ᐅ Load-Bearing Capacity of L-Blocks and Water Drainage on a 45° Slope
Created on: 3 May 2022 15:39
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Nixwill2
Hello everyone,
We are planning to build a retaining wall with 2-meter (6.5-foot) tall L-shaped concrete blocks on the south side of our property. To quickly raise the ground level from 300 meters (984 feet) to 305 meters (1,001 feet) above sea level (the development plan allows a 2-meter (6.5-foot) high retaining wall), we intend to add a slope on top of the wall.
I have attached a simple sketch. It shows a 2-meter (6.5-foot) high L-block at around 300 meters (984 feet) above sea level. To the right is a dashed line representing the house wall, roughly 3.5 meters (11.5 feet) away from the retaining wall. At the top right, the ground floor level of the single-family house is marked at 305 meters (1,001 feet) above sea level. The 45-degree line indicates the slope. I hope this helps to visualize the situation.
Now to my two questions.
We have had a few earthwork contractors inspect the site, and none of them saw any issues. Yesterday, however, I received a preliminary notice from the local building authority that a neighbor downhill has filed an objection. The objection is not against the wall itself but demands that the wall be constructed as a solid concrete structure for static (structural) reasons. Additionally, the neighbor insists that surface water from our property must not flow onto theirs. According to the authority, he cannot enforce the demand for a massive concrete wall, so that request was denied. I fully understand the water issue, which is why it will be part of my second question.
Since this neighbor, who appears to be a builder himself, is already questioning the structural stability, we should expect a thorough review of our wall’s engineering. Therefore, I want to be fully prepared.
As I said, the earthwork contractors see no problem with a 2-meter (6.5-foot) high L-block wall with a 45° slope behind it. What concerns me is that when I searched online, I couldn’t find any L-shaped concrete blocks 2 meters (6.5 feet) high that are certified for a slope steeper than 30 degrees behind them. Is there anyone here who can confirm that such blocks exist? I trust the contractors, but it’s important for me to have some documented proof if needed.
Moving on to question two.
This concerns the water that will certainly run over the remaining part of the wall during heavy rain because of the slope. In my sketch, I marked a dimension ‘X’ at the top end of the L-block. How far down do you think I have to go here to ensure that no water will overflow?
By the way, the scenario in the sketch represents the steepest point. The slope of the property and the wall actually run in the same direction, so in other areas, a much gentler slope will suffice, and I could even create a kind of drainage channel.
The plan is to cover the slope with ground cover plants and hopefully some bushes as well, if they take root. I will create a dedicated thread on this topic in due course.
Do a few plants on the slope provide enough resistance to prevent water from overflowing or at least significantly reduce flow velocity?
We intend to consult a professional landscaper about this issue after the house is built, but for budget reasons, this will have to wait.
I would appreciate any advice or experiences you can share...
We are planning to build a retaining wall with 2-meter (6.5-foot) tall L-shaped concrete blocks on the south side of our property. To quickly raise the ground level from 300 meters (984 feet) to 305 meters (1,001 feet) above sea level (the development plan allows a 2-meter (6.5-foot) high retaining wall), we intend to add a slope on top of the wall.
I have attached a simple sketch. It shows a 2-meter (6.5-foot) high L-block at around 300 meters (984 feet) above sea level. To the right is a dashed line representing the house wall, roughly 3.5 meters (11.5 feet) away from the retaining wall. At the top right, the ground floor level of the single-family house is marked at 305 meters (1,001 feet) above sea level. The 45-degree line indicates the slope. I hope this helps to visualize the situation.
Now to my two questions.
We have had a few earthwork contractors inspect the site, and none of them saw any issues. Yesterday, however, I received a preliminary notice from the local building authority that a neighbor downhill has filed an objection. The objection is not against the wall itself but demands that the wall be constructed as a solid concrete structure for static (structural) reasons. Additionally, the neighbor insists that surface water from our property must not flow onto theirs. According to the authority, he cannot enforce the demand for a massive concrete wall, so that request was denied. I fully understand the water issue, which is why it will be part of my second question.
Since this neighbor, who appears to be a builder himself, is already questioning the structural stability, we should expect a thorough review of our wall’s engineering. Therefore, I want to be fully prepared.
As I said, the earthwork contractors see no problem with a 2-meter (6.5-foot) high L-block wall with a 45° slope behind it. What concerns me is that when I searched online, I couldn’t find any L-shaped concrete blocks 2 meters (6.5 feet) high that are certified for a slope steeper than 30 degrees behind them. Is there anyone here who can confirm that such blocks exist? I trust the contractors, but it’s important for me to have some documented proof if needed.
Moving on to question two.
