Static liquefaction failure of sloping grounds has resulted in significant damages to built structures and even loss of lives. The principal aim of this research is to relate static liquefaction behavior of cohesionless soils to a measurable threshold from the field. Based on a very large number (893) of undrained laboratory shear tests on cohesionless soils collected from the past literature, a threshold triggering excess pore water pressure is introduced in this study above which static liquefaction failure occurs. The effect of variations in the direction and relative magnitudes of principal stresses associated with different modes of shear and ground slopes on static liquefaction failure of cohesionless soils is characterized by empirical relationships of the triggering excess pore water pressure ratio with these variables. The triggering pore pressure ratio can be employed as a more precise criterion for detecting liquefaction triggering and landslide warning in instrumented slopes of saturated cohesionless soils.