The Casagrande spoon, also called Casagrande cup, is a measuring instrument used in geotechnics and civil engineering, to determine the liquid limit of a soil sample.[1] It was invented by Arthur Casagrande.

The instrument is composed of a spherical metal cap, fixed at the edge to a device that, through the operation of a crank, produces the elevation of the cap and its subsequent fall, thus producing a controlled impact against a hard rubber base. The soil mixed uniformly with water is placed on the part of the metal cap opposite the fixed point and is shaped with a template that leaves a uniform groove in the center. With each turn of the crank, a blow occurs on the cap, which tends to make the already wet soil slide, bringing the edges of the groove together.

The liquid limit is defined as the water content with respect to the dry weight of the soil, for which it is verified that the furrow made in a sample closes in a section of 12.7 mm after a sequence of 25 blows of the spoon with 1 cm of free fall, applied with a frequency of two blows per second.