ᐅ Load-Bearing Capacity of L-Blocks and Water Drainage on a 45° Slope

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...

There seem to be problems inevitably arising as soon as the first rain comes...

Is it possible to find an amicable solution with the neighbor, especially since they apparently have expertise in this area...?

Since the neighbor doesn’t actually live there, we only know him as a polite neighbor from phone calls. We were quite surprised by his response to our building application (the kind lady from the office read his entire statement to me—very detailed and intense objections, as if he’s done this before 😉).

We’re planning to call him, not to confront him, but to ask what kinds of services he can provide as a construction company—maybe a wall 😀.

Do you think the water can be stopped here?

Nixwill2 schrieb:

Do the few plants on the slope provide enough "braking" to prevent water from overflowing or to significantly reduce its flow speed?

No. You might, however, consider starting with rice cultivation.

This is really tricky. So what exactly is the starting situation—natural ground slope?

I’m not exactly sure what you mean by natural ground slope, but I’ll take a guess:

The plot extends about 20m (65 feet) wide, from north to south, roughly descending from 306m (1,004 feet) to 300m (984 feet). So, I would say it’s a clearly sloping site.

Just to clarify, the single-family house sits at 305m (1,001 feet). We want to place a terrace here and then have the entire area follow a level line at 305m (1,001 feet), meaning the terrace, the directly adjoining lawn, and a carport and driveway all at the same elevation. Basically, a plateau at 305m (1,001 feet), and the slope should run along the entire southern side, blending into the lawn at the top and meeting a retaining wall at the bottom.

So your house will be perched on a 5 m (16 ft) high artificial hill? 😳
Nothing will grow on the slope, especially if it faces south. Stabilizing it is not a simple task either. I would suggest consulting a landscape gardener beforehand instead of hoping for a quick fix on a tight budget after everything is built. For example, I would also consider child safety. A 45-degree slope and a 2 m (6.5 ft) drop is more like castle construction.
When planning for slope maintenance, it’s also wise to include safety anchors. Try gardening on a 45-degree slope with a 2 m (6.5 ft) steep drop.