Engineering Stratigraphic Columns

Abstract

A dilemma in engineering geologic mapping is how to provide the greatest amount of useful information in the smallest space and in the most visually logical format. Stratigraphic columns, traditionally used to show strictly geologic information, may be adopted to address engineering and hazard identification concerns. Columns can be constructed to show fracture density, rock mass rating, resistance to weathering, soil development, water flow, and other useful parameters for rock cuts, tunnels, foundations, and aggregate potential. Each column summarizes the anticipated engineering behavior of a given vertical section of the local bedrock, and this behavior may be extrapolated, with due care, to other locales where the same section of rock is present. We have used engineering stratigraphic columns for several years as teaching tools (Santi and Gregg, 1999), but they can also serve as concise local or regional reference tools for engineering geologists, much as traditional geologists reference standard stratigraphic columns. Standard stratigraphic columns typically correlate the width of the column to the grain size of each unit, as shown on the example in Figure 1a, or to the resistance to weathering of each unit. A variety of geologic, sedimentological, and genetic information may be displayed, including bedding, nodules, minor mineralogical components, fossils, bioturbation, and other sedimentary structures. If included, the descriptive text adjacent to the column usually details formation name; lithology; range in formation thickness; rock type; minor beds; and information on fossils, color, and sedimentary structures. Standard columns emphasize vertical changes in the rock sequence, at the expense of recognizing lateral variations (Tucker, 1996). They also focus on rock descriptions and ignore soil units. Figure 1. (a) Example stratigraphic column of the Roubidoux Sandstone–Gasconade Dolomite contact near Rolla, MO (prepared by the authors). (b) Example engineering stratigraphic column Very little work has been done on the use …