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Title: Project Palanquin, preshot geologic and engineering properties investigations
Authors: U.S. Army Engineer Nuclear Cratering Group.
Nugent, Robert Charles, 1936-
Girucky, Frank E.
Keywords: Craters and cratering
Geology -- Nevada
Nevada Proving Grounds Area
Nuclear explosions
Pahute Mesa
Palanquin (Project)
Underground explosions
Geophysical survey
Publisher: U.S. Army Engineer Waterways Experiment Station.
Engineer Research and Development Center (U.S.)
Description: Miscellaneous Paper
Abstract: A comprehensive geologic and engineering investigation was undertaken at the site on Pahute Mesa selected for the Palanquin event, a cratering experiment conducted in dry rock at the U. S. Atomic Energy Commission's Nevada Test Site. Foundation conditions were determined by means of five borings satellitic to the emplacement hole: a 5-1/2 inch-diameter boring to a depth of 351.0 feet and four NX-size holes ranging in depth from 199.2 to 270,5 feet. The site is blanketed by a thin soil cover ranging in thickness from 4 to 13 feet, overlying a 575-foot-thick section of the Ribbon Cliff formation. The uppermost flow of this formation is porphyritic trachyte, which is at least 351 feet thick beneath the site. The Ribbon Cliff formation is immediately underlain by the vitric tuffs and welded tuffs of the Timber Mountain Tuff. Although normally a competent material, the porphyritic trachyte is severely altered and decomposed within zones of intense fracturing related to faulting. Subsurface data indicate the presence of a high-angle normal fault that strikes N27°E and dips 64 degrees to the northwest. This fault plane is enveloped by a zone of intense fracturing that is 73 feet thick. The base of this fracture zone parallels the fault plane and intersects the surface at a point 10 feet west of surface ground zero. Field observations indicate that four high-angle joint sets are developed within the Ribbon Cliff formation -- two major sets that have average strikes of N65°W and N55°E, and two minor sets that have average strikes of N16°W and N15°E. Low-angle bedding joints may be present locally. Joints of the dominant sets have an average spacing of less than 2 feet. Minor joint sets have spacings that vary from 3 to 10 feet. A comparison of surface observations with the results of a brief analysis of borehole photographs indicates a significant decrease in the frequency of joints with depth. The average physical property values of the porphyritic trachyte are as follows: dry bulk specific gravity, 2.49; dry bulk density, 155 pcf; static unconfined compressive strength, 13,370 psi; modulus of elasticity, 3.8 X 10⁶ psi; and Poisson's ratio, 0.26. The static unconfined compressive strength tends to increase with increasing density values. The average bulk density in the upper 100 feet was approximately 2 percent lower than average density values below a depth of 150 feet. Geophysical logging consisting of density and caliper logs disclosed that the in situ density values obtained were not quantitatively comparable to values determined in the laboratory due to the erratic variation in borehole diameter indicated by the caliper logs. Although it was not possible to establish the trend of in situ density with depth above a depth of 150 feet due to the erratic variations in borehole diameter, the trend of in situ density with depth below a depth of 150 feet appeared to approximate the trend indicated by physical tests, i.e. no major variation of density with depth. Pressure tests indicated that permeability of the media at the Palanquin site was approximately 35 x 10⁻⁴ cm/sec near the surface and decreased to practically zero at a depth of 110 feet. No pressure test data were obtained between depths of 110 and 139.5 feet. Pressure test data obtained between depths.of 139.5 and 203 feet were unreliable due to excessive head losses in the riser pipe. Between depths of 203 and 213 feet, the permeability was 27 X 10⁻⁴ cm/sec. No pressure test data were obtained below a depth of 213 feet.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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