Question

Brief Research on following :

1) Pile end-bearing capacity in sand considering soil compressibility

2) End-bearing capacity of piles in crushable soils

3). Axial capacity of driven piles in sand

4) Settlement of vertically loaded piles

Answer 1

1) Pile end-bearing capacity in sand considering soil compressibility

In recent years offshore structures located on compressible carbonate sands have een shown to have unusually low pile bearing capacities in spite of the soils having relatively high friction angles. It has been recognized by Semple in 1988 that as a consequence of the high copressibility and strongly contractile nature of these sands, the values of end bearing capacity are likely to be much lower than for siliceous sands at the same relative density. It has become apparent terefore that pile end bearing capacities in sands are dependent on the soil's compressibility as well as its shear stiffness and strength. Compressibility varies widely for different soils, from relatively incompressible silica sands to highly compressible carbonate sands.

These days, non-displacement piles, such as cast in-situ piles are frequently used in urban areas because of noise and vibration considerations. A rational geotechnical method for the evaluation ofthe ultimate end bearing capacity and the load settlement curve of a nonn displacement pile is therefore proposed based on cavity expansion theory and relating to the sol characteristics and the mechanical failure mode below the pile tip.

2) End-bearing capacity of piles in crushable soils

The end-bearing capacity of piles in crushable soils is important for the design of foundations in soils as diverse as offshore skeletal carbonate sediments and highly weathered decomposed granite or residual soils. However, the analysis of foundations in these soils is problematic due to their compressibility and highly curved Mohr Coulomb failure envelopes, with friction angles, decreasing with increasing normal effective stress.

Carbonate sediments are mainly found on tropical and subtropical sea floors in the shallow waters of continental shelves. Since the early 1970s a number of problems have been encountered where offshore oil and gas production facilities have been founded on driven or drilled and grouted piles in these unpredictable carbonate formations. Experience with offshore in skeletal carbonate soils has indicated that conventional methods of geotechnical analysis do not reliably predict the performance of foundations in such soils (Randolph, 1988). In particular, piles driven into deposits containing skeletal sediments give unusually low bearing capacities despite high measured friction angles (Angemeer, Carlson & Klick, 1973). This is largely due to the crushability of the skeletal particles which results in low values of both skin friction and end-bearing capacity. In particular it has been recognized by Semple (1988) that as a consequence of the high compressibility and strongly contractile nature of these sands, the values of end-bearing capacity are likely to be much lower than for siliceous sands at the same relative density.

Problems with foundations in crushable soils are not, however, unique to carbonate sediments. Large areas of western Japan, for example, are covered by deposits of a highly crushable decomposed granite soil (Nishida, 1990; Murata, Hyodo & Yasufuku, 1990). It is likely that low pile endbearing capacities in these soils will also be related to their high compressibility. Coop (1990) demonstrated that the behaviour of carbonate sands when described in a critical state framework was similar to that of less crushable sands at relatively high stresses.

Thus, results from tests on crushable soils at relatively low stress levels is likely to give an indication of the behaviour of less crushable materials such as silica sands at very high stress levels. The limiting end-bearing resistance of a pile is usually related to the effective overburden pressure s_v such that

Q_p =s_v * N_q

Prandtl (1921) suggested that for general shear failure

N_q = tan² (45 + a/2)e ^{pi * tana}

However, the failure mode at a pile tip can be considered to be a local failure, in which case Terzaghi (1943) suggested that a’ should be reduced to tan^-1 [(2/3) tan a’]

Clearly, however, this type of formulation is inadequate for predicting the behaviour of a pile in a crushable soil, since it predicts a higher bearing capacity for soils with higher friction angles.

3). Axial capacity of driven piles in sand

Estimation of the axial capacity of driven piles in sand based on theoretical formulation still involves considerable uncertainty because the number of influencing factors is large and their effects are often interrelated and in some cases difficult to quantify. Correlations with in situ tests are also used. Although the in situ tests reflect to some extent actual soil conditions in the field, they have certain shortcomings. For example, the standard penetration test (SPT) does not reflect actual soil compressibility (Abu Kiefa 1998), and direct extrapolation of the results obtained from the cone penetration test (CPT) may lead to significant size or diameter effects (Meyerhof 1983). To overcome these difficulties, empirical formulae between soil parameters and back-calculated pile bearing capacity factors have been developed by many investigators. Unfortunately, most of the widely used empirical design methods for toe resistance and (or) shaft resistance do not consider the effect of the development of locked-in loads (residual loads during and following driving).

There is a need for new, high-quality field data on pile drivability and axial capacity in sand, particularly from piles of field scale, in order to help resolve some of these uncertainties. However, there is also a need for elucidation of the basic mechanisms that affect pile capacity, and for parametric studies using numerical and laboratory-scale physical models.

4) Settlement of vertically loaded piles

Settlement prediction is a critical issue in design of pile foundations, especially with the increasing use of settlement reduction piles. There are various methods for the prediction. However, the most efficient method is the BEM approach that is based on closed-form solutions for single piles. Settlements of several practical cases have been predicted using a BEM based program GASGROUP which was published a few years ago. The program was developed from closed-form solution for a single vertically loaded pile and use of pile-pile interaction factor that were also developed. Analyses of 12 pile groups are conducted for which measured settlements are available. The predicted settlement is within 7% of measured data for piles installed in normal consolidated soil. With regards to piles in overconsolidated clay, as anticipated, the settlement is overestimated due to negligence of the ground level modulus. The current solutions well capture the predominant impact of depth of underlying rigid layer, profile of shear modulus, etc by utilizing shear modulus profile theoretically deduced from load-settlement curve of a single pile, and/or using reported value directly. They are very efficient both in terms of preparing input data and the computation. Prediction of settlement about a large group (say, 697) piles can be fulfilled within a few minutes. Thereby, the program may be utilized for general design