Does the bottom of a drainage inspection shaft need to be closed?

Hello everyone,

I am currently considering whether it would make sense to omit the bottom of a drainage inspection chamber and instead dig about 30-40 cm deeper to see what the water level in that area actually is. Can this be generally stated, or would the closed bottom (which might also be impermeable) be necessary?

Since I am already on the topic of drainage: Our contractor has laid the drain pipes for the toilet (KG2000 - green) directly into the drainage filter layer (gravel 5/11). Is there no risk of perforation without a sand bed? Although the sand bed in the filter layer would certainly be washed away.
At another location, the rainwater pipes (KG2000 - green) also lie without a sand bed on the drainage filter layer. However, gravel 16/22 was used as the filter layer there.

Best regards

For your information: I have decided on an inspection chamber with a floor. The issue with the rainwater pipes in gravel, or above the drainage filter (16/22 rounded grain), is still on my mind.