3478
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
uniform”. The minimum bearing capacity occurs at
.
3.4
Stochastic soil cases: variation of correlation length in
bottom layer
Comparison of the results of case 9, 10, 1, 11 and 12
indicates the correlation length effect of the bottom layer. It
again shows that increasing horizontal correlation length tends
to increase the standard deviation of the bearing capacity factor
(see Table 1 and Figure 6). However, the largest average
bearing capacity corresponds to the minimum correlation length
case (case 1:
). This differs to what is
occurring in the top layer. The maximum average bearing
capacity corresponds to the largest correlation length case 11,
which is consistent with the results of changing the correlation
length of the top layer.
Figure 7. Cumulative probability curves for variation of
correlation distance in the top layer (cases 1, 5, 6, 7 and 8)
Figure 8. Cumulative probability curves for variation of
correlation length in bottom layer (cases 9, 10, 1, 11 and 12)
4 CONCLUSIONS
In this study, finite element analysis of the vertical bearing
capacity of a strip footing penetrating stiff-over-soft clay was
conducted by taking the spatial variability of undrained strength
into account. The results indicate that with high spatial
variability in the undrained shear strength there is a significant
reduction in the bearing capacity. Mean bearing capacity factors
and statistical distributions were provided for 12 cases of
s
ut
/
s
ub
= 2, COV = 0.1 and 0.3, and
and
= 0.1, 1 and 10.
For the case of top layer thickness equal to the strip footing
width presented it was shown that variation in the top layer had
a greater effect on reducing the bearing capacity (when
correlation distance was held constant). This was due to the
unsymmetric bearing capacity shortening further into the top
layer.
The empirical probabilities of exceedence of the deterministic
bearing capacity factor in the stochastic case differ from case to
case, ranging from on the order of 10
-4
to 0.277, thus attesting
for the influence of the magnitude of spatial variability and
uncertainty on the effects of stochastic modelling. The
maximum value observed for case 7 is well below a “central”
value of 0.5; hence, overall, it is assessed that the deterministic
case is significantly unconservative from an engineering
standpoint.
The conclusions drawn in this paper may be specific for the
geometery and soil conditions analysed. The 12 cases presented
here, however, represent a small subset of 1600 cases analysed
in a more ambitious numerical experiment. Cases of (i)
= 4/3 and 2, (ii) COV = 0.1 and 0.3, (iii)
and
= 0.1, 1 and 10, as well as (iv) a gradient of increasing
undrained shear strength with depth, and (v) a footing
embedded to 0.5B into the top layer, make up the full
programme. The results of the larger study will be published in
due course.
5 ACKNOWLEDGEMENTS
This research is being undertaken with support from the
Australia-China Natural Gas Technology Partnership Fund and
The Lloyd’s Register Educational Trust.
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