Lecturer :
Dyah Ayu Rahmawati Cupasindy, S.ST., M.T.
Frictional (Skin)
Resistance in Clay
Estimating the frictional (or skin) resistance of piles in clay is as difficult a task as estimating that in sand (see Section 12.13) due to the presence of several variables that cannot easily be quantified. Several methods for obtaining the unit frictional resistance of piles are described in the literature. We examine some of them next.
• 𝝀 𝑴𝒆𝒕𝒉𝒐𝒅
• 𝜶 𝑴𝒆𝒕𝒉𝒐𝒅
• 𝜷 𝑴𝒆𝒕𝒉𝒐𝒅
𝝀 𝑴𝒆𝒕𝒉𝒐𝒅
This is a total stress method, proposed by Vijayvergiya and Focht (1972), and is based on the assumption that the displacement of soil caused by pile driving results in a passive lateral pressure at any depth and that the average unit skin resistance is
where
𝜎𝑜′ = mean effective vertical stress for the entire embedment length cu = mean undrained shear strength (Ø’=0)
The value of 𝝀 decreases with the depth of penetration of the pile. (See Table 12.10.) Thus, the total frictional resistance may be calculated as
where A1, A2, A3, Á 5 areas of the vertical effective stress diagrams.
Figure 12.25 Application of l method in layered soil
where A1, A2, A3, Á 5 areas of the vertical effective stress diagrams.
Care should be taken in obtaining the values of 𝜎𝑜′ and cu in layered soil.
𝜶 𝑴𝒆𝒕𝒉𝒐𝒅
This is a total stress method where the unit skin resistance is expressed as
where a is an empirical adhesion factor that lies in the range of 0–1, decreasing with the undrained shear strength cu. Generally, overconsolidated clays have larger cu and hence lower a. Nevertheless, clay deposits at very large depths can be normally consolidated while having high cu due to the overburden stress. Here, a can still be closer to unity in spite of large cu. The stress history (i.e., OCR) comes into play here.
The approximate variation of the value of a is shown in Table 12.11. It is important to realize that the values of a given in Table 12.11 may vary somewhat, since a is actually a function of vertical effective stress and the undrained
cohesion. Sladen (1992) has shown that
where So
𝜎𝑜′ = average vertical effective stress
C ≈ 0.4 to 0.5 for bored piles; and ≥ 0.5 for driven piles
A correlation proposed by Randolph and Murphy (1987) was incorporated into the code of the American Petroleum Institute (API) in 1987 as
and
It was further modified by API (2007) as
Figure 12.26 Variation of a with cu/√𝜎𝑜′ for the NGI-99 method
Karlsrud et al. (2005) proposed an alternate relationship for a that is known as the Norwegian Geotechnical Institute (NGI)-99 method. According to this method
and
The term a has a log–linear relationship with cu/ √𝜎𝑜
′between cu/ √𝜎𝑜
′= 0.25 and 1. This is shown graphically in Figure 12.26. For cu/√𝜎𝑜
′≥ 1,
where C is the correction factor. The interpolated values of a for open-ended and closed-ended piles
are given in Table 12.12. The ultimate side resistance can thus be given as
𝜷 𝑴𝒆𝒕𝒉𝒐𝒅
When piles are driven into saturated clays, the pore water pressure in the soil around the piles increases.
The excess pore water pressure in normally consolidated clays may be four to six times cu.
However, within a month or so, this pressure gradually dissipates. Hence, the unit frictional resistance for the pile can be determined on the basis of the effective stress parameters of the clay in a remolded state (c’=0). Thus, at any depth,
where so
𝜎𝑜′ = vertical effective stress 𝛽 = K tan ∅′𝑅
∅′𝑅 = drained friction angle of remolded clay K = earth pressure coefficient
Conservatively, the magnitude of K is the earth pressure coefficient at rest, or
and
where OCR is the overconsolidation ratio. Combining Eqs. (12.62), (12.63), (12.64), and (12.65) for normally consolidated clays yields
and for overconsolidated clays,
With the value of f determined, the total frictional resistance may be evaluated as
Correlation with Cone Penetration Test Results
Nottingham and Schmertmann (1975) and Schmertmann (1978) found the correlation for unit skin friction in clay (with Ø=0) to be
The variation of 𝛼′with the frictional resistance fc is shown in Figure 12.27. Thus,
Figure 12.27 Variation of 𝛼′ with fc/pa for piles in clay (pa = atmospheric pressure = 100 kN/m²)