What is the function of the thumb penetration test?

The thumb penetration test is primarily used to estimate the unconfined compressive strength of cohesive soils. When performing this test, an operator uses their thumb to apply pressure to a soil sample. The depth to which the thumb can penetrate the soil indicates how strong or weak the soil is. A deeper penetration suggests a lower strength, while less penetration indicates higher strength.

This method is valuable in field conditions, as it provides a quick and easy way to assess the strength of the soil without requiring sophisticated equipment. It plays a significant role in earthwork and excavation projects by helping operators understand the stability and load-bearing capacity of the soil they are working with.

Other options, while relevant to soil analysis, do not pertain specifically to the primary function of the thumb penetration test. For instance, determining soil moisture level and assessing soil temperature involve different testing methodologies, while soil texture classification typically requires a more detailed analysis than what the thumb penetration test provides.