مهندسی عمران شریف (Feb 2019)
NUMERICAL STUDY OF NEGATIVE SKIN FRICTION PHENOMENON ON TAPERED PILES IN COHESIVE SOILS
Abstract
The phenomenon of negative skin friction (NSF) is a friction force downward on the pile surface which occurs as a result of the settlement of the surrounding soil and causes extra load on the pile. Hence, bearing capacity of pile decreases and its settlement increases. Therefore, NSF is considered one of the problems in the design of piled foundations in soft soils. On the other hand, by the advancement of geotechnical engineering science, a new type of piles was introduced called tapered piles.In this type of piles, by the increase of the depth, their section area decreases, and finally, the bottom of the pile will have a smaller cross-section compared to its top side. Hence, in this research, by using ABAQUS finite-element software, a single pile with a uniform section is modeled in a clayey layer under the effect of NSF phenomenon. Afterward, by tapering the pile under various tapering angles and comparing the obtained results, the effect of pile tapering on the NSF phenomena and also neutral plane location have been examined. In addition, in order to validate the numerical modeling, an in-situ pile load test with the measured negative skin friction value on a uniform pile in Bangkok was simulated. Besides, in order to investigate the influence of the pile cross-section shapes on the NSF, modeling of three type piles with a constant volume has been conducted using different shapes including circular, triangular and square cross-sections. The obtained results indicated that using tapered piles is economically beneficial, unlike the granular soils; in cohesive soils, the NSF values and neutral plane location are increased which can noticeably reduce the bearing capacity of piles. In addition, using the tapered piles with triangular cross-section causes a decrease in soil settlement around tapered piles compared to other cross-sections and consequently the NSF could also be decreased. Indeed, using piles with a triangular cross-section which has more contact surface with soil, because of more creation of friction between soil and pile compared to other piles, soil settlement in the vicinity of tapered pile is decreased; following that, the negative skin friction also decreases.
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