DEVELOPMENT OF A MICROCRACKING TECHNIQUE FOR MEASURING IN SITU STRESS AND STRAIN. REPORT 2. FIELD TESTS.
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MISS
Pagination or Media Count:
A laboratory study of the relation between the magnitude of static and dynamic stresses to which hardened portland cement grout specimens were subjected and the degree of resulting internal microcracking in the specimens had indicated that this relation might permit use of grout cylinders as mechanical-type strain gages for measuring free-field strain in in situ material adjacent to high-yield explosions. This possibility was tested in conjunction with a high-explosive cratering shot detonated by the Air Force Weapons Laboratory. Before the test, 11 grout mixtures covering a range of strengths from 50 percent below to 50 percent above the stresses predicted to result from the 16,000-pound field explosion were proportioned in the laboratory, and 330 2- by 4-inch specimens were prepared. Static compressive tests were performed on specimens of each mixture at 7 and 28 days age, and unconfined, uniaxial, dynamic compression tests were performed at various increments of the estimated ultimate stress. The specimens were then sawed slices were prepared and photographed. The photographs were enlarged and examined to determine internal microcracking. In the field, three 5-12-inch-diameter by 42-foot-deep boreholes were drilled 20, 30, and 40 feet from the point of detonation. Specimens of the 11 grout mixtures were placed in the holes in a medium-matching grout, subjected to the blast, recovered except for about a third of the instruments which were lost in the crater, returned to the laboratory, and examined for extent of induced microcracking. The laboratory data were used for standard curves from which a correlation of the field data was made.
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