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Figure 5.10. Results of standard penetration tests in London and Lambeth clays, with engineers' interpretation of the characteristic value

In the Lambeth Clay (below - 21m OD), a variety of interpretations were submitted. Some involve a linear increase in blow count with depth (at a rate approaching 3 blows/m); others assume a constant value between 50 and 70. Figure 5.10 shows that engineers' interpretation of data becomes more scattered where that data is less certain (owing to its greater variability and paucity).

Figure 5.11 shows the results of more than a hundred undrained triaxial compression tests performed in the laboratory on U100 samples taken from nine different boreholes from the site in Holborn discussed above. The undrained shear strength measured in both clays is highly scattered, caused in large part by sample disturbance prior to laboratory testing.

Figure 5.11. Results of triaxial compression tests in London and Lambeth clays, with engineers' interpretation of the characteristic value

The dashed lines on Figure 5.11 indicate theoretical limits to the undrained strength, assuming the following simple relationship between cu, effective overburden pressure ov', and over-consolidation ratio (OCR):

cu

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