U-groove
A joint profile with a rounded bottom, typically machined into very thick components. It requires less filler metal and creates less distortion than a standard V-groove.
The U-groove has a round root and is narrower than a V. This makes the arc melt the root cleanly, you need less filler material, and heat input stays lower. The round shape distributes stresses better than a sharp notch. If the groove gets too narrow, you quickly get lack of fusion at the flanks because the weld pool doesn't reach properly. In the narrow groove the pool cools faster, which increases the risk of cracking in mild steel. It must not be too wide either, otherwise you put in unnecessary heat and get coarse grain in the HAZ. For thick plate, you mill the U-groove with a round root and keep the flanks steep. Set the root face to zero, the flanks must be clean, oil and rust removed. When welding the root, oscillate only a little to ensure both flanks fuse properly. For very thick material, you must preheat, and mechanized welding is the better choice for consistent quality.