated to build. Maybe a professional or someone with a similar problem is reading and could offer a tip?
If our excavation company removes the soil anyway (we only have about 60-100cm (2-3 feet) depth) and prepares a base on which our basement and house (weighing about 100-120 tons on 80 m² (860 sq ft)) can stand, then it shouldn’t be a problem for them to also prepare another 36 m² (6x6 m/20x20 ft) area nearby so that the L-blocks can be placed and supported there. We would use blocks with a height of 300 cm (10 feet), extending 6 meters (20 feet) to the south (nothing behind there because the garden begins) and 6 meters (20 feet) to the east (toward the neighbor’s garage for retaining). The blocks weigh about 2 tons (2.2 U.S. tons) per meter. If we then attach a carport to the house and mount it directly on the L-blocks on the other side, it should be rock solid. In the garage, there could be about 6 tons (6.6 U.S. tons) of cars at worst (two large SUVs; currently we drive a smaller car weighing around 1.5 tons (1.65 U.S. tons)).
Can heavy L-blocks and common soil fill with paving on top actually support this weight without compacting and collapsing if left un-compacted?
Wouldn’t a concrete slab on isolated footings down to natural ground be significantly cheaper than building an entire basement room (6x6 m/20x20 ft), especially since that basement would also require a finished floor?
Are we overcomplicating things, or is the structural engineer maybe just being overly cautious?
I’m grateful for any advice, even links. This garage issue is stressing us out quite a bit at the moment, and we can’t believe we’re the first owners of a sloped plot who want to build a garage without a basement.
Best regards and many thanks in advance from the little dragon, tabaluga
I’m not a structural engineer, but just the thought of building a garage on L-shaped concrete blocks makes my toes curl.
I mean—even if you find an engineer willing to approve that—just go ahead. And in about three years, report back to us about the settlement cracks, because your unconnected L-shaped blocks will each twist individually, and your garage will follow suit. A textbook case of structural damage—and then the engineer is supposed to be the fool in court.
Enough complaining. In my humble opinion as a non-engineer familiar with construction, there are two options:
1. Build a basement. The argument “enough space” doesn’t really hold, since you can redesign for that, and the costs for the basement will obviously be similar to a proper foundation. So why put in all the effort for a ground-level foundation when a foundation including a basement will cost about the same!?
2. Extend the ground floor slab of the main house through the garage, that is, create the garage slab like a cantilevered balcony capable of supporting two Q7 vehicles plus the garage. This comes with two issues: a) thermal bridging/cold spots, meaning likely increased insulation requirements, and b) probably a lot of reinforcement steel needed. Both will increase costs.
Long story short… I would extend the compacted base of the basement all the way to the property boundary and build a garden storage room with a door to the garden beneath the garage. That way, you have ONE continuous foundation under the house, ONE continuous slab, and most likely the lowest cost with the highest benefit and least hassle.
And honestly—listen to your engineer, who signs off on this and would have liability insurance for their work if anything goes wrong.
My 2 cents.
Dirk Grafe
I mean—even if you find an engineer willing to approve that—just go ahead. And in about three years, report back to us about the settlement cracks, because your unconnected L-shaped blocks will each twist individually, and your garage will follow suit. A textbook case of structural damage—and then the engineer is supposed to be the fool in court.
Enough complaining. In my humble opinion as a non-engineer familiar with construction, there are two options:
1. Build a basement. The argument “enough space” doesn’t really hold, since you can redesign for that, and the costs for the basement will obviously be similar to a proper foundation. So why put in all the effort for a ground-level foundation when a foundation including a basement will cost about the same!?
2. Extend the ground floor slab of the main house through the garage, that is, create the garage slab like a cantilevered balcony capable of supporting two Q7 vehicles plus the garage. This comes with two issues: a) thermal bridging/cold spots, meaning likely increased insulation requirements, and b) probably a lot of reinforcement steel needed. Both will increase costs.
Long story short… I would extend the compacted base of the basement all the way to the property boundary and build a garden storage room with a door to the garden beneath the garage. That way, you have ONE continuous foundation under the house, ONE continuous slab, and most likely the lowest cost with the highest benefit and least hassle.
