API REPORT 7 Characterization of Soil Behavior Under Cyclic Loading as Applied to the Foundation Design of Offshore Structures

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API REPORT 7 Document Information:

Title
Characterization of Soil Behavior Under Cyclic Loading as Applied to the Foundation Design of Offshore Structures

American Petroleum Institute

Publication Date:
Mar 1, 1977

Scope:

SYNOPSIS

This report contains the results of a 6 month research program conducted to study the comparison of cyclic strength of saturated soil measured in the cyclic triaxial and the cyclic simple shear tests.

The early part of the study dealt with perfecting a newly acquired cyclic simple shear apparatus. The results of this part of the study are not presented in detail. However, a summary of the results are presented to show that the relative strengths obtained by the cyclic triaxial and the cyclic simple shear test on a sand and on a clay soil are similar to the relative strengths that have been reported by others. The new simple shear apparatus is much easier and faster to operate than other previously used simple shear equipment. Thus, the favorable data which were obtained and the ease of using the apparatus make it an attractive future test apparatus.

The remainder of the study, which is reported here in complete detail concerns cyclic triaxial and cyclic simple shear test results from a special reconstituted silt soil. Comparative triaxial and simple shear tests were performed on samples consolidated to two normal stresses and three anisotropic shear stress conditions. The data showed that the cyclic simple shear strength was about 60 to 80% of the cyclic triaxial strength failure in about 30 cycles. If the cyclic stress was low so that several hundred cycles could be applied before the specimens failed, then the cyclic simple shear strengths were 90 to 100% of the cyclic triaxial strengths.

Static loading tests were conducted on most of the specimens after they had first been subjected to cyclic loading tests. The static-after-cyclic strength data were quite scattered. More strain was required to develop the full static strength after cyclic loading than required to reach the peak strength of an undisturbed sample. However, allowing sufficient static strain, in many cases the static-after-cyclic strength was as large as the static strength of undisturbed specimens.

The static strength of undisturbed specimens in the simple shear test was only about 35 to 45% of the static strength of undisturbed specimens measured in the triaxial test.

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