A study evaluating students’ long term understanding of effective stress and suggestions for its improvement

The effective stress principle is the crucible of soil mechanics; it controls all soil behaviours of interest to the geotechnical engineer. Since its formal postulation by Terzaghi in the 1920’s, it has become core to every course in soil mechanics. Applying the effective stress principle in ground conditions that differ from the simple profiles presented when the subject is first introduced poses many difficulties for students. The simplicity of the principle means it can be covered in just a few pages of a textbook but this does little to promote understanding of the subtleties entwined in the principle. For example, issues such as the influence of static, flowing or capillary water on effective stress are rarely covered in a single location in textbooks, nor are these influences linked to the role they play in geotechnical design. This paper presents the findings of a study that evaluates the ability of undergraduate students to apply the effective stress principle in various geotechnical designs. Students in the study were taught the principle in an introductory soil mechanics module and their understanding was evaluated at the start of a follow-on soils module that commences after the summer recess. The results indicate that only a hazy recollection of effective stress remains and imply that careful attention to the teaching approaches employed are necessary if students are to retain their knowledge and proficiency in applying the effective stress principle in geotechnical design. Suggestions for enhancing learning in this area are provided.