NUMERICAL STUDY OF NEGATIVE SKIN FRICTION PHENOMENON ON TAPERED PILES IN COHESIVE SOILS

The phenomenon of negative skin friction (NSF) is afriction force downward on the pile surface which oc-curs as a result of the settlement of the surroundingsoil and causes extra load on the pile. Hence, bearingcapacity of pile decreases and its settlement increases.Therefore, NSF is considered one of the problems in thedesign of piled foundations in soft soils. On the otherhand, by the advancement of geotechnical engineeringscience, a new type of piles was introduced called ta-pered piles.In this type of piles, by the increase of the depth, theirsection area decreases, and nally, the bottom of thepile will have a smaller cross-section compared to itstop side. Hence, in this research, by using ABAQUS nite-element software, a single pile with a uniform sec-tion is modeled in a clayey layer under the e ect ofNSF phenomenon. Afterward, by tapering the pile un-der various tapering angles and comparing the obtainedresults, the e ect of pile tapering on the NSF phenom-ena and also neutral plane location have been examined.In addition, in order to validate the numerical model-ing, an in-situ pile load test with the measured nega-tive skin friction value on a uniform pile in Bangkokwas simulated. Besides, in order to investigate the in u-ence of the pile cross-section shapes on the NSF, mod-eling of three type piles with a constant volume hasbeen conducted using di erent shapes including circu-lar, triangular and square cross-sections. The obtainedresults indicated that using tapered piles is economi-cally bene cial, unlike the granular soils; in cohesivesoils, the NSF values and neutral plane location are in-creased which can noticeably reduce the bearing capac-ity of piles. In addition, using the tapered piles with tri-angular cross-section causes a decrease in soil settlementaround tapered piles compared to other cross-sectionsand consequently the NSF could also be decreased. In-deed, using piles with a triangular cross-section whichhas more contact surface with soil, because of more cre-ation of friction between soil and pile compared to otherpiles, soil settlement in the vicinity of tapered pile isdecreased; following that, the negative skin friction alsodecreases.