This concerns the water that will certainly run over the remaining part of the wall during heavy rain because of the slope. In my sketch, I marked a dimension ‘X’ at the top end of the L-block. How far down do you think I have to go here to ensure that no water will overflow?
By the way, the scenario in the sketch represents the steepest point. The slope of the property and the wall actually run in the same direction, so in other areas, a much gentler slope will suffice, and I could even create a kind of drainage channel.
The plan is to cover the slope with ground cover plants and hopefully some bushes as well, if they take root. I will create a dedicated thread on this topic in due course.
Do a few plants on the slope provide enough resistance to prevent water from overflowing or at least significantly reduce flow velocity?
We intend to consult a professional landscaper about this issue after the house is built, but for budget reasons, this will have to wait.
I would appreciate any advice or experiences you can share...
Tolentino schrieb:
Have a maintenance shaft installed somewhere on your main sewer line (costs a few hundred euros, but it's worth it). Then, if you let your cistern fill halfway once or twice a year, you can call that sewer maintenance or pipe flushing, and that’s it. At least that is already planned and included in the building permit application.
@x0rzx0rz
The legal case you described on page 4 partly involved the soakaway pit, which caused all the water to pass by the base of the L-shaped retaining walls.
Let’s assume, somehow, it works with a channel to collect the water flowing above the L-shaped walls from the south and lead it east into the cistern. Also, let’s assume I am allowed to pump the excess water into the sewer system.
Where does the water go that has already seeped through the soil from the upper part of the property (lawn, driveway, etc.) down to the L-shaped walls? This water lies below the channel that fills the cistern. Is this water acceptable or is it also causing a problem?
If that is a problem, then anyone working with L-shaped retaining walls would have the exact same issue…
W
WilderSueden4 May 2022 14:50Nixwill2 schrieb:
Where does the water go that has already seeped through the soil from the top of the property (lawn/grass, driveway, etc.) down to the L-shaped concrete blocks?Usually, a drainage system is installed behind the L-shaped blocks. For cast-in-place concrete, a few drainage pipes are often added to direct the water to the lower area. (Would that be your neighbor’s property in your case?) Alternatively, the water can continue to percolate into the subsoil if the volume is not too high (e.g., during steady rain or a pipe leak).Nixwill2 schrieb:
At least this one is already planned and included in the building permit / planning permission.
@x0rzx0rz
The legal case you described on page 4 partly concerned the infiltration shaft, which caused all the water to pass through at the base of the L-shaped retaining walls.
Let’s assume, somehow, it works with a channel to collect the water flowing above the L-shaped walls from the south side and direct it east into the rainwater cistern. Then let’s also assume I am allowed to pump the excess water into the sewer system.
Where does the water go that has already seeped through the soil from the top of the property (lawn, driveway, etc.) down to the base of the L-shaped walls? This water is then below the channel that fills the cistern. Is this water okay, or could it also cause a problem?
If that is a problem, then everyone working with L-shaped walls would have the exact same issue… Well, the L-shaped walls are not just placed on a slope without support. Especially with 2 meters (6.5 feet), and a width of 1 meter (3.3 feet), each block weighs about 1000 kilograms (2200 pounds). If they are reinforced, they may weigh even more. They must not settle, so they need a proper foundation. The foundation design (how much gravel, concrete, etc. in the different layers) is usually specified by the manufacturer or should be correctly calculated by the structural engineer.
Depending on the foundation design, it ideally also requires drainage (in front of the leg of the L-shaped wall). At least, I would install this as an additional safety measure, because this is the most vulnerable area.
Most likely, you can calculate whether the amount of rainfall plus the permeability of the slope would make drainage unnecessary (if all water is collected at the surface). But for that, you need the report from the soil investigation / hydrological assessment.
And the drainage itself should not significantly increase costs if (connection to the sewer at the lowest point is possible) because it can be planned and installed during the foundation work.
In your case, of course, the problem is that this drainage would be at the lowest point of your property… from where the water would still need to be redirected elsewhere 🙁
What exactly do you mean? We don’t have bricks but reinforced concrete walls with a large foundation.
The walls are sealed so that no water enters public areas.
The water soaks into the ground on the property. We also have an old well/spring on the property.
Eventually, the excess water ends up there as well. If needed, we pump it into the cistern.
The walls are sealed so that no water enters public areas.
The water soaks into the ground on the property. We also have an old well/spring on the property.
Eventually, the excess water ends up there as well. If needed, we pump it into the cistern.
rick2018 schrieb:
What exactly do you mean? We don’t have masonry blocks but reinforced concrete walls with a large foundation.
The walls are waterproofed so that no water enters the public area.
The water infiltrates on the property. We also have an old well/spring on the property.
Eventually, the excess water ends up there. If needed, we pump it into the cistern. Ah, I misunderstood you. I thought you had also installed L-shaped concrete blocks on the site.
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