What does the thumb penetration test estimate in cohesive soils?

The thumb penetration test is primarily used to estimate the unconfined compressive strength of cohesive soils. This test involves the application of pressure by thumb or finger on a soil sample and evaluating how easily it deforms or penetrates under that pressure. The degree of penetration is directly related to the soil's strength; a greater penetration indicates lower strength, while less penetration signifies higher strength.

This quick and simple method is beneficial in field conditions, allowing for rapid assessment without the need for complex equipment. While other soil properties such as density, plasticity, and moisture content can be evaluated through various methods, the thumb penetration test is specifically tailored to gauge the unconfined compressive strength, making it a valuable tool for geotechnical engineers evaluating soil behavior under load.