ASDSO Dam Safety Toolbox

Geologic History

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“For appraisal studies, considerable useful engineering information may be obtained from published geological maps. These maps identify the rock units and geologic structures underlying the reservoir and damsite and in the surrounding area."[1]

“The geologic maps and sections should be accompanied by a report describing the various geologic conditions, including bedrock and soil classifications and the types of cementing materials that may occur in the rock and soil. The appraisal stage report should discuss the relationship of the geologic conditions to the present and future permeability of the reservoir and dam foundation, and to the future stability and performance of the dam, spillway, and other structures. Readily apparent geologic problems requiring further investigations also should be discussed, and a tentative program outlining the extent and character of more detailed explorations for the feasibility stage of investigation should be recommended. The reservoir and dam site area should be examined for potential landslides that could be activated by construction."[1]

“During construction, geologic drawings should be revised or new drawings completed to show the conditions actually revealed in construction excavations. Such a map may prove to be an invaluable ‘as-built’ record of operation and maintenance if geologic problems, such as anomalies in foundation behavior or excessive seepage, are encountered later.”[1]

“Data gathered during the reconnaissance are primarily descriptive and should include the following:

  • “General geology of the site: surface and subsurface earth materials
  • “Geologic conditions that may affect erosion and sedimentation
  • “Geologic conditions that may influence groundwater movement and recharge
  • “General character of topography and stream system
  • “Engineering properties of soil and rock materials
  • “Presence and activity of faults
  • “General relationship between the geologic conditions at the site and known or typical design requirements for the proposed conservation practice or structure
  • “Known or projected mining activities and oil or gas extraction” [2]

See also: Geotechnical and Geology > Geotechnical Exploration > Site Reconnaissance

Best Practices Resources

National Engineering Handbook: Chapter 2 - Engineering Geologic Investigations, NRCS

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 5- Geotechnical Investigations and Studies, FERC

Design of Small Dams, USBR


Revision ID: 7101
Revision Date: 07/11/2023