2.2 UNDERSTANDINGS AND UNCERTAINTIES OF NSF
2.2.1 When We Need to Consider NSF
It is the general perception and practice that NSF needs to be accounted for when piles are installed through overlying soft clays with considerable thickness. However, two controversial issues still remain to be resolved, namely,
(1) Do we always need to design for NSF whenever there exists overlying soft clay layer?
(2) Does NSF occur only to pile installed through soft clays? Or is NSF ubiquitous to all piles installed in the ground regardless of soil types?
Some researchers have tried to explore situations when NSF does not exist despite the existence of soft clay deposits so that NSF could be ignored for an economical design of pile foundations. One way to achieve that may be to delay the pile installation until the consolidation of the clay is completed. For example, Milner (1957) reported a project in UK where piles were installed through a soft alluvium deposit approximately 7.6 m thick and founded 1.5 m into the underlying shale to support an access tunnel. The piles were sunk about a year before the backfilling of an earth
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mound of more than 7 m high was started. The ground surface settled about 457~610 mm under the weight of the backfill and dragged the piles to settle as much as 190 mm within 6 months which required costly remedial work. However, on a neighboring site, the sequence of construction was altered such that piles were installed after consolidation of the clay under backfill was essentially completed. It was found that overloading on piles due to negative skin friction had not occurred after this alteration of construction sequence. Ho and Mak (1994) also reported a long-term monitoring on piles driven into saprolite (a soft and friable rock) through an old reclamation fill which has been in place for more than 20 years. Measurements indicated that no significant NSF buildup in the long term after building occupation. They attributed this to the fact that primary consolidation of the marine deposits and alluvium under the old reclamation fill has completed before the installation of piles.
However, Milner’s (1957) successful story by delaying the pile installation to do away with or minimize the NSF is not supported by what was observed by Bjerrum (1969) of a test pile installed in an area with the fill in place for more than 70 years.
Despite the fact that the consolidation of the underlying deep soft clay had been completed 70 years after the filling, significant downdrag loads as large as those observed in a nearby site with large continuing soil settlement was observed to develop on the test pile. The very minor soil settlement due to the reconsolidation of remolded soil during pile driving as well as any minor continued consolidation soil settlement is what it takes to mobilize significant NSF on the pile. Thus, it is questionable whether adjusting construction sequence is effective in alleviating the effects of NSF.
Based on a case study of structural failure of timber piles due to NSF, Kog (1990) proposed a governing criterion when downdrag does not need to be accounted for even though large consolidation settlement is anticipated. He observed from the case history
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that even though substantial consolidation settlement was observed on site six years after the installation of the piles, structural failure of the timber piles was reported only for those lightly loaded piles. No failure was reported on the other piles with heavier loads. He postulated that NSF must be accounted for in the design when the applied load is less than the maximum downdrag load. When the applied load is larger than the maximum downdrag load, no provision needs to be made for negative skin friction.
Engineers may become hesitant to adopt Kog’s (1990) postulation on when the NSF could be disregarded after they looked at the field test results by Fellenius (1977).
As shown in Fig. 2.2, 495 days after the pile installation, a maximum downdrag load of about 570 kN was developed at the neutral point (NP) located at around 43 m below the ground surface. When a dead load of 440 kN was applied on the pile head, significant amount of NSF was cancelled out down to the depth of more than 30 m.
However, with time, significant amount of NSF began to accumulate along the pile shaft again. The maximum downdrag load on 859 days after the pile installation reached as large as 800 kN. Further pile head loading to 800 kN also temporarily cancelled out the NSF. But with time, large downdrag load began to accumulate along the pile shaft again which is well in excess of the applied load. It is thus obvious that large applied load may be able to cancel out the NSF on a pile, as assumed by Kog (1990). However, with time, any further soil settlement, which can be as small as 1 to 3 mm per year can induce very large downdrag load on the pile to well exceed the applied load. The situation would be worsened if any water drawdown or new backfilling occur on or adjacent to the site in the future. Thus, before further understanding of the mechanism of NSF, the proposal by Kog (1990) should be treated with caution.
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Contrary to some researchers who tried to explore scenarios whereby NSF could be ignored even if deep deposit of soft clay exists, Fellenius went to the other extreme and claimed that “all piles experience dragloads!” regardless of soil types (Fellenius, 1984).
His argument is based on the observation that very small displacement is able to mobilized large shear stress at the pile-soil interface. He argued that the pile material is immensely more rigid than that of the soils. With time, there will inevitably be some small settlement of the soil generating a small relative displacement between a pile and the soil that is sufficient to mobilize substantial negative skin friction along the piles.
However, it is noted that such postulation appears to be not well accepted by other researchers and practicing engineers whose perception is that NSF only occurs when there exists an overlying soft cohesive soil which is still consolidating. Terzaghi and Peck (1948), for example, explicitly stated that “if the subsoil consists of loose sand or other highly permeable and relatively incompressible soils, the effect of the fill on the piles can be disregarded.”
It appears that the above two schools of thought on the occurrence of NSF on piles had not been sufficiently substantiated by exploration of underlying fundamental mechanism, but based merely on limited field observations. In particular, the postulation by Fellenius that “all piles experience dragloads” irrespective of soil types appears to be not popular with practicing engineers, and in certain cases may lead to over-conservatism when determining the maximum load on the pile. This will be discussed in more detail later on.