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Sum and mean of depth values z =-383.2 m , mz z =-383.2 m , mz =-=-15.97 m n o

Sum of squared deviations from mean depth ( - mz)2 = 139.1 m2

Sum of cross deviations from mean [( - m^K - mz) = -111.8 m i=1

Slope of best-fit line b i = 1

Standard error is se

5.7.2 Undrained triaxial compression tests on London Clay

Example 5.2 applies statistical analysis to the triaxial test data presented in Figure 5.11 (see p. 145) for the site at Holborn, London. Only data for London Clay is considered here (i.e. the solid black symbols on Figure 5.11). Figure 5.18 shows the outcome of the analysis.

Characteristic values (variance unknown)

I 50 100 150 200 250 300

Characteristic values (variance unknown)

I 50 100 150 200 250 300

Undrained strength (kPa) Figure 5.18. Outcome of statistical analysis of undrained triaxial compression tests in London Clay

Notes on Example 5.2

There is much more data shown in Figure 5.18 than is normally available from typical site investigations in stiff clay. Undrained strengths derived from triaxial compression tests can vary considerably even for samples from the same horizon. Traditional techniques using light cable percussion drilling rigs and U100 tubes do not obtain samples that are truly adequate for strength testing. Since such investigations are common, engineers must be able to judge the soil's characteristic strength from such scattered results.

Example 5.2

Undrained triaxial compression tests on London Clay Determination of characteristic undrained strength

Assuming normal distribution of strength vs depth

Number of tests is n = 91

Sum and mean of strengths cu = 12688 kPa and

Sum and mean of depths X z = -1763.45 m and mz --- -19.38 m n

Sum squared deviations from mean depth (i - mz)2 = 8000m2

Sum cross deviations from mean i = 1 n

Slope of best-fit regression b =

Standard error of regression line

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