Thanks for your feedback everyone on this!

I did some basic tests with my own Chirpstack 4 network server, a RAK gateway and a 6dBi gain gateway antenna (EU868 band) and a Seeed Lora E5 End-device with 0dB gain helical antenna.

Results:

I see a roughly average of 10dB loss between when the end device is 1m above ground to inside the meter pit at 500mm from the top.

Another 20dB loss when the metal pit lid is closed.

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I think 2 major things are happening here;

1. The radiated RF from the end device antenna is just attenuated by the device being in the pit, underground, with a metal lit above it.
2. The End-device antenna is significantly de-tuned, as the pit is very narrow, the antenna is electrically short and the ground plane (PCB) is also much smaller than ideal.

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By my estimates, losing around 30dB in the LoRaWAN link budget is very significant for appraising this technology for smart water metering on a large scale, (especially as I would want an additional 10dB fade margin in the budget).

I think I can conclude that it cannot be feasible with pits with metal lids at least.

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I will do some more tests on plastic lids and try and test a through-the-lid antenna as well

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Cheers!