Weathering of Igneous, Metamorphic, and Sedimentary Rocks in a Semi-Arid Climate - Clay Mineralogy in a Geological Engineering Context

Article excerpt

ABSTRACT

The identification of clay mineral assemblages in soils provides a unique opportunity to demonstrate how basic principles of mineralogy, petrology, and geochemistry are applied to engineering design criteria in construction site preparation. Specifically, the laboratory exercise presented investigates the conditions leading to the formation of smectite in soils and the resulting construction risk due to soil expansion. Students examine soils developed on igneous, metamorphic, and sedimentary rocks near Golden, Colorado. The field locations are areas of suburban growth and several have expansive soil problems. This 2-3 week exercise accomplishes three objectives: First, existing skills in XRD analysis are reinforced and new skills are developed by introducing students to concepts of particle size separation, particle orientation, and sequential analysis steps which are standard practices in clay characterization. Second, lecture material on the geochemistry of weathering of different rock types is applied through a local, practical example. Third, the role of petrologic characterization in site engineering is demonstrated. Students use Atterberg Limits measurements in conjunction with soil mineralogy to assess swelling potential and to design soil treatment needs for each building site. Qualitative and quantitative assessment and evaluation of student learning using a pilot class and two control groups was undertaken using written final reports and final exam questions. Written reports demonstrate proficiency in several lower and higher level concepts encompassed in ABET, Inc. Engineering Criterion 3, however, the use of traditional end-of-semester exams did not capture the knowledge gained by the students completing this problem-based learning exercise.

INTRODUCTION

The geological engineering undergraduate degree at Colorado School of Mines is one of 13 such programs nationwide. The ABET (Accreditation Board for Engineering and Technology)-accredited curriculum and the enrolled student profile are distinctively different from those found in non-engineering earth science degrees. The program's geoscience faculty are challenged to take advantage of the quantitative skills of the students and their interest in applied problem solving yet also accommodate their credit-hour intensive curriculum that has no elective geology components. Over the last 12 years, we have introduced analytical methods to geological engineering students through a multi-year and multi-course approach. Beginning with principles and simple applications of XRD and SEM in sophomore mineralogy and building on these skills in subsequent junior and senior year courses, geological engineers acquire proficiency in analytical methods within the framework of existing courses in geology and geological engineering (Wendlandt and Harrison 1995). Essential workplace sKills in mineral and rock analysis are thus acquired without adding an extra course in the undergraduate program. This is a necessity imposed by the large number of credit hours (136) in the geological engineering undergraduate curriculum and the structured nature of course sequencing in math and sciences, engineering, geology, and geological engineering. Tne majority of geological engineers enter the workplace with a BS degree, thus there is no opportunity for them to acquire familiarity with analytical methods except through subsequent graduate coursework.

The laboratory exercise presented in this paper is completed by juniors in an integrated petrology course (igneous, sedimentary, and metamorphic). This exercise provides a problem-based approach to learning clay mineralogy and substitutes for our traditional lecture-laboratory-homework approach. Students characterize clay mineralogy of soils developed on igneous, metamorphic, and sedimentary rocks in Golden, Colorado. The field locations are areas of suburban growth and several have expansive soil problems. The characterization of clay mineral assemblages in young, immature soils provides a unique opportunity to demonstrate how basic principles of mineralogy, petrology, and geochemistry are applied to engineering properties of soils and design criteria in construction site preparation. …