An investigation of factors affecting interface shear resistance between pipes and soils




Hussien, Ahmed Mustafa

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To develop a better understanding of the behavior at a pipe-soil interface under axial loading, this study investigated the effects of pipe curvature, time, and cyclic loading through the use of model tests. These tests were performed as part of two studies: 1) an investigation of the pipe-soil response for axially loaded flowlines and 2) an investigation on the effects of cyclic loading on pipe piles driven in sand.
To study the flowline-soil interactions, flat and curved model surfaces were installed horizontally in both highly plastic marine clay from the Gulf of Mexico and carbonaceous silty sand from the North West Shelf in Australia. The axial shear resistance between the soils and surfaces was measured using tilt table tests, cyclic medium-scale model pipe tests, and cyclic centrifuge-scale model pipe tests. By analyzing these tests, several conclusions were made: 1) no evidence of a wedging factor was observed, 2) a projected contact area can be used to interpret the stress distribution at the interface, 3) aging can increase the axial resistance but is lost with additional cyclic loading, and 4) the pipe-soil contact area may decrease with cyclic loading. To study the pipe-soil interactions of small-diameter piles in sand, piles were driven at Hornsby Bend in Austin, Texas and at Allen Harbor in North Kingston, Rhode Island. The capacity of the piles was predicted based on cone penetration test results and various methodologies. One group of piles was tested after about one week of aging and the rest were tested after aging for more than 70 days at Allen Harbor, or more than 140 days at Hornsby Bend. Three types of tests were performed: monotonic tensile load tests, low-frequency cyclic tests (to simulate normal operating conditions), and high-frequency cyclic tests (to simulate long-term operational effects). The main conclusions from this work are: 1) two of the five design methods provided estimates that were closest to the measured pile capacities, 2) the piles aged at Hornsby Bend and Allen Harbor experienced aging factors of about two and five, respectively, 3) no cyclic degradation or post-cyclic changes were observed at either site, 4) there were no signs of an interaction between aging and cyclic loading.


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