I would suggest to perhaps rename your forum post to something like “Transmitting LoRaWAN signal from Manhole Cover” or “How to Transmit Signal from Manhole” as it seems to me that the not only do you have to find a durable antenna, you also will have to work out how to get a reliable connection. This is because the elevation is really low and you have a huge metal plate, being the cover right where the antenna is. As metal is a conductor of electricity and magnetism it absorbs electromagnetic waves (radio waves), reducing the strength of the signal and as a result decreasing the distance of a connection.
Before you are to invest resources and purchase antennas, I would first check to make sure that the location where you want a connection to reach will do so by simply placing a node with a normal antenna on the ground near the intended deployment location. Also, if you can source a cover similar to the one where mounting is intended, perhaps try testing a node on one.
If you can get a signal which is strong enough, I would make the assumption that this your best case scenario for a ground mount and begin looking at antennas. If you are unable to get a signal, I would start looking at potential mounting points. As a manhole cover is generally on a road or footpath, a light pole could be nearby where an antenna could be mounted. Perhaps some waterproofed wiring feeding up to the antenna might work in this case.
If the signal is strong enough, there are two antennas I would begin looking at:
Flexible PCB Antenna
These are generally about as thick as a few sheets of paper. My gut feeling is that even though you could place this flat on the ground, I don’t think you would get a connection due to the elevation on top of the fact that the cover is made of metal, unless you could place the antenna on the road or path surface.
Flexible Whip Antenna
This is probably your best bet in my opinion. These have a solid metal base, which would have to be protected somehow, but the top is made of a flexible rubber which surrounds the metal inside that can bend.
I would also consider where this antenna is going to be mounted. If it is on a road, is it in an intersection where would it likely be driven over or is it in the middle of a lane where the only time it would be driven over would in an overtaking situation? Better yet, is the mounting location in the middle of a one way road where it would rarely, if at all, be driven over? The less time it will be driven over the better and would mean that you have some chance of it working for an extended period of time.
Do note that I am still trying to understand about antenna theory and my logic is based on common sense rather than hard facts or experience.