A series of model tests on NSF on pile groups under variety surcharge loading were designed and conducted. During the tests, pile strain, settlement of pile and soil were measured. The results indicate that when side load increases from 20kPa to 120kPa, the neutral plane of the single pile varies from 0.82 L (length of pile in sand) to 0.93 L. Pile head settlement and negative skin friction under side load were smaller compared with the results of the test under uniform loading, while the location of neutral plane was lower.
Soft clay is a kind of soil with the high compressibility and widely located in coastal region. If the soft clay layer is subjected surcharge loading, the settlements of soil next to a pile will be more than those of the pile, negative skin friction (NSF) may occur. In general, NSF will lead to the development of additional compressive force in a pile (Fellenius 1998) and excessive pile settlement which could cause many severe problems, such as, excessive dragload on pile (Fleming, 2009; Kog, 1987 and 1990; Davisson, 1993), excessive pile downdrag settlement (Brand, 1975; Inoue, 1977). To minimize any possible damage, many researchers devoted themselves to the study of the problem of NSF.
According to Terzaghi and Peck (1948), NSF was first discovered in the coastal plains in Holland in which many buildings rest on piles driven through very soft strata to refusal in a bed of sand. The buildings would settle excessively because of the site covered by a thick layer of fill shortly before the piles were driven. Some similar phenomenon were also reported and discussed by Fleming (2009).
Considerable research studies have been conducted to facilitate the understanding of the mechanism of NSF on piles. These efforts include full scale field tests (Leung et al., 1991; Ho & Mak, 1994; Fellenius, 2006), reduced scale model tests at 1g laboratory condition (Shibata et al., 1982; Toma, 1989; Mehmet & Devrim, 1995;) as well as analytical and numerical approaches (Lee et al., 2006; Cheong & MacDonald, 2007). Recently, some centrifuge model studies have been carried out to research NSF (Leung et al., 2004; NG et al. 2008). Compared to traditional model tests with axial loading, the model test for NSF needs to apply load on soil surface and it is difficult to simulate large surcharge loading at 1g laboratory condition except centrifuge. As discussed above, these studies were very helpful for revealing the characteristics of NSF on pile and pile groups.