Figure 13.4. Single pile subject to downdrag

Consider a pile installed through a superficial layer as shown in Figure 13.4. Consolidation of the layer (owing, for example, to fill being placed upon it) will occur after the pile has been installed, resulting in additional loading being applied to the pile.

Soil-structure interaction analysis allows the 'neutral' depth (where the settlement of the consolidating matches that of the pile under load) to be determined, albeit approximately. In many situations, the effort involved in this type of analysis is outweighed by uncertainties in obtaining suitable ground parameters for use in the analysis.

It is more usual to account for downdrag by inclusion of an appropriate upper-bound action. The characteristic vertical compressive action Fck applied to the pile is then:

Fk = Pok + ^ + DGk where PGk and WGk are as defined in Section 13.4 and DGk is the characteristic downdrag acting on the pile (a permanent action). Note that when downdrag is included in this equation, any variable actions may be ignored (hence the absence of PQk). [en 1997-1 ยง7.3.2.2. (7)]

Typically, the consolidating layer is cohesive and downdrag is calculated from:

where a = an appropriate adhesion factor, cuk = the clay's characteristic undrained strength, and AsD = the surface area of the pile shaft in the consolidating layer. In selecting values for a and cuk, it is important to choose upper values so as to maximize the value of DGk.

Some guides8 recommend calculating the design value of downdrag (DGd) from:

where cud = the clay's design undrained strength and Ycu = a partial factor on strength (= 1.4 in Design Approach 1, Combination 1, and Design Approach 3 - see Chapter 6).

However, care must be taken when using this equation not to select the wrong characteristic value of undrained strength, as illustrated in Figure 13.5, which shows the probability of any particular cu value occurring in the field. In this diagram, cuk is a cautious estimate of the clay's 'inferior' undrained strength* (in this example, cuk = 15 kPa). Multiplying cuk by a partial factor Ycu = 1.4 results in a meaningless value for cud (= 21 kPa) indicated by the cross. Instead, it is the 'superior' value of the clay's strength cuk,sup (here = 25 kPa) that must be used, resulting in cud,sup = 35 kPa. In this instance the value of cud,sup appears excessively large, which may reflect the

*See Chapter 5 for discussion of 'inferior' and 'superior' strengths.

fact that the value of Ycu = 1-4 given in Eurocode 7 is used primarily to determine inferior design strengths (e.g. cudinf = 11 kPa).

Undrained shear strength (kPa) Figure 13.5. Selection of design undrained strength for downdrag

In our view, it is more sensible to select the design value of the upper characteristic strength directly (as allowed by Eurocode 7), rather than to calculate it from the characteristic value multiplied by a partial factor.

Although EN 1997-1 suggests downdrag should be considered in ultimate limit states, strictly speaking it is only relevant to serviceability limit states. Downdrag results in additional settlement of piles, which needs to be compared with the limiting total and differential settlements defined for the structure. In rare cases when piles are mainly end-bearing, downdrag may result in excessive compressive loads in the piles, leading to end-bearing failure in the ground or structural failure of the pile.

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