Case studies selecting characteristic values

This section of the book presents a series of case studies that investigate the process of selecting a characteristic value (or values) from field and laboratory tests. The first study (see Section 5.4.1) considers two stiff over-consolidated clays found in London; the second (Section 5.4.2) looks at the soft marine clay found in Singapore; and the third (Section 5.4.3) considers dense, well-graded gravels at Gravesend in Kent.

5.4.1 London and Lambeth clays at Holborn

Figure 5.10 shows the results of more than a hundred standard penetration tests performed at a site in Holborn, London, where over six hundred piles were to be installed by conventional rotary bored techniques.20 The (uncorrected) blow counts increase steadily with depth in the London Clay (solid triangles) and then jump up in value in the Lambeth Clay (open triangles). This data is typical of many sites in London.

As part of our research for this book, we asked more than one hundred engineers and engineering geologists to select a characteristic line (or lines) through this data, on the basis of Eurocode 7's definition of the characteristic value as a 'cautious estimate of the value affecting the occurrence of the limit state'. The lines on Figure 5.10 show the outcome of this study.

In the London Clay (above about - 21m OD), most engineers selected a line that comes within a distinct and narrow band some seven blows wide; the blow count defined by this band increases with depth at a rate just over one blow per metre. Outside of this band are several extreme interpretations that appear difficult if not impossible to justify on the basis of available data.

SPT blow count

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SPT blow count

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