Civil Engineering Reference
In-Depth Information
Figure 18.10
Variability of observed values.
between the two. There is no standard definition for moderately conservative but rea-
sonable ones might be the value for which only 25% of the observations are smaller
or the value which is one standard deviation away from the mean.
The value chosen for design should be considered together with the value of the
factor of safety or load factor which will be used with it. Again there are no hard
and fast rules and these choices are a matter of engineering judgment. If you choose a
worst credible value of
φ
c
then you could choose a partial factor of 1.0 to go with it
but you would probably choose a mean value to determine the water content of your
lorry load of soil.
Remember that, if you are designing for something for which soil must fail, like a
tunnelling machine or an excavator, the worst credible strength is the
largest
value you
have measured reliably.
18.11 Summary
1. Analyses of slopes, foundations and walls require values for soil parameters and
factors of safety or load factors. These depend on the type of structure and on the
ground conditions.
2. A factor of safety is applied to soil strength and its purpose is to ensure that the
structure does not approach its ultimate limit state. A load factor is applied to a
load and its purpose is to ensure that movements are small.
3. A fundamental choice must be made between drained construction requiring
effective stress parameters and undrained construction requiring total stress
parameters.
4. The critical state strength with a factor of safety should be used to calculate an
ultimate limit state unless there is evidence of previous large movements which
have already reduced the strength to the residual. The peak strength with a load
factor should be used to calculate serviceability limit states.
5. Movements can be calculated using soil stiffness. This is non-linear and a value
should be chosen which corresponds to the mean of the strains in the ground.
