OAR 690-020-0036: Design Requirements for New Dams or to Increase Dam Height: Site Suitability and Geotechnical Evaluation

The design to build a new Dam or to increase Dam Height shall characterize the soil and rock at and around the Dam site and shall include the following elements: (1) A description of the general and local geology and geomorphology at and around the proposed Dam and reservoir site; (a) Field investigation by a geotechnical Engineer or engineering geologist or both is required for Dams rated High Hazard. For Dams rated Significant Hazard, field investigation by a geotechnical engineer or engineering geologist or both is required where landslides, faults, dispersive soils, or liquefiable soils could reasonably be expected near or at the Dam site. All such features shall be shown on a map of the Dam site and be described as necessary for design of the Dam; (b) For Dams on located on rock, a drawing must also contain mapping of discontinuities relevant to the safety of the Dam and include evaluation of whether grouting is required; (2) A subsurface investigation to determine the distribution of relevant earth materials, which shall include borings or test pits; identification of springs, seeps, and groundwater encountered at the Dam site; and evaluation of the potential for landslides into the Dam or reservoir; (a) All materials shall be logged by the Unified Soil Classification System; blow counts (for borings only); and include a description of samples taken for testing; (b) Subsurface investigations for High Hazard Dams shall include drilling to a minimum depth of 1.5 times the Dam Height or at least ten feet into bedrock, whichever is less; (3) An evaluation of soil and rock and the testing of relevant materials, which may include: proctor compaction testing from all borrow areas, estimation or testing the permeability of soils to be used in Dam construction, and an assessment for the presence of dispersive soils. There must be a sufficient number of tests to characterize the variability in each borrow area. In addition, an evaluation must contain the following information as applicable and as may be required by the State Engineer: (a) An analysis of materials in the Foundation and proposed Embankment if materials are prone to liquefaction or significant settlement; (b) Where suitable materials can be collected, strength tests shall be required for all High Hazard Dams, and may be required by the State Engineer for Significant Hazard Dams; (c) Testing of dynamic soil properties may be required for High Hazard Dams in areas with large ground acceleration potential from earthquake loading, if soils have potential for significant strength loss upon seismic loading; (4) Borrow area locations. Areas proposed for borrow shall be identified and shown on the drawings; (5) Earthquake considerations. Seismic site characterization is required for High Hazard Dams, and may be required for Significant Hazard Dams. A seismic site characterization shall include earthquake sources, ground motion hazard, peak ground acceleration, and recommended ground motions (time histories); and (6) Site preparation criteria. The site evaluation shall recommend a depth of stripping unsuitable materials, and also a minimum, and where necessary, maximum depth for the Cutoff Trench.