ᐅ Is the proposed dimension for infiltration justified?

Hello!

We are currently in the final stages of our planning. A soil survey has already been completed.
Now our architect needs the dimensions for an infiltration system (most likely as a chamber).
We requested these dimensions from our geotechnical engineer, who is now asking for almost twice as much money for the sizing as for the entire soil survey (more than 800€).

Is this price justified? It seems very, very high to me.
From my research, a calculation according to "DWA A 138" is quite straightforward if you have the necessary data and formulas (there are, for example, Excel templates or online tools available). This should not take a skilled engineer more than 1 to 2 hours, or are we missing something?
Unfortunately, the quote only mentions a flat fee and provides no further details.

Thank you very much!

I can’t say whether the offer is reasonable or not. I would first take a deep breath and then ask myself if there might be a cheaper option.

Have you ever considered a cistern?

Then you could invest the money required for the soil surveyor and the shaft into the cistern instead.

You can certainly build this yourself as well. I did this a few years ago. This (4 m³ (140 ft³) plastic cistern) cost about 1200–1500 euros, including new piping, drainage channels, and so on. Unfortunately, I can’t give an exact figure now.

Since I couldn’t install an underground infiltration system (too many roots), an above-ground infiltration system was used instead. For this, self-slit pipes were placed sideways under plants, more or less well hidden. Excess water is then pumped into these pipes with a pump, distributing it throughout the garden.

In summer, you also have water available to irrigate the garden. Unfortunately, it’s never quite enough.

That really depends on the execution, in this case the depth.

Basically, amounts under €1000 are peanuts in construction. If your expert report is below that, consider yourself lucky.

PS: forget about a cistern; it’s not worth it.

Thanks for the responses! We definitely had to swallow that.

Of course, $1,000 is not much in construction, but just because everything is on a larger scale doesn’t mean you should waste money unnecessarily.

We will probably use a cistern with built-in infiltration (the tank is basically open on the side at the top). However, we need to know how large the infiltration area must be to prevent the tank from overflowing during heavy rain. We are not allowed to discharge rainwater into the sewer system.

Does anyone happen to know a good geologist who could calculate this for us? So we can also get a second opinion.

_pexed_ schrieb:

We will probably use a cistern with an integrated infiltration system (the tank is basically open on the side at the top). However, we need to know how large the infiltration area must be to prevent the tank from overflowing during heavy rain.
We are not allowed to discharge the rainwater into the sewer system.

Calculating the roof area... isn’t that usually done by the general contractor or the site engineer? It’s not complicated, can be done in 5 minutes, and is similar for almost every standard house... so no need to reinvent the wheel.

_pexed_ schrieb:

Does anyone perhaps know a good geologist who could do this kind of calculation for us? That way, we can get a second quote as well.

Isn’t the geologist directory no longer available where geologists register by postal code and can be found?

You could directly contact a company like Graf, for example. I bought the material from them, and they calculated the size of the soakaway for free. The data they needed were already included in the soil report and the drainage plan from the architect.