And honestly—listen to your engineer, who signs off on this and would have liability insurance for their work if anything goes wrong.
My 2 cents.
Dirk Grafe
Hello Dirk
and thank you very much for your message. As a layperson, I still don’t understand why L-shaped blocks would bend. The blocks have a kind of tongue-and-groove joint that interlocks, are frost-resistant (C75 concrete), and can withstand loads of up to 12 tons (axle load) even at the edges. What could possibly "twist" there?
"We don’t need the space" is honestly meant here. We have already planned a similar space for garden tools directly in the house/the basement, and I really can’t imagine what we would do with 50sqm (14sqm inside the house + 36sqm under the garage) of additional space for even more garden equipment. Our house already has more than 25sqm of storage areas aside from the regular living spaces (excluding the garage basement). There is also a utility room, so there is really plenty of space. This was all planned deliberately. We absolutely don’t need another room.
Extending the floor slab is exactly the problem you mentioned. I believe it is called differently, but if the slab is extended (or the basement simply made larger), an additional 36sqm must be insulated to the same standard (KFW70). That is quite extreme in terms of cost—and for a room that nobody needs. If the slab is just left "as is," the different dew points probably destroy the foundation slab over time. So, no gain there. :-(
Still, many thanks from
Tabaluga
and thank you very much for your message. As a layperson, I still don’t understand why L-shaped blocks would bend. The blocks have a kind of tongue-and-groove joint that interlocks, are frost-resistant (C75 concrete), and can withstand loads of up to 12 tons (axle load) even at the edges. What could possibly "twist" there?
"We don’t need the space" is honestly meant here. We have already planned a similar space for garden tools directly in the house/the basement, and I really can’t imagine what we would do with 50sqm (14sqm inside the house + 36sqm under the garage) of additional space for even more garden equipment. Our house already has more than 25sqm of storage areas aside from the regular living spaces (excluding the garage basement). There is also a utility room, so there is really plenty of space. This was all planned deliberately. We absolutely don’t need another room.
Extending the floor slab is exactly the problem you mentioned. I believe it is called differently, but if the slab is extended (or the basement simply made larger), an additional 36sqm must be insulated to the same standard (KFW70). That is quite extreme in terms of cost—and for a room that nobody needs. If the slab is just left "as is," the different dew points probably destroy the foundation slab over time. So, no gain there. :-(
Still, many thanks from
Tabaluga
J
JDoerbecker18 Jul 2014 10:17Hello,
We are currently solving this the way shown in the photo. The blocks are placed on a concrete strip and will then be filled with concrete. Inside, gravel is added and compacted. We will then pave over it and build a carport on top.
Maybe this could be a solution for you as well?
Best regards
Joe
We are currently solving this the way shown in the photo. The blocks are placed on a concrete strip and will then be filled with concrete. Inside, gravel is added and compacted. We will then pave over it and build a carport on top.
Maybe this could be a solution for you as well?
Best regards
Joe
B
Bauexperte18 Jul 2014 10:57Hello,
Best regards, Bauexperte
tabaluga schrieb:At what height did the neighbor build on the boundary? Is it at the original ground level or different?
The structure is planned to be built between our house and the neighbor's garage, which is directly on the property line (building encumbrance exists). Legally, everything is settled; we are also allowed to build up to the boundary, and the neighbor has agreed to an additional encumbrance so that we can build slightly offset.
Best regards, Bauexperte
tabaluga schrieb:
Hello Dirk
and thank you very much for your message. Why L-shaped blocks should bend is not clear to me (as a layman); the blocks have a type of tongue-and-groove joint that interlocks, are also frost-resistant (C75 concrete), and can withstand loads of up to 12 tons ("axle load") even at the edge. What could still "twist" there?The parts are not permanently connected, so the frost resistance doesn’t help you, because water will still work its way between the parts. If I still have photos, I’ll upload some to show what it looks like in case of damage – then you’ll be convinced, I promise.
Best regards
Dirk Grafe
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