angle of repose
Technical term in civil engineering and geology for the steepest angle at which a pile of granular material remains stable.
The angle of repose of sand is about 35 degrees.
If the angle of repose is exceeded, the slope will collapse.
Compound of 安息 (ansoku, 'rest, repose') and 角 (kaku, 'angle'), likely a calque of English 'angle of repose'.