Major exploratory trenches were made at the Power Block Building area of the Pebble Springs Nuclear Project to verify and evaluate the geological conditions interpreted from previous detailed subsurface investigations and geological field studies. Approximately 271,000 cubic yards of material was removed to excavate two long and deep trenches at right angles to each other near their mid-points. The exploratory trenches exposed undeformed late Pleistocene Glaciofluvial deposits and flatlying Tertiary units of middle Pliocene Dalles Formation, early Pliocene Rattlesnake Ridge member (Ellensburg Formation) and the top of the Pomona flow (Yakima Basalt). Volcaniclastic sediments of the Dallas Formation and the Rattlesnake Ridge member showed a variety of depositional features not previously recognized in boring samples. These included: (1) interfingering and lensing of lithologic units; (2) cut and fill structure; (3) grading and cross-bedding; (4) variation in degree of cementation; and (5) rapid change in grain sizes. The trenches did not reveal any tectonic structures detrimental to the nuclear plant siting and they eliminated the possibility of revealing unfavorable geologic conditions during construction. The performance of deep excavation cuts verified previous recommendations concerning the stability of slopes required for plant site construction. The materials obtained from the excavation were used to construct a test-fill along the alignment of the divider dike to evaluate compaction techniques. The trenches were left open so that potential construction bidders could observe and evaluate excavation conditions.

  • Supplemental Notes:
    • This article is an excerpt from the Proceedings of the Fourteenth Annual Engineering Geology and Soils Engineering Symposium that was sponsored by Idaho Transportation Department, Division of Highways; University of Idaho, Department of Geology and Department of Civil Engineering; Idaho State University, Department of Geology and Department of Engineering; and Boise State University, Department of Geology, and Department of Physical Science and Engineering. This symposium was held at the Rodeway Inn, Boise, Idaho, and was hosted by Boise State University.
  • Corporate Authors:

    Idaho Department of Highways

    P.O. Box 7129
    Boise, ID  United States  83707
  • Authors:
    • Farooqui, S M
    • Deacon, R J
    • Keener, Q R
  • Publication Date: 1976-4-9

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Filing Info

  • Accession Number: 00142679
  • Record Type: Publication
  • Created Date: Feb 1 1977 12:00AM