Evaluation of Negative Skin Friction on Bridge Abutment Piles

Piles are used to transfer loads of structures to deeper and stronger soil layers through skin friction and/or end bearing. Surcharge loads, site grading, or dewatering may induce downward movement of soil adjacent to piles installed in a compressible medium. This movement creates negative skin friction stresses acting downward at the pile-soil interface, which applies additional loads “drag forces” to the pile causing a maximum axial load in the pile shaft at the “neutral plane”. To evaluate the development of drag forces, a comprehensive field monitoring program was conducted over four years for three instrumented abutment H-piles as part of a three-span bridge project. The soil settlement and changes in pore water pressure in the soil adjacent to the piles due to the construction of an approach embankment were monitored using multiple-point extensometers and vibrating wire piezometers. The piles’ elastic settlement and strains were measured using single-point extensometers and vibrating wire strain gauges. The field measurements are presented and discussed in terms of responses time histories and load distribution along one pile shaft. In addition, the calculated forces from vibrating wire strain gauges are compared with the unified design method prediction considering the total stress method (α-method) for cohesive soils. The results show that the maximum drag force was developed after the complete dissipation of excess pore water pressure and that the location of neutral plane varied during the embankment construction stages. Employing the total stress method in the unified design method provided a reasonable prediction of the drag force and the neutral plane’s location.

Experimental Study on the Negative Skin Friction of the Pile Group Induced by Rising and Lowering the Groundwater Level

Negative skin friction (NSF) has been one of the important factors in the design of pile foundation; especially, the influence of water level on the pile negative skin friction should be paid attention. In this paper, a series of model tests were carried out to analyze the bearing characteristic of the pile group influenced by groundwater level. The pile axial force and negative skin friction, settlement, and soil pore pressure were investigated. The results showed that both the water level rising and lowering cycle could increase the axial force of the pile along the upper part of the pile, yet reducing it along the lower part of the pile; both the axial force and the negative skin friction of the pile presented a feature of time effect; the value of negative skin friction was positively correlated with that of the pile head load, and the neutral plane ranged from 0.57 L to 0.64 L as the water level changed; the soil featured settling in layers, and the change of pore water pressure was accordant with the water level changing regulation.

ANALISIS MODEL KEGAGALAN RUMAH 2 LANTAI AKIBAT NEGATIVE SKIN FRICTION DAN CURAH HUJAN YANG TINGGI

ABSTRACTOne of the problems that appear in a project is the presence of soft soil. In projects with soft soil types, it is necessary to manage the soil to increase the bearing capacity of soil. Soil subsidence on soft soil causes friction between the soil and the pile blanket, called the negative skin friction. Negative skin friction should not be neglected because it exerts a large enough force on the load that the pile must support. In addition, in designing an engineer must also pay attention to the optimal rainfall that will occur during the construction of a project and after that has an impact on the quality of the soil and foundation used. The topography of a project also needs to be considered, building a house on the edge of a slope can cause problems with slope instability. Increasing the load on the edge of the slope can reduce the safety factor of a slope. In this study, a modeling analysis will be carried out on a 2-story residential building that causes the house to collapse due to failure of the foundation design, the existence of negative frictional resistance on soft soil, and slope stability. ABSTRAKSalah satu permasalahan yang muncul pada suatu proyek adalah adanya tanah lunak. Pada proyek dengan jenis tanah lunak, perlu dilakukan perbaikan tanah untuk meningkatkan daya dukung tanah. Penurunan tanah pada tanah lunak menyebabkan gesekan antara tanah denan selimut tiang yang disebut fenomena tahanan friksi negatif. Tahanan friksi negatif tidak boleh diabaikan karena memberikan gaya yang cukup besar terhadap beban yang harus ditopang oleh tiang. Oleh karena itu, seorang insinyur harus memperhatikan dan mengetahui mengenai perilaku tanah lunak. Selain itu dalam mendesain, seorang insinyur juga harus memperhatikan mengenai curah hujan optimal yang akan terjadi selama pengerjaan suatu proyek dan setelahnya yang berdampak pada kualitas tanah dan fondasi yang digunakan. Topografi suatu proyek juga perlu diperhatikan, membangun rumah di tepi lereng dapat menyebabkan permasalahan pada ketidakstabilan lereng.

Model test study of bending moment and negative skin friction for batter rock-socketed piles under surface load

Batter rock-socketed piles (BRSP) foundation is one of common foundations, such as port engineering or cross-sea bridge, while there are few studies on negative skin friction for BRSP. A series of model tests are conducted to explore negative skin friction of BRSP which are embedded in thick soft clay. The effects of the inclined angle of piles and soil consolidation time to negative friction resistance and the bending moment of BRSP are analyzed. The test results show that: the development of negative friction and bending moment BRSP have pronounced time effect; the longer the consolidation time is, the slower the axial force and bending moment intensify. The ultimate pile shaft axial force and bending moment increases nonlinearly concerning the inclined angle of piles. And the “neutral point” position and peak point of bending moment is always located at 0.9~1.0 times soil depth.