Engineering Geology I GED 355 S Bansah August

Engineering Geology I (GED 355) S. Bansah, August 2010 Engineering Geological Maps & Plans

Definition • examination of natural and manmade exposures of rock or unconsolidated materials • systematic recording of geologic data from these exposures • analysis and interpretation of these data in two- or three-dimensional format (maps, cross sections, and perspective [block] diagrams)

Uses of Geol. Maps & Sections maps and cross sections generated from geologic data 1. serve as a record of the location of factual data; 2. present a graphic picture of the conceptual model of the study area based on the available factual data; and 3. serve as tools for solving three-dimensional problems related to the design, construction, and/or maintenance of engineered structures or site characterization

Responsibilities of the Engineering Geologist in Mapping • Integrating all the available, pertinent geologic data into a rational, interpretive, threedimensional conceptual model of the study area and • presenting this conceptual model to design and construction engineers, other geologists, hydrologists, site managers, and contractors in a form that can be understood

General items to be Included in an Engineering Geological Map 1. Lithology: Differentiate between the various units and state the engineering characteristics of each unit 2. Geologic contacts: Use solid lines to denote contacts where the exposures are excellent and a high degree of accuracy can be prescribed. Use broken lines when unsure of the accurate location of the contact

3. Discontinuities: State the type of discontinuity (eg. Fracture, joint, bedding plane, shear, fault etc) and describe the engineering property (eg. Fracture orientation, spacing, openness, fillings etc) of each 4. Weathering and alteration: Describe the mechanical and chemical alteration of the materials and their engineering properties (eg. Degree and depth of weathering, mineralogical composition of altered material, hardness/strength etc)

5. Water: State the location and amount of groundwater to be expected in an excavation and how the water can be controlled 6. Geomorphology: Describe the landform. This is important in interpreting the geologic history and structure. 7. Vegetation: Describe the vegetation types and patterns. This can provide indirect data on lithologies, discontinuities, weathering, groundwater, and mineralization

8. Cultural features (manmade): Review features such as pipelines of water, sewage, electrical, gas etc. To save money, the utility services can provide such information • The map should be given a final field check by an independent reviewer in order to check the interpretation of geologic concepts used by the mapper

Phases of Engineering Geological Mapping • Mapping prior to construction • Mapping during construction

Mapping prior to construction 1. Preliminary surface geological mapping: • The purpose is to define the major geologic units structures in the site area and the general engineering properties of the units • Serves as pre requisite for other investigative techniques such as trenching and core drilling • Scale can vary between 1: 24000 and 1: 62500

2. Detailed Surface Geologic Mapping: • purpose is to define the regional geology and site geology in sufficient detail • detailed descriptions of the engineering properties of the site geologic units are compiled • Map scales are usually larger than the preliminary map • The preliminary surface geology maps are used to select sites for dozer trenches, backhoe trenches, and drill core holes

3. Trench Mapping • Dozer/backhoe trenches are cut to expose rock or unconsolidated materials below the surface • Structures such as contacts and shear zones must be traceable from wall into floor for optimum determination of their nature and attitude • Scales are larger to reveal critical details. Scales such as 1: 200, 1: 100, 1: 50 are common • Trench holes should be left for re inspection unless it poses a hazard

Trench Mapping

Construction Geologic Mapping Necessary to 1. identify and delineate potential construction-related needs and problems 2. verify and better define geologic interpretations made during design studies 3. determine if the geologic conditions are as interpreted during the design phase. If not, design modifications may be required 4. provide a record of as-built conditions in the event of litigations

Construction Geologic Mapping • Guidelines for mapping of specific features are prepared before construction begins Egs include cutslopes geology maps: for determining slope stability problems and selecting slope support systems Canal and pipeline maps: to determine the final canal/pipeline alignment and all associated requirements for specifications and construction Tunnel maps

Plans and Sections • Vertical sections commonly coincident with a wall of a tunnel or shaft are made • Geologic features are projected to the sections and plotted as they are mapped • two types; measured section and constructed section

Measured/Constructed Sections Measured sections • Data is obtained from actual measurements • the section is drawn to scale Constructed sections • data obtained from pre existing geological and structural maps • actual data could not be obtained due to the complexity of the subsurface • Section lines need not always be single straight lines. They can be deflected to pass through various sites or sections of interest

Geologic Cross Section

Advantages/Disadvantages of the Plan and Section Method Advantages • Produces two dimensional sections and plans that don't require conversion • Easily understood by individuals not familiar with 3 D periphery mapping Disadvantage • Trace of planar features are projected to a plane. - section walls are usually curved in practice

Further readings • Jackson, Julia A. , editor, The Glossary of Geology, 4 th edition, American Geologic Institute, Alexandria, VA, 1997 • Lahee, Frederic H. , Field Geology, 6 th edition, Mc. Graw-Hill, Inc. , New York, NY, 1961