There are many different kinds of underground pipes and each has their own way of joining to the next piece. Many will have a 'neck' that fits into a larger ring on the next piece, but there are other methods too.
The same that is used above ground. Sweat fitting coupler. I would not use copper underground in an area that has winter.
Coated Copper Pipe that is approved for underground only.
Type L or Type K copper pipe is typically used for underground applications. These types of copper pipe have thicker walls than Type M copper pipe, making them more suitable for underground installation where they may be exposed to greater pressure or potential damage. Additionally, this thicker wall helps to prevent corrosion and allows for increased durability underground.
I am a Plumbing/HVAC Contractor and the type of fitting I always used for underground copper service pipe has been 3/4" (standard size for residential) brass flared fittings. A flared connection is a tight, secure connection guaranteed not to leak.
Underground and used for storm/ sanitary disposal
IT IS THE MOST USED ON UNDERGROUNG PIPE WORK
Flex pipe should not have been used in your pool construction underground. It may be against code in most cities. The use of rigid shcedule 40 or sch 80 PVC is standard.
Underground I use K copper exclusively..
Pipe sealant that is often used as a boiler repair by Hercules chemical corp
Teflon tape can be used on any type of liquid joint. However, it is strictly speaking a lubricant, NOT a sealant, and is a cheap and nasty way of sealing joints. - The proper sealant for any liquid pipe joint is a pipe joint compound such as Whitlam's or Oatey's Blue Joint Compound.
galv metal scotch coat taped joints 12" DEEP .......Poly Pipe 18" DEEP
Flush Joint Wash Pipe - used in the oilfield on thin-wall 'Wash Pipe' used in 'fishing' operations - retrieval of broken tools from an well.