Lunar Soil Mechanical Modelling

In September 1968, I found myself in his office at MacLaughin Hall for the first meeting and I was extremely excited to get to know him. Working as a research assistant, I was indeed very lucky to have him as my advisor as well. When it was time to pick two minor areas of study I decided to go with Engineering Analysis and Ground Water Flow Through Porous Media. In fact both areas required very high background of mathematics. Knowing that I had an extremely strong background thanks to the years I spent in Turkey for my highschool and undergraduate education, I reassured Jim by telling I could handle This task easily.

After completing the courses of both minors with great success in April 1970, knowing my strong background in math, Jim asked me if I would be interested in working in a research project on the mechanical modelling of Lunar soils with him. I was delighted and immediately after we started to find the practical means for the solution of this task. It was clear that, to employ astronaut operated mechanical cone was practical to obtain c-O shear strength relationship of lunar soils. Jim has pointed out remarkably that if a trench is excavated with vertical side and fail by a plate pushed down by the astronaut, analysis of this problem knowing the load to cause the failure would yield another c-O relationship. Obviously intersecting two separate c-O relationships obtained by these two experiments would be sufficient to estimate c and O independently.

Our task was to determine the failure mechanism of mechanical cone in c-O soils and based on that develop tip resistance of cone as a function of cone apex angle, cone surface friction (roughness), depth and lateral earth pressure. Using this approach , we have developed the Durgunoglu & Mitchell method (1) and using it was possible to estimate shear strength parameters of lunar soil and their variations with depth. Once compared with USSR reported values obtained previously with a unmanned craft Lunokhod, there was a considerable difference. Evidently, analyzing the vane shear results of Lunokhod, Russians made a frictionless soil assumption in estimating the c value. As a result c values reported by them were highly overestimated. In order to compare USSR and US results in terms of c values we have utilized in the analysis for the Lunokhod vane tests taking soil friction as well. Consequently results of independent missions were remarkably similar.!!!

On April 6, 1973 , NASA awarded Jim with the “NASA Exceptional Scientific Achievement Medal” at the Apollo Honor Awards Ceremony in recognition of Soil Mechanics Experiment. He sent me a letter on April 11, 1973, which I still keep on the wall of my office , stating that my special contributions to the analysis of static penetration resistance were especially valuable for the success of the mission.

(1) Apollo Preliminary Science Report, NASA SP-315, Chapter8. Soil Mechanics, James K. Mitchell and et. al. 1972, Washington, D.C.