Tie-in
Tie-in turns up in 2 fields: welding, geosynthetics. Here is what it means to the people in each one.
Tie-in in welding
How well a weld fuses into the base metal, the bead beside it, or the end of an earlier weld.
Welder's term for proper fusion at bead edges and at starts and stops. Good tie-in means the toes blend smoothly into the base metal or adjacent bead with no undercut or overlap, and a restart melts fully into the crater of the previous rod. On pipe it also names the spot where two halves of a root meet, which is ground and feathered so the bead fuses through.
"Your tie-in at the top is cold, grind the start back to a feather and make it again."
Tie-in in geosynthetics
Connection that joins a new liner to an existing one, as when a landfill cell is expanded.
The existing liner is dug out, cleaned and inspected, then welded to the new sheet, usually by extrusion because exposed sheet is oxidized and often dirty or textured. Tie-ins are often the hardest seams on a job and get extra testing and destructive samples. Old sheet that has aged badly or stress cracked can make a sound tie-in impossible without replacing a strip of it.
"Tie-in to the old cell is all extrusion, so grind back to fresh black before you lay a bead."