Authors: C. P. Wroth and D. M. Wood Authors Info & Affiliations. To a geologist, it is the material in the relative thin zone of the Earth's surface within which roots occur, and which are formed as the products of past surface processes. The tests required to obtain these parameters are also discussed Index Geogrids; friction factors, Grading,chart,effective size,influence on soil properties,standard grading divisions,test methods, 4–7 Group index, Hazen’s formula,Ice segregation, Lateral earth