2.2 UNDERSTANDINGS AND UNCERTAINTIES OF NSF
2.2.2 Movement Required for Mobilization of NSF
There are some contrasting field observations as to the magnitude of soil movement relative to the pile for the full mobilization of NSF on piles. Some field
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observations revealed that full mobilization of NSF were associated with very small soil settlement in the order of a couple of millimeters (see for example Bjerin, 1977;
Fellenius, 1972), while other full scale tests showed that downdrag force continued to increase with increasing soil settlement well beyond hundreds of millimeters (see for example Clemente, 1984; Indraratna, 1992).
Bjerin (1977) observed that NSF was fully mobilized to a depth of about 25 m after a relative displacement of about 5 mm as measured at about 0.12 m away from the pile.
Bjerrum et al. (1969) reported a pile installed in an area in the Harbor of Oslo where a 13-m deep fill has been in place for more than 70 years and the consolidation of the underlying 27-m deep soft clay had been completed with remaining rate of settlement as small as 1 ~2 mm per year. However, due to this small regional soil subsidence as well as the effects of pile driving, downdrag load as large as 2500 kN developed along the pile shaft after the pile installation. He concluded that “negative friction developed very quickly and only small relative movements were required to fully develop its maximum value”. Fellenius (1972) measured the NSF developed in a pile installed through 40 m of soft clay and embedded 15 m into the underlying stiff soil. About 180 days after pile installation, the dissipation of excess pore pressure caused by pile driving generated about 300 kN dragload on the pile. The ground settlements which caused this dragload were as small as 2~3 mm.
The above field observations are in line with Terzaghi and Peck’s (1948) postulation that “an imperceptible downward movement of the fill with respect to the piles is sufficient to transfer onto the piles the weight of all the fill located within the cluster” to induce NSF on piles. In a more fundamental study, Alonso (1984) observed in the laboratory that for low plasticity silty clay, the maximum shear stress is mobilized at a relative displacement of about 2.5 mm, with about 75% of the
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maximum shear stress mobilized at a relative displacement of 1 mm. In another study by Clemence and Brumund (1975) on sand, the mobilization of most of the shear stress occurred at displacements of less than 1 mm.
The observation that a very small relative movement between the soil and the pile is sufficient to mobilize full or major proportion of shear stress along the pile shaft, or even reverse the direction of the shear force from positive to negative has some major implications. For example, Fellenius postulated based on this observation that all piles will experience dragloads regardless of soil conditions and that transient live loads will never superimpose with downdrag loads at NP. More details of these postulations will be discussed later.
However, when it comes to the scenario whereby the negative skin friction on pile is induced by surcharge loading, development of negative skin friction can be associated with very large soil settlement. For example, Indraratna et al. (1992) and Fukuya et al. (1982) revealed that negative skin friction continued to increase with ground settlement well beyond 100 mm. Lee and Lumb (1982) reported that the maximum downdrag load did not achieve until the ground settlement reached about 400 mm. In another field study, Clemente (1981) observed that the downdrag load increased in tandem with soil settlement exceeding 1000 mm.
Recently, Leung et al. (2004) presented centrifuge model tests on a single pile with a diameter of 1.6 m installed through 16 m of soft clay and socketed 2.5 m into the underlying dense sand layer (all in prototype scale). The NSF was induced by the consolidation of the soft clay due to the enhanced self-weight of the soft clay when spun to 100 g. The ground settlement was observed to increase with time and the excess pore water pressure was observed to dissipate after a period of about 83 months (in prototype scale). During this period, the soil settlement keeps on increasing with
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reducing rates. A final soil settlement of about 1060 mm was recorded at the end of consolidation. The downdrag load as well as settlement on the pile continued to increase in tandem with soil settlement and reached about 780 kN and 13 mm, respectively, 30 months after the start of consolidation with a corresponding soil settlement of about 960 mm. Although thereafter the soil continues to settle by another 100 mm, this further soil settlement does not induce any additional dragload and downdrag settlement on the pile.
It can be seen that those who observed a very small magnitude of soil movement for the full mobilization of skin friction are mainly restricted to test conditions that either the soil strength remained unchanged during the test (see for example Clemence and Brumund, 1975; Alonso, 1984), or the NSF was induced by the re-consolidation of remolded soils due to pile driving which were typically confined within several pile diameters around the pile and thus normally did not lead to substantial soil settlement in a large area. On the other hand, in the case of surcharge loading, the dissipation of excessive pore water pressure is normally accompanied with large consolidation settlement. The continuing dissipation of pore pressure lead to ever-increasing effective stress in the soil, which in turn leads to an increase of shear strength at the pile-soil interface and thus an increase in the dragload on the piles.
It appears that in the latter scenario, after substantial soil settlement, the development of NSF tends to stabilize although the soil may continue to settle further (see for example Leung et al., 2004). There may be two plausible explanations for this phenomenon as follows:
(1) One possible reason is that primary soil consolidation has been completed with accompanying large soil settlement. The continued soil settlement is only due to secondary consolidation with constant soil strength. Thus the pile settlement
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as well as downdrag load on the pile remains essentially constant. However, in this regard, it may be worthwhile to mention an unusual test observation presented by Walker and Darval (1973) in a field test in which the NSF was induced by 3 m backfill. At 10 days after the start of filling, the settlement of the ground surface was in linear relationship with log time, indicating that the soil settlement was due to secondary consolidation (soil creep). However, the measured downdrag load has actually increased by about 30% under constant effective stresses with constant peizometer readings. This observation is quite unusual and obviously deviates from the present understanding of the development of downdrag loads. Despite its over-consolidated nature, the conclusion that the primary consolidation of the 15-m thick firm silty clay completed immediately after the completion of the 3-m backfill which took 10 days appears unusual, and thus this case history must be viewed with caution.
(2) Another plausible explanation is that after large soil deformation, strain softening begin to dominate at the pile-soil interface. On one hand, the shear strength at the pile-soil interface tends to reduce towards the residual value;
while on the other hand, the continuing consolidation and increase of effective stress tend to increase the shear strength at the pile-soil interface. The shear strength along the pile-soil interface may reach some point when the net effect of the above two opposite processes turn out to cancel out each other and results in a shear strength at the pile-soil interface more or less constant, despite continuing soil settlement. The strain softening effect and possible reduction of shear strength to the residual value at the pile-soil interface under large soil settlement has been proposed by Tomlinson (1994), but not verified by any test data.